A Transient Stability-Enhanced Method for Grid-Forming VSCs Considering the Current Limiting Algorithm

To address the instability issue in the current limiting method of grid-forming voltage-source converters (GFM-VSCs) during grid sag faults, this paper proposed a transient stability-enhanced method (TSEM) considering the current limiting algorithm. The proposed method limits the steady-state fault current by adaptive virtual impedance (AVI) and adjusts the power reference by detecting the grid voltage magnitude to establish equilibrium points (EPs) of the power curve during grid sag faults. Since this method limits the fault current by reducing the voltage reference while constructing EPs, the voltage source operating characteristics as well as the power synchronization are maintained during faults. Another significant advantage of the proposed method is that the magnitude of grid voltage sag can be detected by variables in the controller without sampling the grid voltage or measuring the line impedance. In addition, a virtual-power-angle-based large-signal model is established in this paper to reveal the transient stability under different controls. Finally, experimental results verify that the proposed control effectively limits the fault current and enhances the transient stability of GFM-VSCs.