An Unbalance and Power Controller Allowing Smooth Islanded Transitions in Three-Phase Microgrids

This article introduces a power controller for three-phase inverters in microgrids that can be used in three-phase three-wire and three-phase four-wire systems. The controller enables active and reactive power tracking and unbalanced current control during grid-tied operation, while also allowing seamless transitions into islanded operation. The proposal is motivated by the compelling need in forthcoming power-electronics-dominated grids to provide ancillary services for the main grid and to support grid-forming functionalities for the microgrid, especially in case of islanded operation. The control is developed on the symmetrical components framework. Power tracking is achieved by dedicated control loops that, exploiting P-f and Q-V droop laws applied on positive-sequence powers, accommodate output-power control during grid-tied operation as well as grid-forming capabilities during islanded operation. The controller also includes synchronous dq-frame control for negative-sequence current regulation for providing unbalanced current compensation. The proposed solution addresses the challenge of simultaneously providing concurrent grid-tied control features, such as output power tracking, during grid-tied operation as well as grid-forming capabilities during islanded operation. The related challenge stems from the intrinsically different control actions involved in the two modes of operation, namely, grid tied and islanded. The proposal is verified on an experimental setup composed of converters rated 3 kW.