SISO impedance modeling and stability comparison of grid-connected inverter control system in different time domains

Abstract

Due to the effects of grid impedance and the negative impedance from the phase-locked loop, the inverter may become unstable during the grid connection process. In order to analyze the small interference stability of grid-connected systems, the analysis method of impedance modeling came into being, which is particularly important for accurately analyzing and judging the oscillation instability of the system under weak grid. However, conventional impedance modeling methods often neglect the discrete characteristics of the control system, resulting in certain limitations when examining stability concerns. To solve this problem, a stability analysis approach based on single-input single-output (SISO) hybrid impedance model is proposed. Firstly, SISO continuous impedance model of system is constructed. Then, the discrete characteristics of the control system are considered, resulting in the derivation of a SISO hybrid impedance model. Furthermore, stable regions of two models are compared under different sampling frequencies. The results indicate that the hybrid impedance model proposed exhibits reduced conservatism, offering a more accurate assessment of stability and stability margins. This finding holds particular significance in guiding the design of controller parameters.