Coordinated Converter Control with Current Differential Protection under Unbalanced AC Fault in Offshore Wind Farm

The modular multilevel converter (MMC) based high-voltage dc (HVDC)-connected offshore wind farm (OWF) is with converters on both sides of the offshore ac grid. After a short-circuit fault on the offshore ac grid, the fault current from the MMC station can be controlled with a high flexibility to coordinate with the current differential protection. In this paper, the converter uncontrollability and overcurrent problems caused by the coordinated control during unbalanced faults are revealed. Thus, a coordinated fault control of the MMC suitable for the unbalanced fault is proposed to solve the uncontrollability problem. In addition, the proposed coordinated control is further modified to solve the MMC overcurrent problem. The simulation results indicate that the proposed coordinated control can ensure the operation of the current differential protection during unbalanced faults without the converter uncontrollability and overcurrent problems.