Dynamic Stabilization of a Centralized Weak Grid-Tied VSC System Considering PV Generator Dynamics

This paper addresses the instabilities and dynamic interactions in a centralized vector-controlled voltage-source converter interfacing a single-stage utility-scale photovoltaic (PV) generator to a high-impedance grid system. The dynamic resistance of the PV generator is considered, and effective active stabilization methods to guarantee the entire system's stability are proposed. Further, a complete state-space linearized model is developed, and different operating conditions alongside the maximum-power operation (MPO) are considered, such as fixed-voltage operation (FVO) and fixed-current operation (FCO). It is found that with the change of the PV generator operation from the FVO or MPO to the FCO region under the weak grid and nominal dc-link capacitance conditions, the dominant eigenmodes are relocated to the open-right-half-plane of the S-plane. This eventually induces inevitable high- and low-frequency oscillation instabilities. Further, it is found that medium-frequency oscillation instability is induced in the FVO region under a weak grid and reduced dc-link capacitance conditions. Active compensators are proposed to reduce the interaction dynamics and stabilize the entire system by reshaping the converter transfer functions; hence, the Nyquist stability criterion is maintained. Finally, detailed nonlinear time-domain simulation results are provided, showing the effectiveness of the proposed method in preserving the system's stability under a wide range of operation conditions.