Doble Testing a Bushing: C2 reading higher than nameplate

[MuniGuy]

Does anyone have an explanation for high C2 readings when Doble testing transformer bushings?

The test was on a three winding transformer. All C2 values were consistent but approximately 40 pf higher than the nameplate value

Are you shorting the bushings together of each winding? This is necessary. Readings can be higher if they are not.
I have seen C2 readings increase bc the xfmr itself was overheating. This caused the C2 to to increase on all 3 primary bushings. Remember,a change in capacitance indicates a physical change.

[rofo42]

Does anyone have an explanation for high C2 readings when Doble testing transformer bushings?

The test was on a three winding transformer. All C2 values were consistent but approximately 40 pf higher than the nameplate value

Since responding to this thread initially, I have tested 100+ bushings; both installed and not installed.

I can now confirm that what you are seeing is simply the addition of the transformer tank and oil being introduced into the equation.

It is completely normal and to be expected. Also, Doble explicitly states to not use rubber insulating mats to influence readings. Doing so only proves that you have an issue that you are masking; whether that issue is actual or as in this case, by design.

The PF should not deviate too much from nameplate values for C2, installed or not. The capacitance is what should be expected to change. Any amount of moisture inside the C2 will greatly affect the readings.

[drobinson]

Hello Bob, you are not alone in experiencing the inconsistent readings for C2 across various bushings. I attended a presentation from ABB at a conference in New Brunswick Canada in 2019 - specifically on C2 testing, construction and value of the C2 test. In the presentation, David M Geibbel - Technical Director ABB Alamo TN presented evidence and statistical data showing that C2 was not a reliable indicator of health of a bushing - in other words, would one "condemn/change" a bushing based on the C2 result. David clearly indicated the answer to be NO. As well, when we asked ABB Bushing Headquarters in Sweden for a statement on C2 testing, they formally wrote back to state that C2 is not considered in failing or passing a bushing.
C2 testing is subject to many influences which are not seen in the C1 testing. Comparison to nameplate PF is contentious leaving only capacitance as a comparison. Trending, although useful in C1 testing is both difficult to keep up (as my experience, most users do not maintain the needed trending history)
In my opinion, C1 testing is your best determinent for health, and one MUST compare to nameplate PF and temperature correction is mandatory if at any extreme temperatures above 25C or below 15C.
There are new methods of testing bushings where the bushings are tested at both 60Hz PF and at lower frequency - down to 1 Hz PF with clear evidence of detecting issues very early in the degradation stages of the bushing - and one can compare the other phase results to determine if one specific bushing is changing.
Hope this is useful to you.

Do you have any links or any documentation? I am getting dragged over the coals for not failing a transformer on bad C2. It is 13.8kV and the bushings had scaffolding surrounding them during the test. any links you could offer would be greatly appreciated! Thanks!