# Generator short circuit fault current calculator

Calculates the short circuit fault current of a 3-phase AC generator.

## See Also

## Parameters

**Rated voltage (V)**. The rated phase-to-phase voltage of the generator in volts.**Rating (S).**The power rating 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.

## What is the impedance of a generator?

Typical impedance values for generator short circuits.

Phase | Equation | Typical reactance (%) | Typical time |
---|---|---|---|

Sub-transient | \(Z_{k}= X_{d}''\) | 10 – 20% | 10ms |

Transient | \(Z_{k}= X_{d}'\) | 15 – 25% | 250ms |

Steady state | \(Z_{k}= X_{d}\) | 200 – 350% | > 250ms |

## How to calculate the fault current for a generator?

The simplified generator short circuit current formula is:

\(I_{fault-actual}= \dfrac{S_{r} \times 100}{\sqrt{3} \times V_{r} \times Z_{\%}}\)

Where:

- \(I_{fault-actual}\) is the fault current in kA.
- \(S_{r}\) is the generator power rating in kVA.
- \(V_{r}\) is the generator rated voltage in V.
- \(Z_{\%}\) is the generator impedance in base percentage.

## How to calculate the fault current for a generator with the per unit system?

### 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:

\(I_{nominal-base}= \dfrac{S_r}{\sqrt{3} \times V_r}\)

### Step 3: Calculate the actual fault current:

\(I_{fault-actual}= I_{fault-pu} \times I_{nominal-base}\)