Stability analysis and impedance shaping of MW-Level photovoltaic energy storage system integrated in traction power supply system

To realize energy conservation and emission reduction of electric railways, it is an effective way to integrate a MW-level photovoltaic energy storage system (PV-ESS) in traction power supply system (TPSS). However, the multi-converter interconnection structure greatly increases the risk of system instability and the complexity of stability analysis. To analyze and solve such instability issue, the small-signal impedance models considering the dynamic characteristics of single-phase grid-connected converter, PV and ESS for multi-converter system integrated in TPSS are developed. Then, the sub-impedance ratio criterion is introduced to evaluate system stability and to identify the dominant units of instability issues. On this basis, the impacts of power flow conditions, circuit parameters, and control parameters on system stability are quantitatively analyzed. Analytical and simulation results show that the negative impedance characteristic of charging ESS is the main cause of instability issue. To solve this, a sliding-mode parallel virtual impedance method is proposed. DC-link voltage feedforward is introduced to shape ESS impedance to a positive characteristic, thus enhancing the system stability margin. Simulation and experimental results confirm that the proposed method can effectively suppress system instability while maintaining reasonable dynamic performance.