Grid Impedance Estimation for Hybrid GFL and GFM Systems

The hybrid systems, including a grid-following inverter(GFL) and a grid-forming inverter(GFM), are extensively utilized in distributed generation. Grid impedance is also critical to controller design and plays an important metric in analyzing system stability. Most impedance estimation methods consider the case of a single inverter, and the methods for multiple inverters ignore the effect of GFM. Meanwhile, they are based on steady-state cases and do not consider dynamic processes in which unstable cases could happen. This paper analyzes the impedance model of the hybrid system and constructs a mathematical model in the α-β frame, capturing the system dynamics. Based on this model, a non-intrusive grid impedance estimation method is proposed using the least squares (RLS) algorithm. The method does not require perturbation injection and can work during system dynamics or in unstable cases, which is instructive for the control and stability analysis of inverters. Simulation results verify the effectiveness of the proposed method.