Mitigation of Subsynchronous Resonance for Grid-Connected Inverters in Series-Compensated Weak Power Grids Through Observed Q -Axis Grid Voltage Feedback.

Series compensations have been widely used in modern power systems to extend the power transfer capacity. Since more and more renewable power generation systems are installed in power systems, the grid-connected inverters (GCIs) used by the renewable power generations may interfere with series-compensated power lines, which introduce subsynchronous resonance (SSR) causing power system failure that has been observed in real cases. The phase-locked loop (PLL) of the GCI's control system is the main factor that can cause the SSR in the series-compensated power grid. To mitigate the impact of the PLL and obtain the stable operation of the power system, a q-axis voltage feedback method has been proposed in literature to damp the harmonic oscillations in the weak power grid where, however, the series compensation is not considered. Because the real q-axis voltage cannot be obtained, the q-axis voltage output from the PLL is used as the feedback variable in literature. However, due to the dynamic response of the PLL, the q-axis voltage output from the PLL is different from the real q-axis voltage, and the direct use of it for the damping of the SSR in the power grid with series compensations is invalid, which is investigated in this article. Besides, an observed q-axis voltage feedback method is proposed in this article to damp the SSR in the series-compensated power grid with GCIs being connected. By putting an observer in the q-axis voltage feedback path, the real q-axis voltage can be estimated and applied to damp the SSR. Simulation and experimental results have been provided to verify the effectiveness of the proposed observed q-axis voltage feedback method in the damping of the SSR. [ABSTRACT FROM AUTHOR]