Singular Value Decomposition Based Pilot Protection for Transmission Lines With Converters on Both Ends

The modular multilevel converter-based high voltage direct current transmission (MMC-HVDC) technology has become an effective integrating method for large-capacity offshore wind farms. However, if AC side faults occur between the MMC and the wind farm, conventional current differential protection methods based on phasor measurements might not detect the large fault current at the wind farm and the MMC, resulting in poor protection performance. In the worst case, MMC blocking may occur during faults near the MMC station, causing relay refusal. The fault currents provided by both converters are analyzed in detail. Due to the fault-ride-through control objectives, it is proved that the singularity of fault current between wind farms and MMC is different. Therefore, a novel singular value decomposition (SVD) based pilot protection is proposed to solve this problem. A Hankel matrix is established for the short-circuit current at each end of the AC line. The Hankel matrix rapidly detects the singularity of the signal and its derivative, and the SVD method uses a short time window to extract the waveform features. This approach enables the detection of the fault features before MMC blocking in the worst-case fault scenario and prevents the incorrect fault identification caused by the minor amplitude of fault current provided by the converters. A detailed model of an offshore wind farm connected to the MMC-HVDC system is established in the real-time digital simulator (RTDS). A protection device compliant with industry standards is used to implement the SVD-based protection algorithm. Hardware-in-loop dynamic experimental tests are conducted to verify the performance of the proposed protection method. The experimental results demonstrate that the SVD-based protection method has excellent performance during various fault scenarios.