A Presentation based on Underground Cables Used In the Transmission And Distribution System.It is a topic covered in the syllabus of B.E. in Electrical Engineering in 5th semester Subject named "Electrical Power System" For more detail you can check the book "Electrical Power System" by Author V.K.Mehta and S.Chand Publication.

1. Active Learning Assignments on
Under Ground Cables
Prepared By :- Guided By :-
PAWAR PARTH J. :-150123109012
BUTANI PARTH D. :-150123109002 Prof. Yogesh Patel
SURANI KISHAN J. :-150123109014
Batch B3
EPS
Electrical Dept. (2015-16)

2. Introduction
• An Underground cable essentially consists of one or more conductor coved with suitable
insulation and surrounded by a protecting layer.
• The types of cable to be used will depend upon the working voltage and service
requement,In general cable must follow following necessary conditions:-

3. NECCESITY CODITIONS
• The conductor used in cables should be tinned stranded copper or aluminium
of high conductivity. Stranding is done so that conductor may become flexible
and carry more current.
• The conductor size should be such that the cable carries the desired load
current without overheating and causes voltage drop within permissible limits.
• 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.
• The cable must be provided with suitable mechanical protection so that it may
withstand the rough use in laying it.
• The materials used in the manufacture of cables should be such that there is
complete chemical and physical stability throughout.

4. Construction Of Cable
•Cores:- A cable may have one or more than one core (conductor) depending
upon the type of service for which it is intended.
•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:- order to protect the cable from moisture, gases or
other damaging liquids (acids or alkalis) in the soil and atmosphere, a metallic
sheath of lead or aluminium is provided over the insulation.

5. Construction Of Cable
•Bedding:- Over the metallic sheath is applied a layer of bedding which
consists of a fibrous material. 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 galvanised 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 similar to bedding is provided over the armouring. This is
known as serving

6. Properties of Insulating Material
High resistivity.
High dielectric strength.
Low thermal co-efficient.
Low water absorption.
Low permittivity.
Non – inflammable.
Chemical stability.
High mechanical strength.
High viscosity at impregnation temperature.
Capability to with stand high rupturing voltage.
High tensile strength and plasticity.

7. Insulating Materials for Cables
 Rubber
 It can be obtained from milky sap of tropical trees or from oil
products.
 It has the dielectric strength of 30 KV/mm.
 Insulation resistivity of 10 exp 17 ohm.cm
 Relative permittivity varying between 2 and 3.
 They readily absorbs moisture, soft and liable to damage due to rough
handling and ages when exposed to light.
 Maximum safe temperature is very low about 38 C
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8. Insulating Materials for Cables
Vulcanized India Rubber
 It can be obtained from mixing pure rubber with mineral compounds i-e
zinc oxide, red lead and Sulphur and heated up to 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.

9. Insulating Materials for Cables
Impregnated Paper
This material has superseded the rubber, consists of chemically pulped paper
impregnated with naphthenic and paraffinic materials.
It has low cost, low capacitance, high dielectric strength and high insulation
resistance.
It has low cost, low capacitance, high dielectric strength and high insulation
resistance.
 Varnished Cambric
This is simply the cotton cloth impregnated and coated with varnish.
As the varnish cambric is also hygroscopic so need some protection.
Its dielectric strength is about 4KV / mm and permittivity is 2.5 to 3.8.

10. Insulating Materials for Cables
 Polyvinyl chloride (PVC)
This material has good dielectric strength, high insulation resistance and high
melting temperatures.
These have not so good mechanical properties as those of rubber.
It is inert to oxygen and almost inert to many alkalis and acids.
Therefore, this type of insulation is preferred over VIR in extreme
environmental conditions.
PVC insulated cables are generally used for low and medium
domestic lights and power installations.

11. CLSSIFICATION OF CABLES
Low tension (L.T) ----- up to
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

12. 3- Core Cables
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
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13. 3-core belted Cable
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14. • 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
i. H-type and
ii. S.L type cables
3- Core Cables
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15. 3- Core Cables
H-TYPE Cables:
Designed by H. Hochstetler.
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.
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16. 3- Core Cables (H-Type)
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17. 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.
3- Core Cables
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18. 3- Core Cables (S.L. Type)
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19. • Pressurized Type Cables
 In these cables, pressure maintained above
atmosphere either by oil or gas.
 Gas pressure cables are used up to 275KV.
 Oil filled cables are used up to 500KV.
3- Core Cables

20. 3- Core Cables
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
i. Self-contained circular type
ii. Self-contained flat type
iii. Pipe Type cables

21. Gas Pressure Cables
In these cables an inert gas like nitrogen is used to exert
pressure on paper dielectric to prevent void formation.
These are also termed as Compression cables
They insulated cores similar to solid type
The cable is inserted in a pressure vessel which may
be a rigid steel pipe, commonly known as pipe line
compression cable.
The nitrogen gas is filled in vessel at nominal
pressure of 1.38 * 10 exp 6 N/ square meter with a
maximum pressure of 1.725 * 10 exp 6 N/ square
meter.

22. Laying of Underground Cables
• The reliability of underground cable network depends to a considerable
extent upon proper laying.
• There are three main methods of Laying underground cables
i. Direct Laying
ii. Draw in system
iii. Solid system

23. Direct Laying
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

24. Draw in System
• 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.

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

26. Insulation Resistance of a Single-Core Cable
Consider a single-core cable of conductor radius r 1 and internal sheath radius
r 2 as shown in Fig. 11.12. Let l be the length of the cable and ρ be the
resistively of the insulation.
Consider a very small layer of insulation of thickness dx
at a radius x.
The length through which leakage current tends to flow
is dx and the area of X-section offered to this flow is
2𝜋𝑥𝑙.
Insulation resistance considered layer =𝜌
𝑑𝑥
2 𝜋𝑥𝑙
Insulation resistance for whole cable is
R = 𝑟1
𝑟2
𝜌
𝑑𝑥
2 𝜋𝑥𝑙
=
𝜌
2 𝜋𝑥𝑙 𝑟1
𝑟2 1
𝑥
dx =
𝜌
2 𝜋𝑙
log 𝑒
𝑟2
𝑟1