Calculation of short-circuit conditions

Performing a calculation

In accordance with the applicable standard, the task of calculating the stable fault run of a network defines fault voltages and currents in the network where one of the four types of line short-circuit – three-phase, two-phase, ground two-phase, and ground one-phase is occurring. The selectable parameters of the calculation include the short-circuit distance from the initial line node and the short-circuit duration. The calculation determines short-circuit current characteristics and consequent or zero sequence impedances at the point of fault. For non-symmetrical faults, fault voltages and currents are determined in the network of an equivalent symmetrical fault, electrically distant. Synchronous or asynchronous machines working in the analysed system are considered as short-circuit current sources.

Practical benefits of the calculation

• checking that the short-circuit resistance of existing LV, HV and VHV switchgears is sufficient when designing new lines/cables in the local distribution system or a plant’s internal consumption. Protection of devices is set according to the sizes of minimum short-circuit currents, devices such as switchgears, switchboards and lines are dimensioned according to maximum currents. In the case of a short-circuit fault, these must withstand short-circuit over-currents until the respective protections are tripped
• a report on the size of short-circuit outputs of the local distribution system for the operator of a regional distribution system to which the calculated local distribution system is connected, or for the needs of checking the safety of a particular system by the state authorities