不对称电网电压下MMC-HVDC系统功率波动抑制策略研究

摘要

电网电压不对称时,在基于模块化多电平换流器的高压直流输电(Modular Multilevel Converter based High Voltage Direct Current,MMC-HVDC)系统中,针对MMC交流侧输出产生的有功功率和无功功率二倍频波动问题,提出了将电网电压、电流均变换至正序dq坐标系下进行电流参考值计算的控制策略,可同时抑制MMC交流侧输出的有功功率和无功功率的二倍频波动.此外,电网电压不对称时,在正序dq坐标系下,电压d、q轴分量中除直流分量外,还将含有由负序分量引起的二倍频分量.若在锁相环的设计中不考虑此二倍频分量,则电网电压不对称时系统整体控制性能将会受到影响.据此设计了基于二倍频陷波器的频率自适应锁相环,在电网电压不对称情况下可准确锁定正序电压相位,并计算电网正序电压幅值及负序电压在正序d、q轴上的分量值,用于所提控制方法中的dq变换及功率波动抑制环节中电流参考值的计算.最后通过仿真验证了所提方法的可行性和有效性.%Unbalanced grid voltage will induce fluctuation twice times the grid frequency of active and reactive power at AC side of MMC in Modular Multilevel Converter based High Voltage Direct Current (MMC-HVDC) system.Aiming at this problem,this paper proposes to transform grid voltage and current into positive sequence dq coordinate system to calculate reference current which can suppress fluctuation of active power and reactive power simultaneously.Moreover,under the unbalanced grid voltage,d-axis and q-axis component of voltage will be comprised of twice times the grid frequency component which is induced by negative sequence component in the positive sequence dq coordinate system in addition to DC component.Overall control performance of system will be influenced without considering twice times grid frequency component informed above in the design of phase-locked loop(PLL).This paper designs frequency adaptive PLL based on twice times grid frequency notch filter,which can track phase and calculate amplitude of positive sequence voltage and positive sequence d-axis and q-axis component of negative sequence voltage,as well as will be used to realize dq transformation in proposed control strategy and calculate reference current in power fluctuation suppression strategy.Feasibility and effectiveness of proposed control strategy have been proven through simulation.