1. Ms. Tahoora Qureshi, Asst. Professor
Underground Cables
Ms. Tahoora Qureshi, Asst. Professor
Department: B.E. Electrical Engineering
Subject: Elements of Power System
Semester: IV
Teaching Aids Service by KRRC Information Section

2. MODULE 5
UNDERGROUND CABLES

3. INTRODUCTION
An underground cable essentially consists of one or more conductors covered with suitable insula-tion and surrounded by a protecting cover.
(i) The conductor used in cables should be tinned stranded copper or aluminum of high conductivity.
(ii) The conductor size should be such that the cable carries the desired load current without overheating and causes voltage drop within permissible limits.
(iii) The cable must have proper thickness of insulation in order to give high degree of safety and reliability at the voltage for which it is designed.
(iv) The cable must be provided with suitable mechanical protection so that it may withstand the rough use in laying it.
(v) The materials used in the manufacture of cables should be such that there is complete chemical and physical stability throughout.

4. CONSTRUCTION OF CABLES
Cores or Conductors
Insulatian
Metallic sheath
Bedding.
Armouring.
Serving.

5. INSULATING MATERIALS FOR CABLES
Commonly used insulating materials
Rubber
Vulcanised India Rubber (V.I.R.)
Impregnated paper
Varnished cambric
Polyvinyl chloride (PVC).
The insulating materials used in cables should have the following properties
(i) High insulation resistance to avoid leakage current.
(ii) High dielectric strength to avoid electrical breakdown of the cable.
(iii) High mechanical strength to withstand the mechanical handling of cables.
(iv) Non-hygroscopic i.e., it should not absorb moisture from air or soil.
(v) Non-inflammable.
(vi) Low cost so as to make the underground system a viable proposition.
(vii) Unaffected by acids and alkalies to avoid any chemical action.

6. CLASSIFICATION OF CABLES
Cables for underground service may be classified in two ways according to
the type of insulating material used in their manufacture
the voltage for which they are manufactured.
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
(i) Low-tension (L.T.) cables — upto 1000 V
(ii) High-tension (H.T.) cables — upto 11,000 V
(iii) Super-tension (S.T.) cables — from 22 kV to 33 kV
(iv) Extra high-tension (E.H.T.) cables — from 33 kV to 66 kV
(v) Extra super voltage cables — beyond 132 kV

7. CLASSIFICATION OF CABLES
Single core low tension cable

8. CABLES FOR 3-PHASE SERVICE
The following types of cables are generally used for 3-phase service :
Belted cables — up to 11 kV
Screened cables — from 22 kV to 66 kV
Pressure cables — beyond 66 kV.
1.Belted cables
3 core belted cable

9. CABLES FOR 3-PHASE SERVICE
2. Screened cables. These cables are meant for use up to 33 kV, but in particular cases their use may be extended to operating voltages upto 66 kV. Two principal types of screened cables are H-type cables and S.L. type cables.
H type cable
3 core H type cable

10. CABLES FOR 3-PHASE SERVICE
3 core S. L. type Cable

11. CABLES FOR 3-PHASE SERVICE
3. Pressure cables For voltages beyond 66 kV, solid type cables are unreliable because there is a danger of breakdown of insulation due to the presence of voids. When the operating voltages are greater than 66 kV, pressure cables are used. Two types of pressure cables viz oil-filled cables and gas pressure cables are commonly used.

12. CABLES FOR 3-PHASE SERVICE
Gas Pressure cables

13. TYPES OF CABLES- PVC INSULATED

14. TYPES OF CABLES- XLPE(cross-linked polyethylene)

15. TYPES OF CABLES-PAPER IMPREGNATED

16. TYPES OF CABLES- MINERAL INSULATED

17. COMPARISION BETWEEN OVERHEAD AND UNDERGROUND TRANSMISSION
FACTOR
UNDERGROUND
OVERHEAD
Public safety
More safe
Less safe
Initial cost
More expensive
Less expensive
flexibility
less
more
Working voltage
Cannot be operated above 66kv due to insulation issues
Designed for operation up to 400kv or above
Maintainance cost
low

18. COMPARISION BETWEEN OVERHEAD AND UNDERGROUND TRANSMISSION
FACTOR
UNDERGROUND
OVERHEAD
Frequency of faults or failures
less
more
Frequency of accidents
low
Voltage drop
Appearance
Good looking
Shabby appearance
Fault location and repairs
Difficult to locate faults and repair
Easier than underground system

19. COMPARISION BETWEEN OVERHEAD AND UNDERGROUND TRANSMISSION
FACTORS
UNDERGROUND
OVERHEAD
Charging current
More capacitance due to less spacing between conductors, so more charging current
Less charging current
jointing
Difficult, so tapping for loads and service not easily possible
Easier than underground syatem
Damage due to lightning and thunderstorm
Free from interruptions due to lightning
Prone to supply interruptions due to lightning
Interference to communication circuits
No interference
More interference