Short Circuit Calculation 3ph transfomer with unequal size tranformers

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xguard

Senior Member
Location
Baton Rouge, LA
3 Phase 120/240 System
Utility Transfomers 2 x 50 KVA and 1 x 167 KVA (lighting pot) connected in closed delta.

None of the examples I've found in Mike Holt articles, UGLY's, or cooper bussman address this situation. Can anyone provide reference to material that shows calculating the short circuit current for three single phase trasformers connected in a delta where one of them is of different size?

(Needed for selecting circuit breaker interrupting rating.)

Thanks!
 
As information you may consider each one of transformer independent. Calculation of the mutual reactance between phase coils is very complicated and it depends on many factors as coil diameter and height and distance between them. The mutual inductance will reduce the short-circuit inductance of each transformer so neglecting the mutual inductance will be not a conservative solution.:happysad:
You have to test the assemble at reduced voltage in short circuit and increase the voltage up to rated current [minimum rated current]. :weeping:
 
Xguard...

The simplest way is to model the 3 single-ph Xfmrs as a delta-circuit. Then transform it into an equivalent-wye. This will result in a 2-loop network. The procedure is similiar to an earlier forum-thread covering the solution for a 3-phase terminal-fault produced by an open-delta arrangement of 3 single-ph Xfmrs!

Contact me with the xfmer parameters and I will send you a single-sheet EXCEL solution. But, only if you reveal your name! No strings attached! GoldDigger should be able to attest to my veracity!

Phil Corso ( Cepsicon@aol.com )
 
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