•A new scenario-based two-stage wind farms planning model is proposed.•The annual installation capacities of wind turbines are optimally planned.•The Levelized Cost of Energy (LCOE) of wind farms is minimized.•Load and wind uncertainties are modeled via scenario based approach.•A desired loading margin is satisfied in the entire planning horizon. Recently, penetration of intermittent power sources has been increased in power systems due to an international drive for clean and sustainable energies; but these alternative sources could encounter power systems with some problems, which need planning and prevention. This paper proposes a two-stage scenario-based planning model for large-scale wind farms development, based on a project management approach. Considering a 10-year project of large wind farms development, the annual installation capacities of wind turbines are first optimally planned in order to minimize the Levelized Cost of Energy (LCOE) of wind farms. Second, as wind power penetration is consistently increasing in the grid, some stability concerns will come up such as voltage instability. To remedy this condition, the optimum dispatch of grid’s conventional power sources and control variables is determined in such a way that not only in any state of operation (taking into account wind and load uncertainties) but also in post-contingency conditions, the prescribed security margin is ensured at the lowest possible cost. This study has been conducted on an actual power system of Iran’s southeast grid, as well as IEEE 118-bus standard test system. Also, modified crow search algorithm (MCSA) is utilized to solve the developed optimization model. The numerical studies substantiate the effectiveness of the proposed method for long-term planning of wind farms.