Download Underground Cables Presented By: Er. H.S. Dhaliwal Assistant Professor,

Underground Cables
Presented By:
Er. H.S. Dhaliwal
Assistant Professor,
Deptt. Of Electrical Engg
BHSBIET, Lehragaga.
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Introduction
• Since the loads having the trends towards
growing density. This requires the better
appearance, rugged construction, greater service
reliability and increased safety. An underground
cable essentially consists of one or more
conductors covered with suitable insulation and
surrounded by a protecting cover. The
interference from external disturbances like
storms, lightening, ice, trees etc. should be
reduced to achieve trouble free service. The
cables may be buried directly in the ground, or
may be installed in ducts buried in the ground.
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Introduction
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CABLE STRUCTURE
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Advantages & 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.
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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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Construction of Cables
• 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
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.
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Construction of Cables
• 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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Construction of Cables
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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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Properties of Insulating Material
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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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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 upto 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.
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Insulating Materials for Cables
• Impregnated Paper
 This material has superseded the rubber, consists of chemically pulped
paper impregnated with napthenic and paraffinic materials.
 It has low cost, low capacitance, high dielectric strength and high
insulation resistance.
 The only disadvantage is the paper is hygroscopic, for this reason
paper insulation is always provided protective covering.
•
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.
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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.
•
XLPE Cables (Cross Linked Poly-ethene)
 This material has temperature range beyond 250 – 300 C
 This material gives good insulating properties
 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.
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XLPE cable
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CLSSIFICATION 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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Extra High Tension Cable
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Low Tension Cable
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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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3-core belted Cable
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3- Core Cables
• 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 and
S.L type cables
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3- Core Cables
H-TYPE Cables:
• Designed by H. Hochstadter.
• 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 Cables (H-Type)
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3- Core Cables
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.
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3- Core Cables (S.L. Type)
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3- Core Cables
• Pressurized Type 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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3- Core Cables (Gas pressure)
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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
a. Self-contained circular type
b. Self-contained flat type
c. Pipe Type cables
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3- Core Cables (Oil filled)
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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.
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 Pressure Cables
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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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Gas Insulated Cables (GIC)
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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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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
a. Direct Laying
b. Draw in system
c. Solid system
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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
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Direct Laying
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Disadvantages of Direct Laying
•
•
•
Localization of fault is difficult
It can be costlier in congested areas where
excavation is expensive and inconvenient.
The maintenance cost is high
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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.
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Advantages of Draw in System
• It is very high initial cost
• Heat dissipation conditions are not good
• This method is suitable for congested areas
where excavation is expensive and
inconvenient
• This is generally used for short lengths cable
route such as in workshops, road crossings
where frequent digging is costlier and
impossible
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Solid System
• In this system the cable is laid in open pipes or
troughs dug out in earth along the cable route.
• The troughing is of cast iron or treated wood
• Troughing is filled with a bituminous after cables
is laid.
• It provides good mechanical strength
• It has poor heat dissipation conditions
• It requires skilled labour and favorable weather
conditions
• It is very much expensive system
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Solid System
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Grading of Cables
• Since the stresses are maximum at surface of
the conductor or inner most part of the
dielectric.
• The stress goes on decreasing as outer most
layer is reached.
• Since the process of achieving the uniform
electrostatic stresses on the dielectric of
cables is known as Grading of cables
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Grading of Cables
• The unequal distribution of stresses is
undesirable because,
• if dielectric is chosen according to maximum
stress the thickness of cable increases or either
this may lead to breakdown of insulation.
• The following are the two main methods of
grading
 Capacitance grading
 Inter sheath grading
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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. However, if a fault does
occur it is difficult to locate and repair the fault
because conductors are not visible. Nevertheless,
the following are the faults most likely to occur in
underground cables
1) open circuit fault
2) short circuit fault
3)earth fault
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• When there is a break in the conductor of a cable,
it is called open circuit fault.
• The open circuit fault can be checked by megger.
For this purpose, the three conductors of the 3core cable at the far end are shorted and earthed.
• The resistance between each conductor and earth
is measured by a megger and it will indicate zero
resistance in the circuit of the conductor that is
not broken.
• However, if the conductor is broken, the megger
will indicate infinite resistance in its circuit
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• When two conductors of a multi-core cable come
in electrical contact with each other due to
insulation failure, it is called a short circuit fault.
• Again, we can seek the help of a megger to check
this fault.
• For this purpose the two terminals of the megger
are connected to any two conductors.
• If the megger gives zero reading, it indicates short
circuit fault between these conductors.
• The same steps is repeated for other conductors
taking two a time.
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EARTH FAULTS
• When the conductor of a cable comes in
contact with earth, it is called earth fault or
ground fault.
• To identify this fault, one terminal of the
megger is connected to the conductor and the
other terminal connected to earth.
• If the megger indicates zero reading, it means
the conductor is earthed. The same procedure
is repeated for other conductors of the cable.
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Physical Limitations of
Underground Lines
The main argument against constructing underground
systems is usually financial. But costs are not the only
limitation.
The laws of physics limit how physically long a power line
can be.
These limits are relatively unimportant on overhead lines
but will severely limit high voltage underground cable
systems
 The higher the voltage the shorter the line length
must be.
 The
limiting effects become very important at
transmission voltages, especially 100,000 Volts and
above.
 Limiting
effects may also be important for
subtransmission voltages, 69,000 Volts and 35,000
Volts.
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Physical Limitations: The Effect
of Capacitance
o Capacitance causes current to flow even when no load is
connected to the cable. This is called “line charging
current”.
o Underground line capacitance for power cables is far
higher than overhead line capacitance.
o Wires are closer to each other
o Wires are closer to the earth (within a few inches).
o Underground lines have 20-75 times the line charging
current that an overhead line has (depending on line
voltage).
o If a line is long enough the charging current could be
equal to the total amount of current the line can carry.
This will severely limit its ability to deliver power.
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Summary of Costs: Overhead
vs. Underground
• Transmission: Underground may be 4-20 times
Overhead.
• Sub transmission: Underground may be 4-20
times Overhead
• Distribution: Underground may be 2-10 times
Overhead
• New underground may be cheaper than
overhead in special conditions and costs vary
greatly from utility to utility and place to place.
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THANKS
Presented By:
Er. H.S. Dhaliwal
[email protected]
098150-74800
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