双极柔性直流系统阀交流侧接地保护研究OA

To address the issue that fault currents caused by grounding faults on the valve AC side of bipolar VSC-HVDC systems have no natural zero-crossing points,the characteristics of fault currents at both the faulty and healthy terminals after converter blocking at the faulty end are analyzed.Analytical expressions are derived,and a criterion and corresponding protection scheme for the healthy terminal to identify grounding faults on the opposite side are obtained.Since arc currents that reverse and pass through a zero crossing are beneficial for restoring the dielectric strength of the arc gap in AC circuit breakers,the impact of reducing DC voltage after a fault on the reverse zero-crossing behavior of valve AC-side fault currents is investigated.Based on this,a protection scheme is proposed in which,after the healthy terminal is taken out of operation,the non-grounded pole on the DC side of the faulty terminal is grounded through a resistor.The expressions for the current limiting resistor and the valve AC-side fault currents are derived.Finally,simulations are performed on the PSCAD/EMTDC platform.The results demonstrate that grounding the non-grounded pole through a resistor can not only enable reverse zero-crossing of valve-side fault current of the converter transformer,but also effectively prevent the occurrence of three-phase short circuits in the converter transformer.