Type of Bonding Sys

Single Core Cable Single Core Cable Screen BondingScreen Bonding

Mitton Consulting Limited


OverviewOverview

IntroductionIntroductionCable screens, induced voltages and currents in the screenCable screens, induced voltages and currents in the screen

Three core cableThree core cableSingle core cableSingle core cable

Different bonding methods for single core cablesDifferent bonding methods for single core cablesSolidly bonded singleSolidly bonded single--core cable systemcore cable systemSpecially bonded singleSpecially bonded single--core cable systemscore cable systems

SingleSingle--pointpoint--bonding systembonding systemSplit singleSplit single--pointpoint--bonding systembonding systemCrossCross--bonding systembonding system

Case studies Case studies -- CDEGSCDEGSTMTM

Desktop study Desktop study –– induced voltage in cable screen for 1 km long 33 kV cableinduced voltage in cable screen for 1 km long 33 kV cablePractical example Practical example –– 27 km 33 kV cable27 km 33 kV cable


Cable screensCable screensCable screen typesCable screen types

Copper tapeCopper tapeCopper or aluminium wireCopper or aluminium wire

Purpose of cable screenPurpose of cable screenTo control the electric field stress in the cable insulationTo control the electric field stress in the cable insulationCable neutral and fault current return pathCable neutral and fault current return pathShielding for electromagnetic radiation Shielding for electromagnetic radiation

–– if the screen is earthed at two endsif the screen is earthed at two endsEnclosing dangerous high voltage with earth potential for safetyEnclosing dangerous high voltage with earth potential for safety

Some cables do not have Some cables do not have ‘‘screensscreens’’Normally cable screens need to be bonded to earth at both endsNormally cable screens need to be bonded to earth at both ends

Provide low impedance fault current return pathProvide low impedance fault current return pathProvide neutral point for the circuitProvide neutral point for the circuitProvide shielding of electromagnetic fieldProvide shielding of electromagnetic field


Induced voltage and circulating Induced voltage and circulating current in cable screencurrent in cable screen

Electromagnetic coupling between the core and screenElectromagnetic coupling between the core and screen

If the cable screen is single point bonded, no electrical continIf the cable screen is single point bonded, no electrical continuity, uity, the mmf generates a voltagethe mmf generates a voltage

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

The circulating current produces an The circulating current produces an opposingopposing magnetic fieldmagnetic field

VCore

Screen

Circulating currentOpposite current direction


Induced voltage and circulating Induced voltage and circulating current in cable screencurrent in cable screen

Steady state induced standing voltage limit for safety Steady state induced standing voltage limit for safety No internationally agreed limitNo internationally agreed limitDifferent countries or utilities have different limit or practiDifferent countries or utilities have different limit or practice (IEEE ce (IEEE Std575Std575--1988 Appendix C).1988 Appendix C).

In the order of 65In the order of 65--150 V, some utility allow for 300150 V, some utility allow for 300--400 V during emergency 400 V during emergency loadloadSome countries only specify voltage limit at exposed metal, someSome countries only specify voltage limit at exposed metal, some specify a fix specify a fix limit that any point along the screen can not exceedlimit that any point along the screen can not exceed

No much evidence to substantiate those limitsNo much evidence to substantiate those limitsEngineering Recommendation C55/4Engineering Recommendation C55/4

65 V for system voltage up to and including 132 kV65 V for system voltage up to and including 132 kV150 V for system voltage 275 kV and 400 kV150 V for system voltage 275 kV and 400 kV

Suitable bonding method should be employed to meet the Suitable bonding method should be employed to meet the standing voltage limit and keep circulating current to an standing voltage limit and keep circulating current to an acceptable levelacceptable levelInduced voltage and circulating current in the cable screen can Induced voltage and circulating current in the cable screen can be studied in CDEGSbe studied in CDEGSTM TM in detailin detail


ThreeThree--core cablescore cablesWell balanced magnetic field from three phasesWell balanced magnetic field from three phases

Induced voltages from three phases sum to zero along the entire Induced voltages from three phases sum to zero along the entire length of the length of the cablecable

Cable screen should be earthed at both endsCable screen should be earthed at both ends

Screen bonding method for threeScreen bonding method for three--core cable is not considered furthercore cable is not considered furthervirtually zero induced voltage or circulating current under steavirtually zero induced voltage or circulating current under steady state operationdy state operation


SingleSingle--core cablecore cableFor HV application, typically for 11 kV and aboveFor HV application, typically for 11 kV and above

SingleSingle--core cables negate the use of ferromagnetic material for core cables negate the use of ferromagnetic material for screen, sheath and armouringscreen, sheath and armouring

Induced voltage is mainly contributed by the core currents in iInduced voltage is mainly contributed by the core currents in its ts own phase and other two phasesown phase and other two phases

If cables are laid in a compact and symmetrical formation, inducIf cables are laid in a compact and symmetrical formation, induced ed voltage in the screen can be minimizedvoltage in the screen can be minimized

A suitable screen bonding method should be used for singleA suitable screen bonding method should be used for single--core core cables to preventcables to prevent

Excessive circulating currentExcessive circulating currentHigh induced standing voltageHigh induced standing voltage


Single core cable bonding methodsSingle core cable bonding methods

Different bonding methods for single core cableDifferent bonding methods for single core cableSolidly bonded singleSolidly bonded single--core cable systemcore cable system

Specially bonded singleSpecially bonded single--core cable systemscore cable systemsSingleSingle--pointpoint--bonding systembonding systemSplit singleSplit single--pointpoint--bonding systembonding systemCrossCross--bonding systembonding system


Solidly bonded singleSolidly bonded single--core cable core cable systemsystem

SimpleSimpleCable screen is bonded to earth grids at both ends (via link boxCable screen is bonded to earth grids at both ends (via link box))Most common methodMost common methodSignificant circulating current in the screenSignificant circulating current in the screen

Proportional to the core current and cable lengthProportional to the core current and cable lengthdede--rates cablerates cable

Could lay cable in compact trefoil formation if permissibleCould lay cable in compact trefoil formation if permissibleSuitable for route length of Suitable for route length of tens of meterstens of meters

R

Y

B

R

Y

B



Very small standing voltage in the order of several volts Very small standing voltage in the order of several volts

The magnitude of the induced voltage and current will be The magnitude of the induced voltage and current will be quantified in the case study laterquantified in the case study later

R

Y

B

R

Y

B

0 V

Standing Voltage Plot

Length

Magnitude

0 V



AdvantagesAdvantagesMinimum material required Minimum material required -- most economical if heating is not an issuemost economical if heating is not an issueProvides path for fault current, minimizing earth return currentProvides path for fault current, minimizing earth return current and and EGVR at cable destinationEGVR at cable destinationDoes not require screen voltage limiter (SVL)Does not require screen voltage limiter (SVL)Less electromagnetic radiationLess electromagnetic radiation

DisadvantagesDisadvantagesProvides path for circulating currentProvides path for circulating currentHeating effects in cable screen, greater lossesHeating effects in cable screen, greater lossesCable therefore might need to be deCable therefore might need to be de--rated or larger cable requiredrated or larger cable requiredTransfers voltages between sites when there is an EGVR at one siTransfers voltages between sites when there is an EGVR at one sitete

Can lay cables in trefoil formation to reduce screen lossesCan lay cables in trefoil formation to reduce screen lossesNormally applies to short cable section of tens of metres longNormally applies to short cable section of tens of metres long

Circulating current is proportional to the length of the cable aCirculating current is proportional to the length of the cable and the nd the magnitude of the load currentmagnitude of the load current


SingleSingle--pointpoint--bonded systembonded system

Cable screen solidly earthed at one end onlyCable screen solidly earthed at one end onlyOpen circuit in cable screen, no circulating currentOpen circuit in cable screen, no circulating currentZero volt at the earthed end, standing voltage at the unearthed Zero volt at the earthed end, standing voltage at the unearthed endendOptional PVC insulated earth continuity conductor required to prOptional PVC insulated earth continuity conductor required to provide path for fault ovide path for fault current, if returning from earth is undesirable, such as in a cocurrent, if returning from earth is undesirable, such as in a coal mineal mineSVL installed at the unearthed end to protect the cable insulatiSVL installed at the unearthed end to protect the cable insulation during fault on during fault conditions conditions Transposition of earth continuity conductor at the mid point of Transposition of earth continuity conductor at the mid point of the sectionthe section

Reduce circulating current in the continuity conductorReduce circulating current in the continuity conductor

SVL installed at unearthed end

R

Y

B

R

Y

B

Earth continuity conductor



Induced voltage proportional to the length of the cable and the Induced voltage proportional to the length of the cable and the current carried in the cablecurrent carried in the cableZero volt with respect to the earth grid voltage at the earthed Zero volt with respect to the earth grid voltage at the earthed end, standing voltage at the unearthed endend, standing voltage at the unearthed endNo circulating current in the screenNo circulating current in the screenCirculating current in the earthCirculating current in the earth--continuity conductor is not significant, as magnetic field from continuity conductor is not significant, as magnetic field from phases are phases are partially balancedpartially balancedThe magnitude of the standing voltage is depended on the magnituThe magnitude of the standing voltage is depended on the magnitude of the current flows in the core, de of the current flows in the core, much higher if there is an earth faultmuch higher if there is an earth fault

R

Y

B

R

Y

B

0 V

Induced Voltage Plot

Length

0 V


SingleSingle--pointpoint--bonded systembonded systemStanding voltage at the unearthed Standing voltage at the unearthed end with normal operating end with normal operating

conditionsconditions


SingleSingle--pointpoint--bonded systembonded systemStanding voltage at the unearthed Standing voltage at the unearthed end during earth fault conditionend during earth fault condition

Voltage at the unearthed end during an earth fault consists of Voltage at the unearthed end during an earth fault consists of two voltage componentstwo voltage components

Induced voltage due to fault current in the coreInduced voltage due to fault current in the coreEGVR of the source site (assuming the screen is single point bonEGVR of the source site (assuming the screen is single point bonded at ded at the source site)the source site)

The voltage due to induction can reach 700 V/kmThe voltage due to induction can reach 700 V/kmFor a 1 kA actual fault currentFor a 1 kA actual fault currentWith 0.05 With 0.05 Ω Ω earth grid impedance at the sourceearth grid impedance at the sourceScreen single point bonded at the source onlyScreen single point bonded at the source onlyCable in flat formation with 150 mm separationCable in flat formation with 150 mm separation

High voltage appears on the unearthed end can cause arcing and High voltage appears on the unearthed end can cause arcing and damage outer PVC sheathdamage outer PVC sheathThe voltage on the screen during a fault also depends on the The voltage on the screen during a fault also depends on the earthing conditionearthing condition


Link box with SVL and cable sectional jointLink box with SVL and cable sectional joint

Protect the outer PVC sheathProtect the outer PVC sheath

Minimizing the joint surge impedanceMinimizing the joint surge impedance



AdvantageAdvantageNo circulating currentNo circulating currentNo heating in the cable screenNo heating in the cable screenEconomicalEconomical

DisadvantageDisadvantageStanding voltage at the unStanding voltage at the un--earthed endearthed endRequires SVL if standing voltage during fault is excessiveRequires SVL if standing voltage during fault is excessiveRequires additional earth continuity conductor for fault currentRequires additional earth continuity conductor for fault current if earth if earth returned current is undesirablereturned current is undesirableHigher magnetic fields around the cable compared to solidly bondHigher magnetic fields around the cable compared to solidly bonded ed systemsystem

Standing voltage on the cable screen is proportional to the lengStanding voltage on the cable screen is proportional to the length th of the cable and the magnitude of current in the coreof the cable and the magnitude of current in the coreTypically suitable for cable sections less than 500 m, or one drTypically suitable for cable sections less than 500 m, or one drum um lengthlength


Split singleSplit single--pointpoint--bonded systembonded system

Variation of singleVariation of single--pointpoint--bondingbondingAlso called double length singleAlso called double length single--pointpoint--bonding systembonding systemCable screen continuity is interrupted at the midpoint and SVLs Cable screen continuity is interrupted at the midpoint and SVLs need to be need to be fitted at each side of the isolation jointfitted at each side of the isolation jointOther requirements are identical to singleOther requirements are identical to single--pointpoint--bonding systembonding system

SVLsSVLsEarth continuity conductorEarth continuity conductorTransposition of earth continuity conductorTransposition of earth continuity conductor

R

Y

B

R

Y

B

0 V



Effectively two sections of singleEffectively two sections of single--pointpoint--bondingbondingNo circulating currentNo circulating currentZero volt at the earthed ends, standing voltage at the Zero volt at the earthed ends, standing voltage at the sectionalising jointsectionalising joint

Induced Voltage Plot

Length

R

Y

B

R

Y

B

0 V

0 V



AdvantagesAdvantagesNo circulating current in the screenNo circulating current in the screenNo heating effect in the cable screenNo heating effect in the cable screenSuitable for longer cable section compared to singleSuitable for longer cable section compared to single--pointpoint--bonding bonding system and solidly bonded singlesystem and solidly bonded single--core systemcore systemEconomicalEconomical

DisadvantagesDisadvantagesStanding voltage exists at the screen and sectionalising insulatStanding voltage exists at the screen and sectionalising insulation jointion jointRequires SVL to protect the unRequires SVL to protect the un--earthed end earthed end Requires separate earth continuity conductor for zero sequence cRequires separate earth continuity conductor for zero sequence current urrent Not suitable for cable sections over 1000 mNot suitable for cable sections over 1000 m

Suitable for 300~1000 m long cable sections, double the length Suitable for 300~1000 m long cable sections, double the length of singleof single--pointpoint--bonding systembonding system


CrossCross--bonded cable systembonded cable system

Ultimate bonding methodUltimate bonding methodConsists of one or more major sections and three minor sections Consists of one or more major sections and three minor sections in each major sectionin each major sectionSumming up induced voltage in sectionalised screen from each phaSumming up induced voltage in sectionalised screen from each phase resulting in neutralisation of se resulting in neutralisation of induced voltages in three consecutive minor sectionsinduced voltages in three consecutive minor sectionsNormally one drum length (500 m approx) per minor sectionNormally one drum length (500 m approx) per minor sectionSectionalising position and cable jointing position should be coSectionalising position and cable jointing position should be coincidentincidentSolidly earthed at major section jointsSolidly earthed at major section jointsTranspose cable core to balance the magnitude of induced voltageTranspose cable core to balance the magnitude of induced voltages to be summed up s to be summed up Link box should be used at every sectionalising jointLink box should be used at every sectionalising jointbalanced impedance in all phases balanced impedance in all phases

Major section

Minor section Minor section Minor section

R

Y

B

R

Y

B

Earthing resistance not

shown in the plot


CrossCross--bonded cable systembonded cable systemwhat if cable cores not transposedwhat if cable cores not transposed

Other than crossOther than cross--bonding the screen, why transpose bonding the screen, why transpose the cables core?the cables core?

If core not transposed, not well neutralised resulting in some If core not transposed, not well neutralised resulting in some circulating currentscirculating currentsCable should be transposed and the screen needs to be cross Cable should be transposed and the screen needs to be cross bonded at each sectionalising joint position for optimal bonded at each sectionalising joint position for optimal neutralisationneutralisation

Inner screen, smaller induced

voltage

R

Y

B

R

Y

B

Earthing resistance not

shown in the plot


Cable bonded system Cable bonded system sectional joint link box diagramsectional joint link box diagram

R

Y

B

R

Y

B

Joint bay earthing system

Joint bay earthing system

Major section joint bay

Lockable link box

Screen voltage limiter

Joints with sectionalising

insulation

Cross bonding connections

Earthing resistance is not shown in the plot

Minor section joint bay

Joints with sectionalising

insulation



Induced voltage magnitude profile along the screen of a major seInduced voltage magnitude profile along the screen of a major section in the crossction in the cross--bonding cable systembonding cable systemVirtually zero circulating currentVirtually zero circulating currentVirtually zero volt to the remote earth at the solidly earthed eVirtually zero volt to the remote earth at the solidly earthed endsndsStanding voltage at the minor section jointsStanding voltage at the minor section joints

R

Y

B

R

Y

B

1~0.867

Minor section 1

Induced Voltage Magnitude Plot

Length

Minor section 2 Minor section 3

Major section

Earthing resistance is not shown in the plot


CrossCross--bonded cable system bonded cable system Interpretation of induced voltage magnitude plot by phasorInterpretation of induced voltage magnitude plot by phasor

1

~0.867

Section 1 Section 2 Section 3

Induced Voltage Magnitude Plot

Induced Voltage Phasor

Length

The induced voltage magnitude profile along the three The induced voltage magnitude profile along the three sections can also be interpreted by induced voltage sections can also be interpreted by induced voltage phasorphasor

Section 1

Section 2Section 3

0V Reference



In order to obtain optimal result, two In order to obtain optimal result, two ‘‘crossescrosses’’ existexistTransposition of cable core Transposition of cable core –– crossing cable core at each sectioncrossing cable core at each sectionCross bond the cable screens Cross bond the cable screens –– effectively no transposition of screeneffectively no transposition of screen

Cross bonding of cable screen Cross bonding of cable screen Cancellation of induced voltage in the screen at every major secCancellation of induced voltage in the screen at every major section jointtion joint

Transposition of cablesTransposition of cablesEnsure voltages to be summed up have similar magnitudeEnsure voltages to be summed up have similar magnitude

Greater standing voltage at the screen of the outer cableGreater standing voltage at the screen of the outer cable

Standing voltages exist at screen and majority of section jointsStanding voltages exist at screen and majority of section jointscable and joints must be installed as an insulated screen systemcable and joints must be installed as an insulated screen system



AdvantageAdvantageNo earthNo earth--continuity conductorcontinuity conductorElectrical continuity of screen for fault currentElectrical continuity of screen for fault currentVirtually zero circulating current in the screenVirtually zero circulating current in the screenStanding voltage in the screen is controlledStanding voltage in the screen is controlledTechnically superior than other methodsTechnically superior than other methodsSuitable for long distance cable networkSuitable for long distance cable network

DisadvantageDisadvantageTechnically complicatedTechnically complicatedMore expensiveMore expensive


Increased cable current carrying capacityIncreased cable current carrying capacitySolidlySolidly--bonded Vs specially bonded cable systembonded Vs specially bonded cable system

Specially bonded cable system effectively reduces circulating cuSpecially bonded cable system effectively reduces circulating current in the rrent in the screensscreensCurrent carrying capacity of specially bonded cable is increasedCurrent carrying capacity of specially bonded cable is increasedExample Example –– 33 kV 630 mm33 kV 630 mm22 cable, 28% more load for cross bondingcable, 28% more load for cross bonding

Conditions based on IEC 60287:

XLPE cableRated Voltage 10-70 kVCopper Conductor 65o C25 or 35 mm2 screenFlat formation, one group onlyLaying depth 1.0mDistance between cable 70mmGround temperature 20o CGround thermal resistivity 1.0 km/W


Case studies Case studies –– with CDEGSwith CDEGSTMTM

Three bonding arrangements:Three bonding arrangements:Cable screens earthed at both ends (flat and trefoil formation)Cable screens earthed at both ends (flat and trefoil formation)Cable screens earthed at one end only (flat formation)Cable screens earthed at one end only (flat formation)Cable screens cross bonded and earthed at both ends (flat formatCable screens cross bonded and earthed at both ends (flat formation)ion)

The cable configurations and operating conditions were as followThe cable configurations and operating conditions were as follows:s:100 100 ΩΩ--m soil resistivity, 5 m soil resistivity, 5 ΩΩ earth grid impedanceearth grid impedanceCable: 33 kV single core XLPE cable, 150 mmCable: 33 kV single core XLPE cable, 150 mm22 copper, with 0.3 mm copper tape screencopper, with 0.3 mm copper tape screenCircuit configuration: Circuit configuration:

Flat @ Flat @ 150 mm centres150 mm centres, 1 m deep, 1 m deepTrefoil @ compact formation, 1 m deep (For solidly bonded case oTrefoil @ compact formation, 1 m deep (For solidly bonded case only)nly)

Cable length: Cable length: 1 km1 kmOperating current: Operating current: 100 A100 A steady state per phasesteady state per phase

1 m

0.017 m

0.015m

1 m

0.150m


Solidly bonded singleSolidly bonded single--core cable core cable system system –– flat formationflat formation

Circulating currents cause earth grid voltage rise at two endsCirculating currents cause earth grid voltage rise at two endsVoltage magnitude is plotted with respect to remote earthVoltage magnitude is plotted with respect to remote earthLess than 1 V induced at the terminationLess than 1 V induced at the termination19~24 A circulating currents in the screen19~24 A circulating currents in the screen

0 15 30 45 60

Section Number

0.00

0.15

0.30

0.45

0.60

Shun

t Pot

entia

l Mag

nitu

de (V

olts

)

LEGEND

GRND_Rscreen: Bus/Line 4. GRND_Yscreen: Bus/Line 5. GRND_Bscreen: Bus/Line 6.

SINGLE COMPUTATION

RunID:Flat 2 Point Term.:Source

0 15 30 45 60

Section Number

0

10

20

30

Sect

ion

Cur

rent

Mag

nitu

de (A

mps

)

LEGEND


SINGLE COMPUTATION



Solidly bonded singleSolidly bonded single--core cable core cable system system –– trefoil formationtrefoil formation

Virtually zero volt in the screenVirtually zero volt in the screen9 A circulating currents in the screen (24 A for flat formation)9 A circulating currents in the screen (24 A for flat formation)Compact trefoil formation reduces circulating current, but does Compact trefoil formation reduces circulating current, but does not facilitate heat dissipationnot facilitate heat dissipation

0 15 30 45 60

Section Number

0.00

0.05

0.10

0.15

Shun

t Pot

entia

l Mag

nitu

de (V

olts

)

LEGEND


SINGLE COMPUTATION

RunID:trefoil 2 Po Term.:Source

0 15 30 45 60

Section Number

0

5

10

Sect

ion

Cur

rent

Mag

nitu

de (A

mps

)

LEGEND


SINGLE COMPUTATION

RunID:trefoil 2 Po Term.:Source


SingleSingle--pointpoint--bonded systembonded systemsteady state conditionsteady state condition

ScreenScreenOnly leakage current flows Only leakage current flows No circulating currentNo circulating current18 V standing voltage at the un18 V standing voltage at the un--earthed endearthed end

Earth conductorEarth conductorInsignificant standing voltage along the insulated earthInsignificant standing voltage along the insulated earth--continuity conductorcontinuity conductorInsignificant circulating current due to transpositionInsignificant circulating current due to transpositionvery low standing voltage at the solidly earthed end very low standing voltage at the solidly earthed end

due to minor circulating current in the earth continuity conductdue to minor circulating current in the earth continuity conductor flow into earth gridor flow into earth grid

0 15 30 45 60

Section Number

0

5

10

15

20

Shun

t Pot

entia

l Mag

nitu

de (V

olts

)

LEGEND

GRND_Rscreen: Bus/Line 4. GRND_Yscreen: Bus/Line 5. GRND_Bscreen: Bus/Line 6. GRND_Earth : Bus/Line 7.

SINGLE COMPUTATION

RunID:Flat 1 Point Term.:Source 0 15 30 45 60

Section Number

0.0

0.5

1.0

1.5

2.0

Sect

ion

Cur

rent

Mag

nitu

de (A

mps

)

LEGEND

GRND_Rscreen: Bus/Line 4. GRND_Yscreen: Bus/Line 5. GRND_Bscreen: Bus/Line 6. GRND_Earth : Bus/Line 7.

SINGLE COMPUTATION


This voltage depends on the capacity current and

the resistance of the earth grid


SingleSingle--pointpoint--bonded systembonded systemfault condition fault condition –– high EGVR at sourcehigh EGVR at source

1000 A earth return current, 51000 A earth return current, 5ΩΩ earth grid at the source siteearth grid at the source siteScreen only bonded at the sourceScreen only bonded at the sourceThe voltage at the screen is dominated by the EGVR of the faulteThe voltage at the screen is dominated by the EGVR of the faulted site due to relatively high d site due to relatively high earth grid impedance of the site (5 earth grid impedance of the site (5 ΩΩ))

5 kV above remote earth potential5 kV above remote earth potentialVoltage due to induction is only a small proportionVoltage due to induction is only a small proportion

0 15 30 45 60

Section Number

0

1500

3000

4500

6000

Shun

t Pot

entia

l Mag

nitu

de (V

olts

)

LEGEND


SINGLE COMPUTATION



SingleSingle--pointpoint--bonding systembonding systemfault condition fault condition ––low EGVR at sourcelow EGVR at source

1000 A actual fault current, 0.051000 A actual fault current, 0.05ΩΩ earth grid at the source siteearth grid at the source siteScreen only bonded at the sourceScreen only bonded at the sourceThe voltage at the screen is dominated by the induced voltageThe voltage at the screen is dominated by the induced voltageApproximately 700 V/kmApproximately 700 V/km

0 15 30 45 60

Section Number

0

200

400

600

800

Shun

t Pot

entia

l Mag

nitu

de (V

olts

)

LEGEND

Rscreen : Bus/Line 4. Yscreen : Bus/Line 5. Bscreen : Bus/Line 6.

SINGLE COMPUTATION


This voltage depends on the resistance of the earth grid and the actual fault current


CrossCross--bonding cable system bonding cable system cable core transposedcable core transposed

Virtually zero volt at earthed ends with respect to remote earthVirtually zero volt at earthed ends with respect to remote earth

About 6 V standing voltage at minor section jointAbout 6 V standing voltage at minor section joint

Virtually no circulating currentVirtually no circulating current

0 50 100 150 200

Section Number

0.0

1.5

3.0

4.5

6.0

Shun

t Pot

entia

l Mag

nitu

de (V

olts

)

LEGEND


SINGLE COMPUTATION



CrossCross--bonding cable system bonding cable system cable core not transposedcable core not transposed

Asymmetric voltage profile along the screens as expectedAsymmetric voltage profile along the screens as expected0.7 V appears at the termination joints, with respect to remote 0.7 V appears at the termination joints, with respect to remote earthearthApproximately 0.5 A circulating currentApproximately 0.5 A circulating currentCable core should be transposed to eliminate imbalanceCable core should be transposed to eliminate imbalance

Induced voltage on outer cable screen

Induced voltage on middle cable screen

0 50 100 150 200

Section Number

0.0

1.5

3.0

4.5

6.0

Shun

t Pot

entia

l Mag

nitu

de (V

olts

)

LEGEND


SINGLE COMPUTATION


This voltage depends on the current

imbalance and the resistance

of the earth grid


Summary Summary -- 11SolidlySolidly--bonded cable systembonded cable system

Inexpensive and simpleInexpensive and simpleSuitable for short length of cable sections, tens of metersSuitable for short length of cable sections, tens of metersTrefoil formation of cables can reduce circulating current (60 %Trefoil formation of cables can reduce circulating current (60 % reduction for the reduction for the case study given)case study given)

SingleSingle--pointpoint--bonding cable systembonding cable systemRelatively inexpensive and simpleRelatively inexpensive and simpleSuitable for cable sections where screen heating could be signifSuitable for cable sections where screen heating could be significanticantGenerally for sections less than 500 m or one drum lengthGenerally for sections less than 500 m or one drum length

Split singleSplit single--pointpoint--bonding cable systembonding cable systemRelatively inexpensive and simpleRelatively inexpensive and simpleDouble the length of singleDouble the length of single--pointpoint--bonding cable system, 300~500 mbonding cable system, 300~500 m

CrossCross--bonding cable systembonding cable systemTechnically complicated and financially expensive Technically complicated and financially expensive Suitable for long cable sections where induced voltage and screeSuitable for long cable sections where induced voltage and screen heating are of n heating are of concernconcern


Summary Summary -- 22

When to use different type of screen bonding method?When to use different type of screen bonding method?Should look at options on a case to case basisShould look at options on a case to case basis

Meet the induced voltage limit (65 V for system voltage up to 11Meet the induced voltage limit (65 V for system voltage up to 110 kV, 150 V 0 kV, 150 V otherwiseotherwiseConsider the circulating current and therefore heating effectConsider the circulating current and therefore heating effectFinancial considerationFinancial consideration

Specially bonded system is more complex and costlySpecially bonded system is more complex and costlySVLSVLLink boxLink boxJoint bayJoint bayEarthEarth--continuity conductorcontinuity conductorFully insulated systemFully insulated system

Only provide specially bonded system when circulating current isOnly provide specially bonded system when circulating current isexcessive or standing voltage is unacceptableexcessive or standing voltage is unacceptable

Type of Bonding Sys

Documents

mitton consulting

actual fault

bonded systemstanding

long cable

shunt potential

earth continuity

solidly bonded

screen bonding