DC electrical transmission growing


Dear Control Engineering: It seems that using dc for electrical distribution is growing. How is this practical?

You shouldn’t expect to have your residential electrical service converted to dc anytime soon. The considerations that made utilities adopt ac almost universally over the last century continue to apply for most uses. Still, there are applications where high voltage dc is being used to great advantage. If you have to move a large amount of electric energy from point A to point B, dc may be the better choice. However, for more typical grid distribution applications, ac still does the job.

For example, ABB just completed a project in China that has pushed out the boundaries. There’s an area in southwest China near Xiangjiaba where there are hydroelectric generating units that put out a lot of power. The problem is that the power is needed in Shanghai which is about 2,000 km (1,200 mi.) away.

Ultra-high voltages require exotic switchgear.This new power line (note that the term is singular) carries 7,200 MW over that distance at 800 kV. ABB says it’s very efficient with less than 7% line loss over that distance. Once the juice gets to Shanghai, it is converted into ac for more traditional local distribution.

(You may remember that in our post last week, we discussed definitions of medium- and high-voltage. Once you’re around 800 kV, you’re definitely in the ultra-high cagegory.)

ABB has been doing a lot of research in this area and has developed the transformers, converters, and switchgear to handle these extreme levels. Earlier efforts in service include 500 and 600 kV systems, but raising the voltage increases efficiency.

Such deployments will likely become more common for applications where generating capacity is removed from consumers, such as hydroelectric and wind generation. It can even help make it more practical to locate coal-fired plants at the minehead, rather than moving coal from a mine to distant plants via rail.

--Peter Welander, pwelander@cfemedia.com 
Control Engineering

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