Harmonic Injection Based Fault Ride-through Control of MMC-HVDC Connected Offshore Wind Farms

Offshore wind farms connected to the grid via MMC-HVDC should have fault ride-through (FRT) ability. However, without high-speed communications, it is difficult to achieve coordinated control between wind power converters and the MMC-HVDC converters. To address this problem, a coordinated control method that is designed based on the injection of harmonics is proposed. During the fault, the offshore MMC converter detects that the DC voltage has exceeded the threshold value, and then injects sequence harmonics into the offshore AC lines, which enables the wind power converters to cooperatively limit the power injected into the grid. Therefore, the coordinated FRT of multiple converters can be realized without additional communication between wind power converters and the offshore MMC converters. A detailed model of MMC-HVDC connected offshore wind farms is constructed in a real time digital simulator (RTDS), and the proposed method is verified by hardware-in-loop dynamic simulation experiments. In addition to the traditional method of only limiting the output voltage of the offshore MMC, the proposed method also coordinately limits the output current of the wind farm to reduce the output power of the wind farm. Therefore, the proposed method allows the DC voltage to be quickly limited within the allowable range, and the system can achieve safe and reliable FRT without communication.