Underground cables

underground cable is now days in urban areas is used and hence it's important to know about it.

1. UNDERGROUND CABLE Presented By :- SIDDHESH SAWANT UNIVERSITY OF MUMBAI Department of Electrical Engineering, A.C . Patil College of Engineering, Kharghar Navi Mumbai-410210 1

2. C ontents Underground system i ntroduction Underground cables Construction of cables Insulating materials for cables Classification of cables Laying of underground cables Types of cable faults Comparision of underground and overhead system Advantages and disadvantages of cables 2

3. Introduction The underground system of electrical distribution of power in large cities in increasingly being adopted. A lthough it is costly system of distribution as compared to overhead system , i t ensures the continuity of supply apart from the following advantages : Its maintenance is less It has a long life Its appearance is good It eliminates hazards of electrocution due to breakage of over head conductors. 3

4. Underground cables An underground cable consists of one or more conductors covered with some suitable insulating material and surrounded by a protecting cover. The cable is laid underground to transmit electric power. 4

5. Construction of Cables Fig . shows the general construction of a 3-conductor cable. The various parts of cable are : Cores or Conductors Insulation Metallic sheath Bedding Armouring Serving 5

6. Various parts of cable Cores or Conductors- A cable may have one or more than one core (conductor) depending upon the type of service for which it is intended. For instance, the 3-conductor cable shown in Figure is used for 3-phase service. The conductors are made of tinned copper or aluminium and are usually stranded in order to provide flexibility to the cable . Insulation - Each core or conductor is provided with a suitable thickness of insulation, the thickness of layer depending upon the voltage to be withstood by the cable. The commonly used materials for insulation are impregnated paper, varnished cambric or rubber mineral compound . Metallic sheath - In order to protect the cable from moisture, gases or other damaging liquids (acids or alkalies) in the soil and atmosphere, a metallic sheath of lead or aluminium is provided over the insulation as shown in Figure. Bedding- Over the metallic sheath is applied a layer of bedding which consists of a fibrous material like jute or hessian tape. The purpose of bedding is to protect the metallic sheath against corrosion and from mechanical injury due to armouring . 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. Armouring may not be done in the case of some cables . Serving- In order to protect armouring from atmospheric conditions, a layer of fibrous material (like jute) similar to bedding is provided over the armouring. This is known as serving 6

7. I nsulating materials for cables XLPE Cables (Cross Linked Poly-ethene) This material has temperature range beyond 250 – 300 C This material gives good insulating properties 7

8. I nsulating materials for cables It is light in weight, small overall dimensions, low dielectric constant and high mechanical strength, low water absorption. These cables permit conductor temperature of 90 C and 250 C under normal and short circuit conditions. These cables are suitable up to voltages of 33 KV. 8

9. Classification of Cables Low-tension (L.T.) cables — up to 1000 V High-tension (H.T.) cables — up to 11,000 V Super-tension (S.T.) cables — from 22 kV to 33 kV Extra high-tension (E.H.T.) cables — from 33 kV to 66 kV Extra super voltage cables — beyond 132 kV 9

10. Single-core Low Tension Cable 10 A cable may have one or more than one core depending upon the type of service for which it is intended. It may be ( i) single-core ( ii) two-core (iii) three-core (iv) four-core etc. Single core low tension cable :- The cable has ordinary construction because the stresses developed in the cable for low voltages (upto 6600 V) are generally small. Single core low tension cable

11. Laying of Underground Cables 11 The reliability of underground cable network depends to a considerable extent upon the proper laying and attachment of fittings i.e. cable end boxes, etc. There are three methods of laying underground cables : direct laying draw-in-system solid-system

12. Types of cables faults Cables are generally laid in the ground or in ducts in the underground distribution system. For this reason, there are little chances of faults in underground cables, the following are the faults most likely to occur in underground cables:- 1) Open circuit fault 2) Short circuit fault 3) Earth fault 12

13. Comparison between Underground & Overhead System 13

14. A dvantages & disadvantages 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. 14

15. 15 THANK YOU…!!!