弱电网下计及锁相环影响的并网逆变器输出阻抗重塑

Reconstruction of the Output Impedance of Grid-connected Inverter Considering PLL Effect in Weak Grids

比例积分+谐振补偿控制器(resonant harmonic compensators,HCs)可以用来抑制电网背景谐波引起的并网电流畸变。然而在电网阻抗较大时,锁相环(phase-locked loop,PLL)、电流控制器与电网阻抗相互耦合,降低了并网逆变器系统的稳定性。为了改善弱电网下锁相环对并网逆变器系统的不利影响,首先建立了考虑锁相环影响的并网逆变器小信号模型,分析了锁相环导致系统稳定性的原因,揭示了电网阻抗影响并网逆变器-电网交互系统稳定性的本质规律。然后,提出一种基于二阶低通滤波器(low-pass filter,LPF)的锁相环优化控制与参数设计方法,以重塑并网逆变器输出阻抗。最后对所提策略进行了仿真与实验验证。研究结果表明,改进的策略可以提高弱电网下并网逆变器系统的稳定性,改善其对电网的适应性,从而证明了理论分析的正确性与所提策略的有效性。

PI+HCs can be used to suppress grid-side current distortion caused by grid background harmonics.However,when the grid impedance is large,phase-locked loop(PLL)and current controller will interact with each other via the grid impedance,reducing the inverter system stability.To improve the adverse impact of PLL on the inverter-grid interaction system,the small-signal model of grid-connected inverter considering the influence of PLL is first established in this paper to explain how PLL deteriorates system stability and reveal the essential law of the grid impedance affecting the stability of the inverter-grid interaction system.Then,an optimization control and parameter design method of PLL based on second-order LPF is proposed to reshape the output impedance of the grid-connected inverter.Finally,the proposed strategy is verified by simulation and experimental tests.The simulation and experimental results show that the adopted method can enhance system stability and improve its adaptability to the grid when the grid impedance is large,thus proving the correctness of theoretical analysis and the effectiveness of the proposed strategy.