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When single-phase-to-ground (SPG) or phase-to-phase (PP) faults occur on collection lines of an offshore wind farm with modular multilevel converter-based high voltage direct current (MMC-HVDC) transmission, the fault current level may be low or even approach zero due to the controlled current at both ends of the collection line. As a result, the traditional current differential protection may malfunction. To address this issue, a method is proposed that utilizes open circuit characteristics of the negative-sequence network to indirectly calculate the fault point negative-sequence voltage by the negative-sequence voltage at the protection installation location. The fault distance can then be determined. The introduction of a twiddle factor eliminates the influence of fault resistance, improving the accuracy of fault distance calculation. Based on this calculated fault distance, a high-sensitivity differential protection scheme is proposed. This proposed protection can maintain sufficient sensitivity and trip correctly during collection line SPG and PP faults. It solves the issue of traditional current differential protection malfunctioning in low-fault current scenarios of transmission lines with inverter-based resources (IBRs) on both ends. Additionally, it can adapt to different reactive power support strategies and operation modes of offshore wind farms. Test results indicate the effectiveness of the proposed protection.
Chao et al. (Mon,) studied this question.
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