A systemic and model-less approach for real-time optimal control of unbalanced AC microgrids dominated by power electronics

This paper presents a two-layer hierarchical control system for power regulation between grid-former converters feeding islanded or grid-connected microgrids. This work considers the inherent unsymmetric nature of microgrids that can be composed of, in addition to three-phase loads, single-phase loads. The proposed secondary control strategy combines the classical automatic generation control (AGC) for active power and frequency regulation with an online feedback optimisation (OFO) method for reactive power and voltage regulation per phase. By coordinating the grid-forming devices using these techniques, an optimal operating point can be attained that minimises the voltage imbalance on target buses. This methodology confers a high degree of robustness to disturbances and model inaccuracies. The efficacy of the methodology is evaluated through the simulation of two case studies. The results demonstrate the suitability of the proposed strategy, which leverages the fast responses of the power converters. • A two-layer hierarchical control steers unbalanced microgrids to optimal operation. • A PLL-free primary layer ensures power sharing and frequency support. • Secondary two-stage layer ensures zero frequency deviation and low imbalance in voltages. • The controller optimises power and voltage without MG models or predictions. • Test cases validate coordination of grid-forming IBRs in 100% RES-based microgrids.