Electric power system suppressing corona discharge from viewpoint of environment

The present invention provides an electric power system for stably supplying power while suppressing corona discharge generating ultraviolet radiation which may cause health problems in human beings. The corona discharge start voltage varies with the weather. Data on weather forecasts is inputted for each district at certain times so as to calculate the corona discharge estimated start voltage for each transmission line with a computer. If the calculation result predicts start of corona discharge at normal transmission voltage, the transmission voltage of the line is lowered or power transmission through the transmission line is stopped.