Difference between electrical and thermal faults?

[ART21]

In case of power transformer,what is the difference between electrical fault and thermal fault?
I want to know that if internal fault is occurred in power transformer then on what basis we can differentiate electrical and thermal fault?

 

[William White]

Okay, to answer this question, you might want to investigate

1) what sort of protection is installed.

2) Why is the protection installed

3) What is is looking for?

You will find that big expensive transformers, or transformers supplying critical equipment, will have lots of sophisticated protection. Low KVA transformers, say supplying residential properties, might only have fuses.

4) What sort of faults can a fuse protect against?
You might want to investigage the diagnostic tests available after a fault. You can test the oil in a transformer

5) what tests can you do
6) what does this tell you

 

[ART21]

Sir,
There is one transformer whose rating is as 25MVA, 132/33KV. Buccholz relay is there to protect transformer against internal fault. One day it was tripped and inspection of that transformer was done. Again Transformer oil testing was carried out and came to know that gas concentration was increasing continuously.Also it was observed that there was problem of loose earthing.
According to analysis it is showing that there may be chances of thermal fault inside transformer.
I have confusion now that how can I correlate this fault detection and fault interpretation?

 

[William White]

If winding temp did not flag or overcurrent protection did not trip then the temperature was not due to load.

Usually these protection systems will only take out the LV side of the transformer.

Bucholz suggest a problem with the transformer, and you'd find that the transformer is taken out completely (LV and HV)

If there was a high resistance fault in the transformer, that would be getting hot, generating gas in the oil.

Did the neutral displacement relay trip?

If you check out the web page of

EOS transformer oils

They provide very good information on the tests they can do on transformer oils - what the tests are, what they mean etc.

 

[Windadct]

In general thermal faults are due to overloading and the Trans just gets too hot and detected by some type of a thermal sensor, though the cause could be other than total power, for example current due to poor PF or circulating current (due to system configuration), harmonics, or failure of cooling fans. This protection is to ideally prevent damage to the transformer due to overheating - a few events like this should not be a problem, but repeated overheating will shorten the life. Whereas as electrical is typically looking for actual electrical problems IN the transformer - short circuits, arcing - etc.

You mention you are having dissolved gas analysis done - this is by far the best diagnostic test. However the description "gas concentration is increasing" does not help, you need analysis of the types of gasses being produced and in what ratios. From this you have a good idea of what is going on. http://www.netaworld.org/sites/default/files/public/neta-journals/NWwtr09_Hamrick.pdf - there are certain levels of gas that can be tolerated, but if too contaminated may need to be vacuum degassed or replaced.

At 25 MVA - not too large - but how was it inspected? If it was opened up ( hopefully NOT drained...), you need to make sure the oil has not absorbed too much moisture - to determine this you can have dew point analysis performed and compare the manufacturers guidelines. If caught in time excessive moisture can be resolved - as long as the tank can withstand full vacuum it can be done relatively easily.

Oh - I have seen more then one case of the core not being properly connected - actually connecting to the case in more than one point, this may not cause arcing but can create a lot of circulating current IN the transformer and then overheating - even whit ZERO load. In larger trans the Core-Case bond should be accessible through the hand-hole at the top. This can be tested by lifting the bond and meggering the core to the case.