Underground cable laying – proposal to lay powercable and OFC in one trench

Matam ManjunathDepartment of Electrical and

Electronics EngineeringNational Institute of Technology

Goa, India–403401Email: manjunath@nitgoa.ac.in

Ankita Singh GaurDepartment of Electrical and

Electronics EngineeringNational Institute of Technology

Goa, India–403401Email:asgaur93@gmail.com

Soumik PissurlenkarDepartment of Electrical and

Electronics EngineeringNational Institute of Technology

Goa, India–403401Email: soumikpissurlenkar@yahoo.com

Abstract—The Objective of this paper is to workout andhighlight the possibilities to operate an AC Power cable andOptical Fibre Cable (OFC) by laying both in one trench.This paper has studied the electrostatic, magnetic and thermalparameters associated with the above proposal. Mathematicalequations, derivations supporting the claim have been presented.At the end, simulation tests were conducted by incorporating theobtained mathematical parameters into simulated environment.Beneficiaries of this proposal range from industry consumer(s),service providing companies, smart grids and the general public.Benefits could include distribution automation, economic aspect,reduced space requirement, reduced manpower and reducedroad digging work. However, experimental tests to conformthe proposal claims, developing standards for installation andoperational purpose etc., all inspire us to work further on thisproposal.

Index Terms—Power cable, OFC cable, underground cablelaying.

I. INTRODUCTION

DEVELOPMENT in the field of technology has led us toprogress in daily life and the levels of communication as

well. Electricity has become the fundamental necessity of allthe sections of the society and so is communication. In earlierdays the transmission of electricity was done over small dis-tances. But now, the transmission lines run for miles together.Same is the case with communication system. Undergroundtransmission lines are preferred over overhead transmissionlines for low power ratings because underground cables areless prone to damage and the transmission of power is donevery efficiently.

Many departments, organizations and companies exist thatproduce, promote, finally install and look after consumerpower cable and OFC operations. In this supplier-consumerchain, interestingly, it is the end consumer who suffers inthe form of huge investments due to these service providers.It is a loss-loss situation for both supplier and receiver ifbetter solutions to reduce expenditure are not discovered.The following example will prove this point. An academicinstitution, in this case consumer, was served with a demandnotice by an under ground power cable and OFC laying

organizations individually to install respective cables for alength of 250 m and 1.2 km. Cable laying work componentslike digging, trench forming, cable laying etc., all were samefor both organizations. Only the cables differ with respect toeach other. It has been found that 30% of consumer investment,in both cable works, goes towards labour work and trenchformation. On other hand, for a length of about 250 m nearthe premises of the consumer, trench belonging to power cableand OFC travel side by side. This is how it was planned andready to be executed to match consumer requirements followedby standard installation procedures of installing organizations.At this stage, a question was put up to both organizations ifthey could lay both cables in one trench instead of two fora length of 250 m from the consumer end. This would savethe consumer from investing an amount equivalent to a 250m length trench digging work and associated manpower andlabour work. In the end, the service providing organizationsrefused to oblige stating that neither of the organizations haveprocedural standards that allow the installation of another cablein its trench. This has inspired the current authors to work ona solution that would help future consumers.

The organizations involved in these installations have notlooked at the merit of such a proposal because of adminis-trative issues and the organizational difficulties they possess.Service providing organizations belonging to Govt. or Privateinstitutions have grown in size and structure tremendously insuch a manner that chances for inter mutual cooperation atground level is low. These organizations have not put anyeffort to come together and co-ordinate with other servingorganizations to benefit the consumer. This paper is an attemptto overcome the barrier and develop a standard benefiting theend consumer. At end, this has inspired the authors to presentthis proposal.

Technically a possibility exists to operate both cables inone trench but no provisions could be found for laying thepower cable and the communication cable together. Given anew cable installation task, both the departments have theirseparate specifications of laying the cables in the undergroundtrench [3], [6]– [9]. After a study of the specifications andthe expenses that occur while laying the cables at any loca-978-1-4799-5141-3/14/$31.00 c© 2014 IEEE

tion, it was observed that most of the expenditure was dueto the labour work in digging, trench formation and cableinstallation. Both the departments create similar trenches to laytheir cables. For any new complex, the electricity departmentand the communication department, do their work separatelythough a possibility may exist to cooperate. Trenching isnot a quick process and is most effective for short distanceapplications. The depth of the cable may vary with application,intended user and construction [7] .The depth of laying thepower cable is about 0.5 meters and for communication cableis about 1.65 meters [8].

This paper would like to search, propose and investigatethe possibilities of laying both the cables simultaneously ina single trench. It presents its findings in order of describingmathematical equations, necessary assumptions in Section II,simulating the practical environment, incorporating mathemat-ical parameters and operational analysis in Section III andconclusions in Section IV.

(a) (b)

Fig. 1: In a domestic locality, (a) OFC and (b). Power cableare installed in trench dedicated to each separately.

II. MATHEMATICAL EQUATIONS

This proposal would like to investigate the electrostatic,magnetic and heat parameters affecting the smooth functioningof both OFC and power cable if lying in one trench. However,studying the consequences of operating a power cable nearan OFC matters most and not vice versa. It is becausethe power cable supersedes OFC in terms of physical size,weight, electric potential and more importantly handling powercapacity. Hence, in this study, power cable is treated as acause and OFC is the consequence. It is understood that OFCis immune to Electro Magnetic Interference (EMI) and itsoperational performance is not affected [1], [2]. Now, the focuswould go to other operational parameters that are likely tointerrupt OFC functioning.

Power cable and OFC working together could create threeoperational possibilities where former would certainly affector damage the latter. They are, first, electrostatic potentialcreated at OFC location caused due to insulation failure ofunloaded power cable. This is no load case where cableconductor carries negligible or no current. In this case, findingthe electrostatic potential or electric field intensity (E), chargeformed at the OFC location due to un loaded power cablevoltage form the first step. Second, magnetic potential created

at OFC location due to loaded power cable. In this case,finding the magnetic field intensity(B) due to current carryingconductor forms a part of this task. Third, find the heator temperature rise at OFC due to a rise in power cabletemperature. As per British Standard BS:6346:1989, powercable operating temperature shall not exceed 70oC. Findingthe OFC temperature when power cable operates at its ratedor exceeded temperatures form final part of this study.

Let E be the electric field intensity at the position of OFCoperated in vicinity of a power cable carrying per phasevoltage V. Let power cable be charged and it carry no or lowcurrent. It causes the electric field intensity to develop aroundthe space of power cable in form of voltage gradient. It isgiven, as in [4], by

E = −∇ · V (1)

Expanding above (1) in cartesian system of coordinates gives

E = −(∂Vx∂x

+∂Vy∂y

+∂Vz∂z

)

Assume that power cable is layed along z-axis and the x, ycoordinates remain same at all positions of power cable andthat of OFC. This sets Vx, Vy both to zero and Vz to be thesupply voltage given by Vmsinωt. In order to find the chargeformed at OFC location due to E (V gradient), it is requiredto find the charge density given by

ρ = ∇ ·D (2)

where ρ is the quantity of charge developed at OFC location,D is the displacement charge density. D is a derived quantitygiven by

D = εE (3)

where ε is the dielectric coefficient of the medium lying inbetween power cable and OFC. It shall consider all materialtypes and their dielectric coefficients that come in betweenthese two cables.

Proceeding to the next step i.e. the task of finding themagnetic potential or flux enclosing the OFC due to loadedpower cable. Maxwell equations which could help in this taskare Ampere Circutal law and Gauss law for magnetic field asgiven by,

∇×H = J +∂D

∂t(4)

∇.B = 0 (5)

where H is the magnetic filed intensity around the OFCperimeter, J is current density enclosed by OFC and D isthe displacement current density enclosed and B is mag-netic flux density at it. These H, J and B are recipientparameters measured in the medium present in between andat the OFC location in particular. It is to be noted herethat this study considers steady state analysis but not timevariant. This sets D as in (4) to zero. Further, expressingthe magnetic flux density B in cartesian system of coordinategives Bx(x, y, z, t)+By(x, y, z, t)+Bz(x, y, z, t) where x,y,zrepresents OFC location and t represents time (t is neglected

as it deals with steady state parameters). Any vector said tobe called as magnetic flux density only if its gradient as in(5) results null. This analysis assumes B at OFC location as aknown quantity. However, relating the magnetic flux density(B) with source the power cable current is given by

B(r, θ, φ) =µI

2πrar (6)

where B(r, θ, φ) is magnetic flux density(in Wb/sq.m) at OFClocation expressed in spherical coordinates, µ is magnetic per-meability of the medium expressed as µ0µr, µ0 is permeabilityof the free space, µr is relative permeability of the medium,r is radial distance(in m) from power cable to OFC and Iis power cable current(in Amps). It has presence of variousmedia hence, the net relative permeability is computed as

1

µr=

1

µins.+

1

µsheath+

1

µsoil(7)

where parameters representing the relative permeability ofcable conductor insulation is µins., cable sheath is µsheath

and soil is µsoil. The thrid and final task is to find the heattransfer related parameters. Heat transfer between two bodiespossessing different temperatures set the heat to flow fromhigh temperature body to low temperature body given, as in[4], by

ρC∂T

∂t−∇(k∇T ) = Q+ h(Tfinal − T ) (8)

where ρ is mass density of the material, C is heat capacity, k iscoefficient of heat conduction, Q is heat source, h is convectiveheat transfer coefficient, Tfinal is final temperature at whichthis heat transfer could settle down and T is body temperature.

III. SIMULATION RESULTS

The possibilities to install, operate, maintain and repair thecables, both when operated in one trench are dependent onsatisfying procedural standards which is otherwise followedby each cable. As per specifications, the depth of laying acable differ with each other and it is about 0.5 m for a powercable, 0.9 m to 1.65 m for an OFC [6]– [8]. Power cables aremade of one or more electrical conductors held together by asheath. Cables consist of three major components: conductors,insulation, protective jacket. The make-up of individual cablesvaries according to application. The construction and materialare determined by three main factors those could determinethickness of insulation, conductor size and type of sheath pro-tection to be given are dependent on operating voltage levelsof the cable, cable current carrying maximum capacity andunderground environmental conditions such as temperature,water, chemical or sunlight exposure, and mechanical impactetc., respectively. In this case, a typical 120 Sq.mm cablewith 11kV operating voltage possessing XLPE type 3.4 mmthickness insulation, 3 core cable, outer sheath of 2.8 mmthickness and finally making up a 57.6 mm overall diametercable was considered for simulation tests.

Single fibre OFC consist at high refractive index core atcentre followed by a lower refractive index cladding and

followed by a primary coating for purpose of mechanicalprotection and at end, strands of aramid yarn which couldprevent any physical damage to fibre glass [7]. At end, likepower cable, it has a cable jacket to match OFC environmentoperating conditions. In this case, single core Fibre cablehaving net dia. of 0.025 sq.mm is considered for purpose ofsimulation.

By theoretical analysis and from point of OFC immunityto EMI, [1], [2], technically it is understood that optical fibrecables can be laid in the vicinity of power cables. This willhelp to save the expenditure and time for completion of work.This study has identified all the possibilities of laying the twocables together and two such possibilities were identified. Thefirst problem identified was that if the strand of the powercable snaps, then there will be contact between the optical fibrecable and the power cable resulting in short circuit and livecontact among the two, which is unsafe. Second possibility, isthat it will create a problem while maintenance as the powercable lies, as shown in Fig. 3, above the optical fibre cable andaccess to the OFC will be a challenging task. However, thisgives installation and operational possibilities to realize theproposal. As per Power cable installation procedures followedby installation organizations, it was installed at 0.5 m deepfrom surface. Similarly, as per OFC installation proceduresfollowed, it was installed at 0.9 m or 1.65 m deep in to earth.Proposal considers a distance of 0.5 m, chosen in relationto trench width, lateral distance to be maintained among twocables shown as in Fig. 3.

Finally, placing both cables diagonally opposite to eachother was found to be the best arrangement. This reduces theprobability for both to come in live contact. The power cablewas placed at a certain distance from the optical fibre cable.This way of laying helps to sort out maintenance problemswhich has been pointed in the above text. The power cablewill not come in contact with optical fibre cable even if thereis a fault and maintenance of both is easier. Adding to this,only a section of few meters requires maintenance or repair atany point of time.

Laying the two cables, some mechanical support must bethere in between so that they don’t come in contact with eachother. Also, the space between the two cables must be filledwith sand so that the cables are geographically stable in theirinstalled positions. In addition, non-conducting material’s likewood may be provided between the two to provide mechanicalsupport. But, this method invites disadvantage. The powercables are robust and can be bent or twisted. This will causeno hindrance in transmission of power. But it is not thesame with the communication cable. Unlike power cables, theOFCs are weak and can be damaged easily. They cannot bebent or twisted. If this is done then the transmission of thecommunication signal is not possible.

For purpose of simulation, typical ratings were chosento simulate practical environment. Operating voltage 11kV,roughly the resistance of cable 0.457 , magnetic relativepermeability of PVC cladding µsheath of power cable is 0.045,and that of free space is 1 and for soil also it is 1 [10].

Horizontal spacing between OFC and power cable is 0.5 m,diagonal spacing is 0.64 m. From point of damage the powercable may cause to the OFC, three possibilities of failures asdiscussed above were simulated and response of parameterswas recorded. OFC is immune to EMI interference but how-ever higher and continuous exposure may have negative effects[1], [2]. Understanding both cables operations from this aspectis vital to the proposal.

Fig. 2: Voltage gradient distribution in a 11kV XLPE type 3core unarmoured cable

Normal functioning power cable is as shown in Fig. 2, hasvoltage gradient curves distributed around conductor insulationand in the outer sheath also. This shows the importanceof insulation and shielding in limiting the voltage gradientdistribution. However, highly concentrated voltage gradientsare a point of concern to the cable health. They increasestress on insulation leading to a physical damage like treeingphenomenon. In addition to this, voltage harmonics are foundto be another source of stress causing phenomenon results intreeing and damage to the cable insulation [5]. The proposed

Fig. 3: One trench accommodating OFC and Power cable bothas per the proposal

laying of trench power cable and OFC, as shown in Fig. 3,has satisfied all standards put up by respective installationprocedural standards. This proposal works well as long as nointerruptions were put by either of them throughout length of

operation. In this regard, to study the parameters discussedas above, the power cable failures have been introduced andresponse was simulated respectively. First, response of an

Fig. 4: Voltage gradient distribution of a punctured 11kVXLPE type 3 core unarmoured cable in vicinity of an OFCand surrounding soil

unloaded and punctured power cable operating at 11 kV,as shown in Fig. 4, was simulated to report voltage gra-dients, electric field vectors respectively. Voltage gradientswere shown in contours surrounding the punctured cable,surrounding space and OFC premises. Its continued operationwould result a charge to accumulate on OFC outer surface.However, the surrounding soil is at ground potential resultingin accumulation of charge if any on OFC surface to dischargeat same rate. Adding to this, low voltage gradients at OFCmake it less prone to the danger even during worse powercable failure. Unlike normal operating condition as shownin Fig. 2, cable voltage gradients as shown in Fig. 4 areless concentrated and radially occupy more space. This isa possible reason to explain less voltage gradient at OFClocation despite the fact that live power cable has punctured.Note, the electric filed vector arrows point radially outwardwith power cable at centre and at this stage power cable wasmere an infinite length charged line placed at a distance fromOFC. Second, loaded and punctured power cable operating at11kV, current density of 1 A/sq.mm was simulated to report, asshown in Fig. 5, the magnetic filed gradients, potential vectorsrespectively. Comparing to Electrostatic potential gradients asin Fig. 4, Magnetic potential contours as shown in Fig. 5appear similar to electric contours and where magnetic fluxdensity vector’s are radial, clockwise directed arrows whichappear dissimilar to electric field vectors. Magnetic potentialgradient appear more than half at places near to power cableand low at places far away from power cable and at thelocation of OFC in particular. This confirms low presence offlux at OFC location during event of cable failure that wasexposed to surroundings completely. The OFC functioningis immune to EMI and hence low quantity magnetic fluxpresence would have negligible effects on its performance[1], [2]. Third, temperature failure of power cable whereits operating temperature exceeds 70oC was simulated and

Fig. 5: Magnetic potential gradient distribution of a punctured11kV XLPE type 3 core unarmoured cable in vicinity of anOFC and surrounding soil

Fig. 6: Heat distribution of a punctured 11kV XLPE type 3core unarmoured cable in vicinity of an OFC and surroundingsoil

then, heat transfer related parameters response was recorded.Heat transfer parameters like coefficient of heat conduction(k),convective heat transfer coefficient are set at 0.3 W/m.K and 1W/(sq.mm K) for insulation, filling around insulation, sheathlayer and at 1 W/m.K and 1 W/(sq.mm K) for soil and Tfinalat 35oC, the surrounding normal temperature respectively. Inthis case, cable temperature was set to 90oC, higher than nor-mal operating voltage and simulation response was recordedhere with as shown in Fig. 6. Unlike power cable components,the high heat conductivity of the soil helps improve heatconduction from soil and is faster than cable components.This could be a possible reason for low temperatures at OFClocation influenced mostly by ground temperature and less bycable operating temperature. Soil heat conductivity results inevaporation of heat from the cable into atmosphere before itreaches the OFC. Adding to this, radial heat transfer of the heatproduced is an added advantage. Though OFC, as shown inFig. 6, possess heat gradient contours, they are low value heatcurves and hence they have less influence in causing damagingto the OFC.

IV. CONCLUSION

The operational possibilities of laying power cable andOFC in one trench have been discussed. Operating conditionswhich could affect OFC performance were simulated andrespective electrostatic, magnetic and heat parameters wereanalysed. Simulation results, influencing parameters responseduring power cable failures were recorded and they foundto have less or negligible affect on OFC performance. Thisproposal has found a solution for reducing the investment costof the consumer. However, field tests conforming the proposalclaims, development of a standard etc. inspire to work on thisproposal further.

ACKNOWLEDGMENT

The authors would like to thank Mr.Saini Singh, A.E.,Mr.Nishant, J.E., Central Public Works Department-Goa,Mr.Kerkar and Mr. Anshul Kumar, Bharat Sanchar NigamLimited- Panaji and Mr.ND Roy, GM Electrical, M/s. FinolexCables Pvt. Limited, Verna-Goa for rendering help duringcourse of study.

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