The Single Equivalent Inverter Admittance Model of Three-Phase Grid-Connected Inverter System Considering Frequency Coupling

The asymmetric inverter controller structure or parameters result in frequency coupling in phase domain for the three-phase grid-connected inverter system. And it is difficult to measure the inverter admittance matrix in engineering practice due to the coupling term. What's more, applying the coupled admittance matrix to analyze the system stability, the generalized Nyquist criterion (GNC) has to be used. Therefore, considering the frequency coupling due to phase-locked loop (PLL), this paper builds the single equivalent inverter admittance model of three-phase grid-connected inverter system. This single equivalent inverter admittance model is convenient for measurement in engineering practice. Besides, by applying this model, the system stability can be analyzed with Nyquist criterion. Compared with the approaches which use GNC, the stability analysis is simpler and it is easy to obtain the stability margin of the system to guide the controller design. The proposed model is verified by simulated measuring results. Experiment results prove the correctness of the stability analysis results.