Future multi-terminal HVDC transmission systems using Voltage source converters

Due to the potential future energy crisis and ever increasing population, wind power, as a renewable energy source producing no emissions and with a sufficient wind resource in many parts of the world, is attracting increasing interest and growing rapidly. Offshore wind strength is relatively much stronger than onshore levels and many large-scale wind farms (greater than 1GW) are planned to be constructed in offshore location and must transmit power over long distances. Voltage-source converter based High Voltage Direct Current (VSC HVDC) transmission system, which enables fast active and reactive power control and has relatively lower losses than conventional AC transmission, is a potential solution for offshore power transmission. Offshore wind farms usually have widely dispersed locations in a strong wind area. Furthermore, VSCs have a limited transmission capacity due to limitations on IGBT and capacitors ratings. For these reasons, a multi-terminal HVDC (MTDC) transmission system, which can extract and deliver power from and to several terminals and provide power to more than one terminal, is an attractive method for offshore wind power transmission. In addition, MTDC has been proposed in other fields such as urban sub-transmission and offshore oil/gas. A detailed description of a MTDC control scheme is presented and its operation demonstrated. The paper concludes with an overview of future research in this field.