In the last few years, microgrids have become a potential solution to improve reliability and security in the supply of electricity to power systems. When a microgrid changes its operation mode from grid-connected to islanded, particular attention must be paid by the operation services. In the islanded mode, the microgrids' frequency and voltage may reach undesirable values, harming the security and quality of the operation. This paper aims to propose global and local strategies of load shedding to preserve the energy supply to high priority loads within quality standards. The focus of this paper is to consider a microgrid operating only with primary control. In this sense, a proper microgrid time-continuous load flow is applied, considering the constant verification of the frequency and the voltage of all buses. Finally, a Monte Carlo Simulation is used to validate the proposal presented and to give some indices that quantify the load shed in each period. The results show the superior performance of the proposed strategy compared to a state-of-the-art load shedding solution that does not consider the priority of loads.

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Electric power systems have undergone substantial changes in their operation. The higher penetration of renewable resources, demand response capability, and generators operating via droop control at the distribution level are the main features resulting in the microgrid concept. Microgrids must operate connected or islanded from the main grid, ensuring reliability and quality in the supply of energy in both operating scenarios. In this sense, the secondary control becomes essential in the system's resilience, since it is responsible for restoring the frequency and voltage within acceptable values. This study proposes a unified frequency and voltage secondary controls for microgrids operating in islanded mode. For this sake, a modification in the load flow algorithm considering a Jacobian matrix takes place, enabling a sensitivity analysis to give the adjustments in the set point of generators. The help of the Levenberg-Marquardt method improves the convergence in the modified load flow. All generators are continuously considered in this process, regarding their capabilities and relative control sensitivities concerning the operation point restoration. The proposed methodology is validated in a modified IEEE-37 node test feeder, showing the efficacy of the centralised secondary control under different scenarios of renewable generation penetration and load levels.

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