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8/12/2019 Cable Screen Bonding
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Single Core CableSingle Core CableScreen BondingScreen Bonding
Mitton Consulting Limited
8/12/2019 Cable Screen Bonding
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Mitton Consulting Limited
OverviewOverview IntroductionIntroduction
Cable screens, induced voltages and currents in the screenCable screens, induced voltages and currents in the screen Three core cableThree core cable
Single core cableSingle core cable
Different bonding methods for single core cablesDifferent bonding methods for single core cables
Solidly bonded singleSolidly bonded single--core cable systemcore cable system Specially bonded singleSpecially bonded single--core cable systemscore cable systems
SingleSingle--pointpoint--bonding systembonding system
Split singleSplit single--pointpoint--bonding systembonding system
CrossCross
--bonding systembonding system
Case studiesCase studies -- CDEGSCDEGSTMTM
Desktop studyDesktop studyinduced voltage in cable screen for 1 km long 33 kV cableinduced voltage in cable screen for 1 km long 33 kV cable
Practical examplePractical example27 km 33 kV cable27 km 33 kV cable
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Cable screensCable screens Cable screen typesCable screen types
Copper tapeCopper tape
Copper or aluminium wireCopper or aluminium wire
Purpose of cable screenPurpose of cable screen To control the electric field stress in the cable insulationTo control the electric field stress in the cable insulation
Cable neutral and fault current return pathCable neutral and fault current return path
Shielding for electromagnetic radiationShielding for electromagnetic radiationif the screen is earthed at two endsif the screen is earthed at two ends
Enclosing dangerous high voltage with earth potential for safetyEnclosing dangerous high voltage with earth potential for safety
Some cables do not haveSome 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 path
Provide neutral point for the circuitProvide neutral point for the circuit
Provide shielding of electromagnetic fieldProvide shielding of electromagnetic field
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Induced voltage and circulatingInduced 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 causeIf the cable screen is bonded at both ends, the mmf will cause aa
circulating current to flow if there is electrical continuity.circulating current to flow if there is electrical continuity.
The circulating current produces anThe circulating current produces an opposingopposingmagnetic fieldmagnetic field
V
Core
Screen
Circulating current
Opposite current direction
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Induced voltage and circulatingInduced voltage and circulating
current in cable screencurrent in cable screen Steady state induced standing voltage limit for safetySteady state induced standing voltage limit for safety
No internationally agreed limitNo internationally agreed limit
Different countries or utilities have different limit or practiDifferent countries or utilities have different limit or practice (IEEEce (IEEEStd575Std575--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 emergency400 V during emergency
loadload
Some countries only specify voltage limit at exposed metal, someSome countries only specify voltage limit at exposed metal, some specify a fixspecify 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 limits Engineering 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 kV
150 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 theSuitable bonding method should be employed to meet thestanding voltage limit and keep circulating current to anstanding voltage limit and keep circulating current to anacceptable levelacceptable level
Induced voltage and circulating current in the cable screen canInduced voltage and circulating current in the cable screen can
be studied in CDEGSbe studied in CDEGSTMTM in detailin detail
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ThreeThree--core cablescore cables Well balanced magnetic field from three phasesWell balanced magnetic field from three phases
Induced voltages from three phases sum to zero along the entireInduced voltages from three phases sum to zero along the entire length of thelength 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 further
virtually zero induced voltage or circulating current under steavirtually zero induced voltage or circulating current under steady state operationdy state operation
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SingleSingle--core cablecore cable For 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 forcore 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 itstsown 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, inducededvoltage 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--corecore
cables to preventcables to prevent Excessive circulating currentExcessive circulating current
High induced standing voltageHigh induced standing voltage
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Single core cable bonding methodsSingle core cable bonding methods
Different bonding methods for single core cableDifferent bonding methods for single core cable
Solidly bonded singleSolidly bonded single--core cable systemcore cable system
Specially bonded singleSpecially bonded single--core cable systemscore cable systems
SingleSingle--pointpoint--bonding systembonding system Split singleSplit single--pointpoint--bonding systembonding system
CrossCross--bonding systembonding system
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Solidly bonded singleSolidly bonded single--core cablecore cable
systemsystem SimpleSimple
Cable 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 method Significant circulating current in the screenSignificant circulating current in the screen
Proportional to the core current and cable lengthProportional to the core current and cable length
dede--rates cablerates cable
Could lay cable in compact trefoil formation if permissibleCould lay cable in compact trefoil formation if permissible Suitable for route length ofSuitable for route length of tens of meterstens of meters
R
Y
B
R
Y
B
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Solidly bonded singleSolidly bonded single--core cablecore cable
systemsystem Very small standing voltage in the order of several voltsVery small standing voltage in the order of several volts
The magnitude of the induced voltage and current will beThe magnitude of the induced voltage and current will bequantified 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
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Solidly bonded singleSolidly bonded single--core cablecore cable
systemsystem AdvantagesAdvantages
Minimum material requiredMinimum material required -- most economical if heating is not an issuemost economical if heating is not an issue
Provides path for fault current, minimizing earth return currentProvides path for fault current, minimizing earth return current andandEGVR at cable destinationEGVR at cable destination
Does not require screen voltage limiter (SVL)Does not require screen voltage limiter (SVL)
Less electromagnetic radiationLess electromagnetic radiation
DisadvantagesDisadvantages Provides path for circulating currentProvides path for circulating current
Heating effects in cable screen, greater lossesHeating effects in cable screen, greater losses
Cable therefore might need to be deCable therefore might need to be de--rated or larger cable requiredrated or larger cable required
Transfers 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 losses
Normally 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 thend the
magnitude of the load currentmagnitude of the load current
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SingleSingle--pointpoint--bonded systembonded system Cable screen solidly earthed at one end onlyCable screen solidly earthed at one end only
Open circuit in cable screen, no circulating currentOpen circuit in cable screen, no circulating current
Zero volt at the earthed end, standing voltage at the unearthedZero volt at the earthed end, standing voltage at the unearthed endend Optional PVC insulated earth continuity conductor required to prOptional PVC insulated earth continuity conductor required to provide path for faultovide 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 mine
SVL installed at the unearthed end to protect the cable insulatiSVL installed at the unearthed end to protect the cable insulation during faulton during faultconditionsconditions
Transposition of earth continuity conductor at the mid point ofTransposition 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 atunearthed end
R
Y
B
R
Y
B
Earth continuityconductor
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SingleSingle--pointpoint--bonded systembonded system Induced voltage proportional to the length of the cable and theInduced voltage proportional to the length of the cable and the current carried in the cablecurrent carried in the cable
Zero volt with respect to the earth grid voltage at the earthedZero volt with respect to the earth grid voltage at the earthed end, standing voltage at the unearthed endend, standing voltage at the unearthed end
No circulating current in the screenNo circulating current in the screen
Circulating current in the earthCirculating current in the earth--continuity conductor is not significant, as magnetic field fromcontinuity conductor is not significant, as magnetic field from phases arephases arepartially balancedpartially balanced
The 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
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SingleSingle--pointpoint--bonded systembonded system
Standing voltage at the unearthedStanding voltage at the unearthed end with normal operatingend with normal operatingconditionsconditions
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SingleSingle--pointpoint--bonded systembonded systemStanding voltage at the unearthedStanding voltage at the unearthed end during earth fault conditionend during earth fault condition
Voltage at the unearthed end during an earth fault consists ofVoltage at the unearthed end during an earth fault consists oftwo voltage componentstwo voltage components Induced voltage due to fault current in the coreInduced voltage due to fault current in the core
EGVR of the source site (assuming the screen is single point bonEGVR of the source site (assuming the screen is single point bonded atded atthe source site)the source site)
The voltage due to induction can reach 700 V/kmThe voltage due to induction can reach 700 V/km For a 1 kA actual fault currentFor a 1 kA actual fault current
With 0.05With 0.05 earth grid impedance at the sourceearth grid impedance at the source Screen single point bonded at the source onlyScreen single point bonded at the source only
Cable in flat formation with 150 mm separationCable in flat formation with 150 mm separation
High voltage appears on the unearthed end can cause arcing andHigh voltage appears on the unearthed end can cause arcing and
damage outer PVC sheathdamage outer PVC sheath The voltage on the screen during a fault also depends on theThe voltage on the screen during a fault also depends on the
earthing conditionearthing condition
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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
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SingleSingle--pointpoint--bonded systembonded system AdvantageAdvantage
No circulating currentNo circulating current
No heating in the cable screenNo heating in the cable screen EconomicalEconomical
DisadvantageDisadvantage Standing voltage at the unStanding voltage at the un--earthed endearthed end
Requires SVL if standing voltage during fault is excessiveRequires SVL if standing voltage during fault is excessive Requires additional earth continuity conductor for fault currentRequires additional earth continuity conductor for fault current if earthif earth
returned current is undesirablereturned current is undesirable
Higher magnetic fields around the cable compared to solidly bondHigher magnetic fields around the cable compared to solidly bondededsystemsystem
Standing voltage on the cable screen is proportional to the lengStanding voltage on the cable screen is proportional to the lengththof the cable and the magnitude of current in the coreof the cable and the magnitude of current in the core
Typically suitable for cable sections less than 500 m, or one drTypically suitable for cable sections less than 500 m, or one drumumlengthlength
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Split singleSplit single
--pointpoint
--bonded systembonded system
Variation of singleVariation of single--pointpoint--bondingbonding
Also called double length singleAlso called double length single--pointpoint--bonding systembonding system
Cable screen continuity is interrupted at the midpoint and SVLsCable screen continuity is interrupted at the midpoint and SVLs need to beneed to befitted at each side of the isolation jointfitted at each side of the isolation joint
Other requirements are identical to singleOther requirements are identical to single--pointpoint--bonding systembonding system
SVLsSVLs
Earth continuity conductorEarth continuity conductor
Transposition of earth continuity conductorTransposition of earth continuity conductor
R
Y
B
R
Y
B
0 V
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Split singleSplit single
--pointpoint
--bonded systembonded system
Effectively two sections of singleEffectively two sections of single--pointpoint--bondingbonding
No circulating currentNo circulating current Zero volt at the earthed ends, standing voltage at theZero volt at the earthed ends, standing voltage at the
sectionalising jointsectionalising jointInduced Voltage Plot
Length
R
Y
B
R
Y
B
0 V
0 V
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Split singleSplit single
--pointpoint
--bonded systembonded system
AdvantagesAdvantages No circulating current in the screenNo circulating current in the screen
No heating effect in the cable screenNo heating effect in the cable screen Suitable for longer cable section compared to singleSuitable for longer cable section compared to single--pointpoint--bondingbonding
system and solidly bonded singlesystem and solidly bonded single--core systemcore system
EconomicalEconomical
DisadvantagesDisadvantages Standing voltage exists at the screen and sectionalising insulatStanding voltage exists at the screen and sectionalising insulation jointion joint
Requires SVL to protect the unRequires SVL to protect the un--earthed endearthed end
Requires separate earth continuity conductor for zero sequence cRequires separate earth continuity conductor for zero sequence currenturrent
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 lengthSuitable for 300~1000 m long cable sections, double the length
of singleof single--pointpoint--bonding systembonding system
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CrossCross--bonded cable systembonded cable system Ultimate bonding methodUltimate bonding method
Consists of one or more major sections and three minor sectionsConsists of one or more major sections and three minor sections in each major sectionin each major section
Summing up induced voltage in sectionalised screen from each phaSumming up induced voltage in sectionalised screen from each phase resulting in neutralisation ofse resulting in neutralisation of
induced voltages in three consecutive minor sectionsinduced voltages in three consecutive minor sections Normally one drum length (500 m approx) per minor sectionNormally one drum length (500 m approx) per minor section
Sectionalising position and cable jointing position should be coSectionalising position and cable jointing position should be coincidentincident
Solidly earthed at major section jointsSolidly earthed at major section joints
Transpose cable core to balance the magnitude of induced voltageTranspose cable core to balance the magnitude of induced voltages to be summed ups to be summed up
Link box should be used at every sectionalising jointLink box should be used at every sectionalising joint balanced impedance in all phasesbalanced impedance in all phases
Major section
Minor section Minor section Minor section
R
Y
B
R
Y
B
Earthingresistance not
shown in the plot
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CrossCross--bonded cable systembonded cable system
what if cable cores not transposedwhat if cable cores not transposed
Other than crossOther than cross--bonding the screen, why transposebonding the screen, why transposethe cables core?the cables core?
If core not transposed, not well neutralised resulting in someIf core not transposed, not well neutralised resulting in somecirculating currentscirculating currents
Cable should be transposed and the screen needs to be crossCable should be transposed and the screen needs to be crossbonded at each sectionalising joint position for optimalbonded at each sectionalising joint position for optimal
neutralisationneutralisationInner screen,
smaller inducedvoltage
R
Y
B
R
Y
B
Earthingresistance not
shown in the plot
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Cable bonded systemCable bonded systemsectional joint link box diagramsectional joint link box diagram
R
Y
B
R
Y
B
Joint bayearthing system
Joint bayearthing system
Major sectionjoint bay
Lockable link box
Screen voltagelimiter
Joints withsectionalising
insulation
Cross bonding
connections
Earthingresistance is notshown in the plot
Minor sectionjoint bay
Joints withsectionalising
insulation
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CrossCross
--bonded cable systembonded cable system
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 system
Virtually zero circulating currentVirtually zero circulating current
Virtually zero volt to the remote earth at the solidly earthed eVirtually zero volt to the remote earth at the solidly earthed endsnds
Standing 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
Earthingresistance is notshown in the plot
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CrossCross--bonded cable systembonded cable systemInterpretation 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 threeThe induced voltage magnitude profile along the three
sections can also be interpreted by induced voltagesections can also be interpreted by induced voltagephasorphasor
Section 1
Section 2Section 3
0V Reference
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CrossCross
--bonded cable systembonded cable system
In order to obtain optimal result, twoIn order to obtain optimal result, two crossescrosses existexist
Transposition of cable coreTransposition of cable corecrossing cable core at each sectioncrossing cable core at each section
Cross bond the cable screensCross bond the cable screenseffectively no transposition of screeneffectively no transposition of screen
Cross bonding of cable screenCross 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 cables Ensure 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
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CrossCross
--bonded cable systembonded cable system
AdvantageAdvantage
No earthNo earth--continuity conductorcontinuity conductor Electrical continuity of screen for fault currentElectrical continuity of screen for fault current
Virtually zero circulating current in the screenVirtually zero circulating current in the screen
Standing voltage in the screen is controlledStanding voltage in the screen is controlled Technically superior than other methodsTechnically superior than other methods
Suitable for long distance cable networkSuitable for long distance cable network
DisadvantageDisadvantage Technically complicatedTechnically complicated
More expensiveMore expensive
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Increased cable current carrying capacityIncreased cable current carrying capacity
SolidlySolidly--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 therrent in thescreensscreens
Current carrying capacity of specially bonded cable is increasedCurrent carrying capacity of specially bonded cable is increased ExampleExample33 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 cable
Rated Voltage 10-70 kV
Copper Conductor 65o C
25 or 35 mm2 screen
Flat formation, one group only
Laying depth 1.0m
Distance between cable 70mmGround temperature 20o C
Ground thermal resistivity 1.0 km/W
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Case studiesCase 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: 100100 --m soil resistivity, 5m soil resistivity, 5 earth grid impedanceearth grid impedance Cable: 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 screen
Circuit configuration:Circuit configuration: Flat @Flat @ 150 mm centres150 mm centres, 1 m deep, 1 m deep
Trefoil @ 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 km
Operating current:Operating current: 100 A100 A steady state per phasesteady state per phase
1 m
0.017 m
0.015m
1 m
0.150m
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Solidly bonded singleSolidly bonded single--core cablecore cable
systemsystemflat formationflat formation Circulating currents cause earth grid voltage rise at two endsCirculating currents cause earth grid voltage rise at two ends
Voltage magnitude is plotted with respect to remote earthVoltage magnitude is plotted with respect to remote earth
Less than 1 V induced at the terminationLess than 1 V induced at the termination
19~24 A circulating currents in the screen19~24 A circulating currents in the screen
0 15 30 45 60
0.00
0.15
0.30
0.45
0.60
ShuntPotentialM
agnitude(Volts)
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
SectionCurrentMagnitude(Amps)
LEGEND
GRND_Rscreen: Bus/Line 4.GRND_Yscreen: Bus/Line 5.
GRND_Bscreen: Bus/Line 6.
SINGLE COMPUTATION
RunID:Flat 2 Point Term.:Source
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Solidly bonded singleSolidly bonded single--core cablecore cable
systemsystemtrefoil formationtrefoil formation Virtually zero volt in the screenVirtually zero volt in the screen
9 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 doesCompact trefoil formation reduces circulating current, but doesnot facilitate heat dissipationnot facilitate heat dissipation
0 15 30 45 60
Section Number
0.00
0.05
0.10
0.15
ShuntPotentialM
agnitude(Volts)
LEGEND
GRND_Rscreen: Bus/Line 4.
GRND_Yscreen: Bus/Line 5.
GRND_Bscreen: Bus/Line 6.
SINGLE COMPUTATION
RunID:trefoil 2 Po Term.:Source
0 15 30 45 60
Section Number
0
5
10
SectionCurrent
Magnitude(Amps)
LEGEND
GRND_Rscreen: Bus/Line 4.
GRND_Yscreen: Bus/Line 5.GRND_Bscreen: Bus/Line 6.
SINGLE COMPUTATION
RunID:trefoil 2 Po Term.:Source
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SingleSingle--pointpoint--bonded systembonded system
steady state conditionsteady state condition ScreenScreen
Only leakage current flowsOnly leakage current flows
No circulating currentNo circulating current
18 V standing voltage at the un18 V standing voltage at the un--earthed endearthed end Earth conductorEarth conductor
Insignificant standing voltage along the insulated earthInsignificant standing voltage along the insulated earth--continuity conductorcontinuity conductor
Insignificant circulating current due to transpositionInsignificant circulating current due to transposition
very low standing voltage at the solidly earthed endvery 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
ShuntPotentialMagnitude(Volts)
LEGEND
GRND_Rscree n: Bus/Line 4.
GRND_Ys creen: Bus/Line 5.
GRND_Bscree n: 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
SectionCurrentMagn
itude(Amps)
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
This voltage
depends on
the capacity
current and
theresistance of
the earth grid
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SingleSingle--pointpoint--bonded systembonded system
fault conditionfault conditionhigh EGVR at sourcehigh EGVR at source 1000 A earth return current, 51000 A earth return current, 5 earth grid at the source siteearth grid at the source site Screen only bonded at the sourceScreen only bonded at the source
The 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 highd site due to relatively highearth grid impedance of the site (5earth grid impedance of the site (5 ))
5 kV above remote earth potential5 kV above remote earth potential Voltage 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
ShuntPotentialM
agnitude(Volts)
LEGEND
GRND_Rscreen: Bus/Line 4.
GRND_Yscreen: Bus/Line 5.
GRND_Bscreen: Bus/Line 6.
SINGLE COMPUTATION
RunID:Flat 1 Point Term.:Source
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SingleSingle--pointpoint--bonding systembonding system
fault conditionfault conditionlow EGVR at sourcelow EGVR at source 1000 A actual fault current, 0.051000 A actual fault current, 0.05 earth grid at the source siteearth grid at the source site Screen only bonded at the sourceScreen only bonded at the source
The voltage at the screen is dominated by the induced voltageThe voltage at the screen is dominated by the induced voltage Approximately 700 V/kmApproximately 700 V/km
0 15 30 45 60
Section Number
0
200
400
600
800
ShuntPotentialM
agnitude(Volts)
LEGEND
Rscreen : Bus/Line 4.
Yscreen : Bus/Line 5.
Bscreen : Bus/Line 6.
SINGLE COMPUTATION
RunID:Flat1 Point Term.:Source
This voltage
depends on the
resistance of theearth grid and the
actual fault current
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CrossCross--bonding cable systembonding 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
ShuntPotentialMag
nitude(Volts)
LEGEND
GRND_Rscreen: Bus/Line 4.
GRND_Ysc reen: Bus/Line 5.
GRND_Bscreen: Bus/Line 6.
SINGLE COMPUTATION
RunID:Flat 2 Point Term.:Source
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CrossCross--bonding cable systembonding cable system
cable core not transposedcable core not transposed
Asymmetric voltage profile along the screens as expectedAsymmetric voltage profile along the screens as expected
0.7 V appears at the termination joints, with respect to remote0.7 V appears at the termination joints, with respect to remote earthearth
Approximately 0.5 A circulating currentApproximately 0.5 A circulating current Cable core should be transposed to eliminate imbalanceCable core should be transposed to eliminate imbalance
Induced voltage on
outer cable screen
Induced voltage onmiddle cable screen
0 50 100 150 200
Section Number
0.0
1.5
3.0
4.5
6.0
ShuntPotentialM
agnitude(Volts)
LEGEND
GRND _Rscreen: Bus/Line 4.
GRND _Ysc reen: Bus/Line 5.
GRND _Bscreen: Bus/Line 6.
SINGLE COMPUTATION
RunID:Flat 2 Point Term.:Source
This voltage
depends on
the current
imbalance and
the resistance
of the earth
grid
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SummarySummary -- 11
SolidlySolidly--bonded cable systembonded cable system
Inexpensive and simpleInexpensive and simple
Suitable for short length of cable sections, tens of metersSuitable for short length of cable sections, tens of meters
Trefoil formation of cables can reduce circulating current (60 %Trefoil formation of cables can reduce circulating current (60 % reduction for thereduction for thecase study given)case study given)
SingleSingle--pointpoint--bonding cable systembonding cable system
Relatively inexpensive and simpleRelatively inexpensive and simple
Suitable for cable sections where screen heating could be signifSuitable for cable sections where screen heating could be significanticant Generally 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 system
Relatively inexpensive and simpleRelatively inexpensive and simple
Double 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 system
Technically complicated and financially expensiveTechnically 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 ofn heating are ofconcernconcern
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SummarySummary -- 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 V0 kV, 150 Votherwiseotherwise
Consider the circulating current and therefore heating effectConsider the circulating current and therefore heating effect
Financial considerationFinancial consideration
Specially bonded system is more complex and costlySpecially bonded system is more complex and costly SVLSVL
Link boxLink box
Joint bayJoint bay
EarthEarth--continuity conductorcontinuity conductor
Fully 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