MMC-HVDC直流侧极间短路暂态特性分析

模块化多电平换流器的高压直流输电（Modular Multilevel Converter based High Voltage Direct Current，MMC-HVDC）系统中极间短路故障是最严重的直流侧故障类型，目前多侧重于分析闭锁前电容放电过程及相应的故障保护策略，而对MMC极间短路故障发展全过程分析不够深入，由于极间短路故障暂态过程发展迅速，故障发展各阶段联系紧密、相互影响，为了更好地对直流断路器进行选型和调整继电保护设备，因此有必要对其故障全过程进行分析。首先，分析了MMC拓扑结构及其子模块的三种工作状态。然后，根据电容、电感充放电机理，给出换流器闭锁前、后各阶段故障电流路径，建立故障各阶段的等效电路及数学模型，研究了故障距离对直流侧故障电流、电压波形及换流器闭锁时间的影响。最后，基于PSCAD/EMTDC仿真软件，搭建71电平MMC-HVDC双端直流输电模型，验证了故障特性分析的正确性和有效性，对保护设备阈值具有工程意义。

Analysis of DC pole-pole fault transient in MMC-HVDC grid

The pole to pole (P2P) fault in the Modular Multilevel Converter based High Voltage Direct Current (MMC-HVDC) system is the most serious DC fault type. At present, several works in literature analyze the capacitor discharge currents and propose the protective methods. The analysis of the whole transient process of P2P fault is not deep enough. The transient process of P2P faults develops rapidly, the stages of the fault are closely related and affect each other, in order to better select the HVDC circuit breakers (CBs) and adjust the protectives devices, it is necessary to analyze the whole process of the fault. This paper analyze the MMC and the three working states of its sub-modules. According to the charging and discharging mechanism of capacitor and inductor, the equivalent circuit of each fault stage are established to analyze the fault current of each stage. The influence of fault distance on DC fault current, voltage waveform and converter blocking time is analyzed. Finally, a 71-level two-terminal MMC-HVDC grid modeled in PSCAD/EMTDC to verify the P2P fault analysis.