Flash in Dry Type Transformer

01-What could be the probable causes of electrical flash or fault in transformer?

You are very close to th answer. Most probable reason seems to be insulation failure.

02-if we assume that Phase C HV winding became open first then What would be its effect on HV side current and LV side current and voltage?

If Phase C HV winding open first, then in no case, there would be any melting on LV side. You draw a delta HV connection, and open a phase winding. You can yourself see that current increases on other phase (A & B) HV windings leading to overload and ultimately tripping on overcurrent/overload for these two phases.

03-If it is assumed that insulation failure happened on LV side first causing interturn fault then what would be its effect on HV side voltage and current?

Yes, it is the most probable cause i think. Why? because, if LV side C phase melts due to insulation failure (inter-turn fault), it would cause excessive current on the same phase HV and LV windings both due to mutual induction. HV side C phase might have got open due to burning at the weakest point. You can check the same.

04-what could be the probable causes of fault on HV winding?

HV side C phase might have got open due to burning at the weakest point

05-what could be the probable causes of fault on LV winding?

Insulation failure, ultimately leading to inter-turn fault.

One more thing, tell in detail about the tripping relay operation. is it numrical type? if it is then, on which phase overcurrent did it tripped? Phase A, B or C ?

That detail could throw some more light on the problem.

Thanks sir for your quick response

some pics have been attached for your analysis

What could be the probable causes of insulation failure?i think it could be due to manufacturing issue.Is there any possiblity of overvoltage on LV side causing insulation failure in our case?

Following protection relay of siemens has been installed:

SIPROTEC 7SJ80 Multifunctional Protection Device

Dear Mr. Shahzad,

It is seen that at the time of occurence of fault, the Trafo is only loaded about 10% of FLA; therefore question of Overloading does not arise. But you did not mention the type/nature of load ; i.e. the harmonic content or THD/THVD etc. High harmonic content in electrical loads may generate additional heating effect which may be harmful to epoxy insulation ( solid epoxy insulation has less expansion coefficient/ elasticity compared to liquid insulation like Transformer oil.). Also if epoxy insulation has some inherent defects like high porosity, due to passage of time, due to moisture ingress, final flashover may occur after initial formation of 'TREE FAULT'. This type of occurence of fault can only be diagonised during routine checking or condition monitoring of insulation of Transformer. The sample of insulation can be taken out to check porosity of epoxy insulations.

Thanks,

Manindra

Thanks for your valuable comments

Please have a look on the load of damaged transformer for your further analysis and comments:

01-400VAC supply for two 80kVA UPS

02-400VAC supply for one 120kVA UPS

03-Tranformers OLTC panels

04-Plant DB supplies

05-Small ventillation supplies

06-Airconditioner supplies

Hi Shahzad,

From the photographs it appears that the failure is most probably due to Partial discharge. This is the most common cause of CRT failure. The reason may be due to manufacturing fault .Blow holes or very fine crack or rough surface may result when the casting is done with improper vacuum. But during final test this could have been located but at times it escape notice of the Inspector..The insulation failure can also be a reason which happens because of accumulation of the dust and moisture and Poor road

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.

It is requested to all to comments against this thread,discussion thread have been closed by admin

06-What will be the unbalanced loading effect on this transformer?

Could it had been caused by a surge or switching?.

It has not happened during surge or switching

Shahzad,

Involving both windings simulataneously makes me think that the failure happened when there is a dead short circuit on LV side. This includes unsynchronised paralleling of generator connected to 400V SwB or failure of a large motor or a cable fault etc.

Due to limitations of design, the dry type transformers cannot handle the surges (current or voltage) that well when compared to conventional oil filled transformers.

raghun,Thanks for your comments,

To have precise analysis from your side,please i am going to share following comments:

01-there is no dead short circuit on LV side

02-No 400V generators are in facility

03-no large motor is the load of damaged transformer

04-No cable fault observed in the system

After looking at the pictures, there is not much melting of the windings, so let me retract my statement that the primary failed first.

The transformer failure could have been caused by corona.
Corona could be caused by transformer design, manufacture, damage, high altitude and overvoltage.

Sometimes solid insulation can withstand corona for long periods of time.

At higher altitudes the air density is less, so the air breaks down at a lower voltage.
Transformers users at high altitudes have more problems with transformers. This observation is from working with different customers and also from theory.

The transformer windings cannot be repaired, so check with the transformer manufacture to see if there is any problem with removing insulation in the area of the failure. (You may be surprised by the results of this failure analysis)

Good Luck

Following an invitation to reply to this thread, I offer the following.

First, I am not experienced with transformers in operation so anything that I offer is to be regarded as "opinion" and not informed response.

Secondly, the OP is insistent that there is no large external load and that the protection devices are correctly operational.

Thus we must conclude the failure happened "inside" the protected zone.

The fact that it is described as "instantaneous" and has created no "spectacular" failure outcomes lead me to suggest the following.

A failure on the secondary (probably caused by an unrestrained piece of debris) has created a significant internal load in the transformer that has then become apparent in the primary leading to phase protection to shut down that phase (and then subsequently the transfore went to 2 phase operation.)

The absence of long term scorching discoloration, the absence of "dripping" encapsulant and other symptoms is why I suggest this has been a short timeframe incident.

Such a failure would also leave all external wiring on secondary unaffected (since it never carried current the fault current) and the OP has not described anything external to the transformer as being at issue.

The primary wiring would have curried the instantaneous current, but since it is not concentrated into a winding was probably able to dissipate the losses.

Transformer core saturation may have provided some protection of the primary (reduced the impact) and it may be of benefit to verify that this portion of the transformer is also in good condition durign rebuild.

Again, I stress that I am not experienced with transformer construction or diagnosis and this advice is from my memory of tranformer theory more than 25 years ago.

Further thought overnight has led to the following addition.

Harmonics in load may have been causing vibration/stress in the material of the secondary coils leading to decline in capability to maintain insulation effectiveness.

You mention UPS devices connected (and other items). I would recommend that you have a waveform analysis of current draw/voltage impact of each device independantly.

You may have a single device that has effectively fatigued the insulation in the secondary.

Again, I stress that I am not close to transformer instalation and diagnosis ..... etc.

Thanks sir for your valuable comments

Did you write to the manufacturer or supplier(agent)?.

Lot of thanks sir for sparing time to compile your valuable times