Extension of pole differential current based relaying for bipolar LCC HVDC lines

Line commutated converter (LCC) based high-voltage direct current (HVDC) transmission is characterized by long-distance power transfer, a complex and harsh corridor environment, and the rapid fault evolution of DC lines. Since high fault currents can potentially cause unalterable destruction to power devices, fast and reliable line protection and isolation are essential to enhance the security and reliability of LCC-HVDC transmission systems. This paper combines an extended relaying using single- and double-ended measurements of the transient current at the boundary of positive and negative polarity DC lines to meet this obligation. During a fault, the DC line equivalent circuit is initially analysed with the time domain response of positive and negative polarity DC line (poles) currents at two ends to define local and global pole differential currents (PDCs). The criterion of relaying is then developed using PDCs for the bipolar and monopolar modes of operation of the LCC-HVDC transmission system. The proposed relaying method is validated through simulation, with results indicating that it can quickly (within 5.1 ms) and reliably detect internal faults and remain stable during external faults, accurately identify faulty poles without requiring data communication synchronization for bipolar operation, and demonstrate strong transition resistance. However, the monopolar operation uses global PDC to detect faults reliably within 15.6 ms. The proposed scheme is implemented on a 2000 MW, ± 500 kV, 900 km bipolar LCC-HVDC transmission system, simulated using PSCAD/EMTDC software. Additionally, the results are analyzed using MATLAB/Simulink.