HotHot--Spots in Underground Spots in Underground Power Cable SystemsPower Cable Systems

R. A. Hartlein, NEETRAC R. A. Hartlein, NEETRAC -- School of ECESchool of ECEW. Z. Black, School of M. E.W. Z. Black, School of M. E.

Georgia Institute of TechnologyGeorgia Institute of TechnologyAtlanta GeorgiaAtlanta Georgia

Basic ProblemsBasic ProblemsThe rating of cables (ampacity) is only as good as the The rating of cables (ampacity) is only as good as the rating calculated for a set of worst case thermal rating calculated for a set of worst case thermal conditionsconditions

There are many practical situations that are not covered There are many practical situations that are not covered by the Ampacity Tables (IEEE Std 835by the Ampacity Tables (IEEE Std 835--1994). 1994).

Guidance is needed for these Guidance is needed for these ““nonnon--standardstandard”” cases. The cases. The Ampacity tables have only two values of soil resistivity Ampacity tables have only two values of soil resistivity and donand don’’t address thermal stability or thermal backfill.t address thermal stability or thermal backfill.

The location of a The location of a singlesingle weak thermal link can lead to a weak thermal link can lead to a localized hot spot that requires derating of the localized hot spot that requires derating of the entireentirecircuit.circuit.

Causes of Weak Thermal LinksCauses of Weak Thermal LinksDirect buried circuits with short sections of cable in Direct buried circuits with short sections of cable in conduit (road crossings)conduit (road crossings)

Locations where underground cables transition to Locations where underground cables transition to overhead lines via overhead lines via ““riser sectionsriser sections””

Cable installations with sections of high thermal Cable installations with sections of high thermal resistivity soil resistivity soil –– sections where thermal backfill is usedsections where thermal backfill is used

Areas where soil is thermally unstableAreas where soil is thermally unstable

Areas where cable route passes near external heat Areas where cable route passes near external heat sources and forced cooling methods are usedsources and forced cooling methods are used

Studies at Georgia Tech were Studies at Georgia Tech were conducted to establish the conducted to establish the significance of these problems significance of these problems and to provide potential and to provide potential solutions.solutions.

Short Sections in ConduitShort Sections in ConduitThe ProblemThe Problem

Direct buried cables are often routed through short Direct buried cables are often routed through short segments of conduit when the cable must cross under a segments of conduit when the cable must cross under a road, sidewalk or similar structure. road, sidewalk or similar structure.

When this situation occurs, the ampacity must be When this situation occurs, the ampacity must be reduced because conduit thermal resistance is higher reduced because conduit thermal resistance is higher than the thermal resistance for the directly buried cable. than the thermal resistance for the directly buried cable.

The important question is: The important question is: ““How much should it be How much should it be reduced?reduced?””–– For long segments in conduit: circuit rating must be equal to thFor long segments in conduit: circuit rating must be equal to the e

rating of the cable in conduit. rating of the cable in conduit. –– For short conduit segments: it is reasonable to expect that someFor short conduit segments: it is reasonable to expect that some

of the heat generated inside the conduit will dissipate along thof the heat generated inside the conduit will dissipate along the e cable axis into the portion of cable directly buried in the eartcable axis into the portion of cable directly buried in the earth. h.

Short Sections in ConduitShort Sections in ConduitThe SolutionThe Solution

The finite element software package ANSYS was used to The finite element software package ANSYS was used to solve for the shortsolve for the short--section desection de--rating factors. rating factors.

The fact that this program has three dimensional The fact that this program has three dimensional capabilities is important, because the decapabilities is important, because the de--rating factors rating factors must include the heat that is conducted along the axial must include the heat that is conducted along the axial direction of the cable. direction of the cable.

The axial conduction promotes cooling of the cable The axial conduction promotes cooling of the cable segment inside the conduit. segment inside the conduit.

Short Sections in ConduitShort Sections in ConduitAssumptionsAssumptions

Steady stateSteady stateAll properties as constantAll properties as constantNo current in cable metallic shield, dielectric losses are negliNo current in cable metallic shield, dielectric losses are negligiblegibleNonNon--metallic layers in the cable construction are lumped into a one,metallic layers in the cable construction are lumped into a one,thermally equivalent layer.thermally equivalent layer.The conduit is thin and its thermal resistance is close to that The conduit is thin and its thermal resistance is close to that of the soil.of the soil.The thermal resistance of the air layer is the only quantity thaThe thermal resistance of the air layer is the only quantity that varies with the t varies with the temperature.temperature.The thermal resistivity of the soil is 90 The thermal resistivity of the soil is 90 cmcm°°CC /W and the ambient soil /W and the ambient soil temperature is 25temperature is 25°°C unless otherwise stated.C unless otherwise stated.The vertical plane through the cable centerline and the verticalThe vertical plane through the cable centerline and the vertical plane plane perpendicular to the cable axis at the center of the conduct areperpendicular to the cable axis at the center of the conduct are adiabatic adiabatic planes.planes.Regions of the soil that are far from the cable are maintained aRegions of the soil that are far from the cable are maintained at the ambient t the ambient soil temperature.soil temperature.The cable is located concentrically within the conduit. The cable is located concentrically within the conduit.

Short Sections in ConduitShort Sections in ConduitThe ModelThe Model

Typical Finite Element Grid for OneTypical Finite Element Grid for One--fourth of the Problem Geometryfourth of the Problem Geometry

H

D

L

r

d

Conductor, T c

Soil, ρsoil

Conduit, T cond, thickness negligibleAir layer, Tair

Cable insulation,

Earth/Air interface, Ta

The ModelThe Model

Short Sections in ConduitShort Sections in ConduitModel VerificationModel Verification

84.3384.3384.3284.32604604604604Three Cables in Three Cables in ConduitConduit

106.4106.4102.4102.4678678666666Three Cables, Three Cables, DirectDirect--BuriedBuried

56.0456.0454.4654.46867867855855Single Cable in Single Cable in ConduitConduit

72.7072.7075.7875.7899399310081008Single Cable, Single Cable, DirectDirect--BuriedBuried

Heat Generated Heat Generated Finite ElementFinite Element

(W/m)(W/m)

Heat Generated Heat Generated CMCAP CMCAP (W/m)(W/m)

Ampacity Ampacity Finite Finite

Element Element (amps)(amps)

Ampacity Ampacity CMCAP CMCAP (amps)(amps)

InstallationInstallation

Short Sections in ConduitShort Sections in ConduitResultsResults

0.85

0.90

0.95

1.00

0 10 20 30 40L/D

DF3 cables in a conduit

1 cable in a conduit

Cable Derating Factor as a Function of Dimensionless Conduit Length, L/D, for Single and Triplexed Cables. 35 kV cable with a 750 kcmil (380 mm2) aluminum conductor buried at a depth of 0.914 m in a soil with a thermal resistivity of 90cmºC/W and an ambient temperature of 25ºC.

Short Sections in ConduitShort Sections in ConduitResultsResults

0.70

0.80

0.90

1.00

0 10 20 30 40L/D

DF

ρsoil/ρinsul = 0.45

ρsoil/ρinsul = 0.25

ρsoil/ρinsul = 0.15

Cable Derating Factor as a Function of Dimensionless Conduit Length, L/D, for Several Soil Resistivities. Single 35kV, 750kcmil (380 mm2) aluminum conductor cable buried in 25ºC soil to a depth of 0.914m. The conduit is 152 mm in diameter and the equivalent thermal resistivity of the cable insulation layers is 350 cmºC/W.

Cables in RisersCables in RisersThe ProblemThe Problem

For many years, the For many years, the ““riser cableriser cable”” heat transfer problem heat transfer problem was not covered by ampacity calculation programswas not covered by ampacity calculation programs

The ampacity of a cable in a riser is potentially reduced The ampacity of a cable in a riser is potentially reduced because:because:–– Cables in risers cannot dissipate heat as readily as other Cables in risers cannot dissipate heat as readily as other

environmentsenvironments

–– The riser is exposed to solar radiation, further increasing the The riser is exposed to solar radiation, further increasing the cable temperaturecable temperature

–– The riser is often not vented, trapping heat around the cable The riser is often not vented, trapping heat around the cable

Three Cases

I Closed Top and Bottom

II Open Top and Bottom

III Open Top and Closed Bottom

Cables in RisersCables in RisersThe ModelThe Model

Cables in RisersCables in RisersAssumptionsAssumptions

Steady StateSteady StateRiser and cable are concentricRiser and cable are concentricRiser length is much greater than the diameterRiser length is much greater than the diameterNo axial or No axial or azimuthalazimuthal temperature variations (One temperature variations (One dimensional problem)dimensional problem)Thermal properties of cable and riser are constantThermal properties of cable and riser are constantThermal & fluid properties of air inside riser are bulkThermal & fluid properties of air inside riser are bulkFor 3 cable case, cables are lumped together into For 3 cable case, cables are lumped together into equivalent single cable.equivalent single cable.Cable conductor is isothermalCable conductor is isothermalIncident solar radiation is normal to the riserIncident solar radiation is normal to the riser100% load factor100% load factor

Cables in RisersCables in RisersConvective AssumptionsConvective Assumptions

Cables in RisersCables in RisersResults Results -- Experimental VerificationExperimental Verification

Case I

Cables in RisersCables in RisersResults Results –– Experimental Verification Experimental Verification

Case II Case III

Cables in RisersCables in RisersResultsResults

Cables in RisersCables in RisersResultsResults

Single Cable Per Riser Three Cables Per Riser

Thermal Stability of SoilsThermal Stability of Soils

Heat generated in cables drives moisture away Heat generated in cables drives moisture away from cable, drying soil next to cablesfrom cable, drying soil next to cablesDry soils have much higher thermal resistivity Dry soils have much higher thermal resistivity than moist soils. With sufficiently high ampacity than moist soils. With sufficiently high ampacity thermal runthermal run--away conditions can occur.away conditions can occur.If current is high enough, generating sufficient If current is high enough, generating sufficient heat and if it is maintained for a long enough heat and if it is maintained for a long enough time, soil will become unstable and the circuit time, soil will become unstable and the circuit will have to be derated or it will overheat. will have to be derated or it will overheat.

Thermal Resistivity of SoilsThermal Resistivity of Soils

0

50

100

150

200

250

300

350

0 10 20 30 40

Moisture Content - %

Res

istiv

ity -

cm C

/W

Georgia ClayDry Density = 80 lb/ft3

Measurement of Thermal StabilityMeasurement of Thermal Stability

To take thermal stability into account, limits of To take thermal stability into account, limits of stable conditions must be identifiedstable conditions must be identifiedStability can be measured with thermal needle Stability can be measured with thermal needle (IEEE Std(IEEE Std--442, 1981)442, 1981)Needle measurement will yield a measure of Needle measurement will yield a measure of both the thermal resistivity and thermal stability both the thermal resistivity and thermal stability of the soilof the soilStability depends on the soil properties, the Stability depends on the soil properties, the ampacity (heat generated) of the cable and the ampacity (heat generated) of the cable and the duration of applied heatduration of applied heat

Data from Thermal NeedleData from Thermal Needle

Slope of log time vs needle temperature is Slope of log time vs needle temperature is measure of thermal resistivity of soilmeasure of thermal resistivity of soilLinear sections of data give dry and moist Linear sections of data give dry and moist thermal resistivitythermal resistivityIf heat generated in needle is sufficient to dry If heat generated in needle is sufficient to dry soil sample, needle test can be used to measure soil sample, needle test can be used to measure thermal instabilitythermal instabilityStability is measured by recording heat input to Stability is measured by recording heat input to needle and time required for sample to dryneedle and time required for sample to dry

01

log time

Prob

e Te

mpe

ratu

re

Sample is moist

Sample is dry

Sample is drying

Dry Resistivity

Moist Resistivity

Heat Input to Probe is Constant

Measurement of Thermal StabilityMeasurement of Thermal Stability

Needle is energized to same power level Needle is energized to same power level as cablesas cablesSmall size of needle concentrates heat Small size of needle concentrates heat and accelerates drying process (stability)and accelerates drying process (stability)The time required for needle to begin The time required for needle to begin drying soil is related to time for cables to drying soil is related to time for cables to dry same soil by: (t/ddry same soil by: (t/d22))needle needle = (t/d= (t/d22))cablecable

Use of External CoolingUse of External Cooling

Localized hot spots may be mitigated by Localized hot spots may be mitigated by several methods including burying heat several methods including burying heat pipes near the cables, using cooling pipes near the cables, using cooling provided by external sources and using provided by external sources and using thermal backfillthermal backfillOne possible means of cooling by external One possible means of cooling by external sources is to use chilled water circulated sources is to use chilled water circulated through pipes placed near the cablesthrough pipes placed near the cables

External Forced Cooling Test ConditionsExternal Forced Cooling Test Conditions

Outdoor, fullOutdoor, full--scale test of three horizontally scale test of three horizontally buried 25 kV, 1000 kcmil aluminum conductor buried 25 kV, 1000 kcmil aluminum conductor cables on 9 inch centerscables on 9 inch centersCables were buried in a wellCables were buried in a well--graded thermal graded thermal backfillbackfillCooling provided by 10,000 Btu/hr capacity Cooling provided by 10,000 Btu/hr capacity chiller which circulated water by a 1/25 hp pump chiller which circulated water by a 1/25 hp pump in two in two ¾¾ in PVC closed loop pipes buried in PVC closed loop pipes buried between the cablesbetween the cablesCable and water temperatures were continually Cable and water temperatures were continually monitoredmonitoredFour tests were completed at 500, 900, 1050 Four tests were completed at 500, 900, 1050 and 1200 amps until steadyand 1200 amps until steady--state conditions state conditions were reachedwere reached

External Forced Cooling ResultsExternal Forced Cooling Results

Average drop in conductor temperatures when Average drop in conductor temperatures when chilled water was circulated: chilled water was circulated:

88°°C for 500 amp testC for 500 amp test1818°°C for 900 amp test C for 900 amp test 1818°°C for 1050 amp testC for 1050 amp test3939°°C for 1200 amp test C for 1200 amp test

Forced cooling is economically feasible for only Forced cooling is economically feasible for only short segments where cables are routed through short segments where cables are routed through a thermal bottleneck (a steam line or other heat a thermal bottleneck (a steam line or other heat source) or for longer segments but only during source) or for longer segments but only during times when cables are loaded beyond their times when cables are loaded beyond their normal rating, perhaps during seasonal peak normal rating, perhaps during seasonal peak loading periods.loading periods.

Use of a Corrective BackfillUse of a Corrective BackfillWhen soil is known to be thermally unstable or When soil is known to be thermally unstable or has a high thermal resistivity, use of a low has a high thermal resistivity, use of a low resistivity, stable backfill is recommendedresistivity, stable backfill is recommendedMost backfills consist of uniformly graded Most backfills consist of uniformly graded granular materials with a weak binder to assure granular materials with a weak binder to assure good thermal contact between grains and good thermal contact between grains and eliminate the possibility of moisture movement in eliminate the possibility of moisture movement in the backfillthe backfillGuidance is needed for determining the rating of Guidance is needed for determining the rating of cables buried in a thermal backfillcables buried in a thermal backfill

Thermal Backfill ProjectThermal Backfill Project

Finite element program used to determine a Finite element program used to determine a single value of the single value of the effective thermal resistivityeffective thermal resistivityof the complex cable environment consisting of of the complex cable environment consisting of native soil, thermal backfill and protection layer.native soil, thermal backfill and protection layer.The The effective thermal resistivityeffective thermal resistivity is designed to is designed to replace the soil resistivity used in the traditional replace the soil resistivity used in the traditional ampacity tables, which then results in a value for ampacity tables, which then results in a value for the ampacity of cables buried in the thermal the ampacity of cables buried in the thermal backfill. Goal was to expand the use of the backfill. Goal was to expand the use of the ampacity tables to the case of thermal backfills.ampacity tables to the case of thermal backfills.

Surface of Earth

Native Soil, ρns Native Soil. ρns

Protection Layer, ρpl

Thermal Backfill, ρbf

Cables

Native Soil, ρns

Thermal Circuit for Finite Element Program

Typical Results for Thermal BackfillTypical Results for Thermal Backfill

Three Three -- 3 in O.D. cables buried to a depth 3 in O.D. cables buried to a depth of 39 in on 6 in centersof 39 in on 6 in centersTrench width 27 in, depth 48 in, backfill Trench width 27 in, depth 48 in, backfill depth 18 in, protection layer depth 6 indepth 18 in, protection layer depth 6 inThermal resistivity of native soil is 150, Thermal resistivity of native soil is 150, backfill is 50 and protection layer is 60 cm backfill is 50 and protection layer is 60 cm °°C/WC/W

Results for Effective ResistivityResults for Effective ResistivityEffective Resistivity for Cables in Corrective

Backfill

108110112114116118120122124126

4 5 6 7 8 9 10 11

Centerline Cable Spacing, inches

Effe

ctiv

e Re

sist

ivity

, cm

C/W

Center Cable

Outer Cables

Experimental VerificationExperimental VerificationFinite element results were verified with test Finite element results were verified with test results from a fullresults from a full--scale outdoor buried cable testscale outdoor buried cable testTest conditions were: Three 25 kV, 1000 kcmil Test conditions were: Three 25 kV, 1000 kcmil aluminum on 9 in centers, 48 in deep. Trench aluminum on 9 in centers, 48 in deep. Trench 36x60 in. Backfill 24 in thick, protection layer 6 in 36x60 in. Backfill 24 in thick, protection layer 6 in thickthick36 T/Cs measured temperatures continually36 T/Cs measured temperatures continuallyWhen effective resistivity was used in ampacity When effective resistivity was used in ampacity tables, predicted and measured interface tables, predicted and measured interface temperatures were within 1temperatures were within 1°°C. C.

ConclusionsConclusions

Previous work has examined five common Previous work has examined five common cases which can result in thermal hot spots cases which can result in thermal hot spots (weak links in the thermal chain) so that cable (weak links in the thermal chain) so that cable ratings can be determined for these complex ratings can be determined for these complex thermal circuitsthermal circuits

1.1. Short sections of high resistivity soilShort sections of high resistivity soil2.2. Transition to overhead lines via riser sectionsTransition to overhead lines via riser sections3.3. Cable routes through thermally unstable soilCable routes through thermally unstable soil4.4. High thermal resistivity segments which can benefit High thermal resistivity segments which can benefit

from external coolingfrom external cooling5.5. Cables buried in regions of multiple thermal resistivity Cables buried in regions of multiple thermal resistivity

(thermal backfills and protection layers)(thermal backfills and protection layers)