Hybrid Power Synchronization Based Transient Stability Enhanced Control for Grid-Forming Inverters Under Symmetrical Grid Fault

The current limiter can effectively limit the overcurrent for grid forming (GFM) inverter, but it also makes the inverter easier to lose the stable equilibrium point (SEP) during grid fault. The untimely fault clearing, inaccurate fault depth detection and grid impedance measurement result may lead to overoptimistic deceleration effect evaluation and transient instability problem. In this article, the hybrid power synchronization method is proposed to increase the robustness in transient stability of the GFM inverter against parameters variation. The reactive power that internally related to fault depth is introduced as the dynamic active power reference. It can make sure that the SEP exist during grid fault without the need of fault depth detection. In the same time, the dynamic power compensation related to fault depth is also achieved. The reactive power feedback gain optimization and equivalent power reference limitation method are further given to increase the robustness against large impedance measurement error. Based on equal area criterion, it is demonstrated that with the proposed hybrid power synchronization method, the transient stability of GFM voltage source converter always can be guaranteed once the SEP exists. The impact of line-to-ground fault on transient stability is analyzed and the robust enhanced method is also given. Finally, the effectiveness of the proposed method is verified on a 5 kW experimental platform.