1.1 Electrical systems and measurements

All electrical systems are individually identified in terms of their typical characteristics namely voltage, current, power factor, energy, power, etc to distinguish each system from others. The above electrical units defining the characteristics of an electrical system depend on the two main basic parameters of these systems namely, voltage and current. The various other electrical units can be derived and ascertained using these basic two parameters.

Table 1.1 gives the various parameters that are commonly used to define the characteristics of an electrical system along with the main inputs used for measuring the values of these characteristics/ units.

There are also some other electrical units like capacitance, inductance, etc that are dependent on the properties of the material of conductors and construction/application of equipment. These affect the main parameters voltage and current and hence all the other parameters indicated above.

It is well known that electrical power is normally consumed in the form of AC or DC (Alternating Current and Direct Current). Hence measurements are required for both AC and DC systems. The following paragraphs give a brief on the fundamentals of these two systems.

1.1.1 Alternating current system

The power generators in power stations generate AC voltages, which follow sinusoidal waveforms alternating between positive and negative values around a zero (X) axis, due to the angular movement of rotors and the magnetic field. In simple terms, if Vp is the maximum voltage attained at a generator terminal then –Vp is the minimum voltage attained at the same terminal with respect to zero in one complete cycle. The magnitude of voltage V at any time t alternating at a frequency ‘f’ cycles/second, can be mathematically expressed as

V = V_p Sin (2π × f × t) ……………………. (1.1)

This follows the curve as shown in Figure 1.1.

Figure 1.1 shows the waveform of a single phase AC system. Three phase systems have their phases exhibit similar waveform, but shifted by 120° from each other. As the characteristic curve is continuously varying over time, it means that the measurements (along the curve) are also continuously varying in all these three phases over time, based on the system behaviour as well as load characteristics.

Each point of the sine wave represents a unique value in terms of its magnitude and angular position, though these values repeat over and over again for each cycle. Hence the measurements made shall be such as to truly represent these constantly varying parameters. In AC systems it is quite common to come across the following parameters to be considered for measurements.

• Voltage and current – Average, peak, Root mean square (RMS)*.
• Power – peak or maximum, average.
• Energy – Active energy, Reactive energy.
• Powerfactor – lagging, leading, unity.
• Frequency – Normal, Harmonics.

* The RMS value is the effective value of a varying voltage or current. It is the equivalent to the steady DC (constant) value which gives the same heating effect.

All the above measurements require a good understanding of electrical systems and the way electricity is distributed to the various consumers – industrial, commercial, etc.

1.1.2 Direct current system

Direct current (DC) system is defined by the polarities positive and negative with a definite, constant difference of potential between the polarities. A DC system can be feeding a load in one of the following combinations.

• Positive and negative not connected to ground potential (Floating).
• Positive connected to ground (zero) potential.
• Negative connected to ground (zero) potential.

Based on the method of connection, the current flow can be different and therefore it is necessary to ensure correct polarity connection methods to perform measurements in a DC system.

In electricity transmission systems, HVDC (High Voltage Direct Current) technology is gaining importance on account of the reduced transmission losses. While the normal DC systems used in many industrial and commercial applications are usually in the range of 3V to 220V, HVDC systems are generally rated above 400kV. HVDC systems consequently require different types of instruments for measurements and monitoring.

Most electrical systems employ AC voltage and therefore we will deal with the measurements carried out in ac systems in this chapter.

1.2 Need for measurement in electrical installations

As already noted, measurements basically help to distinguish the characteristics of various electrical systems. This however is only the very basic purpose. Measurements are also needed by the utility companies as well as the end users for the following reasons:

• Utility companies need to continuously monitor the quality of electricity supplied in order to ensure that the electrical parameters are within the defined limits so that, their consumers are not affected.
• It is necessary for the utility companies to charge the consumers for the amount of electricity supplied to them and hence need to measure the energy supplied to their consumers. The consumers shall also be convinced on the methods used for the measurement of electricity consumption. A well defined and uniform method of measuring the consumption is therefore necessary to help both the producers and consumers of electricity. Energy meters thus constitute the basic instrument in any electrical system for measuring the quantum of energy consumed.
• With increasing electricity consumption, it is also necessary to monitor the increase in demand of electricity consumption so that action plans can be initiated by the utility companies and Governments for enhancement of generating capacities, transmission and distribution systems, etc to meet the future demands. The measurements of consumption patterns at various consumer points help to take decisions on enhancement/ strengthening of the electrical systems.
• Some of the changes in electrical parameters are transient in nature but repetition of such transients can affect the reliability of the system in the long run. Transient changes need to be continuously recorded and analysed to curtail the abnormal factors responsible for such short time adverse phenomenon. This is essential to avoid costly damages to the systems in service.
• The sizing of electrical equipment is affected by the powerfactor of the system. Sizing can be optimized by monitoring and keeping the powerfactor close to unity. Powerfactor also affects the capacity of the generating plants on account of the need to supply reactive power.
• Harmonics is another phenomenon that affects upstream sources and also downstream power systems. Measurement of harmonics is gaining importance due to increasing use of power semiconductor devices in equipments like UPS, VFDs etc.
• Energy conservation is gaining importance, as the energy savings in a system can more than compensate for the efforts needed to generate the additional electricity for energy that is wasted.
• The efficiencies of different systems can be evaluated by correct measurements at the appropriate points.
• Measurements help to check the status of deterioration of equipment due to aging, and help the consumers take decisions on replacements and renovations of equipment.
• Alarms can be generated through detection of abnormal electrical parameters in the system, so that corrective actions can be taken to avoid failure and damage to the connected equipment either by prompt isolation or shut down.

Thus, a well designed good measurement system can contribute to a reliable and optimal electrical system that is beneficial both for the Electricity producers and the consumers.

1.3 Importance of accuracy

Any method of measurement is subject to some degree of error and electrical measuring devices are no exception. While measurements provide the various benefits enumerated above, they may not serve the purpose without adequate accuracy in the measurements. This is particularly true for electricity which is a costly commodity and which has become an integral/ basic need in today’s world.

Nevertheless the level of accuracy can differ from location to location and it is necessary to install instruments of correct accuracy based on the location and the kind of application.

For example, the power consumption of an installation served by the utility company is measured at the service entry point of the consumer. Since this instrument decides the amount payable by the consumer and receivable by the utility, this shall have the highest possible accuracy so that both the parties are reasonably convinced of the reliability of the measurements made by these instruments that decide their expense/ income. However, there can be many other branch consumption points within the consumer premises, which may either be important or unimportant depending on the time period of use, power rating of the equipment, etc. In these branch measurements the user shall judiciously decide the level of accuracy needed to serve the purpose, because higher the accuracy higher will be the cost. Hence the requisite type of instruments shall be selected by the user based on the capital cost and the likely benefits achievable from such instruments.

We saw earlier, that measurements are also performed to give timely alarms to the users so as to take corrective action and avoid shutdown or failure of a costly process or equipment. It goes without saying that the accuracy levels employed for such critical alarm devices shall be quite high to avoid unnecessary shutdown or catastrophic failures.

Energy conservation study is another area that requires reasonably good and accurate measuring devices because the recommendations and decisions on energy conservation will depend on the quality of data collected from these devices. Poor quality measuring instruments may lead to wrong and costly decisions which may finally give no appreciable benefits and savings.

Hence it is necessary to evaluate the benefits and returns from an instrument and decide on the required accuracy specifications for the instrument. An instrument without proper accuracy may remain only as an ornamental piece and may not give any benefit to the user.

1.4 Types of measuring devices

The types of electrical measuring devices can be broadly classified based on the types of outputs derived for a particular measurement. These devices can be further subdivided based on the type of construction.

Based on the types of measurement outputs to be derived from the instruments, following are the broad classifications of the instrument types:

• Indicating instruments.
• Integrating instruments.
• Recording instruments.

1.5 Transducers

Today, many process industries are remotely controlled by monitoring various parameters like pressure, temperature, liquid levels, etc through computer monitors. All these are possible, thanks to the advances in electronics and sensors. The sensors are used in conjunction with instruments called transmitters to basically produce electrical outputs for the convenience of measurements. In some cases the transmitters are integrated with the sensor. A typical process transmitter is shown in figure 1.3.

In a similar way, remote electrical measurements are achieved by use of transducers. A transducer arrangement is analogous to the above and has an input parameter coming from an electrical device to a “black box” called the transducer. The transducer changes the received electrical parameter quantity such as voltage, current, power or frequency into a proportional DC output for measurement, control and protection purposes. The outputs are basically DC signals like 0 to 20mA, 4 to 20mA, 0-5V etc which are directly proportional to the magnitude of the input parameter.

By adopting such transducers in an electrical circuit, a complex electrical quantity, such as active power (watts), reactive power (vars) etc measured at the load point can be converted into a load independent DC current signal and transmitted using just two wires over long distances for display, recording or control. Thus the values obtained from transducers can be used for monitoring as well as for control purposes, so that the desired predetermined output value can be achieved. Apart from the requirement of fewer wires for transmission of the signal, the transducer output wires can have small cross section and need to be insulated only for low voltage.

Transducers can be broadly classified into two types, “Active transducers” and “Passive transducers”. “Active” Transducers need external auxiliary supply for their working. “Passive” Transducers can be powered with the input signal itself without an external auxiliary supply. Modern digital measuring instruments have the additional advantage of communicating the measured data to remote control systems using digital communication networks without the need for transducers.

1.6 Instrument transformers – principle

Ammeters used for current measurements in DC circuit are either connected directly to the circuit or by using shunts. When the measurements involve large magnitudes of current, say of the order of hundreds of amperes, it is a general practice to use low range ammeters in conjunction with suitable shunts. However, the measurement of large currents in AC circuits using shunts does not provide accurate results due to following reasons:

• In DC, resistance is the only parameter while in the case of AC circuits, reactance values also come in the picture. Division of current between a meter and its shunt when used in AC circuits would depend on the ratio of reactance values of the two paths. For proper measurement, the time constant of the meter and shunt should be the same, which is difficult to achieve.
• A shunt can be employed only up to a range of a few hundred amperes, because power consumed by shunts increases with increase in the magnitudes of current.
• Measuring circuit is not isolated from the power circuit.
• Insulation of the instrument and shunt is difficult in high voltage systems.

For measurement of large DC voltages a multiplier is used in conjunction with the measuring instrument. A ‘multiplier’ provides a high resistance connected in series with the instrument coil. A low range DC voltmeter with the help of a multiplier can measure a high value of DC voltage. However when measuring AC voltage using multipliers, the following problems could arise:

• For voltages more than 1000V, the power consumed by the multiplier becomes very high.
• Construction of multipliers for large voltages is very costly and complicated.
• Measuring circuit is not isolated from the power circuit.

Hence, use of shunts or multipliers is not a convenient method for measuring alternating currents and voltages. This is why, Instrument transformers are used when it comes to AC measurements.

The underlying principle of instrument transformers is same as that of power transformers. Transformer is the most common and widely used electrical equipment in AC installations. In its simplest form, a transformer basically consists of two windings for each phase, with magnetic linkage between the two windings. These windings are referred to as primary and secondary windings. The primary winding receives an AC voltage as input, transforms this input and delivers at its output a voltage that is of the same magnitude or of magnitudes that may greater than or lesser than the input magnitude (step up or step down).

Figures 1.4 (a) and (b) indicate in the simplest form, the typical transformation taking place between the two windings and the equations that subsequently follow provide the relationship between the input and output voltage magnitudes, with Ep as the voltage across the primary winding, Ip the current in the primary, Es as the voltage output across the secondary, Is as the current in the secondary and Np, Ns being the number of turns of these respective windings.

The relationship between the primary and secondary number of turns and voltages is given by the equation

There are three main types of Instrument transformers:

• Current Transformer (CT).
• Voltage Transformer (VT) or Potential transformer (PT).
• Combined Voltage and Current Transformer (CVCT).

The rating of instrument transformer is specified in terms of volt-ampere (VA) with values not exceeding 100VA. The latest numeric relays and electronic measuring instruments in fact, pose demands of less than 5 VA, compared to the earlier mechanical type relays and measuring instruments.

A current transformer has its primary winding directly connected in series with the main circuit which carries the main current of the system. A proportional lower magnitude current is produced in the secondary of the CT, which is made to flow through the protection relay or measuring instrument. The standard secondary currents used are 1 ampere and 5 amperes and the CTs producing these are referred to as 1 amp CT and 5 amp CT respectively.

All current transformers used in electrical circuits are basically similar in principle in that they all consist of magnetically coupled primary and secondary windings, wound on a common iron core. The primary winding of current transformers are connected in series with the main circuit and hence the primary of current transformers shall be capable of withstanding the system short-circuit currents.

The basic relationship underlying design of all current transformers is:
AMPERE TURNS in Primary = AMPERE TURNS in secondary
e.g. 100 amps x 1 turn = 1 amp x 100 turns.

Current transformers have much lower secondary currents than their primary currents implying that the number of turns on the CT secondary is always more than in its primary winding. The secondary turns requirements for 1 amp secondary CT is 5 times the corresponding 5 amp CT and that is one main reason for the higher price of 1 amp CT in comparison with a 5 amp CT.

A voltage transformer is basically one, whose primary winding is connected across the main electrical system voltage that is under measurement. A proportionate low magnitude voltage is generated in the secondary winding of the VT. The commonly used secondary voltages are 110 volts to 120 volts (as per local country standards) and 380 volts up to 800 kV or more for primary voltages.

The construction features of current transformers and voltage transformers, the various types used and their methods of connections are covered in subsequent chapters.

1.7 Instrument transformers – advantages

Voltage transformers and current transformers continuously measure the voltage and current of an electrical system and provide corresponding low magnitude signals to protection relays to enable them to detect abnormal conditions. The low magnitude signals are also used by the analogue and digital measuring instruments for purposes of displaying and recording the system parameters. The values of actual currents in modern electrical systems vary from a few amperes in households, small industrial/commercial houses, etc. to thousands of amperes in power intensive plants, national grids, etc. Similarly the voltages in electrical systems vary from few hundreds of volts to many thousands of volts.

It is not practically possible to custom design measuring instruments and relays for every voltage/ current application. Voltage transformers and current transformers help to solve this issue by having standardized secondary output magnitudes. For example, the current transformers usually convert a primary current to either 1 ampere or 5 amperes and similarly voltage transformers convert primary voltages to around 110V. This kind of standardization helps meters to be manufactured for very low voltage and current applications making them light, transportable and cost effective. Hence the main purpose of the instrument transformers for metering applications is for standardization of the meters for all electrical applications with minimum time and cost spent in design, selection and manufacturing. International standards have therefore evolved defining standard outputs for voltage transformers and current transformers.

These facilitate that only a minimum types of relays and meters need to be designed and manufactured for catering to the wide range of requirements of all types of electrical systems and consequently the selection and cost of relays and measuring instruments are brought within manageable means.

The main functions of instrument transformers are:

• Transform currents or voltages from usually high values to values that are easier to handle by relays and instruments
• Insulate the relays and instruments from the primary high voltage system
• Facilitate standardization of relays and instruments etc. to a few rated currents and voltages.

Current transformers and voltage transformers are extensively used for precise measurements as well as for routine measurements. They are also used for control and protection purposes. The main applications of instrument transformers are for the following:

• Metering – for energy billing and transaction purposes.
• Isolation – to insulate the metering circuit from the primary high voltage system.
• Protection – for system protection and protective relaying purposes.

1.8 Summary

Measurement of electrical parameters helps in distinguishing their characteristics and monitoring for any abnormal conditions. Electrical measuring instruments are used both in AC and DC systems. Most of the electrical parameters viz. active power, reactive power, power factor etc are dependent on two basic parameters namely voltage and current. All major electrical measurements require a good understanding of electrical systems and the way the electricity is transmitted and distributed to the various consumers – industrial, commercial, etc.

The purpose of performing measurements in electrical systems is not only for distinguishing their characteristics but also to help Utility companies to plan for their future growth and take strategic decisions regarding improvements/modifications needed for improving system reliability. Measurements also facilitate detection of abnormal behaviours in electrical systems, so necessary corrective steps can be taken to avoid costly interruptions and catastrophic failures. Measurement of energy is the main requirement from the commercial point of sale and purchase of electrical energy between the producer and the consumer of electricity.

The level of measurement accuracy needed, depends on the nature of returns and the benefits to the producer and consumer and hence accuracy shall be decided based on the requirement of the application.

Measuring devices can be broadly classified as indicating, integrating and registering types based on the kind of outputs obtained through them. The measuring instrument can be further sub divided as analogue (electromechanical) type and digital type based on the construction and operating principles. Electromechanical types generally incorporate moving needle mechanism for indication while digital meters provide the readings in the form of numerical displays. Transducers are yet another type of measuring devices used for providing low value signals to relays, control and measuring instruments.

Instrument transformers are essential devices in metering circuits. Voltage transformers and current transformers are used for providing voltage and current inputs to the measuring instruments, relays and control devices. They basically operate on the principle of electromagnetism similar to that of the standard power/distribution transformers. The current and voltage transformer outputs are standardized to ensure standardization of metering, protection and control devices. Typical outputs of current transformers are 1 amp or 5 amperes while voltage transformers usually produce 110V output.

2.1 Basic theory of voltage transformers

As seen in the previous chapter, the principle of operation of voltage transformer (VT) / potential transformer (PT) is same as that of power transformers used for voltage transformation. While the secondary voltage of power transformers is usually one of the utilization voltages required to feed a consumer, the secondary voltage of a potential transformer is usually standardized to 110 volts -120 volts AC. Potential transformers are used for measurement of voltage and providing the input to protection relays and measuring instruments. The term “potential transformer” is generally adopted to differentiate it from the standard power transformer.

There are two types of voltage transformers used in electrical system measurements:

• Electromagnetic type (commonly referred to as a VT).
• Capacitor type (referred to as a CVT).

The construction of a voltage transformer differs from that of a power transformer in the design and construction of cooling, insulating and mechanical aspects, on account of the much lower values of power handled by the voltage transformer.

The electromagnetic type VT is similar to that of a step down transformer whose primary (HV) and secondary (LV) windings are connected using a common magnetic circuit as shown in Figure 2.1. A VT is invariably used for stepping down the voltage (HV to LV) for purposes of measurements, protection and control in HV installations. The primary winding is provided with a large number of turns (proportional to the voltage being applied) and is connected across the voltage to be measured. The connection can be either between phase-to-phase or phase-to-neutral. The secondary winding has lesser turns compared to the primary winding and the turns ratio equals the voltage ratio of the primary and the secondary as seen in the first chapter. The ‘volts per turn’ ratios in both the primary and secondary windings remain the same.

Figure 2.1 shows the schematic of a single phase VT. In the three phase system it is necessary to use three sets of windings, one per phase and then connect them either in star form or in delta form. Figure 2.2 shows a three phase VT connected in star-star configuration.

It is possible to have one common primary winding and two or more secondary windings in one VT unit. The voltage transformers having this kind of arrangement are referred to as two/ three/ four/…. core VT/PT depending on the number of secondary windings.

The electro magnetic type of construction is generally adopted for high voltage systems up to 132 kV. As the voltage level increases, the insulation requirements also demand special considerations. To optimize the insulation and costs, often a different type of voltage transformer called capacitor voltage transformer (CVT) is used for voltages beyond 132kV. The principle of operation of a CVT differs from that of electromagnetic type in that, the coupling between the primary and secondary is achieved by means of capacitor coupling instead of electromagnetic coupling. Figure 2.3 shows the schematic of connection used in a CVT. Here the primary portion is provided with a number of capacitors connected in series to split the primary voltage to convenient values. The total voltage across the capacitors is divided among the capacitors proportionate to their values.

The CVT is usually used in high voltage outdoor installations above 132 kV. The capacitors also allow the injection of high frequency signals onto the power line conductors to provide end-to-end communication between interconnected substations for protection relays, telemetry/supervisory and voice communications.

2.2 Types and categories of voltage transformers

The types of voltage transformers in use can be broadly classified in terms of voltages, insulation used for construction, etc as shown in Table 2.1.

Figure 2.4 shows the typical constructions of the various types / categories referred to in the above table.

Each category of voltage transformer addresses a distinct application in transmission, distribution, or modification of electrical voltage. Though they may be wired differently from each other, the basic electrical relationships and mathematical formulas used are the same.

2.3 Use of VT for measurements

The windings of electro-magnetic type voltage transformers may be connected either inter-phase or between phase and earth. Most VTs used nowadays are connected phase to earth, but phase to phase connection was popular quite some years ago, to reduce the number of VTs using the principle of Blondel’s theory. Capacitor voltage transformers can only be connected phase-to-earth. VTs are commonly used in 3-phase groups, generally in star-star configuration as per Figure 2.5. The secondary voltages provide a complete replica of the primary voltages as shown in the vector diagram. Either phase-to-phase or phase-to-earth voltages may be selected for monitoring on the secondary side.

In the three phase configuration, the voltmeters are either connected across each secondary phase and neutral or across two phases to get the readings of the secondary phase voltage or line to line voltage respectively. It is to be noted that the voltage reading on the secondary side would be 110V or 110V/√3, depending upon how the voltmeter is connected. However it is usual to have the meter dial marked and calibrated to read the primary voltage directly based on measurements on the secondary side. In case of digital meters, the internal programming ensures that the digital display on the voltmeter shows the primary voltage magnitudes directly.

The schematic of a three phase voltage transformer and its connections to meters are shown in Figure 2.6. As we observed earlier in section 2.1, the VT can have multiple cores each designed and specified for different applications.

Single secondary winding may be utilised for protection as well as metering using separate branch circuits. However, separate windings each for metering and protection circuits are also employed as shown in Figure 2.6. The protection winding quite often is connected in open delta formation for polarizing E/F protection.

2.4 VT terminal markings and polarity

According to North American standards, the notation H is used for high voltage winding terminals and X for the secondary low voltage terminals, while in IEC the use of primary alphabets with capital and small letters like A, B, C, a, b, c, etc are common. Numerical suffixes are added to the alphabet to identify different cores. Typical marking of terminals for three phase systems according to North American standards are shown in Figure 2.7.

Figure 2.8 shows the typical identifications adopted in European and now IEC standards.

The direction of the current flow in a voltage transformer does not affect the response of the measuring or control devices that operate with the magnitude of the voltage in single phase systems. It means that the reversal of connections of the devices on the secondary terminals does not affect the operation of the devices. However, this is not the case when interchanging the terminals of the measuring or control devices that operate with the interaction of two or more voltages. In these cases, the correct operation of the devices depends on the relative phase positions of the voltages, in addition to their magnitudes. It is normal practice to show the relative instantaneous directions of voltages with distinctive polarity markers on each primary and secondary terminal, indicating that at the instant when the primary voltage is rising at a marked primary terminal, a similar voltage rise is reflected at the marked secondary terminal. It is normal practice to identify the terminals of voltage transformers with black dots (circles) to indicate the relative polarities of the primary and secondary windings as shown in Figure 2.9.

It is necessary that terminals having corresponding capital and lower-case markings shall have the same polarity at the same instant. Usually, the corresponding high-voltage and low voltage terminals of the equipment are also marked as ‘P1’ and ‘S1’ respectively.

The typical connections of a voltage transformer with two or more secondary windings is shown in Figure 2.10, where ‘P’ stands for the primary and ‘S’ stands for the secondary terminals.

2.5 Characteristics and classes

The voltage transformers are specified by the following characteristics:

• Rated primary and secondary voltages
• Voltage factor
• Accuracy
• Errors related to accuracy
• Burden
• Frequency response
• Transient response

2.5.5 Burden

Power transformer ratings are defined by kVA values giving their loading capacities. The burden of a voltage transformer can be considered at its rated power capacity but defined in terms of VA due to very low magnitudes of power involved in VT circuits.

When several loads are connected in parallel, it is usually sufficiently accurate to add their individual volt-amperes arithmetically to determine the total volt-ampere burden. The standard burdens to be used for testing and comparing voltage transformers are rated at 120 volts and at 69.3 volts.

In other words, a burden can be defined as the load which may be imposed on the voltage transformer secondary windings by associated meter coils, leads and other connected devices without causing an error greater than the stated accuracy classification.

Typical burden’s in VA’s range from 10 VA to 45 VA. For larger capacities, the burden can go as high as 100 VA and above. Today, with digital relays and meters replacing the electromagnetic ones, the burden requirement of an instrument transformer has reduced considerably compared to earlier days.

A burden is specified at a particular load angle/power factor. It is to be note that as per international standards the accuracy of the VT should be within specified limits up to its rated burden. Voltage transformers are designed to maintain the specified accuracies in voltage output at their secondary terminals, and this is achieved by designing the winding’s impedances with a suitably low value.

The use of long secondary lead cables, tend to introduce further resistance and associated voltage drop, which can affect the accuracy of the voltage as it is received at the switchboards. The obvious solution is to consider a distribution box installed close to the voltage transformer and supply the relay and metering burdens over separate leads to each panel. Apart from the basic fact that this keeps all lead lengths to the minimum, this helps in making an allowance for the resistance of the leads of the individual burdens when the particular equipment/meter is calibrated.

For protection purposes, the accuracy of the voltage measurement may be important during fault conditions, as the system voltage might be reduced to a lower value by the fault. Voltage transformers for such types of services must comply with an extended range of performance i.e. the output should be within a specified range during such conditions to enable proper detection of the fault.

Primary voltage has no effect on the ratio error and phase angle error, as there is no wide variation of supply voltage to which the primary winding of a voltage transformer is connected. Table 2.6 shows the values of a transformer ratio correction factor.

Accuracy Class	Limits of transformer ratio correction Factor	Limits of power factor load
1.2	1.012 – 0.988	0.6 – 1.0
0.6	1.006 – 0.994	0.6 – 1.0
0.3	1.003 – 0.997	0.6 – 1.0

Effect of burden on voltage ratio

Whenever the secondary burden increases, secondary current increases and hence primary current also increases. Due to drawl of higher currents, voltage drops in both primary and secondary windings increase. Thus for a given value of primary voltage, the secondary voltage available at the VT terminals decreases. Hence, the actual voltage ratio between primary and secondary windings increases when the secondary burden increases. The variation in ratio error can be found to be almost linear with the change in burden.

With regard to phase angle error, primary voltage is more advanced in phase because of increased voltage drops with an increase in secondary burden. The phasor secondary voltage reversed is retarded in phase owing to an increase in secondary winding voltage drops. Thus, with the increase in burden, the phase angle between the primary and secondary voltage reversed increases, becoming more negative.

Effect of power factor of secondary burden

If the power factor of a secondary burden is reduced, voltage drops remain almost constant and the transformer ratio increases. Secondary voltage advances in phase and primary voltage is retarded in phase. Thus, the phase angle reduces with a decrease in the secondary power factor. The Standard burdens for voltage transformers for rating purposes as per IEEE are given in Table 2.7.

Burden Designation	Voltamperes	Burden power factor
W	12.5	0.70
X	25	0.70
Y	75	0.85
Z	200	0.85
ZZ	400	0.85

Revenue metering accuracy class (IEEE)

Revenue metering accuracy classes require that the TCF of instrument transformers shall be within specified limits. This requirement is specified when the power factor of the load is in the range of 0.6 to 1.0. These shall be met by a voltage transformer for the load of any value (in VA) in the range from zero to the specified standard burden. The range of input voltage magnitudes is 90% to 110% of the rated primary magnitude.

A voltage transformer shall be assigned an accuracy rating for each of the standard burdens for which it is rated. For example, an accuracy rating might be 0.3W, 0.6Y, 1.2ZZ.

0.3W indicates that

• PT will operate with an accuracy between 99.7-100.3 percent if the operating system power factor is greater than 0.6 and
• The secondary connected devices do not exceed 12.5 VA and operate at a power factor greater than 0.7.

0.6Y indicates that

• PT will operate with an accuracy between 99.4-100.6 percent if the operating power factor is greater than 0.6 and
• The connected devices do not exceed 25 VA and operate at a power factor greater than 0.85.

1.2ZZ means that

• PT will operate with an accuracy between 98.8-101.2 percent if the operating power factor is greater than 0.6. and
• The connected devices do not exceed 25 VA and operate at a power factor greater than 0.85.

2.5.6 Voltage drop in voltage transformers

The voltage drop in the secondary circuit is of importance. The voltage drop in the secondary fuses and long connection wires can change the accuracy of the measurement. It is especially important for revenue metering windings of high accuracy (class 0.2, 0.3). The total voltage drops in this circuit must not be more than 0.1 percent.

Typical values of Resistance in fuses are as below:

6A	0.048
10A	0.024
16A	0.0076
25A	0.0042

6-10 A is the typical range normally chosen for safe rupture of the fuses in VT applications. The voltage drop in the leads from the VT to the associated equipment, in practice, can be alarming in case of measuring circuits and shall be suitably controlled by choosing proper cross section of wires. The voltage drop in the leads is an important factor to be considered in case of metering circuits (with low burden) compared to protective circuits (with higher burdens). Figure 2.11 gives typical circuit with voltage drops.

2.5.6 Frequency response

For a constant voltage, flux is inversely proportional to the frequency of operation. An increase in frequency reduces the flux and therefore the magnetising component and the iron loss component of a no load exciting current decreases. This ultimately leads to a decrease in the voltage ratio but the decrease is not directly proportional to the increase in frequency. The leakage reactance increases and the leakage reactance drops are increased, leading to an increase in the voltage ratio. Changes in voltage ratio due to a change of frequency are dependent on the relative value of loss and leakage reactance since the effects produced by them oppose each other. Due to an increase in frequency, primary voltage advances. Secondary voltage gets retarded due to a secondary reactance. Therefore the phase angle increases as frequency increases.

Frequency response is normally evaluated for linear systems only. At the outlook, instrument transformers are not linear devices. However they are usually designed and sized (with various component parameters) to properly operate in a linear region only. This means that most of the time, instrument transformers can be regarded as linear devices, which can be affected by frequency of voltage applied. The frequency response of voltage transformers and CVTs can be evaluated only when the magnetic flux in the core is in the linear region.

With windings predominantly offering the inductive reactance in a circuit, the capacitances of the circuit affect the frequency response. The typical frequency range of signals in voltage transformers is up to 10 kHz. In this range, a voltage transformer acts as a low-frequency filter. The cut-off frequency depends on the following parameters of the voltage transformer.

• Independent stray capacitances associated with primary and secondary windings (C1 and C2 respectively are shown in Figure 2.11). The Linkage stray capacitance between primary and secondary windings is shown as C12 in Figure 2.11.

The frequency response of a typical voltage transformer can be studied using models and simulation software, such as an Alternative Transient Program (ATP), which applies trapezoidal rule of integration to solve the differential equations of system components in the time domain.

2.5.7 Transient response

Following power system transient conditions generally affect the response of the voltage transformers:

• Sudden decrease of voltage at the transformer terminals (due to a fault close to a voltage transformer).
• Sudden overvoltage (on the healthy phases due to phase to ground faults elsewhere in the network).

The first power system condition can produce internal oscillations within the electromagnetic voltage transformer cores. These in turn appear on the secondary winding output in the form of high-frequency oscillations with frequency much higher than the system frequency, sometimes called ringing. The damping time of such oscillations is usually between 15 and 20 milliseconds.

The second power system condition can lead to the saturation of the electromagnetic core affecting the output and the response by the connected devices. This is usually named as subsidence transient, which is defined as an error voltage appearing at the output terminals of a coupling-capacitor voltage transformer, resulting from a sudden and significant drop in the primary voltage.

The transient can be classified as belonging to one of the following three classes:

• Unidirectional.
• Oscillatory, f oscillation > 60Hz.
• Oscillatory, f oscillation < 60Hz.

The major factors that can influence the subsidence transients are:

• Coupling capacitor voltage transformer (CCVT) burden.
• Coupling capacitor voltage transformer design.
• Ferro-resonance suppression circuit (FSC).

The aspects related to the burden of a coupling capacitor voltage transformer that influence the subsidence transient are:

• Magnitude of burden – the influence of the burden is relatively less when the magnitude of the actual burden of the circuit is smaller than the nominal one.
• Burden power factor – Generally a decrease in the power factor in turn leads to lessening of the subsidence transient.

Typical responses in the secondary ends due to transient primary voltage conditions will be generally as per Figure 2.12 with the response varying with the burdens to some extent.

During the sudden application or sudden disappearance of primary voltage, the distribution of energy stored in the CVT is a major factor influencing the effect of the point of wave voltage interruption. The transient performance in those cases is similar with the transients as in the case of the primary short circuit.

2.6 Standards for voltage transformers

Following are the primary standards of voltage transformers followed by manufacturers based on the country of application, though they are closely related with some minor differences in the terminal markings, accuracy class definitions, etc.

• IEEE Std C57.13 – Standard requirements for instrument transformers.
• IEC 60044-2 – Inductive voltage transformers.

The standards usually specify the requirements to be met by the manufacturers and equipments related to:

• Manufacture and designing requirements.
• Testing standards .
• In-service requirements.

2.7 Hazards with voltage transformers

The following factors shall be duly considered in voltage transformer installations, to avoid possible hazards and consequent damages and losses:

• Unearthed neutral terminals on secondary end.
• Short circuiting of secondary terminals.
• Back feed from secondary.
• Explosions.
• Over voltages due to Ferro-resonance and lightning.

2.8 Safety precautions

Some of the important safety precautions related to voltage transformers are listed in the following paragraphs. Due compliance of these factors while working on VT circuits can ensure safety for personnel and equipment.

• An instrument transformer shall always be considered as a part of the circuit to which it is connected. The leads and terminals (or other parts) of the voltage transformer shall never be touched unless they are known to be adequately grounded.
• Even in the case of cast resin/moulded type VTs, the insulation surfaces should never be touched, since a voltage gradient and stress exists across the entire insulation surface from the terminals to the grounded metal parts.
• The secondary neutral should be grounded close to the transformers.
• The application of power fuses in the primary circuits of voltage transformers is a recognised and recommended operating practice confined to panel mounted voltage transformers. To provide the maximum practical protection against damage to other equipment or injury to personnel in the event of a voltage transformer failure, it is necessary to use the optimum and smallest possible fuse ampere rating, which will not result in nuisance blowing. Increasing the fuse ampere rating results in longer clearing time, thereby increasing the possibility of damage to other equipment or injury to personnel. This therefore needs to be avoided.

When performing work on outdoor voltage transformers, the following minimum safety precautions are necessary:

• Only qualified and trained personnel should be allowed to work on the voltage transformer.
• The equipment must be properly de-energised and isolated with an issued permit.
• Work permit must be provided for personnel intending to perform any work on the voltage transformer.
• Before starting work, the PT should be relieved of any residual energy by using an earth discharge rod.
• Earthing connections shall be physically checked to ensure they are in good condition
• Proper PPE and working platforms shall be ensured when working at elevated levels on outdoor PTs.
• Any work on a VT or in its immediate vicinity shall be completely avoided during rain and lightning.
• The VT’s surface must be kept clean to avoid flashovers.
• The secondary terminal box shall be completely sealed to prevent water entry during rains.
• Adequate clearances with respect to ground, to the nearby devices and to the nearby fences shall be ensured while designing the layouts and during installation. The minimum clearances stipulated in various international standards shall be strictly adhered to in order to prevent accidents and failures.
• The secondary terminals of a voltage transformer shall never be short circuited. A secondary short circuit will cause the unit to overheat and fail in a very short period of time.

2.9 Accuracy checks on voltage transformers

The accuracy class indicates the voltage transformer’s performance characteristics. A VTs accuracy is dependent on the number of devices connected to the secondary terminals and the applied voltage. However, estimation of the actual operating voltages and the connected burdens may not be always possible. Hence routine tests are carried out on VTs at a reduced number of voltages and/or burdens that are in the permissible range. Routine tests at a reduced number of voltages and/or burdens are permissible, provided it has been shown by Type tests on a similar transformer that such a reduced number of tests are sufficient to prove its characteristics.

The Type test for the measurement accuracy of a voltage transformer should be made at a voltage between 80% and 120% of rated voltage and with burdens between 25% and 100% of rated burden at a power factor of 0.8 lagging. To prove compliance with this, type tests shall be made at 2%, 5% and at 100% of rated voltage and at rated voltage multiplied by the rated voltage factor, at 25% and at 100% of rated burden at a power-factor of 0.8 lagging.

If the voltage transformer has two separate secondary windings, it is necessary to specify two output ranges, one for each winding. The upper limit of each output range should correspond to a rated output value. Each winding should fulfill its respective accuracy requirements within its output range. If no specifications of output ranges are known, then these ranges are deemed to be from 25% to 100% of the rated output for each winding.

For measuring voltage transformers of accuracy class 0.1 and 0.2 and having a rated burden lower than 10 VA an extended range of burden can be specified. The voltage error and phase displacement shall not exceed the values given in the table, when the secondary burden is any value from 0 VA to 100% of the rated burden, at a power factor equal to 1. This requirement is mostly requested for certified accuracy of energy measurements. The measurement errors shall be determined at the terminals of the voltage transformer and shall include the effects of any fuses or resistors as an integral part of the VT.

2.10 Applications of voltage transformers

Voltage transformers are used mainly for the following applications:

• Metering of voltage, power, frequency, etc.
• Protection of all major equipment through relays.
• Indication of voltage availability by indication lamps.

Although this course is devoted to metering applications, we will briefly outline the features of all these applications to give a better idea on how the characteristics of metering application transformers differ from the other types.

2.10.1 Connection to obtain the residual voltage

It is common to detect earth faults in a three-phase system using the displacement that occurs in the neutral voltage when earth faults take place. The residual voltage (neutral displacement voltage, polarising voltage) for earth-fault relays can be obtained from a VT between neutral and earth, for instance at a power transformer neutral. It can also be obtained from a three-phase set of VTs, which have their primary winding connected phase to earth and one of the secondary windings connected in a broken delta. Figure 2.13 shows the measuring principle for the broken delta connected system.

2.10.2 Ferro resonance in magnetic voltage transformer

While the phenomenon of ferro resonance in a CVT is an internal oscillation between the capacitor and the coil winding, the ferro resonance in a magnetic voltage transformer is an oscillation between the inductive reactance of the voltage transformer and the capacitance of the network. The oscillation can only occur in a network having an insulated neutral. An oscillation can occur between the network’s capacitance to ground and the non-linear electro magnetic inductance of the voltage transformer. Such an oscillation can be triggered by a sudden change in the network voltage e.g. switching or a fault.

Damping of ferro resonance

The electromagnetic type voltage transformer can be protected from ferro resonance oscillation by connecting a resistor across the open delta point in the three-phase secondary winding as shown in Figure 2.14. A typical value of the resistance is 50-60 ohm, 200W.

2.11 Summary

There are basically two construction types of voltage transformers. The electro magnetic type and the capacitor type. The basic operation principle of an electromagnetic type voltage transformer is the same as the standard electromagnetic power transformer. The secondary voltages of typical voltage transformers are standardized as 110V – 120V. Voltage transformers can have multiple cores feeding measuring and relay circuits separately. Voltage transformers are also classified based on the operating voltage, type of insulation, number of phases and the location of installation. The secondary of a VT for measurement purposes is usually connected in Star configuration with the neutral to enable voltage measurements between phase to phase or phase to neutral.

The terminals of a VT are marked with different identification tags in IEEE and IEC standards. IEEE generally uses prefix H for primary (high voltage) and prefix X (Low voltage) for secondary ends. IEC generally uses capital alphabet (A, B….) for primary side and smaller alphabet with core number (1a, 1b, 2a, etc) for the secondary side. The polarity of a VT is usually identified with a small circle to show the primary and secondary voltage relationship.

The technical characteristics of a VT are usually specified in terms of rated voltages of primary and secondary windings, accuracy, burden, rated voltage factor, errors, etc. The required insulation levels are provided in the international standards.

Accuracy is an important factor for measuring circuits. However, accuracy is not a major cost factor for voltage transformers because of the high efficiency of VTs. The errors in the output values of a VT are caused by ratio error and phase angle error. The burden of a VT basically gives its maximum allowable load within which the VT will maintain its accuracy and acceptable errors. The other two factors that can affect a VT are its responses to frequency variations and input transients.

IEC and IEEE standards define the typical operating characteristics of voltage transformers and also list the various tests required to be conducted on VTs for checking their conformance to the standards.

While using voltage transformers it should be ensured that the secondary neutrals are properly grounded and that short circuiting of terminals are avoided. Proper isolation shall be ensured to avoid back feeds and protection from possible over loading / short circuit conditions with the provision of fuses. VTs may flashover and explode in the event of loss of insulation oil, dirty bushings, improper grounding and lightning strikes. VTs are affected by over voltages due to ferro resonance and lightning and hence protection is needed through the provision of proper fuses and lightning protection.

3.1 Basic theory of current transformers

Current transformers are used for producing smaller currents in the range of one to five amperes for metering and relay applications in power circuits having currents ranging up to thousands of amperes. The main purpose is to use these small magnitude currents in relays and meters for the purpose of detection of normal and abnormal currents. The principle of current transformer operation is based on the basic theory of normal transformers used in AC systems considering ampere turn balancing in primary and secondary windings as given by the following equation (primary and secondary currents/turns suffixed with p and s respectively).

In an “ideal” current transformer, the secondary current would be exactly equal to the primary current multiplied by the turns ratio and vectorially opposite to the primary current. However, as in the case of a voltage transformer, some portion of the primary current or primary ampere-turns is utilized for magnetizing the core, thus leaving less than the actual primary ampere turns to be “transformed” into secondary ampere-turns. This naturally introduces an error in the transformation. Theses errors are of two types—the current or ratio error and the phase error.

The following are the important characteristics of current transformers, which distinguish them from standard voltage/ power transformers:

• Current transformer has its primary winding directly connected in series with the main circuit, carrying the full operating current of the system unlike the parallel connection to voltage source adopted in standard transformers.
• The primary current in current transformer is dependant on the current in the main system to which it is connected in series and is not determined by the load connected on the secondary winding of current transformer.
• The primary winding of a current transformer always consists of very few turns (very often only a bar of conductor equivalent to a single turn forms the primary winding) and there is no appreciable voltage drop across it.
• The secondary winding of the current transformer has large number of turns; the exact number is determined by the turns ratio.
• A current proportional to the primary current flows in the secondary winding of a CT based on the turns ratio designed to ensure a standard secondary current of 1A or 5A for the rated primary current. These secondary values are universally standardized.
• The secondary winding of the current transformer should never be left open and shall either be connected to a device with a current coil or shall be just shorted using shorting links, when not connected to a device on the secondary.

A typical current transformer used for measurement purposes is made up of the following main parts which are completely sealed with a suitable insulating cover to form a single unit.

• Magnetic core: The magnetic core is made up of high quality steel which forms a closed magnetic circuit, similar to voltage transformers. At least one of the windings or both (primary and secondary) are wound over the core. Sometimes the transformer can have multiple cores in one assembly, each serving a distinct function.
• Primary winding: The primary winding is made up of copper conductor with adequate insulation. The primary winding design can be dead tank or live tank design.
• Secondary winding: The secondary winding is made up of enameled copper wire.

3.2 Types and categories of current transformers

Current transformers can be broadly categorized as shown in Table 3.1. They are generally not classified on the value of currents being handled.

Table 3.1
Categories/ Types of current transformers

One important method of differentiating the type of a CT is based on the construction method adopted. The constructions basically differ on how the primary winding is formed. There are three types of current transformers based on this categorization:

• Wound primary type
• Bar primary type
• Window type

Both wound primary and bar primary construction are adopted for metering and protection current transformers. However the window type CT is exclusively used for protection purposes.

Typical wound primary type CT is shown in Figure 3.1, where both the primary and secondary windings are wound over a common magnetic core.

Wound primary type CTs have the following limitations:

• Thermal limitations on the maximum currents of primary winding especially while carrying short circuit currents.
• Need for special structural requirements against high magnetic forces.

Wound type CTs are generally adopted for primary currents up to 100 amperes only. The bar primary type has its primary in the form of bus bar so that the above limitations are overcome for higher currents. For currents greater than 100 amperes, bar primary type is used, where the primary is nothing but part of the main conductor of the circuit to be measured/ protected. Typical construction of bar primary type CT is as shown in Figure 3.2.

The major cause for errors in the CT is its leakage reactance which shall be minimized to limit the CT measurement errors. If the secondary winding of the CT is evenly distributed around the complete iron core as per Figure 3.3, its leakage reactance can be almost eliminated.

3.3 Use of CT for measurements

Generally current transformers are used to measure the currents for the following purposes:

• Tariff metering
• Reference Purpose
• Alarm/ Trip purposes

Tariff metering basically refers to the readings, which are directly linked to the charges payable by the consumer for the electricity consumed. It demands very high accuracy because both the consumer as well as the power supplier must be convinced about the correctness of the values recorded. However it is impossible to have meters with zero error. Hence it is generally considered acceptable to have tariff metering CT the lowest practically feasible error which is in the order of ± 0.2% exhibited by tariff meters used globally.

Current transformers are also used for providing reference values of currents for monitoring the consumption patterns. This is necessary because typical electrical distribution systems comprise of many branch consumers whose consumption values can vary either due to inherent load characteristics or due to other factors like poor efficiency, defects in load, fault conditions, etc. Reference purpose CTs are some times provided to cross check the consumption recorded by utility companies (normally referred to as check metering) and hence they also require equivalent accuracy as the tariff metering CT. However majority of the metering CTs are just for reading the primary current to get a broad idea about the performance of the equipment whose current is measured. Normally these requirements do not demand very high accuracy because an error of even 10% may not defeat the purpose for which the CT is used. Hence the reference purpose CTs mostly used in switchgears, motor control centers, Local control stations, etc. do not require very high accuracy as required by tariff metering CTs. An error of 2 to 5% is generally well accepted for these applications.

A CT used for current metering shall be able to provide a fairly accurate value in the secondary winding from 0 to 100% of the full load current. Hence a typical characteristic demanded from a metering current transformer is as per Figure 3.4. However it is practically impossible to have a linear primary- secondary current relationship over such a wide range. Nevertheless the importance of linear relationship cannot be ignored. The accuracy factor may also depend on the actual primary current with lower values introducing higher error percentages in the readings. Hence a CT used for measurements shall have its secondary current to be almost a true reflection of primary current at least for a load range of 50 to 120%.

The point at which the curve reaches steady state or saturation is called knee point voltage of the CT. In the case of metering current transformers, normally the load current is not expected to go beyond the primary rated current of the CT. However under fault conditions, this is not the case and the fault currents may some times be many times the primary current. The secondary currents also tend to increase proportionately and can damage the connected instruments used for measurement purposes. Hence the CT used for measurements shall have its maximum secondary current restricted to little above full load value, which is normally achieved by core saturation. Core saturation offers a high reactance beyond a particular current thereby restricting the secondary current to within 120% of its rated value. After core saturation any further increase in primary current does not have any effect on the secondary current so instruments do not get destroyed under overload or fault conditions. Hence it is common to have metering CTs with a very sharp knee-point voltage at a relatively low value of primary current. A special nickel-alloy having a very low magnetizing current is used for the core in order to achieve the accuracy.

3.4 Representation of a current transformer

A conceptual representation of a CT is shown in Figure 3.5.

In electrical scheme drawings, a common symbol is used for both protection or metering type CTs. However the symbols used to depict current transformers differ in respect of IEC and IEEE standards. Typical symbols adopted in these standards are as given in Figures 3.6(a) and (b).

The terminals are marked with P1, P2 for primary, S1, S2, etc for secondary with prefix numerals for multi core secondary CT terminals. Typical terminal markings of current transformers as per IEC are shown in Figure 3.7 while the markings adopted as per IEEE standards are as shown in Figure 3.8. These markings are also indicative of the polarity of the transformer as discussed later.

The core balance CT or the window CT is used for measuring the vector sum of currents through a three phase balanced load or a three phase and neutral unbalanced load. Under normal conditions the vectorial sum of these currents shall be zero. The secondary windings of the CT will have zero voltage under this condition. Any earth fault in the section beyond the CT will create an unbalance in the vectorial sum, generate a voltage in the secondary winding and drive a current through the relay connected to it. The three phase or three phase neutral cable cores or the busbars or the cables of windings (in case of motors) are passed through the window of the CT for detection of earth faults.

3.5 Typical connections of current transformers

For three phase current measurements, single phase current transformers (one in each phase) are connected in star or delta depending upon the application requirements. Figures 3.9 to 3.11 show the typical CT connections adopted in three phase systems. The use of CTs in one phase only is generally adopted as inputs for ammeters used in local control stations for large size motors. In these local metering applications, usually the CT is connected in the centre phase and is taken as representation of the load current for local monitoring purposes.

3.6 Current transformer characteristics and classes

Similar to the voltage transformers, current transformers are defined by some main characteristics and technical parameters. Though this course mainly focuses on metering CTs, the characteristics of protection CTs are also covered to enable an overall view of the characteristics of the different current transformers and how they differ based on application requirements. Following are the main characteristics of current transformers:

• Magnetization characteristics
• Burden
• Rated voltage, thermal and short circuit currents
• Rated secondary current
• Accuracy
• Errors related to accuracy
• Responses under varying load/ system conditions

3.6.1 CT magnetization characteristics

Under actual operating conditions, when a primary current flows in a CT, its secondary winding generates a voltage depending upon the impedance connected to the winding so that a secondary current proportional to the primary current (as per CT ratio) flows in secondary. If the current is slowly increased from zero to the rated value, the voltage across the CT secondary also increases proportionately. A graph can be drawn to show how the secondary voltage of a CT changes with the change in primary current at specified secondary load impedance (burden). Due to the non-linearity of core, the characteristic follows the B-H loop characteristic of the magnetic core. The graph comprises of three regions:

• Initial region
• Unsaturated region
• Saturated region

These are graphically shown in Figure 3.12.

The magnetization characteristic of a CT is usually obtained using the secondary injection method. This takes the form of a curve showing the voltage variation across the secondary winding with varying current flow. The primary terminals remain open during this process. This curve describes the CTs performance in the best way.

The transition from the unsaturated to the saturated region of the open circuit excitation characteristic is a rather gradual process in most core materials. This transition characteristic makes a CT not to produce equivalent secondary current beyond certain point. This transition is defined by “knee-point” voltage in a CT, which fairly decides its accurate working range.

The knee point voltage is generally defined as the voltage at which a 10% increase in volts at the secondary side of the CT requires more than 50% increase in excitation (primary) current. For most applications, it means that current transformers can be considered as approximately linear up to this point. Beyond this value the CT output current gets saturated and it no longer represents the primary current. The saturation property is particularly useful in metering current transformers, to ensure that the current in metering devices is limited to its maximum allowable value to prevent damage to the instruments.

3.6.6 CT errors

The error in a CT shall be within required limits based on the application requirements. The errors are mainly introduced because the primary current contains two components as given below.

• An exciting current which magnetises the core and supplies the eddy current and hysteresis losses and is usually lost within the CT itself.
• Remaining primary current component which is available for transformation to secondary current in the inverse ratio of turns.

The exciting current is not getting transformed and is therefore the cause of transformer errors, which means it shall be minimized as much as possible to get more accurate secondary currents. The more the exciting current, the lesser would be the accuracy.

The amount of exciting current drawn by a CT depends upon the core material and the amount of flux that must be developed in the core to satisfy the output requirements of the CT. That is, to develop sufficient driving voltage required across the secondary winding to push secondary current through its connected load or burden. Hence the selection of core materials plays an important role in getting higher accuracy.

The error which a CT introduces into the measurement of a current and which arises from the fact that the actual transformation ratio is not equal to the rated transformation ratio is called the Ratio error or current error as per IEC. The current error (or ratio error) expressed in percent is given by the following formula:

Current error % = (Kn × Is – Ip ) × 100 / Ip

Where

• Kn is the rated transformation ratio
• Ip is the actual primary current and
• Is the actual secondary current when Ip is flowing, under the conditions of measurement.

The other factor introducing errors in measurements is the relative phase angle difference in the primary and secondary current vectors. For an ideal transformer the primary and secondary current vectors shall be in phase with zero displacement error i.e. the secondary vector angle shall be exactly the same as the primary vector. However in practice this is not achievable and an error is introduced in phase angle between the primary and secondary currents. The phase displacement is referred to as positive when the secondary current leads the primary current and is expressed in minutes or centi-radians.

Composite Error: The composite error is defined as the rms. value of the difference between the instantaneous values of the primary current and the instantaneous values of the actual secondary current multiplied by the rated transformation ratio under steady-state conditions. The positive signs of the primary and secondary currents are used in defining the composite error corresponding to the convention for terminal markings.

The composite error ε_c is generally expressed as a percentage of the rms. values of the primary current as per following formula, with the parameters as noted earlier and T being the time duration of one cycle.

In the case of current transformers used for metering, the composite error should be greater than 10 %, in order to protect the apparatus supplied by the instrument transformer against the high currents produced in the event of system fault. The rated instrument primary limit current (IPL) is an important factor for measuring transformers, which is defined as the value of the minimum primary current at which the composite error of the measuring current transformer is equal to or greater than 10 %, with the secondary burden equal to the rated burden of the CT. Yet another other factor is the Instrument Security Factor (FS) which is equal to the ratio of rated instrument limit primary current (IPL) to the rated primary current, which basically indicates the factor of safety for the instruments being connected.

3.6.8 Polarity

Polarity is important in ensuring the correct operation of the relays and metering results. For some of the relay applications the direction of current flow is very important. Polarity in a CT may be considered same as identification of positive and negative terminals of a battery. The terminals of CT are usually marked by P1 and P2 on the primary and S1 and S2 on the secondary to match the current flows as shown in Figure 3.13. Standards indicate that at the instant when current is flowing from P1 to P2 in primary, the current in secondary must flow from S1 to S2 through the external circuit and the markings shall be ensured to meet this requirement. This marking is very important for the relays to get true current directions in specific applications like reverse power detection when two or more sources are feeding a common bus in parallel. A reversal of connection will lead to malfunction and can cause undesired tripping.

3.7 Metering and protection type current transformers

A metering CT is usually specified with the following parameters:

• Rating: XXX/ 5 or XXX/1 amps
• Burden: 15 VA
• Accuracy Class: 0.2, 0.5, etc
• Insulation class: B, F, etc

The metering CT requirements are defined by various accuracy classes and the basic requirements of these specifications shall be understood to ensure that the test results are verified before accepting a current transformer. Table 3.2 defines the applicable current error in line with IEC requirements for the different accuracy classes of the metering current transformers.

Table 3.3 gives the acceptable phase displacement errors for the different classes of current transformers, as per IEC.

It is to be noted that a metering CT is expected to have the least error from 100% to 120% of its rated primary current. The allowable phase displacements are more at lower ranges and less at rated current. The protection CT on the other hand is not expected to give an accurate feedback of the current under normal operating ranges. However they shall be able to offer the actual current to the relays under fault conditions. Saturation of core at fault currents may not give the true condition to the relay and hence the relay may not trip when needed, which in turn may damage the costly main equipment for which it is used. Hence the protection CT requires core saturation to occur at much higher values compared to the metering CT.

Typical tolerance values for the accuracy classes for protection CTs are as per Table 3.4.

Higher accuracy is not a major requirement for protection current transformers. Protection CTs are expected to ensure that the error in measurement at the worst condition does not differ by more than 10 to 20% compared to 0.2%, 0.5%, 2%, etc required by metering CT’s. Further, the protective relays are not normally expected to give tripping commands under normal conditions. On the other hand the relays are concerned with a wide range of currents from acceptable fault settings to maximum fault currents which are many times the normal rating. Larger errors may be permitted and it is important that saturation is avoided wherever possible to ensure positive operation of the relays mainly when the currents are many times the normal currents.

Figure 3.15 shows the typical characteristic required for a protection CT.

Table 3.5 summarizes some of the major differences between a metering and a protection current transformer.

3.8 Standards for current transformers

Separate standards (parts) are available in IEC for specifying the requirements of current transformers unlike IEEE which specifies the requirements in a common standard for voltage transformers and current transformers. However both IEEE and IEC have separate standards for protective current transformers, which indicate the importance of current transformers for protective relay applications. Following is the list of main standards adopted for current transformers, with some special standards for protection current transformers.

• IEEE Std C57.13 – Standard requirements for instrument transformers.
• IEEE Std C37.110 – IEEE guide for the Application of Current Transformers used for protective relaying purposes.
• IEEE C57.13.1- IEEE Guide for Field Testing of Relaying Current Transformers
• IEC 60044-1 – Current transformers.
• IEC 60044-6:1992, Instrument transformers – Part 6: Requirements for protective current transformers for transient performance.

Similar to the standards for any electrical equipment, these standards also basically specify the requirements to be met by the current transformers related to:

• Manufacture and designing requirements
• Testing standards
• In-service requirements.

3.10 Hazards associated with current transformers

Following can lead to some hazards associated with CTs and caution is needed to take care of these factors.

• Open circuiting of secondary terminals
• Explosions
• Non grounding

3.10.1 Open circuiting of CT secondary

A current transformer secondary should never be open-circuited while main current is passing through the primary winding. An open circuit can also happen when a connected load is removed from the secondary, like removal of an instrument for repair work, etc. When the secondary is open with the main primary current still flowing, most of the primary current becomes magnetizing current. To maintain electrical stability, the vector angles of the current change in such a way as to keep the total current in the primary the same as before. Since the primary winding is carrying mostly the magnetizing current, the flux in the core shoots up to a high level and a very high voltage appears across the open secondary terminals. Current transformers normally employ very high turns ratios like 5/100, 1/200, 1/1000, etc. Hence, the open circuit voltage across secondary winding under this condition can reach dangerously high levels and can lead to:

• Break down of insulation of the winding
• Safety hazard to personnel
• Flashovers

The high flux also saturates the core and results in strong residual magnetism left in the core, which increases magnetization current during subsequent usage and introducing error in the transformation ratio. Hence it is very vital that the secondary of a CT winding shall never be open circuited during live conditions and it is necessary to short the secondary terminals before removing any secondary load while current is flowing through the primary winding. Shorting links are standards in almost all the relays operating with secondary current from current transformer. Metering instruments do not generally come with such shorting links and hence it is usual practice to specify special shoring links as part of the CT terminals.

It is also recommended to use lugs which are circular in shape so that the cable leads cannot be easily removed from the instrument and relay terminals. A pin type termination is considered as a safety hazard while carrying out switchgear terminations.

3.11 Applications of current transformers

It can be concluded that the basic purpose of CT is to measure current and the measured value is used for metering or relay applications. Following are some of the typical and common applications for which current transformers are used.

• Over-current or under-current protection: The current transformer can continuously monitor the current and drive an over-current relay or an under current relay as a switch, when the main current exceeds or goes below a preset value of current. The output current of the current transformer activates the relay and the protective circuit.
• Measurement of very high currents: A current transformer steps down the current for a low current meter to monitor the high current circuit. Alternatively, a resistor can be connected in series with the current transformer secondary winding to develop a voltage across the resistance so that when the secondary current passing through this resistor falls below or goes above a preset value, an alarm or trip can be initiated.
• Isolation transformer in current metering circuits: A current transformer while stepping down the current also acts as an isolation transformer to keep the metering circuits isolated from the very high voltages of the primary circuit.
• Remote circuit current status monitoring system: A current transformer can also be loaded with an LED as a discrete current indicator to monitor the circuit condition in a remote location. When the remote circuit current is flowing, the current transformer will generate enough current to light an LED to indicate that the current flowing. When the remote current ceases to flow, the LED gets turned off.

Overcurrent relay applications

Figure 3.17 shows an over current relay application. In this application, the fuses bypass the AC trip coils. Under normal conditions, the fuses carry the maximum secondary current of the CT due to the low impedance path. Under fault conditions, I_sec having reached the value at which the fuse blows and operates trip coil TC to trip the circuit breaker. The inverse current characteristic of the fuse enables to achieve a limited degree of grading.

Overcurrent & Earth fault application

There are two possible methods of connection as shown in Figures 3.18 (a) and (b). Figure 3.18 (b) is the most economical arrangement for this application.

3.12 Summary

Current transformers are used to produce smaller currents of 1 ampere or 5 amperes for the purpose of relay and metering applications. The current transformer construction consists of a common magnetic core linking the primary and secondary windings like a standard transformer. The basic theory of current transformer is similar to that of a voltage transformer, with the turns ratio deciding the secondary current for a particular primary current. The ratio of primary to secondary current is inversely proportional to the number of turns in the respective windings. Unlike voltage transformer, the CT secondary winding is connected in series with the current coils of the relays and meters.

The current transformers are categorized based on voltage, insulation, area of use, construction, application and output currents. Construction types basically vary between current transformers based on the magnitude of currents handled. For smaller currents, a wound primary type is adopted whereas for larger currents, bar primary types are adopted. The third type is window construction which is generally used for detecting earth faults in cable circuits.

CTs are mainly used for metering and relay applications. The metering can be either for tariff purposes or for reference purposes. Though accuracy is important in all the applications, the accuracy for tariff metering shall be the highest and general standard is to ensure an error of less than 0.2% for major installations. It is generally expected that the secondary of the metering CT shall be able to provide a linear output current in the range of 50 to 120% of the rated primary current for which the CT is used. However to protect the meters at higher currents, the CT is allowed to get saturated beyond 120%. Accuracy limit factor and instrument security factors shall be duly considered in metering CTs to protect the instruments connected. However the protection CTs are expected to correctly operate under short circuit and fault conditions, which are defined by accuracy class as 5P20, 10P20, etc.

The symbol for CT as per IEC is a small circle with the ratio, accuracy class and burden specified by the side of the symbol. IEEE adopts CT symbol in the form of windings. The terminals are identified with P and S in the case of IEC standards, while they are identified with H and X in the case of IEEE standards. Current transformers are always manufactured as single phase types and are connected externally in star or delta for measuring three phase currents.

The magnetization characteristic of a CT is divided into three regions – initial, unsaturated and saturated regions and these characteristics indicate the typical accuracy maintained by the current transformers for specific ranges. Knee point voltage is an important characteristic, which is used for relay applications, mainly in case of differential CTs to match the two sets of current transformers used for differential protection. The rated burden of a CT is the maximum load that can be connected and it is usual to have CTs with burdens in the order of 15 VA to 30 VA.

Current transformers are also defined with other electrical performance characteristics like rated voltage, rated thermal current, short time current, dynamic current, etc. The rated secondary current is an important factor that decides the requirement of the meters and relays to be connected to a CT.

Current error (ratio error) and phase displacement error affect the outputs of a current transformers and the accuracy of final measured values. The composite error defines the requirements of a metering and protection type CT with a higher composite error of above 10% accepted for a metering application, basically to safeguard the meter. Accuracy of a current transformer indicates the percentage error allowable at a minimum range of 50 to 120% of rated current.

Standards for current transformers specify the CT requirements for a normal ambient of 40 deg C and an altitude of up to 1000 m above sea level. Installations with other site conditions would require suitable corrections and additional construction features. The International standards also specify the routine, type and special tests applicable for the current transformers before they are put into service.

One of the important precautions to be taken while using a current transformer is not to open its secondary when it is in service without shorting the secondary terminals. The open circuit operation of a CT can develop a very high voltage at the secondary terminals which can cause harm to insulation as well as operating personnel. Like any electrical equipment CTs shall be handled, operated and maintained considering all safety precautions.

Learning objectives

• Different types of meters
• Classification of meters
• Measurement principles
• Alternating current system, Peak value, RMS value
• Active and reactive power
• Circuit configurations (direct and using instrument transformers)
• Ammeter and voltmeter
• Frequency meter
• Power factor meter
• Power and energy meter
• Measurement of current, power and frequency
• Integrating Instruments
• Registering Instruments
• Recording Instruments
• Digital Instruments
• Instrument mounting and wiring

4.1 Types of meters

The meters intended to be used in electrical installations must be suited to the system of supply, whether DC or AC, and shall match the voltage, frequency, number of phases and the load to be measured. The continuous current rating of the meter shall not be less than the maximum load of the system to which it is connected. Modern meters have a maximum continuous current rating of at least 120% of the rated current and a voltage rating of at least 110% of the rated voltage.

A voltmeter displaying the value of voltage indicates the presence and magnitude of voltage. An ammeter displaying the value of current helps the operators to know whether the equipment is correctly loaded enabling them to optimize loading and avoiding overloading of any equipment. There are other many derived parameters using the above fundamental parameters of voltage and current viz., power factor, power, energy, etc. An indication of a derived parameter like power factor alerts the operator to take appropriate action when the power factor goes below the prescribed limits. Continuous measurement of frequency, monitoring of the same to control it to be well within the prescribed limits is vital for maintaining the healthiness of a power system. Measurement of electrical power helps the operator to know the extent of loading of electrical equipment viz. power/distribution transformers, motors, lighting loads etc.

Active energy measured in kilo watt hours is the primary parameter based on which electricity is either sold or bought. Other derived parameter like demand expressed in kVA or kW helps the consumer to schedule the electrical loads optimally. In an industrial environment, continuous monitoring of the demand helps the consumer to stagger the operation of various units optimally. Utility companies employ power factor capacitors at their distribution substations to maintain the power factor at an acceptable value so that distribution cable sizes are optimized. The monitoring of reactive power is vital for control of the power factor compensation so that power factor is not over corrected. Leading power factor may lead to over voltages that can be detrimental to the system components. Hence kVAR monitoring is also an important factor for an efficient distribution system.

Based on the operating principle and the type of construction, metering instruments are broadly classified into:

• Electromechanical or Analogue type
• Electronic or Digital or Numeric types

Though electromechanical types have been in use from the time metering was considered necessary, they are slowly getting replaced with digital meters/ relays with electronic circuits.

4.1.1 Electromechanical type

Use of galvanometers is very common in laboratory experiments to detect and measure the flow of current in electrical circuits. Figure 4.1 shows the construction principle of a basic galvanometer adopted in electromechanical meters.

The instrument basically consists of a coil located in a magnetic field and when a current is passed through the coil, the coil experiences a torque proportional to the current. It is possible to have an opposing force like a spring to oppose or balance the coil’s movement so that the amount of deflection of a needle attached to the coil is proportional to the current passing through the coil. Such “needle movements” are the heart of the electromechanical meters.

4.2 Classification of meters

Meters may be further classified into following types based on their method of functioning.

• Indicating meters
• Integrating meters
• Recording meters

Those instruments which directly indicate the value/ quantity of the electrical parameter at the time when it is being measured are called indicating instruments e.g. ammeters, voltmeters and wattmeters. In such instruments, a pointer moving over a graduated scale, directly gives the value of the electrical quantity being measured. These meters help the operator to know the values of circuit parameters like current, voltage and power at any given instance.

An integrating meter, as the name implies, integrates the instantaneous values over a particular time period. A well known example is the energy meter (kilo watt hour meter) that presents the cumulative value of electrical energy over a prescribed interval of time, viz. energy consumed every day, every month etc.

While indicating meters and integrating meters help the user to obtain the desired measurement values at the time of observation, these types of meters do not provide the information pertaining to the past. Recording meters record the values of any measurement over a time range preceding the time of observation either electronically or otherwise, which can be retained for reference at any time in the future.

Electrical power is generated and used in forms of AC or DC (Alternating current and Direct current). Thus measurements are required for both AC and DC. However most electrical systems are predominantly AC based and therefore brief description of the parameters normally measured and monitored in AC systems are given in the following paragraphs.

4.3 Measurement principles

An important requirement in indicating instruments is the arrangement for producing deflecting or operating torque (Td) when the instrument is connected in the circuit to measure an electrical parameter. The deflecting torque causes the moving system (and hence the pointer attached to it) to move from zero position to indicate on a graduated scale the value of electrical quantity being measured. The actual method of producing the deflecting torque depends upon the type of instrument and will be discussed while dealing with the particular instrument.

4.3.2 Damping torque

The degree of damping decides the behaviour of the moving system. If the instrument is under-damped, the pointer will oscillate about the final position for some time before coming to rest. On the other hand, if the instrument is over-damped, the pointer will become slow and lethargic. However, if the degree of damping is adjusted to such a value that the pointer attains the final position quickly without overshooting or oscillating about it, the instrument is said to be dead-beat or critically damped. Figure 4.2 shows the graph of under-damping, over damping and critical damping (dead-beat). Damping torque in indicating instruments can be provided by (i) air-friction (ii) fluid friction and (iii) eddy currents. These mechanisms produce opposing torque depending upon the speed of movement of the pointer assembly.

4.4 Alternating current system parameters

The current in an AC system follows the same sinusoidal waveform as the applied voltage, except that its magnitude is based on the impedance Z of the circuit. Due to the inductive nature of the circuit the resultant current in the conductor will lag the applied voltage by an angle Φ. The instantaneous positions of voltage and current in relation to each other can be vectorialy indicated as shown in Figure 4.3, with both V and I rotating 360º in each cycle.

4.4.1 Peak and average values

While discussing alternating current and voltage, we will often find it necessary to express current and voltage in terms of INSTANTANEOUS values, MAXIMUM or PEAK values, PEAK-to-PEAK values, EFFECTIVE values or AVERAGE values. Each of these values has a different meaning which we will discuss in this section. The INSTANTANEOUS value of an alternating voltage or current is the value of voltage or current at one particular instant. The value is zero if the particular instant is the time in the cycle at which the polarity of the voltage or current is changing. The value is the peak value, if the selected instant is the time in the cycle at which the voltage or current stops increasing and starts decreasing. There are actually an infinite number of instantaneous values between zero and the peak value.

The AVERAGE value of an alternating current or voltage is the average of ALL the INSTANTANEOUS values during ONE alternation. Since the value of the parameter increases from zero to peak value and decreases back to zero during one alternation, the average value must be some value between these two limits. You can determine the average value by adding together a series of instantaneous values of the alternation (between 0° and 180°), and then dividing the sum by the number of instantaneous values used. The computation would show that one alternation of a sine wave has an average value equal to 0.636 times the peak value. The formula for average value of voltage is:

E_avg = 0.636 x E_max

Where Eavg is the average voltage of one alternation, and Emax is the maximum or peak voltage. Similarly, the formula for average current is:

I_avg = 0.636 x I_max

Where Iavg is the average current in one alternation, and Imax is the maximum or peak current.

The above definition of an average value shall not be confused with that of the average value of a complete cycle. Because the voltage is positive during one alternation and negative during the other alternation, the average value of the voltage values occurring during the complete cycle is zero for a balanced wave, whereas in net effect the load current is 0.636.

It is not practically useful for a meter to show instantaneous values as the voltages and currents vary continuously (sine wave) and at an extremely fast rate. Hence for measurement purposes, usually alternating voltage and current are expressed in EFFECTIVE rather than in peak-to-peak, average and instantaneous values.

4.4.2 Effective or RMS value

The effective value of an AC waveform can be stated as its equivalent DC value which will produce the same amount of heating in a resistive circuit. Thus an AC current flowing in a resistive circuit of 1 ohm and producing 1 watt equivalent of heat is said to have an RMS value of 1 ampere. The same concept also applies to voltages.

The effective value of a sine wave of current can be calculated to a fair degree of accuracy by taking equally-spaced instantaneous values of current along the curve and extracting the square root of the average of the sum of the squared values. For this reason, the effective value is often called the “root-mean-square” (RMS) value.

Thus,

Applying integral calculus for the sign wave, the effective or RMS value (Ieff) of a sine wave of current will work out is 0.707 (i.e. 1/√2) times the maximum value of current (Imax). Thus, I eff = 0.707 x Imax. When Ieff is known, Imax can be found by using the formula Imax = √2x Ieff = 1.414 x Ieff.

Since alternating current is caused by an alternating voltage, the ratio of the effective value of voltage to the maximum value of voltage is the same as the ratio of the effective value of current to the maximum value of current. Stated another way, the effective or RMS value (Eeff) of a sine-wave of voltage is 0.707 times the maximum value of voltage (Emax),

Whenever an alternating current or voltage value is specified, the value is always the effective value unless there is a definite statement to the contrary. Similarly, all the meters, unless marked to the contrary, are calibrated to indicate effective values of current and voltage.

4.5 Active and reactive power

The power consumed in a circuit is measured in terms of watts and in a DC circuit it is equal to the product of voltage (V) applied across the load and the current (I) flowing into the load.

i.e. W = V × I

Similarly, the total electric power input to be met by the AC source in an AC circuit is called the apparent power and is provided by the multiplication of voltage and current and is termed as Volt-amperes (VA). This apparent power comprises of two components; active power and reactive power. A certain amount of input electricity is always consumed in the inductive and capacitive components of any electrical equipment fed by AC supply (except heaters which are purely resistive). The inductive power which is consumed internally for generating and sustaining a magnetic field or for maintaining electrostatic charge in a dielectric medium (such as insulation or capacitors) and does not produce any practical output is called the reactive power. The power that actually gets converted into useful output (such as mechanical or thermal energy) is called the active power and is given in terms watts or kilowatts (kW).

In AC circuits the parameter “power factor” is necessary to describe the “active” and “reactive” power consumed. Power factor is given by the cosine of the angle by which a AC current wave is lagging the AC voltage wave in an inductive circuit. Taking this lagging angle as Ø, the active power consumed in a single phase AC circuit is given by:

W = V × I × cos Ø

The reactive power is measured in “volt-ampere reactive” or VAR or kVAR and is given by

VAR = V × I × sin Ø

kVAR = kV × I × sin Ø

The total VA input is equal to the square root of the sum of the squares of the active and reactive powers.

The relationship between Active and Reactive power can be seen in Figure 4.6.

In the case of three phase AC systems the phenomenon is the same, however currents flow through three phases to the star or delta connected load. In either type of connection, the power consumption in kilowatts is given by:

kW = √3 × kV × I × cos Ø

Hence, to measure the active and reactive power consumed in a circuit, it is necessary that the meter shall be able to read voltage applied and current flowing as well as the phase angle between these two. Watt meters and VAR meters have two coils namely as voltage (or potential) coil and current coil. These coils are independently connected. The voltage coil is connected across the load to read the voltage and the current coil is connected in series with load to read the current.

4.6 Typical circuit configurations

Basic circuit configurations in single phase applications are covered below, which can be further extended for three phase applications.

• The current in a single phase AC two–wire circuit, with a phase wire and neutral, is measured by the use of a single phase ammeter having its coil in series with the load.
• The voltage in a single phase two–wire circuit is measured by a voltmeter coil connected across the source terminals or the load. The voltmeter consists of a large resistance which makes a only a small current to flow through the instrument unlike an ammeter.

The above single phase connection principles can be extended to three phase systems also. When the quantities to be measured are large and the voltage of the circuit is very high for the instruments to withstand, the instruments are connected to the supply system through instrument transformers. An example of a three phase measurement scheme using instrument transformers is shown in Figure 4.7.

Basically the instrument transformers help in bringing down the operating voltage and current to values that can be handled by the meters. The standard secondary output voltage of a voltage transformer (VT) is 110V or 120V with the primary of the VT connected across the two phases or across phase and neutral. The standard secondary current of a current transformer is 1 ampere or 5 amperes with the primary of the current transformer connected in in series with each phase carrying the load current. Accordingly the meters in 3 phase systems are rated for 110V, 5 amps or 120V, 1 amp, etc matching with the VT and CT specification.

One more advantage with the use of CT is that the same 5 amp meter can be used to read 100 amperes or 1000 amperes just by changing the multiplication factor (in case of digital meter) or by changing the graduated scale (in case of analogue meters).

The meters in a three phase system can be connected in any one of the following configurations:

• Three phase four wire (with neutral)
• Three phase three wire with three CTs
• Three phase three wire with two CTs

4.7 Ammeters and voltmeters

In a DC circuit an ammeter is connected in series with the circuit under as shown in the Figure 4.8 so that the total current or a fraction of the current to be measured (if the current is large, a shunt is used to divert a major portion of current so that only a small current flows through the instrument) passes through the instrument. The ammeter must be capable of carrying this current without incurring damage and without abnormally increasing the resistance of the circuit into which it is inserted. For this reason, an ammeter is designed to have low resistance.

A voltmeter is used to measure the potential difference between two points of a circuit. It is thus connected in parallel with some part of the circuit as shown in Figure 4.8. The voltmeter must have a high enough resistance so that it will not get damaged by the current that flows through it; and also will not affect the current flows in the circuit to which it is connected.

The basic principle of the ammeter and the voltmeter is the same. Both are current operated devices i.e., deflecting torque is produced when current flows through their operating coils. In the ammeter, the deflecting torque is produced by the current or a certain fraction of that current which we wish to measure. A voltmeter is basically a moving coil galvanometer with a resistor in series. It employs a small coil of fine wire installed in a strong magnetic field. When an electrical current passes through the coil, the galvanometer’s indicator rotates. The angular rotation is proportional to the current through the coil. For use as a voltmeter, a series resistance is added so that the angular rotation becomes proportional to the applied voltage.

4.8 Frequency meters

The measure for describing the back-and-forth alternating rate of oscillation of an AC voltage or current wave is termed as frequency. The unit for frequency is Hertz (abbreviated Hz), which indicates the number of wave cycles completed during a period of one second. Prior to naming the frequency unit as Hertz, frequency was simply expressed as “cycles per second.” Older meters and electronic equipment often bore the term “CPS” (Cycles per Second) instead of Hz. A typical AC wave form is shown in Figure 4.9.

Period of the wave form and the frequency of the wave form are mathematical reciprocals of each other. That is to say, if a wave has a cycle period of 5 seconds (taking 5 seconds to complete one full back-forth oscillation), its frequency will be 0.2 Hz, or 1/5th of cycle per second.

In the United States, the standard power-line frequency is 60 Hz, meaning that the AC voltage oscillates at a rate of 60 complete back-and-forth cycles every second. In Australia, Europe and most of the Asian countries, where the power system frequency is 50 Hz, the AC voltage only completes 50 cycles every second which is equivalent to 20 milli-second time period for each cycle.

There are two basic versions of the standard needle type analogue frequency meter: moving-coil meter and ratio meter. Moving-coil meter consists of two coils tuned to different frequencies, one higher and one lower compared to the measuring frequency, and connected at right angles to each other. Frequencies in the middle of the range equalize the currents in the two coils and allow the needle or pointer to indicate the midpoint of the scale. Changes in frequency create an imbalance between these currents, causing the coils and the needle connected to it move over the graduated scale. Ratio meters use two frequency inputs, one of a known value and the other which is to be measured.

The reed type frequency meter is also a common instrument used for frequency measurements. This frequency meter consist of reeds having natural vibration frequencies 90, 96, 100, 106, 110, etc with the reeds marked as 45, 48, 50, 53, 55 respectively on a scale (in a typical 50Hz meter) and installed on a common bar having a flexible support. An armature is attached to the bar such that the 50Hz current when flowing through the coil exerts maximum attraction on the armature couple of times in every cycle (positive and negative cycle). All the reeds of the bar undergo vibration with this input; but the reed having the natural frequency closest to that of the flowing current (whose frequency is being measured) vibrates more prominently. The frequency is read from the scale value opposite the reed having the greatest vibration.

A digital frequency meter converts the input cycles into pulses and uses counters/ integrated chips for displaying the measured frequency.

4.9 Powerfactor meters

A standard analog type powerfactor meter consists of two voltage coils one with a non inductive series resistance and the other with a series capacitance. The deflecting torque is proportional to the phase angle between the voltage and current vectors and operates the needle to show the power factor value. These instruments have no provision for an opposing torque and therefore when the input is disconnected the needle tends to rest at any random value in the scale.

4.10 Integrating instruments

For electricity utilities, the measurements of kilowatts as well as kilowatt hours are important. The kilowatts recorded over a long period of time or at periodic intervals basically help in ascertaining the maximum input power consumed in the system. This value is used for planning the generation capacity and also to assess the trend of power growth for plans needed to meet the future loads. The “kilowatt hour”, which may be termed as the cumulative energy consumed in the system over a period is the acceptable basis for billing purposes both by the electricity seller and consumer.

The energy in a single phase two–wire circuit is measured by the use of a single phase AC watt hour meter having its current coil in series with the phase wire and its voltage coil connected between the phase wire and neutral. There is a constant integration needed for the varying values of voltage and currents in the system over time.

An electromechanical kilowatt-hour meter essentially consists of:

• A small motor with a current circuit connected in series with the line and a potential circuit connected across the line.
• A brake which opposes the turning of the motor.
• A revolution counter which records the number of turns.

The relation between the motor torque and the opposing torque of the brake is such as to cause the motor to rotate at a speed which is proportional to the power flowing in the circuit. Energy or the product of power and time, is proportional to the number of revolutions made by the motor, and is shown on the revolution counter.

Though digital types are increasingly replacing the electromechanical type, the study on meters is incomplete without an understanding on the functioning of electromechanical type meters. Hence we will devote some time on these electromechanical instruments to have a good knowledge on their working principles.

4.10.1 Watt-hour meter or energy meters

From the commercial viewpoint, the measurement of electrical energy is the most important of all electrical measurements. An instrument which measures electrical energy is called an energy meter or a watt hour meter. Since electrical energy consumed by a load adds up with time (watt-hours = watts × hours), it is evident that the watt hour meter is an integrating instrument.

The electromechanical energy meters employ a phenomenon first noted by Dr. Ferraris in 1884 that a motor torque could be produced electromagnetically by alternating current fluxes. The electromechanical meters are combination of one, two or three single-phase watt-hour meter stators that drive a common shaft (rotor) at a speed proportional to the power of the circuit. Rotor revolutions are transmitted through gearing to the drive shaft and indicated by a clock type or a cyclometer type register.

All energy meters are essentially watt meters with control springs and pointers removed. They do however, have braking torque and a counting mechanism. As in a wattmeter, the driving torque in an energy meter is provided by a shunt circuit (carrying current proportional to supply voltage) and a series circuit (carrying current proportional to the load current). Since there is no controlling torque, the moving system will rotate continuously like an electric motor and hence the name motor meters. The driving torque on the rotor is proportional to the power being supplied. A braking or retarding torque proportional to the disc speed is provided by a brake magnet. Consequently, the disc speed will be proportional to the power and the number of revolutions made by the disc in a given time is a measure of the electrical energy consumed by the load during that time. The counting mechanism is so designed that it counts the revolutions of the disc in terms of kilowatt hours (kWh).

General Operation: Figure 4.11(i) shows the general construction of a watt-hour meter. The driving torque on the rotor is proportional to the power being supplied i.e.

Driving torque (Td) α Power

Braking Torque: Figure 4.11 (ii) shows how braking torque is provided in a standard watt hour meter. An aluminium disc is attached to the spindle and intercepts the flux of permanent magnet (i.e. brake magnet). As the disc rotates, eddy currents are induced in the disc. There eddy currents react with the flux of the permanent magnet to produce a braking (or retarding) torque proportional to the disc speed.

4.10.2 Three-phase watt-hour meter

In a three-phase system, energy can be measured by means of two single-phase watt hour meters. The total energy supplied will be the algebraic sum of the two readings. However, this is never commercially done as it would be more expensive and more troublesome than the use of a single three-phase meter. A three-phase meter is merely a combination of two single-phase meters with their moving elements mounted on the same spindle. The total driving torque is equal to the sum of the toques exerted by both the moving elements. Thus only one counting mechanism is required which will directly indicate the energy being supplied to the three-phase circuit.

Figure 4.12 shows how a three-phase watt hour meter is connected in a three-phase circuit to measure energy. The current coils are connected in series in any two lines and each potential coil is across these two lines and the third line. In fact, the connections are similar to two-wattmeter method used to measure power in a three-phase circuit.

It is very important that the two elements are ‘balanced” i.e. the driving torque of the two elements be exactly equal for equal amounts of power flowing through each. If this is not done, the meter will not indicate correct reading on unbalanced load. The balancing adjustment is most conveniently checked with the potential coils connected in parallel and the current coils in series opposition. If the elements are balanced, there will be no rotation of the disc for this condition. The usual lag, load and power factor adjustments are made independently for each element.

It is possible to connect transducers (covered later) in the circuits to produce one pulse for every 0.1 kWhr (or kVAhr) consumed at the connected load point. A controller can be used to record and add these pulses for every 15 minutes or as desired to get the average power used over the last 15 minutes. This is called as the “sliding window”, as these values change every 15 minutes or adopted time period. The controller compares this 15 minutes running average with a set target load value (which is normally 30 minutes) to operate a set of relays in case the extrapolated demand is likely to exceed the maximum allowable or the target kW value. The relay contacts are subsequently used to switch off non-critical electrical loads in the next 15 minutes so that the average overall load in the 30 minute period can be limited to the prescribed/ target maximum kW value.

4.11 Registering instruments

In this section let us have a look at the various types of registering instruments used for electrical measurements.

4.11.4 Summated metering

In some of the installations it is possible that multiple supplies are provided but for billing purpose it is necessary to use a single meter to sum up the consumptions of the different feeders. Though summing up of individual consumptions may give the overall consumption, the summing up of demands will not give the true demand due to the diversities involved in the different feeders. In these cases, a summation current transformer is used to get the overall demand and consumption pattern of multiple circuits in a single meter.

A summation CT is an interposing Current transformer which has n+1 winding, where n = number of feeders. The “n” secondaries are used to measure the current in the “n” feeders and the extra (+1) winding is used as a single secondary winding summing up all the ‘n’ currents and its current is used as input to a single energy/ demand meter to record the summated current.

When the two load currents flow through the summation CT, the two primary fluxes add up in its core vectorialy resulting in the total primary flux. This total flux is linked with the secondary winding of the summation CT. Due to transformer action, a proportional secondary current is produced in the secondary circuit and flows through the energy meter. The current seen by the meter is thus the vector summation of the two currents.

The summation can also be achieved by using one of the following methods:

• Paralleling of the secondary outputs of all CT leads and connecting them to the meter current coil.
• Independent elements of all CTs mounted on a common shaft of the same meter, so that the shaft gets the total torque to record the total input and consumption.
• Separate circuits, each fitted with a transducer to record the consumption at different points independently in the form of pulses and transmitting the pulses simultaneously to a common meter which can record the total consumption.

4.12 Recording instruments

There are two types of recording devices:

• Analogue recorders
• Digital recorders

Strip chart type: Analogue recorders of strip chart type consist of:

• Long roll of graph paper moving vertically.
• System for driving the paper at some selected speed. A speed selector switch is generally provided. Chart speeds of 1 – 100 mm/s are usually used.
• Stylus for making marks on the moving graph paper. The stylus moves horizontally in proportional to the quantity being recorded.
• Stylus driving system which moves the stylus in a nearly exact replica or analog of the quantity being recorded.
• Range selector switch to ensure input to the recorder drive system is within the acceptable level.

Most recorders use a pointer attached to the stylus. This pointer moves over a calibrated scale thus showing the instantaneous value of the quantity being recorded. An external control circuit for the stylus may be used.

Paper Drive Systems: The paper drive system should move the paper at a uniform speed. A spring wound mechanism may be used but in most of the recorders a synchronous motor is used for driving the paper.

Marking Mechanisms: There are many types of mechanisms used for making marks on the paper. The most commonly used one is marking with “Ink filled Stylus”. The stylus is filled with ink by gravity or capillary action. This requires that the pointer shall support an ink reservoir and a pen, or contain a capillary connection between the pen and an ink reservoir. In general, red ink is used but other colours are available and in instrumentation display, a colour code can be adopted. The stylus, moving over the paper with pre-printed scales, traces the variations of the input signal. This method is most commonly employed as a wide range of recording speeds is possible and also there is little friction between the stylus tip and the paper. The disadvantages of this method are that ink spatters at high speeds, batches at low speeds and clogs when the stylus is at rest. These recorders can only be used for slow variations as the ink flow cannot support quick movements of the pointer mechanism. For example, transient changes such as large voltage dips cannot be accurately read from the paper records of a recording voltmeter.

4.13 Digital instruments

Digital instruments display the numerical quantity to be measured in the form of a digital number. A panel displaying digits represents the value that is measured.

Meters are available for every measurement like voltage, current, frequency etc. But their versatility is in terms of possessing multiple measurement capability in a single meter measuring and recording active, reactive and apparent demand/ energy in addition to the basic and standard parameters. Static multi function meters are microprocessor based and have non-volatile memory to record, store and communicate the data. The multi function electronic meter in its basic form has a power supply, a metering engine, a microcontroller, other add-on modules such as Real Time Clock (RTC), LED/LCD display, communication ports/modules, etc. A block diagram of a digital voltmeter is shown in Figure 4.15. The analogue to digital converter converts the analog input into the digital code that is proportional to the magnitude of the incoming analogue signal. The system also incorporates a feedback circuit and successive approximation register for continuous measurements.

Electronic energy meters are replacing the traditional electromechanical meters in many residential, commercial and industrial applications because of the versatility and low-cost afforded by electronic meter designs. The measurement circuits are electronic with LED (Light Emitting Diode) display or LCD (Liquid Crystal Display). The accuracies of electronic meters are typically class 0.2, 0.5, 1.0. They can be used to cater to multiple types of connections like CT-VT operated, CT operated, 1-phase or 3-phase Direct connected segment. The block diagram in Figure 4.16 shows the major components of a typical electronic energy meter.

The electronic meters are difficult to tamper with and if somebody does attempt to tamper them, they can be programmed to send alarm signals and record the information. The programmability of microprocessor has become a very useful tool for incorporating different features like Tamper data, Import- Export, TOU metering, load pattern analysis, etc. They are increasingly getting applied in SCADA (Supervisory Control and Data Acquisition) systems with communication ports to support various communication platforms as well as for downloading of data.

4.14 Instrument mounting and wiring

There are many ways of mounting the instruments for measurement purposes. The energy meter at consumer premises is usually connected before the consumer’s isolating device (Main Switch) in a separate enclosure either in a separate lockable room or in an enclosure having locking provisions, to avoid the possibilities of tampering. Meters are normally flush mounted on the front of an enclosure which requires suitable cut-outs on the front door.

Meters can also be mounted on DIN rails but in such cases it is necessary to have the glass window to enable direct reading of the meters. The transducers that do not directly provide any displays on their face are DIN rail mounted inside the panels.

Tamper proofing is necessary for energy billing by adopting seals at one or more locations as shown below.

• CT Secondary Boxes (in addition to locking arrangement)
• PT Secondary Box (in addition to the locking arrangement)
• Meter Cabinet
• Meter Test Block
• CT Primary Links and Top Covers
• MD Reset feature
• Meter reading port

In metering installations using current and voltage transformers, the size and the length of the CT and VT wires play a major role in measurement accuracy. For example, large voltage drops can take place in undersized PT wires which can reduce the voltage being measured, thereby causing lower energy being registered by the meter. Such errors in some installations can be as large as 1% or more in energy recording and hence the additional investment made by the Utility companies for the high accuracy meters and VTs would go wasted.

Most metering circuits use fuses in the VT outputs backed up by primary fuses upstream. The failure of any of these fuses, if not detected will affect the measurement reliability. Normally fuse failure relays are a necessary in such cases. Modern meters have self checking facilities that can indicate major mistakes in the input circuit.

Another problem associated with CT metering is the resistance of the CT lead wires adding burden to the CT thereby affecting the accuracy of measurements. In long distance metering higher cross section wires are needed in CT circuits to minimize the burden.

It is also preferable to route the metering cores of the CT and VT in suitable metal conduits to protect them from physical damage and tampering

4.15 Summary

The meters chosen for an installation shall match the electrical system conditions like voltage, frequency, AC or DC, etc. Based on the operating principle of the meters they are broadly classified as electromechanical or digital (analog and numeric type). Electromechanical meters usually have a needle pointer which moves under the influence of the measured electrical parameter. Preset calibrated values are marked on a dial over which the pointer moves to indicate the measured value. Digital meters on the other hand, sample the inputs and integrate the samples over a time period to give the digital display of the measured value.

Based on application, meters may be further classified as indicating type, integrating type and recording type. While indicating and integrating type meters give the values of the measured parameters at a particular instant of time or over a period of time, recording meters help to keep the past data for future reference for study of system growth, improvements etc.

In DC systems, the main electrical power measurements are usually voltage, current and power. However in AC systems there are some more important electrical parameters to be measured viz., peak and average values, peak to peak value, RMS value, frequency, power factor, Active/ reactive power, etc.

Three phase AC measurements are carried out done with single phase PTs and single phase CT’s connected in different configurations to match the system connections. There are many ways of measuring the energy consumption and one popular method is to measure the power consumption in AC circuits by connecting current transformers in two phases only.

Ammeters, voltmeters, frequency meters and energy meters are the other common meters used in AC systems. Energy meter of analogue type has an induction disc which rotates when power is consumed. Mechanical dials are used to give the energy consumption readings in analogue types. Most of the installations in the world still employ these electromechanical type energy meters which are quite economical and proven to be reliable with reasonably good life.

Registering meters are used to record power consumption. Trivector meters measure the active, reactive power and help to assess power factor of a system, so that improvements can be implemented in the system. Multiple tariff meters record the energy consumptions at peak and non-peak periods to enable utility companies charge differential rates based on the power demand of the grid and to limit the power consumption by industries during peak periods.

Learning objectives

• Types of transducers depending on principle, active and passive
• How measurement is carried out with transducers
• Average responding transducers
• R.M.S. responding transducers
• Power Transducers and their use
• Use of interposing transformers
• Applications of Transducers

5.1 Basics of transducers

A transducer is a device which converts one form of energy to another. Typically, one of these forms can be mechanical, optical, or thermal while the other is mostly electrical or mechanical.

While the term transducer commonly implies use as a sensor/detector, any device which converts energy can be considered a transducer. Transducers may be categorized by application: Sensor, actuator, or combination. Sensors are used to provide signals that are measurable and recordable. A sensor can be defined as “a device which can detect a change in a physical stimulus and turns it into a signal which can be measured or recorded”. For example, mercury in a glass thermometer expands and contracts with the temperature being measured and the contraction/ expansion of the liquid can be read on a calibrated glass tube. A thermocouple converts temperature to an output voltage due to the temperature gradient which can be read by a voltmeter. For accuracy, all sensors need to be calibrated against known standards. In short, a sensor is used to detect a parameter in one form and report it in another form of energy (usually an electrical and/or digital signal). For example, a pressure sensor might detect pressure (a mechanical form of energy) and convert it to electricity for display at a remote gauge.

An actuator accepts energy and produces movement (action). The energy supplied to an actuator might be electrical or mechanical (pneumatic, hydraulic, etc.). An electric motor and a loudspeaker are both transducers, converting electrical energy into motion for different purposes.

Combination transducers have both functions- they both detect and create action. For example, a typical ultrasonic transducer switches back and forth many times a second between acting as an actuator to produce ultrasonic waves, and acting as a sensor to detect ultrasonic waves.

The transducers gain more importance in any continuously running plant, which is monitored and controlled by acquiring data on the operating status. Hence in any process plant or a power plant or any similar applications, where it is always a necessity to measure the various parameters of the plant at various stages, the transducers are employed wherever possible. The transducers are employed to monitor all critical parameters of a plant. These parameters give an indication on how the process is functioning and help the operators to take corrective actions at appropriate times in case these parameters deviate from the normal or established requirements. A common monitoring and control system is in place in such installations to monitor the status of the various parameters at various stages. It is also normal to control most of these parameters form this remote station by giving inputs to the plant areas to ensure that the outputs at each stage meet the plant process requirements. The parameters that are of interest and which can affect the system outputs can be pressure, temperature, levels of liquids, vibrations, etc with each one critical for some specific process and application and may not be critical in some other cases. It is also to be noted that all parameters can be controlled from remote though all can be monitored. Some can be controlled directly and others have to be controlled by manual or by controlling another connected system.

A transducer is often referred to as a transmitter when used for measuring and reporting process parameters to a monitoring and control system. A transducer can have its input directly from the source being measured or from the sensor output and converts to another signal that can be connected to a calibrated device to indicate the value of the inputs. In the early days of instrumentation, the sensors were directly connected to instruments/indicators. Thus most measuring instruments were located in the field. A more elegant method was required to convey the measurements to a remotely located control area. This gave rise to the development of transmitters. The output of these devices was invariably a current or a voltage which made it possible for connecting to the control room in a convenient manner using signal wiring. It should be noted that the accuracy of measurement should not be compromised by the length of wiring from the transducer to the control point. While this may not be a problem with transducers having a voltage output, those using a current signal need to be designed so that the output current is independent of the length of wiring. Current transducers therefore need a power source which can drive he required current irrespective of the burden (resistance of output circuit) and behave like a current source.

Usually, sensors are employed at specific stages to directly measure the required parameters and convert to a measurable signal. The outputs from such sensors are then converted to equivalent electrical signals and taken via shielded cables to a remote end for the convenience of measurement as well as control of these sensor outputs. The monitoring and control is normally done at a remote place and can be a single system having an overall common control over various stages of the plant. A typical process transmitter set-up is as per Figure 5.1.

Transducers provide electrical isolation by built-in transformers, also termed as ‘Galvanic Isolation’, between the input and the output ends. This ensures that the cabling from the output terminals is isolated from the fault level and insulation requirements of the input end. Some of the major advantages of using galvanic isolated transducers are:

• The transducer is mounted close to the source end ensuring substantial reduction in the burdens and cabling.
• The monitoring and display can be mounted remote from the transducer location.
• It is possible to use single output for multiple measurement purposes.
• The burden on the CT’s/VT’s can be reduced considerably.

While the need for transducers in process control systems can be understood in the above context, its use for electrical measurement has other purposes. Measurements in electrical circuits often involve current transformers which though are electrical devices, have limitations of output burden and cannot be used for transmission of measurements to remote monitoring locations. This is also true of any complex measurements which involve current as a measured parameter. An electrical transducer is thus a device which accepts an input electrical quantity such as voltage, current, power or frequency into a proportional dc output for the measurement purpose as shown in Figure 5.2. The design of such a transducer overcomes the burden limitations to a great extent.

By adopting transducers in a circuit, a complex electrical quantity, such as watts, vars, etc measured at the load point can be converted into a load independent dc current signal and transmitted with just two wires over any distance for display, recording or control. The values so obtained can be used to monitor as well as to give corrective signals to the upstream system so as to get the desired predetermined output value. A well known application of transducers is for the variable frequency drives, where speed can be varied or maintained as per the needs. The remote data access system or data control system acquires the actual speed of a motor through a 4 to 20mA signal to monitor the speed of a process drive. The same data system can also transfer a current signal to the variable frequency drive panel as an input to maintain or vary the speed to the desired value, whenever needed.

Through using transducers for indication of watts or vars, the number of signal wires to be laid between source and indicator can be reduced from as many as nine to two. Further, the transducer output wires can have small cross section and insulated for low voltage.

It is common to use protection class transformer outputs as inputs for the transducers which enables transducers to be able to withstand significant short-term overloads. A typical requirement for the current input circuits of a transducer suitable for connection to protection-class instrument transformers is to withstand:

• 300% of full-load current continuously
• 2500% for three seconds
• 5000% for one second

It is common to consider input impedance of any current input circuit to be kept as low as possible, and that for voltage inputs to be kept as high as possible. This considerably reduces errors due to impedance mismatch.

5.2 Types of transducers

Transducers are broadly classified into different types. These types represent the methods adopted or principle of operation for the measurements. The following paragraphs describe their basic functional requirements.

The transducers may be called as input device or output device, depending on how they transfer communication to the other end i.e. whether just for monitoring alone or whether for control of an upstream component or part of a system.

Input transducers are those which convert some property or effect into an electrical signal. Output transducers usually have electrical input and generate a mechanical or some other output.

“Active” Transducers require external auxiliary power supply for their working. An active transducer thus can introduce a power gain using its auxiliary power source. On the other hand, “Passive” Transducers use the power of the input signal for their operation and will work without an external auxiliary power supply. A typical example is a piezoelectric crystal which converts pressure (more exactly strain) to a small signal voltage. Passive transducers cannot introduce any gain to their outputs and require other devices such as amplifiers to be useful.

In electrical systems, the major transducers used are:

• Voltage transducer (Input from VT)
• Current transducer (Input from CT)
• kW transducer (Inputs from VT and CT)
• kVAR transducer (Inputs from VT and CT)
• Powerfactor transducer (Inputs from VT and CT)
• Frequency transducer (Input from VT)

Most of these electrical transducers are provided with external DC auxiliary supply for their correct operation and are usually designed to give an output varying between 4 to 20 mA with 4mA produced at zero measurement and 20mA produced at the full scale measurement.

Figure 5.3 shows three basic transducer types commonly adopted. Type 2 is a two-wire transducer energized by the loop current in which the loop source voltage is included as part of the receiving end. The transmitter signal floats and the grounding is done at the processor end. The Type 3 transmitter has the auxiliary supply voltage fed locally and sources the loop current using this auxiliary supply. In this case, transmitter common is connected to receiver common. Type 4 is a four-wire transmitter energized by an independent auxiliary supply at the transmitter. The transmitter sources the loop current to a floating receiver load.

The output signal of the transducer is dc analogue signal of a definite range but obtained from the ac input, which means that DC analogue signal produced by the transducer will contain a certain amount of alternating component, or ripple. Ripple is generally defined as the peak-to-peak value of the alternating component of the output signal although. It is also named as ‘mean-to-peak’ or ‘r.m.s.’ values. It is normally necessary to indicate the conditions under which the value of the ripple is measured and shall be provided as an important technical data, e.g. 0.25% r.m.s. = 1.0% peak-to-peak ripple.

Transducers with a current output will have a maximum output voltage, known as the compliance voltage. If the load resistance is too high and hence the compliance voltage is exceeded, the output of the transducer will no longer be accurate.

Transducer outputs may be both digital and analogue. Digital outputs are typically in the form of a communications link with RS232 and/or RS485 type links. The response times of digital transducers are comparatively longer than analogue types. This usually is linked on the rate at which values are transferred to the communications link and the delay in processing data at the receiving end.

5.3 Measurements using transducers

Typical transducers with sensor inputs generate 4 to 20mA loop current to the processor or controller where this current is converted to an equivalent voltage across a resistance to provide the calibrated readings. Figure 5.4 shows a typical arrangement used for such measurements. It can be noted that for a current of 0 to 20mA, it is possible to generate up to around 5V to 10V across a 250 to 500 ohm resistance, which is sufficient to provide necessary sensitivity for the output readings.

Though there are many types of transducers, our study will be focused on electrical transducers which are mainly used to read and monitor the electrical parameters of a power generating and distribution system. The main internal parts of a transducer are the digital circuits that samples and analyses the input value to produce the equivalent pre-calibrated output current. With the advent of digital technology, the transducers are built to provide the required accuracy levels of 0.2% to 0.5%, ensuring linear relationship, which are well accepted in most of the installations.

The electrical transducer inputs are similar to the inputs required in the analogue and digital meters depending on the measuring parameter. For example a voltage transducer may just need voltage as its input while transducers for kW, kVA, power factor, etc will require both voltage and current of the system under measurement. The transducers generally require an auxiliary voltage supply as an additional input to ensure uninterrupted operation.

Most electrical transducers accept direct connection up to a maximum of 600 volts AC only. For AC voltages greater than 600 volts, potential transformers are required and hence most of the transducers are normally rated for 110 V AC inputs that can be used for systems up to hundreds of kilo volts. Similarly for AC applications most of the electrical transducers do not accept current input beyond 20 amperes directly into their circuits. For higher amperages, it is necessary to employ current transformers. All AC inputs to the transducers are isolated and level shifted by adopting transformers inside the transducers. Transducer outputs are independent of the connected loads and their variations, meaning they are not directly connected to the load circuit and are isolated. This offers the following specific advantages:

• The transducer can be open-circuited or short-circuited without causing any problem in the load circuit.
• Can transmit the measured data over very long distances using thin wires which make the wiring simple and inexpensive.
• Multiple indicating or recording instruments may be connected to the same transducer without any adjustments or precautions.
• Can be connected or disconnected without affecting the other instruments in the circuit and their outputs.

The majority of present day transducers in the use of electrical measurements produce load independent current signal, in the 4-20mA range, though other ranges of this current as well as voltage ranges are possible. The outputs are usually standardized as current signals like 4 to 20mA, 0 to 10mA, etc, and voltage outputs like 0 to 10V, etc. As stated earlier, the transducers act as current sources for the measuring device at the other end. The transducer’s output voltage adjusts itself to drive the required current regardless of the loop impedance of the output circuit. This current output remains constant up to a specified maximum load resistance which ensures that the type and length of cabling needed are little significant to affect the accuracies. The receiving instrument shall typically have a low input resistance (10-250 ohm) to make the current signal immune to electrical interference.

When electrical control signals were introduced at an early stage in Europe, 0-20mA output range was adopted to ensure conversion from a current to voltage signal by feeding the current through a suitably sized resistor without any offset. Scaling is made easy by direct ratio calculations. Though zero based signals are still available, the 4 to 20mA output range is mostly used in present day transducers as this range has a number of features different from zero based signals. It ensures that the transducer failure condition is clearly identified in the 4-20mA range vis-à-vis 0-10mA current output from the transducer. It shall however be noted that since the signal has a 16mA swing it requires deduction of 4mA while scaling calculations are carried out for the output.

The outputs provide a linear scale with maximum output corresponding to 100% of the measured value, with minimum output corresponding to zero measurement. It means that a 4-20mA transducer measuring 0 to 1000 amperes will provide 4mA output at zero input, 12mA at 500 amperes input and 20mA at 1000 amperes input on a linear scale for the 16mA span as shown in Figure 5.5.

The use of digital technology in the transducers provides the following advantages.

• Improved long-term stability
• Accurate r.m.s measurements
• Better communications facilities
• Possibility of programming the scale factors.
• Multiple functions
• Compact transducer size that can be rail mounted.

5.4 Average responding transducers

As we are aware, the AC measurements can be average or RMS values of the AC wave form. The standard method for measuring the average magnitude of an ac signal is to use a precision rectifier followed by a filter as per Figure 5.6. This measures the “average” value of the sinusoidal AC wave in the rectified wave form with positive cycles. A scale factor can be calibrated into the measurement to show the rms value directly (say by using 1.111 factor which is the ratio of r.m.s. to average for pure sine wave).

The transducers measuring voltages and frequency typically have one input from voltage transformers as shown in Figure 5.7 for measuring the values.

The above simple measurement is sufficient as long as the input is an undistorted sine wave. The rms value may still be closer to the actual value as long as the distortion is limited, within reasonable values to meet the accuracy levels demanded in the measurements. The results are considered acceptable as long as the waveform to be measured has no greater than a few percent distortions, say less than 5%. If the input has higher distortions and consequently more harmonics with higher amplitudes, the form factor of 1.111 for pure sine wave does not hold good any longer. Hence direct reading of rms value will no longer be correct, though the average value may still be reasonably accurate. Accordingly it becomes necessary to measure the quantities with an r.m.s. responding instrument for ac wave forms having more harmonics. However considering the advent of digital loads like PCs, UPS, etc , which are prone for harmonics, it is becoming a normal practice to use r.m.s. technique whether measuring the rms value of pure sinusoidal or non-sinusoidal signals to ensure better accuracy in the measurement results.

5.5 RMS responding transducers

True R.M.S. measurement of ac wave forms is needed for higher accuracy taking care of the amplitudes of the distortions/ harmonics in the ac wave form. The rms responding transducers include an averaging square root circuit as an additional system component to the standard average rectifier circuit, which calculates and reads the rms value of the input wave. Figure 5.8 shows the block diagram of a transducer incorporating the square circuits for measuring the rms value.

The square root circuit samples the true values of the average circuit outputs to provide the mA output in proportion to the true rms values without any need for multiplication or correction factors.

5.6 Power transducers

A watt or power transducer measures true electrical power delivered to a load and converts that measurement to a dc voltage or current signal proportional to the power measured. As we are aware the true power output is dependent on three parameters – voltage, current and powerfactor of the system under measurement. Hence the transducer shall be able to measure the phase shift between the voltage and current to arrive at the power factor of the system. Typically the transducers have two input channels, one for the voltage and the other for the current, when powerfactor values are needed. They may also be provided with two voltage inputs from two different sources, like generators to be operated in parallel, to monitor and control the phase angle of the two sources precisely and adjust their end power factor by sending appropriate signals to the sources.

Considering typical wave forms of the two channels A and B as per Figure 5.9, the phase angle difference between the channel inputs is 360 × P1/P. In case the two channel inputs represent two voltages of two independent supplies, the phase-shift between these two sources can be detected which is useful for paralleling/synchronising applications. In a power transducer, these two channels are basically the voltage and current inputs typically from a distribution feeder feeding a load which is used to arrive at the power factor for calculating the active and reactive power values.

Phase angle is determined by timing the crystal clock pulses over a specified gate time which is selectable from say 10 milli seconds to 199.99 seconds. By selecting the minimum gate time of 10 milli second, the update rate of measurements can be up to 20 per every second for a 50/60 Hz ac line frequency. Improved accuracy is obtained by making the gate time long enough so that multiple cycles can be averaged.

The transmitters determine the frequency by taking the inverse of period as measured with a calibrated quartz crystal time base, which are then processed in software. The analog output is generated by an ultra-linear 16-bit digital-to-analog (D/A) converter for 0.02% output accuracy. The power transducers are used in both single phase and three phase applications. A typical single phase application requires voltage and current transformers to provide the inputs to the transducers as shown in Figure 5.10.

Three phase power measurements require more than one VT and CT for better accuracy. Following are the different combinations of transducers adopted for power measurements in electrical systems.

• 3 phase 3 wire Delta (1-1/2 element)
• 3 phase 3 wire Delta (2 element)
• 3 phase 4 wire star (2-1/2 element)
• 3 phase 4 wire star (3 element)

The delta and star in the above types indicate how the instrument transformers are connected to the transducer inputs. Figures 5.11 to 5.14 give these different types and the connections involved. Basically the ½ type represents use of lesser number of VT compared to the number of CT in the circuit connections/ inputs for measurement purposes. All these types give almost identical output values for a single system.

For a three phase balanced load like a motor, it is usual to connect one centre phase CT for power measurements as this can save considerable cost in a process plant with multiple motor loads of different ratings. Figure 5.15 shows a typical arrangement adopted.

5.7 Use of Interposing transformers with transducers

Interposing transformers are commonly used in protection circuits to match the phase angles of the currents in the current transformers used at two different circuits, for comparison purposes. In traditional transformer differential schemes, the requirements for phase and ratio correction were met by the application of external interposing current transformers (ICT’s), as a secondary replica of the main winding connections, or by a delta connection of the main CT’s to provide phase correction only.

Transducers generally do not use interposing transformers for measurement purposes. However some times, it is possible that the transducer is rated for 1 ampere while the CT’s are rated for 5 ampere outputs OR vice versa. In such cases, it would be necessary to introduce a 5/1 A CT or a 1/5A as interposing between the main CT and the transducers. Figure 5.16 shows typical transducer connection using interposing transformers.

5.8 Applications of transducers

Today the transducers are finding applications in many process industries for their simple connection requirements, cost, sensitivity, etc. In application of transducers, it is necessary to ensure the following.

• Accuracy – The output shall be close to the “true” value of the variable being measured.
• Sensitivity- The measured results shall show consistent values with no random variability in the measured variable.
• Operating range – Shall be able to cover the full operating range of the system being measured with accuracy and precision maintained over the entire measuring range.
• Calibration – It shall be possible to calibrate the transducer easily with minimum efforts.
• Drift – The transducer does not lose accuracy over time.
• Reliability with zero failure chances.

The modern transducers are able to provide most of the above features which had also proven results in many process industry applications as well as SCADA applications in power plants. The transducers are widely used in the following applications.

• Electro magnetic applications like photo detector (light levels to resistances), magnetic tapes, optical disks, etc.
• Thermo electric applications like Resistance temperature detectors, thermistors, thermocouples, etc.
• Photo electric applications like LED, photo diodes, etc.
• Electro chemical applications like pH probes, gas analyzers, gas detectors, etc.
• Electro mechanical applications like galvanometer, load cells, strain gauge, etc.
• Electro acoustic like microphone, loud speaker, etc.

Figure 5.17 shows a typical block diagram of a data control system adopted in modern process units with multiple inputs with a few demanding auxiliary supply and some being passive type transducers. The CPU monitors and also controls a few inputs through actuators which control the inputs of the selective channels to maintain desired outputs.

5.9 Summary

Transducers are becoming important components in today’s continuous process plants. The plants are provided with monitoring and controlling system to maintain the process and plant critical which are nowadays measured by sensors and transducers with suitable feedback controls. Electrical transducers are finding increased applications in measurements of electrical parameters like voltage, current, power, etc. Electrical transducers are mostly provided with auxiliary external supply. Most of the transducers are standardized in many applications with 4-20mA dc current outputs which offer specific advantages and ensure failure conditions are known at measuring end.

For ac wave measurements it is necessary to have rms measuring transducers to provide better results even with distorted wave forms. The electrical transducers are provided with inputs from VT and CT to measure the voltage and current and to produce equivalent milli amperes for transfer to remote monitoring systems. The kW/ kVAR power transducers require inputs from both voltage and current transformers and many types of connections are followed in the three phase systems.

Continuous process plants transfers inputs from various transducers with electrical, mechanical and other measurements in the form of linear dc milliamperes which are then analyzed and controlled by the Data control systems.

Learning objectives

• Tariff metering and control and indication metering
• Metering code
• Parameters monitored and purpose
• Tariff Metering Arrangements
• Importance of accurate and reliable metering
• Standards applicable

6.1 Tariff metering and control / indication metering

Power utility companies generally employ two broad categories of metering to monitor, record and bill the power consumption by the consumers. These are

• Tariff metering
• Statistical metering – control/ indication

Tariff Metering is adopted for electrical energy billing by utility companies for all types of consumers – residential, commercial, industrial, etc – whereas the control/ indication metering is used by utility companies as their internal data for monitoring and recording purposes to meet the present and future needs. The accuracy demanded for the tariff metering is quite high whereas lesser accuracies are considered adequate for the control and indication monitoring since these are basically used to provide statistical data.

Tariff metering is invariably installed at the service connection point so as to measure the energy consumed from this point onwards for charging to the consumer. Each utility company establishes different standards for the tariff metering to the different consumers based on the power generation adequacy and consumption patterns of the various consumers in a particular country.

The types of external components used for tariff metering like current transformers, voltage transformers, transducers, meters, etc are also specified and installed with higher accuracies matching the accuracy expected for tariff metering. However, the accuracies for control and indication metering for statistical purposes need not have the same kind of accuracies adopted for tariff metering. Some of the practices generally followed by many utility companies are as below.

• The consumer is responsible for the supply and installation of all service equipment like local isolation switches, enclosure box, etc. The revenue meter is normally supplied by the utility company and remains as its sole property.
• It is mandatory that the utility companies shall be given free access to these tariff meters to check the performance of the meters, to take the readings and also to arrange replacements, etc., as may be needed, during the normal operation. This is applicable for single as well as multi occupancy buildings

Control and indication metering is mainly adopted at the major transmission and distribution substations of the utility companies for their internal use to monitor and control the power being distributed and also for future growth projections.

It is to be noted that the CT error can vary with CT magnetization current based on its characteristics. Some CTs having with low knee point voltage may have an error greater than 0.5% while others with relatively higher knee points (e.g. line CTs) will have comparatively lower errors. However even the ones with errors greater than 0.5% may still be usable provided that the accuracy of the measuring instrument is chosen to get a higher accuracy.

Generally the values recorded by tariff metering could be just kWH alone in case of smaller consumers while for large consumers (commercial and industrial establishments) the metered values can include maximum demand and the kVAH to take care of the power factor to be maintained at these installations as per the utility regulations. On the other hand, the control and indication metering try to collect the maximum data at various points in the distribution system including voltage, current, etc which are not normally metered for tariff purposes. The details collected help the utilities to see the trend of power quality, power consumption growth, etc so that long term plans can be made to meet future growth and also to help cut down transmission and distribution losses, based on the actual consumption recorded by tariff meters. Table 6.1 gives some basic comparisons between the two types of metering.

6.2 Metering code

Many countries have adopted or established separate companies for distribution of Electricity and metering practices are established by these independent companies for providing connections and charging the consumers. These distribution companies are generally under the purview of regulatory authorities to ensure uniform practices are adopted by the companies without any preferential treatments to the various consumers.

In any installation, it is necessary that the installation is certified by a professional electrical engineer who is normally provided with license to provide such services. The professional is expected to follow the codes established by the utility companies and give all basic data needed for supply connection at the appropriate voltage.

All the utility companies have established codes for metering purposes which define the types of metering adopted for a particular installation and the basic provisions expected from the consumers. The metering codes can be broadly classified as per Table 6.2 based on the magnitudes of voltage and/or loads.

6.3 Parameters monitored and purpose

Meters are also adopted for monitoring and control purposes without using them for tariff purposes, mainly to achieve the long term goals and objectives of the utility/ distribution companies and the regulatory authorities. With electricity consumption increasing in most of the countries, the national grids of many countries see unexpected demands during economical growth conditions and also see fall in power demand during some bad times. The utility companies generally prefer to keep their systems flexible to meet any variation in the power consumption patterns happening in different parts of their country due to socio-economical reasons. Following are some of the major parameters monitored by distribution companies with installation of suitable meters at select places owned by them.

• Grid network Control: Along the grid there are many substations at different voltages that are established to serve different regions and different types of consumers. These networks are provided with meters to record Voltages, frequency and loads at different nodes so that they can be constantly monitored and be ensured that network operation criteria are met (e.g. plant approved load, voltage profile, generator capability, transmission stability, switchover operations, etc.). These parameters help the companies to know how stable the distribution system is and also to plan for improvements before it is too late.
• Each regional grids are usually provided with a centralized master station, which has records of MW, MVAR inputs and also have the data on the installed power generation capacity. These stations provide the services for long term planning studies and security analysis. It also helps to fill the gaps in power demands and also tries to identify possible metering errors.
• Generation business unit is provided with values of system frequency and generation loads. These generation dispatch and control metering values are constantly monitored and assessed with available reserves. They help the Government to take correct investment decisions in power sectors to meet the expected demand growth and augmentation needed in the existing power plants.
• Historical metering information of the various areas at substations and receiving ends and metered consumption are used by utility companies to calculate the transmission and distribution losses so as to take corrective steps for minimizing the losses as part of energy conservation efforts. This also helps to substantially save on the investments needed for new power plants and associated infrastructure requirements.

Most of the metering values are continuously logged in modern computer systems on a instantaneous, hourly, daily weekly and monthly basis depending on the importance of the parameters. This helps the data to be retrieved whenever required for analysis and future projections.

6.4 Tariff metering arrangements

Tariff metering for residential and commercial LV installations up to a maximum load of around 30 amperes are invariably connected by direct reading energy meters and beyond these loads it is done through CT operated meters. Normally smaller installations of less than 3kW to 5kW are provided with single phase direct reading single phase energy meters. Table 6.3 gives some of the common types of tariff metering arrangements followed in many countries. (Assuming 230V single phase distribution)

HV metering is connected using Voltage and current transformers. Generally the metering equipment (current and voltage transformers) is supplied and installed by the utility companies. The consumer is expected to provide space and some basic facilities for the installation of metering equipment in accordance with the utility operator’s requirements. The meter enclosure shall be located at a convenient location which normally requires approval by the utility distribution company and shall be close to the entry point of the consumer premises.

6.5 Importance of reliable and accurate metering

The metering helps on the following important factors

• For tariff purposes
• For ensuring quality distribution
• To facilitate future growth plans at appropriate times.

Hence the meters installed in the power system shall guarantee that the system is properly functioning and also shall ensure that the investment decisions taken by the Government and utility companies based on the meter readings serve the real purpose of meeting the consumer’s expectations and are also economically viable. It means that the accuracy of meters and the associated components is very much important to get authentic and dependable values all the times. Another issue is related to the reliability of the metering instruments because an energy meter recording lower values can affect the income of the utility companies and instruments reading higher values can create protests from consumers. It is also possible that some transformers along the distribution lines may be over loaded due to wrong ammeters having high negative errors. Some of the important design and installation features to be considered in the installations are as below.

• The accuracy of a meter, whether ammeter or energy meter is likely to fall when currents much lower than the nominal ratings and especially when connected by current transformers. Hence it is very important to consider the CT ratio to match with the actual load current of the system to ensure that the secondary output currents from the CT are not too low in the measuring range.
• It is further to be realized that the tariff meters provide the main source of income for the utility and distribution companies. The tariff rates are fixed based on the capital cost and operating cost of the distribution systems and it is vital that energy meters that provide the consumption figures are reliable and accurate to ensure customer satisfactions and to comply with the Electricity Act and local electricity codes adopted by the Government.

6.6 Standards applicable

The metering codes of the utility companies normally set standards to be followed in regard to the procedures for power supply applications, the type of components to be installed, the accuracy classes of the meters to be adopted for the various tariff classes, etc. It is a standard practice to provide meters with high accuracy (0.2) for high end customers, who are the major contributors to the exchequer of the utility companies. Lower accuracy classes in the order of around 1.0 are generally accepted for single phase and three phase LV installations, while an accuracy of 0.5 is adopted for medium industries and commercial establishments having HV metering.

International standards like NEC define detailed descriptions related to consumer installations, which always include local meters. The test requirements of the CT, VT are defined in IEC and IEEE standards, which are universally adopted by many utility companies and the manufacturers of these items.

• IEC 62052-11:2002, Electricity metering equipment (a.c.) – General requirements, tests and test conditions – Part 11: Metering equipment.
• IEC 62053-11:2003, Electricity metering equipment (a.c.) – Particular requirements – Part 11: Electromechanical meters for active energy (classes 0,5, 1 and 2).
• IEC 62053-22:2003, Electricity metering equipment (a.c.) – Particular requirements – Part 22: Static meters for active energy (classes 0,2 S and 0,5 S).
• IEC 62053-23:2003, Electricity metering equipment (a.c.) – Particular requirements – Part 23: Static meters for reactive energy (classes 2 and 3).
• IEC 62053-31:1998, Electricity metering equipment (a.c.) – Particular requirements – Part 31: Pulse output devices for electromechanical and electronic meters.
• IEC 62053-61:1998, Electricity metering equipment (a.c.) – Particular requirements – Part 61: Power consumption and voltage requirements.
• IEC 62054-11: Particular requirements for electronic ripple control receivers.
• IEC 62054-21: Particular requirements for time switches.
• IEC 62059-11 Electricity metering equipment – Dependability – Part 11: General concepts.
• IEC 62059-21 Electricity metering equipment – Dependability – Part 21: Collection of meter dependability data from the field.
• IEC 62059-41 Electricity metering equipment – Dependability – Part 41: Reliability prediction.

IEC-62056 is the series of standards to be issued defining data exchange requirements for electricity tariff metering.

• IEC 62056-21, Electricity metering. Data exchange for meter reading, tariff and load control Part 21: Direct local data exchange.
• IEC 62056-46, Electricity metering. Data exchange for meter reading, tariff and load control. Part 46: Data link layer using High level Link Data (HDLC) protocol.
• IEC 62056-53, Electricity metering. Data exchange for meter reading, tariff and load control. Part 53: Companion Specification for Energy Metering (COSEM) application layer.
• IEC 62056-61, Electricity metering. Data exchange for meter reading, tariff and load control. Part 61: OBIS – Object identification system.

IEC 61107 (presently IEC 62056-21) defines the standards for computer protocol to read utility meters. The meter can send data in predefined formats to a hand held unit (HHU) using serial port communication link. The data can be exchanged with modulated light with photodiode, or hard wires with 20mA current loop. A sign-on procedure with HHU addresses a particular meter by number. The data from the meter are generally grouped to various security settings like ‘no security group’, ‘ low security groups’ and ‘ the high security groups’. For data in low security group, a password authentication of the HHU is required before information can be read. For high security parameters, a cryptographic password exchange is needed between the HHU and the meter before data can be downloaded. These modes are usually protected by anti-tampering features such as switches that sense if the meter enclosure has been opened.

6.7 Summary

Utility companies throughout the world incorporate metering for two main purposes, one for the tariff to be collected from the consumers and the other for internal review mainly for condition monitoring as well as for future planning and to ascertain the growth projections. The tariff metering is adopted at the point of power supply termination to the consumer, with the data shared with the consumers. The control metering is adopted at many parts of the transmission and distribution systems along the grid with data used for internal purposes of the companies only. While tariff metering is mainly record the total energy consumed including total demand in some cases, the control metering reads various electrical parameters including voltage, frequency, current, etc at regular predefined intervals to continuously monitor the condition of the grid and also retrieve old data as and when needed.

Most of the countries now have the concept of independent distribution companies with a Government regulatory authority monitoring their functions. The consumers are protected with established metering codes of the distribution companies which define the classification of metering based on consumption pattern and the purpose of use like residential, commercial, industrial, etc. The tariff metering is adopted with meters having accuracies of 0.2 to 0.5 while control and indication metering are accepted with accuracies in the order of 0.5 to 1.5. The metering codes also define rules related to import/ export metering based on the captive power availability at consumer end and the supplies are governed by separate agreements between consumer and the distribution companies.

The meters for tariff shall be properly sealed and protected to avoid pilferage and tampering. Depending on the power consumption, the metering can be LV single phase, three phase or HV/EHV 3 phase system. While direct metering is adopted for LV systems up to around 30 amperes, CT meters are adopted for loads beyond 30 amperes per phase and also for HV metering.

IEC and IEEE standards define the metering accuracies and the tests to be conducted on the metering components. Today, many of the countries are collecting the metering data from remote by having data communication links with local meters. This considerably saves time and also ensures availability of data at any time. The data exchanges follow HDLC/COSEM protocol with requirements being defined in new IEC standards under draft stage.

Learning objectives

• Testing of a voltage transformer
• Testing of current transformers
• Testing of meters and transducers
• Commissioning of instrument transformers
• Commissioning of meters and transducers
• Maintenance of current transformers, voltage transformers and transducers

7.1 Requirements of testing

Any electrical installation is considered incomplete unless it is visually checked, tested and commissioned in a well defined manner adopting procedures and test equipments that are reliable and are safe to use. The correct operation of electrical equipments require all the protection system components like CT, VT, etc shall be tested as defined by international standards. Similarly, metering plays an important role for the utility companies to ensure that fair charges are collected through the tariff meters and also to ensure that the future demand projection figures are reliable and useful. IEEE and IEC are the two major international standards, which are followed for the specification and testing requirements of the electrical equipments and components by many electrical installations in the world. IEC requirements are analyzed in this chapter considering that many countries adopt the IEC standards for their electrical installations, which is expected to be the main standards to be followed in future.

7.2 Testing of voltage transformers

Following are the tests recommended on voltage transformers as per IEC 60044 Part 2

7.3 Test procedures

7.3.12 Transmitted over-voltages measurement

A low-voltage impulse (U1) shall be applied between one of the primary terminals and earth. For single-phase current transformers for GIS metal-enclosed substations, the impulse shall be applied through a 50 ? coaxial cable adapter with the enclosure of the GIS section connected to earth as to be done in service. The terminal(s) of the secondary winding(s) intended to be earthed shall be connected to the frame and to earth.

The transmitted voltage (U2) shall be measured at the open secondary terminals through a 50 Ω coaxial cable terminated with the 50 Ω input impedance of an oscilloscope having a bandwidth of 100 MHz or higher which reads the peak value. If the current transformer comprises more than one secondary winding, the measurement shall be successively performed on each of the windings. In the case of secondary windings with intermediate tappings, the measurement shall be performed only on the tapping corresponding to the full winding. The over voltages transmitted to the secondary winding (Us) for the specified over voltages (Up) applied to the primary winding shall be calculated as follows:

Us = (U2 / U1) × Up

In the case of oscillations on the crest, a mean curve should be drawn, and the maximum amplitude of this curve is considered as the peak value U1 for the calculation of the transmitted over voltage.

The voltage transformer is considered to have passed the test if the value of the transmitted over voltage does not exceed the limits given in Table 7.5.

7.4 Testing of current transformers

IEC 60044 Part 1 specifies the testing requirements for the current transformers. The major tests recommended as per the standard are as below. It may be noted that though the current transformers operate on the principle of currents flowing through them, the tests are mostly related to their insulation levels applicable for a particular application. Hence most of the type tests are done with voltages applied across the windings, similar to the voltage transformers, except for test verifying short circuit characteristics, temperature rise, etc.

7.5 Test procedures

7.5.13 Polarity test

Figure 7.1 shows the simple testing arrangement for verifying the CT polarity markings at the time of commissioning electrical systems. The factory test is similar in principle except for the large power source.

Connect the battery negative terminal to the current transformer P2 primary terminal. This arrangement will cause a current to flow from P1 to P2 when + ve terminal is connected to P1 till the primary gets saturated due to the DC Voltage. If the polarities are correct, a momentary current will flow from S1 to S2.

A centre zero galvanometer is connected across the secondary of the current transformer. Touch or flick the +ve battery connection to the current transformer primary terminal P1. If the polarity of the current transformer is correct the galvanometer should flick in the +ve direction.

7.5.14 Test for CT magnetizing curve

It is necessary to test the characteristics of a CT before it is put into operation, since the results produced by the relays and meters depend on how well the CT behaves under normal and fault conditions. Figure 7.2 shows a simple test connection diagram that is adopted to find the magnetic curve of a CT.

In the circuit shown in Figure 7.2, the current is passed through the secondary from zero to the full rated current across S1 and S2. Hence a milli-ammeter is used to measure the currents, and the corresponding voltages across S1 and S2 are measured. This basically indicates the voltage generated at the secondary terminals corresponding to the currents flowing in the winding.

The readings shall be taken until the effect of increase in the current does not generate a proportionate in the voltage. The curve is to be drawn and the exact knee point is decided where the current increase of 50% causes less than 10% change in the excitation voltage.

7.6 Tests on meters and transducers

The meters and transducers are used in low voltage circuits after the necessary step down of the primary voltages and currents using VT and CT. Hence the meters and transducers do not undergo exhaustive high voltage tests even when they are used in HV and EHV systems. They are basically tested for the following before they leave manufacturer’s place.

• Dielectric voltage tests as applicable for LV systems (maximum 2kV)
• Verification on the calibration to meet the desired voltage/ current range with specified accuracies for the range from 20 to 120% of the rated voltage or current.
• The transducers are tested for their ability to produce the current in linearity to the range of measurements for which they are designed, with external auxiliary supplies.

Transducers are basically tested to check for the current loop characteristics with external auxiliary supply when the actual input varies between 0 to 120%. The linear characteristic of the output current values corresponding to input values are measured and ascertained to be within the accuracy class of 0.5, adopted for most of the transducers.

Most of the modern transducers of established manufacturers are calibrated using special test equipment, having current and voltage dispensers which are stable to within 0.05% and have total harmonic distortion levels of less than 0.02%. They use precision voltmeters and watt meters, with 0.02% accuracy to calibrate to within 0.2% of full scale for current and voltage transducers and 0.2% of reading for power transducers. This accuracy is generally applicable for a complete range of operating conditions including a temperature range of 0 to 50°C and hence calibration and testing is very critical for transducers.

Modern power systems are subject to distortion due to use of electronic control systems and use of non-linear loads, which are not in the control of utility companies. The level of harmonic distortion up to 5% for 3^rd harmonic is becoming common. Hence the errors resulting from harmonic distortion should be duly taken into consideration, while evaluating the use of measuring equipment.

As shown in Figure 7.3, even low levels of 3rd harmonic distortion can easily result in a 1% error on the output of a mean sensing transducer. It should be noted that calibrating or checking a mean sensing voltage or current transducer using a high stability source with low total harmonic distortion, can give erroneous results.

True r.m.s. current and voltage transducers utilize square-law circuitry to measure the true r.m.s. value of the input waveform. Hence these devices are not affected by guaranteed levels of distortion as shown in Figure 7.3. Power transducers respond correctly up to tenth harmonic because they use pulse height-width modulation of a square wave as the basic measurement technique.

Once the metering equipments leave the factories, they are tested in the field before put into service and also at regular intervals to establish their characteristics during the life cycle.

7.7 Need for field tests

The equipments are normally tested at manufacturer’s works and transported to site once the test results are satisfactory. Invariably there would be considerable time elapse between the factory tests and the readiness of site to charge the equipment. The time elapsed may vary from at least a month to several months depending on many factors. The installation and providing necessary connections also take considerable time, even if the site is waiting for the equipment. Following factors are unavoidable from the time the equipment is ready at manufacturer’s place till it is ready to get charged in the place of use.

• Time for packing and arranging for the transportation after completing all the commercial formalities.
• Transportation time from the manufacturer’s works to the place of installation depending upon whether the item is imported or locally available. Even in case of locally available equipment in the country of the project, transportation, distance and the facilities may make the delivery to go from a week to few weeks.
• Possible damages during the transportation either directly on the equipment or indirectly due to unknown reasons. Rough handling and improper packing can also lead to unknown damages.
• The delay in readiness of foundation or building may result in the equipment being kept in unfavorable climatic conditions in the filed that can affect the internal insulation of the equipment. Some times the environmental conditions in a construction site or an existing nearby plant can also result in some deterioration to the internal condition of the components and oil used in the equipment.

All the above reasons plus many other possible human mistakes are the general reasons for delay in energizing the electrical equipment. Hence it is necessary to ensure that there are no internal damages that can affect its life and performance. For example, the oil dielectric strength might have gone down over a period of time that would require filtration before charging a transformer. If not, an internal flashover or short circuit may make the whole project to wait for the transformer getting repaired. Similarly in a switchgear panel, some internal links or shorting may lead to similar issues.

The above reasons are basically related to the delay in energisation due to unavoidable reasons. Once the equipment is energized, it is necessary to have periodical maintenance to keep the equipment in healthy conditions. The maintenance activities may result in replacement of some components, some adjustments in the internal mechanism, some rewiring, etc. All these need also to be checked for correctness before the equipment is put back into service.

Some basic tests are prescribed especially for HV/MV equipment that are to be conducted before restoring the service or charging the equipment for the first time. These tests are named as pre-commissioning tests / checks, commissioning tests, maintenance tests, etc and since most of these tests are done in the field of service, these are referred to as field tests in this chapter. It should be recognized that many of the tests described in the earlier tests call for special test equipment/facilities and therefore are not feasible to be done at the installation site. While the factory tests are done with the object of ensuring manufacturing quality and establishing conformity with the relevant specifications, the aim of field testing is to ensure the health of equipment before energizing (after the first installation or subsequent maintenance work).

Field tests are usually performed by independent contractors, the installation contractor or the manufacturers. The individuals who perform the acceptance tests should preferably be certified and/or licensed for the equipment under test. The system should be initially checked for damage, deterioration and component failures using specific component checks, inspections, and tests defined by the equipment manufacturer. Then the interconnection of the system components should be checked using de-energized and energized methods, to verify the proper interconnection and operation of the components, ON/OFF control, system interlocks and protective relaying functions. It is recommended that all the field tests are witnessed by a person who could be the operator of the plant or in case of shortage of skilled man power, a commissioning engineer who is not associated professionally with the agency/ person performing the tests. Once the above tests are complete, the system can be energized and operational tests and measurements should be performed. All steps and results of the field tests should be carefully documented for review and for use in the future for comparison with the results of future tests. Considerable variation in the results of present tests compared to earlier tests is indicative of problems like deterioration of insulation, dirty environmental conditions, etc.

7.8 Safety precautions

The safety procedures given below are from IEEE Standard 510-1983 which stipulates the safety practices to be followed by all personnel dealing with high voltage applications and measurements so that any possible accidents are avoided due to the presence of electrical hazards while conducting the tests..

Considerations of safety in electrical testing apply not only to personnel but to the test equipment and apparatus or system under test. These recommended practices generally cover the practices needed while testing in laboratories, in the field and of systems incorporating high voltage power supplies, etc. A voltage of approximately 1,000 volts has been assumed as a practical minimum for these types of tests. Individual judgment is necessary to decide if the requirements of these recommended practices are applicable in cases where lower voltages or special risks are involved.

7.9 Commissioning of instrument transformers

Commissioning of instrument transformers need to be done with sequential checks, pre-commissioning tests and final commissioning tests as per established practices and as recommended by applicable standards. Before we review the tests adopted, we will have a review on the importance of these tests and how they are critical for the performance in the long run.

The following paragraphs brief the various steps and procedures for commissioning tests related to the instrument transformers (CT/VT).

7.10 Maintenance of CT, VT and metering devices

Generally maintenance is not a major requirement for voltage transformers and current transformers as they are static equipments. However they are likely to fail due to the following reasons and hence corrective steps and periodical cleaning are to be taken as below.

• Excessive dust or other kinds of pollution can reduce the creepage distance and affect the instrument transformers. The surface dust shall be brushed off the instrument transformer surfaces after taking all safety precautions. Polluted transformers can be cleaned with spirit, petrol or toluene, after availing a small shut down, which does not affect its purpose.
• Traces of arcs and minor surface damages can be easily removed with sandpaper, after which the surface should be treated by applying a thin layer of silicone paste to put back into service.

Meters shall be separated into groups having common physical attributes and the average performance of each group will be determined based on the weighted average of the meter’s percentage registration at light load (LL) and at full load (FL) giving the full load registration a weight factor of four (4).

Analysis of the test results for each group evaluated shall be done in accordance with the statistical formulas outlined in ANSI/ASQC Z1.9 – 1995 Formulas B-3, Tables A-1, A-2 and B-5. The minimum sample size shall be 100 meters when possible.

It is necessary that the meters installed are checked and calibrated at established intervals to minimize errors and to maximize profits.

7.11 Summary

Testing of electrical equipments is necessary to prove their performance, technical capability, and to give confidence to the users on the accuracy and safety factors. The tests are broadly classified as routine tests, type tests, sample/ acceptance tests, special tests and field tests. Since the insulation forms the main requirement of the VT and CT windings, these are tested by different combination of tests like power frequency voltage tests, switching impulse tests, etc to ensure that the equipment can safely operate at the design voltages, which can range from a few volts to several kilo volts. Temperature rise test, short circuit test and ratio tests prove the basic design requirements of the instrument transformers for the chosen application. The ratio test and accuracy test results ensure that the outputs from the equipments provide reliable metering and relay applications. Some of the tests like partial discharge tests, RIV tests are applied for instrument transformers above around 72.5kV ratings. In regard to meters and transducers, the tests are limited to dielectric test at low voltage and calibration/ accuracy checks.

Though factory tests are done on all electrical equipment, it is necessary that field checks and tests are done prior to putting them into service. Field tests require lot of safety practices to be adopted to avoid equipment damages and personnel accidents/ shocks. These include proper grounding, safety clearances, caution notices, proper isolations, providing CT shorting links, etc. Insulation resistance test, polarity test and ratio test are the important tests normally conducted on metering transformers as part of commissioning tests before putting them into service.

Maintenance of metering devices can be followed up with proper schedule based on the location of the devices and the nature of the surroundings (for cleaning the surfaces) as well as the metered readings. Meter calibrations shall be decided based on the accuracy levels and the consumption patterns, so that the errors do not affect both the utility companies and the consumers.

Learning objectives

• Smart metering
• Technology behind smart metering
• Benefits of smart metering
• AMR and metering for smart grids

8.1 Smart metering

As the name implies, “smart metering” shall be able to provide a level of service above and beyond the measurement of consumption of a measurable item and provision of other useful technical parameters, which are already available in many modern meters. The name “smart metering” not only refers to the metering of electrical measurements but also refers to future metering of many public distribution services like water, gas, steam, etc. Smart metering has a range of special functions:

• Measurement consumption over the fixed periods in line with the metering codes and local requirements.
• Storing the measured data for multiple time periods, with the time period adjustable by the service provider and having tamperproof access.
• To provide ready access to the stored data by consumers as well as by the utility companies or their representatives, with suitable interfacing devices like video display unit, hand held equipment (HHE), etc.

In addition to the above, smart metering also provides at least one of the following functions:

• The local display of the data shall be available in a user friendly and meaningful form to the consumers in order to analyze and control their internal consumption.
• The data measured and recorded shall cover broad parameters such that the Distribution Company can use such as parametric details for the purposes of system monitoring, operation and control, future planning as well as loss assessment.
• Shall allow adjustment of billing for different time periods (to facilitate time-of-the day tariffs) as may be fixed by utilities for the purposes of demand control.

The Italian utility ENEL are the pioneers in installation of smart meters since 2001. Important reasons for ENEL to take up this exercise were the expected savings or revenues in the areas purchasing, logistics, customer services and theft prevention. ENEL has chosen smart electricity meters that communicates through Power Line carriers to the nearest substation. Immediately thereafter, centralized control rooms are able to read the data through GSM. By the end of 2005, ENEL had 27 million smart meters installed, of which 24 million meters are being remotely managed and read bimonthly.

8.1.1 Present day smart meters

“Smart” meters have undergone many improvements over time, and today smart meters with the following capabilities are available in many European countries, North America and Australia.

• Possibility to read the meter both locally and remotely, whenever needed.
• Remote control of the output through the meter including isolation of the electricity supply to the customer as an extreme step, in case of defaults.

The data from smart meters are usually collected by any of the following systems.

• Using handheld equipment (HHE) with an infrared optical probe carried by the meter readers.
• Through a public communications infrastructure like telephone, cellular phone or two-way paging.
• Over a fixed private communication network owned by the utility like power line carrier (PLC).

The electricity meter usually communicates by means of a modem or an optical link. Meters are available that can continue to accumulate data at fixed intervals of 15, 30, 60 minutes for almost two to three months continuously. These smart meters are commonly used in commercial and industrial applications and are also available in single phase, 200-ampere versions for residential applications. Data is usually collected manually by connecting the HHE to an infrared optical probe in the front of the meter. The HHE then collects the accumulated data which can be transferred to the utility data systems by the meter readers area wise, for billing and monitoring purposes. Typical smart meters with interface ports are shown in Figure 8.1

8.2 Technology behind smart metering

The smart technology communication infrastructure combines two modern technologies to transfer readings and to achieve management of control between the central control area and the various smart meters, spread across a country. Figure 8.2 shows the basic technological features of a typical smart metering system.

Firstly, communication is achieved from a set of meters which are all interconnected to a common nearby “data concentrator” through the Power Line Carrier. The modem linked to each meter and the data concentrator allows the recorded data to be encoded and decoded and transmitted as an electro magnetic signal much above the 50/60 Hz mains voltage frequency, similar to the internet technology. This PLC technology basically uses the existing network of electrical cables which ensures a convenient and economical solution which is well suited in densely populated areas. However the signal strength deteriorates with the transfer distance and hence requires careful planning for locating the data concentrators so as to effectively collect all data. Subsequently the data concentrators encode the collected data in digital format and transfer to the upstream central control area using the GPRS (General Packet Radio Service) network, which is much more reliable compared to PLC for transfer of data over very long distances. It requires some additional communication equipment like antennas and towers in the selected areas to ensure efficient data transfer. By use of off-peak bandwidth time and efficient data compression, the operating costs for the transfer of information by the GPRS network can be optimized.

Secondly an Information System for data management is needed to complete the full cycle of operation and to achieve the purpose of smart metering. Since the system always involves millions of permanently connected smart meters (to achieve breakeven) it results in a very large volume of data to be transferred, stored and processed. The daily transmission of load data read (on a 15 or 30 minutes intervals) by millions of meters corresponds to many terabytes of data that shall have to be effectively received and handled each day. The management of this data normally involves a powerful and highly complex information system to store and process all these data. The information system must be highly efficient, totally reliable, fully secure to achieve proper readings, the system shall also be able to match several million of devices, which are expected to be in service for many years.

Figure 8.3 shows the internal components of a smart meter (Courtesy: Texas Instruments) currently in the market for a three phase system.

8.3 Benefits of smart metering

Smart metering helps many of the stakeholders in power generation, distribution, metering and actual consumers. A brief list of benefits applicable to different stakeholders is given in the following paragraphs.

8.4 Automated meter reading (AMR)

The advancement of smart metering is paving the way for automated meter reading and smart grid concepts in advanced countries, with companies and Governments implementing policies and regulations to effectively upgrade their power networks.

8.4.2 Technical features of AMR

AMR is a process of digitally recording the energy meter figure values from a location away from the metered point, in automatic set intervals. This process eliminates the traditional manual reading/ recording/processing of the meter data by meter readers. Automatic Meter reading also makes the data recording to be achieved in a fast manner thereby saving considerable time. The AMR can be classified into two types based on the distance between the meter reading station and the target meter.

• Local reading by a hand held device communicating with the meter for collecting the data and then transferring the collected data to the remote control station.
• Collection of data directly from a central office located far away by using appropriate modems to interface with the meters.

Refer to Figure 8.4 for the block diagram.

The typical components adopted in AMR system are:

• Numerical Energy Meter: The measurements are done by a modern electronic meter without any moving parts which is usually provided with an auxiliary UPS to ensure communication even during power outages. The smart meters described in the earlier section belong to this category.
• Interface Device on the meter: Interface device that is capable of communicating with HHE or the remote station, based on the system adopted and the communication media selected.
• Communication Media: This one includes a set of complex sub elements like the local-loops at each end, intelligent switches, and data transfer lines like micro-wave, fiber optics or VSATS.
• Interface Device at remote location: It is functionally similar to the interface device on the meter normally GSM compatible.
• PC with software suite: The software suite shall have facility to dial the target number, establish a data call, collect the meter reading and close the session at selected intervals as programmed.

The various communication media that can be used for AMR are:

• Public Switch Telephone Network (PSTN)
• Wireless in local loop (WLL), which includes Global system for Mobile communication (GSM) or code division multiple access or CDMA.
• Dedicated radio link which includes Low Power Radio (L.P.R.) and High Power Radio (H.P.R.) based AMR.
• Fibre optic/coaxial cable.
• Power line communication and VSats.

In France, eRDF, the new subsidiary of EDF which operates the electricity distribution networks, is having plans to replace the entire residential electricity meters under its control in France with smart meters by 2015. This is expected to install a total of 35 million smart electricity meters which is presently the biggest AMR scheme under adoption.

8.5 Smart grid

A lot of discussion in the USA on electric power networks these days centers around ‘smart grids’. Smart grids are power grids which make effective and innovative use of available mature technologies, particularly the information and communication technology (ICT) to achieve significant economies for the power utilities and appreciable savings to the consumers.

One of the definitions of a smart grid goes as follows: A smart grid is a comprehensive and innovative way of adapting our current grid to fit the electric needs of our culture. A smart grid will proactively allow utilities to reduce peak demand by actively managing production and consumer demand. It will also identify efficiency opportunities ensuring that consumers and utilities have accurate, timely and detailed information on the energy being used. A smart grid is inherently safer than our current grid. With its two-way communication, a smart grid can quickly identify, respond to, and recover from threats and interruptions. The smart grid will seamlessly integrate all clean energy technologies from roof-top solar systems to wind farms to electric vehicles. These benefits are also a step toward becoming more fossil-energy independent.

However, smart grids may mean different things to different people. For example, Tuscan Electric Power Co., Arizona looks at smart grid as six separate concepts/technologies that allow for remote control and monitoring of equipment. Each item offers potential benefits to the utility and to its customers. The concepts are comprised of:

• Substation automation
• Distribution circuit remote switching
• Distribution capacitor switching with communications
• Advanced meters and meter data management (MDM)
• Demand response (DR)
• Home area network (HAN)

Naturally, one of the key movers of smart grids will be the way electricity tariffs are formulated to encourage consumption during lean periods of the daily load cycle (and discourage usage during peak or heavy load periods); in other words, the introduction of ‘time-of-the day’ or demand-based tariffs. One has to be however aware of the pitfalls such as cost of implementation, justification of cost vis-à-vis the benefits accruing and most importantly methods of addressing security issues that may arise while using public datacomm infrastructure for its operation.

A detailed description on smart grid is provided in a report titled Modern Grid Initiative, published by the U.S. Department of Energy (DOE)/ National Energy Technology Laboratory (NETL). The report includes a comparison between today’s grid and modern grid (smart grid) by analyzing key issues through seven major characteristics noted below:

• Self-healing
• Motivation and participation of the consumer
• Resists attack
• Provides power quality for 21st century needs
• Accommodates all generation and storage options
• Enables markets
• Optimizes assets and operates efficiently.

The comparison between services in the current scenario and the smart grid foreseen in the next few years is shown in Table 8.1.

The implementation of the smart grid is enabled with the following drivers in a country and can be delayed in case these driving forces are ineffective or non existent.

• Liberalization of energy markets (in particular full competition at retail level as practiced in the EU since July 2007).
• Evolution of Government regulatory frameworks in supporting competitive market environments.
• Technology developments, mainly in the field of ICT.
• Growing consumer interest on possibilities of reducing their electricity bill and willingness to absorb a part of the expenditure.
• The need to curb energy consumption levels, dictated by international policies and commitments related to energy efficiency and greenhouse gas emissions.

8.6 Summary

Today’s technological advancements are reflected in electrical metering systems in the form of smart metering and automated meter reading. Smart metering refers to metering system that deploys specific communication links in a user friendly manner to collect data from various meters and transfer the collected data to a remote station for billing and monitoring purposes. Smart metering is now being considered for adoption in many public distribution systems like water, gas, steam, etc. The smart meter shall have suitable communication links to facilitate transfer of data and for direct control of the system operation through the meter.

Today’s smart metering systems commonly adopted in US and European countries use local data concentrators for a group of local meters which collect data from the meters through power line carriers thereby saving considerable investment cost. The PLC necessitates the data concentrator to be located close to the metering points, to avoid loss of data and error in readings. The data collected at the concentrators shall then be transferred to a remote location for data management and control having PC and suitable software suites.

Smart metering helps utilities, distribution companies and consumers in many ways. Notable among them are the improved service quality due to competition and reliable measurements with modern instruments. The system efficiency can be improved drastically with reduced complaints on billing errors. Prepaid services would also be possible with smart metering.

The automated meter reading (AMR) is an extended form of smart metering and provides many benefits to the utility and consumers. The automated meter reading almost cuts down the human interface for riding the meter data and instead a remote device directly communicates with local meters for collection of required data at specific intervals of time without need to go to the service locations. Today’s cellular technology AMR in addition to other communication methods like telephone lines, fibre optic cables, etc. can be effectively used to have an efficient system.

The ultimate aim of smart metering is to achieve a smart grid which is expected to have more knowledgeable consumers who will have active participation in load management thereby saving considerable peak grid demands in the long run. The smart grid is aimed for implementation in the coming decade and is expected to provide faster response to the consumers with minimum faults and disturbances and better power quality.

Chapter 1

1. Reactive power in an electrical circuit is given by VA or KVA (True/ False)
2. The instantaneous value of voltage in a DC system is given by Vmax Sin (2πft) – (True/ False)
3. Instrument transformers are based on the principle of electro magnetism (True/ False)
4. Power tariff instruments need not have high accuracy as the cost of investment is high (True/ False)
5. Transducers can be used to provide isolation between high voltage power circuits and low voltage instrumentation circuits (True/ False)

Chapter 2

1. The primary of a VT offers high impedance (True/ False)
2. Measuring instruments on the secondary side of a VT are connected in parallel (True/ False)
3. The secondary of a VT has less number of turns than its primary winding (True/ False)
4. A capacitor type VT cannot be used for very high voltages (True/ False)
5. Ferro resonance is a phenomenon caused by the high voltages of lightning strikes (True/ False)

Chapter 3

1. A CT primary is always connected in parallel with the main circuit (True/ False)
2. CTs used for protection should get saturated at lower currents than those of metering CTs (True/ False)
3. The ratio of currents in primary and secondary of a CT is directly proportional to the ratio of the number of turns in the primary and secondary (True/ False)
4. Opening of the primary of a CT is hazardous (True/ False)
5. The rated burden of the CT should be less than the connected burden of the CT (True/ False)
6. Calculate the percentage ratio error of the CT with the following details:

CT ratio = 100/1

Actual current in secondary for a actual primary current of 100A = 0.87 A

Chapter 4

1. The impedance offered by a voltmeter is low (True/ False)
2. Power factor is the ratio of real power to reactive power (True/ False)
3. A moving coil ammeter can be used to measure AC current (True/ False)
4. A single phase watt meter can be used to measure power in an unbalanced three phase circuit (True/ False)
5. Modern digital ammeters offer very high burden (True/ False)
6. Calculate the peak value and average value of a voltage supply whose RMS value is 415 V
7. A 90 kW, 415V, three phase induction motor draws a current of 110 amps at a lagging power factor of 0.85. What is the power drawn by the motor?

Chapter 5

1. The output of a current transducer depends on the load connected to the secondary of the transducer (True/ False)
2. An active transducer needs auxiliary power supply for functioning (True/ False)
3. The standard output current ratings for transducers are 1 A and 5 A (True/ False)
4. Open circuiting of the secondary of a transducer is hazardous due to inducement of very high voltages (True/ False)

Chapter 6

1. Billing of consumers in based on control metering (True/ False)
2. ____________ metering is used to obtain the total consumption of several feeders using a single meter
3. 5% and 10% are the normal accuracy limits used for tariff metering (True/ False)
4. Utility companies employ _______ for regulating the metering used for various types of consumer installations

Chapter 7

1. Type tests have to be conducted on each device manufactured by the manufacturer before dispatch (True/ False)
2. Routine tests are those tests performed on a routine basis during maintenance to ensure the healthiness of equipment (True/ False)
3. Name any three electrical tests to be conducted on instrument transformers _____________, ____________, ________________.
4. Tests to be conducted in the field before energizing the equipment are called _________

Chapter 8

1. List two advantages of Smart metering

   _______________________________________________________

   _______________________________________________________

2. Cost-Benefit analysis is not as important for Smart metering when compared to normal tariff metering (True/ False)
3. AMR metering enables Utility personnel who visit the consumers to quickly collect metering data using hand held portable data collection devices (True/ False)
4. List two advantages of Smart grids

   ____________________________________________________________________________________________________________________________

Chapter 1

1. Reactive power in an electrical circuit is given by VA or KVA (True/ False)
2. The instantaneous value of voltage in a DC system is given by Vmax Sin (2πft) – (True/ False)
3. Instrument transformers are based on the principle of electro magnetism (True/ False)
4. Power tariff instruments need not have high accuracy as cost of investment is high (True/ False)
5. Transducers can be used to provide isolation between high voltage power circuits and low voltage instrumentation circuits (True/ False)

Chapter 2

1. The primary of a VT offers high impedance (True/ False)
2. Measuring instruments on the secondary side of a VT are connected in parallel (True/ False)
3. The secondary of a VT has less number of turns than its primary winding (True/ False)
4. A capacitor type VT cannot be used for very high voltages (True/ False)
5. Ferro resonance is a phenomenon caused by the high voltages of lightning strikes (True/ False)

Chapter 3

1. A CT primary is always connected in parallel with the main circuit (True/ False)
2. CTs used for protection should get saturated at lower currents than those of metering CTs (True/ False)
3. The ratio of currents in primary and secondary of a CT is directly proportional to the ratio of the number of turns in primary and secondary (True/ False)
4. Opening of the primary of a CT is hazardous (True/ False)
5. The rated burden of the CT should be less than the connected burden of the CT (True/ False)
6. Calculate the percentage ratio error of the CT with following details:

   CT ratio = 100/1

   Actual current in secondary for a actual primary current of 100A = 0.87 A

   Percentage CT ratio error = ((Kn × Is) – Ip ) × 100 / Ip

Where

• Kn is the rated transformation ratio
• Ip is the actual primary current and
• Is is the actual secondary current when Ip is flowing, under the conditions of measurement

  Percentage CT ratio error = ((100 x 0.87) – 100/ 100

  = 0.13 %

Chapter 4

1. The impedance offered by a voltmeter is low (True/ False)
2. Power factor is the ratio of real power to reactive power (True/ False)
3. A moving coil ammeter can be used to measure AC current (True/ False)
4. A single phase watt meter can be used to measure power in an unbalanced three phase circuit (True/ False)
5. Modern digital ammeters offer very high burden (True/ False)
6. Calculate the peak value and average value of a voltage supply whose RMS value is 415 V

Peak value = RMS value/ 0.707

= 415/0.707

= 586.9 V

Average value = 0.637 x Peak value

= 373.85 V

7. A 90 kW, 415V, three phase induction motor draws a current of 110 amps at a lagging power factor of 0.85. What is the power drawn by the motor?

Power = 1.732 x V x. I x Cos φ

V = 415 V

I = 110 A

Power factor = 0.85

Therefore power drawn by the motor = 1.732 x 415 x 110 x 0.85

= 67.2 kW

Chapter 5

1. The output of a current transducer depends on the load connected to the secondary of the transducer (True/ False)
2. An active transducer needs an auxiliary power supply for functioning (True/ False)
3. The standard output current ratings for transducers are 1 A and 5 A (True/ False)
4. Open circuiting of the secondary of a transducer is hazardous due to the inducement of very high voltages (True/ False)

Chapter 6

1. Billing of consumers in based on control metering (True/ False)
2. Summation metering is used to obtain the total consumption of several feeders using a single meter
3. 5% and 10% are the normal accuracy limits used for tariff metering (True/ False)
4. Utility companies employ Code for regulating the metering used for various types of consumer installations

Chapter 7

1. Type tests have to be conducted on each device manufactured by the manufacturer before dispatch (True/ False)
2. Routine tests are those tests performed on a routine basis during maintenance to ensure the healthiness of equipment (True/ False)
3. Name any three electrical tests to be conducted on instrument transformers during commissioning. Ratio test, Winding resistance test, Insulation resistance test.
4. Tests to be conducted in the field before energising the equipment are called _Commissioning tests_

Chapter 8

1. Mention any two advantages of Smart metering
   Possible to read meters both locally and from remote location
   Remote control of output including isolation of electricity
2. Cost-Benefit analysis is not as important for Smart metering compared to normal tariff metering (True/ False)
3. AMR metering enables Utility personnel who visit the consumers to quickly collect metering data using hand held portable data collection devices (True/ False)
4. List two advantages of Smart grids
   Self healing, Distribution circuit remote switching