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Topic Title: Closure for family at last
Topic Summary: EDF substation explosion 2008
Created On: 23 June 2011 09:25 PM
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 23 June 2011 09:25 PM
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slittle

Posts: 3374
Joined: 22 November 2007

Chelmsford Weekly News


3 years of waiting must have been terrible for his family, I'm sure everyone's thoughts are with them.


Stu
 23 June 2011 10:08 PM
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peteTLM

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Was that the tap changer accident as the article discusses oil? Either way a horrible way to go, and peace for the family i hope.


When i was a child, the substation opposite our house exploded, tearing a hole through the outer plating and pebbledashing everything around it. I cant imagine what it would have been like being next to something similar.

P

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 23 June 2011 10:13 PM
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alancapon

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Yes, it was the catastrophic failure of the tapchanger.

Regards,

Alan.
 23 June 2011 10:13 PM
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ant1uk

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What I don't understand is why it happened in the first place? and I really don't understand why it has taken 3 years for a hearing to tell what the cause of death was, why has it taken so long? it is obvious that the explosion was the cause so why so long? and what has the Health and safety been doing? why have they not given a report yet?

A horrible story to read really.
 06 January 2013 01:07 AM
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alancapon

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The final closure on this one by the HSE: UK Power Networks fined over worker death

Not really anything to add to this.

Regards,

Alan.
 06 January 2013 05:51 AM
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Jaymack

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Was this incident an accident in the sense that it was unavoidable, or was it caused by an error in procedures?

This was most probably an off load tap changer, that are usually padlocked in the OFF position. If this was operated without de-energising the transformer, that could cause rupture of the tap changer housing. I suspect that this was the actual cause of the incident.

I have seen the after effects of an automatic tap changer failure on a 26MVA transformer, this was caused by failure of a diverter rod on this 6 year old unit by G.E.C. The tap changer tank ruptured, and the burning oil gorged out of this unit, and into the adjoining indoor bays for identical rectifier transformers, fortunately with no other damage. This transformer was one of 12 and I didn't loiter around these unnecessarily, or any other similar units!

The divertor rod had sheared, due to an insufficient number of threads on the rod!

Regards
 06 January 2013 09:01 AM
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John Peckham

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As I understand it was an on load tap changer controlled from their operations centre at Ipswich. It was not responding so an AP was sent to hand wind the tap changer. No mention in the report then that there was a known problem with the device and a modification kit to overcome the problem had not been fitted and was found in an employees garage?

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John Peckham

http://www.astutetechnicalservices.co.uk/
 06 January 2013 01:57 PM
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alancapon

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The transformer concerned was a 132/11kV step-down with an on-load tap changer. For whatever reason, the transformer did not tap automatically when required by the voltage control scheme. The AP sent to site to investigate is believed to have intended to hand-wind the tap changer as part of his investigation into the fault, when it failed catastrophically while he was beside it.


Regards,

Alan.
 27 March 2013 04:01 PM
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dlane

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For those that are interested there is now a video from the HSE on the cause of this unfortunate incident that identifies what they believe to be the likely cause.

Chelmsford HSE Video

Should remove any rumours that are circulating of what happened.

Kind rgeards

Donald Lane
 27 March 2013 06:37 PM
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Legh

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Most informative - thank you Donald.
A tragic case indeed !

I have some thoughts, for what they're worth.

What might be the timing interval between a change in internal operating conditions of the tap changer and catastrophic danger? - a couple of seconds?
How might this be detected ? - a sudden change in temperature affecting the oil dielectric breakdown...

So I assume that some method of sensing these changes could operate an emergency shut-down relay.
Other possibilities might be the use of Flash suits and a possible second set of eyes, CCTV monitoring

This stuff makes me twitchy just thinking about it so its extremely unlikely that I will ever end up inside a HV compound performing this operation, but for someone it will pose an interesting problem.

My sympathies to the family

Legh

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 27 March 2013 09:03 PM
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dlane

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They have done an excellent job with the video and what happened is very sad considering that the the victim was only trying to solve a fault to the best of his ability and would never have known what was going on inside the tapchanger.

The timings for a fault will vary depending upon the circumstances, for this kind of fault of a poorly made contact heating the oil I would speculate it to be in the region of 5 to 10 seconds.

This was an old transformer so I don't know what protection it had on it. Our selector tanks have a device called a Buchholz relay thatdetects the prescense of gas or a surge in the oil level up to the conservator tank. The Buchholz will trip the transformer breakers, but still may not prevent a catastophic failure under extreme circumstances.

We also have pressure relief devices that will open on a pressure increase within the tank, the most modern ones I have seen of these depressurise into an expansion tank that collects the oil and vents off any gas and do prevent the tank from rupturing.

We do have sequencing relays that activate an alarm if a tapchange hasn't been completed withn a specified time, but these don't usually cause a trip. Maybe that needs to be re-thought.

Arc flash and flame resistant clothing is my pet subject. It can certainly aid in reducing injuries but it has to be specified correctly and other clothing worn needs to be compatible with it.

The commentary said that he was covered in burning oil, the problem with FR clothing is that it has a protection of 3 to 5 seconds. If the clothing is subjected to flames longer than this then it will either start to burn, breakup or shrink depending upon what type it is. If you are covered in a burning substances then FR clothing will eventually fail and to my knowledge EDF did provide FR clothing for access switchyards.

We actually have different procedures for tapchangers that were developed after an accident in Teeside around 10 years ago and we actually prohibit manual operation of any tapchanger unless it is isolated, the manual operating handles are removed from the transformer pens and only issued with the permits. Under our rules we may have reset the motor breaker and tried remote operation with everyone in a safe area. If that failed we would have had the transformer taken out of service. If we believed it to be safe, we may have left it inservice until a more convenient time to isolate it. Unfortunately this methodolgy creates issues around availability, but whilst our management are happy with it we can enforce it.

I was always taught never to operate any HV aparatus that was showing signs of distress, it should be isolated and investigated. The problem is how you define that distress, on initially approaching that transformer it would not have looked as though it was in distress, so his actions were understandable if their procedures allowed it. In some instances maybe that term 'distressed' needs to be widened to include certain malfunctions.

It is a very sad case and I don't want to say too much that may upset others. Apologies if I have already managed to do that.

Kind regards

Donald Lane
 27 March 2013 10:07 PM
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Martynduerden

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Only £275,000 fine absolute bloody disgrace.

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Regards

Martyn.

Only a mediocre person is always at their best



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 27 March 2013 11:23 PM
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Alexg

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Very sad incident.

Could someone explain to me something that has been bugging me.

The risk of transformer exploding has been thought about, that's why they was enclosed in a building as a containment. Why was it not thought of then to make the manual operation of the tap changer from outside the building by the use of a longer rod through the wall so there was a barrier between the operator and the equipment? - That might of saved his life as the burning oil would of been dispersed and not poured directly onto him, it seems obvious to me, but have I missed something?

Alex.

 28 March 2013 08:22 AM
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Thripster

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Commiserations to the poor family. The HSE video seems to say that a reduction in the
number of turns results in an increase of voltage - is that correct?

Regards
 28 March 2013 11:30 PM
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alancapon

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Originally posted by: Thripster
. . . The HSE video seems to say that a reduction in the number of turns results in an increase of voltage - is that correct? . . .

Usually. The effect of increasing or reducing the number of turns depends on whether the tap changer is fitted to the HV or LV winding. It would be normal on this type of transformer to fit the tap changer to the HV winding, meaning that a reduction in turns will increase the output voltage. Fitting to the HV side results in lower currents in the tap changer contacts, giving increased life to the contacts as well as allowing them to be physically smaller. The downside, is that a problem will lead to an increased risk of a flashover occurring due to the compact nature of the mechanism.

Regards,

Alan.
 28 March 2013 11:55 PM
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alancapon

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Originally posted by: Alexg
. . . The risk of transformer exploding has been thought about, that's why they was enclosed in a building as a containment. Why was it not thought of then to make the manual operation of the tap changer from outside the building by the use of a longer rod through the wall so there was a barrier between the operator and the equipment? . . .

On a site with multiple transformers, it is usual to build blast walls between them, such that a catastrophic failure of a transformer will not affect other transformers on the site. The usual reason for totally enclosing a transformer is reducing its audible noise, rather than failure containment.

With what appears to be a stuck mechanism, or a mechanism that may not have completed its operation, I would have wanted the transformer isolating on the HV and LV sides before I attempted something that may have caused the tap changer mechanism to move. It is conceivable that a partially complete operation would have led to excessive heating of the insulating oil in the selector compartment, leading to catastrophic failure as soon as the selector contacts separated. The total enclosure round the transformer would have minimised options for escape.

Regards,

Alan.
 29 March 2013 08:23 AM
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Thripster

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Thanks Alan. I'll have to study in more detail.

Regards
 29 March 2013 10:04 AM
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alancapon

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The output voltage of a transformer is dependant on the turns ration between the primary and secondary windings. Adding turns to the primary winding will have exactly the same effect as removing turns from the secondary winding.

Regards,

Alan.
 29 March 2013 01:17 PM
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potential

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I have a query about the video of the fault in the link posted by alancapon.
At the end of the video, about 10 minutes in, it demonstrates how they think the fault occurred.
There is something I don't understand about how the mechanism worked after the main selector drive shaft broke.

If you look at the depiction of the selector chamber after the main selector drive shaft breaks, the lower left and centre contacts continue to move while the operating handle is turned.
I can understand how the lower centre contact continues to move because it is connected to the main selector drive shaft via gearing before the broken part.
However can anyone explain how the lower left contact continues to move while the operating handle is turned when it is disconnected by the break in the main selector drive shaft?
 29 March 2013 01:34 PM
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Thripster

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Yes, understood Alan - just didn't know the tap changer is fitted to the primary.

Regards
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