A hybrid HVDC converter based on MMC and diode rectifier for offshore wind farm integration

Aiming at the occasion of DC transmission of offshore wind power, a new type of hybrid converter topology is proposed in which a diode rectifier is connected in parallel with the AC side of the MMC converter and the DC side is connected in series. MMC converter provides a stable voltage source for offshore wind farms and serves as power of black start. The DC side of the diode rectifier has a bypass circuit breaker and a thyristor as the starting circuit. Due to the application of diode rectifier with higher integration and MMC converter with lower DC operating voltage in this topology, this HVDC transmission system has a higher power density and lower cost, and has some certain utilities. This paper analyzes the principle of the hybrid converter, the power distribution ratio, and the start-up and steady-state control methods. Finally, the PSCAD/EMTDC simulations for the HVDC transmission system are conducted to validate the proposed hybrid converter and control strategy.