The short circuit current produced during a fault in a system is limited by the impedance of the system up to that fault point. So the impedance of the system provides a limitation to fault current up to the fault point.

Considering the following figure where a fault occurs at the feeder denoted by F.

Limitation to Fault Current

Figure: Fault at Feeder

Here the fault current or the short circuit current generated from the generator or the generating station is limited by the impedance of the generator, transformer, and impedance of the line between the generating station up to the fault point.

Dependence of Short Circuit Current on System Size

The fault current or the short circuit current depends upon the size of the power system which will supply the fault current. The greater the system greater will be the fault level or the greater will the value of the short circuit current. As in a larger system, larger generators or larger generating stations will supply huge current to the fault point.

We can summarize the increase in short circuit current or fault level with an increase in system size in the following points.

  1. With a larger size system, the generator or generating stations will have larger kVA which will supply a huge fault current (short circuit current) to the fault point in a fault condition.
  2. Large generators will have smaller impedance. Hence the limitation to the fault current by such small impedance will result in a larger value of short circuit current.
  3. For a larger system, the current carrying capacity of the line is large. For larger current carrying capacity lines, the impedance of the line is small and hence the limitation to the fault current by such small impedance will result in a larger value of short circuit current.

Illustration of Limitation to Fault Current

Here we will illustrate the fact that for a larger system, the short circuit current or the fault level will be higher. For this, we will consider two cases and compare the short circuit current value.

Case-1:

Consider a 3 phase 400 V, 75 kW induction motor. This motor is connected to an infinite bus-bar through a 100 kVA transformer whose impedance is ZT = 0.1 Ω.

Limitation to Fault Current

Figure: Case-1

Assuming the power factor to be 0.8 lagging. The full-load current if the motor is;

Here, we consider a fault occurring at the point ‘F’. The figure above shows this fault point clearly.

For this fault, the impedance of the transformer only limits the fault current.

The short circuit current is

Case-2:

Consider a 3 phase 400 V, 75 kW induction motor which is connected to an infinite bus-bar through a 4000 kVA transformer whose impedance is given by ZT = 0.015 Ω.

Limitation to Fault Current

Figure: Case-2

Similarly, assuming the power factor to be 0.8 lagging. The full-load current if the motor is;

Here, we consider a fault occurring at the point ‘F’. The figure above shows this fault point clearly.

For this fault, the impedance of the transformer only limits the fault current.

The short circuit current is

Conclusion:

Here we considered two cases. Hence it is clear that the short circuit current for the larger systems in Case-2 has a higher short circuit current i.e. ISC2 > ISC1.


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