Recovery Control for Hybrid MTDC Systems with Offshore Wind Farms Supplying Weak Grids

Multi-terminal direct-current (MTDC) grids are projected to be deployed for the large-scale grid integration of offshore wind farms. Hybrid systems simultaneously considering line commutated converter (LCC) and voltage source converter (VSC) technologies have been proposed. However, the output power fluctuation and the lack of reactive power compensation at the onshore receiving end may compromise voltage stability during the transient recovery process of the hybrid system upon faults. This paper presents a recovery control strategy for hybrid VSC-LCC systems. The system under study consists of an LCC and two VSCs and is used to transmit bulk power from a wind farm to both an isolated island and a mainland ac system. A sensitivity analysis is conducted to assess the voltage stability of the ac bus on the LCC side. A coordination recovery control strategy is designed and a simulation model is built in PSCAD/EMTDC to verify the strategy. Simulation results show that the presented strategy is effective to maintain transient voltage stability of the hybrid system and can prevent LCC communication failures.