Transformer Meggering

[infinity]

We recently had a new transformer go up in smoke when it was first energized (all field connections were checked and found to be correct). No one knows the real cause of the problem yet but some have raised the issue of meggering the transformer before it's energized.

Since it's made up of coils how would a megger test really prove anything and if so how would one actually test it before energizing? We're getting all sorts of conflicting opinions.

 

[nakulak]

the best answer to this always comes from the manufacturer - try to get their engineering dept. to give you a str8 answer on how to check the transformer before energizing (the next one) - this also may provide you with some needed ammo so that you can have them replace the unit (rather than burning it up) at their expense (not yours)

 

[infinity]

The manufacturer hasn't been much help. We asked for a field rep to come out and take a look and they wouldn't send one. Their response was to "send it back". It seems that there was an insulation failure between some of the windings. Molten bits of copper were pouring out of the bottom of the A phase winding along with some nasty smoke.

The core has been removed and sent back for testing and replacement. Before it's reinstalled there has been some talk of testing it in the field. There are varying opinions as to how to do this testing.

 

[hillbilly1]

I had a buck boost transformer that I had let the smoke escape from, it was wired correctly according to the diagram sent with it, turned out the manufacture had sent the wrong diagram for it and replaced it free of charge.

 

[zog]

We recently had a new transformer go up in smoke when it was first energized (all field connections were checked and found to be correct). No one knows the real cause of the problem yet but some have raised the issue of meggering the transformer before it's energized.

Since it's made up of coils how would a megger test really prove anything and if so how would one actually test it before energizing? We're getting all sorts of conflicting opinions.

Megger testing transformers before energizing is very important and usually done as part of an acceptance test, but your little cheapo fluke megger wont do you much good you will need one of the big boys.

# megger tests are performed, Pri to Sec (With the secondary winding grounded), Sec tp PRimary (Pri grounded) and both windings to ground. A test is done for 10 minutes for each setup, a DAR and PI are calculalted.

Also a TTR test is performed, winding resistance, and maybe some other tests depending on what type and size the transformer is.

Now you are screwed, all these aceptance tests are done to prevent this type of thing, in fact insuranance companies will require these tests are performed for new installations by a NETA or NICET certified testing company.

 

[zog]

There are varying opinions as to how to do this testing.

ANSI/NETA Acceptance testing standard has it all, do yourself a favor and hire a certified testing company to do this for you.

 

[petersonra]

The manufacturer hasn't been much help. We asked for a field rep to come out and take a look and they wouldn't send one. Their response was to "send it back". It seems that there was an insulation failure between some of the windings. Molten bits of copper were pouring out of the bottom of the A phase winding along with some nasty smoke.

The core has been removed and sent back for testing and replacement. Before it's reinstalled there has been some talk of testing it in the field. There are varying opinions as to how to do this testing.

What would the field rep be able to do? I think your only option is to send the melted one back for replacement.

A while back I read an article that claimed that most large transformer DOAs were due to damage incurred during shipping caused by high G shocks (potholes). IIRC, the suggestion was made that only air ride trucks be used to ship large transformers and testing be done before energization.

 

[infinity]

What would the field rep be able to do? I think your only option is to send the melted one back for replacement.

A while back I read an article that claimed that most large transformer DOAs were due to damage incurred during shipping caused by high G shocks (potholes). IIRC, the suggestion was made that only air ride trucks be used to ship large transformers and testing be done before energization.

The prevalent thought is that the damage happened during shipping. Having been personally involved with the handling and installation I can say that it likely didn't happen after it came off of the truck at the site.

 

[cadpoint]

Megger testing transformers before energizing is very important and usually done as part of an acceptance test, but your little cheapo fluke megger wont do you much good you will need one of the big boys.

# megger tests are performed, Pri to Sec (With the secondary winding grounded), Sec tp PRimary (Pri grounded) and both windings to ground. A test is done for 10 minutes for each setup, a DAR and PI are calculalted.

well ge wally, My fluke 1507 has Dar and PI, you mean my new(tool) toys no good?

Where is the finite cut-off of my meter in an aspect of appling a test as you decribed above, One's meggaring the winding, Gezz I guess I did buy to much meter!

Also a TTR test is performed, winding resistance, and maybe some other tests depending on what type and size the transformer is.

I'll give you that!

There's alot of jobs that go in that don't include any pricing in the job to cover testing. Unless presented as per part of the project it'll just never happen.

 

[zog]

well ge wally, My fluke 1507 has Dar and PI, you mean my new(tool) toys no good?

Where is the finite cut-off of my meter in an aspect of appling a test as you decribed above, One's meggaring the winding, Gezz I guess I did buy to much meter!

Does it go to at least 5,000V? Might need to be 10kV depending on the transformer specs. Also my experience has been thise small handheld battery operated meggers just dont have the ooomph necessary to charge the windings on a large transformer, or at least not very quickly. there is a lot of absorbtion and polarization current needed, until those are met you wont get a true Insulation resistance (Or leakage current) reading.

I'll give you that!

There's alot of jobs that go in that don't include any pricing in the job to cover testing. Unless presented as per part of the project it'll just never happen.

Too bad, and the risk you take by not testing new equipment is what started this post, kaboom!

 

[cadpoint]

...just dont have the ooomph

Thats what she said!

No only a mear - 1000V ...

 

[eric9822]

Out of curiosity, what is the KVA?

 

[infinity]

out of curiosity, what is the kva?

500 kva....

 

[bth0mas20]

500 kva....

Who is the manufacturer? What is the design of the windings? We just had 3 ABB xfmrs...brand new self destruct upon energizing. Two of them were paper wound dry type 3mva and one was a encapsulated widing that faulted internal. The two paper wound were easy to spot the faults. The encapsulated one was not noticable, it passed the TTR and megger but was found to have a fault in C phase with a surge test.

 

[infinity]

Who is the manufacturer? What is the design of the windings? We just had 3 ABB xfmrs...brand new self destruct upon energizing. Two of them were paper wound dry type 3mva and one was a encapsulated widing that faulted internal. The two paper wound were easy to spot the faults. The encapsulated one was not noticable, it passed the TTR and megger but was found to have a fault in C phase with a surge test.

Not sure who the manufacturer is but I can take a look next time I'm down there.

 

[mxslick]

Got any pics?

 

[aja21]

The xformers we are using are in the area of 25MVA. I know alot bigger than what we are talkin about here but we hi pot em before energizing then twice a year preventative.

 

[infinity]

Got any pics?

Not now the transformer has been removed. Here's one before energizing: