6545(Print), ISSN (Online) Volume 4, Issue 2, March April (2013), IAEME & TECHNOLOGY (IJEET)

Transcription

1 INTERNATIONAL International Journal of JOURNAL Electrical Engineering OF ELECTRICAL and Technology (IJEET), ENGINEERING ISSN 0976 & TECHNOLOGY (IJEET) ISSN (Print) ISSN (Online) Volume 4, Issue 2, March April (2013), pp IAEME: Journal Impact Factor (2013): (Calculated by GISI) IJEET I A E M E COMPARATIVE STUDY OF POWER CABLES AND TESTING AS PER INDIAN STANDARDS Neha Niharika 1, Debasish Basak 2 and Bubun Das 3 1 Baba Hira Singh Bhattal Institute of Engineering and Technology, Lehragaga , Punjab, India, 2 Senior Principal Scientist & Head, Electrical Laboratory, CSIR-Central Institute of Mining and Fuel Research, Barwa Road, Dhanbad , India 3 Project Assistant, Electrical Laboratory, CSIR-Central Institute of Mining and Fuel Research, Barwa Road, Dhanbad , 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. POWER CABLES Modern power cables come in a variety of sizes, materials, and types, each particularly adapted to its uses. Large single insulated conductors are also sometimes called power cables in the industry [3]. Cables consist of three major components: conductors, insulation, and protective jacket. The makeup of individual cables varies according to application. The construction 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 sunlight exposure, and mechanical impact, determining the form and composition of the outer cable jacket. Cables for direct burial or for exposed installations may also include metal armor in the form of wires spiraled around the cable, or a corrugated tape wrapped around it. The armor may be made of steel or aluminum, and although connected to earth ground is not intended to carry current during normal operation. Power cables use stranded copper or aluminum conductors, although small power cables may use solid conductors. The cable may include uninsulated 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 cables may have a plastic or metal sheath enclosing all the conductors. The materials for the sheath are selected 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 smoke if burned. Cables intended for underground use or direct burial in earth will have heavy plastic or 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 because of copper, only aluminum is allowed to be used in power cable in India. Copper conductor is only used for cables used in control circuit. The stranding is necessary to provide flexibility to cables. 220

3 3. INSULATING MATERIAL FOR CABLES The satisfactory operation of the cables depends to a great extent upon the characteristic of insulation used. In general, the insulating materials used in cables should have the following properties: 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 h. Gutta percha: It is similar to rubber but it becomes soft at about 65 C. It is nonhygroscopic 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 mechanical considerations and the voltage at which it is required to operate. According to voltage, the cables 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 in power transmission and distribution, control system, utility network, railway signaling installation, switching station, textile mill, chemical plants, machine shops, etc. Cross linked polyethylene cables abbreviated as XLPE cables have been developed to overcome the susceptibility of PILC cables to ingress of moisture. Polyethylene which has the best electrical properties, but poor thermal properties on account of its thermoplastic nature in comparison to impregnated paper, is cross-linked by dry Nitrogen curing process to improve its thermo mechanical properties without loss of electrical properties [5-12]. Elastomeric insulated cables have a long history and variety of application in industries, mines, ships, aircraft, earth moving equipment, wagon tripplers, stackers, reclaimers, welding equipment, electric traction, etc. Comparative study between PVC, XLPE and Elastomer insulated flexible power cables are shown in Table

5 Table 1: Comparison of cables SL. NO. PVC XLPE ELASTOMER INSULATED FLEXIBLE CABLE 1. Definition PVC is Polyvinyl Chloride, a thermoplastic material which can be melted again and again. These materials can be heated to a certain temperature and will harden again as they cool. 2. Characteristic s 3. Indian Specification for testing Properties of PVC: - Toughness, strength. - Ease of blending, ease of processing - Flame resistant and fire prevention properties. Additives can provide PVC clear or colored, rigid or flexible, etc. - Excellent electrical insulation properties. - Impact strength and resistant to bad weather conditions. Resistance to grease, oil and chemicals - PVC is chemically stable and does not depolymerize - Density: g/cc. IS: 1554 (Part-1) used for working voltage up to and including1100volts and (Part-2) for working voltage from 3.3kV up to and including 11kV. IS-694/ used for working voltage up to and including 1100 volts (Unarmoured PVC Insulated Cable) The polyethylene crosslinked is a method called the Catenary Continuous Vulcanizations (CCV) process by Peroxide. This process improves the properties of cables and makes it suitable for power transmission. These compounds can be used at high temperature above 110 C where the normal Polyethylene compounds are thermally unstable. - Higher current carrying capacity with high permissible continuous conductor temperature - Extremely low dielectric losses - Higher short circuit ratings - Light in weight -Trouble-free in maintenance and simple in terminating and jointing. IS: 7098 (Part-1) used for working voltage up to and including 1100 volts and (Part-2) for working voltage 3.3 kv upto and including 33 kv. These are flexible cables made of elastomeric compound used in coal mines, quarries, and other miscellaneous mines for the following voltage grades: 1.1, 3.3, 3.8/6.6, and 6.35/11 kv. -Flexible in nature -In ambient temperature and temperature not exceeding 90 C under normal operation and 250 C under shortcircuit condition. IS:14494 used for the following voltage grades: 1.1, 3.3, 3.8/6.6 and 6.35/11 kv IS: 9968 (Part- 1) /1988 used for voltages up to and including 1100 volts. 223

6 4. Insulation PVC insulation applied extrusion 5. Physical test for insulation and outer sheath a) Tensile strength, As per IS:5831 elongation at break b) Ageing in air oven Cross-linked polyethylene insulation applied by extrusion As per table 1 of IS:7098 for insulation and IS:5831 for sheath c) Insulation resistance test As per IS:5831 As per table 1 of IS:7098 Elastomeric compound. As per IS:6380 As per IS:6380 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: Not applicable. f) Ageing in air bomb g) Tear Resistance h) Hot Set Test i) Oil Resistance test Not applicable. Not applicable. As per IS:6380 j) Loss of mass in air oven k) Heat shock test l) Thermal stability Test 6. Type tests (Test on Conductor) a) Wrapping test as per IS: 8130 Common Tests b) Annealing test (for copper) c) Conductor Resistance test d) Tests for armoring wires/ strips f) Flammability test IS:8130 IS:8130 As per IS: As per table 1 of IS: Not applicable. a) Wrapping test as per IS: a) Not applicable. As per in table 2 and IS:3975 Period of burning after removal of the flame shall not exceed 60 seconds and the unaffected portion from the lower edge of the top clamp shall be at least 50 mm. g) High voltage Tes Apply 3 KV AC and increased to 6 KV within 10 secs and should withstand 6 KV for 5 minutes duration Method as per Part no. 1 of IS:10810 [13] Method as per Part no. 5 of IS:10810 Method as per Part no. 53 of IS:10810 Method as per Part no 45 of IS: NOTE: 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.

7 Following tests are only applicable for Elastomer Insulated Power Cables: a) Partial discharge test- As per of clause no of IS:14494 b) Bending test followed by Partial discharge test- As per of clause no of IS:14494 c) Dielectric power factor test- As per of clause no of IS:14494 d) Heating cycle test followed by Partial discharge test- As per IS:14494 of clause no e) Water absorption test As per IS: 6380 and As per clause no of IS:14494 The similarities of three standards mentioned earlier for testing of electric power cables are described below: Tests can be carried out on armoured and unarmoured single core, twin core, three core and multi-core cables for electric supply and control purposes. These cables are used in mines also. 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 into account additional information given in standards. The manufacturer shall be identified throughout the length of the cable by means of a tape bearing the manufacturer s name or trade-mark, or by manufacturer s name or trade mark being indented, printed or embossed on the cable. In case none of these methods can be employed, or if the purchaser so desires, color identification thread in accordance with a scheme to be approved by the Bureau of Indian standards (BIS) shall be employed. The indentation, 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 the different flexibility classes given in IS: The conductor shall be provided insulation of PVC, XLPE, or ELASTOMER by extrusion. The thickness of insulation shall not be less than nominal value as specified in Table 2 of IS: 1554, IS: 7098, IS: respectively for the 225

8 above power cables. The insulation shall be so applied closely on the conductor and it shall be possible to remove it without damaging the conductor. Armoring is made up of galvanized round steel wire. Armouring shall be applied over the insulation in case of single core cable and over the inner sheath in case of twin, three and multi-core cables. It should be applied as closely as practicable. The direction of lay of the armour shall be left hand and that of individual strand shall be right hand. The resistance of armour shall not exceed 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: 5. CONCLUSION 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 Black A power cable is an assembly of two or more electrical conductors, usually held together with an overall sheath. There are various types of cables. An effort has been made to carry out a comparative study of XLPE, PVC and Elastomeric insulated power cables. Different Indian Standards used for testing of these cables have been studied. Similarities in material construction, uses, and voltage grades, methods of testing and range of values of different tests as per relevant Indian standards have been reported. Test methods for carrying out specific tests as per Indian standards are available in IS: in different parts. The power cable must be capable of carrying the required normal full load current continuously under the site conditions throughout the year. Therefore, the current ratings specified must be corrected to site conditions by applying suitable derating/uprating factors depending upon: (a) ground or ambient air temperature (max.), (b) Thermal resistivity of soil during 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 with required current carrying capacity and laying condition has to be selected. Thus the selection of particular type of cable i.e. PVC, XLPE, Elastomeric insulated and the particular material for screening, sheathing, bedding, armouring or serving etc. out of many choices available depends upon 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 single overall goal to ascertain a trouble-free operation under specified conditions. The large variety of cable related tests can be categorized according to their purpose and stage during qualification, manufacturing and installation of the cables. The development of new cable types, or the extension of existing cable types to new sizes or ratings, may require comprehensive testing of materials, components, and production processes. 226

9 REFERENCES 1. 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 May, 2006, pp 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 Jan. 2010, Section 4.2, pp J. B. Gupta A Course in Power System, S. K. Kataria & Sons, 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, IS: 5831, Indian Standard Specification for PVC Insulation and Sheath of electric Cables, BIS, IS: 6380, Indian Standard Specification for Elastomeric Insulation And Sheath of electric Cables, BIS, IS: 7098 (Part I, II), Indian Standard Specification for Crosslinked polyethylene Insulated PVC sheathed Cables, BIS, IS: 8130, Indian Standard Specification for Conductors for Insulated Cables and Flexible Cords, BIS, 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, IS: (Part 5, 7, 10, 11, 12, 14, 15, 30, 43, 45, 53), Indian Standard Specification for Methods of Test for Cables, BIS, 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 Jan. 2004, pp 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 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 , ISSN Print : , ISSN Online:

10 AUTHORS Neha Niharika was born in 1987 in India. She is at present Assistant Professor in Electrical Engineering Deptt., Baba Hira Singh Bhattal Institute of Engineering and Technology, Lehragaga , Punjab, India. Debasish Basak was born in West Bengal, India. He received B.E. (Electrical Engg.) in 1986 from National Institute of Technology (formerly, Regional Engineering College), Durgapur, M.E. (Electrical) with specialization in System Engg. & Operations Research in 1988 from I.I.T. Roorkee (formerly, University of Roorkee), MBA in Operations Management in 2002 from IGNOU, New Delhi and Ph.D. (Engg.) in 2009 from Jadavpur University. He has worked nearly one year at IIT Kharagpur as Junior Research Engineer in a project sponsored by 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 international conferences/journals. His research interest includes NDT of wire ropes, rural electrification works, 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.Roy Memorial College, Dhanbad, Jharkhand, India. She has been working since March, 2010 in Electrical Laboratory of Central Institute of Mining & Fuel Research, Barwa Road, Dhanbad as Project Assistant Level- I in the project entitled as Third Party Inspection & Monitoring of projects under RGGVY (Rajiv Gandhi Grameen Vidyutikaran Yojana) for 10 (ten) districts Nagaland, sponsored by Dept. of Power, Govt. of Nagaland, India. She has published 3 papers in International journals/conferences. Her research interest includes rural electrification, testing of electric cables and NDT of steel wire ropes. 228