Distributed Secondary Control in Microgrids

The recent changes in the structure of power generation towards a distributed generation have motivated the increasing interest in the control of so-called microgrids. Microgrids typically consist of an AC electrical network connecting a set of loads and droop-controlled inverters. While the decentralized and proportional primary droop-control ensures the stabilization and the synchronization of the inverters frequencies, the purpose of secondary control strategies is to regulate the frequencies to a rated value. In this work, we build on recent necessary and sufficient stability conditions for an inductive microgrid and propose several analysis and design results for two distributed secondary control schemes: a Distributed Averaging PI (DAPI) controller and a Centralized Averaging PI (CAPI) controller. We will show that both controllers achieve frequency regulation in the presence of power losses in network, even when the secondary control action is restricted to a subset of inverters. We will also prove that the controllers minimize a quadratic cost function of the secondary-power generation for a proper selection of the droop coefficients.