基于电压收敛性预测的MMC低开关频率均压控制策略

Reduced Switching Frequency Voltage Balancing Control Strategy Based on Voltage Convergence Prediction for Modular Multilevel Converters

随着基于模块化多电平换流器(modularmultilevel converter,MMC)的柔性直流输电技术逐渐向高电压等级、高传输容量发展,MMC子模块数量迅速增加,由此带来换流器运行效率低下的问题日益凸显。为实现MMC内部悬浮电容能量均衡,传统均压算法控制下的各桥臂子模块高频投切,会产生大量无效开关损耗。针对这一问题,提出一种低开关损耗的MMC调控方法。首先,基于MMC运行原理,推导了子模块投切、轮换与电容电压暂态增量的数学关系,从而提出对电容电压序列收敛性的预测方法。基于此,提出一种改进的低开关频率均压控制策略。该策略根据对桥臂内子模块电压序列的收敛性的预测结果,在每个离散控制周期初始时刻动态调控子模块轮换数,以消除非必要的子模块轮换。与现有均压策略相比,该策略能在不降低均压效果的前提下,有效降低开关频率,有助于提高系统的运行效率。基于实际柔性直流输电工程参数,在PSCAD/EMTDC上搭建仿真模型,对所提出改进型方法和现有低开关频率控制方法进行了对比验证。

With the development of modular multilevel converter(MMC)based DC power transmission technology to high voltage levels and high transmission capacity,the sub-module number has increased rapidly,which has led to the increasingly prominent problem of low operation efficiency.However,in the process of achieving the capacitors energy balance,the traditional voltage balancing method brings a large amount of invalid switching losses.In this paper,the mathematical relationship between module switching and transient increment of capacitor voltage is derived,and a prediction method for the convergence of capacitor voltage sequences is proposed.Based on this method,an improved voltage balancing control strategy is presented.According to the prediction result of the sub-module voltage convergence of each arm,the adjusting switching number is modified dynamically to eliminate unnecessary adjusting switching.Compared with the current voltage balancing strategy,the switching frequency is effectively reduced without degrading the voltage balancing effect.The proposed method is validated on the practical engineering parameters based MMC system.