Analysis and Mitigation of Subsynchronous Resonance in Series-Compensated Grid-Connected System Controlled by a Virtual Synchronous Generator

Analogizing the subsynchronous resonance (SSR) phenomenon of the series-compensated grid-connected system based on synchronous generator, the virtual synchronous generator (VSG) also has the low-frequency oscillation problem owing to the impedance interaction between VSG and series-compensated network. Stability of the series-compensated grid-connected system controlled by VSG is analyzed by the impedance-based Nyquist stability criterion. Due to the inductive impedance of VSG, VSG is easy to couple with the series-compensated network in the low-frequency areas, which can lead to the SSR. With the increase of the series compensation level, the stability of VSG becomes worse and the induced oscillation frequency becomes higher. Therefore, a modulation signal feedforward active-damping control strategy with time-delay compensation is proposed to suppress this SSR, which directly feeds the active-damping control signal to the pulsewidth modulation modulator for reshaping the output impedance of VSG from the inductance to the resistance–inductance. Moreover, a value range of virtual resistance is given with considering the influence on the regular operation of VSG. Finally, the simulation and experimental results verify the correctness of the SSR analysis and the feasibility of the proposed active-damping control strategy.