A Pilot Protection Principle for MMC-HVDC Transmission Lines Based on Charge Balance

Current differential protection based on Kirchhoff's law has sensitivity problem due to the long-distance DC transmission lines with distributed parameter effect. In this paper, the charge of the DC transmission line under the situations of external and internal faults is analyzed. The feature is that the charge will be injected into the fault point from both sides of the DC line when the internal faults occur, and if the external faults occur, the distributed capacitance of the DC line will be discharged. Then a pilot protection principle based on charge balance is proposed using the polarity of the sum of the charge in a time period. The proposed principle is simple and independent of the parameters of the DC line, and it theoretically suits for both transient process and steady-state process. Finally, an MMC-HVDC transmission system is built in PSCAD/EMTDC, and the simulation results show that the method is reliable and not affected by fault resistance.