Active and Reactive Power Multi-Objective Control of Multi-Microgrid in On- and Off-Grid Modes

In this paper, a multi-microgrid (MMG) system consisting of three microgrids (MGs), each with three nano grids (NGs) and one central battery storage unit, is modeled to pursue multiple objectives in on-grid and off-grid modes by implementing control strategies in the d-q frame. When the MMG is connected to the upstream grid, all three MGs are responsible for injecting active power into the grid while supplying their internal loads. On the other hand, MG 1 can inject reactive power into the grid to mitigate voltage sags, while MG 2 can absorb reactive power from the grid to address voltage swells. Additionally, MG 3 focuses on recovering fault voltage by improving its low voltage ride-through (LVRT) capability during a single-line-to-ground (SLG) fault. In off-grid mode, the MGs pursue additional objectives to ensure that the load of MG 2, considered the critical MG, remains powered under all operating conditions. If the central battery of MG 2 becomes disconnected, the central batteries of MGs 1 and 3 will provide support, thereby improving the resilience of the DC bus. Furthermore, the AC bus in MG 2 is designed to ensure that if the inverter supplying the internal loads of any of NGs 1 to 3 encounters an outage, the inverters of NGs 1 to 3 in MGs 1 and 3 will provide support. Results obtained from nonlinear simulations in MATLAB/SIMULINK show that the proposed control strategies in on-grid mode successfully injected active powers of 15 kW, 20 kW, and 20 kW for MGs 1, 2, and 3, respectively. During a voltage sag, MG 1 injected 65 kVAr of reactive power into the grid, while MG 2 absorbed 60 kVAr of reactive power to stabilize the voltage profile during a voltage swell. In the event of an SLG fault, MG 3 recovered the voltage by injecting 90 kVAr of reactive power, with a maximum three-phase current of 200 A into the fault location. When an SLG fault coincided with voltage sags, the voltage profile improved from 0.6 per unit (pu) to over 0.9 pu due to the simultaneous operation of MGs 1 and 3. Also, in off-grid mode, the proposed control strategies successfully injected 20 kW through MG 1 and 25 kW through MG 3 to support MG 2 when its central battery went out of circuit. Additionally, when the inverter of NG 1 in MG 2 was disconnected, 13 kW of auxiliary power was transferred from NG 1 of MG 1 to NG 1 of MG 2, ensuring that the three-phase load remained powered.