No Load Loss Test Single Bushing Transformer

[Tscott8201]

All,
I'm having a weird result when trying to No load loss test a new batch of 75KVA 24.9/14.4x120/240 single phase pad mount transformers. These units are essentially a single bushing pole mount transformer in a box with with H1 being the primary connection and H2 being grounded to the case. When we perform a no load loss test (open circuit test) we are having this particular batch trip out the test equipment as if an internal short is present. Test connections are as follows.

Test equipment H1 lead to TX H1A lead
Test equipment H2 lead to TX Ground lug
Test equipment X1 lead to TX X1
Test equipment X2 Lead to TX X3

Test equipment applies a voltage to the X1 and X3 bushings and H1 and Ground are open with voltage being measured.

When these units are put in the test cage and energized via the H1 bushings, we measure 120/240 volts as expected and the units appear to operate as intended. The only instance in which we are having issues is the no load loss test. What could be causing the issue?

My running theory is that they are wound backwards (additive versus subtractive) causing the ground connected side of the primary winding to become energized instead of the H1 bushing when a secondary voltage is applied. I plan to test this theory today by swapping the test connections on the X1 and X3 bushings but I wanted put this up here to see if this theory holds water or if I am misunderstanding the transformer polarity concept.

Any help would be appreciated.

Thanks,
Tom

 

[waross]

Your test set may not be able to test such a low impedance transformer. 1.8% imp Volts may be overloading the test set.
A 75 KVA transformer at 1.8 imp. Volts may be similar to a 208 KVA transformer at 5% imp Volts under test conditions. KVA x 5/1.8 imp Volts.
And just a suggestion: If polarity is an issue, I would swap the high voltage leads to the instrument rather than the low voltage leads.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Tscott8201]

Waross,
If my theory is correct, then the case of the unit is becoming energized and shorting to the grounded track the unit sits on. Swapping the H1 and H2 leads won't affect this as the tank will still be energized and these leads in this test mode are only measuring voltage.

Interesting thought on the impedance though, I'll have to see what else we are testing and see if these are unusually low or not.

Tom

 

[waross]

You may have a connection to ground but you will not have a ground fault until the circuit is completed.
If the only connection to the HV bushing is your test set, then your test set may be grounding one lead for safety.
You may reverse the polarity of the HV potential that your test set sees by reversing either HV or LV leads, but not both.


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[stevenal]

Does X2 have a ground strap on it? I believe X2 should be floating so the two windings are kept isolated.

 

[Tscott8201]

Stevenal,
Yes, we are floating X2.

Tom

 

[thermionic1]

You could easily test your polarity theory with a turns ratio test set (TTR), or a number of similar methods.

I was Doble testing a new GSU years ago and when I was doing the exciting current test on H2 (after doing H1 and other tests successfully), H2 would trip out the test set (300mA). I hypothesized that the core was still magnetized from factory testing. I backfed the 13.8kV secondary with 480V from a strong source, deenergized and tried again. That was the trick. Maybe you can backfeed from a variac, slowly up to nominal voltage and see if that helps.