This paper is aimed at presenting a methodology leading to the incorporation of large wind farms to the transmission planning of hydrothermal systems. We resorted to a simulation program for an optimum operation of a generation-transmission system based on stochastic dual dynamic programming and incorporated the wind energy features as well as undertook the simulations. This work was applied to an existing hydrothermal system; chiefly for evaluating the wind farm impact in three specific areas: the network expansion, the energy cost, and the effect on the reservoir water level.