This part of IEC 61660 describes a method for calculating short-circuit currents in d.c. auxiliary systems in power plants and substations. Such systems can be equipped with the following equipment, acting as short-circuit current sources:
rectifiers in three-phase a.c. bridge connection for 50 Hz;
stationary lead-acid batteries;
smoothing capacitors;
d.c. motors with independent excitation.
NOTE Rectifiers in three-phase a.c. bridge connection for 60 Hz are under consideration. The data of other equipment may be given by the manufacturer.
This standard is only concerned with rectifiers in three-phase a.c. bridge connection. It is not concerned with other types of rectifiers.
The purpose of the standard is to provide a generally applicable method of calculation which produces results of sufficient accuracy on the conservative side. Special methods, adjusted to particular circumstances, may be used if they give at least the same precision. Short-circuit currents, resistances and inductances may also be ascertained from system tests or measurements on model systems. In existing d.c. systems the necessary values can be ascertained from measurements taken at the assumed short-circuit location. The load current is not taken into consideration when calculating the short-circuit current. It is necessary to distinguish between two different values of short-circuit current:
the maximum short-circuit current which determines the rating of the electrical equipment;
the minimum short-circuit current which can be taken as the basis for fuse and protection ratings and settings.