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 Post subject: 3P Fault current Vs SLGPosted: Sat Feb 21, 2009 5:28 pm

Joined: Thu Jan 29, 2009 2:06 am
Posts: 29
So I was looking over a study done by another firm and I noticed something that seemed odd. The single line to ground (SLG) fault current was almost as much as the 3 Phase fault current.

To me this does not make sense. For the sake of discussion let's say the available 3 Phase fault current was 53,000 they show a SLG of 50,000. Is there any thing that could possible cause this? The system is 480 and the utility feeds are 25kV into three 2500kVA TX to 480 with three utility input feeds at 4000A to the facility on three different busses.

thoughts?

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 Post subject: Posted: Sat Feb 21, 2009 5:39 pm
 Sparks Level

Joined: Sun Dec 23, 2007 1:44 pm
Posts: 348
Location: Charlotte, NC
Abaolutely makes sense. If the supply transformer is delta-wye, the line to ground fault duty would be higher than the three phase at the secondary. This occurs because the only zero sequence impedance you have at the secondary of the transformer is the transformer Z. None of the zero sequence from the primary transfers from the delta connection.

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 Post subject: Posted: Sat Feb 21, 2009 5:57 pm

Joined: Thu Jan 29, 2009 2:06 am
Posts: 29
I thought three phase bolted fault would be the worse case scenario?

And would therefore be able to supply a higher available fault current. Which it does, I suppose I just assumed there to be a greater difference.

I mean isn't the 3 Phase fault based on LLLG or is it just LLL

I think I confused myself...lol

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 Post subject: Posted: Sat Feb 21, 2009 6:12 pm
 Sparks Level

Joined: Sun Dec 23, 2007 1:44 pm
Posts: 348
Location: Charlotte, NC
Well, it is actually true that a delta-wye without a resistance grounded secondary will have a higher L to G than 3Ph. fault current. It will generally be higher downstream of the transformer until the negative sequence Z builds to the point that it equals the positive sequence.

If it is a wye-wye connection then this will not be the case.

IE doesn't necessarily follow that trend though if you are expecting a three phase fault instead of a single phase fault.

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 Post subject: Posted: Sat Feb 21, 2009 6:48 pm

Joined: Thu Jan 29, 2009 2:06 am
Posts: 29
Hence symmetrical fault. I just figured that more "power" would be involved in a LLL than a SLG fault. I guess I was confusing IE with available fault current.

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 Post subject: Posted: Sun Feb 22, 2009 1:11 pm
 Arc Level

Joined: Wed Jun 04, 2008 9:17 am
Posts: 428
Location: Spartanburg, South Carolina
SirSpark wrote:
Hence symmetrical fault. I just figured that more "power" would be involved in a LLL than a SLG fault. I guess I was confusing IE with available fault current.

There probably will be more energy involved in a LLL than SLG arcing fault even if the SLG fault current is higher. There are at least two arcs involved in a LLL fault and only one in a SLG fault.

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 Post subject: Posted: Sun Feb 22, 2009 3:04 pm

Joined: Thu Jan 29, 2009 2:06 am
Posts: 29
Yeah it all makes perfect sense now I dont know what I was thinking...

I mean It is not Ia+Ib+Ic so why would fault current be any different....

So then if we are talking delta-wye and similar conditions as previously stated one should suspect something is wrong if the SLG was significantly lower than the 3P....

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 Post subject: Posted: Sun Feb 22, 2009 5:06 pm
 Sparks Level

Joined: Sun Dec 23, 2007 1:44 pm
Posts: 348
Location: Charlotte, NC
You should suspect something is wrong with a delta - solidly grounded wye if the L to G is not the highest available.

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 Post subject: Posted: Mon Feb 23, 2009 4:17 pm

Joined: Thu Jan 29, 2009 2:06 am
Posts: 29
acobb wrote:
You should suspect something is wrong with a delta - solidly grounded wye if the L to G is not the highest available.

Sure enough I had my neutral impedance values incorrect on the transformers...

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 Post subject: Posted: Mon Feb 23, 2009 6:12 pm
 Sparks Level

Joined: Sun Dec 23, 2007 1:44 pm
Posts: 348
Location: Charlotte, NC
Good to get it figured out now instead of later!!! Glad it helped.

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 Post subject: Posted: Tue Feb 24, 2009 4:46 pm

Joined: Thu Jan 29, 2009 2:06 am
Posts: 29
acobb wrote:
Good to get it figured out now instead of later!!! Glad it helped.

Yeah I don't know what I was thinking, I was probably thinking to many things at once, the more I look at it the more it totally makes sense.

At least I knew enough that something was odd in my calculations to further research it....

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