schematic knowledge

Let us first illustrate the calculation of the fault current using the impedance diagram with the help of the following examples.

Consider the power system of Fig. 6.8 in which a synchronous generator supplies a synchronous motor. The motor is operating at rated voltage and rated MVA while drawing a load current at a power factor of 0.9 (lagging) when a three phase symmetrical short circuit occurs at its terminals. We shall calculate the fault current that flow from both the generator and the motor.

We shall choose a base of 50 MVA, 20 kV in the circuit of the generator. Then the motor synchronous reactance is given by

Also the base impedance in the circuit of the transmission line is

Therefore the impedance of the transmission line is

The impedance diagram for the circuit is shown in Fig. 6.9 in which the switch S indicates the fault. 

The motor draws a load current at rated voltage and rated MVA with 0.9 lagging power factor. Therefore

Then the subtransient voltages of the motor and the generator are

Hence the subtransient fault currents fed by the motor and the generator are

and the total current flowing to the fault is

Note that the base current in the circuit of the motor is

Therefore while the load current was 1603.8 A, the fault current is 7124.7 A.