Without a shadow of doubt, with traditional power transmission systems, home and factory power systems and motors – good old alternating current (ac) is the name of the game with electricity. Direct Current (dc) has always been the poor cousin.

Dear Colleagues

EIT Stock ImageWithout a shadow of doubt, with traditional power transmission systems, home and factory power systems and motors – good old alternating current (ac) is the name of the game with electricity. Direct Current (dc) has always been the poor cousin.

Electrical engineering students through the ages have devoted huge amounts of effort to considering the generation, distribution and transmission of energy in the form of alternating current. The "War of the Currents" started in the 1880s with a fight between ac and dc and was firmly resolved then in favour of ac.

However the old maxim of dusting off the old approaches and trying them again in new applications is certainly true of direct current today where it is seeing a resurgence particularly in power generation (e.g. photovoltaic cells) and most recently – in long distance power transmission systems.

Highly Successful Alternating Current
As most of you would well know and I apologise for telling you ‘how to suck eggs’ – but the success of ac has been in the easier conversion to different voltages – either ‘stepping up’ or ‘stepping down’. Especially for transmission over long distances where it is important to increase the voltage to extraordinarily high values. Use of these high voltages (and resultant low currents) means that the losses over the cables are very low and thus it is very effective economically to transport the energy.

Where required the voltages are then stepped down for the local consumers - again using the miracle of the transformer. This ease of conversion between different voltages was not possible with direct current. Even though, in the 1920s we had the mercury arc valve and then the solid state thyristor - these really only provided a modicum of advantage in being able to change the voltage levels as they were particularly expensive and difficult to apply. Prices have now started to fall though as the technology becomes more effective.

Many of the new sources of power these days are wind and solar in remote locations which are often extraordinarily far from the consumers in the far off cities and this has propelled us into using dc power for long distance transmission.

Over Extreme Distances There are Problems with Alternating current
Over extreme distances, alternating current does have disadvantages as you would imagine – perhaps, intuitively. Having to swing the voltage quickly from a positive to a negative peak 50 times a second (for Europe, Africa, Asia and Australia) is hard work and there are losses as a result. This effectively means the direction of the current has to be reversed - all exceptionally ‘exhausting’ for the generators.

High voltage dc lines experience a lower loss than equivalent high voltage ac lines over significant distances.

This has led to strong development in the use of direct current (dc) systems today.

UHVDC Projects are Mind Bogglingly Big
Recent examples of these ultra high voltage direct current (UHVDC) projects range from an Oklahoma transmission line 1100km long connecting Oklahoma to Western Tennessee with 9 million consumers. This will carry 4000 MW using dc at 600kV. Considerably higher than the traditional alternating current ones at ~400kV.

But UHVDC lines are really big in Asia with China leading the pack with some huge projects.

For example, China has a 800kV line from a dam in Yunnan province to Shanghai with a capacity of 6400MW. An even larger one is carrying 12,000 MW over 1400 km from the coal and wind sources of Xinjiang to Anhui province at 1.1 million volts.

Point to Point is not the Only Game in Town
One of the challenges with current UHVDC cables is that they are generally point to point. This makes it difficult to fan out and have the traditional grid arrangement. However, there are new technologies allowing for new switch gear to isolate faulty cables and to make this a workable option.

So Consider the Way Forward
When looking to a new project – remember that older technologies can often be resurrected and are somewhat more effective than existing approaches.

And certainly having know-how in direct current has advantages these days and it may be that this surpasses alternating current because of its ease of transport.

Especially when one considers that most electronic equipment is powered by low voltage direct current (incl. photovoltatic cells and batteries).

Thanks to the Economist for an interesting article and for Wikipedia for their succinct summary of the virtues of ac and dc.

A clever quotation for direct current from Donald Sadoway: In a battery, I strive to maximize electrical potential. When mentoring, I strive to maximize human potential.
Yours in engineering learning


Mackay’s Musings – 3rd Jan’17 #633
780, 293 readers – www.eit.edu.au/cms/news/blog-steve-mackay

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