Calculating Shield Resistance of Tape Shield Cable

[sdugre]

I am in the process of calculating sequence impedances for a triplexed 750MCM, 34.5kV, EPR cable with copper tape shield, and am running into some problems.

The cable vendor, Prysmian, provided all the relevant cable data, including a shield resistance of 34.4 ohm/mi, which sounded high to me. I questioned this, and he responded that it was calculated "using contact resistance method from ICEA P-45-482 for helically applied tape, not overlapped" which is A = 1.27*n*w*b, where A is the effective cross sectional area of the shield in cmil, n is the number of tapes, w is the width of tape in mils, and b is thickness of the tape in mils. Since the result is in terms of area, it seems you need the resistivity of the tape shield to derive the resistance. The formula also seems to imply that the shield resistance is independent of the diameter of the shield, which seems wrong to me.

To satisfy my own curiosity, I checked a few text books to see if I could corroborate this. Every source I found used a different formula which does take the shield diameter into account. Here is an example from "Distribution System Modeling and Analysis" by W. Kersting: R = 7.94x10^8*p/(d*T) where R is the shield resistance in ohm/mi, p is the resistivity in ohm-m @ 50C, d is the outside diameter of the shield in inches, and T is the shield thickness in mils. Using the same cable data provided by Prysmian, and a resistivity of 2.3715x10^-8 ohm-m (which is suggested as average in the text), I come up with a shield resistance of 3.36 ohm/mi, which is an order of magnitude smaller than the value provided by Prysmian.

So which method is more correct? Any input on how you calculate shield resistance would be appreciated.

Thanks!

 

[7anoter4]

The formula you picked up from P-45-482 is destined for:
"Helically applied tape ,not overlapped."
For overlapped tape:
A=4*b*dm*sqrt(100/2/(100-L))
You are right : this standard does not calculate resistance but the short-circuit current the shield. So, A is not an actual area-in order to calculate the tape resistance.
These formulas are shown also in :
PRYSMIAN’S WIRE AND CABLE ENGINEERING GUIDE.
A good approximation of tape resistance is in:
EPRI-POWER PLANT ELECTRICAL REFERENCE SERIES VOL. 4 WIRE AND CABLE
PP 4-16 SHIELD IMPEDANCES:
Rs=rs/8/rm/t /10^6[ ohm/ft]
For copper tape rs=30 ohm/cmil at 50oC.t[tape thickness]usually 5 mils.
From the attached sketch one may calculate the resistance as following:
R=4*(1-k/100)/t/pi/DIA*ro [ohm/m]
ro=1/58*(234.5+50)/(234.5+20) [ohm.mm^2/m]
k=overlap %
DIA =ins.shield diameter[mm]
t=tape thickness [usually 0.127 mm]
For k=18% this R match above Rs.

 

[sdugre]

Thank you 7anoter4 for your detailed response.

The EPRI formula you provided is in the same form as the Kersting formula, the difference being attributable to the resistivity assumption. I came up with 7.07 ohm/mi using the 30 ohm-cmil/ft you sited, versus 3.36 ohm/mi if Kersting's assumed resistivity is used. Do you have any insight on the rather large discrepancy between these resistivity assumptions?

Interestingly, I noticed that the EPRI formula is also in the Prysmian Engineering Guide, including the 30 ohm-cmil/ft resistivity assumption for overlapped copper tape. Now to find out why Prysmian isn't following their own guide...

 

[ScottyUK]

At what temperature is the resistivity stated?

 

[sdugre]

50C in each case.