# Generator short circuit fault current calculator

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

## 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:

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