Dear Friends,

when switching on the HT breaker it is tripping immediately and showing winding temp and oil temperature fault on annunciator.Also the oil level of transformer is low. At the same time i removed the LV side connection of transformer and check the continuity between phase and earth and its showing short between phase and earth.

What could be the reasons????

hmmm... please also check the insulation power factor, insulation resistance and polarization index, oil dissolved gas analysis, oil di-electric breakdown voltage test, and other typical transformer tests... its quite unreliable to conclude based on the LV-gnd resistance check and alarms prompting in the annunciator alone... can i ask, do you have oil leaks? when was the last time that you conducted PM in that transformer? what was the oil level in the conservator before you energized your transformer? is your secondary bushings damaged? how about your buchholz relay condition? so that you can figure out the real root cause of the problem, i suppose that your transformer secondary windings had already burned out or winding insulayion had already deteriorated...

Yes laster, i agree with u.......might be something wrong with LT side transformer winding.There is no oil leakage...the transformer is new and within 3 months the oil is gone to minimum level...because of overheating the winding eaten..lots of oil....after that..may be the winding is damaged or touched to body...but what could be the reason for overheating??..i checked the load ...the transformer is loaded only 50%

hmmm... if there's overheating in the transformer, then your buchholz relay will activate due to gas formations in your transformer oil. that means your conservator is filled with gas bubble build-ups from your transformer (resulted to low oil level at conservator, since the oil goes inside the transformer tank brought by high oil or winding temp) i guess you should perform electrical tests on your transformer to address that temp rise... or maybe, check the mechanical operations of your buchholz for clogged and trapped floats... (unwanted tripping)

What is the vector group ?
Is the neutral isolated also ?
What are your megger values ?

You may have a fault to ground on your transformer.

What's are the relay targets ?
What is the fault levels as captured by the relay(s) ?

Have you done any other test ?
Ratio, resistance, capacitance dissipation. Oil dielectric test ... ?

Dear Lester...but why the phase and body is showing short??? Can i open the transformer and check??? or which test should i carry before open the transformer to find out the cause...The HT breaker is tripping and showing the fault Buchholz-WTI-OTI fault.

i suppose you have checked the insulation resistance/ megger test on the secondary windings, together with oil di-electric breakdown voltage test... if the transformer fails to the first test, then you should rewind your transformer... if your transformer fails in the latter, then purify/ filter or replace your oil... the fault that happened to your transformer was only the result of oil loss in the transformer tank... that's an ouch! if you can rewind your transformer or purify/ filter your transformer oil in-house, then you can open it... otherwise, seek your transformer manufacturer for warranty claims or for rewinding services... (to minimize your downtime)

regarding issue in the conitnuity readings between secondary windings and transformer casing, perhaps windings were burned out prior to failure (could be there's leaking current that had created overheating in the unit then high oil evaporation rate or the transformer oil filling during commissioning was not properly enough) but insulation resistance test and insulation power factor test are more reliable since we're dealing with milliohm winding resistance... (you may easily assume that it could be line-gnd fault depending on the resolution of your tester) if you can retrieve your transfomer commissioning data, then you can evaluate if your provider had checked the winding resistance and other related electrical tests...

It is very obvious that your transformer has been low on oil for a long time. It is also clear that there is no planned maintenance your facility equipment. If your CB trips on no load, the unthinkable has happened.

So guess what, you have lost a transformer due to lack of maintenance.

Take oil samples from top oil & bottom oil of Power transformer main tank as wells OLTC tank are to be collected and tested to insure the breakdown voltage test (BDV test)
To improve the BDV value the oil is filtered and to remove moisture from winding it is kept in heat chamber.

Irrelevant protections acted for an earth fault. Needs thorough investigation of your protection system.

Lift the neutral, Top up oil,Test resistance between LV Windings and HV Windings then
HV to LV.Test oil by test set

Your question may be right but insufficient information. Whether this transformer is step up or step down. How does LV star is connected. According to you Trans. is new and oil leakage is not there, than how the oil level goes down. Before commissioning traped air was removed from Trans. after filtration? You have to check throughly your protection & annunciation system as irrelavent protections operated.

The buchholz relay will operate for low oil if the oil level is allowed to fall below the location of the relay. Did you remove the neutral before carrying out the IR test on the winding?
It could just be low oil level that’s causing the problem but before re-energising you need to disprove all other possibilities.

Good luck

@ Garry, yes i removed both neutral and earth before IR.

Dear Randive sir,

The trafo is 11/415V, 50HZ,Hermetically sealed,indoor type.

Delta/star connected.