全桥型模块化多电平变流器低冲击电流可控充电策略研究

Research on controllable charging strategy with low impulse current of full-bridge modular multilevel converter

模块化多电平变流器(MMC)具有模块化程度高,交流输出电压谐波小,控制灵活等优点。全桥型MMC可以通过闭锁来阻断直流故障电流,具有良好的故障穿越能力。并且单个的全桥型MMC模块可以输出正、负和零三种电平,桥臂具有输出负电压的能力,具有较高的直流电压利用率。由于MMC子模块含有储能电容,启动前必须先对子模块电容充电,否则系统无法平稳安全解锁。通过分析全桥型MMC交流侧充电过程,指出了由不控充电到模块解锁后可控充电产生电流冲击的原因。在此基础上提出全桥型MMC低冲击电流可控充电策略,可使子模块电压在解锁后平滑地上升到额定值。实验验证了提出的控制策略的可行性和有效性。

Modular multilevel converter(MMC)has advantages of high degree of modulization,low AC output harmonic current,flexible control and so on.Full-bridge MMC can block DC fault current by locking sub module,which provides it good ability of fault ride-through.Moreover,the sub module of full-bridge MMC has three output voltage levels of positive,negative and zero which increases utilization of DC voltage.MMC sub module capacitor needs pre-charging before starting,otherwise the system would not unblock steadily and safely.In this paper the reason of appearing the impulse current when switching from uncontrolled charging to controlled charging is pointed out by analyzing the AC charging procedure.On this basis a kind of controllable charging strategy of full-bridge MMC is put forward by which the sub module capacitor voltage can increase to the reference level smoothly.Experiment result shows the validity of this strategy.