Abstract: This paper proposes a novel method to evaluate and optimize the performance of single-phase grid-connected inverter system with VPI controllers through Nyquist diagrams. This approach allows to remove the limitation presented by Bode diagrams and its stable criterion that the crossover frequency must be higher than resonant frequency. A systematic method is proposed to obtain the highest stability and avoidance of closed-loop anomalous peaks: it is achieved by the maximization of the Nyquist trajectory distance to the critical point. The parameter design process of the improved VPI control method is simplified. It is no longer difficult to achieve a tradeoff between stability and transient response speed. The effectiveness and feasibility of the proposed control strategy are verified by results from theoretical analysis and experiments.

Key words: Nyquist diagrams, Vector Proportional Integral（VPI）, closed-loop anomalous peaks, phase compensation, parameter design

摘要： 在单相光伏并网逆变控制中，针对谐振频率高于截止频率时Bode图及其稳定判据的使用受到限制等问题，提出一种基于 Nyquist图的谐波补偿方法。该方法将Nyquist曲线到临界稳定点（－1，j0）的最短距离作为系统稳定程度的表征量，通过对矢量比例积分控制器（VPI）设计合适的相角补偿使该距离在Nyquist图中达到最大，从而提高奇次谐波补偿次数，同时避免闭环异常峰，闭环系统的稳定性和抗干扰能力得到增强。改进后的VPI控制使系统保持较高的稳定性，参数变化对系统稳定性的影响可忽略不计，因而简化了参数设计过程。理论分析和仿真结果验证了基于Nyquist图进行改进后的VPI控制策略的有效性和可行性。

关键词: Nyquist图, 矢量比例积分控制（VPI）, 闭环异常峰, 相位补偿, 参数整定