Research on Improvement of AC Line Distance Protection in AC-DC Compound Power Grid

In AC-DC compound power grid, the DC commutation failure due to AC fault can cause change in characteristics of AC grid fault, thus causing AC distance protection to operate incorrectly. The current measurement and the traditional distance protection impedance calculation result of the pure AC system and the AC-DC compound power system are compared, and the influence of DC feed on the distance protection is analyzed. The result shows that during the commutation failure, the DC system injects a large nonperiodic component into AC fault current, which causes the improper protection. Then, an improved distance protection combining distributed parameter line model and differential equation algorithm is presented. Finally, through the simulation experiment of PSCAD/EMTDC, it is proved that the optimization scheme can accurately and quickly distinguish the internal and external faults of the fixed point, and significantly reduce the influence on the ranging error of the transition resistance.