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"Defect Intensity" for Substation Grounding System (IEEE 80-2000)

"Defect Intensity" for Substation Grounding System (IEEE 80-2000)

"Defect Intensity" for Substation Grounding System (IEEE 80-2000)

(OP)
Hi!

I'm designing a grounding system for a substation. I'm using IEEE 80-2000.
As initial data, I've "If" (defect intensity) to substation ground, wich is to 30 metres from where the grounding system will be placed. If=34,86kA  

"I'm doing numbers" and the grounding system I get, is disproportionate...

Is this "If" which I must use for calculations, or I need the "If" to that 30 metres??


I have two paths:

A) "If" I need is "If" to substation ground, but I do something wrong, because the grounding system is disproportionate...

B) "If" I need is "If" to that 30 metres where grounding system will be placed, but I don't know calculate it, because I don't have the impedance of the transformer, detailed into "x" "y"...

Someone could help me?
All comments will be wellcome.

Best regards and thanks in advance!

RE: "Defect Intensity" for Substation Grounding System (IEEE 80-2000)

I'm not familiar with the term "defect intensity."  I also don't know what you mean by the grounding system is "disproportionate."  I also cannot figure out what is 30 meters from the grounding system.  I don't know what you mean by the "impedance of the transformer, detailed into 'x' 'y'...".

I'm guessing that you are wanting to determine the current division between the ground grid and neutrals or shield wires.  See Chapter 15 of IEEE Std 80-2000.  If you have any specific questions, please frame them in terms used in IEEE Std 80.
 

RE: "Defect Intensity" for Substation Grounding System (IEEE 80-2000)

(OP)
Thanks and sorry for my technical english!!

OK, let me see, maybe with a pic:


RE: "Defect Intensity" for Substation Grounding System (IEEE 80-2000)

I see now.

If you have a grounding analysis program that will do it, the best way is to analyze the substation grid and the new grounding system as a single system.  The presence of the new grounding system will change the portion of If that flows through the earth (Ig).  Besides this complicating factor, the current density in the new grounding system will be different than in the substation grid.

If you cannot analyze the combined system as one system, then you need to estimate the mutual influences of the separate parts.  See Design of Switchyard Grounding Systems Using Multiple Grids, Stephen W, Kercel, IEEE Transactions on Power Apparatus and Systems, Vol PAS-100, No. 3, March 1981.
 

RE: "Defect Intensity" for Substation Grounding System (IEEE 80-2000)

I agree with jghrist suitable software will be better.
The "DESIGN OF SWITCHYARD GROUNDING SYSTEMS USING MULTIPLE GRIDS"-by Stephen W. Kercel was published in 1981 and never the less was deducted from Schwartz formulae published in 1954 the factor as K1 , K2, no  suffered a lot of  change so compared with the last IEEE-80 from 2000 the result will be at least 20% different.
Also the Kercel method permits the total grid assemble [up to 3 grids together but not far apart] resistance and GPR. The current has to be shared according to other criteria as:
-constant current density per meter length of conductor
-the same GPR for all grids and other.
 

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