Control-Based Distance Protection for Tie Line Between Offshore Wind Farms and MMC During Single-Phase Grounding and Interphase Faults

Offshore wind farms (OWFs) are often connected to onshore systems via a modular multilevel converter (MMC)-high voltage dc (HVDC) line. However, for the tie line between the OWF and the MMC, the unique fault behaviors will threaten the correct operation of conventional distance relays. Therefore, a coordination scheme between protection and control is proposed in this article. The proposed method can make the distance relay on the OWF terminal be triggered within 0.3 s. After that, the zone I distance relay on the MMC terminal can safeguard the entire line, allowing the fault to be switched off in less than 0.7 s. The proposed method can make the OWF-MMC system compatible with the traditional distance relays, while the overvoltage can be suppressed. Furthermore, the typical fault ride-through (FRT) demands such as current limitation and reactive current injection are still satisfied. In addition, the suggested approach possesses a robust capacity to tolerate fault resistance. These advantages avoid investment in modifying the protection algorithm while effectively protecting the inverter. PSCAD simulation and hardware-in-the-loop (HIL) testing validate its performance under various fault conditions.