Analysis of Persistent Oscillation Mechanism and Low Frequency Coupling Characteristics of Power System From the Perspective of Grid-Connected Converter

The voltage disturbance in the weak grid will cause frequency coupling and even persistent oscillation of the grid current of grid connected converter in power system. From the perspective of grid-connected converter, this paper establishes a unified complex variable model of disturbance frequency / coupling frequency in grid connected system, analyzes the evolution mechanism from SIDO (Single Input Double Output, SIDO) to DIDO (Double Input Double Output, DIDO) of grid connected converter, clarifies the relationship among voltage disturbance, grid impedance and frequency coupling. Then a robust current control strategy based on improved PLL structure has been proposed. Compared with the traditional method, this paper demonstrates that the coupling frequency component is the main cause of instability or oscillation, and the improvement of low frequency characteristics is helpful to enhance the adaptability of the converter to the weak grid. Finally, the correctness of the theoretical analysis is verified by the experimental platform.