A distributed multi-agent supervision strategy (S-CG) based on Command Governor ideas is proposed to solve constrained voltage regulation problems in electrical Medium Voltage (MV) power grids in the presence of Distributed Generation (DG). The idea is that an active management of the set-points of some controllable variables of the grid, e.g. the power either provided by the distributed generators or consumed by the loads, can be an effective tool not only for maintaining relevant system variables within prescribed operative constraints in response to unexpected adverse conditions, but also for implementing some smart-grid functionalities. The supervision problem is formulated as a distributed constrained optimization problem by imposing that the voltages at certain nodes have, compatibly with all prescribed constraints and changed conditions, minimal deviations from their nominal values. Simulation results show that the proposed approach ensures, under certain conditions, feasible evolutions to the overall power grid under critical events.