Zero Sequence Impedances of Cables

[jstickley]

Can anyone provide a quick tutorial on how cable installation affects the zero sequence impedances of three-phase cables?

I've been referencing the Westinghouse Transmission and Distribution Reference Book in an attempt to figure out how to appropriately model a 4160 V three-phase underground cable for a new start-up source to a generator (I need to determine available fault current at the start-up bus). The following excerpt spurs my question:

"The method of bonding and grounding... has an effect upon the zero-sequence impedance of cables. An actual cable installation should approach one of these three theoretical conditions:

1. Return current in sheath and ground in parallel.
2. All return current in sheath, none in ground.
3. All return current in ground, none in sheath."

I'd appreciate any guidelines forum members can provide to help me determine which assumption is true for the particular installation I'm trying to model (examples of installation practices that correspond to each theoretical condition would be wonderful). I have cable impedance information from the manufacturer, but want to ensure that our application matches the assumptions used in their calculations before blindly using the data provided.

Thanks....

 

[jbartos]

Suggestion: Often, 4.16kV cables have the shield grounded at both ends, which forms the shield and ground in parallel.