Performance of multi-agent secondary control for reconnecting a microgrid of grid-forming converters to the main grid

Electrical microgrids (MGs) are meant to make access to electricity more flexible. In this regard, they are required to work, alternatively, either connected to or disconnected from (forming an island) the main grid. Therefore, seamless re-synchronisation is a relevant feature to be built in MG controllers. A centralised secondary MG controller is the obvious choice for re-synchronisation, but recently, distributed multi-agent secondary controllers have been proposed in the literature because they would simplify the communication system required. However, several aspects of this functionality need to be investigated further. In this paper, the implementation of a multi-agent secondary control that re-synchronises an island MG with the main grid is presented and analysed. The importance of communication delays for the overall MG stability, together with the instantaneous power flows in the MG during re-synchronisation, are the focus of this paper. Results show that the influence of the communication delays and the power flow control during transients can be tackled by an appropriate design of the control parameters. The results of this work were validated in a real MG comprising four 15 kVA converters, a 75 kVA grid emulator, and an industrial communication infrastructure.