基于MMC的光伏直流升压并网系统故障分析及限流控制策略

MMC-based fault analysis and current-limiting control strategy for a photovoltaic DC boost grid-connected system

基于模块化多电平换变流器(modular multilevel converter,MMC)的光伏直流升压并网系统为大容量并网提供了更多的可能。而光伏升压系统直流侧发生故障时暂态过程复杂,故障电流上升迅速且峰值过高。针对此问题,首先对系统直流侧双极短路故障时的直流升压变换器与MMC换流站进行了故障过程分析,并通过故障回路分别计算出了可靠闭锁下流经短路点的故障电流。然后针对MMC换流站的故障电流,依据其控制原理,提出基于电压变化的主动限流控制策略。该控制通过引入电压变化量动态改变桥臂参考电压,从而限制故障电流。最后通过PSCAD仿真模型验证了故障分析结果与限流效果,经检验,该控制策略可以有效减小断路器的开断电流以及桥臂过流峰值。

A modular multilevel converter(MMC)-based PV DC boost grid-connection system provides more possibilities for large-capacity grid-connection.However,the transient process of the DC side fault of a PV boosting system is complicated,and the fault current rises rapidly and the peak value is too high.To address this problem,first,the fault process of a DC boost converter and MMC converter station in the case of a bipolar short-circuit fault on the DC side of the system is analyzed,and the fault current flowing through the short-circuit point under reliable blocking is calculated through the fault circuit.Then for the fault current of the MMC converter station,based on its control principle,an active current limiting control strategy based on voltage variation is proposed,one which dynamically changes the reference voltage of the bridge arm by introducing voltage variation to limit the fault current.Finally,the fault analysis results and current limiting effect are verified by a PSCAD simulation model,and it is verified that the control strategy can effectively reduce the opening current of the circuit breaker and the peak overcurrent of the bridge arm.