Transformer Primary Injection Testing Calculation

[Vernon]

Thanks for the input. Here are some answers

Original Question: A primary injection is to be performed on a transformer 138kV/25kV, 2500kva, 8%Z using a 480V generator connected to the high side with the low side grounded. The expected current in the high side is?


7. You did not specify the xfmr connection but assuming it is Delta/Wye my calculation is 3007.12 A primary



Didn't read all the answers but this one jumped out and just slapped me as very high. I think that if no three phase connection is given one assumes a single phase transformer. If I tell you I am standing here with a 480V to 120V transformer in my hand you would assume one transformer with a turns ratio of 4 to 1 and a primary voltage of 480 volts.

Ok - so the full load current at 138,000V and 2,500,000 VA is 18.116 A. At 8%Z with a shorted secondary you would multiply that by 12.5 so at rated voltage you have 226.45 amps Isc. But you are only applying 480V so you would get, proportionately, 0.787 AMPS. Generally, if I specify that a transformer is (for example) 10%Z that means that with the secondary shorted it will take 10% of the rated primary voltage to get full load current. www.cttestset.com

[Inglefles897]

Ok - so the full load current at 138,000V and 2,500,000 VA is 18.116 A. At 8%Z with a shorted secondary you would multiply that by 12.5 so at rated voltage you have 226.45 amps Isc. But you are only applying 480V so you would get, proportionately, 0.787 AMPS. Generally, if I specify that a transformer is (for example) 10%Z that means that with the secondary shorted it will take 10% of the rated primary voltage to get full load current. www.cttestset.com

Hi Vernon, can you explain why we multiply by 12.5 and why we don't use three phase voltage in the calculation? Thank you.

[roberts311]

Didn't read all the answers but this one jumped out and just slapped me as very high. I think that if no three phase connection is given one assumes a single phase transformer. If I tell you I am standing here with a 480V to 120V transformer in my hand you would assume one transformer with a turns ratio of 4 to 1 and a primary voltage of 480 volts.

Ok - so the full load current at 138,000V and 2,500,000 VA is 18.116 A. At 8%Z with a shorted secondary you would multiply that by 12.5 so at rated voltage you have 226.45 amps Isc. But you are only applying 480V so you would get, proportionately, 0.787 AMPS. Generally, if I specify that a transformer is (for example) 10%Z that means that with the secondary shorted it will take 10% of the rated primary voltage to get full load current. www.cttestset.com

I can follow all the math except where the 12.5 number came from. Can anyone fill me in on what I missed?

[roberts311]

Didn't read all the answers but this one jumped out and just slapped me as very high. I think that if no three phase connection is given one assumes a single phase transformer. If I tell you I am standing here with a 480V to 120V transformer in my hand you would assume one transformer with a turns ratio of 4 to 1 and a primary voltage of 480 volts.

Ok - so the full load current at 138,000V and 2,500,000 VA is 18.116 A. At 8%Z with a shorted secondary you would multiply that by 12.5 so at rated voltage you have 226.45 amps Isc. But you are only applying 480V so you would get, proportionately, 0.787 AMPS. Generally, if I specify that a transformer is (for example) 10%Z that means that with the secondary shorted it will take 10% of the rated primary voltage to get full load current. www.cttestset.com

I see it now...

(kVA x 100)/(kV x %Z)


Therefore,

(2,500,000VA x 100)/(138,000V x 8) = 226.449A


But at 480VAC vice 138,000VAC so,

138,000/480 = 287.5


So dividing the short-circuit primary amps by the voltage ratio difference:

226.449A/287.5 = 0.787A

[young_dad]

7. A primary injection is to be performed on a transformer 138kV/25kV, 2500kva, 8%Z using a 480V generator connected to the high side with the low side grounded. The expected current in the high side is?

Assuming that the answer from the practice tests is correct, I believe I have finally satisfied myself with a reasonable solution to this problem.

- Assume the xfmr is single-phase since no other information is given.
- Using SC calculation, we find that the secondary fault current in a SC would be 1,250A.
- Knowing the secondary voltage and current, we can solve for the reactance in the secondary coil, giving 20 ohms @ 60Hz (another assumption).
- Changing gears; we now have a known secondary impedance since the secondary is grounded (20 ohms).
- With the fixed turns ratio of 5.52, we find that applying 480 to the primary gives us 86.9V on the secondary.
- Apply Ohm's Law to find secondary current over our 20 ohm impedance (4.35A).
- Again, use the TR of 5.52 to find what level of primary current will cause 4.35A in the secondary.

With all these assumptions, I come up with 0.787A of primary current.

This reference pointed me in the right direction. Until now, all the solutions I have seen on here were as clear as mud. https://www.eaton.com/ecm/groups/public/@pub/@electrical/documents/content/wp009001en.pdf

[young_dad]

I can follow all the math except where the 12.5 number came from. Can anyone fill me in on what I missed?

It just popped into my head the other night: 100% ÷ 8% = 12.5. Vernon multiplies by 12.5 rather than dividing by 0.08.

[mmacdonell]

A primary injection is to be performed on a D-Y transformer rated 138kV/25kV, 2500kva, 8%Z using a 480V generator connected to the high side with the low side grounded. What is the expected current on the high side?

0.262A
0.455A
0.787A
1.36A

[alexeic]

A primary injection is to be performed on a D-Y transformer rated 138kV/25kV, 2500kva, 8%Z using a 480V generator connected to the high side with the low side grounded. What is the expected current on the high side?

0.262A
0.455A
0.787A
1.36A

Answer: 0.455A

[carl21]

i understand the math, but have never seen or heard of this done. hopefully this is just an old method to something we currently do in its place for testing.

[jrm5116]

I got 1.364A on the high side.

So using a power base of 2500kVA and a voltage base of 138kv, the actual impedance seen by the high side would be

(.08)*(138kV^2)/(2500kVA) = 609.41 Ohms seen by the high side.
percent imp*(Base Voltage Squared)/(Base KVA Squared)

the equivalent circuit would be a 480V source to a three 609.41 impedances connected in a Delta.

the single phase equivalent using ohms law would be 480V = (Line Current/Square Root of 3)* 609.41 Ohm

Or

I = 480/609.41*1.732 = 1.364A

If I use a Y connection. I would get the .45476A.
If I solve for phase current instead of line current, I'd get .7876A

Did anyone else get this answer?

[jrm5116]

i understand the math, but have never seen or heard of this done. hopefully this is just an old method to something we currently do in its place for testing.

Not quite the same, but at HV to MV substations prior to initial energization, they'll short out the high side of a transformer and put a generator on the low side at a lower voltage to verify all instrumentation through out the new system. You can search Through Fault Testing to get more information on it.

[Kamal_]

Answer: 0.455A

I agree, Answer 0.454A

[EricGoetz92]

There is a question on the practice test that reads:

A primary injection is to be performed on a transformer rated 138kV/25kV, 2500kVA, 8%Z using a 480V generator connected to the high side with the low side grounded. What is the expected current on the high side?

The "correct" answer is 0.787 A and the hint is "(kVA x 100)/(kV x %Z)". The hint is incorrect because it doesn't include the square root of three in the denominator. The hint should be "(kVA x 100)/(sqrt(3) x kV x %Z)". And this hint gives you the maximum short circuit current for that winding at the rated voltage, so you then need to take this number and multiply it by the applied voltage/rated voltage. So in this problem it would be:

I short circuit @ V rated = (2500 x 100)/(sqrt(3) x 138 x 8) = 130.7406 A
I short circuit @ 480V = I short circuit @ V rated x (480/V rated) = 130.7406 x (480/138000) = 0.45475 A

Ultimately, the "correct" answer on the quiz is incorrect and needs to be divided by sqrt(3).