transformer low side fault current calculation 3

[alone021]

Hello everyone,

I read a thread here before, which said utility fault level was given, and utility connects to the high side of a transformer, nameplate information is 20 MVA, 8.2%, 161:13kV, asked how to calculate the transformer low side fault current...below was the solution in that thread

To solve this problem you have to know the utility (source) 3-phase fault current or MVA. Assume that the source 3-phase fault is 2500 MVA. Then use the transformer MVA as the MVA base.
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.

my question is what the type of this transformer fault current at low side? is it a LG, or LL, or LLG or LLL or LLLG fault? or this 9880 amp is just the highest one, doesn't matter which type of the fault?

Thank you

 

[xnuke]

LLL.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[RRaghunath]

9880A thus calculated is three phase symmetrical fault current at the secondary terminals of the transformer.

 

[alone021]

Thank you.

so it is a LLL fault current, is it because the source 3-phase fault level at 2500MVA is given?

and if the source LG or LLG is given, we could use the same method to get the transformer secondary LG or LLG fault current, right?

 

[jghrist]

For the case of a delta-wye transformer, the high side LG or LLG fault doesn't matter because there is no zero-sequence path through the transformer. LG fault = 3*Isecondary / [2*Zpu(source)+3*Zpu(transformer)]. This assumes all impedances are pure reactance and that the transformer zero-sequence impedance is the same as the positive-sequence impedance.

 

[cranky108]

If you want zero-sequence fault impedance, that needs to be tested. Good luck, if it wasn't done tested when it was new.