Calculating transformer fault currents HI/LO side
Calculating transformer fault currents HI/LO side
(OP)
I am trying to learn some more about fault and fault currents. If a utlilty has a known fault current on the high side of a power transformer (161kv) of x amount of amps three phase. With the nameplate information given (kva,voltage,impedance) what is the forumla to determine the fault current on the low side bushings (13kv)?? I have looked for examples and all I find are ones that pertain to the utility as being bolted. All the new arc flash rules have got me wanting to learn a little more about these procedures. I understand it may not be as easy as a b c, but I would appreciate any direction someone could give for me to find the proper formulas and procedures for the calculations. I have an example in mind of a 20 MVA, 8.2%, 161:13kV transformer.






RE: Calculating transformer fault currents HI/LO side
If you take the utility fault current and divide it by the line-to-ground voltage, that gives you the utility impedance.
The trick is the fact that the transformer also transforms impedances in addition to voltages and currents.
Power engineers use the per-unit system for this stuff. You should be able to Google up some good tutorials on per-unit.
RE: Calculating transformer fault currents HI/LO side
DPC, Am I missing something?
V=IZ not I/V = Z
I really need to brush up on this stuff but is that correct?
RE: Calculating transformer fault currents HI/LO side
First you need to calculate the available FLA on the secondary. This is going to be, in your case based on the info given:
20MVA/.0138MV = 1449A
Then take 1449A/.082 %Z
And you get:
17674A
( I assumed the 13kV you had listed was 13.8kV on the secondary side)
That should do it
RE: Calculating transformer fault currents HI/LO side
1) Source (utility) p.u. impedance = MVA base/SC MVA source = 20/2500 = 0.008.
2) Transformer p.u. impedance = 8.2/100 = 0.082
3) Total Zp.u. = 0.008 + 0.082 = 0.090
4) Transformer Secondary fault current = I(secondary)/ total Zp.u.
5) Transformer secondary current = 20 MVA x 1000/(1.73 x13 kV) = 889.28 Amps.
6) Transformer fault current at 13 kV side = 889.28/0.090 = 9,880 Amps.
Hope this will help.