新型模块化多电平变换器电容电压波动规律及抑制方法

Voltage Ripple Principle and Restrain Method of Floating Capacitors in a New Modular Multilevel Converter

模块化多电平变换器以其结构模块化、不需要变压器以及易于扩展到高电平等优点,成为近年来研究的热点。本文针对一种新型的模块化多电平变换器拓扑研究了其悬浮电容电压波动规律。首先从半桥臂的输出功率入手,分析了其功率脉动和一个周期内的能量变化,给出了电容电压波动幅值表达式。针对电容电压在低频下波动较大的问题,提出了一种电容电压波动抑制方法。采用叠加高频零序电压和相间环流的方式重构桥臂电压和电流,通过提高半桥臂输出功率的波动频率来减小悬浮电容的能量波动幅值,进而减小电容电压波动幅值。设计了一台三相五电平的实验样机并对上述控制方法进行了实验验证。

Modular multilevel converter（MMC） is a highly attractive multilevel converter topology for medium and high voltage applications during recent years.In this paper,a new MMC topology and the associated voltage ripple problem of floating capacitors are studied.Based on the instantaneous output power of a half bridge,power ripple and energy variation in a period can be derived.Then the mathematic expression of voltage ripple amplitude is acquired.This problem becomes more severe when the fundamental frequency is lower.This paper proposes a method to restrain the lower frequency voltage ripple.By adding high frequency zero-sequence voltage and phase circulating current to increase the power ripple frequency,the energy ripple amplitude can be reduced and then voltage ripple is also reduced.A three-phase five-level prototype is built up to verify the validity of the method by simulation and experimental results.