Y-Delta Fault Current Calculation, two 50's and one 75
Y-Delta Fault Current Calculation, two 50's and one 75
(OP)
One of our customers has requested fault current data for the secondary side of their transformer bank. They have a Y-Delta, 240 volt, three phase, four wire service, with two 50 kVA and one 75 kVA, 7200 to 120/240 transformers. The center tap of the 75 kVA provides the load side neutral. My question is, how to calculate the fault current at the secondary spades. I have read different practices including the worst case scenario of treating all three transformers as if they were 75 kVA.
The single phase to ground primary fault current is 4326 amps. Three phase to ground is 5710 amps.
High side: R1=0.3853, X1= 1.2038, R0=0.8923, X0=2.3132
Z= 1.65% for the 50 kVA's and 1.6% for the 75 kVA.
I have done many of these for single phase services and for grounded wye three phase services. However the split secondary of the different size transformers has me wondering how to incorporate the differences in impedances into a common secondary bus.
Any help would be appreciated. My boss has already denied my request for a subscription to the IEEE library.
The single phase to ground primary fault current is 4326 amps. Three phase to ground is 5710 amps.
High side: R1=0.3853, X1= 1.2038, R0=0.8923, X0=2.3132
Z= 1.65% for the 50 kVA's and 1.6% for the 75 kVA.
I have done many of these for single phase services and for grounded wye three phase services. However the split secondary of the different size transformers has me wondering how to incorporate the differences in impedances into a common secondary bus.
Any help would be appreciated. My boss has already denied my request for a subscription to the IEEE library.
Happiness is a way of travel, not a destination.






RE: Y-Delta Fault Current Calculation, two 50's and one 75
Dave Smith of Power Technologies Inc. wrote a program to do this some years ago. It’s a DOS-based program which I managed to hold onto and it still works.
Lighting transformer is connected between b-n on primary and B-C on secondary.
Maximum fault currents are given for the following fault conditions:
Fault Amps
A-N 11,951
B-N 13,256
C-N 11,850
A-B 14,988
B-C 17,727
C-A 15,219
A-B-C 19,849
A-B-C-N 19,887
This may feed you, but it doesn't teach you how to fish! I'll try to see if I have any write-up on the methodology involved.
Note that you don't need to know the available phase-to-ground fault current since the neutral of the transformer bank is floating.
RE: Y-Delta Fault Current Calculation, two 50's and one 75
RE: Y-Delta Fault Current Calculation, two 50's and one 75
RE: Y-Delta Fault Current Calculation, two 50's and one 75
Happiness is a way of travel, not a destination.
RE: Y-Delta Fault Current Calculation, two 50's and one 75
RE: Y-Delta Fault Current Calculation, two 50's and one 75