New Principle of Hybrid Bipolar DC Transmission Line Protection Based on Longitudinal Impedance

Because traditional HVDC transmission and flexible HVDC transmission have their own advantages and disadvantages, in order to give full play to their advantages, hybrid HVDC transmission has become a research hotspot. In this paper, the protection principle of hybrid bipolar DC transmission lines consisting of traditional HVDC transmission and flexible HVDC transmission is studied. The concept of impedance is extended, and a fault protection principle based on the longitudinal impedance of the rectifier side and the inverter side is proposed. The longitudinal impedance is the ratio of the difference between the transient voltage across the rectifier side and the inverter side and the sum of the transient currents. The longitudinal impedance is used to distinguish the internal and external faults. Finally, simulation modeling is put up with PSCAD, protection algorithm is carried out with MATLAB, a large number of simulation results verify the correctness of the proposed protection principle, and can achieve fault pole selection.