Stability Evaluation of Grid-Connected Microgrid Clusters in Asymmetrical Grids

Abstract

Stability of grid-connected microgrids could be to a high extent guaranteed by designing a robust control system for inverters. However, this is not necessarily the case when microgrids are connected to an asymmetrical grid, where the grid impedance could have a different value in each phase. In this condition, the asymmetry of the grid impedance causes couplings between <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">αβ</i> - or <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, and therefore, the control system could not be simplified as a single-input single-output system anymore. In this case, the system turns into a multi-input multi-output system, and therefore, stability evaluation of such a system consisting of several microgrids that form microgrid clusters with multiparallel inverters could be a challenging task, which needs heavy mathematical derivations. In this article, a straightforward method is used for modeling the grid-connected microgrid clusters, in which the asymmetry of the grid impedance has been considered. Based on this approach, all the inverters in a microgrid are lumped in a frequency-domain-based model using the inverters’ Norton equivalent models. Then, based on the derived model, the generalized Nyquist criterion is used to evaluate the stability of the whole system. The proposed method considerably reduces the mathematical burden, while accurately predicts the system stability. The experimental results in a small-scale test bench in the laboratory validate the accuracy of the proposed method.