Transient Stability Enhancement of Current Limited-GFM Inverters Based on Varying Virtual Impedance

Due to the limited overcurrent capability of semiconductor components, the grid-forming (GFM) inverters require additional overcurrent protection schemes. This article analyzes the interaction between the circular current limiter and different types of virtual impedance from the perspective of transient stability. The established unified equivalent circuit model reveals the mechanism of current limitation by the circular limiter, which is based on both the feedforward-virtual impedance (FVI) method and embedded-virtual impedance (EVI) method. Although with the same current-limiting effects, the power-angle characteristics of these two methods are different. The dominant factor influencing transient stability is demonstrated by the analysis of power-angle trajectory. This study observes that the EVI method exhibits superior transient stability compared to the FVI method under the same parameters. Further, a parameter-switching approach based on the EVI method is proposed, which aims to achieve an extended fault ride-through duration. Finally, theoretical analysis is validated through simulations and control hardware-in-the-loop experiments.