Short-circuit analysis of the onsite electric power system at Ringhals unit 4

Abstract

Because of an upcoming modernization of the electric system in unit 3 and 4 at Ringhals AB, new short-circuit current calculations have to be made. This report focuses on the development of a model of the electric power system in a nuclear power plant in order to perform short-circuit current calculations. In this model, different components can be exchanged in order to investigate future adjustments in the system. The IEC 60909-0 standard is used in order to calculate various types of short-circuit currents for the power system. The usage of the standard IEC 60909-0 is in some cases quite conservative, which is positive when calculating currents for a system of this type. For the standard IEC 60909-0, some correction factors are introduced. The factor KT is used for correction of the impedance of the transformers and KG is used for the correction of the generator impedance. For the voltage, two factors are introduced to increase/decrease the voltage at the fault location. The power system is divided into several different voltage levels, where calculations are made for short circuits on the 6,6 kV and 500 V levels. Nine different cases are set up in order to evaluate the system, so that the maximum and minimum currents can be determined. The different currents examined appear when a balanced three-phase fault occurs. The calculated currents are the maximum initial short-circuit current, the peak short-circuit current, and the maximum and minimum steady-state short-circuit currents. Among the cases that have been examined, the real case that gives the highest maximum initial current is case 3 for all buses. In the same case, the highest peak current and maximum steady-state current appear. The lowest minimum steady-state current is found in case 8 for the buses H4x-y6 and VHC4x, and in case 9 for the buses D4x-y6 and DHC4x. In the performed sensitivity analysis, the new calculations have been compared to previous results. The deviations are small for all cases, which means that the new calculations have been performed with great satisfaction.