Transformer Overall tests

[buddy91082]

Not familar with test results for transformer capacitance and power factor % and what values are in acceptable ranges for dry type transormers. Does NETA have acceptable ranges?
Any good artciles to read up on?

Thanks.
b

 

[DTR2011]

From NETA MTS-2011 (Large Dry Type) - optional test.

"3. CH and CL power-factor or dissipation-factor values will vary due to support insulators and bus work utilized on dry transformers. The following should be expected on CHL power factors:
Power transformers: 2.0 percent or less Distribution transformers: 5.0 percent or less
Consult transformer manufacturer’s or test equipment manufacturer’s data for additional information.
4. Power-factor or dissipation-factor tip-up exceeding 1.0 percent should be investigated."

From my experience, there are a few other things you can run into. Negative Power Factor Readings - this can come up in dry types. Consult the test set OEM for advice.

The other thing you need to be careful about is when you are tying the HV connections together. On an Oil filled, it is easier as the bushings are brought out. On a dry type, you have to be careful to watch your clearances from ground (and other things such as temperature probes / leads). Safety can not be overstated here!

I have also found on lower voltage units (4160V Pri), these can be high capacitance units, which may trip your test set. Settle on a lower test voltage and document as such.

Additionally, the type of insulating material (and its age), will have an effect on your results. If you get very high readings on a new unit that is one thing. On a very old unit, best to check if there are historical tests on the unit in question