Re-synchronisation of a Microgrid to the Main Grid Using Multi-Agent Secondary Control

The idea of having self-powered microgrids has often been proposed to take full advantage of distributed generation resources. These microgrids can work either isolated from or connected to the main grid, which brings a great deal of flexibility to the operation of electric power systems. Disconnection and reconnection of these microgrids to the main system are expected to be frequent operations that need detailed consideration. The connection can be carried out without the same frequency, using back-to-back converters or switching off the microgrid and re-energising it from the larger grid directly, but these approaches have some obvious drawbacks, like the cost of extra back-to-back converters or the loss of service during the blackout. A synchronisation that leads to no voltage difference between both grids at the point of connection solves these issues. However, if the microgrid includes more than one unit imposing the voltage and frequency, those units must be properly communicated and coordinated during the synchronisation. This paper presents a synchronisation control for a microgrid, where energy is fed through electronic power converters, using distributed multi-agent secondary control. The control structure presented does not require an electronic power converter at the point of connection, only measurements. The analysis of some ideas to mitigate the synchronisation transients are also included. Detailed nonlinear simulation results are provided to show the control performance in a case study.