混合型模块化多电平换流器启动冲击电流特性分析及控制策略

The Inrush Current Characteristics and Control Strategies of HybridModular Multilevel Converter Systems During Start-Up Processes

启动控制是模块化多电平换流器(MMC)正常运行的基础。半桥型和全桥型子模块构成的混合型MMC,在预充电阶段存在电容欠电压且电压不均衡现象,导致换流阀受控充电解锁后出现桥臂过调制和冲击电流。该文首先针对混合型MMC的拓扑结构,分析半桥型和全桥型子模块预充电电压特性及其等效电路,提出两类模块电容电压计算方法。利用全桥型子模块负电平输出特性,提出一种交流侧受控充电启动策略,能够在电容欠电压条件下避免换流器过调制从而抑制冲击电流。PSCAD/EMTDC仿真结果验证了换流阀启动特性的正确性和所提出的启动控制策略的有效性。

Start-up control is the basic conditions of modular multilevel converter(MMC)system operation.However,the capacitor voltages of hybrid-MMC,which consists of full-bridge sub-modules and half-bridge sub-modules,are relatively low and unbalanced after precharge,resulting in inrush current and over modulation.The precharge voltage characteristics of hybrid-MMC and the equivalent circuit are analyzed,based on the converter topology.A method to calculate the precharge voltage of two kinds of sub-modules,respectively,is presented.Then,based on the capability of producing negative level of full-bridge sub-modules,an ac-side start-up control strategy is proposed in this paper,to suppress the inrush current and over modulation.The theory analysis and the proposed strategy are demonstrated by the PSCAD/EMTDC simulation.