The fault analysis of flexible DC distribution grids is the basis of designing the protection scheme. Up to now, the fault characteristic of DC fault, considering unblocked IGBT in converters, has not been systematically investigated. Firstly, the grounding mode and the control strategy of the DC distribution grid were determined. Secondly, on the premise that IGBT is unblocked, this paper analyzed fault characteristics of pole-to-pole fault and pole-to-ground fault, as well as the influence of fault resistance. The impacts of DC faults on other sections were also discussed. The results show that, with regard to the pole-to-pole fault, whether IGBT is blocked or not does not affect the fault characteristic evidently in the first two transient stages. However, when the fault reaches the steady state, compared with the condition that the IGBT is unblocked, the DC voltage and fault current are larger when the IGBT is blocked. In terms of the pole-to-ground fault, the unblocked IGBT would result in DC unbalance voltage, which will further generate DC components in the ac-side current of the converter. Finally, the simulation model was built in PSCAD/EMTDC, and the result confirms the validity of the theoretical analysis.%柔性直流配电网故障特性分析是保护方案设计的基础.当前,直流系统故障期间,换流器 IGBT 不闭锁时的故障特性尚未深入探讨.首先,确定直流配电网的接地方式和控制策略.其次,分析 IGBT 不闭锁时直流极间短路和单极接地短路的故障特性及过渡电阻等因素的影响,并讨论故障对其他区域的影响.结果表明,对于极间短路,除故障达到稳态后 IGBT 闭锁较之不闭锁会出现极间电压和故障电流增大的现象外,IGBT 闭锁与否对故障特性影响不大;对于单极接地短路,IGBT 不闭锁会使直流侧出现不平衡电压,导致换流器交流侧电流出现直流分量.最后,在PSCAD/EMTDC中搭建仿真模型,证明了理论分析的正确性.
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