Buried Power Cables 2

[SETlabguy]

I am in the geotechnical field and was curious as to where electrical engineers are finding testing for the thermal resistivity of soil, and how they found their lab of choice...search engine, etc.

Thanks much

 

[AusLee]

Hi, i hope this helps:

IEEE Standard 81 is the IEEE Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Ground System

 

[dpc]

I don't think you'll find much field testing being done for thermal resistivity. Normally, typical values are used.

The electrical resistivity of soil is often tested, however.

 

[SETlabguy]

DPC,

I am curious as to where the typical values come from. Are they based on soil classification, atterberg limit tests on the soil, density, watercontent? If not field tests, is it common practice to take specimens from the field and send to a lab for actual numbers?

Again, thanks much for your input

 

[dpc]

The National Electrical Code Annex B has some notes regarding typical values. For "average soil" they cite a rho of 90 (degC-cm/watt)and state (with no justification) that this covers 90% of USA. For "very dry soil" they cite a rho = 120.

The technical source for all of this comes from one paper - AIEE Paper 57-660 (aka Neher-McGrath).

So basically, everyone defaults to rho = 90 in soil.

(This is based on the 2002 NEC - I haven't checked the 2005 version on this)

 

[cgrodzinski]

For the purpose of designing of an electrical underground circuit soil studies must be done. It is true that if the time is limited somone may default value of the "rho" to the most often used by others. Unfortunately, this leads to overdesign the cable size - "just to be on a safe side" or cable fault when "rho" value is bigger than anticipated. I found areas where resistivity was rho=6.0[K-m/W] as well as rho=0.4. Usually, before the cable is ordered field and laboratory tests are performed and analyzed, and cable size calculated based on these values. As you are aware, the moisture and compaction play significant role in thermal resistivity value and should be specified as a part of parameters for the cable installation. For the purpose of the transmission voltage level cables the most often used standard for cable calculation is IEC60297. If you would like to discuss more you may contact Geotherm Inc (

http://www.geotherm.net).

They are the best known company that provides such services in this field for electrical utilities. They are located in Aurora, Ontario, Canada.

 

[cuky2000]

The requirement of soil thermal resistivity varies depending of the importance of the UG line, voltage, the amount of power to be delivered and live cycle cost of the installation.

The NEC assumption of average thermal resistivity of 90°C-cm/W in 90% of the USA soil is based on a soil studies performed in the 1950s. The NEC also provide typical values of rho of 120 for very dry soil (rocky & sandy) and rho=60 for damp soil (coastal areas & high water table).

The soil thermal resistivity average values in the NEC appear to be satisfactory to size low voltage and MV feeders and most applications is less expensive to over design the feeder than justify economically a thermal resistivity study.

However, for HV transmission line other driving factors require to determine the soil thermal resistivity as accurate as possible an often use backfill other than native soil to maximize the heat dissipation capability of the cable to improve the line ampacity. Utilities and TL owner often spec one soil sample location every 1000 ft or less at each depth the cable(s).

[sub]Some of standard and reference guide typically used in the USA are:
IEEE Std 442- IEEE Guide for Soil Thermal Resistivity Measurement
EPRI Report TR-108919: Soil Thermal Properties Manual for Underground Power Transmission: Soil Thermal Property Measurements, Soil Thermal Stability, and the Use of Corrective Thermal Backfills
NOTE: There is in progress a draft standard in progress “IEEE P1254- Guide for Soil Thermal Stability Measurements & Data Evaluation”[/sub]

 

[mc5w]

Backfilling with concrete will both establish good thermal conductivity and provide superior protection against machanical damage. The thermal conductivity is more critical near the ducts and once the heat has a chance to spread out the conductivity is not quite so critical.

Please add red dye to the concrete. The concrete should extend up at least a foot above the top duct so that if a backhoe operator should try to dig through he would have to take multiple bites to reach a duct.

 

[cuky2000]

Hi mx5w,

I concur with you that concrete is a good material with low thermal resistivity around 55 °C-cm/W. However, if the rest of the backfill has poor rho this will provide limited help.

Direct buried with low thermal resistivity medium is a lower cost option with superior performance in term of maximizing the cable ampacity.

 

[tinfoil]

cuky2000:
Although some materials other than native backfill do have poorer thermal performance, you are sometimes required to replace with native soil for political/environmental reasons.

Using concrete (and I like the 'red die' suggestion) will at least increase the effective contact area with the native soil to a much larger area, aiding in heat dissipation.

 

[EEJaime]

I was asked to evaluate a lawsuit in which our company was named by the owner of a plastics plant whom claimed that we had undersized a major feeder due to very high local soil rho values. After looking at all the data, I determined this was not the cause, but the vagueness of the soil resistivity issue caused such confusion we almost lost the case. It turned out that an inexperienced installer, omitted a pull box and tried to pull six sets of 500kcmil conductors about a thousand feet, through four ninety degree bends and stressed the insulation to point of failure, but they hired a thermal resistivity lab whom proved that the local rho was slightly above the 90 degree standard we used. I cannot imagine having to test every single project, but if there are any important installations, especially high voltage and high amperage feeders, I would recommend a test or the concrete encased duct bank solution. Another thing this does, (I specify 3" seperation between ducts), is provide adequate spacing so that the ducts do not radiate added heat to each other in multiple conduit banks. We had specified concrete encasement, but the owner deleted it for cost reasons. That in itself got our legal fees payed.

 

[BJC]

The Army Core of Engineers cold regions research lab has some data on the thermal conductivity of soils ( mostly frozen soils and perma frost).
There was a paper published about 20 years ago on thermal conductivity by a fellow at SCE. He used a device that formed a cylinder of soil with a heater in the middle. Both ends of the device were well insulated so all the heat essentially traveled out raidially through the soil. He measure temperature at the inside and outside of the soil cylinder. He developed the test because there was not good data and accurately calculated the rho value. The soils were he was working were unknow and the utility was experiencing cable failures.
It's a very good paper and one should be able to find it by searching the IEEE archives. I am traveling at the moment and cannot get a copy I have at the home 40.
Soils also change seasonially. I know of one case where a duct run in a wet climate was backfilled with pea gravel. The installation never had any trouble untill on dry year in August cable started poping. The duct had been essentially under water untill the water tabel dropped. Then it was in dry gravel, a very good insulator.

 

[mc5w]

For duct banks that more than 1 row of ducts I would use duct spacing of 10 or 12 inches, not 7.5 inches. Spreading out the ducts can make a big difference.

Also, placing a red ribbon 1 or 2 feet above the duct bank does not prevent dig ins. Excavating contractors never ever see those things and keep right on digging. Of course, we charge them through the nose for repairs. About the only way to stop a dig in is to stand around with a Thompson or an Uzi.

 

[cgrodzinski]

I agree that in some cases soil tests may not be economically feasible. However, in reality the “rho” (please note that I use the unit as in the cable calculation standard [K*m/W]) value of concrete can vary depends on its composition and moisture content. It can be as low as 0.4 when moist or as high as 1.0 when dry. Assigning arbitrarily rho=0.55 for cable ampacity calculation can lead to severe overheating when the concrete and surrounding soil dry out caused by i.e. heat from the cable or solar radiation. The same applies to any backfill. Both concrete and backfill have “instability” point based on their moisture contents. Crossing this point could lead to the circuit destruction (famous thermal runaway). For this reason it is nice to know not only the thermal resistivity at the time of measurement but also the seasonal variation of the moisture content.

Since until a few years ago when the cable was buried it was forgotten as there was no reliable equipment to monitor its temperature where and when necessary. Because of this, the only factor that could prevent cable failure caused by overheating was to thoroughly measure the thermal resistivity of the soil surrounding u/g cable circuits. Applying the distributed temperature measuring system can “correct” inadequacy of the project initial phase.

[sub]NOTE: a draft standard in progress “IEEE P1254- Guide for Soil Thermal Stability Measurements & Data Evaluation” - Do you know when it will be finished?[/sub]

 

[cuky2000]

Dear cgrodzinski,

As you pointed out, the correct unit of the thermal resistivity (rho) in SI units is K.m/W (Degree Kelvin. meter per Watts). In US often the rho units are in oC.cm/W also called as “thermal-ohm-feet” in some literature and computer program such as AmpCalc. [sub](Please notice that K.cm/W = oC.cm/W = K.m/100W.)[/sub]

It is also truth the thermal resistivity has significant changes in value not only with moisture but also with soil density (compaction), surface conditions, etc. Typically the worst likely scenario considered in the conventional underground design.

[sub]This lead to roughly estimate the cable temperature during the operation of the underground line. As the technology progress, there is available more accurate temperature monitoring system some of them based in the property of fiber optic reflection with the temperature generated from the power cable. That will help the engineer to minimize the risk especially in hot summer peak demand.[/sub]

Regarding your question about the status of the draft standard P1254 I understand that may be subjected to changes. I do not have any better info in this matter. You may check the IEEE standards administrator, Jodi Haasz at j.haasz@ieee.org

 

[mc5w]

Some places such as Summit County, Ohio and City of Parma, Ohio have banned direct burial electrical services. One problem is the Tree Root Circuit Breaker method where either the tree root puntures the cable or if God or an errant drive knocks over the tree the tree uproots the cable.

Under residential property I use schedule 80 PVC conduit which is a lot stronger than schedule 40. Concrete encasement is not practical in this application and invites use of a digging bar.

Direct burial is a poor economy. By the time that I remove rocks from the trench and backfill, level and compact the bottom of the trench, and so forth I can stick in a conduit and be done with it.

Another issue is that direct burial cables need to conform to the bottom of the trench necessitating a cable length 1.5 to 2 times the trench length. For a 3 pair telephone cable this is easy but for power cables a bit hard to achieve.

 

[k2panman]

We bury electric cable in 4 inch schedule 40 PVC and backfill with pea rock. We space conduits 2" apart. However, we have a lot of wet soil when we dig 4' down for primary (25kV) cable, as most of our service territory is about 6' above sea level.

K2ofKeyLargo