模块化多电平换流器子模块对地电压分析及其dv/dt抑制方法

Analysis on Voltage to Ground of Submodule and Its dv/dt Reduction for MMC

模块化多电平换流器在开关过程中,子模块对地电压变化产生的dv/dt对杂散电容充放电形成共模电流,造成严重的传导电磁干扰。文中推导子模块对地电压跳变值与桥臂中子模块位置的关系,分析不同跳变值出现的概率,指出某些位置会出现数倍电容电压(UC)的跳变。为将所有位置子模块的最大对地电压跳变抑制为UC,文中提出一种抑制方法,该方法基于上一控制周期的子模块投切状态、桥臂电流方向、电容电压及当前控制周期需要投入的子模块个数,来确定当前周期的子模块投切状态,并限制两个周期之间子模块投切状态改变的数目为一个或一对,从而达到抑制对地电压跳变的目的。最后通过实验验证子模块对地电压跳变规律及所提方法的有效性。

A modular multilevel converter(MMC)usually consists of many submodules(SMs).During the commutation transient,the voltage to ground of SMs will suddenly change,and high dv/dt causes common-mode current to flow into the ground through parasitic capacitors,resulting in severe conducted EMI.This paper analyzes the voltage jump rules for SMs at different locations and also concluded that SMs at some positions jumped several times the capacitor voltage(UC).In order to limit the maximum voltage jump value of SMs at all positions to UC,a suppression method is proposed.The trigger pulses of each SM in current control cycle are determined by the arm current direction,the capacitor voltages,the trigger pulses in last control cycle and the number of SMs switched at on-state in current control cycle.Therefore,the number of state-changed SMs could be limited to one or a pair between two cycles.Finally,the experimental results reveal that the analysis of the voltage jump rules is correct and the proposed method is effective.