Sequence Admittance Model Decoupling Method for Grid-Connected Converters

Impedance method is one of the classical methods for system stability analysis. In order to simplify the complexity of the model, it usually ignores the anti-diagonal elements of the sequence impedance matrix. However, the asymmetric control structure of the converters will lead to frequency coupling. When the frequency coupling is serious, ignoring the anti-diagonal elements will cause a large error. In this paper, the converter sequence admittance matrix considering phase-locked loop(PLL) and control system is established in d-q frame, and the reason of frequency coupling phenomenon is analyzed. Then, based on the characteristics and the constraints of the admittance matrix, the transfer function of the converter in the full band decoupling is solved, and the low and middle band decoupling method is designed. Finally, a simulation model in MATLAB verifies that the decoupling method of middle and low frequency band can effectively eliminate the frequency coupling, and the results of the sequence impedance method are consistent with the generalized Nyquist criterion after decoupling, which can accurately judge the stability of the system.