Improved scheme for differential protection of UHVDC transmission lines

An improved scheme of differential protection was proposed, which utilized the relationship of equivalent electrical triangle balance among the specific voltage data, current data and line resistance, combined with the low-pass filtering effect of the integral algorithm. To construct improved criterion, the differential currents at both ends of the line was taken as the action amount, and the ratio of the difference of the voltage at both ends of the line to the line resistance was used as the braking amount. Theoretical analysis shows that, for external faults, the movement is significantly less than the brake; for internal faults, the movement is greater than the brake, the criterion can be set in a simple and clear way. Meanwhile, the proposed scheme is insensitive to the sampling frequency and independent of distributed capacitive current. The simulation model makes reference to a ±800 kV UHVDC in China. Three typical faults were set on the simulation model to prove correctness of the criterion. The simulation results show that the recommended scheme has the characteristics of high reliability and sensitivity, and can recognize the external or internal fault precisely and rapidly, and has good engineering application prospect. © 2019, Zhejiang University Press. All right reserved.