Power Transformer Secondary Resistance

[dpenbert]

I'm not sure if this is the right place for this question but here goes..
I am working with a transformer repairman on resistance issues with the secondary side of some 3phase transformers. We have had resistance readings that are greater than 5% between X12, X23, X13. After analysis if appears that the source of the problem was "loosely" torqued connections.
Here is one example:
Resistance found: X12=.003902, X23=.002968, X13=.004036
Torque found: X1=20,19 ; X2=22,21 ; X3=15,17

All LV connections (from coil to bushing) were torqued to specification (26 ft-lb). The unit then tested fine and resistances were X12=.003213, X23=.003171, X13=.003226.

So my question is.. How would the secondary resistance of a transformer be effected if torque on X1, X2, X3 were inconsistent from bushing to coil, or from phase to phase? The above example was inconsistent phase to phase and here is an example of one inconsistent from bushing to coil.

Resistance found: X12=.003402, X23=.002824, X13=.003420
Torque found X1=25,30 ; X2=27,30 ; X3=24,31

All LV bushing connections were torqued to 30 ft.lb to be consistent with the coil connections torques. The unit tested fine and resistances were X12=.003220, X23=.003167, X13=.003255. Would inconsistency in torque between the coil and bushing connections cause resistance from phase to phase to vary that much?

Thanks for any input,

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David P. - QE

 

[LarryFine]

It sounds like you've answered your own question. If all you did was re-torque, and brought everything into desired parameters or specs, your correction was what caused the new readings.

When troubleshooting a problem, and you need to locate the problem, it's important to make only one change at a time between checks, so you know which change made the difference.

 

[dpenbert]

I have found that when every connection is torqued consistently, then the resistance readings are consistent which solves the resistance problem, but I am in need of an explanation of this:

- If the bushing connections are within a close window to the spec 25,27,24 but the coil connections are outside (above) spec 30,30,31, why would this cause inconsistent resistance readings, or should it (spec is 26)??

Sometimes you can solve a problem by doing what you think is right, but to be able to explain it is another thing, I am looking for an explanation.. This could have been a resistance reading error if the above torques were fine, so I need to justify spending the time to fix the connections.

Thanks,

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David P. - QE