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 insulation 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.

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 irrelevant 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.

Analyzing the info that you have initially provided:
1. Breaker trips immediately when energized
2. Winding and oil temperature fault indication
3. Oil level was low

and the later info you have indicated:

4. That the secondary neutral earthing connection was removed when the phase and the earth test indicated a short

Then this is clearly an indication that your secondary winding is faulted.

You can verify and confirm the secondary winding fault by doing a transformer winding ratio test, using a TTR tester.

Thanks,

Am sure that secondary winding is faulted.But at the same time am interested to know what could be the reason behind that...the transformer is new and jst commissioned before three months ago...let me explain the problems once again.

1) Transformer 1250 KVA, Hermetically sealed,11kv/415v,50hz,indoor type,with ventilation.
2) Within three months the oil level is gone to minimum level.
3) MV breaker is tripped and showing the WTI/OTI fault.
4) At the same time the main HT breaker is tripped and showing Earth fault.
5) After that i removed the LV side conection and checked the continuity between phase and body and it showing short.It means that something wrong is happened with LV winding.
6) there is no oil leakage and know one is stolen the oil.
7) I checked the client consumption and it's only 700 KVA.So no question of overloading.
8) The load is paper mill.


Am interested to know the cause behind transformer failure within three months.

Even from last two days i put power analyser to study our customer plant.Let see after report.

Did you document the oil levels during your commissioning?
Is this a new installation?

Based on your explanation that no oil has been stolen, It is obvious that there has been a manufacture issue since day 1; Meaning, you installed a transformer with a low oil levels.

Initially, if oil levels were not checked then it would be difficult to determine low oil levels until the transformer is gradually loaded, and heat increases. at this point you would start having the problems you explained. i.e. Winding failure and so on. Hence your current problems.

Now you can no longer seek redress from the manufacture because your commissioning was questionable.

Because LT side of transformer is earthed through neutral grounding. So that you are getting short between phase and neutral and phase and body.

For getting open then you have to remove neutral connection with neutral grounding. Then you will get open at the LT side of transformer.

This is my personal opinion / observation based on the facts that has been described. If the oil level dried down to an abnormal low level level and assuming that no oil leak or pilferage has taken place and the transformer is hermetically sealed there is a strong possibility that there had been some partial discharge that eventually progressed to a slow but destructive arcing fault on the transformer winding. That may be the cause of slowly building-up heat and drying-up the transformer until the arcing fault developed into a full blown line- to ground fault.
An arcing fault will not trip the primary breaker because the arcing fault current is very small. It is also possible that the arcing fault started as winding turn-to-turn fault. In this case an oil dielectric test will say something on the test result. A TTR test will also provide some abnormalities in the TTR test ratio of the windings. Those may probably be the cause of the shorted winding.

First we need the eliminate the suspected problems.

1. Is the oil level at the minimum mark or out of sight below the mark? If the oil level is at the minimum mark, I will consider that the oil circulation still exist and transformer is still being cooled. A winding or oil should not be heated up within few seconds of fault current before the protection relay activated.

2. Earth fault was detected at the HV circuit breaker. That means there was an earth fault at the HV side of the transformer. The earth fault at LV side of transformer is not detectable by the HV side earth fault protection relay, due to the delta winding.

3. WTI/OTI, are these non-electronic type of sensors. ie capillary tube type? If the temperatures are measured using electronic relays, I would like to know how the temperature relays are powered. If it is powered by AC voltage tapped from the transformer LV side, it may be just a failsafe alarm due to loss of AC power after the HV side was tripped by the earth fault relay. If there are 2 separate alarm indications, ie, alarm and trip, it will be easier to distinguish whether the transformer has tripped by the high temperatures.

Dear Meng thanks for your positive feedback.It is agreed that the HT breaker is tripped because of the earth fault on HV side. But the same time lv side phase and body is showing short and the oil level is minimum.What could be the reason for short between phase and body on lv side after removing of all the connections???

It's difficult to tell why LV has an earth fault. Have you performed insulation check between HV and LV windings? May be there is a short between HV and LV windings.

Have you check whether the transformer tank walls are deformed? If the tank walls had bulged outwards due to increase in internal pressure, than the tank internal volume has increased and therefore the oil level drops.

I removed all the transformer connection and checked the continuity on both (HV and LV) side.But is showing phase and body short only on LV side.There is no phase to body short on HV side.