Stability Analysis of Primary Plug-and-Play and Secondary Leader-Based Controllers for DC Microgrid Clusters

In this paper, we propose a new hierarchical control scheme for microgrid (MG) clusters, given by the interconnection of atomic dc MGs with ZIP loads, each composed by both grid-forming and grid-feeding converters. In the primary level, we develop a new Plug-and-Play (PnP) voltage/current controller in order to achieve simultaneous voltage support and current feeding function with local references. The coefficients of each stabilizing controller are characterized by explicit inequalities, which are related only to local electrical parameters of the MG. Moreover, we provide a sufficient condition on the ZIP loads to guarantee passivity and asymptotic stability of electric system. The robustness of performance to system uncertainties is also demonstrated. In the secondary level, a leader-based voltage/current controller is proposed to achieve both voltage and current regulation for the MG cluster without specifying the individual setpoints for each MG. The proposed distributed controller requires a communication network where each regulator exchanges information with its communication neighbors only. With the proposed scheme, each MG can plug-in/out seamlessly, irrespective of the power line parameters and models of other MGs. Closed-loop stability proof of MG clusters is formally proved independently of the cluster topology. Moreover, theoretical results are validated by extensive hardware-in-loop tests showing robustness of the closed-loop cluster against perturbations in the loads, PnP operations, and noises/delays in the communication network.