Multi-terminal DC transmission systems for connecting large offshore wind farms

This paper describes the use of voltage source converter (VSC) based, multi-terminal (MT) HVDC transmission system technology for connecting large doubly-fed induction generator (DFIG) based wind farms, over long distance. The operational principles of the proposed system are described and control strategies for coordinating various grid side converters are proposed, to ensure smooth system operation and power sharing between various AC networks. DC voltage control and DC droop characteristics are implemented for the grid side VSC which transmit wind farm generated power to AC grids. PSCAD/EMTDC simulations for a four-terminal DC system for connecting two wind farms to two different AC networks are presented to demonstrate robust performance during wind speed and power variations, and large disturbances caused by the trip of one of the wind farms.