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Offshore wind farms (OWFs) with modular multilevel converter high-voltage dc (MMC-HVdc) have become an important form of renewable energy utilization. However, if a fault occurs at the tie line between the MMC and the OWF, the fault steady-state current at the fault point will be equal to zero when the negative-sequence current is suppressed, so traditional differential protection may fail to operate. To cope with this issue, this article proposes a new pilot protection method based on Minkowski distance. For internal faults, OWF and MMC will have different transient currents, so the Minkowski distance will be much higher than 0, but it will be equal to 0 for external faults and normal operation since the fault currents on both sides are the opposite. Therefore, the internal fault can be detected reliably. The proposed method does not depend on the power frequency phasor extraction, so it is not affected by the frequency offset in the transient process. In addition, this method operates quickly and has a strong ability to withstand fault resistance and environmental noise. Moreover, since there is always a transient process when a fault occurs, the proposed method applies to different fault ride-through strategies. PSCAD simulation and real-time digital simulator experiments show that the proposed method is suitable for different fault locations and types.
Yang et al. (Mon,) studied this question.