Transformer Excitation Test. 2

[SAMBOY]

Good Day to you guys!

We have a situation in one of our project : Relocation of a 30MVA 69kV/13.8kV Power Transformer, The utility company is requiring us to have an excitation test on the Transformer.
Is it still necessary to conduct the excitation test? If yes, why?

FYI, We have already conducted all the standard test on the transformer, except of course for the excitation test.

Thanks

 

[bradh01]

Samboy,

If I was the owner of this transformer, I would require a field test for the excitation current. The excitation test is one of many tests typically used to verify the overall "health" of the transformer. It is typically done each time a large power transformer is tested. As with most routine testing results, it is compared to previous results to verify no major changes have occured, and in the case of the excitation test, it gives an indication that the transformer core is electrically and mechanically sound.

Other field tests typically performed are: DC insulation resistance, winding resistance, power factor, turns ratio, bushing tests, lightning arrestor tests, core ground, oil analysis,etc.

 

[mpparent]

I'm surprised they only asked for excitation. I would have thought a Doble test would be asked for as well.

Mike

 

[buzzp]

I can understand the excitation test since the transformer was moved. However, I would never do this test on a transformer that has been out of service less than a few days. This is because the core still has some remnant flux remaining and the excitation test will be all over the map, unless you degauss the transformer prior to the test which can be very dangerous.
I do agree that other tests should be done as well but it sounds like these were done already.

 

[mpparent]

By the way...a Doble test set allows you to degauss a transformer (or so I'm told).

Mike

 

[buzzp]

mpparent,
No a Doble tester will not degauss a transformer. I contacted them once and asked them what they suggest we do to accomplish this. There suggestion was to hook up a battery to the transformer and hit'er that way. Well, the electricians did this and the sparks were a flying. Don't listen to Doble when they tell you how to degauss. We simply skip this test since we don't want to kill someone unnecessarily, LOL.

 

[electricpete]

The excitation test checks among other things integrity of the magnetic flux path. It's not too difficult to imagine the rough handling while moving a transformer might cause might disturb the lamination pack such that a gap would form at the joint between core legs. This would not show up under insulation tests and not under TTR unless severe. It would only show under excitation test (or possibly leakage reactance test, but that is not a common test).

Generally, remnant flux problems are created by dc testing. Therefore any dc testing if required would be done after excitation tests. Also it may be possible thatcertain types of faults/trips may cause remnant magnetism. For a ct (different animal), opening the secondary under load can cause remnant magnetism problems. I'm not sure if there is a comparable scenario for power transformers. Once remnant magnetism occurs it generally must be removed electrically. I have never heard of time since deenergization as an important factor in discussing residual mangetism.

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[mpparent]

Thanks for the info. buzzp,

Mike

 

[electricpete]

I don’t know a lot about the specifics of using a battery for degaussing a transformer. But I can tell you I would trust Doble above pretty much anyone else as a source for information regarding transformer testing. I would suspect if you asked Doble that they could pull out of their vast literature a detailed procedures for degaussing a transformer including suitable precautions to avoid any dangers.

They do recommend excitation test as part of the routine battery of power transformer tests. We have rarely had problems with that test.

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[buzzp]

Electricpete,
I don't know all the particulars of what Doble suggested but I believe it went something like hooking up a regular car battery to the transformer (as far as primary, secondary, I dont know). Well when its hooked up, it draws a lot of current. The problem comes in breaking the battery to the transformer. You can imagine the arc that was pulled off from this battery. We almost had an electrician get seriously injured because of this. I went to another Doble session (two of them actually) after this event and asked them what they recommend we do to degauss a transformer, they still hold to the battery. At least, they should be telling people to use a suitable disconnecting device rather than having a poor electrician pull off the cable from the battery or transformer.
So yes, Doble was asked three times and each time they suggested the battery with no mention what so ever of a suitable disconnecting device (which an engineer should know but an electrician would not). I was also told that Doble does not put a lot of credibility in the excitation test (by one of their client engineers). You can believe all of this or not but when it comes to excitation tests and Doble, they are not the experts. Everything else about a transformer they are pretty good with.
There is almost always a DC component (albeit small) on the AC (at least in our facilities (can't explain but maybe you can)) and this is responsible for the remnant flux in our transformers since no DC tests are done before the excitation tests.

 

[electricpete]

I believe the car battery method will involve several steps of reversing polarity and applying reduced voltage each step. Don't know much more than that. It's unfortunate that Doble didn't mention precautions. As I said, I think Doble has a detailed procedure for this. I looked on the website but couldn't find the procedure... there appears no search feature availble for the proceedings like there used to be.

"You can believe all of this or not but when it comes to excitation tests and Doble, they are not the experts."

We will have to agree to disagree on that. I don't know who would possibly know more than an organization which provides the test equipment, assists with testing problems, assists in test data interpretation, has a huge database of customer test results, has twice-yearly meetings with papers/presentations from Doble engineers and customers throughout the country (including quite a few on excitation testing). (who knows more?)

If they have stopped recommending excitation testing, then that is a new development since I stopped working on transformers around 9 years ago. I know that leakage reactance testing was at that time a new development.... I can speculate that perhaps they now recommend leakage reactance testing in place of excitation testing (just speculation).

I have not heard much about dc voltage on an ac power system. I did hear there is a solar cycle every 11 years (1989, 2000, 2011) where increased solarmagnetic activity causes small dc voltage gradients on the earth's surface. This affects mostly long transmission lines since the voltage difference increases with distance. Transformers directly connected at the ends of long transmission lines can have overheating problems due to the dc. This would affects generator stepup transformers and transmission system transformers but not distribution customers since the dc does not pass through from primary to secondary of a transformer and the distances associated with distribution systems are too short to generate any dc voltage

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[buzzp]

I know Doble is pretty much the experts on transformers. I can only tell you what they recommended. You can choose to believe me or not, as it sounds like your questioning the truth about what they told us to do. I'll let you develop your own conclusion and I won't bother with this any more.

 

[electricpete]

Fair enough. I don't disbelieve you. I was just trying to provide balance to the discussion and stand up for the Doble organization with which I personally have had a lot of positive interaction. Different experiences lead to different conclusions.

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[Milko]

The most important test before a relocation that must be done is the FRA Test (Frecuency Response Analizer). This test checks the mechanical integrity of the transformer and is performed before and after the relocation.

The FRA and Excitation test, together, will give you the change of health (if occur) of the power transformer.

I recommend, performance all transformer test: DOBLE, FRA, Ratio, Excitation, Insulation, Oil. May be expensibe but it is better.



Melquiades
Protective Relay
Testing and Commissioning

 

[prc]

Let me confess that like electricpete Iam an unabashed admirer of Doble ,not alone for their products,but more for their technical services and for the famous Doble client conferences that they are conducting for more than 70 years.

Transformer excitation current measurement at low voltage was first proposed by Doble in 1967 as a diagnostic test and there is no body in the world more authoritative than them on this subject.Regarding demagnetisation of core they have published papers in their Doble conferences in 1970(Paper 6-1501)1971 (Paper 6-1001) and 1973(paper6-601).Demagnetisation will be required when you have done any tests with Dc (meggering,winding resistance measurement )or when some current breaking transients are seen by transformer(transformer tripping after a fault ).The trick to avoid arc flash (while demagnetising using 12 V battery ) is to connect a high resistance across the winding .
Excitation current measurement is the most sensitive diagnostic electrical test available to transformer engineers for detecting not only core faults ,but also interturn faults in winding ,wrong connections of winding/tap changer ,displacements in core /winding during transport etc.Engineers are using this during manufacturing of trfs as a stage inspection test to find out manufacturing defects ,during final tests for fingerprinting,as a commissioning test ,as a diagnostic test after a major fault tripping ,adverse DGA results etc.

Leakage reactance test is done to detect incipient winding defects (buckling/tilting /displacement )from short circuit
current flows .

Both above tests can be done with Doble measuring equipment .

 

[Chad12]

A very simple way to accomplish this is to use a Megger brand digital 3 phase TTR. The turns ratio test, of course must be performed anyway, and this unit also displays excitation current data at the same time. 2 birds, 1 stone.

Chad Snyder
Director of Operations
American Power Technology

http://www.ampowertech.com