Single Core Cable Screen Bonding Mitton Consulting Limited Overview Introduction Cable screens, induced voltages and currents in the screen Different bonding methods for single core cables 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 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 Purpose of cable screen 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 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 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 Mitton Consulting Limited 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). 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 CDEGSTM in detail Mitton Consulting Limited Three-core cables Well balanced magnetic field from three phases Induced voltages from three phases sum to zero along the entire length of the cable 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 steady state operation Mitton Consulting Limited Single-core cable 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 Mitton Consulting Limited Single core cable bonding methods Different bonding methods for 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 Mitton Consulting Limited 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 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 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 Mitton Consulting Limited Solidly bonded single-core cable system Advantages Disadvantages 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 and the magnitude of the load current Mitton Consulting Limited Single-point-bonded system 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 Reduce circulating current in the continuity conductor R R Y Y B B Earth continuity conductor SVL installed at unearthed end Mitton Consulting Limited Single-point-bonded system 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 Mitton Consulting Limited Single-point-bonded system Standing voltage at the unearthed end with normal operating conditions Mitton Consulting Limited 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 The voltage due to induction can reach 700 V/km 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 Mitton Consulting Limited Link box with SVL and cable sectional joint Protect the outer PVC sheath Minimizing the joint surge impedance Mitton Consulting Limited Single-point-bonded system Advantage Disadvantage 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 Mitton Consulting Limited Split single-point-bonded system 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 Mitton Consulting Limited 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 Mitton Consulting Limited Split single-point-bonded system Advantages Disadvantages 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 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 Mitton Consulting Limited Cross-bonded cable system what if cable cores not transposed Other than cross-bonding the screen, why transpose the cables core? 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 Mitton Consulting Limited Cross-bonded cable system 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 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 Mitton Consulting Limited Cross-bonded cable system In order to obtain optimal result, two ‘crosses’ exist Cross bonding of cable screen 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 Ensure voltages to be summed up have similar magnitude 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 Mitton Consulting Limited 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 Disadvantage Technically complicated More expensive Mitton Consulting Limited Increased cable current carrying capacity Solidly-bonded Vs specially bonded cable system 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 Mitton Consulting Limited Case studies – with CDEGSTM Three bonding arrangements: 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: 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: 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 Mitton Consulting Limited 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 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 Mitton Consulting Limited Solidly bonded single-core cable system – trefoil formation 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) 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) 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 Screen Only leakage current flows No circulating current 18 V standing voltage at the un-earthed end Earth conductor 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 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 Mitton Consulting Limited Single-point-bonded system fault condition – high EGVR at source 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) 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) 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 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) : 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 Virtually zero volt at earthed ends with respect to remote earth About 6 V standing voltage at minor section joint 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 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 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 Mitton Consulting Limited Summary - 1 Solidly-bonded cable system Single-point-bonding cable system 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 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 Summary - 2 When to use different type of screen bonding method? Should look at options on a case to case basis Specially bonded system is more complex and costly 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