Comparative study of power cables and testing as per indian standards


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Comparative study of power cables and testing as per indian standards

  1. 1. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –INTERNATIONAL JOURNAL OF ELECTRICAL ENGINEERING 6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 2, March – April (2013), © IAEME & TECHNOLOGY (IJEET)ISSN 0976 – 6545(Print)ISSN 0976 – 6553(Online)Volume 4, Issue 2, March – April (2013), pp. 219-228© IAEME: IJEETJournal Impact Factor (2013): 5.5028 (Calculated by GISI) ©IAEME COMPARATIVE STUDY OF POWER CABLES AND TESTING AS PER INDIAN STANDARDS Neha Niharika1, Debasish Basak2 and Bubun Das3 1 Baba Hira Singh Bhattal Institute of Engineering and Technology, Lehragaga – 148031, Punjab, India, 2 Senior Principal Scientist & Head, Electrical Laboratory, CSIR-Central Institute of Mining and Fuel Research, Barwa Road, Dhanbad–826015, India 3 Project Assistant, Electrical Laboratory, CSIR-Central Institute of Mining and Fuel Research, Barwa Road, Dhanbad–826015, India ABSTRACT Power cables play a major role in industrial growth of a country. Testing of Power cables as per relevant Indian standards is very essential. An attempt has been made in this paper to study different testing standards for power cables and to evaluate the comparative relevance of the testing standards. Keywords: Power cables, Indian standard, Testing 1. INTRODUCTION Electricity is the key to economic developments and India’s current shortages of electricity have hampered industrial growth. The growth and demand for energy is increasing at a very fast rate, especially in the industrial sector, the transport sector and the household sector, thereby putting a great deal of pressure on the available resources. By selection of proper power cables, energy efficient systems can be improved [1]. A power cable is an assembly of two or more electrical conductors, usually held together with an overall sheath. The assembly is used for transmission of electrical power [2]. Whether being used to convey electric power or signals, it is the purpose of a wire or cable to convey the electric current to the intended device or location. Electrical insulation (dielectric) is provided to largely isolate the conductor from other paths or surfaces through which the current might flow. Therefore, it may be said that any conductor conveying electric signals or power is an insulated conductor. Power cables may be installed as permanent wiring within buildings, buried in the ground, run overhead, or exposed. 219
  2. 2. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 2, March – April (2013), © IAEME2. POWER CABLES Modern power cables come in a variety of sizes, materials, and types, each particularlyadapted to its uses. Large single insulated conductors are also sometimes called power cables inthe industry [3]. Cables consist of three major components: conductors, insulation, andprotective jacket. The makeup of individual cables varies according to application. Theconstruction and material of the power cables are determined by three main factors: • Working voltage, determining the thickness of the insulation; • Current-carrying capacity, determining the cross-sectional size of the conductor(s); • Environmental conditions such as temperature, water, chemical or sunlightexposure, and mechanical impact, determining the form and composition of the outer cablejacket. Cables for direct burial or for exposed installations may also include metal armor in theform of wires spiraled around the cable, or a corrugated tape wrapped around it. The armormay be made of steel or aluminum, and although connected to earth ground is not intended tocarry current during normal operation. Power cables use stranded copper or aluminumconductors, although small power cables may use solid conductors. The cable may includeuninsulated conductors used for the circuit neutral or for ground (earth) connection [4]. Some power cables for outdoor overhead use may have no overall sheath. Other cablesmay have a plastic or metal sheath enclosing all the conductors. The materials for the sheath areselected for resistance to water, oil, sunlight, underground conditions, chemical vapors, impact,or high temperatures. Cable materials may be specified not to produce large amounts of smokeif burned. Cables intended for underground use or direct burial in earth will have heavy plasticor metal, most often lead sheaths, or may require special direct-buried construction.The various parts of cable are:1. Cores or conductor: A cable may have one or more cores depending upon the type of service. The conductors are made of tinned copper, solid copper or aluminum.2. Insulation: Each core or conductor is provided with a suitable thickness of insulation. The commonly used materials for the insulation are impregnated paper, varnished cambric or rubber mineral compound.3. Metallic sheath: In order to provide the cable from moisture, gases or other damaging liquids in the soil and atmosphere, a metallic sheath of lead or aluminum is provided over the insulation.4. Bedding: Over the metallic sheath is applied a layer of bedding which consists of a fibrous material like jute or Hessian tape.5. Armouring: Over the bedding, armouring is provided which consists of one or two layers of galvanized steel wire or steel tape. Its purpose is to protect the cable from mechanical injury while laying it and during the course of handling.6. Serving: In order to protect the armouring from atmosphere condition, layer of fibrous material similar to bedding is provided over the armouring. This is known as serving.For all cables, the conductor was, at one time, almost universally stranded copper but becauseof copper, only aluminum is allowed to be used in power cable in India. Copper conductor isonly used for cables used in control circuit. The stranding is necessary to provide flexibility tocables. 220
  3. 3. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 2, March – April (2013), © IAEME3. INSULATING MATERIAL FOR CABLES The satisfactory operation of the cables depends to a great extent upon the characteristicof insulation used. In general, the insulating materials used in cables should have the followingproperties: 1. High insulating resistance to avoid leakage current. 2. High dielectric strength to avoid electrical breakdown. 3. High mechanical strength to withstand the mechanical handling of cables. 4. Non-hygroscopic i.e., it should not absorb moisture from air and soil. The moisture tends to decrease the insulating resistance and hastens the breakdown of the cable. In case the insulating material is hygroscopic, it must be enclosed in a waterproof covering like lead sheath. 5. Non-inflammable. 6. Low cost so as to make the underground system a viable proposition. 7. Unaffected by acids and alkalies to avoid any chemical action. 8. Low permittivity. 9. Capacity to withstand high rupturing voltage.The various insulating materials used for manufacturing of cables are: a. Rubber: Rubber may be natural and synthetic. Its relative permittivity is between 2 and 3; dielectric strength is 30 KV/mm. b. Vulcanized India Rubber (VIR): It is prepared by mixing India rubber with mineral matter such as sulphur, zinc oxide, red lead etc. though vulcanized process improve the non-hygroscopic and other properties like strength, durability, resilience, elasticity etc; but it still reduces its specific insulation resistance. The main drawback is that owing to sulphur content it attacks copper. c. Impregnated Paper: It is prepared from wood pulp rags or plant fibers by a suitable chemical process. It is quite cheap, has low capacitance, and high insulation resistivity. The only disadvantage is that it is hygroscopic, and even it is impregnated with compound, it absorbs moisture, which lowers insulation resistance. d. Polyvinyl Chloride (PVC): This is a synthetic compound and is processed with certain material known as plasticizers. It is inert to oxygen and almost inert to alkalies and acid and therefore its use is preferred over VIR e. Polyethylene: Polyethylene is a straight chain polymer derived from ethylene. Its electrical property is very good. The maximum operating temperature under the short circuit is 100˚C. It has limited use in power cables. f. Cross-linked polyethylene: Low density polyethylene, when vulcanized under controlled conditions, results in cross-linking of carbon atom and the compound produced is a new material having high melting point with light weight, small dimension and good mechanical strength. g. Varnished Cambric (or Empire Tape): This is cotton cloth impregnated and coated with varnish. The cambric is lapped in the form of tap on to the conductor and its surface is coated with petroleum jelly compound to give easy sliding of the surface. 221
  4. 4. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 2, March – April (2013), © IAEME h. Gutta –percha: It is similar to rubber but it becomes soft at about 65˚C. It is non- hygroscopic but cannot withstand even medium voltages. i. Silk and cotton: This is used in low voltage cables. The conductor may have a single layer or double layer covering depending upon the requirements of service. Silk and cotton covered wire are usually used for instruments and motor winding. j. Enamel Insulation: These wires are cheaper than silk and cotton covered wires and therefore for low voltage machine and instruments enameled wires are used. Enamel insulation is liable to crack.4. CLASSIFICATION OF CABLES The type of cable to be used at a particular location is determined by mechanicalconsiderations and the voltage at which it is required to operate. According to voltage, thecables are classified as: Low voltage (LV) cables for operating voltage up to 1,000 V. High voltage (HV) cables for operating voltage up to 11,000 V. Super tension (ST) cables for operating voltage up to 33,000 V. Extra high tension (EHT) cables for operating voltage up to 66,000 V. Extra super-voltage cables for operating voltage beyond up to 1,32,000 V. Types of power cables on basis of material used is PVC Cables XLPE Cables Elastomeric Cables.5. COMPARATIVE STUDY PVC insulated Power and Control Cables are widely known for their suitability inpower transmission and distribution, control system, utility network, railway signalinginstallation, switching station, textile mill, chemical plants, machine shops, etc. Cross linked polyethylene cables abbreviated as XLPE cables have been developedto overcome the susceptibility of PILC cables to ingress of moisture. Polyethylene whichhas the best electrical properties, but poor thermal properties on account of its thermoplasticnature in comparison to impregnated paper, is cross-linked by dry Nitrogen curing processto improve its thermo mechanical properties without loss of electrical properties [5-12]. Elastomeric insulated cables have a long history and variety of application inindustries, mines, ships, aircraft, earth moving equipment, wagon tripplers, stackers,reclaimers, welding equipment, electric traction, etc. Comparative study between PVC, XLPE and Elastomer insulated flexible powercables are shown in Table – 1. 222
  5. 5. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 2, March – April (2013), © IAEME Table – 1: Comparison of cablesSL. ELASTOMERNO. PVC XLPE INSULATED FLEXIBLE CABLE1. Definition PVC is Polyvinyl The polyethylene cross- These are flexible Chloride, a thermoplastic linked is a method called the cables made of material which can be Catenary Continuous elastomeric melted again and again. Vulcanizations (CCV) compound used in These materials can be process by Peroxide. This coal mines, heated to a certain process improves the quarries, and other temperature and will properties of cables and miscellaneous harden again as they cool. makes it suitable for power mines for the transmission. These following voltage compounds can be used at grades: 1.1, 3.3, high temperature above 3.8/6.6, and 6.35/11 110˚C where the normal kV. Polyethylene compounds are thermally unstable.2. Characteristic Properties of PVC: - Higher current carrying -Flexible in nature s - Toughness, strength. capacity with high -In ambient - Ease of blending, ease permissible continuous temperature and of processing conductor temperature temperature not - Flame resistant and fire - Extremely low dielectric exceeding 90˚C prevention properties. losses under normal Additives can provide - Higher short circuit ratings operation and PVC clear or colored, - Light in weight 250˚C under short- rigid or flexible, etc. -Trouble-free in circuit condition. - Excellent electrical maintenance and simple in insulation properties. terminating and jointing. - Impact strength and resistant to bad weather conditions. Resistance to grease, oil and chemicals - PVC is chemically stable and does not de- polymerize - Density: 1.32-1.42 g/cc.3. Indian IS: 1554 (Part-1) used for IS: 7098 (Part-1) used for IS:14494 used for Specification working voltage up to working voltage up to and the following for testing and including1100Volts including 1100 volts and voltage grades: and (Part-2) for working (Part-2) for working voltage 1.1, 3.3, 3.8/6.6 and voltage from 3.3kV up to 3.3 kV upto and including 33 6.35/11 kV and including 11kV. kV. IS: 9968 (Part- 1) IS-694/ used for working /1988 used for voltage up to and voltages up to and including 1100 volts including 1100 (Unarmoured PVC volts. Insulated Cable) 223
  6. 6. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 2, March – April (2013), © IAEME4. Insulation PVC insulation applied by Cross-linked polyethylene Elastomeric extrusion insulation applied by compound. extrusion5. Physical test for insulation and outer sheath a) Tensile strength, As per IS:5831 As per table 1 of As per IS:6380 elongation at break IS:7098 for insulation b) Ageing in air oven and IS:5831 for sheath c) Insulation resistance test As per IS:5831 As per table 1 of As per IS:6380 IS:7098 d) Shrinkage test As per IS:5831 As per table 1 of Not applicable. IS:7098 e) Hot deformation test As per IS:5831 As per IS: 5831- Not applicable. f) Ageing in air bomb Not applicable. Not applicable. As per IS:6380 g) Tear Resistance h) Hot Set Test i) Oil Resistance test j) Loss of mass in air oven As per IS:5831 As per table 1 of IS: Not applicable. k) Heat shock test 5831. l) Thermal stability Test6. Type tests (Test on Conductor) a) Wrapping test as per IS: a) Wrapping test as per IS: 8130. a) Not applicable. 8130 Common Tests b) Annealing test IS:8130 Method as per Part (for copper) no. 1 of IS:10810 [13] c) Conductor IS:8130 Method as per Part Resistance test no. 5 of IS:10810 d) Tests for As per in table 2 and IS:3975 armoring wires/ strips f) Flammability Period of burning after removal of the flame shall Method as per Part test not exceed 60 seconds and the unaffected portion no. 53 of IS:10810 from the lower edge of the top clamp shall be at least 50 mm. g) High voltage TestApply 3 KV AC and increased to 6 KV within Method as per Part no 10 secs and should withstand 6 KV for 5 minutes 45 of IS: 10810 durationNOTE:The protective coverings used in Elastomeric Insulated Flexible Cable are as follows: a) Semiconductor compound: It shall be so formulated as to be suitable for the operating temperature of the cable both during sustained operation and during short circuit and shall have no harmful or damaging effect on the cable insulation. b) Tape: The proofed tape shall be closely woven textile without slevage proofed with rubber. c) Protective screen: It shall be applied either spirally or in the form of braid consisting of tinned annealed copper wires not exceeding 0.51 mm in diameter. This shall not preclude the use of cord or other similar material. 224
  7. 7. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 2, March – April (2013), © IAEMEFollowing tests are only applicable for Elastomer Insulated Power Cables: a) Partial discharge test- As per of clause no. 26.2 of IS:14494 b) Bending test followed by Partial discharge test- As per of clause no. 26.3 of IS:14494 c) Dielectric power factor test- As per of clause no. 26.4 of IS:14494 d) Heating cycle test followed by Partial discharge test- As per IS:14494 of clause no. 26.5 e) Water absorption test – As per IS: 6380 and As per clause no. 26.8 of IS:14494The similarities of three standards mentioned earlier for testing of electric power cables aredescribed below: Tests can be carried out on armoured and unarmoured single core, twin core, three coreand multi-core cables for electric supply and control purposes. These cables are used in minesalso. Following tests are normally performed in power cables: a) Routine test for demonstration of the integrity of the cable. Following tests constitute routine tests: 1. Conductor resistance test, 2. High voltage test, 3. Armour Resistance b) Type test for demonstration of satisfactory performance characteristic to achieve the intended application. c) Acceptance tests for accepting or rejecting the sample in a certain lot. Following tests constitute acceptance test: 1. Annealing test (for copper), 2. Tensile test (for aluminum), 3. Wrapping test (for aluminum), 4. Conductor resistance test, 5. Test for thickness of insulation and sheath, 6. High voltage test and 7. Insulation resistance test. d) Optional test: It is the special test as per the agreement between the purchaser and the supplier. Following tests shall constitute optional tests: 1. Cold bend test, 2. Cold impact tests, 3. Armour Resistance tests The tests shall be carried out in accordance with methods as per IS: 10810; taking intoaccount additional information given in standards. The manufacturer shall be identified throughout the length of the cable by means of atape bearing the manufacturer’s name or trade-mark, or by manufacturer’s name or trade markbeing indented, printed or embossed on the cable. In case none of these methods can beemployed, or if the purchaser so desires, color identification thread in accordance with ascheme to be approved by the Bureau of Indian standards (BIS) shall be employed. Theindentation, printing or embossing shall be done only on the outer sheath. The conductor used in these standards is made up of copper wire according to thedifferent flexibility classes given in IS: 8130. The conductor shall be provided insulation ofPVC, XLPE, or ELASTOMER by extrusion. The thickness of insulation shall not be less thannominal value as specified in Table 2 of IS: 1554, IS: 7098, IS: 14494 respectively for the 225
  8. 8. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 2, March – April (2013), © IAEMEabove power cables. The insulation shall be so applied closely on the conductor and it shall bepossible to remove it without damaging the conductor. Armoring is made up of galvanized round steel wire. Armouring shall be applied overthe insulation in case of single core cable and over the inner sheath in case of twin, three andmulti-core cables. It should be applied as closely as practicable. The direction of lay of the armour shall beleft hand and that of individual strand shall be right hand. The resistance of armour shall notexceed that of the conductor as specified in IS: 8130 by more than 33 percent. The identification of core for various types of cable shall be as follows: No. of cores Power cores Earth cores Pilot cores 1 Red, Black, Yellow, Blue or Neutral 2 Red, yellow 3 Red, Yellow Green 3 Red, Yellow, Blue 4 Red, Yellow, Blue Green 5 Red, Yellow, Blue Green Black5. CONCLUSION A power cable is an assembly of two or more electrical conductors, usually heldtogether with an overall sheath. There are various types of cables. An effort has been made to carry out a comparative study of XLPE, PVC andElastomeric insulated power cables. Different Indian Standards used for testing of these cableshave been studied. Similarities in material construction, uses, and voltage grades, methods oftesting and range of values of different tests as per relevant Indian standards have beenreported. Test methods for carrying out specific tests as per Indian standards are available in IS:10810 in different parts. The power cable must be capable of carrying the required normal full load currentcontinuously under the site conditions throughout the year. Therefore, the current ratingsspecified must be corrected to site conditions by applying suitable derating/uprating factorsdepending upon: (a) ground or ambient air temperature (max.), (b) Thermal resistivity of soilduring dry season, (c) Depth of laying, (d) Total number of cables/circuits in groups. Naturally, the most economical construction and the size of the cable consistent withrequired current carrying capacity and laying condition has to be selected. Thus the selection ofparticular type of cable i.e. PVC, XLPE, Elastomeric insulated and the particular material forscreening, sheathing, bedding, armouring or serving etc. out of many choices available dependsupon the usages, laying and climatic conditions [14,15]. Power cables are subjected to comprehensive tests during development, qualification,manufacturing and installation. The various tests serve different purposes with the singleoverall goal – to ascertain a trouble-free operation under specified conditions. The large varietyof cable related tests can be categorized according to their purpose and stage duringqualification, manufacturing and installation of the cables. The development of new cabletypes, or the extension of existing cable types to new sizes or ratings, may requirecomprehensive testing of materials, components, and production processes. 226
  9. 9. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 2, March – April (2013), © IAEMEREFERENCES1. Debasish Basak, “Choice and Evaluation of Electric Cables for Energy Efficient System in Mines”, Executive Training Programme on “Latest Trends and Techniques of Energy Conservation for Cost-effective Production in Mining and Allied Industries” at CMRI, Dhanbad, India during 15-19 May, 2006, pp 87-92.2. Debasish Basak, “Testing of Power Cables and its Conformity”, National Workshop on Instrumentational & Safety Aspects in Electrical & Environmental Testing of Electrical Appliances, organized by NRTC, Parwanoo, H.P., India during 28-29 Jan. 2010, Section 4.2, pp 1-8.3. J. B. Gupta “A Course in Power System”, S. K. Kataria & Sons®, 2010-114. IS: 694:1990, Indian Standard specification for PVC Insulated Cables for working voltages up to and including 1100 volts.6. IS: 1554 (Part I, II), Indian Standard Specification for PVC Insulated Electric Cables,© BIS, 19887. IS: 5831, Indian Standard Specification for PVC Insulation and Sheath of electric Cables, © BIS, 19848. IS: 6380, Indian Standard Specification for Elastomeric Insulation And Sheath of electric Cables,© BIS, 19849. IS: 7098 (Part I, II), Indian Standard Specification for Crosslinked polyethylene Insulated PVC sheathed Cables, © BIS, 198810. IS: 8130, Indian Standard Specification for Conductors for Insulated Cables and Flexible Cords,© BIS, 198411. IS: 9968 (part-1) – 1988, Indian Standard specification for Elastomer Insulated cables.12. IS: 14494, Indian Standard Specification for Elastomer Insulated Flexible Cables For Use in Mines,© BIS, 199813. IS: 10810 (Part 5, 7, 10, 11, 12, 14, 15, 30, 43, 45, 53), Indian Standard Specification for Methods of Test for Cables, © BIS,1984.14. D. Basak, “Electrical Energy Aspects vis-à-vis Its Conservation in Mining Sector”, Executive Training Programme on “Latest Techniques and Practices of Energy Conservation for Cost-effective Production in Mining and Allied Industries” at CMRI, Dhanbad, India during 19-23 Jan. 2004, pp 51-55.15. D. Basak, “Electrical Energy Conservation in Mining Sector: Some Areas”, Executive Development Programme on “Recent Trends and Practices of Energy Conservation for Cost-effective Production in Mining and Allied Industries” at CMRI, Dhanbad, India during 9-12 April, 2002, pp 51-56.16. Anuradha Tomar and Dr. Yog Raj Sood, “A New Approach for Power Factor Improvement in Cable Industry” International Journal of Electrical Engineering & Technology (IJEET), Volume 3, Issue 2, 2012, pp. 242 - 249, ISSN Print : 0976-6545, ISSN Online: 0976-6553. 227
  10. 10. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 2, March – April (2013), © IAEMEAUTHORS’Neha Niharika was born in 1987 in India. She is at present Assistant Professor in ElectricalEngineering Deptt., Baba Hira Singh Bhattal Institute of Engineering and Technology,Lehragaga–148031, Punjab, India.Debasish Basak was born in West Bengal, India. He received B.E. (Electrical Engg.) in1986 from National Institute of Technology (formerly, Regional Engineering College),Durgapur, M.E. (Electrical) with specialization in System Engg. & Operations Research in1988 from I.I.T. Roorkee (formerly, University of Roorkee), MBA in Operations Managementin 2002 from IGNOU, New Delhi and Ph.D. (Engg.) in 2009 from Jadavpur University. He hasworked nearly one year at IIT Kharagpur as “Junior Research Engineer” in a project sponsoredby ISRO, Bangalore. In 1989, he joined CMRI as Scientist “B”. Presently, he is Scientist “F”(Senior Principal Scientist) and head of Electrical Laboratory of CIMFR (erstwhile CMRI),Dhanbad. He has published about 45 technical papers in national and internationalconferences/journals. His research interest includes NDT of wire ropes, rural electrificationworks, reliability, and testing of electric and telephone cables.Bubun Das was born in 1985 in India. She received M.Sc in Physics in 2008 from P.K.RoyMemorial College, Dhanbad, Jharkhand, India. She has been working since March, 2010 inElectrical Laboratory of Central Institute of Mining & Fuel Research, Barwa Road, Dhanbad asProject Assistant Level- I in the project entitled as “Third Party Inspection & Monitoring ofprojects under RGGVY (Rajiv Gandhi Grameen Vidyutikaran Yojana) for 10 (ten) districtsNagaland,” sponsored by Dept. of Power, Govt. of Nagaland, India. She has published 3 papersin International journals/conferences. Her research interest includes rural electrification, testingof electric cables and NDT of steel wire ropes. 228