[Help] Transformer Test Results & Conflicting ELEC/EE Opinions

[MrMoe]

I'm looking to purchase six transformers. Three 167kVA ABB and three 333kVA MEVENCA single phase transformers (Nameplates attached; serials blacked out given that they're not my property, yet).

During the due diligence process, a licensed electrician performed insulation tests and determined that three of the six were defective. However, the EE responsible for the site from which the transformers were pulled is disputing the test results on the basis that he personally oversaw the replacement of the six transformers (now a 2MVA substation) and they were all operating satisfactorily prior to being removed.

In order to provide as much background as possible, I have attached the test results. These were pretty much "informal" tests; they're purpose was to establish whether the transformers worked or not in order to kick off purchase negotiations. In addition, I'd like to add that (and please pardon the redundancy; the nameplates already cover this), the transformers are multi-winging, 2 HV terminals in, 4 LV terminals out (the intention is to connect X2 & X3 in series). They have been stored in a sheltered (albeit, not completely insulated) environment for approximately 5 months; level concrete floors and relative humidity between 50-80% throughout according to my best estimate.

I greatly would appreciate any feedback from this community regarding the following:

1) Would you consider these test results as a reliable indicator as to whether I should move on and look at other transformers (this is with regards to the ones labeled as "bad"). I know that there is a myriad of other tests to be performed on the "good" ones, but would you completely disregard the "bad" ones and look elsewhere?

1) There is always room for human error during testing procedures and, given the fact that these transformers are a "tad" more intricate than your typical overhead distribution equipment, do you believe that there may be an increased possibility that a licensed electrician unfamiliar with these specific transformers may have committed an error?

I can provide as much information as possible if you have any requests for it; I'm simply trying to figure out whether I should have them checked again by another qualified professional or if I should simply avoid the transformers that tested poorly.

Thank you very much for all your help!

 

[petersonra]

Hard to tell about the qualifications of either the EE or the electrician from here.

Personally I would not be spending a ton of money on something I paid to have tested and it tested as bad.

Even if they are bad it might be cost effective to have them rebuilt.

 

[MrMoe]

Hard to tell about the qualifications of either the EE or the electrician from here.

Personally I would not be spending a ton of money on something I paid to have tested and it tested as bad.

Even if they are bad it might be cost effective to have them rebuilt.

Thank you for your input.

I agree, I would generally walk away from anything that tested bad but given the available equipment shortage in my neck of the woods and the very very decent asking price, I'm trying to figure out if they're worth a second opinion/test or if I should just treat them as faulty and try to negotiate them based on the rebuild costs.

Once again, thank you for the feedback!

 

[EmagSamurai]

We have lots of equipment that was working fine but fails testing once removed from service. I know that doesn't really help answer your question, but I'm always surprised when I see that as a justification for putting equipment back in service. Rather than looking at it as possibly dodging a bullet by catching it before something disastrous happened.

All that said, if you are in a position where you need the equipment, then you're a bit stuck. My first pass would be to have the equipment retested by someone else. If it fails the second time that gives you a solid basis for asking for a further reduced price based on needing the equipment refurbished. But whether or not your just throwing away your money is really dependent on the situation. If they're already selling this stuff for scrap prices, then you're probably wasting your money trying to negotiate further, and you need to decide if the price is low enough to cover the refurb costs.

 

[MrMoe]

We have lots of equipment that was working fine but fails testing once removed from service. I know that doesn't really help answer your question, but I'm always surprised when I see that as a justification for putting equipment back in service. Rather than looking at it as possibly dodging a bullet by catching it before something disastrous happened.

All that said, if you are in a position where you need the equipment, then you're a bit stuck. My first pass would be to have the equipment retested by someone else. If it fails the second time that gives you a solid basis for asking for a further reduced price based on needing the equipment refurbished. But whether or not your just throwing away your money is really dependent on the situation. If they're already selling this stuff for scrap prices, then you're probably wasting your money trying to negotiate further, and you need to decide if the price is low enough to cover the refurb costs.

This is exactly the type of advice I'm looking for (THANK YOU!). If I'm interpreting your reply correctly, you're basically saying that the EE's argument that the equipment was working fine "pre-tests" doesn't mean that it wasn't faulty prior to removal.

 

[EmagSamurai]

This is exactly the type of advice I'm looking for (THANK YOU!). If I'm interpreting your reply correctly, you're basically saying that the EE's argument that the equipment was working fine "pre-tests" doesn't mean that it wasn't faulty prior to removal.

That is correct. I tried not to put it in quite so few words because I didn't want it to sound like I was being inflammatory. But you pretty well summed it up.

 

[petersonra]

If I'm interpreting your reply correctly, you're basically saying that the EE's argument that the equipment was working fine "pre-tests" doesn't mean that it wasn't faulty prior to removal.

It doesn't mean it was faulty either.

Could be faulty tests or bad interpretation although the numbers on his test report look pretty awful for the ones denoted as bad. I am surpirsed with 10 kohm readings that it did not trip something.

 

[MrMoe]

It doesn't mean it was faulty either.

Could be faulty tests or bad interpretation although the numbers on his test report look pretty awful for the ones denoted as bad. I am surpirsed with 10 kohm readings that it did not trip something.

Thank you for the feedback. I do get that the fact that the equipment was working properly (as per the EE responsible for supervising it) isn't conclusive evidence that it's in or out of satisfactory operating conditions.It seems as if it could go either way with regards to your comments; could be a bad test or a bad transformer.

You do mention that at 10Khoms you would have expected something to trip. I apologize for my lack of knowledge regarding transformer testing but just wanted to point out that the transformers were tested on a standalone basis; nothing else hooked up to them other than the testing rig. I mention this because I'm not entirely sure if your mention of a potential "trip" was related to the tests or the transformer's performance while previously in service.

 

[synchro]

You do mention that at 10Khoms you would have expected something to trip. I apologize for my lack of knowledge regarding transformer testing but just wanted to point out that the transformers were tested on a standalone basis; nothing else hooked up to them other than the testing rig.

The identical measurements of 10.00 kohms on three different pairs of terminals would almost certainly be indicative of a measurement error. Not only do they match within 1 part in 1000, but they just happen to be at a nice round number and that's very unlikely to be just a coincidence.
Perhaps, though, there could be a 10K low limit on their test setup and the actual resistances were even much lower.

 

[MrMoe]

The identical measurements of 10.00 kohms on three different pairs of terminals would almost certainly be indicative of a measurement error. Not only do they match within 1 part in 1000, but they just happen to be at a nice round number and that's very unlikely to be just a coincidence.
Perhaps, though, there could be a 10K low limit on their test setup and the actual resistances were even much lower.

Thank you for your feedback.

I'm going to followup with the tester regarding these particular results. I recall him mentioning a reworked pressure relief valve in a unit and wouldn't be surprised if he actually dismissed the unit in its entirety and just market everything down as bad (i.e. 10.00Kohms all around).

This is why I resort to this forum when I can't find answers by myself online. I don't have the expertise to back up such a claim but when emboldened by your suggestions, I do have some ground to bring certain things up with the tester and/or seller. I sincerely appreciate the help.

 

[zbang]

Were the transformers moved between service and testing? It's possible that shook something loose which caused the test failures. (Also- that 10k reading does look suspicious, and I'll suggest that none of them were in service with that low a reading.)

 

[MrMoe]

Were the transformers moved between service and testing? It's possible that shook something loose which caused the test failures. (Also- that 10k reading does look suspicious, and I'll suggest that none of them were in service with that low a reading.)

1) Yes, they were moved from service to storage. Not a very long trip but definitely taken out of a vault (not sure if my translation is correct regarding that term; steel enclosure comprising the transformers, disconnects, etc.) so we can assume that some movement took place since their service period.

2) Correct, the transformers were tested on a standalone basis; completely disconnected.

 

[MrMoe]

Perhaps, though, there could be a 10K low limit on their test setup and the actual resistances were even much lower.

This quote sparked my curiosity. I looked up the test equipment, a Megger MIT1025 and, indeed, the resistance range is from 10K to 20T ohms so your assumption is certainly plausible.

 

[Hv&Lv]

Don’t assume just because they worked when they were taken out of service that means anything.
I have three 833kVA XFs now that are working fine at a step down station.
once we get them out of service and store them for a while I would be scared to re-energize them.

I would ask if he has any doble tests available.
with the Doble database, they probably have one on file.

 

[mivey]

re-test. no more informal stuff.

if it fails the formal test, move on unless you want to spend the money to have it shipped and for a re-build evaluation.

the ee can assure all he wants but a doble test is mandatory.

 

[cpickett]

I have three 833kVA XFs now that are working fine at a step down station.
once we get them out of service and store them for a while I would be scared to re-energize them.

Just out of curiosity, what sort of failures would you expect after storing a transformer and then re-energizing?

 

[Hv&Lv]

Just out of curiosity, what sort of failures would you expect after storing a transformer and then re-energizing?

Moisture ingress during storage for one. Especially as old as mine are. (1952 GEs)
Yes, transformers are supposed to be sealed with a nitrogen blanket, but many are not and need a constant nitrogen supply to make up for the leaks. When a transformer is stored the coil isn’t warmer than ambient any longer.
With leaks and no N2 supply, they will essentially “breath” and draw in moisture.

Before I would purchase and energize an older XF that was removed from service for SOME reason, I would demand a battery of tests, Doble, SFRA, DGA, etc..

 

[paulengr]

Just out of curiosity, what sort of failures would you expect after storing a transformer and then re-energizing?

It arcs internally, ruptures the tank, lights the oil on fire, then you have a huge pool oil fire to contend with.

With these small ones nobody is going to put a lot of protection on them.

I’d say 10-16 weeks construction time new depending on demand. Closer to 10 with dry and shorter on cast coil (days) if they weren’t so small. Might be able to get much faster. If you need faster generally at those sizes it’s pretty easy to get used or already rebuilt. Rebuilding saves very little time on construction.

DGA on transformers that sit with no heat on them is largely useless. The oil stratifies and the moisture sits hidden on the bottom. Trust me I’ve been burned a lot this way on even a transformer that tests good.

As to N2 blankets only on larger units when you start to see conservators 10 MVA plus and regionally depending on weather. In the Southeast N2 bottles are pretty rare. If you don’t do this you might have to desiccant filter maybe once every 10-20 years so maybe once in it’s lifetime but you’ll be in it for other reasons and filtering more often than that if you do annual DGA and keep up with it. As an example though even though they have and maintain their own testing lab Duke, largest utility in the US, doesn’t test the majority of their transformers. To me that’s nuts because you get early warning like a year or more in advance before things go bad.

 

[Hv&Lv]

It arcs internally, ruptures the tank, lights the oil on fire, then you have a huge pool oil fire to contend with.

With these small ones nobody is going to put a lot of protection on them.

I’d say 10-16 weeks construction time new depending on demand. Closer to 10 with dry and shorter on cast coil (days) if they weren’t so small. Might be able to get much faster. If you need faster generally at those sizes it’s pretty easy to get used or already rebuilt. Rebuilding saves very little time on construction.

DGA on transformers that sit with no heat on them is largely useless. The oil stratifies and the moisture sits hidden on the bottom. Trust me I’ve been burned a lot this way on even a transformer that tests good.

As to N2 blankets only on larger units when you start to see conservators 10 MVA plus and regionally depending on weather. In the Southeast N2 bottles are pretty rare. If you don’t do this you might have to desiccant filter maybe once every 10-20 years so maybe once in it’s lifetime but you’ll be in it for other reasons and filtering more often than that if you do annual DGA and keep up with it. As an example though even though they have and maintain their own testing lab Duke, largest utility in the US, doesn’t test the majority of their transformers. To me that’s nuts because you get early warning like a year or more in advance before things go bad.

I’m in the southeast. All ours and the Duke ones I see have N2 blankets.
We test ours on a five year rotation.

I’ve never understood Duke’s “run it to failure” mentality.