Congestion management of power flow in transmission lines using hydroelectric power generation is examined in the scenario where new wind power generation is installed in a power grid. A method is proposed for determining the optimal dispatch of generation output that maximizes transmission margin and minimizes transmission loss under the constraint that the frequency variation is kept within a tolerable frequency range. The optimal dispatch of generation output is determined by optimal power flow calculations based on particle swarm optimization. To check the validity of the proposed method, numerical simulations are carried out on EAST 30-machine 117-bus system of an Institute of Electrical Engineers of Japan. The effectiveness of the proposed method is demonstrated by the results of simulations. Basic analysis of which generators should be utilized to output how much power is performed based on these results.