Within the context of ancillary services provided to active distribution networks by distributed storage systems (DSSs), the paper focuses on the problem of optimally locating DSSs in order to maximize their support to the network voltage control. In particular, the paper proposes an optimal planning procedure that accounts specifically for the minimization of the network voltage deviations based on the formulation of a mixedinteger linear programing problem. In view of the large size of the problem, its solution relies on the so-called benders decomposition technic. The paper discusses the performance and the sensitivities of the proposed method as function of different scenarios accounting for various loads and distributed generation (DG) profiles as well as for different number of DSSs. An application example, referring to the IEEE 13 busses test feeder, is finally included to demonstrate, and discuss, the efficiency of the proposed method.