Transformer Primary Injection Testing Calculation

[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).