TR: China Wind Energy Potential, HVDC

(Just taking notes, trying to understand what HVDC is, what it has to do with variable power sources such as wind, and why it makes buried transmission lines convenient.)

Technology Review: China's Potent Wind Potential:

"The major grid upgrades already under way in China are making extensive use of continental-scale high-voltage direct-current (HVDC) lines, which remain the stuff of supergrid blueprints in Europe and the United States. 'They are leading the world in implementing long-distance transmission schemes,' says Bjarne Andersen, director of U.K.-based consultancy Andersen Power Electronic Solutions and an expert in the ultra-efficient HVDC technology."

Technology Review: Europe Backs Supergrids:

"This summer [2008], for example, a negotiator appointed by the EC convinced France to accept a new transmission connection with Spain, breaking a 15-year impasse over expanding power exchanges between the countries. Use of high-voltage DC (HVDC) technology will enable planners to bury the new line and thereby overcome local opposition to conventional overhead AC transmission lines."

Wikipedia explains how HVDC can have low power losses:

"Power in a circuit is proportional to the current, but the power lost as heat in the wires is proportional to the square of the current. However, power is also proportional to voltage, so for a given power level, higher voltage can be traded off for lower current. Thus, the higher the voltage, the lower the power loss."

"The advantage of HVDC is the ability to transmit large amounts of power over long distances with lower capital costs and with lower losses than AC."

Lots of interesting stuff in the Wikipedia article. This excerpt seems to explain the connection between HVDC and variable power sources:

"Because HVDC allows power transmission between unsynchronised AC distribution systems, it can help increase system stability, by preventing cascading failures from propagating from one part of a wider power transmission grid to another. Changes in load that would cause portions of an AC network to become unsynchronized and separate would not similarly affect a DC link, and the power flow through the DC link would tend to stabilize the AC network. The magnitude and direction of power flow through a DC link can be directly commanded, and changed as needed to support the AC networks at either end of the DC link. This has caused many power system operators to contemplate wider use of HVDC technology for its stability benefits alone."