Effect Of Dry Zone Formation Around Underground Powercables On

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EFFECT OF DRY ZONE FORMATION AROUND UNDERGROUND POWER CABLES ON THEIR RATINGS Ossama E. Gouda Cairo University, Egypt [email protected]

Ghada M. Amer Benha University, Egypt [email protected]

ABSTRACT As it is known there are many factors affecting underground power distribution cables loadings. Such these factors are ambient temperature, cable depth laying, and number ofcable parallel circuits and thermal resistivity of the soil. One important factor usually ignored is the formation of dry zones around the underground power cables due to cable loading. Dry zones are usually formed around underground power cables under loading condition due to the migration of soil moisture content. In this paper the effect of dry zone formation on the underground power cables ampacity is investigated. De-rating factor for the formation of dry zone around underground power cables is suggested and calculated for different types ofnatural backfill soils. lEe 60287-1-3 is taken as reference. Experimental work is done to study the dry zone phenomena of each type of soil.

INTRODUCTION

EXPERMINETALSTUDTY 1. Soil Samples Used in Testing Several experiments are carried out on different types of

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natural soils to study the dry out zone formation under different loadings conditions. Six types of natural soils are investigated for studying the drying out phenomena and the thermal behavior of the soil around the power cables. These types of soil can be classified in composition as given in table 1. Table 1 CI assiifilea tiIon flor mves tl12at e d SOl·1 types Soil type

Weight percentage (%) Gravel Sand Silt Clay

Classification

Sand l

1.5

88.5

10

Very coarse sand, poor in gravel, moderately poor in silt

Sand2

2

88.5

9.5

Moderately fine sand, poor in gravel, moderately poor in silt

Sand3

13

84

3

Sand4

8

92

8

60

30

3

37

30

Silty sand Clayey Silty sand

2.

The current ratings of buried cables are determined by the characteristics of surrounding soils and cable properties as given in IEC 60287 -1-3[1]. In this standard the soil thermal resistivity of the surrounding soil is supposed to be varied from 0.5 C''m/w to 1.2 C''m/w, but under loading the heat dissipated from underground power cables increases the soil thermal resistivity and this may lead to cable thermal failure and thermal instability of the soil around the underground cables[2], [3]. For this reason de-rating factors for cable loading taking the dry zone formation into consideration has to be considered during distribution cable network design. Several approaches have been adopted to establish current ratings of buried cables based on constant values of soil thermal conductivities [4 -7]. Mathematical models are suggested by many researches to study the drying out phenomena around underground power cables [8 - 14]. In this paper de-rating factor for underground power cables taking dry zone formation into account are calculated depending on IEC 60287 -1-3[1]. This paper also contains an experimental work carried out on different types of soils to investigate the formation of dry zone phenomena under loading by heat source simulating the underground cables.

Adel Z. EI Dein Valley University, Egypt azeinm2001 @hotmail.com

-

Medium to coarse sand, some gravel and traces of silt Medium to coarse sand, some gravel Medium to coarse sand, some gravel

30

Medium to coarse sand, some gravel

Thermal Test for Studying the Drying Out Phenomena in Sandy Soils 2.1 Experimental setup

Fig. 1 shows a sketch for the arrangement used in this test. The sample under testing is contained in a cylinder of material with a diameter of 100 mm. The height of the soil sample is 100mm. In the top part, a heat flux of known magnitude is introduced in a downward direction' this flux is measured by means of a calibrated heat flux meter. The bottom of the sample is in contact with a porous slab of sintered Pyrex glass with small pores (pores diameter 5 mm). This filter plate is glued on to a vessel of transparent plastic material completely filled with water a flexible tub connects the vessel with a leveling bottle, the water level in this bottle function as an artificial ground water table. The cylinder containing the soil sample has been sealed off by an o-ring against the top wall of the insulated level. By this arrangement the moisture tension and thus water content can be adjusted. A number of their couples are placed with the walls at the axis of the sample that provide a possibility of measuring the temperature distribution at different points of the soil sample. 2.2 Test results The temperature distribution at different points in the investigated samples, sandI, sand2, sand3, sand4, silty sand and clayey silty sand against distance are given in

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20th International Conference on Electricity Distribution

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Fig's 2-7. The samples under testing are heated under the stated condition for heat flux density Qh and suction tension PF 00, as shown in figures there are two slopes for the temperature distance relationship with respect to time, i.e. there are two zones, zone I near the heat source represents the cable and this is the drying out zone and zone2 which is usually start from the end of zone I and it is known as the wet zone. The discontinuity in the curves indicates the separation between dry zone and moist zone. It is noticed also that the slope of each zone gives indication to the increase in the thermal resistivity that could be calculated as following [I]:

(~~) Qh

dT. dZ

- - IS the

Table 2: The Thermal resisitivities and velocity dry band of different soil types under testin Soil type

Qr w/m2

Pr

(I)

(J"=--

Where

finally the dry zones reached to steady state after time between 24 to 48 hours for the different soils under testing. Also it is noticed that the velocity of dry band formation decreases with time until reaching to very small value at steady state. But it is noticed that the time and the velocity of dry band formation depend on the loading w/m 2 and the Pr values.

Sandl

.

728

00

temperature gradient CO/m

c Is the soil resistivity COm/Wand Qhis the heat flux density w/m 2 The velocity of the dry band formation can be calculated by using the relation:

Sand2

728

00

X 1 - X2

= velocity of dry band X j> X 2 (2) 11 - 12 Where X, is the position of dry band at any point recorded at tJ, and X2 is the position of dry band at any point recorded at t2•

Sand3

728

~-+-Tl1 e rm o ~ou p ' es

Fe ·orat ed plat e se a l - --t- Glass fi er plat e - --t- Cooler

~--f- O · r i n g

Wat er w be

-+

Sand4

00

~

Silty Sand

Clayey Sand

Fig I Arrangement used in drying out experiments Table2 gives the thermal resisitivities of different soil types under testing when loading by 728 w/m2 at suction tension Pf = 00 . From this table it is noticed that for sand I the dry band is partially formed after 3 hours, 3.5 hours for sand2, 2 hours for sand3, 2.7 hours for sand4, 4 hours for silty sand and 3 hours for clayey silty sand and

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728

728

728

for dry zone

COm/w

for wet zone

COm/w

Velocity of dry band formation em/hrs 0.45 between I to 3 hours

(J

(J

1

0.137

0.137

3

1.136

0.471

5

1.2

0.543

24

1.67

0.766

48

1.64

0.749

I 3.5

0.188 1.089

0.188 0.484

6

1.244

0.6

24

1.648

0.763

48

1.737

0.686

2

0.549

0.374

4

0.869

0.549

6

1.010

0.597

24

1.751

0.789

48

1.537

0.795

1 5

0.477 0.986

0.12 0.670

24

1.770

0.784

48

1.654

0.534

00

E l ect r · ~ heet e r Heat ux met er Tl1e rmal insulat ion r - --t- Soil samp e

~-rlr,----t-

Time in hou rs

I

0.223

0.223

4

1.098

0.4995

6

1.226

0.554

24

1.590

0.883

48

1.609

0.732

3 6

0.565 0.8360

0.283 0.48 1

24

1.694

0.824

48

1.648

0.549

00

00

O.1 between 5 to 9 hours 0.00416 between 24 and 48 hours 0.36 between I to 3 hours 0.016 between 6 to 24 hours 0.004 1 between 24 to 48 hours 0.25 between 2 to 4 hours 0.2 between 4 to 6 hours 0.033 betwee n 6 and 24 hours 0.0085 between 24 and 48 hours 0.6 between 1 to 3 hours 0.2 between 3 to 5 hours 0.004 1 between 24 to 48 hours 1.66 between I to 4 hours O.15 between 4 to 6 hours 0.055 betwee n 6 and 24 hours 0.012 between 24 and 48 hours 0.2 between 3 to 6 hours 0.38 between 6 to 24 hours 0.01 between 24 to 48 hours

DE-RATING FACTOR DUE TO THE DRY BAND FORMATION

By de-rating factor we mean the ratio between current ampacity 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 current ampacity taking the dry band into consideration. To use

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th is formula the ratio betw een the dry and moi st zones resisitiv ities of the backfill soil (u) and the difference between the critical temperature of boundary between the moist and dry zones CO and ambient temp erature (Sx-Sa) have to be obtained. Table 3 gives thes e values for the soil und er testing wh en Qh equals 728w/m 2. Some tests are carried out by varying Qh to be 468 w/rrr' and 344 w/rn'' respectively but it is noticed that the re is no essentia l variation in (Sx-Sa) and also in (u),

are of equal size and car rying the same load), Dielectr ic loss per unit length for the insulation surrounding the conductor per phase , R';. ~ A lternati ng curren t resistance per unit length of the conductor at its maxim um operati ng temperature H'~

( !?f'~rJ ,

Ther ma l resistanc e per unit length per core between conductor and sheath (C''m/w) ,

T~

120 J8 hrs 2J hrs l> 6 hrs + 5 hrs 3 hrs
"*

100

§: ~ ~

~

~

I-

+

"0

100

*

90

"* c:

110

o

J8 hrs 2J hrs 6 hrs 3.5 hrs 1 hrs

90

§:

80

80

~

Movement of dry band formation

~

70

ro

60

I-

70

~

~

60 50

50

JO

JO

30 30 20 20

0

Dista nce (em)

Fig 2 Temperature versu s distance for sand I when PF and Qh =728 w/rrr'

00

Sand I Sand2 Sand3 Sand4 Silty sand Clayey silty sand

e,

ea

63 65 58 76 57

25 27

60

22 22 21 18

u =~

e,-ea

2. 179 2.16 2.2 1 2.257 2. 1962

38 38 36 34 38

· ~f · ·

~q r

2.055

42

From the so man y tests carried out on different soils used as backfill materials it is noticed that the critical temperature for dry band formation dep end s on the soil component s but its ind epend ent on the cable loading. Also the ratio between the dry and wet thermal resisiti vities depend on the soil typ e and indep endent on the cabl e loadin g but it is noticed that th e time required to form the dry band around underground pow er cabl es depends on the cabl e loading, soil typ e and soil moi sture content. The ampacity of cable loading can be calculated by IEC 60287-1-3 equations [1] without and with dry band formation for different distribution cables. IEC 60287-1-3 are listed below: The current carrying capa city of a buried cable is: 1

=

J 5 Dist an ce (em)

Fig 3 Temperature versus distance for sand2 wh en PF and Qh =728 w/m 2 • f:;

T a bl e 38 x- 8a an d u tior SOl'1sa m n es un d er testing, Type of soil

0

5

J

"*+

100

"0

90

J8 hrs 2J hrs 6 hrs J hrs 2 hrs

80

§: c ~

70

ro

c

~

I-

60 50 JO 30 20

0

J 5 Dist ance (em)

Fig 4 Temperature versus distanc e for sand3 wh en PF and Qh =728 w/m 2. 120

c: J8 hrs 110

"* +

2J hrs 5 hrs

100

:({

3 hrs

o

1 hrs

90

§:

80

~ ~

ra

70

~

~

I-

M ovement of dry band formation

60 50 JO 30 20 0

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00

J 5 Distance (em)

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20th International Conference on Electricity Distribution

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Fig 5 Temperature versus distance for sand4 when PF 00 and =728 w/m". T. Thermal resistance per unit length of bedding between sheath and armour (C''m/w), t : Thermal resistance per unit length of the external serving 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, and A: Ratio of armouring losses to conductors total losses in that cable.

o,

And the modified equation for cable rating calculation is: 1

=

determine the cables under study de-rating factor with dry zone formation. From the tabulated results it is clear that soil type's sand 2 and sand 1 have higher de-rating factor than the others. 1:0

48 hrs

o

3 hrs

+ 24 hrs + 6 hrs

90 80

Q:

70

'" ~

60

~

50

E

r'Jlovement of dry band formation

40 30 20;-0--:----;;---:---:-----::---:----:----o--~ 4 5

e:". - 6:;, the difference between the critical

:s.~".

temperature and ambient temperature Co, The ratio between the thermal resistivities (of dry and moist zones) 8x-8. and u are taken from table 3 for different types of sands and their thermal resistivity plotted in Fig's from 2 to 7 and tabulated in table 2. A computer program to calculate the de-rating factor for 11, 33, 66 and 132 kY cables using the tested soils as backfill materials is used. Fig 8 shows sample of dry band formed around the directly buried three cables 33kY. Table 4 gives sample of the obtained results. It is concluded that de-rating factor due to dry zone formation is ranged between 0.88 and 0.98 depending on the backfill soil and cable ratings. The cables depth and spacing are taken as 1 m and 0.4 m respectively for cables higher than 33 kY and for cables rated less than 33 kY the laying depth is taken as 0.8m. Fig. 8 shows the surface temperature distribution around 33 kY cables. The dry zones are formed at 63, 65, 58, 56, 57 CO and 60 CO respectively depending on the soil type. l'

110 1:0

48 hrs

+

6 hrs 4 hrs 1 hrs

Distance (em)

Fig 7 Temperature versus distance for Clayey 2 Silty sand when PF 00 and Qh =728 w/m . They have approximately the same dry to moist thermal resistivity and same differenc e between critical and ambient temperature as given in table 3, also they have approximately the same components as given in table 1 , there are little differences in weight percentage of gravel and silt. Sand 4 has the lowest de-rating factor, the reason may be due to it has the highest value of dry to moist thermal resistivity as given in table 3 and also it does not contain any amount of clay or silt as given in table I.Silty sand and clayey silty sand have also good de-rating factors but they may cause corrosion for cable sheathing due to the high amounts of silt.

+ 24 hrs

100 90

'"

o

80

Q: c

~

"'"

~ f-

70

Movement of dry band formation

60 50 40

Fig. 8 temperature distribution within and around the directly buried three cables (33kY) , three phases, three cores in flat formation

30 20 10

o 0

4

5

Distance (em)

Fig 6 Temperature versus distance for silty sand when PF 00 and Qh =728 w/m". Table 4 gives summary of the calculated results to

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Figure 8 gives the temperature distribution around 33kY, three phases' three core cables when loaded by I106A and 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 at temperature 63°C.

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Table 4 De-rating factor of single-core cables In flat configuration Type of soil

Sand1

Sand2

Moist thermal resistivity

0.766

0.763

1.67

1.648

63

65

Clayey silty sand

Sand3

Sand4

Silty sand

0.7898

0.784

0.732

1.77

1.609

1.694

56

57

60

0.8241

(C''rn/w) Dry thermal resistivity

1.7513

(C''m/w) Drying out zone temperature

58

CO

Ampacity without dry band formation Amp. Ampacity with dry band formation Amp. De-rating factor

687

688

132 kV cable 678

680

699

666

643

652

615

0.935

0.9477

0.9071

609

0.895

634

0.907

615

0.9234

841

842

830

832

858

767

777

734

726

758

734

De-rating factor

0.912

0.9228

0.8834

0.9017

Ampacity without dry band} Amp. Ampacity with dry band} Amp. De-rating factor

1106

33 kV cable 1108 1092

1095

1127

1082

1024

1037

980

970

1010

979

0.925

0.9359

0.897

0.8858

0.8962

0.9048

674

675

668

686

656

639

647

607

631

612

Ampacity without dry band} Amp. Ampacity with dry band} Amp. De-rating factor

REFERENCES For a Conference citation: publication 60287-1-3 "Calculations of the continuous current rating of cables (100% loadfactor", 1982. [2] Koopmans G., Gouda O.E. "Transport of heat and moisture in soils with hysteretic moisture potential" 4th. International conference on

[1] IEC

66 kV cable Ampacity without dry band .Amp. Ampacity with dry band .Amp.

cables decreases cables capacity by factor defined in this paper by de-rating factor depending on the soil type 2- From the so many tests carried out it is noticed that drying out phenomena in backfill soil started at different temperatures with different velocities depending on the soil type and the weight percentage of silt 3- The time required for dry zone formation around buried cables is longer for the sand samples contain silt than samples do not contain silt. While the velocity of dry zone movement around the cables buried in sand contain silt is slower than that do not contain silt

0.948

0.8843

11 kV cable 666

0.9585

613

0.872

814

around underground cables loaded by peak loadings". 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 Cable Installations ", IEEE. Transactions on Industry Applications, Vol. 24, No.1 January-February 1988, pp.99-1 05. [5] M.A. Hanna, A.Y. Chikhani and M.M.A. Salama , "Thermal Analysis of Power Cables in Multi-Layered Soil" Part 3: Case of Two Cables in a Trench, IEEE Transactions on Power Delivery, Vol. 9, No.1, January 1994, pp. 572-578. [6] G. 1. Anders, H. S. Radhakrishna, "Power

Cable Thermal Analysis with Consideration of Heat and Moisture Transfer in the Soil", IEEE Transactions on Power Delivery, Vol. 3, No.4, October 1988, pp. 1280-1288. [7] G. 1. Anders, A.K.T. Napieralski, and W. "Calculation of the Internal Zamojski

Thermal Resistance and Ampacity of 3-Core Unscreened Cables with Fillers "IEEE 0.920

0.908

0.9198

0.9329

CONCLUSIONS From the experimental study and analysis carried out in this paper, it is concluded that: 1-The dry zones formation around underground

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numerical methods in thermal problems. 15-18 July 1985, Swansea, U.K. [3] Gouda O.E., "Formation of the dried out zone

Transactions 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 Ampacity Analyzed With the Finite Element Method' IEEE Transactions on Power Delivery, 23,No. 2, April. 2008, pp. 537-543.

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Vol.

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20th International Conference on Electricity Distribution

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[9] Charis Demoulias, DimitrisP. Labridis, Petros.S. Dokopoulos, and Kostas Gouramanis "Ampacity of Low-Voltage Power Cables Under 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 "Theoretical Model to Calculate Cidras Steady-State and Transient Ampacity and Temperature in Buried Cables" IEEE Transactions on Power Delivery, Vol. 18, No. 3, July 2003, pp. 667-678. [11] Michael R. Yenchek, and Gregory P. Cole, "Thermal Modeling of Portable Power Cables ", IEEE Transactions on Industry Applications, Vol. 33, No.1, January/February 1997, pp. 72-79. [12] G.J. Anders, and A. Napieralski and Z. Kulesza "Calculation of the Internal Thermal Resistance and Ampacity of 3-Core Screened Cables with Fillers" IEEE Transactions on Power Delivery, Vol. 14, No.3, July 1999, pp. 729-734. [13] Neil P. Schmidt "Comparison between 1.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 and R.WD. Ganton "New Approach to Ampacity Evaluation of Cables in Ducts Using Finite Element Technique" IEEE Transactions on Power Delivery, Vol. PWRD-2, No.4, October 1987, pp. 969-975.

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