Calculation of three-phase and single-phase short circuits in 0.4 kV electric networks for checking the sensitivity of protective devices

When designing and operating electrical networks with a voltage of 0.4 kV, it is necessary to calculate the parameters of their operating modes. One of the operating modes of 0.4 kV electrical networks is an emergency short circuit mode, the most important design parameters of which are three-phase and single-phase short circuit currents. Without calculating the short-circuit currents, it is impossible to make the correct choice of protective devices for protecting transformers installed at transformer substations with a voltage of 20 ... 6 / 0.4 kV, as well as wires and cables outgoing from them overhead and cable lines with a voltage of 0.4 kV. The article shows a methodology for calculating single-phase and three-phase short-circuit currents in 0.4 kV electrical networks and provides examples of calculations for 0.4 kV lines made with A-25 wire, powered by TM-160/10 / 0.4 transformers. The choice of an example of calculations was carried out based on an analysis of the structure and composition of electrical networks in one of the districts of the Oryol region, which showed that the networks include lines made with A-25 wire. For these lines, the problem of ensuring the sensitivity of protective devices to single-phase short-circuit currents is especially urgent.