DC line fault is one of the key problems that must be solved in a flexible HVDC system. During quite a long time between the existing main protection and backup protection of the HVDC line, there is no line protection method to detect the fault, which may lead the protection at the AC side to act before the backup protection of the DC line. To solve the problem, a novel two-step distance protection for flexible HVDC lines is proposed in this manuscript. Firstly, based on the uniform distributed parameter model, the equivalent lumped parameter model of the HVDC transmission line at low frequency is analyzed. Secondly, according to the time domain differential equation and the least squares algorithm, novel distance protection based on the iterative calculation of fault distance is proposed, which can eliminate the influence of distributed capacitive current and improve the precision of calculation. To improve the rapidity and reliability of the distance protection, low pass filters with two different cut-off frequencies are used to process the electrical quantities. Finally, simulation results show that the proposed distance protection can respond to metallic poleto-ground faults and pole-to-pole faults rapidly and reliably.
A Novel Two-step Distance Protection for Flexible HVDC Transmission Lines