Intersystem fault between MMC-HVDC and AC systems and its impact on DC/AC protection

Abstract

Flexible DC and AC transmission lines can be arranged across vertically or sharing transmission corridor. Under this condition, the possibility of intersystem fault increases significantly, which endangers the safety of both DC and AC systems. Here, the intersystem fault between bipolar half-bridge MMC-HVDC system and AC system is analysed. The reliability of DC wavefront protection after intersystem fault is analysed based on initial traveling wave, while the reliability of DC intersystem fault protection and AC protection is analysed based on 50 Hz component characteristics. Since wavefront protection cannot be guaranteed to operate reliably, intersystem fault protection based on 50 Hz components is applied to DC system to reliably detect intersystem faults and isolate the fault by commanding DC circuit breaker to trip. Although current differential relaying and zero-sequence overcurrent relaying in AC system can reliably detect intersystem faults, a large DC offset prevents effective fault isolation of AC circuit breaker. This highlights the necessity of MMC-HVDC system to reliably isolate intersystem faults.