应用于STATCOM的级联U-Cell拓扑直流电压控制策略

王颖杰 薛道路 焦岚轶 廖国强 齐东迁

Abstract：Compared with traditional cascade H bridge topology， cascaded U-Cell topology of static synchronous compensator （STATCOM） has few switch devices， low loss and good economic value. However， cascaded U-Cell also exists the problem of unbalanced capacitor voltage， which affects the system reliable operation in severe cases. A mathematical model of cascaded U-Cell is built for this problem， and the principle of capacitor charging and phase-capacitor voltage balance are analyzed in detail. Based on this， a control strategy combining redundant switching states rotation and active voltage vector superimposed by switching state is put forward in the paper， of which is extended to arbitrary odd number level. This strategy can make the device loss more uniform， achieved the balance of the capacitor voltage in the cascade U-Cell， and improve the reliability of the system. Finally， the simulation and experiment platforms of STATCOM based on five-level cascaded U-Cell topology are built to verify the correctness of the theory and control strategy.

Keywords：cascaded U-Cell;STATCOM;redundant switch state;switch state rotation;active voltage vector

0 引 言

無功功率补偿是电力系统的重要组成部分，能够校正功率因数，改善电压调整率，提高系统动静态稳定性[1]。无功功率补偿常用的拓扑是级联H桥[2-3]。级联U-Cell结构是在级联H桥结构的基础上改进所得，输出相同电平数时，级联U-Cell可以使用更少的开关器件，且其通态损耗比级联H桥少很多，因此总损耗明显降低[4]。

级联U-Cell中直流侧电压均衡控制直接关系到变流器交流侧输出波形质量和变流器的动态响应速度[5]。目前级联U-Cell电压均衡控制方法主要有以下两种[6]：一种是调整变换器输出电压和相电压相位差;另一种是脉冲轮换调制算法。文献[4]通过改变电网相电压和变换器输出电压之间的相位差实现无功补偿，此种方法可以稳定直流电压但未能实现电容电压的均衡。文献[6]采用脉冲轮换的调制策略，在两个调制波周期轮换对电容充放电。因为轮换周期较长，所以并未完全实现相内电容平衡控制，同时此种方法只适用于五电平级联U-Cell，普适性较差。

级联U-Cell每个模块为二端口，这与级联H桥一端口模块相比，相内电容电压平衡控制相对较复杂。本文从开关状态轮换的SPWM调制算法和基于开关状态叠加有功电压矢量控制策略入手，分别解决由拓扑本身造成电容电压不平衡，和各U-Cell单元损耗差异等造成的电容电压不平衡问题。首先，建立五电平级联U-Cell电容充电数学模型，从数学公式推导出开关状态轮换的SPWM调制算法实现相内电容电压平衡的可行性，使器件损耗更均匀化。……