Soil Thermal Conductivity

[dik]

We have a series of direct burried cables (800 sq.mm.) type for a 138 kV Switchyard.

The cables will be bedded in bedding sand; is there anyone that can provide guidance about calculating or determining the thermal conductivity of the soil enclosure. It has an impact on the heat generated within the soil enclosure.

Dik

 

[jghrist]

For 138 kV cable, I recommend getting the thermal conductivity tested or use a thermal backfill with a specified thermal conductivity.

 

[dpc]

Maybe start here.

http://tdworld.com/mag/power_underground_cables_need/

We have worked with Okonite in the past to calculate conductor ampacities for sand backfills. I'm not sure where they get their numbers.

The original Neher McGrath paper may have some information - I don't remember.



David Castor

http://www.cvoes.com

 

[DLite30]

Dy sand is about 0.27 W/m-K, topsoil is about 0.52 W/m-K. Depending on what you're trying to protect, worst case could be either one of these.

 

[EEJaime]

This issue was discussed in cvirgil's post 238-117975 to some extent although that set of conductors was in concrete encased duct banks. I don't think there was ever a definitive response to his query there, but one item I noticed is in one response from cuky2000-he listed a few values of rho for various materials. Perhaps it would be advantageous to utilize another material to backfill around the cables as noted there and also in this thread.

As an aside to this, cuky2000 noted that the rho of concrete is between 60-85 degC cm/watt. If this is a verifiable value, would you feel that on alternative to direct buried cables in sand would be to use concrete encased PVC conduit in which to route the cable?

Just a thought.

Regards,
EEJaime

 

[rcw retired EE]

Cables in conduit may not do as well as direct buried cables due to the heat transfer from the cable through the air to the PVC conduit, conduit to concrete, concrete to soil. I haven't run the scenarios in our underground cable ampacity programs for a while so I don't recall which way is better.

UG ducts do provide other advantages, but it costs more.

 

[dik]

I tried searching for the following thread with no luck... maybe by posting the name here...

thread238-117975

 

[dik]

Yes... it worked...

thanks, Dik