Analysis and Active-Impedance-Based Stabilization of Voltage-Source-Rectifier Loads in Grid-Connected and Isolated Microgrid Applications

This paper addresses the stability analysis and active-impedance-based stabilization of pulsewidth modulated voltage-source rectifier (VSR) loads in grid-connected and isolated voltage-source inverter (VSI)-based microgrid systems. Using small-signal analysis, detailed and coupled admittance-based models of the VSR-interfaced load and the source side are obtained in the rotating reference frame by considering all control loops and phase-locked loop dynamics. Therefore, a multi-input-multi-output (MIMO) impedance-admittance network is yielded. Using the generalized Nyquist stability criterion, it is shown that a VSR-interfaced load can negatively interact with the source-side dynamics resulting in unstable voltage response at the point of interconnection. To suppress these instabilities, proposed active-impedance-based compensators are implemented in the control structure of the VSR-interfaced load to satisfy the generalized Nyquist stability criterion and provide seamless integration of VSRs into both grid-connected and isolated VSI-based ac systems. A theoretical analysis, time-domain simulations, and experimental results are presented to evaluate the effectiveness of the proposed techniques.