Due to the growing importance of wind farms in the electric power generation matrix in many countries, for example, in Brazil, this paper proposes voltage preventive control actions as an activity to support the programming of the operation of the electric system, based on the ranking of wind power units that most significantly impact the system's loading margin through the control of its power factor. A sensitivity index is proposed to obtain this ranking, whose mathematical formulation is based on a linear approximation of the power balance equations in the vicinity of the maximum loading point. The system used to test this proposal is a 56-bus system prepared with current real data from the Northeast subsystem of the National Interconnected System of Brazil that includes 22 wind farms with 234 wind turbines, which comprise a total of 600 MW of installed capacity. The results obtained for the 56-bus test system show the applicability of the proposed voltage preventive control actions by indicating the wind farms that can contribute most significantly to the increase in the system's loading margin from an adequate adjustment of the power factor of these units.