1.0 SCOPE 2.0 BONDING METHODS 2.1 Introduction 2.2 Design 2.3 Single-Point Bonding 2.4 Cross Bonding 2.5 Sheath Sectionalizing Joints 2.6 Sheath Standing Voltage 2.7 Sheath Voltage at Through Fault 2.8 Sheath Overvoltages 3.0 ACCESSORIES AND THEIR TESTING FIGURES 3.1 Sheath Sectionalizing Insulator 3.2 Ground Continuity Conductor 3.3 Link Box 3.4 Bonding Lead 3.5 Sheath Voltage Limiter 3.6 Grounding Pit TABLE OF CONTENTS Figure 08-1: Sheath Bonding Arrangements at Ungrounded Terminations Figure 08-2: Sheath Bonding Arrangements at Solidly Bonded Joints in Cross Bonded Systems Figure 08-3: Sheath Bonding Arrangements at SVL Protected Joints in Cross Bonded Systems Figure 08-4: Single-Point Bonding Diagrams for Circuits Comprising One Cable Length Only Figure 08-5: Single-Point Bonding Diagram for Circuits Comprising Three Cable Lengths Figure 08-6: Transposition of Parallel Conductor to Reduce Induced Voltage with Power Cables in Flat Formation or Trefoil Figure 08-7: Sectionalized and Continuous Cross Bonded Cables Figure 08-8: Termination of Cross Bonded System with Single-Point Bonded Length Figure 08-9: Typical Insulating Shroud for Base of Oil Filled Cable Outdoor Sealing End Figure 08-10: Link Box Grounding Arrangement Figure 08-11: Typical Details of Grounding Pit 4.0 BIBLIOGRAPHY PAGE NO. 2 OF 23
1.0 SCOPE This Standard provides the guidelines for selection of the bonding and grounding of the insulated metallic sheath of single core power cables of voltage 69kV thru 380kV in the electrical systems of Saudi Electricity Company (SEC), Saudi Arabia. 2.0 BONDING METHODS 2.1 General 2.2 Design Bonding and grounding of the sheath must perform the following functions: 2.1.1 Reduce or eliminate the sheath losses thereby optimizing conductor current carrying capacity & size and prevent undue jacket heating. 2.1.2 Limit sheath voltages to a required value. 2.1.3 Maintain a continuous sheath circuit to permit fault current return for protective relay operation, and adequate lightning and switching surge protection. 2.2.1 All requisite direct inter-sheath and sheath-to-ground connections shall be made through approved disconnecting links, bonding leads and sheath voltage limiters(svls), within the link-box. 2.2.2 Single-core 69kV and 110/115/132 kv cable circuits of length upto 1.5 km in trefoil or flat formation shall be either single-point bonded or cross bonded with prior approval from SEC. For circuit lengths above 1.5km the metallic sheath of the power cables shall be cross-bonded. PAGE NO. 3 OF 23
2.2.3 Single-core 230kV and 380kV cable circuits of length upto 1km shall be single-point bonded or cross bonded with prior approval from SEC. For circuit lengths above 1km the metallic sheath of the power cables shall be cross-bonded. 2.2.4 The design of sheath bonding arrangements shall be per ANSI/IEEE 575 (Guide for the Application of Sheath-Bonding Methods for Single Conductor Cables and the Calculation of Induced Voltages and Currents in Cable Sheath) and considerations must be given to the following: a. Any safety aspect that may arise and any limiting values of sheath voltage as specified subsequently in this document. b. Residual sheath circulating currents in cross bonding system due to a large variation in the cable section lengths or spacings and their effect on the cable ampacity. c. Coordination of the sheath insulation levels in relation with the over voltage due to system transients and faults to which the sheath insulation will be subjected. d. The selection of SVL rating and monitoring and maintenance of the complete sheath insulation system in operation. 2.2.5 Bonding lead connections to the underground cable sheath shall be preferably soldered. Lead sheath(if present) shall not be injured by hot solder. The area of contact shall be sufficient to prevent the current from melting the solder. In order to prevent corrosion, copper bonding wires connected directly to the lead sheath(if present) shall form a 60 angle with the sheath at the point of soldering as recommended by Underground Systems Reference Book by Edison Electric Institute. 2.2.6 The sheath bonding bay ground of each cable circuit in two or more circuit scheme shall be separate; and there shall be no metallic connection between them. This requirement shall apply even if these circuits are using a common joint bay. A separate link box shall be used for each circuit as shown in Figure 08-10. PAGE NO. 4 OF 23
2.2.7 For circuits comprising of two or more cables per phase a separate link box shall be provided for each group of three cables. 2.3 Single-Point Bonding 2.3.1 In single-point bonding system the sheath of the single-core cables shall be solidly bonded and grounded at one(sealing) end of the cable route. The other end of the cable sheath shall be connected to ground through the SVLs as shown in Figure 08-1. The bonding arrangements shall be as shown in Figure 08-4. Single-point bonding system comprising three cable lengths shall be per Figure 08-5. 2.3.2 The single-point bonded cable installation shall be provided with a parallel ground continuity conductor, which shall be solidly grounded as shown in the above-mentioned figures. The spacing of the parallel ground continuity conductor from the cable circuit shall be sufficiently close to limit the voltage rise of the cable sheath during through fault conditions and as shown in Figure 08-6. The ground continuity conductor shall be transposed at the mid point of each minor section, if the power cables are not transposed (The ground continuity conductor need not to be transposed for cables laid in trefoil formation). 2.3.3 In single-point bonding system it shall be preferable to ground the sheath at the end which is subjected to the highest incoming transient voltages. 2.4 Cross Bonding 2.4.1 In cross bonding system the cable route shall have at least three minor sections/drum lengths of cable and cross bonding of the cable sheath shall be as shown in Figure 08-7. Cross bonding of cable sheath without transposition is permitted only in case of cables in trefoil formation. When the number of minor sections are exactly divisible by three (the length of the sections are equal) the sectionalized cross-bonding of the cable sheaths shall be adopted. When the number of minor sections are not exactly divisible by three, the cable sheaths shall be continuous cross bonded. PAGE NO. 5 OF 23
2.4.2 In cross bonding system (sectionalized and continuous type) the sheaths of the cables in any one minor section shall be electrically isolated from the sheaths of the cables in the adjacent sections(whether or not they are in the same major section) of the same phase. No parallel ground continuity conductor is needed here and sheath loss due to unequal cable lengths/minor sections shall not exceed the limits specified below: a. 3% for cables laid direct buried. b. 5% for cables installed in ducts. 2.4.3 The lengths of the minor sections of any one major section shall be as nearly uniform as practicable. Otherwise, cable center spacing is to be varied to adjust the induced sheath voltage in a manner to counteract the effect of length mis-match and to meet required cable ampacity. 2.4.4 In the event of relocation of terminations on existing circuits which cannot be re-arranged within the constraints of mis-match or continuous cross bonding, single-point bonded sections may be incorporated in otherwise cross bonded circuits as shown in Figure 08-8. with prior approval from SEC. 2.4.5 Except for cables in trefoil formation, in cross bonded system the cables shall be regularly transposed at each major and minor sectionalizing position. 2.4.6 The grounding at the junctions of major sections in the sectionalized cross bonding shall be only by local ground rods as shown in Figure 08-10. The sheath bonding arrangements at the cable joints in cross bonded system shall be as shown in Figure 08-2 and 08-3. SVLs shall be connected by the bonding leads of not exceeding 10m of length across the sheath sectionalizing insulators. 2.5 Sheath Sectionalizing Joints 2.5.1 Any cable joint, where sheath sectionalizing is required shall incorporate an insulating section and be so arranged that the requisite insulation level is provided both to ground and between the core screens and sheaths of the cables entering and leaving the joint. PAGE NO. 6 OF 23
2.5.2 All sheath sectionalizing joints shall be so arranged as to accommodate bonding leads of the concentric type, and the necessary bonding lug shall be located as closely as practicable to the sectionalizing insulation. 2.5.3 Before a sheath sectionalizing joint is used in the bonding scheme the insulation strength required at the sheath sectionalizing joint must be evaluated by calculating the maximum voltage appearing across the joint due to through faults and lightning and switching surges. 2.6 Sheath Standing Voltage at Emergency Load Condition 2.6.1 In a practical installation having variable lengths of minor sections, the sheath standing voltage shall be calculated for the longest minor section. 2.6.2 The sheath standing voltage shall be calculated for the maximum (emergency) load condition and most stringent portion of installation (duct banks etc.) per the applicable methods and formulae as recommended in clause D2 of Appendix D of ANSI/IEEE575 for single-point and cross bonding systems. 2.6.3 The sheath bonding and grounding arrangement shall be such that the standing sheath voltage at maximum (emergency)load current shall nowhere exceed the following voltage to ground. a. 65V rms for system voltage upto and including 132 kv. b. 110V rms for system voltage 230 kv and 380 kv. 2.6.4 When the sheath standing voltage at a sealing end/termination of the cable exceeds 10Vrms, to prevent hand contact with the sheath or metal parts connected to it, resin bonded glass fiber shrouds as shown in Figure 08-9 shall be provided. PAGE NO. 7 OF 23
2.7 Sheath Standing Voltage at Through Fault 2.7.1 In order to find out the ratings of the SVLs the maximum magnitude of power frequency sheath voltage induced under through fault conditions(3- phase, phase to phase and single phase to ground fault) shall be calculated by the applicable methods and formulae as recommended in clause E3 and E4 of Appendix E of ANSI/IEEE575 for single-point and cross bonding systems. The limiting values of sheath voltage at through fault shall nowhere exceed the following voltage to ground: a. 5kV rms for system voltage upto and including 132 kv. b. 10kV rms for system voltage 230 kv and 380 kv. 2.7.2 The SVL ratings shall be based on the ultimate short circuit current. 2.7.3 In case of single-point bonding system, having two cables per phase, and where system may be operated in abnormal condition with only one cable per phase, the sheath voltage calculation shall take into account the division of any ground fault return current between the two ground continuity conductors. 2.8 Sheath Overvoltages 2.8.1 It shall be ensured that the insulation requirement of sheath sectionalizing insulators or jacket of the cable shall be sufficient to withstand the sheath overvoltages resulting from steep fronted transient voltage wave (due to switching operations, flashover on or near the cable terminations or lightning impulse in cable circuits connected to overhead lines). To restrict these voltages to acceptable levels as mentioned in clause 3.1.3.c and 3.1.4, SVLs shall be installed for single-point and cross bonding systems. 2.8.2 The travelling transient voltage arising across the sheath and joint sleeve sectionalizing insulation shall be calculated per document no.3 under bibliography. PAGE NO. 8 OF 23
3.0 ACCESSORIES AND THEIR TESTING 3.1 Sheath Sectionalizing Insulator 3.1.1 These insulators shall consist of two short sections of brass or copper sleeving separated by a non-conducting material having sufficient creepage to prevent current leakage across them. 3.1.2 Sheath sectionalizing insulators shall be externally insulated to permit 10kV D.C. high voltage testing of the jacket of the cable at site. 3.1.3 The sheath sectionalizing insulators shall be tested to withstand: a. 10kV D.C. for 1 minute. b. the maximum power frequency voltage arising between sheaths during a system through fault (see clause 2.7.1). c. 1.2/50µsec. impulse voltage as below: Rated Lightning Impulse Withstand Voltage for Cable Insulation(kVpeak) 1.2/50µsec. Impulse Test Voltage (kvpeak) Upto 325 60 550 75 900 95 1300 125 3.1.4 Impulse withstand voltage between the sheath sectionalizing insulator and ground shall be 50% of the impulse test voltage mentioned above. PAGE NO. 9 OF 23
3.2 Ground Continuity Conductor The ground continuity conductor shall be of 600/1000V, class 2 stranded copper conductor, XLPE insulated (with minimum average thickness of 3.3mm) conforming to IEC 60502 and IEC 60228. The minimum size of the conductor cross-section shall be computed per IEC 60949 for the ultimate fault current to be carried by the ground continuity conductor for 1 sec. and maximum copper temperature of 250 C. The color of the PVC outer jacket shall be green with yellow stripe. 3.3 Link Box Link box shall be provided and tested per 12-TMSS-11. 3.4 Bonding Lead 3.4.1 The bonding lead shall be a coaxial cable with concentric type, class 2 stranded copper conductors, XLPE insulated ( with minimum average thickness of 3.3mm) conforming to IEC 60502 and IEC 60228. The minimum size of the conductor cross-section shall be computed per ICEA P- 45-482 for the system fault current to be carried by the bonding lead conductor for 1 sec. and maximum copper temperature of 250 C. The PVC outer jacket shall be graphite coated and embossed with the legend: ELECTRIC CABLE ---BONDING LEAD 3.4.2 The length of the bonding lead shall be preferably within 3m, but in no case will exceed 10m. Joints in the bonding lead are not acceptable. The surge impedance of the bonding lead shall not exceed 30Ω. 3.4.3 Bonding leads shall be marked to indicate sheaths of the concerned cable sections. 3.4.4 Bonding leads shall be routine tested per IEC 60502. The bonding leads shall also be tested per clause 3.1.3. The (outer sheath) insulation between outer screen and ground shall be tested per 3.1.4. PAGE NO. 10 OF 23
3.5 Sheath Voltage Limiter(SVL) Sheath Voltage Limiter(SVL) shall be provided and tested per 12-TMSS-11. 3.6 Grounding Pit The grounding pit details shall be as shown in Figure 08-11. Separate grounding pit shall be provided for each cable circuit and for each group of cables (R, Y, B), when two cables per phase are laid. The equivalent resistance(re) of grounding rods for each pit shall be not more than 2Ω. PAGE NO. 11 OF 23
4.0 BIBLIOGRAPHY 1. ANSI/IEEE 575, Guide for the application of sheath-bonding methods for single conductor cables and the calculation of induced voltages and currents in cable sheath. 2. ICEA Pub.P-45-482, Short Circuit Performance of Metallic Shields and Sheaths of Insulated Cable(Second Edition). 3. IEEE Transactions on Power Apparatus and Systems Vol.PAS-84, No.10, Sheath Overvoltages in H.V. Cables Resulting from Special Sheath- Bonding Connections. 4. Engineering Recommendation C.55/4, Cables Consultancy Group The Electricity Council Engineering Management. 5. Electra (128) CIGRE Study Committee 21, Working Group 7, Guide to the Protection of Specially Bonded Cable Systems against Sheath Overvoltages. 6. Electra (47) CIGRE study committee 21, Working Group 7, The Design of Specially Bonded Cable Circuits (Part II). 7. IEC60228, Conductors of Insulated Cables. 8. IEC60287, Electric Cables-Calculation of Current Rating. 9. IEC60502, Extruded Solid Dielectric Insulated Power Cables for Rated Voltages from 1kV upto 30kV. 10. IEC60949, Calculation of thermally permissible short-circuit currents, taking into account non-adiabatic heating effects. 11. 12-TMSS-11, Link Boxes for Sheath Grounding of Underground Power Cables. PAGE NO. 12 OF 23
DWG # TE-0408-0100-00 PAGE NO. 13 OF 23
DWG # TE-0408-0200-00 PAGE NO. 14 OF 23
DWG # TE-0408-0300-00 PAGE NO. 15 OF 23
DWG # TE-0408-0400-00 PAGE NO. 16 OF 23
DWG # TE-0408-0500-00 PAGE NO. 17 OF 23
DWG # TE-0408-0600-00 PAGE NO. 18 OF 23
DWG # TE-0408-0700-00 PAGE NO. 19 OF 23
DWG # TE-0408-0800-00 PAGE NO. 20 OF 23
DWG # TE-0408-0900-00 PAGE NO. 21 OF 23
DWG # TE-0408-1000-00 PAGE NO. 22 OF 23
DWG # TE-0408-1100-00 PAGE NO. 23 OF 23