calculating single phase fault

[PwrCntEE]

I'm trying to reflect a fault level through a single phase transformer winding. I know I can use the %Z and the rated current to get the maximum unlimited secondary fault current. But If I know the actual level on the primary, how do I relect it thru to the secondary? I cannot just use the turns ratio cause I'm looking at current....or can I?

Data:
12,470V pri, 240/120V (center-tap), 15Kva, 1.3%Z

Available on the 3ph line (steady state)
bolt=1653A, L-L=1494A, L-N=2107A, X/R=2.1

 

[stevenal]

How is primary connected? If connected line to line, use the formula for a line to line fault with fault impedance, inserting the transformer impedance (reflected to primary) in place of the fault impedance. Use turns ratio to convert to secondary.

If connected line to neutral, do the same for a line to ground fault.

 

[PwrCntEE]

Thank you, I use the point-to-point method and many times forget about the P.U. method. The underlying question was posed by a co-worker. He is trying to model (enter) a 3ph, 12.5kV system in EDSA (software package). However, it must have balanced loading. While it will calculate 1ph and 3ph, it must do them seperately....

He is trying to model the 1ph transformer using a 3ph transformer. Then running the 3ph fault results to then convert back to 1ph results.....thus he keeps the system in one (3ph) file. So he was trying the different 3ph configurations and getting results, but did not know what was correct. As I like to calculate by hand, he asked me and I drew a blank....again thanks for the jog.

After all this, he gave up on his model.

 

[mersin33]

I don't know how (if) this can be done in EDSA (I know it is very difficult to model a single phase load or trf in a three phase system using EDSA), but I think the following should work:

-Calculate the fault on the 120V (or 240V) side of the dist transformer.

-Reflect this faulted transformer's seen impedance to primary side. For a solid fault, this would be 1.3%

-Convert this impedance value to system base.

-Then do a manual single-phase to ground fault analysis, using the impedance value you found before as fault impedance (any fault on the secondary of the single phase trf would be a single phase trf for the system)

-You can use EDSA to convert amp values of short circuits to impedance values.

Hope it helps

 

[rbulsara]

On a three phase program, the single phase 15 kVA transformer can be simulated as 45kVA(3x15), 415Y/240V -grounded secondary transformer with 1.3%Z and look up SLG fault current. (single line to ground fault).

This should come out to same value if you follow what stevenal said. (Around 4600A for 240V L-L fault or half of that for 120V L-N fault)