"01 November 2007
Peter jones, Bo Westman
Finding efficient, cost-effective and environmentally friendly ways of transmitting electricity generated can be almost as important as generating the electricity in the first place. Peter Jones and Bo Westman look at HVDC Light.
For well over 100 years, AC (Alternating Current) has been regarded as the natural choice for electrical power transmission. However, HVDC (High Voltage Direct Current) is now emerging as a practical and economical alternative that is not only very efficient for transmitting large amounts of power over long distances, it also offers an elegant solution for a number of reliability and stability issues associated with connecting sustainable energy schemes – especially wind farms – to the main power grid.
HVDC is also something that people involved in large scale CSP (Concentrating Solar Power) stations are tentatively looking at, as the potential to generate utility-scale solar generated electricity (and transport it vast distances) moves from a pipe dream to an area of serious consideration.
What is HVDC?
ABB pioneered HVDC, and developed the first commercial scheme over 50 years ago. This was a link between the Swedish mainland and the island of Gotland in the Baltic sea. The power rating was 20 MW and the transmission voltage 100 kV. Since then, a total of about 70,000 MW of HVDC transmission capacity has been installed in more than 90 projects worldwide.
A key advantage of HVDC is that long distance transmission is more efficient, as there is no need to charge the capacitance of a transmission line with the alternating voltage. In addition, the power flow can be controlled rapidly and accurately, as to both the power level and the direction. This possibility is often used in order to improve the performance and efficiency of the connected AC networks.
Originally, the HVDC converters were equipped with mercury arc valves. Later thyristor valves were introduced that made the design of HVDC systems more flexible, and increased the amount of power they could transfer.
Why use HVDC?
HVDC has a number of properties that make it different from AC transmission:
- The two converter stations can be connected to networks that are not synchronised, or do not even have the same frequency;
- Power can be transmitted over very long distances without compensation for reactive power; reactive power is power that does not add to the transmitted power, but is a by-product of AC transmission resulting from charging the line or cable capacitance at 50 or 60 times per second. Since HVDC operates at constant voltage it does not generate reactive power;
- Only two conductors are needed (possibly just one if the ground or sea is used as the return) compared with three conductors for AC. (...)
About the Authors:
Peter Jones and Bo Westman work for ABB;"
Ver artigo completo (HVDC Light Caracteristics, where has HVDC light been used, HVDC and CSP...)
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