effect of dry zone formation around underground powercables on

8/14/2019 EFFECT OF DRY ZONE FORMATION AROUND UNDERGROUND POWERCABLES ON

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CI RED 20th International Conference on Electricity Distribution Prague, 8-11 June 2009

Paper 0120

EFFECT OF DRY ZONE FORMATION AROUND UNDERGROUND POWER CABLES ONTHEIR RATINGS

Ossama E. GoudaCairo University, [email protected]

Ghada M. AmerBenha University, [email protected]

Adel Z. EI DeinValley University, Egyptazeinm2001 @hotmail.com

natural soils to study the dry out zone formation underdifferent loadings conditions. Six types of natural soilsare investigated for studying the drying out phenomenaand the thermal behavior of the soil around the powercables. These types of soil can be classified incomposition as given in table 1.

tl t d 1able 1 CI ifi ti flssi lea Ion or mves 12a e SOl typesSoil Weight percentage (%)

type Gravel Sand Silt ClayClassification

Sand l 1.5 88.5 10Very coarse sand, poor in

gravel, moderately poor in silt

Sand2 2 88.5 9.5Moderately fine sand, poor ingravel, moderately poor in silt

Sand3 13 84 3Medium to coarse sand, some

gravel and traces of silt

Sand4 8 92 - Medium to coarse sand, somegravelSilty

8 60 30Medium to coarse sand, some

sand gravelClayey

Medium to coarse sand, someSilty 3 37 30 30sand

gravel

As it is known there are many factors affectingunderground power distribution cables loadings. Suchthese factors are ambient temperature, cable depthlaying, and number of cable parallel circuits and thermalresistivity of the soil. One important factor usuallyignored is the formation o f dry zones around theunderground power cables due to cable loading. Dry

zones are usually formed around underground powercables under loading condition due to the migration o fsoil moisture content. In this paper the effect of dry zoneformation on the underground power cables ampacity isinvestigated. De-rating factor for the formation o f dryzone around underground power cables is suggested andcalculated for different types o f natural backfill soils. lE e60287-1-3 is taken as reference. Experimental work isdone to study the dry zone phenomena of each type o fsoil.

ABSTRACT

INTRODUCTION

The current ratings of buried cables are determined by thecharacteristics of surrounding soils and cable propertiesas given in IEC 60287 -1-3[1]. In this standard the soilthermal resistivity of the surrounding soil is supposed tobe varied from 0.5 C''m/w to 1.2 C''m/w, but underloading the heat dissipated from underground powercables increases the soil thermal resistivity and this maylead to cable thermal failure and thermal instability of thesoil around the underground cables[2], [3]. For thisreason de-rating factors for cable loading taking the dryzone formation into considerat ion has to be consideredduring distribution cable network design. Severalapproaches have been adopted to establish current ratings

of buried cables based on constant values of soil thermalconductivities [4 -7]. Mathematical models are suggestedby many researches to study the drying out phenomenaaround underground power cables [8 - 14]. In this paperde-rating factor for underground power cables taking dryzone formation into account are calculated depending onIEC 60287 -1-3[1]. This paper also contains anexperimental work carried out on different types of soilsto investigate the formation of dry zone phenomena underloading by heat source simulating the undergroundcables.

2. Thermal Test for Studying the Drying Ou tPhenomena in Sandy Soils

2.1 Experimental setup

Fig. 1 shows a sketch for the arrangement used in thistest. The sample under tes ting is contained in a cylinderof material with a diameter of 100 mm. The height of thesoil sample is 100mm. In the top part, a heat flux ofknown magnitude is introduced in a downward direction'this flux is measured by means of a calibrated heat fluxmeter. The bot tom of the sample is in contact with aporous slab of sintered Pyrex glass with small pores(pores diameter 5 mm). This filter plate is g lued on to avessel of transparent plastic material completely filledwith water a flexible tub connects the vessel with aleveling bottle, the water level in this bott le function asan artif icial ground water table. The cylinder containingthe soil sample has been sealed of f by an o-ring againstthe top wall of the insulated level. By this arrangementthe moisture tension and thus water content can beadjusted. A number of their couples are placed with thewalls at the axis of the sample that provide a possibilityof measuring the temperature distribution at differentpoints of the soil sample.

2.2 Test results

EXPERMINETALSTUDTY

1. Soil Samples Used in Testing

Several experiments are carried out on different types of

The temperature dist ribution at different points in theinvestigated samples, sandI, sand2, sand3, sand4, siltysand and clayey silty sand against distance are given in

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CI RE D 20th International Conference on ElectricityDstribution Prague, 8-11 June 2009

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Fig's 2-7. The samples under testing are heated under thestated condit ion for heat flux density Qh and suct iontension PF 00 , as shown in figures there are two slopesfor the temperature distance relationship with respect to

time, i.e. there are two zones, zoneI near the heat sourcerepresents the cable and this is the drying out zone andzone2 which is usually start from the end of zoneI and itis known as the wet zone. The discontinuity in the curvesindicates the separa tion between dry zone and moistzone. It is noticed also that the slope of each zone givesindication to the increase in the thermal resistivity thatcould be calculated as following [I]:

(~~ )( J " = - - (I )

Qhd T . .

Where - - I S the temperature gradient CO/mdZc Is the soil resistivity C Om/WandQhis the heat flux density w/m 2

The velocity of the dry band formation can be calculatedby using the relation:X 1 - X 2 = velocity of dry band X j> X 2 (2)

11 - 12Where X, is the position of dry band at any pointrecorded at tJ, and X 2 is the position of dry band at anypoint recorded at t 2

E l e c tr ~ heeterHe at ux m et erTl1ermal insulation

r - - - t - Soilsamp e~ - + - Tl 1e r m o ~ o u p ' e s

~ - r lr , - - -- t - Fe orated plate~ - - f -O r i n g seal- -

- t - Glassf i er p la te- -- t - Cooler

Waterw be - + ~

Fig I Arrangement used in drying out experiments

Table2 gives the thermal resisitivities of different soiltypes under testing when loading by 728 w/m 2 at suctiontension P f = 00 . From this table it is noticed that for sandIthe dry band is partially formed after 3 hours, 3.5 hoursfor sand2, 2 hour s for sand3, 2.7 hours for sand4, 4hours for silty sand and 3 hours for clayey silty sand and

finally the dry zones reached to steady state after timebetween 24 to 48 hour s for the different soils undertesting. Also it is noticed that the velocity of dry bandformat ion decreases with time until reaching to very

small value at steady state. But it is noticed that the timeand the velocity of dry band formation depend on theloading w/m 2 and the Prvalues.

Table 2: The Thermal resisitivities and velocity dry bandof different soil types under testin

Time(J for (J for Velocity of

Soil Qr in dry wet dry bandtype w/m 2 Pr formationhours zone zoneCOm/w CO m/w em/hrs

1 0.137 0.137 0.45 between I

3 1.136 0.471to 3 hours

5 1.2 0.543O1 between 5

Sandl 728 00 to 9 hours24 1.67 0.766

0.00416between 2448 1.64 0.749 and 48 hours

I 0.188 0.188 0.36 between I3.5 1.089 0.484 to 3 hours

6 1.244 0.6 0.016 betweenSand2 728 00 6 to24 hours24 1.648 0.763 0.004 1

48 1.737 0.686between 24 to

48 hours

2 0.549 0.374 0.25 between 2

4 0.869 0.549to4 hours

6 1.010 0.5970.2 between 4

to 6 hoursSand3 728 00

0.033 between24 1.751 0.789

6 and24 hours

0.008548 1.537 0.795 between 24and 48 hours

1 0.477 0.12 0.6 between 15 0.986 0.670 to 3 hours

24 1.770 0.7840.2 between 3

Sand4 728 00 to 5 hours0.0041

48 1.654 0.534 between 24 to48 hours

I 0.223 0.223 1.66 between I

4 1.098 0.4995to4 hours

6 1.226 0.554 O15 betweenSilty 4 to 6 hours

728 00Sand 0.055 between24 1.590 0.883

6 and24 hours0.012 between

48 1.609 0.732 24 and 48hours

3 0.565 0.283 0.2 between 36 0.8360 0.48 1 to 6 hours

Clayey728 00 24 1.694 0.824

0.38 between 6Sand to 24hours

48 1.648 0.5490.01 between24 to48 hours

DE-RATING FACTOR DU E TO TH E DRY BAND

F O RM ATI O N

By de-rating factor we mean the ratio between currentampacity of the cable with dry band formation and the

cable ampacity assuming there is no dry band is formed.lEC 60287-1-3 [I] gives formula to calculate the currentampacity taking the dry band into consideration. To use




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CI RED 20th International Conference on Electricty Distribution Prague, 8-11 June 2009

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th is formula th e rat io betw een th e dry and moi st zo ne sresisi tiv it ies o f the backfill soil (u) and the d iffe rencebetween the critical temperature of boundary between them oist and d ry zones CO and ambient temperature (Sx-Sa)

hav e to b e obtained. Tab le 3 gives thes e va lues for th esoil under test ing when Qh equals 728w /m 2. Some testsare carri ed out by vary ing Qh to be 468 w/rrr' and 344w/rn'' respectively but it is noticed that the re is noessential variation in (Sx-Sa) and also in (u),

are of equal size and carrying the same load),H '~ Dielectr ic lo ss per un it length for the insu lation

surrounding the conductor per phase,R ' ; .~ Alternating current resistance per unit length of the

co nd uctor at its maxim u m op erati ng te mperat ure( ! ?f '~r J ,T~ Thermal resis tance per unit length per core between

conductor and sheath (C''m/w),

120"* J8 hrs

110 c: 2J hrs+ 6 hrs

100 " 3.5hrs0 1 hrs90

: 80~

~

ro 70~

~ 60I-

50

JO

30

200 J 5

Distance (em)

Fig 3 Temperature versus distance for sand2 when PF 00and Qh =728 w/m 2

o J8 hrs"*

2J hrsl> 6 hrs+ 5 hrs* 3 hrs


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CI RED 20th International Conference on Electricity Distribution Prague, 8-11 June 2009

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1:0 48 hrs+ 24 hrs+ 6 hrso 3 hrs

rJlovement of dry band formation

4 5Distance (em)

Fig 7 Temperature versus distance for ClayeySilty sand when PF 00 and Q h =728 w/m 2 .

20 ;-0- - : - - - - ; ; - - - : - - - : - - - - - : : - - - : - - - - : - - - - o - -

90

40

30

80

Q: 70

'"~ 60

E~ 50

They have approximately the same dry to moist thermalresistivity and same difference between critical andambient temperature as given in table 3, also they haveapproximately the same components as given in table 1 ,there are little differences in weight percentage ofgravel and silt. Sand 4 has the lowest de-rating factor, thereason may be due to it has the highest value of dry tomoist thermal resistivity as given in table 3 and also itdoes not contain any amount of clay or silt as given intable I.Silty sand and clayey silty sand have also goodde-rating factors but they may cause corrosion for cablesheathing due to the high amounts of silt.

Fig. 8 temperature distribution within and around thedirectly buried three cables (33kY), three phases, three

cores in flat formation

determine the cables under study de-rating factor with dryzone formation. From the tabulated results it is clear thatsoil type's sand 2 and sand 1 have higher de-rating factorthan the others.

Figure 8 gives the temperature distribution around 33kY,three phases' three core cables when loaded by I106Aand directly buried in soil type sand 1. The spacing

between each phase is 0.4 m and the buried depth is 1 m.It is noticed that there is dry band zone formed attemperature 63C.

1:0 48 hrs+ 24 hrs+ 6 hrs'" 4 hrso 1 hrs

Movement of dry bandformation

o

110

100

90

80

Q: 70c~

" 60'"~ 50

f-

40

30

20

100 4 5

Distance (em)

Fig 6 Temperature versus distance for silty sand when PF00 and Q h =728 w/m".

Fig 5 Temperature versus distance for sand4 when PF 00and o, =728 w/m".

T. Thermal resistance per unit length of bedding betweensheath and armour (C''m/w),

t : Thermal resistance per unit length of the externalserving of the cable (C''m/w),T Thermal resistance per unit length between the cable

surface and the surrounding soil (C''m/w),J 1 Ratio of losses in the metal sheath to total losses in all

conductors in that cable, andA: Ratio of armouring losses to conductors total losses in

that cable.

Table 4 gives summary of the calculated results to

: s . ~" . e:". - 6:;, the d ifference between the cr it ica ltemperature and ambient temperature Co,

l' The ratio between the thermal resist ivit ies (o fdry and moist zones)

8x-8 . and u are taken from table 3 for different types ofsands and their thermal resistivity plotted in Fig's from 2to 7 and tabulated in table 2. A computer program tocalculate the de-rating factor for 11, 33, 66 and 132 kYcables using the tested soils as backfill materials is used.Fig 8 shows sample of dry band formed around thedirectly buried three cables 33kY. Table 4 gives sample

of the obtained results. It is concluded that de-ratingfactor due to dry zone formation is ranged between 0.88and 0.98 depending on the backfill soil and cable ratings.The cables depth and spacing are taken as 1 m and 0.4 mrespectively for cables higher than 33 kY and for cablesrated less than 33 kY the laying depth is taken as 0.8m.Fig. 8 shows the surface temperature distribution around33 kY cables. The dry zones are formed at 63, 65, 58, 56,57 CO and 60 CO respectively depending on the soil type.

And the modified equation for cable rating calculation is:1 =




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Table 4 De-rating factor of s ingle-core cables In flatconfiguration

Silty ClayeyType of soil Sand1 Sand2 Sand3 Sand4 silty

sandsand

Moistthermal 0.766 0.763 0.8241

resistivity 0.7898 0.784 0.732

(C''rn/w)Dry thermal

resistivity 1.67 1.648 1.7513

(C''m/w)1.77 1.609 1.694

Drying outzone 63 65

temperature 58 56 57 60

CO

132 kV cableAmpacity 687 688 678 680 699 666

without dryband

formationAmp.

Ampacity 643 652 615 609 634 615with dry

bandformation

Amp.De-rating 0.935 0.9477

0.9071 0.8950.907

0.9234factor

66 kV cable

Ampacity 841 842 830 832 858 814

without dryband .Amp.Ampacity 767 777 734 726 758 734with dry

band .Amp.

De-rating 0.912 0.9228 0.872

factor0.8843 0.8834 0.9017

33 kV cableAmpacity 1106 1108 1092 1095 1127 1082

without dryband} Amp.Ampacity 1024 1037 980 970 1010 979with dry

band} Amp.De-rating 0.925 0.9359

0.897 0.88580.8962 0.9048

factor11 kV cable

Ampacity 674 675 666 668 686 656without dryband} Amp.Ampacity 639 647 613 607 631 612with dry

band} Amp.De-rating 0.948 0.9585

0.920 0.908 0.9198 0.9329factor

CONCLUSIONS

From the experimental s tudy and analysi s carried out inthis paper, it is concluded that:1-The dry zones formation around underground

cables decreases cables capacity by factor definedin this paper by de-rating factor depending on thesoil type2- From the so many tests carried out it is noticed

that drying out phenomena in backfill soil started atdifferent temperatures with different velocitiesdepending on the soil type and the weightpercentage of silt3- The time required for dry zone formation aroundburied cables is longer for the sand samples containsilt than samples do not contain silt. While thevelocity of dry zone movement around the cablesburied in sand contain silt is slower than that do notcontain silt

REFERENCES

For a Conference citation:

[1] IEC publication 60287-1- 3 "Calculations ofthe continuous current rating o f cables (100%loadfactor", 1982.

[2] Koop mans G., Gouda O.E. "Transport of heatand moisture in soils with hysteretic moisturepotential" 4 th. International conference onnumerical methods in thermal problems. 15-18July 1985, Swansea, U.K.

[3] Gouda O.E., "Formation o f the dried out zonearound underground cables loaded by peakloadings". Modeling, Simulation & Control,

ASME Press, vol. 7, No.3, 1986, pp. 35-46.[4] 1. Hegyi and A. Klestoff "Current-Carrying

Capability for Industrial Underground CableInstallations ", IEEE. Transactions on IndustryApplications, Vol. 24, No.1 January-February1988, pp.99-105.

[5] M.A. Hanna, A.Y. Chikhani and M.M.A.Salama , "Thermal Analysis of Power Cablesin Multi-Layered Soi l" Part 3: Case of Tw oCables in a Trench, IEEE Transactions onPower Delivery, Vol. 9, No .1 , January 1994,pp. 572-578.

[6] G. 1. Anders, H. S. Radhakrishna, "Power

Cable Thermal Analysis with Consideration ofHeat and Moisture Transfer in the Soil", IEEETransactions on P ow er Delivery, Vol. 3, N o . 4 ,October 1988, pp. 1280-1288.

[7] G. 1. Anders, A.K.T. Napieralski, and W.Zamojski "Calculation o f the InternalThermal Resistance and Ampacity of 3-CoreUnscreened Cables with Fillers "IEEETransactions on Power Delivery, Vol. 13, No.3, July 1998, pp.-699-705.

[8] Francisco de Leon, and George J. Anders"Effects of Backfilling on Cable AmpacityAnalyzed With the Finite Element Method'

IEEE Transactions on Power Delivery, Vol.23,No. 2, April . 2008, pp. 537-543.




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[9] Charis Demoulias, DimitrisP. Labridis,Petros.S. Dokopoulos, and Kostas Gouramanis"Ampacity of Low-Voltage Power CablesUnder Non-sinusoidal Currents" IEEE

Transactions on Power Delivery, Vol. 22,No.1,January 2007, pp. 584-594[10] Carlos Garrido, Antonio F. Otero, and Jose

Cidras "Theoretical Model to CalculateSteady-State and Transient Ampacity andTemperature in Buried Cables" IEEETransactions on Power Delivery, Vol. 18, No.3, July 2003, pp. 667-678.

[11] Michael R. Yenchek, and Gregory P. Cole,"Thermal Modeling o f Portable PowerCables ", IEEE Transactions on IndustryApplications, Vol. 33, N o . 1 , January/February1997, pp. 72-79.

[12] G.J. Anders, and A. Napieralski and Z.Kulesza "Calculation of the Internal ThermalResistance and Ampacity of 3-Core ScreenedCables with Fillers" IEEE Transactions onPower Delivery, Vol. 14, No.3 , July 1999, pp.729-734.

[13] Neil P. Schmidt "Comparison between1.E.E.E. and ClORE Ampacity Standards"IEEE Transactions on Power Delivery, Vol.14, No.4, October 1999, pp. 1555-1562.

[14] GJ.Anders, M. Chaaban, N. Bedard andR.WD. Ganton "New Approach to AmpacityEvaluation o f Cables in Ducts Using Finite

Element Technique" IEEE Transactions onPower Delivery, Vol. PWRD-2, No.4 , October1987, pp. 969-975.


effect of dry zone formation around underground powercables on

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