1. Underground Cables 1 Presented By: Rathod Vedant( 130030109096) ATMIYA INSTITUTE OF TECHNOLOGY & SCIENCE

2. Introduction 2 Since the loads having the trends towards growing density. This requires the better appearance, rugged construction, greater service reliability and increased safety. An underground cable essentially consists of one or more conductors covered with suitable insulation and surrounded by a protecting cover. The interference from external disturbances like storms, lightening, ice, trees etc. should be reduced to achieve trouble free service. The cables may be buried directly in the ground, or may be installed in ducts buried in the ground.

3. Introduction 3

4. Advantages & Disadvantages 4 Advantages Better general appearance Less liable to damage through storms or lighting Low maintenance cost Less chances of faults Small voltage drops Disadvantages The major drawback is that they have greater installation cost and introduce insulation problems at high voltages compared with equivalent overhead system.

5. Construction of Cables Punjab EDUSAT Socity 5

6. Construction of Cables 6 Core or Conductor A cable may have one or more than one core depending upon the type of service for which it is intended. The conductor could be of aluminum or copper and is stranded in order to provide flexibility to the cable. Insulation The core is provided with suitable thickness of insulation, depending upon the voltage to be withstood by the cable. The commonly used material for insulation are impregnated paper, varnished cambric or rubber mineral compound.

7. Construction of Cables 7 Metallic Sheath A metallic sheath of lead or aluminum is provided over the insulation to protect the cable from moisture, gases or others damaging liquids Bedding Bedding is provided to protect the metallic sheath from corrosion and from mechanical damage due to armoring. It is a fibrous material like jute or hessian tape.

8. Construction of Cables 8 Armouring Its purpose is to protect the cable from mechanical injury while laying it or during the course of handling. It consists of one or two layers of galvanized steel wire or steel tape. Serving To protect armouring from atmospheric conditions, a layer of fibrous material is provided.

9. Insulating Materials for Cables 9

10. Properties of Insulating Material 10  High resistivity.  High dielectric strength.  Low water absorption.  Non – inflammable.  Chemical stability.  High mechanical strength.  Capability to with stand high rupturing voltage.  High tensile strength.

11. Insulating Materials for Cables 11 Vulcanized India Rubber It can be obtained from mixing pure rubber with mineral compounds i-e zinc oxide, red lead and sulphur and heated upto 150 C. It has greater mechanical strength, durability and wear resistant property. The sulphur reacts quickly with copper so tinned copper conductors are used. It is suitable for low and moderate voltage cables.

12. CLSSIFICATION OF CABLES 12 Low tension (L.T) ----- up to 1000V High tension (H.T) ----- up to 11, 000V Super tension (S.T) ---- from 22KV to 33KV Extra high tension (E.H.T) cables --- from 33KV to 66KV Extra super voltage cables ------beyond 132KV

13. 3- Core Cables 13 Belted Cables  In these cables the conductors are wrapped with oil impregnated paper, and then cores are assembled with filler material. The assembly is enclosed by paper insulating belt.  These can be used for voltages up to 11KV or in some cases can be used up to 22KV.  High voltages beyond 22KV, the tangential stresses becomes an important consideration.  As the insulation resistance of paper is quite small along the layer, therefore tangential stress set up, hence, leakage current along the layer of the paper insulation.  This leakage current causes local heating, resulting breaking of insulation at any moment

14. 3-core belted Cable 14

15. 3- Core Cables 15 Screened Cables These can be used up to 33kv but in certain cases can be extended up to 66kv. These are mainly of two types  H-type and  S.L type cables

16. 3- Core Cables 16 H-TYPE Cables: Designed by H. Hochstadter. Each core is insulated by layer of impregnated paper. The insulation on each core is covered with a metallic screen which is usually of perforated aluminum foil. The cores are laid in such a way that metallic screen make contact with one another. Basic advantage of H-TYPE is that the perforation in the metallic screen assists in the complete impregnation of the cable with the compound and thus the possibility of air pockets or voids in the dielectric is eliminated. The metallic screen increase the heat dissipation power of the cable.

17. 3- Core Cables (H-Type) 17

18. 3- Core Cables 18 S.L - Type: (Separate Lead) Each core insulation is covered by its own lead sheath. It has two main advantages, firstly the separate sheath minimize the possibility of core-to-core breakdown. Secondly the, bending of cables become easy due to the elimination of over all sheath. The disadvantage is that the lead sheaths of S.L is much thinner as compared to H-Type cables, therefore for greater care is required in manufacturing.

19. 3- Core Cables (S.L. Type) 19

20. 3- Core Cables 20 Pressurized Type Cables In these cables, pressure is maintained above atmosphere either by oil or by gas. Gas pressure cables are used up to 275KV. Oil filled cables are used up to 500KV.

21. 3- Core Cables 21 Oil Filled Cables Low viscosity oil is kept under pressure and fills the voids in oil impregnated paper under all conditions of varying load. There are three main types of oil filled cables a. Self-contained circular type b. Self-contained flat type c. Pipe Type cables

22. 3- Core Cables (Oil filled) 22

23. Laying of Underground Cables 23 The reliability of underground cable network depends to a considerable extent upon proper laying. There are three main methods of Laying underground cables a. Direct Laying b. Draw in system c. Solid system

24. Direct Laying 24 This method is cheap and simple and is most likely to be used in practice. A trench of about 1.5 meters deep and 45 cm wide is dug. A cable is been laid inside the trench and is covered with concrete material or bricks in order to protect it from mechanical injury. This gives the best heat dissipating conditions beneath the earth. It is clean and safe method

25. Direct Laying 25

26. Disadvantages of Direct Laying 26 Localization of fault is difficult It can be costlier in congested areas where excavation is expensive and inconvenient. The maintenance cost is high

27. Draw in System 27 In this conduit or duct of concrete is laid in ground with main holes at suitable positions along the cable route. The cables are then pulled into positions from main holes.

28. Advantages of Draw in System 28 It is very high initial cost Heat dissipation conditions are not good This method is suitable for congested areas where excavation is expensive and inconvenient This is generally used for short lengths cable route such as in workshops, road crossings where frequent digging is costlier and impossible

29. Solid System 29 In this system the cable is laid in open pipes or troughs dug out in earth along the cable route. The troughing is of cast iron or treated wood Troughing is filled with a bituminous after cables is laid. It provides good mechanical strength It has poor heat dissipation conditions It requires skilled labour and favorable weather conditions It is very much expensive system

30. Solid System 30

31. Grading of Cables 31 Since the stresses are maximum at surface of the conductor or inner most part of the dielectric. The stress goes on decreasing as outer most layer is reached. Since the process of achieving the uniform electrostatic stresses on the dielectric of cables is known as Grading of cables

32. Grading of Cables 32 The unequal distribution of stresses is undesirable because, if dielectric is chosen according to maximum stress the thickness of cable increases or either this may lead to breakdown of insulation. The following are the two main methods of grading  Capacitance grading  Inter sheath grading

33. EARTH FAULTS 33 When the conductor of a cable comes in contact with earth, it is called earth fault or ground fault. To identify this fault, one terminal of the megger is connected to the conductor and the other terminal connected to earth. If the megger indicates zero reading, it means the conductor is earthed. The same procedure is repeated for other conductors of the cable.

34. Summary of Costs: Overhead vs. Underground 34 Transmission: Underground may be 4-20 times Overhead. Sub transmission: Underground may be 4-20 times Overhead Distribution: Underground may be 2-10 times Overhead New underground may be cheaper than overhead in special conditions and costs vary greatly from utility to utility and place to place.

35. 35 THANKS