Generator Short Circuit Calculation

We will learn to calculate the value of short circuit current contributed by an industrial generator in case of a fault in the power network. It is important to mention that these calculations provides the base for the selection of power network design.

Whenever the fault occurs in the system, electrical components connected to the system will start to contribute towards the fault point. In our discussion, we are only looking at the contribution from the generator side.

To calculate the generator contribution, we need some parameters which can be found in the generator technical datasheet. The technical values in the datasheet varies from manufacturer to manufacturer and it is important for the system designer to have the datasheet of the generator which is going to install in the system. Most important parameters to calculate the short circuit contribution are the rated current and the sub-transient reactance of the generator as the fault current is simply rated current divided by the sub-transient reactance. If the rated current is not available than we will need parameters like Rated Power, Rated Voltage and the Rated Power Factor.

Now the calculation, in the first scenario, we have the rated current of the generator from the technical data sheet which is 806 amperes. We simply divide the rated current with sub-transient reactance of 16.3% and get the generator short circuit contribution, which is 4945 amperes.

Scenario two is where we do not have the value of rated current but rated apparent power in volts-ampere and rated voltage is available to us. First, we calculate the rated current using the common formula and then the short circuit contribution.

Scenario three is a case where we are calculating the rated current from real power in watts therefore we also need the value of rated power factor. Again, first we will calculate the rated current and then the short circuit contribution.

It is very important to note that the value of short circuit contribution from generator is largely dependent of the value of sub-transient reactance. The value of sub-transient reactance largely varies among different manufacturers. Usually it varies from 9 % to 18%, large value of sub-transient reactance is difficult to achieve and generators with high value of sub-transient reactance generates lesser value of short circuit currents. Therefore, before starting the short circuit calculations please check with your manufacturer actual value of sub-transient reactance that we will delivered.

In the last part of our discussion we will try to relate the numerical calculation we have learned with the theoretical concepts. When a fault occurs in the system and we look at the waveform we will find four important components across the current and time axes.

First is the DC component or DC offset which is the natural occurring phenomenon due to the inception of the fault at particular time interval. DC component dies out after few cycles and its value largely depends on the value of inductance and resistance ratio in the circuit. Since generators have high value of inductance therefore if the fault occurs near the generator then DC component will have high value.

Second is the symmetrical component, as name indicates the wave form is symmetrical around the time axis and is without the DC component.

Third is the asymmetrical component which is simply the sum of DC component and symmetrical component.

Fourth is the peak value of the asymmetrical component and is the value at the first half cycle. This is the most destructive force which system needs to bear. We can say that whenever short circuit occurs first system will experience the mechanical impact due to asymmetrical peak and later the system will experience the thermal effect.

For more details or to develop better understanding please visit below video!!!