Small Signal Modeling and Stability Analysis of Photovoltaic and VSG Energy Storage Systems Under Weak AC Grid

[Objective] Grid-forming control （GFM） energy storage converters based on virtual synchronous generator （VSG） control can effectively provide voltage and frequency support for the system； however， current research has rarely considered the support capability of VSG energy storage converters in the scenario of new energy devices connected to weak power grids. [Methods] This study analyzed the stability laws between photovoltaic systems， VSG energy storage systems， and weak electricity grids. First， small signal models for photovoltaic and VSG energy storage systems under weak AC grids were established. Second， the eigenvalue analysis method was used to comprehensively analyze the impact of changes in photovoltaic output and SCR on the system's eigenvalues. Third， we identified the high-sensitivity control parameters that affect system stability by studying the oscillation modes and participation factors. Finally， a method for adjusting control parameters to improve system stability was proposed by studying the root locus of the dominant mode at the critical steady state. Finally， the accuracy of the stability impact law of the system was verified through a MATLAB/Simulink simulation under different operating conditions. The time-domain simulation model constructed using MATLAB/Simulink verified the effectiveness of the theoretical analysis. [Results] The simulation results showed that the small signal model established in this paper can accurately identify the control parameters that significantly impact system stability under various operating conditions， such as photovoltaic output fluctuations （0.5~2.0 p.u.） and changes in grid short-circuit ratio （SCR=0.6~2.0）. By implementing the proposed parameter adjustment scheme， the photovoltaic output of the system increased from 1.2 p.u. to 2.0 p.u. and remained stable at SCR=0.6. [Conclusions] When the output of the photovoltaic system changes， the q-axis pi parameters of the VSG energy storage system and the photovoltaic system have high sensitivity； when SCR changes， the internal and external q-axis pi parameters of the VSG energy storage system have high sensitivity. Orderly adjustment of the control parameters with different sensitivities improved the static stability of the system， thereby providing an effective solution for improving the stability of a high proportion of new energy grids.