A Power-Angle-Based Adaptive Overcurrent Protection Scheme for Grid-Forming Inverter Under Large Grid Disturbances

Abstract

As the capacity of renewable energy generation increases, grid-forming (GFM) inverters are deemed as promising solutions for low inertia power grids. However, power-electronic-based inverters have limited overcurrent capability, so additional overcurrent protection schemes are necessary. More importantly, the stability should not be jeopardized by using additional overcurrent protection schemes. However, GFM inverters with the conventional current reference limiting method tend to be unstable under large grid disturbances such as grid frequency drop. To address this problem, this article proposes a virtual power angle limiting method. The main idea of this method is to limit the output power by limiting the power angle instead of directly limiting the current reference. Thus, the power synchronization control law is still satisfied when the overcurrent protection is triggered, so that the stability can be maintained. Besides, after redesigning the power angle limiting value, the proposed method is also suitable for grid voltage sag cases. A significant advantage of the proposed method is that the output current can be limited automatically during grid voltage sag or frequency drop events without the need of fault detection or tuning control structures and parameters. Simulation and experimental results have verified the effectiveness of the proposed method.