LVRT Capability Enhancement of Grid-Forming Inverters Under Current Saturation: A Dual-Reference Dynamic Cooperative Control Strategy

While transient stability enhancement for grid-forming inverters (GFMs) considering the impact of current-reference limiter has been widely studied, it remains insufficiently explored for GFMs employing circular current limiter due to the heightened complexity of power-angle curves under this limiting scheme. Meanwhile, based on the timing of grid voltage sag and current saturation, there are two types of current saturation model (CSM) for GFMs. A unified approach enhancing low voltage ride through (LVRT) capability across both CSMs has not been studied. To this end, this paper proposes a dual-reference dynamic coordination control strategy for GFMs to enhance its LVRT capability. Firstly, the explicit expression of output active power of GFMs incorporating dq-axis components of voltage reference is derived. Then, the transient synchronous stability control mechanism via power/voltage reference modulation is revealed by using EAC and phase-portrait analysis. Secondly, based on the obtained stability mechanism, a dual-reference dynamic coordination strategy is proposed, which can automatically trigger mode-specific strategies for two types of CSM. It dynamically adjusts power reference to guarantee the existence of equilibrium points (EPs) and modulates voltage reference to ensure the EP is stable and maximize transient stability margin. Finally, experimental results verify the effectiveness of the proposed strategy.