High penetration of Distributed Generation (DG) in Medium Voltage (MV) power grids usually leads to abrupt voltage raises in the presence of either low demand conditions or high power production from renewable sources. In order to cope with the possibly resulting voltage limits violation, a typical adopted approach is the disconnection of the distributed generators or the curtailment of the generated power leading to several disadvantages. To address this issue a Command Governor (CG) approach is proposed for the online management of the reactive power of distributed generators. The method is aimed at solving a constrained voltage control problem in both centralized and distributed ways. The approach envisages an active coordination between some controllable devices of the grid, e.g. distributed generators, MV/HV transformers, in order to maintain relevant system variables within prescribed operative constraints in response to unexpected adverse conditions. In the final simulation example, the proposed approach presents an increased level of effectiveness with respect to heuristic methods imposed by the current Italian norms on reactive power compensation.