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 Post subject: 3P Fault current Vs SLG
PostPosted: Sat Feb 21, 2009 5:28 pm 
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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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PostPosted: Sat Feb 21, 2009 5:39 pm 
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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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PostPosted: Sat Feb 21, 2009 5:57 pm 
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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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PostPosted: Sat Feb 21, 2009 6:12 pm 
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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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PostPosted: Sat Feb 21, 2009 6:48 pm 
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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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PostPosted: Sun Feb 22, 2009 1:11 pm 
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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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PostPosted: Sun Feb 22, 2009 3:04 pm 
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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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PostPosted: Sun Feb 22, 2009 5:06 pm 
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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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PostPosted: Mon Feb 23, 2009 4:17 pm 
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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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PostPosted: Mon Feb 23, 2009 6:12 pm 
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Good to get it figured out now instead of later!!! Glad it helped.


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PostPosted: Tue Feb 24, 2009 4:46 pm 
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acobb wrote:
Good to get it figured out now instead of later!!! Glad it helped.


Thanks for your help...

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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