On occurrence of a short circuit at any point in a system, the short circuit current is limited by the impedance of the system up to the point of fault. Thus referring to Fig. 4.8, if a fault occurs on the feeder at point F, then the short circuit current from the generating station will have a value limited by the impedance of the generator and transformer and the impedance of the line between the generating station and the point of fault.
Also, the short circuit current depends on the size of the power system which supplies the short circuit current, larger the supply source the greater will be the short circuit current.
The reasons of it are as follows:
ADVERTISEMENTS:
(i) Large kVA of the generating plant.
(ii) Smaller impedance of the generating plant, larger the size of the generating plant the smaller will be the impedance.
(iii) Smaller impedance of the line between the generating station and the point of fault—larger the current carrying capacity of the line, the lower will be the impedance.
In order to illustrate the above, let us consider a 3-phase, 400 V, 75 kW induction motor connected to infinite bus-bar (i) through 100 kVA transformer (ii) through 4,000 kVA transformer as shown in Figs. 4.9 (a) and 4.9 (b) respectively.
Full-load current of motor,
Assuming pf to be 0.8 lagging.
When the fault occurs, say at point F, the short circuit current is limited by the impedance of the transformer only.
ADVERTISEMENTS:
The short circuit current, which can be supplied by the source in each case, is worked out as follows:
In first case, i.e., with 100 kVA transformer,
In second case, i.e., with 4,000 kVA transformer,
Thus, it is observed that as the system grows, the capability of supply of short circuit will be increased along with it.