Grounding

[cruiser700]

Hello All,

I have a question about power distribution grounding. Ok so i have a 4 mile run and per NESC i am to place a ground point every quarter mile or 4 per mile. So then if i want to calculate my total system ground resistance wouldn't that just be all my ground point resistances in parallel? The specs i have are each driven ground rod is gonna be 25 ohms, and the substation ground grid is 2 ohm, #4 ACSR has a resistance of .519 ohm per 1000 ft.

 

[3I0]

It wouldn't be purely a parallel connection as that would imply your equivalent distribution line grounding impedance approaches zero with the more shunt connections to ground, which isn't the case in reality.

The line would form a series-parallel impedance network, with shunt impedances to ground and series interconnecting line impedances. With a typical distribution line grounding application such as this, after a certain number of segments where the series/shunt impedances are the held constant, adding additional segments results in a minimal change to the terminal driving point impedance.

There are closed-form formulas you can use to calculate the driving point impedance if the line is long enough (which should be verified). Also note that the inductive reactance of the overhead line shouldn't be ignored in this calculation.

 

[7anoter4]

I agree with va3mnr.
In IEEE 80/2000 ch.15.9 Computation of current division it is an example using Endrenyi method.
Endrenyi's formula [was in IEEE-80/86 pp. 102 ]:
Zeq=0.5*Zs+sqrt(Rtg*Zs)
See Annex C ch.C.4 Equations for computing lines impedances
Where Zs=simple self impedance with earth return [see Annex C formula C7]
Rtg=tower footing ground resistance
or more developed[for instance]:

http://www.doiserbia.nb.rs/img/doi/0353-3670/2006/0353-36700601071V.pdf