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3P Fault current Vs SLG

Discussion in 'General Discussion' started by SirSpark, Feb 21, 2009.

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  1. SirSpark Junior Level

    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.

  2. acobb Sparks Level

    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.
  3. SirSpark Junior Level

    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
  4. acobb Sparks Level

    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.
  5. SirSpark Junior Level

    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.
  6. jghrist Sparks Level

    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.
  7. SirSpark Junior Level

    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....
  8. acobb Sparks Level

    You should suspect something is wrong with a delta - solidly grounded wye if the L to G is not the highest available.
  9. SirSpark Junior Level

    Sure enough I had my neutral impedance values incorrect on the transformers...
  10. acobb Sparks Level

    Good to get it figured out now instead of later!!! Glad it helped.
  11. SirSpark Junior Level

    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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