Underground Cable Impedance-in Duct 1

[Humble2000]

I know how to calculate underground cable impedances if they are direct buried.
What factors needs to be considered if they are run inside a Duct or RPVC conduit?
The first factor will be P( Soil resistivity) which cable is not in contact with it anymore.
The second factor that I can think of is the affect of barrier(duct or conduit) in mutual impedance calculation as the GMR is different now.
I appreciate any feedback on this subject.
Regards,

 

[davidbeach]

I don't have it at hand to check, but I believe that the IEEE Brown Book has some material on this.

 

[CFinley]

The best source of information I've come across to calculate impedance for underground cables is the Cooper Power System's
Electrical Distribution System Protection book.

The book cost me $50, but you might be able to obtain a free copy from your local Cooper Power Systems sales rep.

Pirelli also has a ready-made list of duct or direct-buried cable impedances (Z1 & Zo) as well as ampacities. The U.S. factory has a different name now since Pirelli sold it to a UK company.

 

[rcw retired EE]

Check the Okonite website. It has the calculations for cable impedances in duct.

Am I missing something? Wouldn't the impedance in duct underground be the same as the impedance in open air? Assuming that the duct is the same PVC or steel material in both cases and that the surrounding earth is non-magnetic.

The thermal ampacity and operating temperature at a given ampacity will be different, but the impedance should be the same. Right?

 

[dpc]

I think he is asking about the zero sequence impedance, but that isn't too clear.

 

[Humble2000]

Calculation for Positive and Zero sequence will consider the distance between Phase conductors as well as neutral to phase conductor distances.

Well, according to formulas, the ground resistivity is a factor in calculating cable impedances, so impedance in air and underground would be different.

 

[jghrist]

If the distances are the same between direct buried and non-metallic conduit installation, there will be no difference in the positive- or zero-sequence impedance. The conduit does not change the soil resistivity. There is a difference if the cable is grounded at both ends or not. There is no difference in the calculations between having the shield/neutral grounded at both ends or continuously in contact with earth. Most direct buried installations have the shield under a jacket anyway and not in direct contact with the earth.

 

[Humble2000]

Book says they are different. Z0 and Z1 are calculated based on ZII and ZIJ .Z1=Zii-Zij and Z0=Zij+2Zij

 

[jghrist]

So which of the variables in your linked equations are different for cable in conduit and direct buried cables if the distance between cables is the same? The presence of conduit does not change the soil resistivity.

 

[Humble2000]

To my understanding, when a cable is in a duct or PVC conduit it’s not in direct contact with soil. Now, Dij is calculated based on Concentric neutral and phase conductor distances from each other.
I have a feeing that since they are two different resistivity exists between them the Dij might be different.
Well, these are just assumptions and my understanding of the situation and the purpose of this post is to get an expert opinion.

Thanks

 

[jghrist]

D_ij is calculated based on the distances between conductors regardless of whether they are in air or soil.
I suppose you could take the difference in relative permeability into account. I can't find the relative permeability of soil, but it might make a difference of a few thousanths of a percent in the reactance if there were a large spacing between conductors. Between the phase conductor and the concentric neutral is insulation anyway, not air or soil.