Calculates the short circuit fault current of a 3-phase AC generator.
The generator short circuit fault current calculator uses a simplified method to calculate the fault current from the following parameters:
- Rated (Ur). The rated phase-to-phase voltage of the generator in V.
- Rating (Sr). The rating power of the generator in kVA.
- Impedance (Zk). The short circuit impedance of the generator as a percentage. The short circuit impedance can be specified for the sub-transient, transient or steady state phase of the generator fault. For simplification the resistance can be ignored and only the reactance can be considered. Typical reactance values are shown below.
| Phase | Equation | Typical reactance (%) | Typical time |
| Sub-transient |  |
10 – 20% | 10ms |
| Transient |  |
15 – 25% | 250ms |
| Steady state |  |
200 – 350% | > 250ms |
The generator short circuit current is calculated as follows:
\(I_{fault-actual}= \dfrac{S_{r} \times 100}{\sqrt{3} \times U_{r} \times Z_{\%}}\)Where,
- \(I_{fault-actual}\) is the fault current in kA.
- \(S_{r}\) is the generator power rating in kVA.
- \(U_{r}\) is the generator rated voltage in V.
- \(Z_{\%}\) is the generator impedance in base percentage.
This is basically a combination of the steps in the per-unit calculation method:
Step 1: Calculate the per unit fault current:
\(I_{fault-pu}= \dfrac{1}{Z_{pu}}\)Note that \(Z_{pu} = \dfrac {Z_{\%} }{100}\)
Step 2: Calculate the nominal base current:
\(I_{nominal-base}= \dfrac{S_r}{\sqrt{3} \times U_r}\)
Step 3: Calculate the actual fault current:
\(I_{fault-actual}= I_{fault-pu} \times I_{nominal-base}\)