A Variable Virtual Impedance Current Limitation Strategy of Grid-Forming Energy Storage-STATCOM

Compared to grid-following STATCOM, grid-forming STATCOM possesses voltage-mode characteristics, enhancing its stability and proactive voltage support capability in weak grid conditions. Configuring STATCOM with energy storage enables it to provide inertia support and assist in primary frequency regulation as well. In this paper, the structure and overall grid-forming control of a cascaded H-bridge STATCOM configured with supercapacitors are presented. But it's prone to engendering overcurrent during the grid faults. To overcome this critical issue, this paper proposes a variable virtual impedance current limiting strategy that can maintain the voltage-mode characteristics of energy storage STATCOM (ES-STATCOM). Besides, the voltage magnitude control is blocked to avoid the influence of the reactive power-voltage droop coefficient. Based on that, the ratio of virtual reactance to resistance in the steady state is preferred through the analysis of current control bandwidth and reactive power step response. To reduce the power coupling and facilitate the dynamic response, the transient switch of the virtual impedance ratio is proposed. Finally, the effectiveness and superiority of the proposed strategy are validated through simulations.