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#135850 - 02/10/03 10:40 AM Question from Brazil  
rmiell  Offline
Member
Joined: Nov 2000
Posts: 242
La Junta, Co. USA
Hey guys, I received the following question recently. Anyone want to give me some input on this? I will forward any replies to the individual who is asking the question.

Thanks in advance.

Rick Miell

"I have an additional question:
Here in Brazil we use in rural areas a piece of metal over the wood structure that support distribution transformers ( until 70 kVA - 13,8 kV - 380-220V - connection delta -Y earthed) in order to give a extra protection against lightning. It works like a Franklin arrester.The question is about the grounding (earthing) of this piece of metal. Do I need to connect this grounding wire to the neutral of transformer ? : what is the correct: to make separate eathing circuits or link together ? What option give the best transformer protection ?

By your attention I previously thank.

A. Kipper "

Rick's take on this: The transformer is connected, now, to the ground (earthed), so that is not a problem.

I would not think one should connect the metal frame directly to the transformer neutral, but to the grounding electrode system.

I believe that multiple wires to a grounding electrode systems is ok, as is one wire that is then looped between the electrodes. Any thoughts on this.

Rick Miell
rmiell@hotmail.com


[This message has been edited by rmiell (edited 02-10-2003).]


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#135851 - 02/11/03 02:22 PM Re: Question from Brazil  
C-H  Offline
Member
Joined: Sep 2002
Posts: 1,497
Stockholm, Sweden
I have to agree with you Rick. I can't see any benfit of having the lightning strike go via the neutral. This seems to work just like an ordinary lightning conductor on a building. What does the code have to say on these?


#135852 - 02/11/03 09:41 PM Re: Question from Brazil  
pauluk  Offline
Member
Joined: Aug 2001
Posts: 7,520
Norfolk, England
Hmm, interesting question.

My instinct tells me to run the grounding lead from this protective device separately down to the electrode(s) in the straightest, shortest route possible. However, even with a very low ground impedance, if the xfmr neutral is bonded to the same electrode system (or even a separate system close by) then the secondary side is still going to rise to a high potential if lightning strikes.

How is lightning protection arranged in places like the Mid-West where lightning storms are quite frequent and violent?


#135853 - 02/14/03 12:13 PM Re: Question from Brazil  
C-H  Offline
Member
Joined: Sep 2002
Posts: 1,497
Stockholm, Sweden
This site www.lightningsafety.com contains a lot of info. This appears to be a rather complex issue.

For Brazil, there is this group http://www.cea.inpe.br/webdge/elat/

Quote

My instinct tells me to run the grounding lead from this protective device separately down to the electrode(s) in the straightest, shortest route possible. However, even with a very low ground impedance, if the xfmr neutral is bonded to the same electrode system (or even a separate system close by) then the secondary side is still going to rise to a high potential if lightning strikes.


Yes. I did give this some thought, and also looked up how much power there is in a lightning strike and got the figure "up to 100kA". (Don't quote me on this!) This means that even if one has a 1 ohm impedance to earth, the potential of the neutral will jump 1ohm x 100kA = 100 kV (!!!) If the neutral is earthed again between the user and the transformer, the voltage to earth will be reduced as there is a voltage drop in the conductor. But, this also means that you get a DC voltage between the phase and neutral. I suppose that it is this voltage that the surge arrestors are designed to handle.

From the above site:
"Radial horizontal arcing has been measured at least 20 m. from the point where lightning enters the earth."

In theory, you could use the earth to "filter out" the lightning, if the lightning conductor and the secondary side of the transformer are on separate earth electrodes. But they need to be far apart, as can be seen from the above quote.

As we all know, it's preferable and often required to bond all earth conductors to the neutral in LV systems as a fault from phase to unbonded metalwork will not blow the fuse, even if earthed. (I think we can rule out the use of Earth leakage devices in this case.) This is in line with Ricks suggestion.

However, I came to think of a possible option. If the lightning conductor is earthed via an insulated wire, the risk of fault between this wire and phase is considerably limited. This would allow for a earth conductor run to an earth electrode some distance away without bonding to the neutral. (The rest of the transformer setup is bonded to neutral as usual.) The wire needs to be insulated to a voltage at least as high as that which will arise between the two electrode systems. As a first approximation, this will be the impedance to earth times lightning current, which has already been calculated to 100kV DC.

Opinions?


#135854 - 02/15/03 11:44 PM Re: Question from Brazil  
Trumpy  Offline


Member
Joined: Jul 2002
Posts: 8,217
SI,New Zealand
Rick,
Is this in an area, that has a system, like the Multiple Earthed Neutral system, if it is, the Neutral should be Earthed by a seperate conductor( 16mm2 or more) and a seperate conductor for earthing the Transformer, itself.
There should be no need to Earth the metal around the X-former, as long as it is bonded to the transformer.
This sole wire, should be run down the pole and to at least 2 earth stakes driven to at least 2.5-3metres, deeper if the soil conditions are poor.
Hope this helps. [Linked Image]


Let's face it, these days if you're not young, you're old - Red Green grin

#135855 - 05/19/03 04:06 PM Re: Question from Brazil  
C-H  Offline
Member
Joined: Sep 2002
Posts: 1,497
Stockholm, Sweden
I know it's long ago this was current, but I stumbled upon a scientific paper dealing with this matter. It's from Sri Lanka, but the theory should work for the whole world.

(Don't open this if you aren't really interested: It is a scientific paper)
http://elect.mrt.ac.lk/Lightning_IEE_99.pdf

It turns out that Trumpy was right. I was wrong. [Linked Image] You need two earth rods, which are separate from the neutral. This means that the the transformer insulation is exposed to the lightning voltage and must withstand this voltage.

Apparently, lightning is reposible for many transformer failures.


#135856 - 05/19/03 11:26 PM Re: Question from Brazil  
Bjarney  Offline
Moderator
Joined: Apr 2002
Posts: 2,527
West-Southern Inner-Northeast ...
Some USDA/RUS practices are referenced at: http://www.usda.gov/rus/regs/bulls/1728f803.pdf sections G & H
[>10MB]

Manufacturer literature—one of many http://www.hubbellpowersystems.com/powertest/catalog_sections/PDF_ohio/31_dynavar.pdf
[1MB]

US utilities, and maybe some in other countries, are starting to put their construction drawings and specs online accessible to others, but you’ll have to search.

Try posting at http://www.powerlineman.com/

Information is available more easily than it used to be, but it’s critical to decide what best fits your application.

If lightning performance/minimization of damage and outages are critical to your operation, small details can have a big effect on service continuity {and profits.} There is no way to eliminate 100% of lightning problems, but using resources and techniques to best limit its effects, and expenses that follow.



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