Control interaction analysis of hybrid system with grid-following and grid-forming inverters based on admittance decomposition

In high renewable penetrated power systems, both grid-forming (GFL) and grid-following (GFM) inverters play an important role in maintaining the system stability and economic operation. However, the two kinds of inverters exhibit distinct dynamic characteristics; thus, interconnecting them in a close electrical distance may cause the stability concern. In this paper, the small-signal stability of a hybrid system with GFM and GFL inverters is investigated. Based on the idea of admittance decomposition, this paper first decomposes the overall admittance of two inverters into several sub-admittances corresponding to different control loops and circuit components. Then an in-depth analysis is conducted to reveal the dynamic interaction between multiple time-scale control loops based on decomposed admittances. Moreover, the impact of the line impedance, the grid strength, and the power flow on the system stability is analyzed and a security region is further developed. The accuracy and effectiveness of the analysis are validated through eigenvalue analysis, simulations and hardware in the loop based experiments.