Voltage Transformer Blast/Failure

What were the results of the failure analysis of the transformer.

The transformer could have failed due to excessive load on the secondary. Was the secondary overheated? Could there have been a temporary short in the transformer secondary wiring?

Are you using vacuum contactors? Vacuum contactors are hard on transformers.

Could the transformers have had a manufacturing defect?

Are you applying excessive voltage to transformers? Transformers designed for line to ground and are connected line to line or ? Does transformer show signs of excessive temperature rise?

Are there any indications that any of the bushings arced over? Melted metal or black deposits deposits near one bushing.

What does the transformer manufacture say the problem is?

Hi

In all instances of blasting the top cover is removed and the bottom oil tank is deformed. WE could not ascertain whether the failure is in secondary or primary side.

The VT had not blasted instantaneously, hence we ruled out short circuited secondary. As far as loading is concerned, the protection core is showing a resistance of 450ohms and the metering core is showing a resistance of 290ohms.

hence both are well within the rated burden limit.

No arcing found in the VT bushings.

Connections are OK since we are monitoring the measured voltage in the control room. The voltage measured by the damaged VTs are correctly matching with the incomer voltage as measured by a CVT.

Thanks for the response dear people. Your suggestions are welcome.

N V M

What is the brand,BS/IEC of the transformer,is it oil filled,have you got the test certificate from manufacturer?.

The details of the VT are as follows:

Rating: 220KV

Spec: IS-3156

Ratio: Winding 1:220kV/110V ; Winding 2:220kV/110V ; Maximum System Voltage: 245kV Insulation Level: 460kV/1050kVp; Creepage Distance: 6125mm

Voltage factor: 1.2 Continuous and 1.5 for 30 sec.

Burden: 150VA for winding 1 and 50VA for winding 2Insulation level: 0.2 for winding 1 and 3P for winding 2

Test certificate available. Factory Test attended by APTRANSCO team and no problem in the result.

You seem to be confused and in turn trying to confuse others or else you are playing games;

When you say there is wave trap, the PT ought to be Coupling Capacitor Voltage Transformer (CCVT) this serves as a combined voltage transformer and coupling capacitor for the PLC Carrier system.

You say that two more PT's after the bus were also in the verge of failure - just be aware that two winding PT's are selected upto 66 KV your's are capacitive type for sure.

Dear A.A.Khi:

With due respect to your wisdom and experience, I would like to mention that I need not prove to YOU that I am not playing games.

A wavetrap without CVT is unimaginable for an Engineer. That is why I never mentioned separately about the presence of CVTs. After crossing CVTs we have 3 sets of PTs in our Switchyard for different set of feeders branching out from the same incomer.

And whatever spec i have given is taken from the nameplate of damaged PTs. This explanation is given so that others are not mislead by your SOLUTION/SUGGESTION.

Another piece of information Mr. A A. Khi; If the problem is as basic as you are estimating, I would never have asked this problem in this forum. We would have solved ourselves.

Nothing personal.

Regards

N V M

From your posts I would like to quote:
"And whatever spec i have given is taken from the nameplate of damaged PTs." "Ratio: Winding 1:220kV/110V ; Winding 2:220kV/110V ;"
Name plate would give you, primary 220kV/√3 secondary 110v/√3.
Dear kaavalan I wish you good luck in quest for an appropriate reply.

I wonder if the transformer oil is contaminated: water, PCB, metal, organic fiber, etc.:
https://www.satcs.co.za/Transformer_Oil_Analysis.pdf
Mark Bingham
Relativity PL

Did you check for harmonics generated by VT as well as from load or Power supply?.

Please don't get rude! If your problem is not basic, your knowledge definitely is. In your first posting (question), you have mentioned the PT's insulation class as 3P. Please note that 3P is the Protection Class of the VT and NOT the insulation class. If you cannot even understand the name plate of a VT, how are you expected to understand our suggessions, leave alone solving the problem by yourself?

A first step in learning would be to fully understand the name plate details of your equipment.

No offence!

Now, coming to your query, long storage could have weakened the insulation and it might have causec the failure. Did you check the oil BDV before charging the VTs? Or could there be voltage transients?

If the top of the transformer was blown off, there was some serious arcing or overheating inside the transformer. Or it may be possible that transformer design was defective and normal internal pressure caused top to blow off.

Either you are the transformer manufacture should unwind the transformer and determine where the arcing or overheating occured.

Is there a pressure relief valve on the transformer?

You say that other transformers have indications of failure. Was this determined by gas analysis? Does gas analysis say problem is caused by overheating or arcing?

If its blowing its top off then there is some serious high pressure being built up in that trafo casing really fast.

Pressure increase due to heating is the root cause. What is getting (too) hot? An arc explosion is fast heating.

If your coolant starts to boil the pressure increase would be very rapid. Do you have the correct coolant (oil) in the trafo? Water contamination per post #9 also springs to mind as a possible cause.

Time to put your forensics hat on kaavalan.

What did the manufacturer say?

What did the manufacturer say?

Your first and last stop should have been the manufacturer-- who guaranteed Product.

mm

Dear All,

The Transformer manufacturer is supposed to come to site next week with this team for analysis.

Another piece of information is that the transformers are purchased four years back and kept in storage all these times. Now our team is anlayzing the condition of oil in other similar transformers erected. Another approach we are attempting is to check the oil charactersitics of the transformer oil every one hour after charging again. We planned to replace the PTs with new one due to the extreme pressure from state govt.

We are suspecting that the transformer oil lost its properties due to long storage time. The perfect tan(delta) values before charging the PT might have deceived us. After charging the PT the properties of oil might have gradually degraded.

Another theory proposed by one of our superiors is that one of the wavetraps might have failed resulting in mixing of the 50HZ frequency with the 500kHz frequency of PLCC, which could have been a cause for overheating.

Our team is overlooking manufacturing defect, justifying that it will create immediate failure of PTs. Out PTs have failed after being charged with 220kV for three to six hours.

Due to the pressure from top mgmt. we are againg going to make the crime of charging the line with new PTs without ascertaining the reasons for earlier failures.

Regards

N V M

Pressure to continue......

Better you than me mate!! Who wears the can if they blow up again?

Is it possible to energise the PTs with their top covers removed? Might prevent an explosion or allow useful observations. Some remote temperature logging might be prudent.

PLCC is Power Line Carrier Communications right? Very low energy levels associated with this (typically injected at 10-80W max rf level). I cant see how this could be the source of the problem.

......and it will be pressure that blows them up again!!!

Your next installment should be interesting.

Based on the 4 years in storage, I will make a guess the units were not properly set up for long term storage or that the long term storage requirements were not completely followed for the full 4 years. That would be covered storage, in a conditioned environment, with heaters applied to the oil to mitigate effects of temperature swings and humidity.

I will also guess that no special procedures were followed for bringing the transformers out of storage. These would include testing the oil, pre-installation cleaning, and following manufacturer's recommendations for bringing the equipment out of long term storage.

But I'm just a mechanical engineer. However, I strongly advise looking at an oil sample visual inspection and test before starting any additional transformers.

he Transformer manufacturer is supposed to come to site next week with this team for analysis. Just listen in . Let them solve.

Another piece of information is that the transformers are purchased four years back and kept in storage all these times.

Now our team is anlayzing the condition of oil- WHAT- PETROCHEMICAL/breakdown volt/Turbidity-- and may we see results

n other similar transformers erected(And Running well?).

Another approach we are attempting is to check the oil charactersitics WHAT ?of the transformer oil every one hour after charging( Directly from the Live transformers? )again. We planned to replace the PTs with new one due to the extreme pressure from state govt.OWNER-ENGINEER-VENDOR Triangle!

We are suspecting that the transformer oil lost its properties due to long storage time. The perfect tan(delta) values before charging the PT might have deceived us. After charging the PT the properties of oil might have gradually degraded. BLAME your Lab

Another theory( NOW YOU ARE GROPING) proposed by one of our superiors is that one of the wavetraps might have failed resulting in mixing of the 50HZ frequency with the 500kHz frequency of PLCC, which could have been a cause for overheating. A FEW VOLTS +SKIN EFFECT- magnetostiction !

Our team is overlooking( Ruling Out?-- you cannot) manufacturing defect, justifying that it will create immediate failure of PTs. Out(OUR?) PTs have failed after being charged with 220kV for three to six hours.

Due to the pressure from top mgmt. we are again(g) going to make the crime of charging the line with new PTs without ascertaining the reasons for earlier failures.

SUGGEST( Put all new PTs to a bank test at 220KV in a reserve circuit FILTERED FROM plcc-- using maximum burden loads -- and observe heating by the hour )

mm
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