Stability Analysis of Parallel Grid-Forming Inverters in Asymmetrical Conditions
Ali Akhavan, Juan C. Vásquez, Josep M. Guerrero
Abstract
The stability of inverters can be greatly improved by designing them with a robust control system. However, even if the system remains stable under symmetrical conditions, the presence of asymmetrical load/grid impedance can still pose a threat to its stability. Under this circumstance, the coupling between the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">αβ</i> - and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dq</i> -frames in the average model leads to the creation of a multi-input multi-output (MIMO) system. Consequently, simplifying the system into a single-input single-output (SISO) form is no longer feasible. This situation becomes even more complex when multiple inverters are connected in parallel, where the interaction among inverters complicates the modeling of the system. This paper presents a simple yet accurate approach to address these challenges. To this end, the models of the inverters in the frequency domain are lumped to investigate the stability of the entire system in light of the asymmetrical condition using the generalized Nyquist criterion.