Flash in Dry type transformer

01) short circuit caused it, possibility is away from transformer since it is epoxy cast, attribute the failure to not accurate setting of the protection relay.

02) HV will be open delta or "V" configuration, this could not cause flash. Still a dry type is epoxy set chances of this happening are very rare.

03) Voltage at healthy phase rise, cannot cause flash / meltdown, at the hv side unbalance will cause harmonics some heating, the protection relay should trip the CB. interturn fault is a rare possibility.

04) You are after a wild goose chase.

05) Again you are after a wild goose chase.

Bottom line is that the protection relay is there to isolate the T/F in case of fault, if the relays are accurately set, battery power sourcing is healthy to activate the relay and trip coil circuit of CB is functional the unit should not have melted.

check the battery, check that tripping schematic of CB is functional, confirm the relay settings, and each time when there is a tripping investigate the cause, carry out megger test and other test before putting back in servic - My observations are that you failed in this regard and over the period of time due to repeated stressing of the transformer time and again finally it said good be to you - a sad ending to the saga.

Thanks for your comments:

following is my observations against your comments which may help me to get your more expert comments

It is confirmed that tere was no short circuit fault away from transformer.Transformer was at almost 15% loading before fault.All of sudden it happened.There was no fault in the system asautomation scheme shifted the load on other transformer by closing the bus coupler just after tripping of effected transforme.which is running smoothly.the effected transformer always remained at minimum load in comparision with other two installed here

HV side protection relay operated and isolated the transformer from system.

pleae comments against my input to reach near to conclusion

Failure could be caused by:
Poor transformer design.
Poor transformer manufacture.
Damage to transformer during shipping/installation.
Operating transformer at higher altitude than it was designed for.
Overvoltage transient.
Long overvoltage.
Contamination-Salty atmosphere-Dirt or chemicals in the air-Moisture condensating on the transformer from the air.
Overloading transformer.

The low voltage side probably failed first and the ions/gas caused the high voltage side to fail.
If high voltage side failed first, most likely the overload would trip and the low voltage side would not fail (or at least would not melt).

Dear Mr. Carl Pugh,

please see the comments below for ypur further analysis

Failure could be caused by:
Poor transformer design.

agreed but transformer is in operation since 1 year
Poor transformer manufacture.

agreed but transformer is in operation since 1 year

Damage to transformer during shipping/installation.

agreed but transformer is in operation since 1 year

Operating transformer at higher altitude than it was designed for.

can u please elaborate your comments and its effects?

Overvoltage transient.

Transient effect have not been observed anywhere else in the system if it happened
Long overvoltage.

No long overvoltage observed before and after incident in the system
Contamination-Salty atmosphere-Dirt or chemicals in the air-Moisture condensating on the transformer from the air.
the atmosphere is dry here and room temperature is maintained well around 30C

Overloading transformer.

Mostly this transformer was being loaded around 35% and before incident it was around 15%

dear Shahzad,

How can u assume that Phase C HV winding became open first .........???