半桥三电平逆变器输出不对称交流时直流侧电容电压的分析

多电平逆变器能够产生多阶梯、低失真的电压波形,尤其适用于高压大功率的场合.但是如果输出的电流波形不对称,那么中点电位就会严重偏移.本文以全超导托卡马克核聚变实验装置(EAST)中的快速控制电源为背景,首先对三电平PWM逆变器以及二极管整流器进行分析,根据开关器件的不同状态总结出三电平PWM电路的四种工作模式,然后针对感性负载通过带有直流分量的正弦电流的情况,采用电流充电法和能量守恒法分析出的逆变器直流侧电容电压的变化情况并推导出单位周期内电容电压变化的公式,最后通过仿真与试验进一步验证两个公式的合理性和正确性.

Analysis of capacitor voltage of DC side on half-bridge three-level inverter with asymmetrical AC output

Multilevel inverters which can provide more than two voltage levels and less distorted waveform offer many benefits for high voltage and large power applications.But neutral point potential will offset badly when the output current is not symmetrical.The object of this paper is to fastly control power of Experimental Advanced Superconducting Tokamak(EAST).The power whose load is nearly a pure inductance is required to output an asymmetrical current.There are two respective diode rectifiers for the capacitors on the DC side.The main purpose of this paper is to analyze quantitative the change of the capacitor voltage on the DC side in different output condition to offer the power protection basis.At first,the paper analyses the theory of three-level inverter and the characteristic of diode rectification and explains four kinds of working modes about three-level PWM inverter.Then,based on the current charge method and energy conservation method,the paper analyses the change of the voltage of the capacitor on the DC side and gives out the formula of voltage change in unit cycle when a sine current with DC component passes through the inductive load.The two methods verified each other well and deduced to a consistent conclusion.The simulation and experiment verify further the rationality and correctness of two formulas.