Single Core Cable
Screen Bonding
Mitton Consulting Limited
Overview
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Introduction
Cable screens, induced voltages and currents in the screen
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Different bonding methods for single core cables
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Three core cable
Single core cable
Solidly bonded single-core cable system
Specially bonded single-core cable systems
„ Single-point-bonding system
„ Split single-point-bonding system
„ Cross-bonding system
Case studies - CDEGSTM
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Desktop study – induced voltage in cable screen for 1 km long 33 kV cable
Practical example – 27 km 33 kV cable
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Cable screens
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Cable screen types
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Purpose of cable screen
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Copper tape
Copper or aluminium wire
To control the electric field stress in the cable insulation
Cable neutral and fault current return path
Shielding for electromagnetic radiation
– if the screen is earthed at two ends
Enclosing dangerous high voltage with earth potential for safety
Some cables do not have ‘screens’
Normally cable screens need to be bonded to earth at both ends
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Provide low impedance fault current return path
Provide neutral point for the circuit
Provide shielding of electromagnetic field
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Induced voltage and circulating
current in cable screen
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Electromagnetic coupling between the core and screen
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If the cable screen is single point bonded, no electrical continuity,
the mmf generates a voltage
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If the cable screen is bonded at both ends, the mmf will cause a
circulating current to flow if there is electrical continuity.
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The circulating current produces an opposing magnetic field
Circulating current
Opposite current direction
Core
Screen
V
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Induced voltage and circulating
current in cable screen
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Steady state induced standing voltage limit for safety
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No internationally agreed limit
Different countries or utilities have different limit or practice (IEEE
Std575-1988 Appendix C).
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In the order of 65-150 V, some utility allow for 300-400 V during emergency
load
Some countries only specify voltage limit at exposed metal, some specify a fix
limit that any point along the screen can not exceed
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No much evidence to substantiate those limits
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Engineering Recommendation C55/4
„ 65 V for system voltage up to and including 132 kV
„ 150 V for system voltage 275 kV and 400 kV
Suitable bonding method should be employed to meet the
standing voltage limit and keep circulating current to an
acceptable level
Induced voltage and circulating current in the cable screen can
be studied in CDEGSTM in detail
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Three-core cables
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Well balanced magnetic field from three phases
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Induced voltages from three phases sum to zero along the entire length of the
cable
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Cable screen should be earthed at both ends
Screen bonding method for three-core cable is not considered further
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virtually zero induced voltage or circulating current under steady state operation
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Single-core cable
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For HV application, typically for 11 kV and above
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Single-core cables negate the use of ferromagnetic material for
screen, sheath and armouring
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Induced voltage is mainly contributed by the core currents in its
own phase and other two phases
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If cables are laid in a compact and symmetrical formation, induced
voltage in the screen can be minimized
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A suitable screen bonding method should be used for single-core
cables to prevent
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Excessive circulating current
High induced standing voltage
Mitton Consulting Limited
Single core cable bonding methods
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Different bonding methods for single core cable
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Solidly bonded single-core cable system
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Specially bonded single-core cable systems
„ Single-point-bonding system
„ Split single-point-bonding system
„ Cross-bonding system
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Solidly bonded single-core cable
system
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Simple
Cable screen is bonded to earth grids at both ends (via link box)
Most common method
Significant circulating current in the screen
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Proportional to the core current and cable length
de-rates cable
Could lay cable in compact trefoil formation if permissible
Suitable for route length of tens of meters
R
R
Y
Y
B
B
Mitton Consulting Limited
Solidly bonded single-core cable
system
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Very small standing voltage in the order of several volts
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The magnitude of the induced voltage and current will be
quantified in the case study later
Magnitude
Standing Voltage Plot
Length
0V
R
R
Y
Y
B
B
0V
Mitton Consulting Limited
Solidly bonded single-core cable
system
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Advantages
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Disadvantages
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Minimum material required - most economical if heating is not an issue
Provides path for fault current, minimizing earth return current and
EGVR at cable destination
Does not require screen voltage limiter (SVL)
Less electromagnetic radiation
Provides path for circulating current
Heating effects in cable screen, greater losses
Cable therefore might need to be de-rated or larger cable required
Transfers voltages between sites when there is an EGVR at one site
Can lay cables in trefoil formation to reduce screen losses
Normally applies to short cable section of tens of metres long
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Circulating current is proportional to the length of the cable and the
magnitude of the load current
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Single-point-bonded system
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Cable screen solidly earthed at one end only
Open circuit in cable screen, no circulating current
Zero volt at the earthed end, standing voltage at the unearthed end
Optional PVC insulated earth continuity conductor required to provide path for fault
current, if returning from earth is undesirable, such as in a coal mine
SVL installed at the unearthed end to protect the cable insulation during fault
conditions
Transposition of earth continuity conductor at the mid point of the section
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Reduce circulating current in the continuity conductor
R
R
Y
Y
B
B
Earth continuity
conductor
SVL installed at
unearthed end
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Single-point-bonded system
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Induced voltage proportional to the length of the cable and the current carried in the cable
Zero volt with respect to the earth grid voltage at the earthed end, standing voltage at the unearthed end
No circulating current in the screen
Circulating current in the earth-continuity conductor is not significant, as magnetic field from phases are
partially balanced
The magnitude of the standing voltage is depended on the magnitude of the current flows in the core,
much higher if there is an earth fault
Induced Voltage Plot
Length
0V
R
R
Y
Y
B
B
0V
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Single-point-bonded system
Standing voltage at the unearthed end with normal operating
conditions
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Single-point-bonded system
Standing voltage at the unearthed end during earth fault condition
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Voltage at the unearthed end during an earth fault consists of
two voltage components
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The voltage due to induction can reach 700 V/km
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Induced voltage due to fault current in the core
EGVR of the source site (assuming the screen is single point bonded at
the source site)
For a 1 kA actual fault current
With 0.05 Ω earth grid impedance at the source
Screen single point bonded at the source only
Cable in flat formation with 150 mm separation
High voltage appears on the unearthed end can cause arcing and
damage outer PVC sheath
The voltage on the screen during a fault also depends on the
earthing condition
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Link box with SVL and cable sectional joint
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Protect the outer PVC sheath
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Minimizing the joint surge impedance
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Single-point-bonded system
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Advantage
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Disadvantage
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No circulating current
No heating in the cable screen
Economical
Standing voltage at the un-earthed end
Requires SVL if standing voltage during fault is excessive
Requires additional earth continuity conductor for fault current if earth
returned current is undesirable
Higher magnetic fields around the cable compared to solidly bonded
system
Standing voltage on the cable screen is proportional to the length
of the cable and the magnitude of current in the core
Typically suitable for cable sections less than 500 m, or one drum
length
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Split single-point-bonded system
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Variation of single-point-bonding
Also called double length single-point-bonding system
Cable screen continuity is interrupted at the midpoint and SVLs need to be
fitted at each side of the isolation joint
Other requirements are identical to single-point-bonding system
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SVLs
Earth continuity conductor
Transposition of earth continuity conductor
R
R
Y
Y
B
B
0V
Mitton Consulting Limited
Split single-point-bonded system
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Effectively two sections of single-point-bonding
No circulating current
Zero volt at the earthed ends, standing voltage at the
sectionalising joint
Induced Voltage Plot
Length
0V
R
R
Y
Y
B
B
0V
Mitton Consulting Limited
Split single-point-bonded system
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Advantages
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Disadvantages
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No circulating current in the screen
No heating effect in the cable screen
Suitable for longer cable section compared to single-point-bonding
system and solidly bonded single-core system
Economical
Standing voltage exists at the screen and sectionalising insulation joint
Requires SVL to protect the un-earthed end
Requires separate earth continuity conductor for zero sequence current
Not suitable for cable sections over 1000 m
Suitable for 300~1000 m long cable sections, double the length
of single-point-bonding system
Mitton Consulting Limited
Cross-bonded cable system
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Ultimate bonding method
Consists of one or more major sections and three minor sections in each major section
Summing up induced voltage in sectionalised screen from each phase resulting in neutralisation of
induced voltages in three consecutive minor sections
Normally one drum length (500 m approx) per minor section
Sectionalising position and cable jointing position should be coincident
Solidly earthed at major section joints
Transpose cable core to balance the magnitude of induced voltages to be summed up
Link box should be used at every sectionalising joint
balanced impedance in all phases
Earthing
resistance not
shown in the plot
R
R
Y
Y
B
B
Major section
Minor section
Minor section
Minor section
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Cross-bonded cable system
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what if cable cores not transposed
Other than cross-bonding the screen, why transpose
the cables core?
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If core not transposed, not well neutralised resulting in some
circulating currents
Cable should be transposed and the screen needs to be cross
bonded at each sectionalising joint position for optimal
neutralisation
Inner screen,
smaller induced
voltage
Earthing
resistance not
shown in the plot
R
R
Y
Y
B
B
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Cable bonded system
sectional joint link box diagram
Minor section
joint bay
Major section
joint bay
R
R
Y
Y
B
B
Joints with
sectionalising
insulation
Lockable link box
Earthing
resistance is not
shown in the plot
Joints with
sectionalising
insulation
Cross bonding
connections
Joint bay
earthing system
Screen voltage
limiter
Joint bay
earthing system
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Cross-bonded cable system
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Induced voltage magnitude profile along the screen of a major section in the crossbonding cable system
Virtually zero circulating current
Virtually zero volt to the remote earth at the solidly earthed ends
Standing voltage at the minor section joints
Induced Voltage Magnitude Plot
1
~0.867
Minor section 1
Minor section 2
Minor section 3
Length
Major section
Earthing
resistance is not
shown in the plot
R
R
Y
Y
B
B
Mitton Consulting Limited
Cross-bonded cable system
Interpretation of induced voltage magnitude plot by phasor
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The induced voltage magnitude profile along the three
sections can also be interpreted by induced voltage
phasor
Induced Voltage
Magnitude Plot
Induced Voltage
Phasor
1
~0.867
Section 3
Section 2
Section 1
Section 1
Section 2
Section 3
Length
0V Reference
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Cross-bonded cable system
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In order to obtain optimal result, two ‘crosses’ exist
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Cross bonding of cable screen
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Transposition of cable core – crossing cable core at each section
Cross bond the cable screens – effectively no transposition of screen
Cancellation of induced voltage in the screen at every major section joint
Transposition of cables
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Ensure voltages to be summed up have similar magnitude
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Greater standing voltage at the screen of the outer cable
Standing voltages exist at screen and majority of section joints
cable and joints must be installed as an insulated screen system
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Cross-bonded cable system
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Advantage
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No earth-continuity conductor
Electrical continuity of screen for fault current
Virtually zero circulating current in the screen
Standing voltage in the screen is controlled
Technically superior than other methods
Suitable for long distance cable network
Disadvantage
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Technically complicated
More expensive
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Increased cable current carrying capacity
Solidly-bonded Vs specially bonded cable system
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Specially bonded cable system effectively reduces circulating current in the
screens
Current carrying capacity of specially bonded cable is increased
Example – 33 kV 630 mm2 cable, 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 70mm
Ground temperature 20o C
Ground thermal resistivity 1.0 km/W
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Case studies – with CDEGSTM
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Three bonding arrangements:
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Cable screens earthed at both ends (flat and trefoil formation)
Cable screens earthed at one end only (flat formation)
Cable screens cross bonded and earthed at both ends (flat formation)
The cable configurations and operating conditions were as follows:
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100 Ω-m soil resistivity, 5 Ω earth grid impedance
Cable: 33 kV single core XLPE cable, 150 mm2 copper, with 0.3 mm copper tape screen
Circuit configuration:
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Flat @ 150 mm centres, 1 m deep
Trefoil @ compact formation, 1 m deep (For solidly bonded case only)
Cable length: 1 km
Operating current: 100 A steady state per phase
1m
1m
0.150m
0.015m
0.017 m
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Solidly bonded single-core cable
system – flat formation
Circulating currents cause earth grid voltage rise at two ends
Voltage magnitude is plotted with respect to remote earth
Less than 1 V induced at the termination
19~24 A circulating currents in the screen
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SINGLE COMPUTATION
SINGLE COMPUTATION
LEGEND
LEGEND
30
Section Current Magnitude (Amps)
0.60
Shunt Potential Magnitude (Volts)
GRND_Rscreen: Bus/Line 4.
GRND_Yscreen: Bus/Line 5.
GRND_Bscreen: Bus/Line 6.
GRND_Rscreen: Bus/Line 4.
GRND_Yscreen: Bus/Line 5.
GRND_Bscreen: Bus/Line 6.
0.45
0.30
0.15
0.00
0
15
30
45
60
20
10
0
0
Section Number
15
30
45
60
RunID:Flat 2 Point
RunID:Flat 2 Point
Term.:Source
Term.:Source
Section Number
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Solidly bonded single-core cable
system – trefoil formation
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SINGLE COMPUTATION
SINGLE COMPUTATION
LEGEND
LEGEND
10
0.15
GRND_Rscreen: Bus/Line 4.
GRND_Yscreen: Bus/Line 5.
GRND_Bscreen: Bus/Line 6.
GRND_Rscreen: Bus/Line 4.
GRND_Yscreen: Bus/Line 5.
GRND_Bscreen: Bus/Line 6.
Section Current Magnitude (Amps)
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Virtually zero volt in the screen
9 A circulating currents in the screen (24 A for flat formation)
Compact trefoil formation reduces circulating current, but does
not facilitate heat dissipation
Shunt Potential Magnitude (Volts)
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0.10
0.05
0.00
0
15
30
45
60
5
0
0
Section Number
15
30
45
60
RunID:trefoil 2 Po
RunID:trefoil 2 Po
Term.:Source
Term.:Source
Section Number
Mitton Consulting Limited
Single-point-bonded system
steady state condition
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Screen
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Only leakage current flows
No circulating current
18 V standing voltage at the un-earthed end
Earth conductor
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Insignificant standing voltage along the insulated earth-continuity conductor
Insignificant circulating current due to transposition
very low standing voltage at the solidly earthed end
due to minor circulating current in the earth continuity conductor
conductor flow into earth grid
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SINGLE COMPUTATION
SINGLE COMPUTATION
Section Current Magnitude (Amps)
Shunt Potential Magnitude (Volts)
LEGEND
GRND_Rscreen: Bus/Line 4.
GRND_Yscreen: Bus/Line 5.
GRND_Bscreen: Bus/Line 6.
GRND_Earth : Bus/Line 7.
GRND_Rscreen: Bus/Line 4.
GRND_Yscreen: Bus/Line 5.
GRND_Bscreen: Bus/Line 6.
GRND_Earth : Bus/Line 7.
2.0
20
This voltage
depends on
the capacity
current and
the
resistance of
the earth grid
LEGEND
15
10
5
0
0
15
30
45
60
1.5
1.0
0.5
0.0
0
RunID:Flat 1 Point
Section Number
15
30
Term.:Source
45
60
RunID:Flat 1 Point
Term.:Source
Section Number
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Single-point-bonded system
fault condition – high EGVR at source
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1000 A earth return current, 5Ω earth grid at the source site
Screen only bonded at the source
The voltage at the screen is dominated by the EGVR of the faulted site due to relatively high
earth grid impedance of the site (5 Ω)
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5 kV above remote earth potential
Voltage due to induction is only a small proportion
SINGLE COMPUTATION
LEGEND
6000
GRND_Rscreen: Bus/Line 4.
GRND_Yscreen: Bus/Line 5.
GRND_Bscreen: Bus/Line 6.
Shunt Potential Magnitude (Volts)
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4500
3000
1500
0
0
15
30
45
60
RunID:Flat 1 Point
Section Number
Term.:Source
Mitton Consulting Limited
Single-point-bonding system
fault condition –low EGVR at source
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1000 A actual fault current, 0.05Ω earth grid at the source site
Screen only bonded at the source
The voltage at the screen is dominated by the induced voltage
Approximately 700 V/km
SINGLE COMPUTATION
LEGEND
800
Rscreen
Yscreen
Bscreen
Shunt Potential Magnitude (Volts)
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: Bus/Line 4.
: Bus/Line 5.
: Bus/Line 6.
600
This voltage
depends on the
resistance of the
earth grid and the
actual fault current
400
200
0
0
15
30
45
60
RunID:Flat 1 Point
Term.:Source
Section Number
Mitton Consulting Limited
Cross-bonding cable system
cable core transposed
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Virtually zero volt at earthed ends with respect to remote earth
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About 6 V standing voltage at minor section joint
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Virtually no circulating current
SINGLE COMPUTATION
LEGEND
6.0
Shunt Potential Magnitude (Volts)
GRND_Rscreen: Bus/Line 4.
GRND_Yscreen: Bus/Line 5.
GRND_Bscreen: Bus/Line 6.
4.5
3.0
1.5
0.0
0
50
100
150
200
RunID:Flat 2 Point
Section Number
Term.:Source
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Cross-bonding cable system
cable core not transposed
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Asymmetric voltage profile along the screens as expected
0.7 V appears at the termination joints, with respect to remote earth
Approximately 0.5 A circulating current
Cable core should be transposed to eliminate imbalance
SINGLE COMPUTATION
Induced voltage on
outer cable screen
GRND_Rscreen: Bus/Line 4.
GRND_Yscreen: Bus/Line 5.
GRND_Bscreen: Bus/Line 6.
6.0
Shunt Potential Magnitude (Volts)
This voltage
depends on
the current
imbalance and
the resistance
of the earth
grid
LEGEND
4.5
3.0
Induced voltage on
middle cable screen
1.5
0.0
0
50
100
150
200
RunID:Flat 2 Point
Term.:Source
Section Number
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Summary - 1
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Solidly-bonded cable system
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Single-point-bonding cable system
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Relatively inexpensive and simple
Suitable for cable sections where screen heating could be significant
Generally for sections less than 500 m or one drum length
Split single-point-bonding cable system
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Inexpensive and simple
Suitable for short length of cable sections, tens of meters
Trefoil formation of cables can reduce circulating current (60 % reduction for the
case study given)
Relatively inexpensive and simple
Double the length of single-point-bonding cable system, 300~500 m
Cross-bonding cable system
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Technically complicated and financially expensive
Suitable for long cable sections where induced voltage and screen heating are of
concern
Mitton Consulting Limited
Summary - 2
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When to use different type of screen bonding method?
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Should look at options on a case to case basis
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Specially bonded system is more complex and costly
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Meet the induced voltage limit (65 V for system voltage up to 110 kV, 150 V
otherwise
Consider the circulating current and therefore heating effect
Financial consideration
SVL
Link box
Joint bay
Earth-continuity conductor
Fully insulated system
Only provide specially bonded system when circulating current is
excessive or standing voltage is unacceptable
Mitton Consulting Limited