Under ground Cables

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Under ground
Cables
AneequeQaiser

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Underground Cables
 A cable that is buried below the ground.
 They distribute electrical power or telecommunications.
 Such cables are an alternative to overhead cables.

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Introduction
 Increasingly being adopted, although it is costly system of distribution as
compared to overhead system
 It ensures the continuity of supply apart from the following advantages:
It ensures non-interrupted continuity of supply
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.

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Advantages
 Better general appearance
 Less li able to damage
through storms or lighting
 Low maintenance cost
 Less chances of faults
 Small voltage drop
Disadvantages
 Greater installation cost and
introduce

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Construction of Cables
 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.

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 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: 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. 7Professional Associates
Limited.

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 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.

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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.

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Classification Of Cables
 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

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Single-core Low Tension Cables

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Single-core Cables
 It consists of one circular core of tinned stranded copper or alo, insulated
by layers of impregnated paper.
 lead sheath….. prevents the entry of moisture
 Serving……to protect the lead sheath from corrosion

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Cables for 3-phase Services
 For voltage up to 66KV 3 core cable is preferred.
Following types of cables are generally used for 3 phase services
Belted cables---upto 11KV
Screened cables---from 22KV to 66 KV
Pressure cables---beyond 66KV

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3-Core Cables(Belted Cables)

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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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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
S.L type cables

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3- Core Cables H-TYPE Cables
 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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3-Core (S.L Type) Cables
 S.L - Type: (Separate Lead)
 Each core insulation is
covered by its own lead
sheath

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Separate Lead Type Cables
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.
Disadvantages
 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

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Limitations of solid type cables
 Above cables are referred to as solid type cables because solid insulation
is used and no gas or oil circulates in the cable sheath.
 The voltage limit for solid type cables is 66 kV due to the following reasons :

z Reasons
a) As a solid cable carries the load, its conductor temperature increases and the
cable compound (i.e., insulating compound over paper) expands. This action
stretches the lead sheath which may be damaged.
b) When the load on the cable decreases, the conductor cools and a partial
vacuum is formed within the cable sheath. If the pinholes are present in the
lead sheath, moist air may be drawn into the cable.
c) In practice, voids are always present in the insulation of a cable. Under
operating conditions, the voids are formed as a result of the differential
expansion and contraction of the sheath and impregnated compound.
The breakdown strength of voids is considerably less than that of the insulation. If the
void is small enough, the electrostatic stress across it may cause its breakdown

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Pressure Cables
 When the operating voltages are greater than 66 kV and up to 230 kV,
pressure cables are used. In such cables, voids are eliminated by
increasing the pressure of compound and for this reason they are called
pressure cables.
 Two Types
1. Oil-filled cables
2. Gas Pressure Cables

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3-Core Cables (Pressurized 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.

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Oil Filled Cables
 In such types of cables, channels or ducts are provided in
the cable for oil circulation. The oil under pressure (it is the same
oil used for impregnation) is kept constantly supplied to the channel
by means of external reservoirs placed at suitable distances (say 500
m) along the route of the cable.
 Low viscosity oil is kept under pressure and fills the voids in oil
impregnated paper under all conditions of varying load

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Advantages of Oil Filled Cables
 Greater operating dielectric stresses
 Greater working temperature and current carrying capacity
 Better impregnation
 Impregnation is possible after sheath
 No void formation
 Smaller size of cable due to reduced dielectric thickness
 Defect can easily be detected by oil leakage

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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.

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…..
 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.

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Gas Insulated Cables (GIC)
 In GIC cables high pressure sulphur hexaflouride (SF6), fills the
small spaces in oil impregnated paper insulation and
suppresses the ionization.
 Most EHV and UHV lines insulated with sulphur hexaflouride
(SF6) gas are being used extensively for voltages above 132 KV
up to 1200 KV.
 These cables are very popular for short lengths, river crossings
and high way crossings.

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Advantages of GIC
 Gas Insulated Cables have several advantages over oil filled
cables, Efficient heat transfer hence can carry more current.
 Low dielectric loss and low capacitance
 SF6 gas is non-toxic, chemically stable and non-inflamable.
 Terminations of GIC cables are simpler and cheaper.

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Comparison b/w Underground and overhead
S.No. Particular Overhead system Underground system
1.
2.
3.
4.
5.
6.
7.
8.
Public safety
Initial cost
Faults
Appearance
Flexibility
Location of fault
Repair
Working voltage
It is less safe.
It is less expensive.
Faults occur frequently .
It gives shabby look.
It is more safe.
It is more expensive.
Very rare chances of faults.
Its appearance is good as wires are
not v1sible.
It is more flexible as new It 1s not flexible, as new
conductors can be laid along the conductors arc to be laid in new
existing conductors. channels.
Fault point can be easily located. Fault point cannot be easily
located.
Can be easily repaired. Cannot be easily repaired.
It can work upto 400 kV. It can work only upto 66 kV due
to insulation "difficulty.

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9.
10.
11.
12.
13.
14.
15.
Lightning thunder
Supply interruption
Frequency of accidents
Interference with
communication system
Insulation cost
Erection cost
Uses
More chances of being subjected Very little chances of being to
lightning . subjected to lightning .
More chances of supply Very little chances of supply
interrruption. interruption.
More chances of accidents. Little chances of accidents.
It interferes with communication No interference with communi-
systems. cation system.
Less. The overhead conductor More insulation cost. Under are
bare. Supported on steel ground cables are provided with towers,
insulated from the various wrappings of high grade towers through
insulators. tape etc. Lead sheath is also
provided.
Much less comparatively Erection cost of high voltage
cable is quite high .
This is used for long distance The large charging current on
transmission high voltage limits the use of
long distance transmission .

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