Fault Identification Method for Teed Lines of Flexible DC Power Grid Based on Modulus Phase Plane

Since the flexible DC power grid with teed lines is only installed with DC circuit breakers (DCCBs) at the sending and receiving ends, the DCCBs at each end need to be disconnected when an internal fault occurs. Compared with the double-terminal DC power grid, its fault current characteristics are more complicated, and conventional DC power grid protection methods cannot be directly applied. In order to meet the high protection requirements of the flexible DC power grid with teed lines, a fault identification method based on modulus phase plane for flexible DC lines is proposed in the paper. Firstly, the network topology is studied, and based on the equivalent circuit of internal and external faults before the converter is blocked, the characteristics of fault current modulus are derived. Then, combined with fault locations and types, the protection operation principle and threshold setting are analyzed. At the same time, the adaptive setting of protection is realized by constructing a threshold scaling factor. Finally, the simulation model of the flexible DC power grid with teed lines is established in PSCAD/EMTDC platform to verify the rapidity and reliability of the proposed method. The proposed method is simple in algorithm with low computational complexity, only uses single-end information to quickly identify faults, and has a strong ability to withstand system disturbances. © 2021 Automation of Electric Power Systems Press.