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Power cables In general, Cables are classified according to their uses into the following categories: 1- Power Cables o Single core o Double cores (often Line &Neutral). o Triple cores (often 3-phase). o Four cores (often 3-phase &Neutral). o Five cores (often 3-phase, Neutral &Ground). 2- Control & measurement cables (multi-core with the same color). 3- Telephone cables (in pairs). Electrical power could be transmitted via two methods: • Overhead lines. • Underground cables. & we can compare them as a following: Underground cable Overhead line Feature Higher. Difficult. Used in congested industrial areas & urban areas. Up to 33 kV XLPE up to 420 kV. Lower. Easy. Used in open country for long distance. Up to highest voltage. Cost Maintenance Use Rated voltage When we use underground cables? Whenever possible, overhead line is preferred. Underground cable where overhead line can not be used i.e. we use underground cable for the following: • Mandatory, used inside cities. • For crossing Rivers &Roads. • Recommended in architecture designs. • Used for short distances. • It is safe from the above ground weather & traffic problems, so it gives fewer interruptions than the overhead line. Construction of power cable: • Conductor The main function of the conductor is to carry the transmitted electrical power. The factors affect choice of the conducting material: o Receptivity. o Specific weight. o Thermal properties. o Mechanical properties. o Chemical properties. o Oxidation. o Corrosion. o Soldering problems. o Cost & Availability. • Internal semiconductor layer It is a thin layer of carbon black paper. The main function of it is to provide regulation of internal electrical & magnetic fields. • Insulator The main function of it is to provide insulation for the cable. It is classified into three main categories: o Natural type Such as: Fabrics – Rubber – Wood – Papers. o Synthetic materials (Polymers) Such as: - Polyethylene (PE) - Cross linked Polyethylene (XLPE) - Polyvinylchloride (PVC) - Polytetraflouoroethylene (PTFE) - Polymethlemethacrelate (PMM) -Polypropylene (PP) -Polyamide -Ethylene Polypropylene Rubber o Oil-Impregnated Paper • External semiconductor layer The main function of it is to provide regulation of external electrical & magnetic fields. • Metallic sheath Materials used for cable sheathing are lead layer, aluminum layer or copper wire. The main function of it is to provide: o Good method of cable earthing. o Good path to the fault current. • Bedding (Filling) It consists of plastics materials which provide: o Round cross-section-area of cables to take largest volume in least space area. o Separate the metallic sheath from the armored layer to prevent galvanic corrosion. • Armored It is a protective non-corrosion layer which provides: o Carrying earth fault current. o Gives some protection against mechanical damage both during installation & in service. • External jacket It is a protective jacket which saves the cable from the surrounding solid. It could be made of: o PVC material. o Jute impregnated with bitumen. Laying of cables There are different methods for laying depended on cable type, cable importance & soil surrounding cable. Such as: • Cable supported using brackets above ground level. • Cables laid directly in dug in trenches below ground level (direct burial), the depth of trench may be vary between 1 to 1.5m depends on voltage level. Up to 11 kV h=0.5m 66 kV h=0.75m 132 kV h =1.00m 220 kV h=1.5m The bottom of the trench is leveled freed from stones had sharp edge of rock, a layer 10cm thick of clean sand is laid at the bottom of the trench .After laying The cable it is covered once again with a 10cm thick layer of sand. Where the soil conditions not good. A layer of bricks is usually used for protection against mechanical damage and for identification of the cable route. • Cables laid in ducts: this method for vital cable and for crossing ways. Man holes are made to maintain & repair the cable easily when a fault occurs. Metallic mesh shield: it used when the cable is near to railways or duct of pipe lines to prevent galvanic corrosion. • Conduits (pipes) which used for buildings (walls & ceilings). • The cables laid in bitumen compound. This method is used where the soil is chemically corrosive to the cable. For most industrial applications, cables may be installed through: Race way: This method is used in important place like labs, hospitals and in some applications it will be wall mounted. Cable trays: This method is used in industrial loads, it is sealing mounted. Troughs: this method is very high expensive and used in labs. Notes: • Pipes or ducts in which cables are installed may be made of: ceramic, cast iron, galvanize iron, or cement. These are used for crossing streets or under railway tracks, the size should be sufficiently large to put in additional cables later if required. The cable can be drawn out and replaced without disturbing the earth above. • For conduits (pipes) which used for buildings, we have to take the following into consideration: o It is prohibited to lay conduits in finished of building floors. o When we use any pipe we must have space factor which given by: In order to providing: -Ventilation. -Expansions &extensions. o It is not recommended to use the flexible conduits as possible. Cable specification The cable specification depends on type of conduction, insulation and covering. The following is a checklist that can be used in preparing a cable specification: o Number of conductors in the cable. o Conductor size (mm2). o Conductor material. o Voltage rating and insulation level requirement. o Shielding system requirement. o Outer finish. o Insulation approvals requirements. o Test voltage and partial discharge voltage. o Ground-fault current value and the duration time. Cable ratings • Voltage Rating: The selection of cable insulation rating is made on the basis of the phase to phase voltage of the system in which the cable is to be applied. The design voltage for the cable may be expressed in the form of (Uo/U). U : power frequency voltage between conductor and earth. Uo : power frequency voltage between conductors for which the cable is designed. • Conductor Selection: The selection of conductor size is based on the following considerations: o Voltage-drop limitation. o Load current criteria. o Fault current criteria. o Emergency overload criteria. o Frequency criteria. o Hot-spot temperature criteria. o Length of cable in elevated ambient temperature areas. Selection of cable size To achieve maximum economy in first cost and subsequent appertain of cables, an important aspect is the selection of the optimum size of conductor. Several factors are involved which are: o Continuous current carrying capacity. o Ability to carry short circuit current. o Cost of losses. o Voltage drop. These factors are based on certain standard conditions such as: o Type of conductor material: copper or aluminum o Core construction: single or multi-core. o Insulation material: PVC, XLPE … o Method of installation: in air or in ground. o Surrounding ambient temperature. o Soil thermal resistively. o Depth of laying. o Method of cable grouping. Classification of cables according to voltage Cables can be classified according to rated voltage as follows: 1- Low voltage cables 2- Medium voltage cables 3- High voltage cables Low voltage cables 300/500 V & 450/750 V (Stranded Copper Conductor) Description: Soft annealed solid or stranded copper conductors insulted with PVC compound rated 70° C or 85° C. Application: For indoor fixed installations in dry locations, laid in conduits, as well as in steel support brackets. Single core cable with stranded and PVC Insulated 300/500 V & 450/750 V (Flexible Copper) Description: Soft annealed Copper fine wires bunched in sub - units or stranded bunched together groups into a main unit, which forms the flexible conductor. Insulated with soft PVC 70 °C or 85 °C Compound. Application: For indoor fixed installations in dry locations, where particular flexibility is required for electrical panels' connection or for electrical apparatus they can be laid in groups around steak sheets. Single Core Cable and PVC Insulated 300/500 V (Flexible Copper) Description: Soft annealed Copper fine wires bunched together in sub - units or stranded bunched groups into a main unit, which forms the flexible conductor. These conductors are insulated with PVC compound rated 70 °C and sheathed with PVC compound layer. Application: For indoor movable installation in dry location connecting to source power portable electrical appliances operating under unfavorable conditions, such as portable lamps, fans, refrigerators, washing machines, vacuum cleaners, TV & house hold heating and ventilating apparatus. Multicore Cable with PVC Insulated and PVC Sheathed 0.6/1 (1.2) kV (Single core Copper or Aluminum Conductor) Description: Soft annealed stranded copper or Aluminum conductor, Insulated with PVC compound rated 70 °C and sheathed with PVC Compound layer. Application: For outdoor and indoor installations in damp and wet locations. They are normally used for power distribution in urban networks, industrial plants, as well as in thermo power and hydropower stations. Single Core Cable, with Stranded Circular Copper Conductors, PVC Insulated and PVC Sheathed 0.6/1 (1.2) kV (Multicore Stranded Copper Conductors) Description: Multicore cables of stranded Copper conductors are insulated with PVC compound, assembled together, covered with overall jacket of PVC compound. Application: For outdoor and indoor installations in damp and wet locations. Multicore Cable, with Stranded Copper Conductors PVC Insulated and PVC Sheathed 0.6/1 (1.2) kV (Multicore Stranded Aluminum Conductors) Description: Multicore cables of stranded Aluminum conductors are insulated with PVC compound, assembled together, covered with overall jacket of PVC compound. Application: For outdoor and indoor installations in damp and wet locations. Multicore Cable, with Stranded Aluminum Conductors PVC Insulated and PVC Sheathed 0.6/1 (1.2) kV (Multicore, cu, Armored with steel tape) Description: Multicore cables of stranded Copper conductors are insulated with PVC compound, assembled together, armored with steel tape and covered with overall jacket of PVC compound. Application: For outdoor installations in damp and wet locations, where mechanical damages are expected to occur. Multicore Cable, with Stranded Copper Conductors, PVC Insulated, Steel Tape Armored and PVC Sheathed. 0.6/1 (1.2) kV (Multicore, Al, Armored with steel tape) Description: Multicore cables of stranded Aluminum conductors are insulated with PVC compound, assembled together, armored with steel tape and covered with overall jacket of PVC compound. Application: For outdoor installations in damp wet locations, where mechanical damages are expected to occur. Multicore Cable, with Stranded Aluminum Conductors, PVC Insulated, Steel Tape Armored and PVC Sheathed 0.6/1 (1.2) kV (Multicore , Copper , Armored with steel wires) Description: Multicore cables of stranded Copper conductors are insulated with PVC compound, assembled together, armored with steel wires and covered with overall jacket of PVC compound. Application: For outdoor installations in damp wet locations where mechanical damages are expected to occur. Multicore Cables, with Stranded Copper Conductors, PVC Insulated, Steel Wire Armored and PVC Sheathed 0.6/1 (1.2) kV (Aluminum Conductors Armored with Steel Wires) Description: Multicore cables of stranded Aluminum conductors are insulated with PVC compound, assembled together, armored with steel wires and covered with overall jacket of PVC compound. Application: For outdoor installations in damp wet locations where mechanical damages are expected to occur. Multicore Cables, with Stranded Aluminum Conductors, PVC Insulated, Steel Wire Armored and PVC Sheathed 0.6/1 (1.2) kV (XLPE Insulated and PVC Sheathed) Description: Soft annealed stranded Copper or Aluminum conductor, Insulated with XLPE compound covered with a layer of PVC compound to form the overall jacket. Application: For outdoor and indoor installations in damp and wet locations. They are normally used for power distribution in urban networks, in industrial plants, as well as in Thermo power and Hydropower stations. Single Core Cables, with Stranded Circular Copper Conductors, XLPE Insulated and PVC Sheathed. 0.6/1 (1.2) kV (Multicore Copper Conductors with XLPE Insulated) Description: Multicore cables of stranded Copper conductors are insulated with XLPE compound, assembled together and covered with an overall jacket of PVC compound. Application: For outdoor and indoor installations in damp and wet locations. They are normally used for power distribution in urban networks, in industrial plants, as well as in Thermo & Hydro power stations. Multicore Cables, with Stranded, Copper Conductors, XLPE Insulated and PVC Sheathed 0.6/1 (1.2) kV (Multicore with Stranded Aluminum Conductors, XLPE Insulated) Description: Multicore cables of stranded Aluminum conductors are insulated with XLPE compound, assembled together and covered with an overall jacket of PVC compound. Application: For outdoor and indoor installations in damp and wet locations. They are normally used for power distribution in urban networks, in industrial plants, as well as in Thermo power and Hydropower stations. Multicore Cables, with Stranded, Aluminum Conductors, XLPE Insulated and PVC 0.6/1 (1.2) kV (Multicore Cables, with Stranded Copper Conductors, XLPE Insulated, Steel Tape Armored and PVC Sheathed) Description: Multicore cables of stranded Copper conductors are insulated with XLPE compound, assembled together, armored with steel tape and covered with an external Jacket of PVC compound. Application For outdoor installations in damp wet locations where mechanical damages are expected to occur. Multicore Cables, with Stranded Copper Conductors, XLPE Insulated, Steel Tape Armored and PVC Sheathed. 0.6/1 (1.2) kV (Multicore Cables, with Stranded Aluminum Conductors, XLPE Insulated, Steel Tape Armored and PVC Sheathed) Description: Multicore cables of stranded Aluminum conductors are insulated with XLPE compound, assembled together, armored with steel tape and covered with an overall jacket of PVC compound. Application: For outdoor installations in damp wet locations where mechanical damages are expected to occur. Multicore Cables, with Stranded Aluminum Conductors, XLPE Insulated, Steel Tape Armored and PVC Sheathed 0.6/1 (1.2) kV (MulticoreWith Stranded Copper Conductors, XLPE Insulated steel Wire Armored and PVC Sheathed) Description: Multicore cables of stranded Copper conductors are insulated with XLPE compound, assembled together, armored with steel wires and covered with an overall jacket of PVC compound. Application: For outdoor installations in damp wet locations where mechanical damages are expected to occur. Multicore Cables, with Stranded Copper Conductors, XLPE Insulated, Steel Wire Armored and PVC Sheathed 0.6/1 (1.2) kV (Multicore with Stranded Aluminum Conductors, XLPE Insulated Steel Wire Armored and PVC Sheathed) Description: Multicore cables of stranded Aluminum conductors are insulated with XLPE compound, assembled together, armored with steel wire and covered with an overall jacket of PVC compound. Application: For outdoor installations in damp wet locations where mechanical damages are expected to occur. Multicore Cables, with Stranded, Aluminum Conductors, XLPE Insulated, Steel Wire Armored and PVC Sheathed Medium voltage cables 6/10 (12) kV (Multicore with Stranded Copper Conductors, XLPE Insulated, Steel Wire Armored and PVC Sheathed) Description: Stranded circular compacted Copper conductor, semi conducting layer as conductor screen, XLPE insulated, semi conducting layer as non metallic insulation screen, Copper tape or wire as metallic insulation screen, three cores assembled together with non-Hygroscopic Polypropylene fillers, wrapped with polyester tape and PVC sheathed. Application: These cables are generally suitable for direct burial or for installation on trays or ducts. Multicore Copper Conductors XLPE Insulated and PVC Sheathed 6/10 (12) kV (Multicore with Stranded Aluminum Conductor, XLPE Insulated, Steel Wire Armored and PVC Sheathed) Description: Stranded circular compacted Aluminum conductor, semi conducting layer as conductor screen, XLPE insulated, semi conducting layer as non metallic insulation screen, Copper tape or wire as metallic insulation screen, three cores assembled together with non hygroscopic Polypropylene fillers, wrapped with polyester tape and PVC sheathed. Application: These cables are generally suitable for direct burial or for installation on trays or ducts. Multicore Aluminum Conductors, XLPE Insulated and PVC Sheathed 6/10 (12) kV (Multicore Copper or Aluminum Conductors, XLPE Insulated, Steel Tape Armored and PVC Sheathed) Description: Stranded circular compacted Copper or Aluminum conductor, semi conducting layer as conductor screen, XLPE insulated, semi conducting layer as non metallic insulation screen, Copper tape or wire as metallic insulation screen, three cores assembled together with non hygroscopic polypropylene fillers, wrapped with polyester tape, covered with a layer of PVC compound as a bedding, steel tape armored and PVC sheathed. Application: These cables are generally suitable for direct burial or for installation on trays or ducts. Multicore Copper or Aluminum Conductors, XLPE Insulated, Steel Tape Armored and PVC Sheathed. 6/10 (12) kV (Multicore Copper or Aluminum Conductors, XLPE Insulated, Steel Wire Armored and PVC Sheathed) Description: Stranded circular compacted Copper or Aluminum conductor, semi conducting layer as conductor screen, XLPE insulated, semi conducting layer as non metallic insulation screen, Copper tape or wire as metallic insulation screen, three cores assembled together with non hygroscopic Polypropylene fillers, wrapped with polyester tape, covered with a layer of PVC compound as a bedding, steel wire armored and PVC sheathed. Application: These cables are generally suitable for direct burial or for installation on trays or ducts. Multicore Copper or Aluminum Conductors, XLPE Insulated, Steel Wire Armored and PVC Sheathed High voltage cables 38/66 (72.5) kV (Single Core Copper Conductor, XLPE Insulated, Copper Screened and HDPE Sheathing) Description: Stranded circular compacted copper conductor, semi conducting layer as conductor screen, XLPE insulated, semi conducting layer as non metallic insulation screen, copper tape, wire or both as metallic insulation screen to achieve the required cross sectional area to withstand the earth fault current, longitudinal water blocking tapes to protect the screen area from any longitudinal water penetration, copolymer aluminum tape to protect the cable from any radial water penetration and HDPE sheathed. Single Core Copper Conductor, XLPE Insulated, Copper Screened and HDPE Sheathing. 38/66 (72.5) kV (Single Core Copper Conductor, XLPE Insulated, Lead Screened and HDPE Sheathing) Description: Stranded circular compacted copper conductor, semi conducting layer as conductor screen, XLPE insulated, semi conducting layer as non metallic insulation screen, semi conducting water blocking tape applied helically, lead screened with suitable thickness to withstand the earth fault current and HDPE sheathed. Single Core Copper Conductor, XLPE Insulated, Lead Screened and HDPE Sheathing Design of underground cables under study Several factors are considered in selecting cable size, such as: o Continuous current carrying capacity. o Ability to carry short-circuit current. o Cost of losses. • Continuous current carrying capacity: Current carrying capacity or Ampacity of cable is defined as the maximum allowable current, which may flow through cable conductor without damaging the conductor or its insulation. Where : Rating factor of soil temperature. : Rating factor of soil thermal resistivity. : grouping factor. : Rating factor of buried depth. Note: For cables installed in air there is only one rating factor ( ) which is called 'the air temperature rating factor' Most cable manufacturers issue catalogues which contain ratings for the cables which they supply. However these ratings are based on certain standard conditions such as: o Type of conductor material, copper or aluminum. o Core construction, single-core or multi-core depends on the rated current in cable. o Insulation material, PVC, XLPE … o Method of installation, in air or in ground (direct buried or in duct). o Surrounding ambient temperature. o Soil thermal resistivity. o Depth of laying. o Method of cable grouping. All of these previous factors should be taken into consideration in order to get the current carrying capacity of the cable under actual operating conditions. • Short-circuit current : The most important factor for the cable selection is the capability of cable to carry the short circuit current. The permissible rise in temperature during period of short circuit is 150°C in PVC cable and 250°C in XLPE cables. The electrodynamics load induced due to short circuit reduces the time that cable can be withstood. In addition the symmetrical short circuit increases the armored and the outer sheath losses of the cable. • Cost of losses: One important factor for cable selection is the cost of losses, which means that the cost of power which dissipated in the cable core resistance. Cable manufacturers issue catalogues which contain resistance, inductance & capacitance for the cables which they supply.