CT explosion 3

[RalphChristie]

We've had a CT explosion last week on one of our 66kV-lines, worst I've seen. Investigation still going on.

Many of you have probably seen similar incidents before, but I thought it would just be interesting to show/share it. Porcelain pieces were laying in a radius of approx. 50 meters around the CT.

Luckily no one was hurt in the incident, although it easily could have happened.

(Note that even the structures above are black)

Regards, remember safety first
Ralph


[red]Failure seldom stops us, it is the fear for failure that stops us - Jack Lemmon[/red]

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[SphincterBoy]

RalphChristie, how old is the equipment shown, and had it been power factor tested? Also, don't you do FLIR scans?

We had a similar incident about six years ago, but an old, untested (power factor) PT was the bad boy. The other PTs were tested right away, and found to be in sad shape, and were replaced.

 

[RalphChristie]

First of all I just want to state that the aim of my post is not to ask any question, but just to show and share some images of a CT explosion. (or the remains of it)


SphincterBoy:

To answer your questions,
I am not familiar with all the test "names" you mention (sorry, English is not my first language) but:
Age: Manufactured in the late seventies, round about 1978
Power factor test = tan delta test? No, not been done.
FLIR scan - Infrared scan? Yes, it is done on regular intervals, but it indicated no problem.

Regards
Ralph




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[BJC]

Strange looking CTs> Who made them?

 

[davidbeach]

Ralph, sorry to hear about your CT problem, but it's good to hear from you again. I haven't noticed any posts from you for quite a while.

 

[jraef]

That must have been a spectacular sight... from a good distance.

JRaef.com
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[itsmoked]

Yeah jraef I was thinking the same thing.. I would've loved to have seen that!

Thanks Ralph.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Alex68]

Very interesting images. Thank you very much.
My star for you.

Please let us know the results of investigation.

 

[RalphChristie]

BJC:
The CTs were manufactured by Conelectric, a company located in South Africa. I am not sure, but I think the firm was taken over by Alstom.


davidbeach:
David, thanks for the almost "re-welcome." Little busy in the last few months, but I did peek in from time to time - I do not think I will ever be able to leave this site again!


Jraef, Itsmoked:
One of our contractors (we are busy expanding some of our subs) was at that time sitting in his bakkie (pick-up) maybe 200 meters away. According to him it was a rather spectacular site with all the flames and smoke. The CT gave no indication it was going to explode, he just heard the bang and saw the explosion. When I asked him afterwards what did he do, he just told me "I just stayed in my bakkie, I do not know if the line is going to reclose!"


Alex68:
Thanks for the star - although I do not think I really deserve it!



I've put this images here because:
1. I have seen damaged CTs before, but this one was the worst I've seen.
2. To show other people not familiar with it how it looks like. I noticed more any more that the new guys - fresh from school, college, technicon, university, etc - did all the theoretical stuff regarding a subject, but they do not even know how, those things they learned about, look like! Because they just learn it like a parrot, they do not understand the underlying principles, do not understand why certain things are done and how does certain things work. In the end they unlearn to think! I hate it when maintenance guys just replace stuff and do not search for reasons why it failed.




Regards
Ralph


[red]Failure seldom stops us, it is the fear for failure that stops us - Jack Lemmon[/red]

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[ScottyUK]

Hi Ralph,

Welcome back.

I've uploaded a few shots of the massive explosion which destroyed a 48MVA unit auxiliary transformer at our site in 2001. Three colleagues were killed in this incident.

I've toyed with doing this before, but the reasons you've given above struck a chord with me. Too many people in our industry don't realise just how big the quantities of energy we deal with really are when they casually talk about a 'few MW' or 'couple of kA'. Hopefully this might make one or two realise what can happen when a major HV fault occurs and that energy gets loose.

Those users not on broadband might not want to bother with this - the files are quite large for a slow connection.

http://img241.imageshack.us/img241/317/transformerearthresistorig9.jpg

http://img157.imageshack.us/img157/3816/transformeroverallview1kq5.jpg

http://img227.imageshack.us/img227/8310/transformeroverallview2ur2.jpg

http://img84.imageshack.us/img84/6301/transformertopisophaseduct1ub0.jpg

http://img95.imageshack.us/img95/2821/transformertopisophaseduct3jt3.jpg

http://img223.imageshack.us/img223/9747/unittransformerisophaseductterminalcubicles1on0.jpg

http://img224.imageshack.us/img224/2091/cablesfromtr871aam9.jpg

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Sometimes I only open my mouth to swap feet...

 

[oldfieldguy]

The local (big) electrical utility had a bad run of luck with 230-kV instrument transformers a few years back. A 230-kV CT blew up in one of my client's switchyards.

Pieces of porcelain the size of my hand flew 300 yards from the site, and other pieces hit and pinctured the radiators of two nearby transformers, adding several hundred gallons of spilt oil to the festivities.

Nobody was injured. It was after normal working hours. It gave the emergency squad boys something to talk about. The explosion resulted in the shutdown of a major oil refinery for a few hours as redundant power was routed around the failure.

What fun stuff we play with...

old field guy

 

[itsmoked]

Scotty what's that oval duct on the right side with the round cover on the end?

What exactly blew? The wiring to the transformer or the transformer innards? Looks like that one big duct took the brunt of the blast.

Oh, thanks for the pics!

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[ScottyUK]

Hi Keith,

That's not a duct, it's the conservator tank. Holds, or in that photo it used to hold, about 1600 litres of insulating oil. We lost about 4 tonnes of oil in the fire, just gone completely. The rest was either in the bund, in the remains of the tank, or made its way into the oily water drains.

The accident was due to human error, much as it hurts to say it. Through a gross procedural breakdown, a decision was made to operate the off circuit tapping selector while the transformer was energised. The design of the selector is such that adjacent positions of the switch are bridged by the moving contact as it rotates, causing a short between two tap positions. The fault current in the shorted winding was immense - hundreds of kA - and the selector switch disintegrated, evolving a large volume of ionised gas as it broke up. This ionised gas bubble initiated an interphase flashover between the three phases causing a second, much more violent, explosion which ruptured the tank, ejecting jets of burning oil and gases from the gaps where the tank wall burst away from the top plate. I have a couple of images of the remains of the tap selector somewhere too.

The transformer weighs about 60 tonnes and it moved about 6" off its plinth, shearing the cooler bank from the tank body. The top plate is about 40mm thick, to give some idea of the forces involved in the rupture. You'll notice there are two brown objects on the tank top just in front of the grey ducts. They are interposing current transformers. There used to be three; the third landed maybe 20 metres away. They weigh nearly as much as I do. Loss of this transformer is believed to have tripped the overall differential relay protecting the generator and its auxiliaries, tripping the unit.

The second from last photo shows the neutral bushing. The tapchanger was directly behind that, beneath the conservator tank. That's why the damage is so heavy at that end. The lads were standing between the fire wall and the end of the transformer when it blew up. One reason I have often hesitated before posting those images is that the accident has always been quite a private thing among us who were there. It's impossible for people seeing a few photos to share the pain of the human loss from this accident.

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Sometimes I only open my mouth to swap feet...

 

[Alex68]

Ralph
please verify the ground connection of the exploded CT.
I asked a friend and he said that often a CT explosion is caused by a bad connection of the neutral terminal to ground. This bad connection may be caused by acid rain or bad maintenance, etc...
So one terminal of the CT can be isolated and the big overvoltage caused by the flowing current can cause the explosion.

 

[itsmoked]

Scotty thank you for that added info and RalphChristie for the thought of this thread. Very interesting! Scotty I recognize the human angle and appreciate your position.

It is too bad that the disasater was fairly easy to get into via a few very bad decisions.. There are a lot of traps out in the world that are probably similar and awaiting victims.

I am at my Brother's new house in the CA, Sierras. He has a Jenn-Air stove top. This morning as I went to heat a large rectangular pan for cooking pancakes (28in by about 14in), I turned on, fully, the two gas(propane) burners under it while talking to several people. After about a minute and a half it dawned on me the burners weren't burning!!! I hadn't turned the knobs far enough to turn on the spark generators. Recognizing my mistake I promptly turned one burner to "ignite". As I was actually turning the knob it came to me that "wait!!", I should turn them off and wait a bit. It was too late to get the command to my hand. It ignited - went off - WOOF!.. A yellow flame shot out about a foot from under the entire perimeter of the big pan. Took all the hair off my hand/arm for about 8 inches.

1) So there I was running a gas range. (I only own electric)
2) The propane was absolutely oderless.
3) I'm talking to people (distracted)
4) There is no safety feature to save me from this bad move.
5) I made a bad 500mS decision that was not revokable.

I think we have all done things like this, just some people are caught on really serious ones.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[DTR2011]

Generally poor electrical testing / maintenance is the cause for CT failures of this type.

CTs of this type are generally oil filled.

Testing the insulation via PF/Tan Delta - a change in value=bad
Testing the Ratio = a change in value=bad

Testing the oil for moisture / dielectric strength, etc. There are published specs for go/no-go.

A through visual inspection, including cleaning the porcelan is also recommended.

 

[powerjunx]

Welcome back, Ralph! Nice pics! I'm gald you post and share it.


I seen such damage but not severely damage as yours. It all started when a "hissing sound" on a "phase A" CT was detected in our substation.

Calling out, technical servicemen, since it was installed on critical feeder. They suggested and conducted test such Noise level instrument(i known), getting data on it and leave.


8 hours after the leave.... BANG!!



I never catch up the root cause failure as i never seen those service men again. The rest is history.



greek: i guess your right. It's about maintenance.





regards,

bil

 

[RalphChristie]

Some feedback:

Bad maintenance. Or might I say it? No maintenance at all. This specific batch of CTs are filed with sand (seasand?) and oil. It seems as if the oil can't be changed without taking the sand out, therefore the CTs have to be taken down and send to a company who can change both. (sand and oil) That is the reason for the "no-maintenance"
Well, what can I say...?

First time I heard about sand in a CT. Any comments on it?

Regards
Ralph

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[scottf]

Called a "minimum oil" design. Came about in the 1970's when oil was so expensive and there were more environmental worries (PCB's, etc...).

Not aware that they're used anymore. At least not in developed countries.

 

[prc]

To my knowledge,sand was first used in CTs by GE,USA .I have seen a AIEE paper of 40's vintage on this topic.Taking cue from that paper self used glass marbles(the one used by children to play!) to fill inside CTs to save oil during 70's( Iam not from developed country,but from India.At that time I found cost of marble less than oil -volume to volume basis) .GE used river sand not for oil saving,but to get mechanical with stand strength from high short time fault current.To day also,I know a leading market leader MNC using quartz to fill inside CTs for mechanical strength,oil saving and as a moisture trap.