How are the pre-fault voltages for the different Short-Circuit analysis methods determinated?


Pre-fault voltage is determined by a factor called the c-factor, which is a multiplication factor that is multiplied by the nominal p.u. voltage to arrive at a pre-fault voltage. In the IEC calculation method this c-factor varies depending on the voltage level that the fault is at, and also according to which of the fault current criterions is chosen (Max or Min). To determine the maximum fault currents the pre-fault voltage is increased by a stipulated percentage and transformer impedances are adjusted by a stipulated factor. These percentages and factors can be found in the IEC60909 standard.

These c-factors need to be specified for an external grid so that fault current calculations accurately reflect the source driving the fault currents.

The ANSI short circuit calculation method allows the user to stipulate a c-factor directly. In the complete method the pre-fault voltage of the shorted busbar is determined by a preceding load flow, which the Short-Circuit calculation algorithm performs internally; thus you have a pre-fault voltage that reflects the network setup. The full method also adds the load current to the fault current by superposition which is an important consideration in weaker networks where the fault current may be low and the load current large enough to be significant with respect to the fault current.

By disabling the load flow option on the advanced options page of the Complete SC command, a c-factor can be entered manually. Refer to User's Manual for more details.