基于MMC子模块两级主动控制的直流短路限流方法

DC Short Circuit Current-limiting Method Based on MMC Submodule Two-stage Active Control

模块化多电平换流器(modular multilevel converter,MMC)直流侧故障后短路电路急剧上升,严重影响直流电网安全。为限制故障电流,提出一种基于子模块两级主动控制的直流短路限流控制方法(submoduletwo-stage active control,STAC),通过两段故障检测判据和预设最大短路电流,构造关于直流电流的分段函数K,其输出决定减投子模比例,故障后降低直流电压抑制短路电流,同时设计适应于不同运行条件换流站MMC的控制参数,并且仅通过控制动作限流不产生额外成本。STAC不影响系统正常运行,限流过程维持功率传输。最后在四端直流电网中对STAC限流效果及其性能进行仿真分析。结果表明,所提限流方法能有效抑制故障电流,流经直流断路器故障电流降低49.7%,桥臂电流峰值降低23.15%,故障后100 ms恢复直流电压。

The short circuit current rises sharply after the DC-link fault of the modular multilevel converter(MMC),which seriously affects the safety of the DC grid.In this paper,a submodule two-stage active control(STAC)is proposed for this issue.According to the two-stage fault detection criterion and the preset maximum short-circuit current,the piecewise function K of the direct current is constructed,which output decides to reduce the proportional coefficient of the cast sub-mode to reduce the DC voltage and suppress the short-circuit current.In addition,the control parameters of MMC converter which under various conditions are designed for economize the cost of current limiting control strategy.The STAC does not affect the normal operation of the system and it can maintain power transmission during current limiting.Finally,the current limiting effect and performance of STAC are simulated and analyzed in the four-terminal DC grid.The results show that the current limiting method proposed in this paper effectively suppresses the fault current.The fault current flowing through the DC circuit breaker is reduced by 49.7%,the peak current of the bridge arm is reduced by 23.15%,and the DC voltage is restored 100ms after the fault.