Current constraints require limiting the power transmitted from wind farms to power grids. Hence, wind turbine systems should be capable of regulating and limiting the generation capacity for a wide range of operating wind speeds. In addition, wind power plants must be endowed with active power and frequency control capabilities. In a single wind farm, wind turbines can be installed at different heights or altitudes, consequently exposing them to variable environmental conditions that can cause variations among their surrounding wind speeds. Thus, we propose a de-loading control strategy that integrates over-speeding and pitch control of wind turbines operating at variable wind speeds. This strategy allows not only storing power to satisfy the constraint demand of the power grid, but also contributing to primary frequency control. In fact, the proposed control strategy can adjust the static frequency difference coefficient of wind turbines, which is based on the proportion of variable-speed wind turbines operating under high wind speeds. Moreover, the proposed control strategy, which supports frequency control of the power grid under power constraints, suitably supports the frequency regulation of wind turbines that work at different wind speeds. Simulation results suggest that the proposed control strategy meets the power grid constraints, enhances the performance of primary frequency control, alleviates the frequency control pressure from thermal power plants, and appropriately generates curtailed wind power under variable wind speeds. (C) 2017 Elsevier Ltd. All rights reserved.