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.
For short circuit fault simulations, fault buses in EasyPower using the 3I0 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.
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.
- IBase 1-Phase = (1 kVA) / (0.120 kV) = 8.333 A.
- ZBase 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.