Stability Analysis of Grid-Connected Inverters during the Transient of Grid Voltage Fluctuations in Weak Grid Cases

In the distributed power generation system (DPGS), the dynamic performance and stability of grid-connected inverter are endangered by the non-ideal factors at the point of common coupling (PCC), including the grid impedance, the grid voltage fluctuation and the grid fault. This study focuses on the instability issue under the grid voltage fluctuations or faults. It is found that the reduction of voltage value can affect the loop gain of the phase-locked loop (PLL). Consequently, the inverter output impedance under the non-nominal grid voltage is affected, and the closed-loop poles of the overall grid-connected system with the grid impedance can move toward the unstable region. Hence, the inverter can be unstable if the magnitude of the grid voltage sags deeply in the weak grid. As a result, the protection can be triggered before the voltage supporting strategy comes into effect, and yields the failure of low-voltage ride-through. Test results have been provided to demonstrate the correctness of the presented analysis.