MMC-HVDC系统故障性涌流的产生机理及影响

Mechanism of Fault-induced Inrush Current of MMC-HVDC and Its Influence

针对模块化多电平换流器高压直流输电(modular multilevel converter high-voltage direct-current,MMC-HVDC)阀侧单相接地故障可能引发的换流变故障性涌流问题,该文研究了MMC闭锁前后不同阶段的换流变阀侧接地故障特征,并在此基础上分析了故障性涌流的产生机理及特征,及其对网侧交流断路器开断时间与换流变差动保护的影响。研究结果表明:故障发生后,上桥臂子模块电容持续充电并引发严重过电压,而下桥臂故障回路中幅值较大且衰减缓慢的直流分量持续流向换流变,导致换流变铁心发生饱和并产生故障性涌流。故障性涌流的产生加快了网侧电流过零点的出现,可改善阀侧单相接地故障引起的网侧交流断路器延时断开的问题。同时大量直流分量的侵入将引发换流变铁心深度饱和,可能导致换流变阀侧区外接地故障时差动保护误动。最后,以±320kV厦门柔直工程为背景,利用PSCAD/EMTDC仿真验证了理论分析的准确性。

Following a single-phase-to-ground(SPG) fault on the valve side in the modular multilevel converter(MMC)high-voltage direct-current(HVDC) system,the fault-induced inrush current(FIIC) of the converter transformer may occur.Focusing on the FIIC,the pre-and post-blocking characteristics of the valve-side SPG fault are studied.On this basis,the generation mechanism and characteristics of the FIIC are analyzed,as well as its impacts on the opening time of the grid-side alternate-current(AC) circuit breaker(CB) and the converter transformer differential protection.According to the research results,the upper-arm submodule capacitors may suffer a severe overvoltage due to continuous charging after the fault.Meanwhile,the DC components in the lower-arm fault currents with a large amplitude and slow attenuation continues to penetrate the converter transformer,leading to the saturation of the converter transformer and the generation of FIIC.The FIIC speeds up the occurrence of the grid-side zero-crossings,which can alleviate the challenge of the grid-side ACCB being hard to operate.In addition,the penetration of the significant DC components causes a deep saturation of the converter transformer,which may result in the misoperation of the differential protection under the condition of external SPG faults.Finally,simulations in the PSCAD/EMTDC based on the ±320kV Xiamen MMC-HVDC project have been conducted to verify the accuracy of the theoretical analysis.