Unbalanced Transformer Fault Currents
Unbalanced Transformer Fault Currents
(OP)
We had a fault on a feeder off the the secondary of a Delta-Wye transformer that caused the primary overcurrent relays to operate. The relays that operated on the primary are electromechanical relays, so they don't provide any indication as to the fault current values. The secondary relays are SEL and had event captures so we know what the secondary currents were during the fault. I'm trying to determine what the primary currents were based on the secondary fault currents. Below is the system information and fault data.
Transformer Information:
Primary: 138 kV - Delta
Secondary: 13.8 kV - Wye Resistance Grounded (400 A)
Rating: 30 MVA
Z% = 8.2%
Fault Currents on the Secondary:
Ia = 11392 @ 180.8 degrees
Ib = 10411 @ 6.4 degrees
Ic = 1438.6 @ 316.2 degrees
How do I determine what the primary currents are for each phase?
Transformer Information:
Primary: 138 kV - Delta
Secondary: 13.8 kV - Wye Resistance Grounded (400 A)
Rating: 30 MVA
Z% = 8.2%
Fault Currents on the Secondary:
Ia = 11392 @ 180.8 degrees
Ib = 10411 @ 6.4 degrees
Ic = 1438.6 @ 316.2 degrees
How do I determine what the primary currents are for each phase?






RE: Unbalanced Transformer Fault Currents
RE: Unbalanced Transformer Fault Currents
RE: Unbalanced Transformer Fault Currents
Can you take it from there?
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Unbalanced Transformer Fault Currents
RE: Unbalanced Transformer Fault Currents
RE: Unbalanced Transformer Fault Currents
IA = 1259A 183.4deg
IB = 552A 13deg
IC = 720A -3.8deg
But I've probably stuffed it somewhere :)
RE: Unbalanced Transformer Fault Currents
RE: Unbalanced Transformer Fault Currents
RE: Unbalanced Transformer Fault Currents
Thank you for the help.
RE: Unbalanced Transformer Fault Currents
RE: Unbalanced Transformer Fault Currents
RE: Unbalanced Transformer Fault Currents
If I'm stuck on a site in the middle of the night, with a pen and paper handy, I can work through this quite simple problem. A simple piece of circuit analysis shouldn't require the use of a computer program, unless we're talking the old school meaning of computer and that means me, and the little bit of grey matter I have stuck between my ears.
I'll admit to signs of laziness in that I used a maths program to do the 'complex arithmetic' for me, but to even write the simple equations I had to have a basic understanding at what I was looking at, perchance an understanding of Mr Kirchoff's current law.
Maybe it is my old and curmudgeonly ways but I'm often vexed by dumbing down that can occur in the use of programs in a case such as this. I was a technician well before I became an engineer and this would have been a second year problem in trade school.
*rant off*
RE: Unbalanced Transformer Fault Currents