低开关频率对并网逆变器控制环节的影响及补偿方法

为了防止大功率并网逆变器网侧LCL滤波器所导致的谐振问题,通常采用无源阻尼方法进行谐振抑制。但是传统的无源阻尼方法未考虑数字延时对系统稳定性的影响,通过在离散域和连续域建模结合奈奎斯特稳定判据分析,得出采用准比例-谐振(QPR)控制时,低开关频率下的数字延时角会导致无源阻尼方法在谐振频率附近出现负穿越。这个特性会导致电流环不能稳定运行,并进一步得出QPR控制下的无源阻尼系统稳定性不受电网阻抗变化的影响。为了保证低开关频率下电流环的稳定性,提出采用具有低通特性的超前环节补偿QPR控制器谐振频率相位滞后的方法,给出了超前补偿的设计步骤。最后,仿真和实验验证了所提方法的正确性和有效性。

Effect of Low Switching Frequency on Control Link of Grid-connected Inverter and Compensation Method

In order to prevent the resonance problem caused by the grid-side LCL filter of high-power grid-connected inverter, the passive damping method is usually used to suppress the resonance. However, the conventional passive damping method ignores the effect of digital delay on the system stability. Through the modeling in the discrete domain and the continuous domain and Nyquist stability criterion analysis, it is concluded that, when quasi-proportional-resonant (QPR) control is adopted, the digital delay angle at the low switching frequency will lead to negative crossing near the resonant frequency. This characteristic will lead to the unstable operation of current loop. And it is further concluded that the stability of the passive damping system under the control of QPR will not be affected by the change of grid impedance. In order to ensure the stability of current loop at the low switching frequency, this paper proposes a method to compensate the phase lag of resonant frequency of QPR controller by using phase lead compensation networks with low-pass characteristics and gives the design steps of lead compensation. Finally, the correctness and effectiveness of the proposed method are verified by simulation and experiment.