Cable Screen Bonding

8/12/2019 Cable Screen Bonding

1/38

Single Core CableSingle Core CableScreen BondingScreen Bonding

Mitton Consulting Limited


2/38


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


3/38


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


4/38


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


5/38


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


6/38


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


7/38


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


8/38


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


9/38


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


10/38



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


11/38



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


12/38


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


13/38


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


14/38


SingleSingle--pointpoint--bonded systembonded system

Standing voltage at the unearthedStanding voltage at the unearthed end with normal operatingend with normal operatingconditionsconditions


15/38


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


16/38


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


17/38


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


18/38


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


19/38



--pointpoint


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


20/38



--pointpoint


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


21/38


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


22/38


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


23/38


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


24/38


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


25/38


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


26/38


CrossCross


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


27/38


CrossCross


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


28/38


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


29/38


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


30/38



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.


SINGLE COMPUTATION



31/38



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




SINGLE COMPUTATION

RunID:trefoil 2 Po Term.:Source

0 15 30 45 60

Section Number

0

5

10

SectionCurrent

Magnitude(Amps)

LEGEND


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

SINGLE COMPUTATION

RunID:trefoil 2 Po Term.:Source


32/38



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


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 _Bscreen: Bus/Line 6.

GRND _Earth : Bus/Line 7.

SINGLE COMPUTATION


This voltage

depends on

the capacity

current and

theresistance of

the earth grid


33/38



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




SINGLE COMPUTATION



34/38


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


35/38


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_Ysc reen: Bus/Line 5.


SINGLE COMPUTATION



36/38



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


This voltage

depends on

the current

imbalance and

the resistance

of the earth

grid


37/38


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


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


38/38


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

Documents

Cable Screen Bonding