A Staged Strategy for MMC Offshore Converter Station Under Asymmetric AC Fault

At present, offshore wind power has become an important development direction for wind power generation. Offshore wind farms often use high-voltage direct current transmission technology with modular multi-level voltage source converters (MMC-HVDC) to transmit electricity to the grid. However, the fault response characteristics of offshore wind farms through the MMC-HVDC transmission system are different from those through AC transmission way. Therefore, it is significant that studying the fault ride through (FRT) technology of offshore wind farms through the MMC-HVDC grid connection system for practical engineering. An electromagnetic transient simulation model for an offshore wind farm composed of a direct drive permanent magnet synchronous generator (D-PMSG) connected to the grid is established through MMC-HVDC transmission technology. The mechanism and suppression strategy of overvoltage generation in the transmission network are provided during the fault clearance and recovery phase of the offshore wind farm HVDC transmission system under AC asymmetric faults. A two-stage improved control strategy for MMC-HVDC offshore converter station in response to asymmetric faults has been proposed, which is beneficial for the joint system to quickly recover from the post fault system voltage and suppress the overvoltage risk during the fault clearing phase. Finally, the proposed strategy is validated based on a domestic ± 500kV project in China, demonstrating its effectiveness.