Ducts, cables : influence of soil thermal characteristics

[Foudrette]

Dear All,

I work in the area of underground HV cable links design.
For underground cable ampacity calculation, it is necessary to take into account the thermal behaviour of the soil (this is one of the major parameters). IEC 60287 standard is commonly used to calculate cable ampacities. This standard is suitable for most of the ways of laying. But when you have to cross a road,a railway or a river the burial depth can reach important values (40 feet or 75 feet) for example.

If ampacity calculations are made according IEC 60287, results obtained are very pessimistic because it is assumed there is no change in soil thermal resistivity what is surely false. This leads to choose cable with very big cross-section (to reduce heat due to Joule's losses) what of course dramatically increases the cost of the cable and thus that of the cable link.

Maybe some of you have already studied these problems of soil thermal characteristics simulation.

Thanks for all of your help,
Marie-Hélène

 

[KRSServices]

Foudrette,

I have some experience in the calculation and determination of thermal properties of soils and their respective behaviors. My experience is oriented within the context of municipal utilities and frost protection and therefore may not be exactly what you are referring to. That being stated, one important consideration is the thermal characteristics of the soils.

KRS Services

http://www.krs-services.com

 

[BabbageGeo]

You may find the following of some use:
Thermal Resistivity of Soil


1- Silica at 27 ºC k = 1.3 W/m.ºC, then the Resistivity = 0.77 ºC.m/W
[ 3-263 Perry’s Chemical Engineering Handbook]

2- Soil at 27 ºC k = 0.52 W/m.ºC, then the Resistivity = 1.92 ºC.m/W
[ 3-263 Perry’s Chemical engineering Handbook ]

3- Dry Sand at 20 ºC k = 0.329 W/m.ºC , then the Resistivity = 3.04 ºC.m/W .
[ 3-260 Perry’s Chemical Engineering Handbook ]

4- Sandstone at 40 ºC k = 1.834 W/m.ºC, then the Resistivity = 0.545 ºC.m/W .
[ p21 Heat Transfer, Alan J. Chapman ]

5- Natural ground Resistivity =150 ºC.cm/W = 1.5 ºC.m/W
Backfill Resistivity = 50 ºC.cm/W = 0.5 ºC.m/W
[ IEEE Transaction on Power and Systems, Vol. PAS-98 July/Aug 1979 ]

6- Thermal Conductivity of “mother” soil in Cables trenches = 0.67 W/m.ºC , then the Resistivity = 1.5 ºC.m/W
Thermal Conductivity of the Trench = 2 W/m.ºC , then the Resistivity = 0.5 ºC.m/W
[ IEEE Transactions on Power Delivery, Vol. 8, No. 3 July 1993 ]

These need to be treated with some care. Soil type, moisture content and groundwater flow if any can significantly influence thermal conductivity. In our review of the shallow oil cooled cables in Auckland (NZ) that failed several years ago backfill around trenched cable was more critical than the ground through which the cable was laid. Cable drawn through a pre-drilled hole that is not grouted could result in a significantly different thermal condutivity for the installed system than for a cable in initmate contact with the soil.

Regards

 

[DBNodurf]

These values aren't necessarily useful as defacto values; the properties of real soils are much too variable.

D. Bruce Nothdurft, MSCE, PE, PG, M.ASCE, etc, etc,...
Principal Engineer/Geologist
Atlantic Geoscience & Engineering
Charlotte, NC

 

[Focht3]

Yup. Those are "ballpark" values only.



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