A power-system management system and a method thereof, which generally manage congestion of a power system by relieving a power transmission line from being overloaded when the power transmission line is overloaded due to a rapid increase in load or the occurrence of credible accidents in the power system. To relieve the overload of the transmission line, the amount of power generated by a generator may be reduced, energy may be supplied from an energy storage device to the transmission line, or those two methods may be used together. There is provided a method of quantitatively calculating the amount of a generator's generated power desired to be reduced and the amount of power to be supplied from the energy storage device to the transmission line, in order to relieve the overload of the transmission line.