含直流断路器的双极接线柔性直流输电系统MMC交流侧单相接地故障研究

Single-phase-to-ground Faults at AC Side of MMC in Bipolar HVDC System with DC Circuit Breakers

双极接线柔性直流输电系统采用中性母线接地,换流器交流侧对地电压含有直流偏置,换流器交流侧发生单相接地故障特性不同于对称单极接线系统。柔性直流输电系统配置直流断路器时,换流器交流侧接地故障后可利用直流断路器的直流电流分断能力快速隔离故障。针对含直流断路器的双极接线柔性直流输电系统,分析了模块化多电平换流器(modular multilevel converter,MMC)交流侧单相接地故障机理,阐述了MMC保护闭锁前后故障相和非故障相上、下桥臂的故障电流通路,推导了故障状态下桥臂的电压、电流解析方程。分析了保护动作、直流断路器对MMC子模块过电压的影响,为换流阀子模块过电压保护设计、定值整定提供指导。最后,通过搭建的±500 kV柔性直流输电系统PSCAD/EMTDC仿真模型,仿真验证了理论分析的正确性。

Bipolar modular-multilevel-converter-based high voltage direct current(MMC-HVDC)transmission systems use neutral bus grounding,and the AC-side-to-ground voltage of a converter contains DC bias.The characteristics of single-phase-to-ground faults at the AC side of the converter in such systems are different from those in symmetric monopolar systems.When an MMC-HVDC system is equipped with DC circuit breakers,grounding faults at the AC side of the converter can be rapidly isolated using the ability of DC circuit breakers to interrupt DC current.In this work,the mechanism of single-phase-to-ground faults at the AC side of an MMC is analyzed for a bipolar MMC-HVDC system with DC circuit breakers.The fault current paths before and after blocking the MMC owing to protection are elaborated for the upper and lower bridge arms in faulted and nonfaulted phases.In addition,the analytical equations of the voltages and currents of the upper and lower bridge arms under fault conditions are derived.The impact of protection actions and DC circuit breakers on the overvoltage of MMC submodules is analyzed.Finally,the validity of the theoretical analysis is verified through the PSCAD/EMTDC simulation model of a±500 kV MMC-HVDC system.This work provides guidance for the over-voltage protection design and setting for converter valve submodules.