USE OF AERIAL BUNDLED CABLES FOR LOW VOLTAGE DISTRIBUTION Ali Hirji, Raycap Corporation, Athens Summary: Compared with conventional bare conductor overhead systems, worldwide experience proves that the use of Low Voltage Aerial Bundled Cables (LV ABC) is cheaper, safer, more reliable, requires less tree clearing and pruning and is more aesthetic, less labour intensive, requires lesser maintenance and eliminates supply interruptions and forest fires being initiated by conductor clashing. A Brief Description of LV ABC System: A LV ABC Mains Cable has four insulated conductors, each comprising of an aluminium conductor, insulated with cross linked polyethylene, twisted together to form a bundle. Electrical connections are made by the use of insulation piercing connectors and the bundled cores are supported at intermediate poles and strain poles with easy to install mechanical fittings. Ganged insulated switchgear to which fuses can be added is used for electrical protection and isolation. 1. Useage: LV ABC is used worldwide in a total of approximately 80 countries, and the various motives for adopting LV ABC include: Prevention of conductor clashing and initiation of fires. Reduction of tree cutting and improvement of the environmental impact of overhead distribution. Improvement of safety and reliability. Achievement of overall economy. LV ABC is used for new extensions as well as for replacing existing bare overhead lines and has found application in Urban areas on poles Urban areas on facades of buildings Rural areas on poles 2. Choice of System: There are at least five designs of LV ABC in common use thoughout the world. These follow either of the two basic forms: Fully supported (Refer Fig I) – where all the equal-sized phase and neutral cores share the mechanical load. This type is used in Norway, Sweden, Austria, Germany, Ireland, U.K., New Zealand and Australia. Neutral supported – the three phase cores are laid up around an aluminium alloy neutral which acts as a catenary to support the whole bundle. In the Middle East, India and Finland the neutral is bare, whereas in France the neutral is insulated. In Italy, a second protective outer sheath is added to the three phases with the neutral having only single insulation – (Refer Figures 2 & 3). Self-supporting LV-ABC LV-ABC Lines with insulated LV-ABC Lines with bare Lines neutral messenger neutral messenger Fig 1 Fig 2 Fig 3 For a 4-wire, low voltage system with a solidly earthed neutral, the fully supported cable system provides the following benefits: The fully sized neutral is important in a multiple earthed neutral system and to avoid extensive losses due to unbalanced loading. The neutral is at less mechanical risk Longer spans are achievable – an important factor when existing poles are retained for reconstruction work and for rural applications. Easier erection because all cores, rather than just the neutral are supported at suspension and anchor clamps. The cable is cheaper to manufacture 3. Cable: Fully supported and neutral supported cables are manufactured to Harmonisation Document HD 626 by CENELEC and Australian Standard AS3560. The compacted, stranded circular aluminium conductor (material alloy 1350), is insulated with cross-linked polyethylene whose resistance to ultra-violet light degradation is provided with an appropriate percentage of carbon black pigment dispersed in it. The combined thermal and mechanical stress (method 6 –self supporting cables) to determine the cable behavior is performed in accordance with 2.3.6 of HD626 and an onerous adhesion Test of the compound to the conductor is included in AS3560. Because the cable has to be compatible with Insulation Piercing Connectors, the maximum thickness of insulation at any point on the circumference of the conductor and the maximum and minimum conductor diameter are important criteria in the cable design. The physical size and spacing of the rib profiles on both the neutral and phase cores is very important to enable linesmen to clearly identify the neutral and to differentiate between all phase cores. The neutral is fully ribbed to make it similar to aerial service cables with the heights of the ribs on the phase cores noticeably greater than those on the neutral core. In addiction the phase cores are required to have an embossed or indented number. Please see Fig 4. Fig 4 The system can consist of additional 1 or 2 insulated aluminium conductors with cross sections of 16sq mm or 25sq mm as pilot wire for street lighting. The service lines of all LV ABC systems are usually also of the self supporting type, composed of 2 to 4 factory bundled insulated aluminium conductors with cross sections of 16sq mm, 25sq mm or 35sq mm. 4. POLE HARDWARE: Pole hardware consists of Open Eye Bolts and Hook Brackets. These fitting are designed to have: Adequate strength A close range of opening forces in two directions – vertically downward and horizontally away from the pole. The open eye bolt as shown in Fig 5 is suitable for fixing to either a timber pole or a concrete pole with a preformed hole. The hook bracket as shown in Fig 6 is suitable for clamping or banding to a concrete pole where a preformed hole does not exist. Fig 5 Fig 6 5. Mechanical Fittings: Mechanical fittings for supporting and anchoring LV-ABC cables consist of : - Suspension clamps - Yokes - Strain clamps Suspension Clamps : These support the LVABC cables at intermediate and angle poles. The traditional J- Clamp with a rubber bush and wing bolt can be used for self supporting as well as cables with bare and insulated neutral messenger. The clamp as shown in Fig 7 can be used without any special tools since it is equipped with a wing nut and has no loosable parts. The rubber bush is based on a weather and UV resistant elastomer and the clamp is made from hot galvanized steel. The line can be in a straight direction or a deviation angle up to 300. Standard Type Twisted Type Type with Bolts Fig 7 Fig 8 Yokes : The Yoke, made from hot dip galvanized steel is used when the cable has a deviation angle from 30 0 to 600. Fig. 8 shows a typical yoke with two standard suspension clamps. For large line deviations i.e. greater than 600, use of two strain or anchor clamps becomes necessary. Strain/anchor clamps : A typical clamp designed to anchor a self-supporting LV-ABC cable is shown in Fig 9. The strap of the clamp can be made of hot dip galvanized steel or extruded aluminium for improved corrosion resistance and lighter weight. The straps are shaped to provide a sufficiently large window area so that the cable tails have adequate space to exit the clamp without contact with the straps. The risk of abrading the insulation of the cores is further reduced by the use of straps with rounded edges. Fig 9 A single M12 bolt and self locking nut allows the strain clamp to be fixed to pole brackets with closed eyes or also screws with closed eyes. The clamp is made of weather and UV resistant glass fibre reinforced polymer. Figure 10 shows an example of a pole structure using both suspension and strain clamps for making a T – connection and Fig 11 shows an LV-ABC anchoring while connecting to an underground cable . Fig 12 shows the connection of LV-ABC to a bare overhead line. Similarly anchor clamps and angled clamps for service cables are also available. Fig 10 Fig 11 Fig 12 6. Insulation Piercing Connectors: The technology for insulation Piercing Connectors used for joining mains cables and main cables to branch has evolved over twenty five years in Europe, passing through three main steps : 1. The connector had two separate bolts; one for the main and one for the tap. Insulation piercing occurred on the main cable only and the service line was stripped. Bolts were in direct electrical contact with the conductor and the insulation over the connector was a rigid cover and did not provide any resistance to water ingress. The disadvantages of this included the risk of failure due to nicking of the strands of the service conductor due to the stripping operation carried out by the linesmen with a knife, risk of accident because of the energized bolt during live line installation, corrosion and oxidation due to lack of water and air tightness to the connection and the possibilities of over and under tightening of the bolts. 2. The connector had two separate bolts with shear off heads to provide the correct torque during tightening. The bolts were in direct electrical contact with the conductors and the connector had a flexible cover. The risk of accidents due to energized bolts and corrosion/oxidation problems still remained. This led to the development of a one bolt connector with simultaneous connection to the main and tap cores. The bolt was insulated from the main and tap cores and the piercing was achieved through insulation piercing blades, with the bolt being integrated into the plastic body of the connector. However, the risk of accident due to the exposed blades and corrosion still remained. 3. The connector has one bolt with simultaneous tightening of the main and tap cores. The bolt was insulated from the insulation piercing blades and designed with a shear head device. The connectors were fully insulated and sealed using elastomer seals which firmly conform to the cable insulation and with silicone grease on the aluminium alloy/brass/copper contact blades, to prevent any moisture ingress. This design met with all customer needs and complied with safety requirements. The features of the Insulation Piercing Connectors evolved in this third stage of evolution include: a. Easy and safe to fit on energized cables. b. Have no life metal parts that are accessible. c. The plastic components are made from strong UV and weather resistant glass reinforced nylon. d. Elastomeric inserts seal around the insulation piercing contact blades. e. Cold applied integral caps seal the end of the branch cable, preventing corrosion and ensuring reliable electrical contact. f. Fastener, nut and washer are protected against corrosion. g. The bolts are fitted with shear heads. The range of Insulation Piercing connectors available today include. Insulation Piercing connectors for main to main connections. Insulation Piercing connectors for main to service connections. Insulation Piercing connectors for connecting bare aluminium/copper main conductors to insulated service conductors. Insulation Piercing connectors for connecting LV-ABC cores to underground cable lines. The fitting of a typical IPC is shown in Fig 13. Fully insulated hexagon socket wrench with ratchet is used for tightening and shearing the bolt heads. Fig 13 7. Water Proof Pre-Insulated Compression Lugs and Connectors : While bare compression lugs and connectors can be used along with adhesive coated heat shrinkable tubings for insulation and sealing on LV ABC, pre insulated connectors and lugs are preferred since they avoid the need for carrying a heat shrinking torch to the top of a pole and doing shrinking at a height. Three versions of the Pre-insulated and sealed connectors are available to meet with the different mechanical load requirements for self-supporting cables, for phase conductors of LV-ABC with neutral messenger and for the insulated neutral conductors of LV-ABC. Pre-insulated lugs are suitable for any insulated, stranded aluminium conductors. Stripped cables, using insulating stripping tool for LV-ABC as shown in Fig 14, are inserted in the connector up to the block in the connector or lug end as the case may be, and crimped according to the marks with appropriate crimping die over the insulation. The electrical contact and the sealing by the elastomer ring is achieved during the crimping process. The lugs are available with aluminium palm and also as bimetallic lugs with an aluminium tubular portion friction welded to a copper palm. The insulation material is made of weather and UV resistant polymer. A typical pre-insulated connector is shown in Fig 15 and a typical pre-insulated lug is shown in Fig 16. Fig 14 Fig 15 Fig 16 8. Switchgear : Insulated switchgear especially designed for LV ABC is available for mounting on : a concrete or wooden pole, no cross arm required (Fig 17) or the wall of a building There is a preference for three-pole rather than four pole units to avoid the possibility of incorrectly connecting the neutral core of the main cable. Switchgear can be fitted with blades or fuses. Fuses accommodated are the NH Type to DIN Specifications with blade type contacts and sizes as shown in Fig 17, of 160A, 250A, 400A. A typical disconnecting-fuse switch comprises a fully insulated frame made of impact resistant weather-proof plastic, a pivoted removable switching flap and cover hoods for the terminals. The Fig 17 frame holds the tap access conductor terminals which are connected to the fuse contacts. The switching flap has fuse holders suitable for the DIN spec fuses. The opening eye is integrated into the flap which can be operated by means of operating rods. 9. Wall Mounted Saddles and Cable Ties for LV-ABC Lines : Wall mounted saddles as shown in Fig 18 are designed to install LV-ABC lines (self supporting and insulated neutral types) along – side walls and poles. The LV-ABC cable is fixed to the saddle by a cable tie. A second cable can be installed on the same support by hanging it to the bottom side with an additional cable tie. The body and the cable tie are made of weather and UV resistant polymer material. Usually, every 0.7m, a saddle is installed on a wall. Fig 18 10. Weak-Link Hooks : These are used as hangers between the pole support fitting and the strain/ anchor or suspension clamp in areas where damage to the LV ABC line could be expected from falling trees. Weak links withstand normal working loads but the controlled failure mechanism releases the cable in the event of overloads, enabling the cable to drop to the ground. A typical weak link is shown in Fig 19. Fig 19 11. Construction of LV-ABC Lines: Labour costs involved in the construction of LV ABC lines can be reduced by using equipment and work practices that : - Suit all situations encountered - Are safe - Are easy to apply - Ensure reliable installations The main cause for failures is poor workmanship during installation or connecting. Training to all linesmen to ensure awareness of equipment, work practices and materials is essential. The tension stringing method, Fig 20, has been proven to be the best. Fig 20 This method is: Safe because there is no lifting of the cable at any stage. Economical as a minimum cost equipment is used such as drumstand, running sheaves and hydraulic winch. Reliable. The cable insulation cannot be damaged by other aerial cables, trees, vehicles, rough ground and rocks. Low cost since it is quick and requires minimum size of crew. Flexible as setting up of rollers, of running ropes and stringing of cable can be carried out as separate tasks to suit available resources, vehicles, weather and available time. Suitable for running LV-ABC under bare live LV or HV lines. Stocking: A nylon stocking as shown in Fig 21 is used. This has no splinters (unlike metal stocking) and enables LV-ABC to be strung under live bare LV and HV lines. Fig 21 Fig 22 Rope : A polypropylene rope is required of the appropriate diameter. Swivel : A swivel as shown in Fig 22 is used with the pulling stocking to eliminate twist. Rollers : The stringing block consists of a plastic coated roller, suspension assembly and a strap with clamping device or a plastic roller, a steel loop and a swivelling suspension hook. Please refer to figs 23 & 24 respectively. Fig 23 Fig 24 Fig 25 Running Cable: The step by step procedure is as follows : 1. Pull out rope through the rollers fixed to the poles. 2. Attach stocking and rope to the cable. 3. Pull through rope and cable. 4. Apply brake at drum end to keep cable from being abraded by trees, rough ground aerial services and telephone cables. 5. Get cable up to near sag with winch 6. Grab cable with Comealong. The tensioning device is attached to the pole and to the Comealong and the ratchet operation of the tensioning device gets cable up to sag 7. Attach one strain clamp 8. Grab cable at the other end with Comealong 9. Attach second strain clamp – cable is now up to sag 10. Fit suspension clamps 11. Remove rollers. For an installation where a LV ABC is used for replacing an existing bare LV line the procedure will be as follows: a. Run LV ABC cable b. Energize LV ABC cable c. Change over service connections from bare to LV ABC d. Remove cross arms The tension used while stringing LV ABC and the maintenance of a defined sag are governed by pole spacing, ground clearance, structure strengths, construction limits and fitting limits. The maximum working tension of the bundled cores is the peak assigned tension allowable for various values of temperature and wind, such as 50C and no wind or 150C and 500MPa wind. Taking into account that mechanical stresses have to be transferred through the XLPE insulation, particularly at strain/anchor fittings, it is considered to adopt 40MPa as the maximum working tensile stress of the conductor. On this basis, the maximum working tension of a 4x95sqmm bundled cable is 4x95x40 = 15.2KN However, the practicality of opening the bundle to fit service connectors usually needs to be taken into account and so for a 4x95sqmm cable an Every Day Tension of approximately 6KN has been chosen as the most appropriate for the majority of the situations. The Straining Tension (kg) is related to the weight of the cable (kg) per metre, the Span length in m and the sag in the middle of the span by the Formula : PxL2 T= 8F Where T = Tension in kg P = Weight of cable per m in kg L = Span length in m F = Sag in the middle of the span in m For example a 4x95sqmm cable has a weight per metre i.e. P = 1 • 35 kg/m. If the span length is 40m and sag in the middle of the span (after taking into account ground clearance and circuit to circuit clearances etc) is 0 • 51m at 25ºC, 1,35 x 40 x 40 Therefore T = 8 x0,51 T = 529 kg In case the span between poles varies along the route length, then the “equivalent” span is calculated as : ( span1) 3 ( span2) 3 ( span3) 3 ... Equivalent Span = ( span1 span2 span3...) See Fig 26 for clearance recommendations. Fig 26 12. Possible Strategies to consider when changing from bare overhead lines to LV ABC : The possible strategies a Utility could consider include : Having a clear, committed company policy for usage Developing an implementation plan Communicating frequently Producing documentation Introducing specific sizes and types of cables Conducting training for: - All linesmen - Supervisors - Technical staff Co-ordinating purchase of: - Cable and fittings - Construction equipment - Tools Developing some champions within the organizations Save time and money – outsource work to organizations having the total integrated system knowledge 13. Economic evaluation : An economic evaluation after taking into account the costs of : Material Labour Operating & maintenance expense Secondary available expense Set – up, inclusive of training, tools and construction equipment. An economic evaluation of a utility showed that LV ABC was cheaper than bare overhead lines for both new and retrofit work. Payback period for adopting LV ABC was only one year. ECONOMIC EVALUATION Operating & Maintenance Costs Bare LVABC 76 sqmm 95sqmm $/km/year $/km/year Energy Losses 2507 2006 Breakdown – maintenance 46 5 Breakdown – foregone revenue 110 2 Routine maintenance 2670 2014 Tree clearing 7000 250 Fines 264 44 Electrical accidents 16 2 Savings are due to lower costs in a) Tree clearing b) Maintenance c) Labour and these offset the higher material cost for the cable. 14. Conclusion: LV ABC System provides electricity supply authorities with tremendous opportunities to lower cost and to improve safety and reliability. BIOGRAPHY Ali Hirji was born in Mumbai (India) on 18th August 1946. He graduated in Electrical Engineering, from the College of Engineering, Pune affiliated to the Pune University. His employment experience includes Mahindra Engineering and Chemical Products Ltd., Raychem Corporation, Tyco Electronics and Raycap Corporation. His special fields of interest include Power Cable Accessories, Connectors and Fittings for Cables and Overhead Lines. Ali Hirji was a Member of the Bureau of Indian Standards Committee for the Drafting of the Indian Standard IS13573 covering the Performance Requirements for Power Cable Accessories upto 36KV and also a Faculty Member of IEEMA (Indian Electrical & Electronics Manufacturers Association) for Training Utility Engineers under the Government of India’s Electrification Schemes.
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