基于模块化多电平换流器的统一潮流控制器桥臂电容电压平衡控制策略

A Balance Control Strategy for Bridge Arm Capacitor Voltage of MMC Based UPFC

建立了基于模块化多电平换流器(modular multilevel converter,MMC)拓扑的统一潮流控制器(unified power flow controller,UPFC)的详细数学模型,讨论了串联式接入系统和并联式接入系统对MMC内部特性的影响,结果显示串联式接入更易出现上下桥臂电容电压不平衡现象;探讨了低频环流的本质,揭示了单相系统功率与环流的关系,得到通过控制桥臂电容电压可抑制低频环流的结论。以理想工况下桥臂电容电压为控制目标,设计了基于基频和负序二倍频旋转坐标系的电压平衡控制器,该控制器可同时适用于并联侧和串联侧。RTDS仿真结果证明在最近电平逼近调制下所提控制策略可有效控制桥臂电压平衡,并能抑制负序二倍频环流。

Based on the topology of modular multilevel converter（MMC） a detailed mathematical model of unified power flow controller（UPFC） is established, and the impacts of the parallel grid-connection of UPFC and the series grid-connection of UPFC on internal characteristic of MMC are researched, and the research results show that under the series grid-connection of UPFC the unbalance, which is between the capacitor voltage of upper bridge arm and the voltage of lower bridge arm, is likely to appear. The essence of low-frequency circulating current is analyzed and the relationship between the power of single-phase system and the circulating current is revealed, thus, a conclusion that low-frequency circulating current can be suppressed by controlling bridge arm capacitor voltage, is attained. Taking the bridge arm capacitor voltage under ideal operating condition as control objective, based on fundamental frequency and negative sequence second harmonic frequency rotating frames a balance controller of upper and lower bridge arm capacitor voltages, which is suitable for the parallel grid-connection side and series gird-connection side simultaneously, is designed. RTDS simulation results show that under the nearest level modulation the capacitor voltage balance between upper and lower bridge arms can be effectively controlled by the proposed control strategy, and the negative-sequence second harmonic frequency circulating current can be suppressed.