Modeling and Stability Analysis of Two-stage Photovoltaic Grid-connected System Based on Phase-by-phase Angle Analysis

Cascaded DC-DC converter and DC-AC inverter systems (CDCACS) are widely used in renewable energy grid-connected systems. The stability of these converters is the basis for ensuring the stable and reliable operation of the entire system. High-precision modeling of the system is a prerequisite for carrying out system stability analysis. The existing impedance or average model cannot analyze complex nonlinear behavior such as period-doubling bifurcation. And the discrete time mapping (DTM) model can, but cannot be established for cascade systems with different switching frequencies and the modeling process is complex. In addition, the state equations of the CDCACS are time-varying and there is no equilibrium point in the general sense, which means that the Lyapunov indirect method cannot be used to evaluate system stability. Therefore, this study proposes a discrete time mapping model for AC system (ACDTM) utilizing phase-by-phase angle analysis (PAA) for the inverter in the CDCACS. It not only transforms complex time-varying systems into time-invariant systems for the DC-AC inverter, but also greatly reduces the complexity of traditional DTM model. The complex nonlinear behavior such as period-doubling bifurcation and Hopf bifurcation of the system can be predicted by the proposed method in the CDCACS. Then, taking the two-stage photovoltaic grid-connected system as an example, an ACDTM is given, and the influence of control parameters on system stability is analyzed. Lastly, experiment is used to confirm the effectiveness and accuracy of the method.