1. A method for determining reliability in an electric power system, comprising the cyclically performed steps of: inputting various data including data representing generated energy and load amount and connecting status of the power system; executing a first calculation process with respect to a Jacobian; executing a second calculation process for converging calculation of power flow; executing a reliability decision process for determining the reliability of the power system from the result of said first and second calculation processes; executing a first variation decision process for determining whether any variation shown in the data of the generated energy and load is less than a predetermined value and carrying out said first calculation process if an affirmative result is obtained; if a negative result is obtained in said first variation decision process, executing a second variation decision process for determining whether any change in the connecting status for the power system has occurred and carrying out said second calculation process if a negative result regarding the connecting status is obtained in said second decision process; if an affirmative result is obtained in said second variation decision process, executing a third calculating process for partially correcting of the calculation result of said first calculation process regarding the Jacobian in a preceding cycle and then carrying out said second calculation process; and executing a process for indicating a fault in the power system in response to a negative result in the reliability decision process.