DFR & ITC of Dry Type Xfmrs?

[nowwearesuckingdick]

Using a Megger Delta 4000 power factor test set and PowerDB automated form 93500 for two winding transformer to test for example a 2500 kVA dry type transformer. Is there any point of attempting a narrow band dielectric frequency response (NB DFR) test or the affiliated individual temperature correction (ITC) which also uses NB DFR for it's algorithm?

I have been through at least one AVO/Megger transformer testing class and several previously recorded webinars talking about DFR and ITC as well as read several technical articles on the subject but no one really addresses whether or not this technology adequately applies to dry type transformers as well. Even the AVO instructor was unsure on this and all the info is really only on liquid filled which seems to be Megger's main focus anyway.

Would love to hear some more experienced techs'/engineers' thoughts on the subject along with any links to supporting technical information/literature you could provide for further reading.

Thanks,
Jesse

[nowwearesuckingdick]

I have also just emailed Megger directly asking this same question so if they provide a definitive answer I will be sure to post up.

[SecondGen]

I would think its the same as PF on a dry-transformer - not all that effective but it can be done. Very interested to see what Megger says.

[anonymous]

I had never heard of DFR until I saw this post, so take this with a grain of salt. After looking at IEEE C57.161-2018, I believe DFR is strictly for oil immersed transformers. Oil transformers have two different insulating media, paper and oil, with different power factor responses. I believe the basic idea is that at low frequency the power factor is mostly due to oil, and at high frequency the power factor is mostly due to cellulose. So by testing at different frequencies and plotting the results, some fancy math can estimate the moisture content in the paper. If I'm understanding this right, then there wouldn't be much point to doing this test for a dry transformer since the power factor would be basically the same at different frequencies. In other words doing DFR testing on a dry type transformer wouldn't be any different from doing a power factor test.

[rofo42]

Using a Megger Delta 4000 power factor test set and PowerDB automated form 93500 for two winding transformer to test for example a 2500 kVA dry type transformer. Is there any point of attempting a narrow band dielectric frequency response (NB DFR) test or the affiliated individual temperature correction (ITC) which also uses NB DFR for it's algorithm?

I have been through at least one AVO/Megger transformer testing class and several previously recorded webinars talking about DFR and ITC as well as read several technical articles on the subject but no one really addresses whether or not this technology adequately applies to dry type transformers as well. Even the AVO instructor was unsure on this and all the info is really only on liquid filled which seems to be Megger's main focus anyway.

Would love to hear some more experienced techs'/engineers' thoughts on the subject along with any links to supporting technical information/literature you could provide for further reading.

Thanks,
Jesse

Megger doesn't state that you can't or shouldn't run a NB DFR test on dry type transformers. There is still some extra benefit on a dry type testing down to 1HZ, mainly from comparisons to possible sister units.

A tip-up test is what is recommended for dry type transformers.

Around the 54 minute mark in Meggers "Benefits of 1 Hz Power Factor Testing for Insulation Condition Assessment" webinar they discuss this briefly.

[nowwearesuckingdick]

Megger doesn't state that you can't or shouldn't run a NB DFR test on dry type transformers. There is still some extra benefit on a dry type testing down to 1HZ, mainly from comparisons to possible sister units.

A tip-up test is what is recommended for dry type transformers.

Around the 54 minute mark in Meggers "Benefits of 1 Hz Power Factor Testing for Insulation Condition Assessment" webinar they discuss this briefly.

Thanks for that lead. I am awaiting a more detailed response from Dr. Robalino who appears to be Megger's top source of info on this subject but the preliminary response was just a partial answer to my full question and indicated that the software can't accurately/reliably apply the ITC feature to dry types due to lack of oil.

[nowwearesuckingdick]

Here is Megger's official response on this subject matter:

"Below is the collective response from Diego, Sanket, and I:

I will try to address this question in two parts:

ITC correction for Dry Type transformers
We are using the activation energy of oil filled transformers to determine ITC for dry type transformers.
ITC correction will be removed for dry type transformers in a future release since the correction provided is not accurate
Question for you: what temperatures are you testing dry type transformers? At or near 20°C?
NB DFR & 1 Hz
Are these results accurate on a dry type transformer? Yes!
Are these results meaningful? Only if they are displayed in a way that provides the user with useful information.
NB DFR interpretation, while possible, was met by most customers with confusion. Do I focus on the frequencies at the end of the spectrum? What about the beginning? What does PF improve at some points and worsen at others? Lots of questions, and the answers provided would rarely improve a customer’s ability to interpret graphs for subsequent results
1 Hz interpretation is considerably easier. It is one data point and can be compared to line frequency, sister assets, etc.
For dry type (and FR3 filled) transformers, Megger continues to investigate and update insulation rating assessment

One note we want to make is that a more informative insulation evaluation test method on a dry type transformer would be a tip-up test.

With all that being said, you are correct that the majority of NB DFR/1 Hz/ITC insulation analysis information involves oil-paper systems. I hope the explanation provided above sheds light on what Megger has done and will do in the future to improve our assessment capabilities of dry type transformers."

Regards,

Ken Petroff
Product Manager
Megger Valley Forge
2621 Van Buren Ave.
Norristown, PA 19403

T 610.676.8573
T 610.676.8500
M 610.764.0388
E Ken.Petroff@megger.com
www.megger.com