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Distributed generation, which is installed to exploit renewable energy sources, can also be used as a reactive power resource and contribute to tackle the voltage regulation problem in smart distribution grids. Adopting a decentralized approach with off-line coordination, the paper proposes an optimal set-point design for the voltage/reactive power control scheme of distributed generation. The objective is to improve the voltage profile along the feeders of a distribution system. Using only local measurements, the actual operating conditions of the feeder are firstly estimated; then, the set-point is evaluated by solving an optimal voltage profile problem. The off-line coordination avoids any modification to the architecture of existing control systems and requires communication only in the case of significant changes of the distribution system topology. The results of numerical simulations are presented for MV feeders with wind and photovoltaic generations. A comparison with standard control schemes is reported, as well as considerations about the off-line coordination among the distributed generations and the tap-changer of the HV/MV transformer.