Transformer-Infrared shows much higher temperature than oil & winding 1

[Beengineer]

We loaded the 75MVA transformer up to .92PU and performed infrared scanning and meantime observed changes of top oil and winding hottest spot temperature gauge.

4 hours after the uploading, the top oil and winding hottest spot readings are 51C and 71C respectively. It's consistent with the heat run calculation based on IEEE 57.91. But the the infrared showed that the hottest spot was at the lower part of the tank (at the side), it's 84C, while infrared readings of other parts were less than 45C.

The ambient temperature at the time of testing was about 60F, and it's overcast.

I thnik in the normal situation, the the infrared scanning temperature reading should be less than the winding hottest spot temperature reading.

I would like to hear your opinion.

THanks

 

[jghrist]

Maybe a blocked radiator tube causing a local hot spot.

 

[ANDY197]

hot spot inside the transformer, loose connection, partial discharge,electrical arcing, there is lots of things to go bad.
Pull the oil sample (dga) monitor combustible gases...

 

[Beengineer]

DGA of this transformer shows no problem.

 

[oldfieldguy]

Obvious question, but I have to ask: Do you have the correct emissivity setting on your IR scanner? It makes a great difference.

If you have adequate access, you might want to corroborate your IR temperature with a contact temperature reading at the site of the hot spot.

A hot spot on the case can indicate factors as Dandorian indicated, as well as something causing stray flux to interact with the case resulting in heating due to currents induced in the case wall.



old field guy

 

[electricpete]

Good comments above. A lot of possibilities. Note the flux tends to cancel for windings wrapped on the core where we have high and low wrapped together, but in the area between low voltage bushings and winding, those three low-voltage (high current) leads are separated, creating high fluxes.

Can you actually post the thermal image - there may be some clues.


=====================================
(2B)+(2B)' ?

 

[Beengineer]

Oldfieldguy -The thermographer took the infared scan. He used default emissivity setting(.92?). He used this setting for all images. The weather was overcast. SO the inaccuracy caused by the sun glare was minimal.


Electripete - Sure, I will send the image a little later.

THanks

 

[afroeeng]

Oil temperature and winding temperature indication won't show you true hot spot temperature. oil temperature will always be less than any hot spot temperature because of it's mass and effect of the cooling system. Winding temperature is based on a combination of oil temperature and a derivative of the current flowing in the windings( based on design modelling). Therefore it will give you a general indication of the average winding temperature. The thermographic scan is therefore the most suitable way of picking hot spots that are not severe enough to raise the general temperature measured by the instruments.however , like on the other contributions, it is susceptable to error readings as well especially the less advanced scanners that show readings only.

 

[prc]

Most probably the reason for tank heating will be the leakage flux from the windings.Where is the tank joint for this unit? At top or at the portion where the overheating is observed?Transformer designers normally take care of this by providing suitable magnetic (silicon steel) or electromagnetic (aluminum or copper) shields.When the eddy current generated in tank plate is broken by a joint,excessive heating can ocurr at that point.Then this heating will be there on both sides of tank. Another possibilty will be from the flux from high current LV leads of transformer as electricpete pointed out. This can be a chance if it is a Generator step up transformer and the heating is on LV side,near to bushings or leads from bottom end of windings.

You can differentiate this by following the below procedure.When putting on full load from cold condition, watch the temperature in the observed area-it will rise with in 10-15 minutes to 70-80 C while the oil temperature is still in cold condition.

Anyway 84 C is not a cause for concern,provided it is not going above 110 C in peak summer under full load.

 

[Ceast]

Was the unit commissioned? Baseline test data? SFRA?

 

[TurbineGen]

Was your load balanced or at least reasonably close? If your load was unbalanced your tertiary winding could be overloaded causing the hot spot. If you were loaded more or less balanced, then I would tend to agree with PRC.

One more thought: is this an LTC? If so, what tap position was it in? It could have been the preventative auto heating up.

------------------------------------------------------------------------
If it is broken, fix it. If it isn't broken, I'll soon fix that.

 

[bacon4life]

Here is a recent factory heat run test where the bottom of the transformer was almost as hot as the top oil temperature. The hot spot at the bottom corresponds to where the tank shielding stopped. The manufacturer insisted this was fairly typical. It definitely showed me the importance of documenting the factory heat run tests.

http://files.engineering.com/getfil...a0-a167-24fdb5f31f0b&file=Heat_run_test23.pdf

 

[Beengineer]

Attached are infrared images.

No SFRA done. Not likely it has the loose winding issue.

The load is fairly balanced, and it's not with LTC.

 

[ANDY197]

Is this shell type transformer, forced oil?
On the picture looks like only one pump is running.

 

[Beengineer]

Yes, it's shell form, FOA.

I will go to check the pump tomorrow.

How did you identify that one pump was not working?

 

[ANDY197]

Flow gauge on the pipe.

 

[Beengineer]

The pipe worked fine. The gauge shown "off" because the control image was taken before the infrared scan, and before the 2nd set cooling fan/pump was kicked-on.