交流系统不对称时模块化多电平换流器的控制

Control of Modular Multilevel Converter Under Imbalance of AC Power System

在交流系统不对称的情况下,三相模块化多电平换流器(modular multilevel converter,MMC)内部会出现零序性质的2次谐波环流,该零序分量将进入MMC直流侧,加剧直流电压、电流和功率的2倍频波动。为抑制直流侧功率的2倍频波动,以瞬时功率理论和比例谐振调节器为基础,设计了两相静止坐标系下MMC的控制策略。该控制策略由交流回路控制和直流回路控制2部分构成,前者实现MMC与交流系统之间的功率调节;后者实现MMC内特性的调节,主要抑制MMC环流中的2次谐波成分,使桥臂电流波形接近正弦。仿真系统采用不控整流站,逆变站采用MMC,结果表明:在交流系统不对称的情况下,该控制策略有效抑制了环流中的2次谐波成分以及MMC直流侧电压、电流和功率的2倍频波动。

During the imbalance of AC power system,the second-order harmonic circulating current,which behaves just like zero-sequence current does,appears inside three-phase modular multilevel converter（MMC）,and such a circulating current enters into DC side of MMC and the double frequency fluctuation of DC voltage,current and power will be intensified.To suppress the double frequency fluctuation of DC power,a control strategy for MMC in stationary α-β frame,based on instantaneous power theory and proportional-resonant regulator is designed.This control strategy is composed of two parts,i.e.,AC-loop control and DC-loop control,the former is used to implement the power adjustment between MMC and AC system and the latter is used to regulate the internal performance of MMC,that is,it mainly suppresses the second-order harmonic circulating current inside MMC to make the current in bridge arms close to sinusoidal.To verify the proposed controller,an inexpensive high voltage direct current transmission system,in which a non-controlled rectifier is at one side and MMC at the other side,is built for simulation.Simulation results show that under imbalance of AC power system the proposed control strategy can effectively suppress the second-order harmonic in circulating current as well as the double frequency fluctuation of voltage,current and power at DC side of MMC.