HVDC- high voltage direct current

Two years ago, when a person whose major is medicine asked me about the reason to use HVDC(high-voltage direct current) lines to transmit long-distance power from West China to East China, I could not give a very clear explanation, which quite embarrassed me since no excuse could be found for my own limited and obscure understanding of the concept.(It is funny that actually today when I have not finished this article, there is another guy who asked exactly the same question, and I think this time my answer made him satisfied.)

At the time, I was already a second-year PhD student in power engineering, but I still had some trouble in basic knowledge and concepts of power systems (even now I am still learning them). The phenomenon is not rare among PhD students, I believe, at least at some informal discussions with my fellow colleagues.

Therefore, I decide to write down some basic knowledge that a power engineering student should have, and hopefully I can accumulate and update them in this blog regularly as I learn. It may not  be very scientific, but it conveys the basic principle of the concepts and issues.

Well, go back to the question about HVDC, I had known that high voltage line has the capability of reducing the power loss, but why direct current?
Actually several reasons can justify the use of DC lines:
(1) The materials required in DC lines are about 1/3 less than AC lines (Two lines one phase vs Three lines three phases). Look at these two pictures from Chinese state grid website (a is AC lines, b is DC lines):

However, we should also realize the required AC/DC/AC converter station is quite expensive. Due to the trade off between the materials saving (and also the saving from less power loss in DC lines) and the additional cost of converters, a cost-benefit analysis is usually conducted for determining the equivalent distance for both AC and DC lines.  If a transmission needs to be built within the determined equivalent distance (short lines), then AC is more beneficial; DC is more attractive in long distance power delivery. HVDC lines are usually used for long distance, great than 400 miles or 600km.

(2) Additional losses exist in AC lines because of inductance and capacitance, compared to DC lines. The electricity current that is used to charge capacitance in AC lines, can instead be used for customers' power usage in DC lines. In other words, DC lines are more efficient.

(3) DC lines can connect two unsynchronized AC systems, which may not be achievable by AC lines. It thanks to the AC/DC/AC converters that separate the operation of two connected AC systems.

There may be more advantages of DC lines than articulated here, like the capability to survive in fault conditions and so forth. Certainly DC lines have disadvantages, but the technology is still promising in long-distance power delivery (especially for renewable energy integration)!