Impedance Modelling and Stability Analysis of Joint Photovoltaic and Battery Based Energy Storage System Under Weak Grid

In conventional grid-following (GFL) photovoltaic (PV) power generation systems, power fluctuations can be smoothed by attaching grid-forming (GFM) energy storage (ES) converter. In heterogeneous systems containing GFL and GFM converters, the output impedance model and the cascade system stability criterion can be utilized to study the grid-connected stability of the system. However, most of the existing impedance models are single-machine impedance models or simple parallel models that do not fully consider the frequency coupling effect between different converters and grid impedance, making it difficult to accurately assess the stability of heterogeneous systems under high grid impedance. In view of this, taking the two-machine heterogeneous grid-connected system as an example, this paper analyzes the coupling effect between the output impedance of the system and the grid impedance, and finds that the grid impedance has an additional effect on the output impedance of the paralleled system. Based on this, the small disturbances stability of the paralleled system under weak grid is discussed. The results of the simulation experiments show that the stability analysis based on the improved impedance model in this paper realizes the accurate judgment of small disturbance stability of the system under weak grid.