1. Under Ground Cables By Dr. P. Bala Chennaiah, Associate Professor,
EEE Department,
AITS, Rajampet

2. Known to Unknown Can you identify the things shown below? F1G.1
Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

3. Introduction to Underground Cables
Electric power can be distributed by underground cables.
Advantages of underground cables :
Less liable to damage through storms and lightening.
Low maintenance cost.
Less chances of faults.
Better voltage regulation.
Good general appearance.
Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

4. Definition of a Cable
An underground cable essentially consists of one or more conductors covered with suitable insulation surrounded by a protecting cover.
F1G.2.
A simple cable
Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

5. Definition of a Cable
In a simple way, a cable can be seen as a combination of conductor, insulator and protective cover.
Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

6. Requirements of Underground Cables
Conductor used in cables should be copper or aluminium.
Conductor must be stranded and tinned.
Conductor size should be such that the cable carries the desired load current.
Cable should cause voltage drop within permissible limits.
(Contd…)
Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

7. Requirements of Underground Cables
Cable must have proper thickness of insulation to give high degree of safety.
Cable must be provided with suitable mechanical protection.
Materials of a cable must be free from chemical and physical throughout.
Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

8. Properties of Insulation materials used for Underground Cables
High insulation resistance to avoid leakage current.
High dielectric strength to avoid electrical breakdown.
Good mechanical strength to have flexibility.
Low coefficient of thermal expansion.
Non-inflammable.
(Contd…)
Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

9. Properties of Insulation materials used for Underground Cables
Low permittivity.
Immune from acid and alkalies.
Non-hygroscopic i.e. should not absorb moisture.
Low power factor.
Cost should be economical.
Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

10. Most commonly used Insulating materials in Underground Cables
Impregnated paper
Poly Vinyl Chloride
Butyl Rubber
Polythene
Cross linked Polyethylene
Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

11. Comparison between Overhead Lines and Underground Cables
S.no
Aspect
Overhead Lines
Cables 1
Place of use
Distribution and Transmission
Only in Distribution 2
Cost of installation
Cheap
Costly 3
Requirement of towers…etc
Compulsory
Not required
Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

12. Comparison between Overhead Lines and Underground Cables
S.no
Aspect
Overhead Lines
Cables 4
Spacing between conductors
In metres (more)
In centimetres (less) 5
Charging current
Less
More 6
Cross section
Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

13. Comparison between Overhead Lines and Underground Cables
S.no
Aspect
Overhead Lines
Cables 7
Insulation requirement
Less
More 8
Safety 9
Appearance
Not good
Better
Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

14. Comparison between Overhead Lines and Underground Cables
S.no
Aspect
Overhead Lines
Cables 10
Voltage Regulation
Good
Poor 11
Radio interference
More
Less 12
Chances of Fault occurrence
Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

15. Comparison between Overhead Lines and Underground Cables
S.no
Aspect
Overhead Lines
Cables 13
Voltage stress on Insulation
Less
More 14
Heating 15
Efficiency
Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

16. Classification of Cables
(a) According to Number of conductors
(i) Single-core cables
(ii) Two-core cables
(iii) Three-core cables
(iv) Four-core cables
Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

17. Classification of Cables
(b) According to Voltage
(i) Low-tension (LT) cables (up to 1 KV)
(ii) High-tension (HT) cables (up to 11 KV)
(iii) Super-tension (ST) cables (up to 33 KV)
(iv) Extra High-tension (EHT) cables (up to 66 KV)
(v) Extra super voltage cables (132 KV and above)
Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

18. Classification of Cables
(c) According to Insulation and lead sheathing
(i) Belted cables
(ii) Horchstadter (H)- type cables
(iii) Separate Lead (SL)- type cables
(iv) HSL-type cables
Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

19. Classification of Cables
(d) According to the methods of improving dielectric stress
(i) Solid Type cables
(ii) Oil Filled Type cables
(iii) External Oil Pressure Type cables
(iv) Gas Pressure Type cables
Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

20. Construction of Low Voltage Cables
Low Voltage Cables are of two types
(i) Single-core cables
(ii) Multi-core cables
Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

21. Construction of Low Voltage Cables
(i) Construction of a Single-Core Cable:
Fig.1.
Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

22. Construction of Low Voltage Cables
Construction of a Single-Core Cable:
These are used for voltages up to 1 KV.
It consists of one circular core.
Its core is made of tinned stranded copper or aluminium.
Impregnated paper layers are used as insulation for this cable.
A lead sheath is provided around the insulation to protect it from moisture.
Jute is provided around the lead sheath as serving to avoid corrosion.
Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

23. Construction of Low Voltage Cables
Advantages of a Single-Core Cable:
It is very simple in construction.
Electrostatic Stresses developed are very small.
Large conductor cross-section is available.
Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

24. Construction of Low Voltage Cables
(ii) Construction of a Multi-Core Cable:
Fig.2.
Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

25. Construction of Low Voltage Cables
Construction of a Multi-Core Cable:
These are used for voltages up to 6.6 KV.
It consists of either circular cores or shaped cores.
Its core is made of tinned stranded copper or aluminium.
Impregnated paper layers are used as insulation between the cores.
Paper belts are provided around the insulation.
Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

26. Construction of Low Voltage Cables
Construction of a Multi-Core Cable: (contd…)
A lead sheath is provided around the insulation to protect it from moisture.
Around the lead sheath jute is provided to protect it from corrosion and mechanical injuries.
Galvanized steel wire is provided as armouring.
Fibrous material like jute is provided around the armouring as serving to avoid corrosion.
Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

27. Construction of Low Voltage Cables
Advantages of a Multi-Core Cable:
A single multi-core cable can be used as replacement for the group of single-core cables.
Cost of installation is low when compared with single-core cables’ installation.
Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

28. Construction of H.T. Cables
High Tension (H.T.) Cables have a belt of insulation
around the cores’ group.
So, H.T. Cables generally called as Belted Cables.
Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

29. Construction of H.T. Cables
Construction of a High Tension Belted Cable:
FIG.1
Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

30. Construction of H.T. Cables
Construction of a High Tension Belted Cable
These are used for voltages up to 11 KV.
It consists of group of circular cores.
Its core is made of tinned stranded copper or aluminium.
Cores are separated by Impregnated paper insulation layers.
Another layer of impregnated paper tape called Belt Paper is wound around the insulated cores.
Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

31. Construction of H.T. Cables
Construction of a High Tension Belted Cable:(contd…)
The gap between the insulated cores is filled with jute so as to give circular cross-section to the cables.
The belt is covered with lead sheath to protect the cables from the entry of the moisture.
The lead sheath is covered with a layer of galvanized steel wire as armouring.
Finally Steel armouring is surrounded by a layer of serving.
Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

32. Construction of S.T. Cables
Super Tension (S.T.) Cables can be classified into three types.
H – Type Cables
SL – Type Cables
HSL – Type Cables
Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

33. Construction of S.T. Cables
(i) Construction of a Hochstadter (H) Type Cable:
Fig.1
Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

34. Construction of S.T. Cables
(i) Construction of a Hochstadter (H) Type Cable:
These are used for voltages up to 33 KV.
The cores are insulated by layers of impregnated paper.
This insulation on each core is covered with metallic screens like Aluminium foils.
The cores are laid in such a way that metallic screens make contact with each other.
An additional conducting belt is wrapped around the cores .
Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

35. Construction of S.T. Cables
(i) Construction of a Hochstadter (H) Type Cable:
The cable has no insulating belt.
This cable is further provided with lead sheath, bedding, armoring and serving.
All the core sections and conducting belt are kept at earth potential .
The electric stresses are radial there fore the dielectric losses are reduced .
Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

36. Construction of S.T. Cables
Advantages of a Hochstadter (H) Type Cable:
The perforations in the metallic screen will gives complete impregnation to the cable to avoid entry of air.
Metallic screens increase the heat dissipating capacity of cables.
Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

37. Construction of S.T. Cables
(ii) Construction of Separate Lead (SL) Type Cable:
Fig.2
Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

38. Construction of S.T. Cables
(ii) Construction of Separate Lead (SL) Type Cable:
These cables can be used for voltages up to 66 KV.
This type of cable is basically a H-type cable but the screen around each core is replaced by its own lead sheath.
Each core can be treated as a separate single core cables.
In this type of cables there is no overall lead sheath.
Only armouring and serving are provided.
Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

39. Construction of S.T. Cables
(ii) Advantages of Separate Lead (SL) Type Cable:
Bending of cables becomes easy due to the elimination of over all lead sheath.
It reduces the core to core break down due to the use of separate sheaths.
Oil leakages are less on hilly routes due to elimination of filler spaces containing compound.
Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

40. Construction of S.T. Cables
(iii) Construction of a HSL Type Cable:
Fig.3
Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

41. Construction of S.T. Cables
(iii) Construction of a HSL Type Cable:
This cable is a combination of both H and SL type cables.
In this type of cables, each core is rounded with impregnated paper, metallic paper screen and then separate lead sheath.
Then the cores are provided with filler, bedding, armouring and finally serving.
Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

42. Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet