Cable Screen Bonding

Single Core Cable Screen Bonding Mitton Consulting Limited

Overview   

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 - CDEGS TM  

Desktop study – induced voltage in cable screen for 1 km long 33 kV cable Practical example – 27 km 33 kV cable Mitton Consulting Limited

Overview   

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 - CDEGS TM  

Desktop study – induced voltage in cable screen for 1 km long 33 kV cable Practical example – 27 km 33 kV cable Mitton Consulting Limited

Cable screens 

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  safet y 

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

Provide low impedance fault current return path Provide neutral point for the circuit Provide shielding of electromagnetic field Mitton Consulting Limited

Induced voltage and circulating current in cable screen 

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Electromagnetic coupling between the core and screen If the cable screen is single point bonded, no electrical continuity, the mmf generates a voltage If the cable screen is bonded at both ends, the mmf will cause a circulating current to flow if there is electrical continuity.  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 

Steady state induced standing voltage limit for safety  

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

No much evidence to substantiate those limits 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 CDEGS TM in detail Mitton Consulting Limited

 Three-core cables 

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 Well balanced magnetic field from three phases 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 

 virtually zero induced voltage or circulating current under stea dy state operation

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Single-core cable 

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For HV application, typically for 11 kV and above Single-core cables negate the use of ferromagnetic material for screen, sheath and armouring  Induced voltage is mainly contributed by the core currents in its own phase and other two phases If cables are laid in a compact and symmetrical formation, induced  voltage in the screen can be minimized  A suitable screen bonding method should be used for single-core cables to prevent  

Excessive circulating current High induced standing voltage

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Single core cable bonding methods 

Different bonding methods for single core cable 

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Solidly bonded single-core cable system 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    

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

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Solidly bonded single-core cable system 

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 Very small standing voltage in the order of several volts  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

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Solidly bonded single-core cable system 

 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  

Circulating current is proportional to the length of the cable cable aand the magnitude of the load current Mitton Consulting Limited

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 to pr provide path for fault current, if returning from earth is undesirable, such as in a co al mine SVL installed at the unearthed end to to protect protect the the cable cable insulati insulation during fault conditions  Transposition of earth continuity conductor at the mid point of the section 

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 field from phases are partially balanced  The magnitude of the standing voltage is depended on the magnitude 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 

 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 bon ded 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 Mitton Consulting Limited

Link box with SVL and cable sectional joint 

Protect the outer PVC sheath

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Minimizing the joint surge impedance

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Single- point-bonded system 

 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 solidly bonded system

Standing voltage on the cable screen is proportional to the leng th th of the cable and the magnitude of current in the core  Typically suitable for cable sections less than 500 m, or one drum length Mitton Consulting Limited

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   

SVLs Earth continuity conductor  Transposition of earth continuity conductor

R

R

Y

Y

B

B

0V

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Split single- point-bonded system   

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

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Split single- point-bonded system 

 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 insulat ion joint Requires SVL to protect the un-earthed end Requires separate earth continuity conductor for zero sequence c urrent 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

R

R

Earthing resistance not Y shown in the plot

Y

B

B

Major section Minor section

Minor section

Minor section

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Cross-bonded cable system  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

Mitton Consulting Limited

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 major se section in the crossbonding cable system  Virtually zero circulating current  Virtually zero volt to the remote earth at the solidly earthed e nds Standing voltage at the minor section joints Induced Voltage Magnitude Plot Minor section 1

Minor section 2

Minor section 3

1 ~0.867

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 

 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 

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 sec tion joint

 Transposition of cables 

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 

 Advantage 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  

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Disadvantage  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 cu rrent 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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 TM Case studies –  with CDEGS  with 

 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 follow s: s:   

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, 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 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 GRND_Rsc reen: Bus/Line 4.

GRND_Rscreen: Bus/Line 4.

GRND_Yscreen: Bus/Line 5.

GRND_Yscreen: Bus/Line 5.

0.60

   )   s    t    l   o    V    (   e    d   u    t    i   n   g   a    M    l   a    i    t   n   e    t   o    P    t   n   u    h    S

   )   s   p   m    A    (   e    d   u    t    i   n   g   a    M    t   n   e   r   r   u    C   n   o    i    t   c   e    S

0.45

0.30

0.15

0.00 0

15

30

45

60

GRND_Bsc reen: Bus/Line 6.

GRND_Bscreen: Bus/Line 6.

20

10

0 0

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   

 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 SINGLE COMPUTATION

SINGLE COMPUTATION 10

0.15

   )   s    t    l   o    V    (   e    d   u    t    i   n   g   a    M    l   a    i    t   n   e    t   o    P    t   n   u    h    S

 LEGEND

 LEGEND

   )   s   p   m    A    (   e    d   u    t    i   n   g   a    M    t   n   e   r   r   u    C   n   o    i    t   c   e    S

0.10

0.05

0.00 0

15

30

45

60

GRND_Rsc reen: Bus/Line 4.

GRND_Rscreen: Bus/Line 4.

GRND_Ys creen: Bus/Line 5.

GRND_Yscreen: Bus/Line 5.

GRND_Bsc reen: Bus/Line 6.

GRND_Bscreen: Bus/Line 6.

5

0 0

Section Number 

15

30

45

60 RunID:trefoil 2 Po

RunID:trefoil 2 Po

Term.:Source

Term.:Source

Section Number

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Single- point-bonded system steady state condition 

Screen   

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Only leakage current flows No circulating current 18 V standing voltage at the unun-earthed end

Earth conductor   

Insignificant standing voltage along the insulated earthearth-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 conduct or flow into earth grid

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SINGLE COMPUTATION

SINGLE COMPUTATION

   )   s    t    l   o    V    (   e    d   u    t    i   n   g   a    M    l   a    i    t   n   e    t   o    P    t   n   u    h    S

 LEGEND

GRND_Rscree n: Bus/Line 4.

GRND _Rscreen: Bus/Line 4.

GRND_Ys creen: Bus/Line 5.

GRND_Yscreen: Bus/Line 5.

GRND_Bscree n: Bus/Line 6.

GRND _Bscreen: Bus/Line 6.

GRND_Earth : Bus/Line 7.

GRND _Earth : Bus/Line 7.

2.0

20

This voltage depends on the capacity current and the resistance of the earth grid

  LEGEND

   )   s   p   m    A    (   e    d   u    t    i   n   g   a    M    t   n   e   r   r   u    C   n   o    i    t   c   e    S

15

10

5

0 0

15

30

45

60

1.5

1.0

0.5

0.0 0 RunID:Flat 1 Point

Section Nu mber

15

30

Term.:Source

45

60 RunID:Flat 1 Point

Term.:Source

Section Nu mber

Mitton Consulting Limited

Single- point-bonded system fault condition – high EGVR at source   

1000 A earth return current, 5 Ω 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.

   )   s    t    l   o    V    (   e    d   u    t    i   n   g   a    M    l   a    i    t   n   e    t   o    P    t   n   u    h    S

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 low EGVR at source    

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

   )   s    t    l   o    V    (   e    d   u    t    i   n   g   a    M    l   a    i    t   n   e    t   o    P    t   n   u    h    S

: Bus/Line 4.

Yscreen

: Bus/Line 5.

Bscreen

: 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:Flat1 Point

Term.:Source

Section Number

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Cross-bonding cable system cable core transposed 

 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 GRND_Rscree n: Bus/Line 4. GRND_Ysc reen: Bus/Line 5. GRND_Bscree n: Bus/Line 6.

   )   s    t    l   o    V    (   e    d   u    t    i   n   g   a    M    l   a    i    t   n   e    t   o    P    t   n   u    h    S

4.5

3.0

1.5

0.0 0

50

100

150

200 RunID:Flat 2 Point

Term.:Source

Section N umber

Mitton Consulting Limited

Cross-bonding cable system cable core not transposed    

 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

  LEGEND

Induced voltage on outer cable screen GRND _Rscreen: Bus/Line 4.

This voltage depends on the current imbalance and the resistance of the earth grid

GRND _Ysc reen: Bus/Line 5.

6.0

   )   s    t    l   o    V    (   e    d   u    t    i   n   g   a    M    l   a    i    t   n   e    t   o    P    t   n   u    h    S

GRND _Bscreen: Bus/Line 6.

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 

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 signif icant 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  

 Technically complicated and financially expensive Suitable for long cable sections where induced voltage and screen heating are of concern Mitton Consulting Limited