This article describes how to use EasyPower to calculate short circuit current for single-phase tap-off transformers attached to a 3-phase system. It allows for the determination and coordination of TCC damage curves for these single-phase transformers. This method resolves the analysis for the single-phase model only. The method described below may be used in short circuit analysis as well as coordination and arc flash studies.

## Model (in EasyPower)

This method assumes a single-phase transformer tapped off of a 3-phase system, with a 50 foot cable extended off of the secondary side of the transformer.

For the transformer:

- Multiply the transformer kVA size by 1.732.
- Keep the transformer impedance as-is.
- Keep the bus voltage as-is.

For the cable, multiply the length by 1.15.

## Solution

For short circuit fault simulations, fault buses in EasyPower using the 3I_{0} fault type. This properly accounts for the single phase to ground faults.

For the TCC damage curve of the transformer, use the 100% 3-phase curve in EasyPower, and not the unbalanced de-rating curve.

## Example

The following example demonstrates the model described above. To simplify the hand calculations, some assumptions are made. For example, resistance is neglected and the incoming 3-phase source is assumed infinite.

Hand calculations for a single-phase system modelled in EasyPower:

- Ignore the infinite source.
- Transformer 1-phase impedance is 2% on a 1.0 kVA base. Use 1.0 kVA base for calculations.
- I
_{Base} 1-Phase = (1 kVA) / (0.120 kV) = 8.333 A. - Z
_{Base} 1-Phase = ( 120 V ) / (8.333 A) = 14.4 Ω. - Transformer impedance is already on a 1 kVA base.
- To simplify hand calculations, only use reactance in cable impedance.
- Cable reactance is 1.0 Ω per 1000 feet.
- Cable reactance is then ((1.0 Ω)/(1000 feet))*(100 feet) = 0.1 Ω.
- Per-unit cable reactance is then (0.1 Ω) /(14.4 ΩBase) = 0.006944 pu.
- Total reactive impedance for fault calculations is (0.02 pu)+(0.006944 pu) = 0.026944 pu.
- Confirm that in
**Short Circuit Options**, the **Driving Point Voltage** is set to 1.0 pu. - Fault current = (1.0 pu) / (0.026944 pu) * 8.333 A = 309.278 Amps.
- Performing a 3-Phase fault on the bus single-phase load shows the same answer as in in the previous bullet.