Analysis and Suppression Study for Instability Phenomenons in Weak Grid-Inverter System

Abstract

To improve the system stability composed of the weak grid and grid-connected inverter, this paper proposes a DC current feedforward control strategy for the inverter. First, the output impedance model of grid-connected inverter and the input impedance model of weak grid are built by small-signal model and the Laplace transformation. Then, adopting the impedance matching method, the system stability is analyzed based on general Nyquist criterion (GNC). The parameters of grid side and the parameters of inverter side are analyzed to estimate the impact on system stability based on Bode diagrams and Nyquist curves. In addition, the influence of proposed control strategy on system stability is analyzed and the effect is compared by Nyquist curves without or with proposed control strategy. This strategy adopts band-pass filter to select specific frequency range, and uses proportional controller to eliminate the impact of DC link current on DC link voltage. The proposed link can equivalently change the output impedance of the grid-connected inverter to meet GNC and improve the system stability. Finally, the system model composed of the weak grid and grid-connected inverter is established in MATLAB/Simulink. And the results verify the correctness and validity of the proposed control strategy.