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EHV/HV Cable Sheath Earthing:

Introduction: In urban areas, high voltage underground cables are commonly

used for the transmission and distribution of electricity. Suchhigh voltage cables have metallic sheaths or screenssurrounding the conductors, and/or armour and metallic wiressurrounding the cables. During earth faults applied to directlyearthed systems, these metallic paths are expected to carry asubstantial proportion of the total fault current, which would

otherwise flow through the general mass of earth, whilereturning to system neutrals. These alternative return pathsmust be considered when determining the extent of the gridpotential rise at an electrical plant due to earth faults.

For safety and reliable operation, the shields and metallicsheaths of power cables must be grounded. Without grounding,shields would operate at a potential considerably above ground.Thus, they would be hazardous to touch and would cause

rapid degradation of the jacket or other material interveningbetween shield and ground. This is caused by the capacitivecharging current of the cable insulation that is on the order of 1mA/ft of conductor length.

This current normally flows, at power frequency, between theconductor and the earth electrode of the cable, normally theshield. In addition, the shield or metallic sheath provides a faultreturn path in the event of insulation failure, permitting rapidoperation of the protection devices.

In order to reduce Circulating current and electric potentialdifference between the sheathings of single core three-phasecables, the sheathing is grounded and bonded at one or bothends of the cables. If the cable is long, double bonding has tobe carried out which leads to circulating currents and increasedtotal power loss. Raising the sheath’s resistance, by decreasingits cross section and increasing its resistivity, can reduce thisalmost to the level of the core losses.

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However, in case of an earth fault, a considerable portion of thefault current flows through the increased sheath resistance,

creating much higher power in the sheaths than in the faultycore. A simple solution, a conductor rod buried into the soilabove or under the cable can divert this power from thesheaths.

Cable Screen:

(1) Purpose of cable screen:

Cable screen controls the electric field stress in the cableinsulation.

Cable Screen Provides return path for Cable neutral and faultcurrent.

If the screen is earthed at two ends than it provides Shieldingfor electromagnetic radiation.

Enclosing dangerous high voltage with earth potential for safety.

(2) Purpose of bonding cable screens at both ends:

The electric power losses in a cable circuit are dependent onthe currents flowing in the metallic sheaths of the cables so byreducing the current flows in metallic sheath by differentmethods of bonding we can increases the load current carryingcapacity (ampacity) of the cable.

It provides low impedance fault current return path and providesneutral point for the circuit.

It provides shielding of electromagnetic field.(3) Induced voltage & circulating circulating current in cablescreen:

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Electromagnetic coupling between the core and screenElectromagnetic screen.

If the cable screen is single point bonded, no electricalcontinuity and mmf generates a voltage.

If the cable screen is bonded at both ends, the mmf will causecirculating current to flow if there is electrical continuity.

The circulating current produces an opposing magnetic field. Suitable bonding method should be employed to meet the

standing voltage limit and keep Circulating current to anacceptable level.

Laying Method of Cable:

The three Single core cables in a 3-phase circuit can be placedin different formations. Typical formations include trefoil(triangular) and flat formations.

(1) Trefoil Formation:

To minimize the electromechanical forces between the cablesunder short-circuit conditions, and to avoid eddy-current heatingin nearby steelwork due to magnetic fields set up by loadcurrents, the three single-core cables comprising the threephases of a 3-phase circuit are always run clamped in ‘Trefoil’

formation. Advantage:1. This type of Formation minimizes the sheath circulating currents

induced by the magnetic flux linking the cable conductors andmetallic sheath or copper wire screens.

2. This configuration is generally used for cables of lower voltages(33 to 132kV) and of smaller conductor sizes.

Disadvantages:

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1. The trefoil formation is not appropriate for heat dissipationbecause there is an appreciable mutual heating effect of the

three cables.2. The cumulated heat in cables and cable trench has the effect of 

reducing the cable rating and accelerating the cable ageing.(2) Flat Formation:

This is a most common method for Laying LT Cable. This formation is appropriate for heat dissipation and to

increase cable rating. The Formation choice is totally deepened on several factors like

screen bonding method, conductor area and available space for installation.

Type of Core and Induced Voltage:

(1) Three Core Cable:

For LT application, typically for below 11 kV. Well balanced magnetic field from Three Phase. Induced voltages from three phases sum to zero along the

entire length of the cable. Cable screen should be earthed at both ends Virtually zero induced voltage or circulating current under 

steady state operation.(2) Single Core Cable:

For HV application, typically for 11 kV and above. Single–core cables neglects the use of ferromagnetic material

for screen, sheath and armoring. Induced voltage is mainly contributed by the core currents in its

own phase and other two phases.If cables are laid in a compactand symmetrical formation, induced in the screen can beminimized.

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 A suitable screen bonding method should be used for single–core cables to prevent Excessive circulating current, high

induced standing voltage.igh voltage.Accessories for HT Cable Sheath Bonding:

(1) Function of Link Box?

Link Box is electrically and mechanically one of the integralaccessories of HV underground above ground cable bondingsystem, associated with HV XLPE power cable systems.

Link boxes are used with cable joints and terminations to

provide easy access to shield breaks for test purposes and to

limit voltage build-up on the sheath Lightning, fault currents and switching operations can cause

over voltages on the cable sheath. The link box optimizes lossmanagement in the cable shield on cables grounded both sides.

In HT Cable the bonding system is so designed that the cablesheaths are bonded and earthed or with SVL in such way as toeliminate or reduce the circulating sheath currents.

Link Boxes are used with cable joints and terminations toprovide easy access to shield breaks for test purposes and tolimit voltage build-up on the sheath. The link box is part of bonding system, which is essential of improving current carryingcapacity and human protection.

(2) Sheath Voltage Limiters (SVL) (Surge Arrestors):

SVL is protective device to limit induce voltages appearing onthe bonded cable system due to short circuit.

It is necessary to fit SVL’s between the metallic screen andground inside the link box. The screen separation of power cable joint (insulated joint) will be protected against possibledamages as a result of induced voltages caused by shortcircuit/break down.

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Type of Sheath Bonding for HT Cable:

There is normally Three Type of Bonding for LT/HT Cable Screen.(1) Single Point Bonded.1. One Side Single Point Bonded System.2. Split Single Point Bonded System.(2) Both End Bonded System(3) Cross Bonded System(1) Single point bonded system:

(A) One Side Single Bonded System:

 A system is single point bonded if the arrangements are suchthat the cable sheaths provide no path for the flow of circulatingcurrents or external fault currents.

This is the simplest form of special bonding. The sheaths of thethree cable sections are connected and grounded at one pointonly along their length. At all other points there will be avoltage between sheath and ground and between screens of adjacent phases of the cable circuit that will be at its maximum

at the farthest point from the ground bond. This induced voltage is proportional to the cable length and

current. Single-point bonding can only be used for limited routelengths, but in general the accepted screen voltage potentiallimits the length

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The sheaths must therefore be adequately insulated fromground. Since there is no closed sheath circuit, except through

the sheath voltage limiter, current does not normally flowlongitudinally along the sheaths and no sheath circulationcurrent loss occurs.

Open circuit in cable screen, no circulating current. Zero volt at the earthed end, standing voltage at the unearthed

end. Optional PVC insulated earth continuity conductor required to

provide path for fault current, if returning from earth is

undesirable, such as in a coal mine. SVL installed at the unearthed end to protect the cable

insulation during fault conditions. Induced voltage proportional to the length of the cable and the

current carried in the cable . Zero volt with respect to the earth grid voltage at the earthed

end, standing voltage at the unearthed end. Circulating current in the earth–continuity conductor is not

significant, as magnetic fields from phases are partiallybalanced. The magnitude of the standing voltage is depended on the

magnitude of the current flows in the core, much higher if thereis an earth fault.

High voltage appears on the unearthed end can cause arcingand damage outer PVC sheath.

The voltage on the screen during a fault also depends on theearthing condition.

Standing voltage at the unearthed end during earth faultcondition. During a ground fault on the power system the zero sequence

current carried by the cable conductors could return bywhatever external paths are available. A ground fault in theimmediate vicinity of the cable can cause a large difference inground potential rise between the two ends of the cable system,posing hazards to personnel and equipment.

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For this reason, single-point bonded cable installationsneed a parallel ground conductor , grounded at both ends of 

the cable route and installed very close to the cable conductors,to carry the fault current during ground faults and to limit thevoltage rise of the sheath during ground faults to an acceptablelevel.

The parallel ground continuity conductor is usually insulated toavoid corrosion and transposed, if the cables are nottransposed, to avoid circulating currents and losses duringnormal operating conditions.

Voltage at the unearthed end during an earth fault consists of two voltage components. Induced voltage due to fault current inthe core.

Advantage: No circulating current. No heating in the cable screen. Economical.Disadvantage:

Standing voltage at the un–earthed end. Requires SVL if standing voltage during fault is excessive. Requires additional earth continuity conductor  for fault

current if earth returned current is undesirable. Higher magneticfields around the cable compared to solidly bonded system.

Standing voltage on the cable screen is proportional to thelength of the cable and the magnitude of current in the core.

Typically suitable for cable sections less than 500 m, or onedrum length.

(B) Split Single Point-bonded System:

It is also known as double length single point bondingSystem.

Cable screen continuity is interrupted at the midpoint and SVLsneed to be fitted at each side of the isolation joint.

Other requirements are identical to single–point–bondingsystem like SVL, Earth continuity Conductor, Transposition of earth continuity conductor.

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Effectively two sections of single–point–bonding. No circulating current and Zero volt at the earthed ends,

standing voltage at the sectionalizing joint.

Advantages: No circulating current in the screen. No heating effect in the cable screen. Suitable for longer cable section compared to single–point–

bonding system and solidly bonded single-core system. Economical.Disadvantages: Standing voltage exists at the screen and sectionalizing

insulation joint. Requires SVL to protect the un–earthed end. Requires separate earth continuity conductor for zero sequence

current. Not suitable for cable sections over 1000 m. Suitable for 300~1000 m long cable sections, double the length

of single–point–bonding system.(2) Both End Solidly Bonded (Single-core cable) systems.

Most Simple and Common method. Cable screen is bonded to earth grids at both ends (via link

box). To eliminate the induced voltages in Cable Screen is to bond

(Earth) the sheath at both ends of the cable circuit.

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This eliminates the need for the parallel continuity conductor used in single bonding systems. It also eliminates the need to

provide SVL, such as that used at the free end of single-pointbonding cable circuits

Significant circulating current in the screen Proportional to thecore current and cable length and de rates cable.

Could lay cable in compact trefoil formation if permissible. Suitable for route length of more than 500 Meter . Very small standing voltage in the order of several volts.

Advantages: Minimum material required. Most economical if heating is not a main issue. Provides path for fault current, minimizing earth return current

and EGVR at cable destination. Does not require screen voltage limiter (SVL). Less electromagnetic radiation.

Disadvantages: Provides path for circulating current. Heating effects in cable screen, greater losses .Cable therefore

might need to be de–rated or larger cable required. Transfers voltages between sites when there is an EGVR at

one site. Can lay cables in trefoil formation to reduce screen losses .

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Normally applies to short cable section of tens of meters long.Circulating current is proportional to the length of the cable and

the magnitude of the load current.(3) Cross-bonded cable system.

 A system is cross-bonded if the arrangements are such that thecircuit provides electrically continuous sheath runs from earthedtermination to earthed termination but with the sheaths sosectionalized and cross-connected in order to reduce thesheath circulating currents.

In This Type voltage will be induced between screen and earth,but no significant current will flow.

The maximum induced voltage will appear at the link boxes for cross-bonding. This method permits a cable current-carryingcapacity as high as with single-point bonding but longer routelengths than the latter. It requires screen separation andadditional link boxes.

For cross bonding, the cable length is divided into threeapproximately equal sections. Each of the three alternating

magnetic fields induces a voltage with a phase shift of 120° inthe cable shields.

The cross bonding takes place in the link boxes. Ideally, thevectorial addition of the induced voltages results in U (Rise) = 0.In practice, the cable length and the laying conditions will vary,resulting in a small residual voltage and a negligible current.Since there is no current flow, there are practically no losses inthe screen.

The total of the three voltages is zero, thus the ends of thethree sections can be grounded.

Summing up induced voltage in sectionalized screen from eachphase resulting in neutralization of induced voltages in threeconsecutive minor sections.

Normally one drum length (500 m approx) per minor section. Sectionalizing position and cable jointing position should be

coincident.

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Solidly earthed at major section joints. Transpose cable core to balance the magnitude of induced

voltages to be summed up. Link box should be used at every sectionalizing joint and

balanced impedance in all phases. Induced voltage magnitude profile along the screen of a major 

section in the cross–bonding cable system. Virtually zero circulating current and Voltage to the remote earth

at the solidly earthed ends. In order to obtain optimal result, two ‘‘crosses’’ exist. One is

Transposition of cable core crossing cable core at each sectionand second is Cross bond the cable screens effectively notransposition of screen.

Cross bonding of cable screen: It is cancelled inducedvoltage in the screen at every major Section joint.

Transposition of cables:It is ensure voltages to be summedup have similar magnitude .Greater standing voltage at thescreen of the outer cable.

Standing voltages exist at screen and majority of section jointscable and joints must be installed as an insulated screensystem.

Requirement of transposefor cables core. If core not transposed, not well neutralized resulting in some

circulating currents.

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Cable should be transposed and the screen needs to be crossbonded at each sectionalizing joint position for optimal

neutralization

Advantage: Not required any earth continuity conductor. Virtually zero circulating current in the screen. Standing voltage in the screen is controlled.

Technically superior than other methods. Suitable for long distance cable network.Disadvantage: Technically complicated. More expensive.Bonding Method Comparison:

EarthingMethod

Standing

Voltageat CableEnd

Sheath

VoltageLimiter Required

Application

Single EndBonding

Yes Yes Up to 500 Meter  

Double EndBonding

No No Up to 1 Km andSubstations short

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connections, hardlyapplied for HVcables, rather for MVand LV cables

CrossBonding

Only atcrossbondingpoints

Yes Long distanceconnections where

 joints are required

Sheath Losses according to type of Bonding:

Sheath losses are current-dependent losses and are generatedby the induced currents when load current flows in cableconductors.

The sheath currents in single-core cables are induced by“transformer” effect; i.e.by the magnetic field of alternatingcurrent flowing in cable conductor which induces voltages incable sheath or other parallel conductors.

The sheath induced electromotive forces (EMF) generate two

types of losses: circulating current losses (Y1) and eddy currentlosses (Y2), so the total losses in cable metallic sheath are: Y=

 Y1+Y2 The eddy currents circulating radially and longitudinally of cable

sheaths are generated on similar principles of skin andproximity effects i.e. they are induced by the conductor currents, sheath circulating currents and by currents circulatingin close proximity current carrying conductors.

They are generated in cable sheath irrespective of bondingsystem of single core cables or of three-core cables

The eddy currents are generally of smaller magnitude whencomparing with circuit (circulating) currents of solidly bondedcable sheaths and may be neglects except in the case of largesegmental conductors and are calculated in accordance withformulae given in the IEC60287.

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Circulating currents are generated in cable sheath if thesheaths form a closed loop when bonded together at the

remote ends or intermediate points along the cable route. These losses are named sheath circulating current losses and

they are determined by the magnitude of current in cableconductor, frequency, mean diameter, the resistance of cablesheath and the distance between single-core cables.

Conclusion:

There is much disagreement as to whether the cable shieldshould be grounded at both ends or at only one end. If grounded at only one end, any possible fault current musttraverse the length from the fault to the grounded end, imposinghigh current on the usually very light shield conductor. Such acurrent could readily damage or destroy the shield and requirereplacement of the entire cable rather than only the faultedsection.

With both ends grounded, the fault current would divide andflow to both ends, reducing the duty on the shield, with

consequently less chance of damage. Multiple grounding, rather than just grounding at both ends, is

simply the grounding of the cable shield or sheath at all accesspoints, such as manholes or pull boxes. This also limits possibleshield damage to only the faulted section.