Transformer Oil Pressure when Power Factor Testing

[ElectricalTestTech]

Does the pressure in a oil filled transformer matter when power factoring the transformer?

For some reason, I recall a topic at a NETA conference or Doble conference that mentioned something along the lines of "you should power factor under positive pressure and not a vacuum"...

This statement could be totally false and I cannot find any literature to back the statement up. Has anyone else heard anything along these lines?

[mask12900]

Any one knows how to perform Tan Delta Test on Dry Type Power Transformer of 2000 Kva and how to connect instrument with Transformer bushings please help me out .

Regards

[wooddy]

Does the pressure in a oil filled transformer matter when power factoring the transformer?

For some reason, I recall a topic at a NETA conference or Doble conference that mentioned something along the lines of "you should power factor under positive pressure and not a vacuum"...

This statement could be totally false and I cannot find any literature to back the statement up. Has anyone else heard anything along these lines?

cannot do hv power factor test(doble) under vacuum as you run a risk of flashover, pressure should not make a difference other than the moisture content of the dry air or nitrogen or sf6 in a breaker

[Kerry4407]

Does the pressure in a oil filled transformer matter when power factoring the transformer?

For some reason, I recall a topic at a NETA conference or Doble conference that mentioned something along the lines of "you should power factor under positive pressure and not a vacuum"...

This statement could be totally false and I cannot find any literature to back the statement up. Has anyone else heard anything along these lines?

I'm not aware of anything regarding the pressure of the transformer that would affect power factor results. Power factor is measuring the capacitance of the insulation and has little to do with the internal conditions beyond the bushings.

[Chipperk]

I'm not aware of anything regarding the pressure of the transformer that would affect power factor results. Power factor is measuring the capacitance of the insulation and has little to do with the internal conditions beyond the bushings.

Doble always recommends positive nitrogen pressure before powerfactoring xfmrs.

[Kalbi_Rob]

Doble always recommends positive nitrogen pressure before powerfactoring xfmrs.
As quoted from Megger "Application guide for power factor testing of power & distribution transformers" and attached to this post for reference:
"Never perform electrical tests of any kind on a unit under vacuum. Flashovers can occur at voltages as low as 250 volts."

As quoted from "Transformers: Basics, Maintenance, and Diagnostics"
"Transformers must not be sampled if there is a negative pressure(vacuum) at the sample valve. If there is a vacuum at the bottom, air will be pulled in when the sample valve is opened. Wait until the pressure gauge reads positive before sampling. Pulling in a volume of air could be disastrous if the transformer is energized."

258

[rofo42]

As quoted from Megger "Application guide for power factor testing of power & distribution transformers" and attached to this post for reference:
"Never perform electrical tests of any kind on a unit under vacuum. Flashovers can occur at voltages as low as 250 volts."

As quoted from "Transformers: Basics, Maintenance, and Diagnostics"
"Transformers must not be sampled if there is a negative pressure(vacuum) at the sample valve. If there is a vacuum at the bottom, air will be pulled in when the sample valve is opened. Wait until the pressure gauge reads positive before sampling. Pulling in a volume of air could be disastrous if the transformer is energized."

258
Your quoted piece from Transformers: Basics, Maintenance, and Diagnostics sounds like a quote from an oil sampling procedure, not a Doble testing procedure.

I also do not see it in the attachment.

[Kalbi_Rob]

Your quoted piece from Transformers: Basics, Maintenance, and Diagnostics sounds like a quote from an oil sampling procedure, not a Doble testing procedure.

I also do not see it in the attachment.

First, Doble does not own the sole rights for performing Power Factor testing on transformers, and actually have one of the worst test sets on the market due to being outdated from good competition (Megger and Omnicron are better from other techs I've talked to, but I have never had the opportunity to verify). They are the pioneers, and were the best on the market until they decided to start focus on performing the testing themselves.

Secondly, you are correct the second half of my quote is from oil sampling procedure that I did not attach, but the first part
"Never perform electrical tests of any kind on a unit under vacuum. Flashovers can occur at voltages as
low as 250 volts."
was taken directly from the
"Two-Winding Transformers Test Connections
For all transformer testing, including spare transformers, ensure the following safety conditions are observed:"
section, fourth bullet down.
I do not attach documents unless I can easily find them online due to previous legal issues with IEEE standards.

That being said, I included it because it is one of 2 reasons you do not test a transformer under vacuum. The other being the other quote. Pulling oil samples is also a "Doble" testing procedure by the way.

[backpack96]

Does the pressure in a oil filled transformer matter when power factoring the transformer?

For some reason, I recall a topic at a NETA conference or Doble conference that mentioned something along the lines of "you should power factor under positive pressure and not a vacuum"...

This statement could be totally false and I cannot find any literature to back the statement up. Has anyone else heard anything along these lines?

Testing under a vacuum is often misunderstood by the tester. If the transformer has oil that is at the normal level a vacuum will not affect the tests. Sealed transformers often go from a positive pressure to a negative pressure when there is a change in oil temperature due to being unloaded or a cold front moving thru.

It is recommend not to test under a vacuum as the dielectric constant of a vacuum is 1.0 while mineral oil has a dielectric constant of 2.2. A high vacuum, such as a vacuum bottle, has a good dielectric but you cannot achieve this type of vacuum on a transformer. When testing a transformer without fluid one must greatly reduce the test voltage and make sure that explosive gasses are not present.

Normally working on a transformer that is being placed under a vacuum is not recommended for safety as the tank is under stress,