TRANSMISSION
UNIT – IV AND
DISTRIBUTION
UNDER GROUND
CABLES
Underground Cables
Underground Cables
Types of Cables
Construction of Single Core and 3 Core Cables
Insulation Resistance
Potential Gradient
Capacitance of Single Core and 3 Core Cables
Grading of Cables
Power Factor and Heating of Cables
DC Cables .
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Under Ground Cables
2
Cables
An insulated wire or wires having a
protective casing and it used for
transmitting Electricity or Tele-
communication signals from one place to
another place.
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Underground Cables
An underground cable essentially
consists of one or more conductors
covered with suitable insulation and
surrounded by a protecting cover.
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Underground Cables
REQUIREMENTS :
1) The conductor used in cables should be tinned stranded
copper or aluminum of high conductivity. Stranding is
done so that conductor may become flexible and carry
more current.
2) The conductor size should be such that the cable carries
the desired load current without overheating and causes
voltage drop within permissible limits.
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Underground Cables
REQUIREMENTS :
3) 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.
4) The cable must be provided with suitable mechanical
protection so that it may withstand the rough use in laying
it.
5) The materials used in the manufacture of cables should be
such that there is complete chemical and physical
stability throughout.
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Underground Cables
CONSTRUCTION OF CABLES :
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Underground Cables
CONSTRUCTION OF CABLES :
1) Cores or Conductors – One or more than one core
(1Φ & 3Φ)
2) Paper Insulation – Suitable thickness of insulation
3) Metallic sheath – Protection from moisture, gases
or other damaging liquids
4) Bedding - Fibrous material like jute or hessian tape
5) Armoring - One or two layers of galvanized steel
wire or steel tape
6) Serving - Protect armoring from atmospheric
conditions 8
Underground Cables
INSULATING REQUIREMENTS FOR CABLES
1) High insulation resistance to avoid leakage current.
2) High dielectric strength to avoid electrical
breakdown of the cable.
3) High mechanical strength to withstand the
mechanical handling of cables.
4) Non-hygroscopic i.e., it should not absorb
moisture from air or soil.
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Underground Cables
INSULATING REQUIREMENTS FOR CABLES
5) Non-inflammable.
6) Low cost so as to make the underground system a
viable proposition.
7) Unaffected by acids and alkalies to avoid any
chemical action.
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Underground Cables
INSULATING MATERIALS USED IN CABLES
a) Rubber.
b) Vulcanized India Rubber (V.I.R.).
c) Impregnated Paper.
d) Varnished cambric.
e) Polyvinyl chloride (PVC).
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Types of Cables
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Types of Cables
1) Low-tension (L.T.) cables — upto 1kV
2) High-tension (H.T.) cables — upto 11kV
3) Super-tension (S.T.) cables — from 22kV to 33kV
4) Extra high-tension (E.H.T.) cables — from 33kV
to 66kV
5) Extra super voltage cables — beyond 132kV
A cable may have one or more than one core
depending upon the type of service
(i) Single-core (ii) Two-core
(iii) Three-core (iv) Four-core etc
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Types of Cables
SINGLE-CORE CABLE
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Types of Cables
SINGLE-CORE CABLE
The cable has ordinary construction - (upto 6600 V)
It consists of one circular core of tinned stranded
copper (or aluminum) insulated by layers of
impregnated paper.
The insulation is surrounded by a lead sheath which
prevents the entry of moisture into the inner parts.
In order to protect the lead sheath from corrosion, an
overall serving of compounded fibrous material (jute
etc.) is provided.
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Types of Cables
SINGLE-CORE CABLE
The cable has ordinary construction - (upto 6600 V)
Single-core cables are not usually armored in order
to avoid excessive sheath losses.
The principal advantages of single-core cables are
simple construction and availability of larger
copper section.
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Types of Cables
CABLE FOR 3-PHASE SERVICE
For voltages upto 66 kV, 3-core cable is preferred due
to economic reasons. However, for voltages beyond
66 kV, 3-core-cables become too large and unwieldy
and, therefore, single-core cables are used.
The following types of cables are generally used for
3-phase service :
1. Belted cables — upto 11 kV
2. Screened cables — from 22 kV to 66 kV
3. Pressure cables — beyond 66 kV.
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Types of Cables
I) BELTED CABLES
These cables are used for voltages upto 11kV but in
extraordinary cases, their use may be extended upto
22kV.
The belted type construction is suitable only for
low and medium voltages as the electrostatic stresses
developed in the cables for these voltages are more or
less radial i.e., across the insulation.
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Types of Cables
I) BELTED CABLES
However, for high voltages (beyond 22 kV), the
tangential stresses also become important. These
stresses act along the layers of paper insulation.
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Types of Cables
I) BELTED CABLES
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Types of Cables
II) SCREENED CABLES - H TYPE
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Types of Cables
II) SCREENED CABLES - H TYPE
H-type cables.
This type of cable was first designed by H. Hochstadter and hence
the name.
Each core is insulated by layers of impregnated paper.
The insulation on each core is covered with a metallic screen
which usually consists of a perforated aluminum foil.
The cores are laid in such a way that metallic screens make contact
with one another.
An additional conducting belt (copper woven fabric tape) is
wrapped round the three cores. 22
Types of Cables
II) SCREENED CABLES - H TYPE
H-type cables. - Advantages
Two principal advantages are claimed for H-type cables. Firstly,
the perforations in the metallic screens assist in the complete
impregnation of the cable with the compound and thus the
possibility of air pockets or voids (vacuous spaces) in the
dielectric is eliminated. The voids if present tend to reduce the
breakdown strength of the cable and may cause considerable
damage to the paper insulation.
Secondly, the metallic screens increase the heat dissipating power
of the cable. 23
Types of Cables
II) SCREENED CABLES – S.L. (SEPARATE LEAD) TYPE
24
Types of Cables
II) SCREENED CABLES – S.L. (SEPARATE LEAD) TYPE
S. L. (Separate Lead) - type cables.
It is basically H-type cable but the screen round each core
insulation is covered by its own lead sheath. There is no overall
lead sheath but only armouring and serving are provided.
The S.L. type cables have two main advantages over H-type
cables.
Firstly, the separate sheaths minimize the possibility of core-
to-core breakdown.
Secondly, bending of cables becomes easy due to the
elimination of overall lead sheath. 25
Types of Cables
II) SCREENED CABLES – S.L. (SEPARATE LEAD) TYPE
S. L. (Separate Lead) - type cables.
However, the disadvantage is that the three lead sheaths of S.L.
cable are much thinner than the single sheath of H-cable and,
therefore, call for greater care in manufacture.
All the cables of above construction are referred to as solid type
cables because solid insulation is used and no gas or oil circulates
in the cable sheath.
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Types of Cables
III) PRESSURE CABLES
1. Oil-filled cables.
2. Gas pressure cables.
• In such types of cables, channels or ducts are
provided in the cable for Oil or Gas
circulation.
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Types of Cables
III) PRESSURE CABLES - OIL-FILLED CABLES.
1. In such types of cables, channels or ducts are
provided in the cable for Oil circulation.
2. Oil-filled cables can be used for higher voltages,
the range being from 66 kV upto 230 kV.
• Oil-filled cables are of three types viz.,
a) Single-core conductor channel,
b) Single-core sheath channel and
c) Three-core filler-space channels.
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Types of Cables
III) PRESSURE CABLES - OIL-FILLED CABLES.
Single-core
conductor
channel
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Types of Cables
III) PRESSURE CABLES - OIL-FILLED CABLES.
Single-core
sheath
channel
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Types of Cables
III) PRESSURE CABLES - OIL-FILLED CABLES.
Three-core
filler-space
channels
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Types of Cables
III) PRESSURE CABLES - GAS PRESSURE CABLES
The voltage required to set up ionization inside a
void increases as the pressure is increased.
Therefore, if ordinary cable is subjected to a
sufficiently high pressure, the ionization can be
altogether eliminated.
At the same time, the increased pressure produces
radial compression which tends to close any
voids.
This is the underlying principle of gas pressure
cables. 32
Types of Cables
III) PRESSURE CABLES - OIL-FILLED CABLES.
1. In such types of cables, channels or ducts are
provided in the cable for Oil circulation.
2. Oil-filled cables can be used for higher voltages,
the range being from 66 kV upto 230 kV.
• Oil-filled cables are of three types viz.,
a) Single-core conductor channel,
b) Single-core sheath channel and
c) Three-core filler-space channels.
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Laying of Underground
Cables
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Laying of Underground Cables
DIRECT LAYING
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Laying of Underground Cables
DIRECT LAYING
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Laying of Underground Cables
DRAW IN SYSTEM
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Laying of Underground Cables
DRAW IN SYSTEM
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Laying of Underground Cables
SOLID SYSTEM
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Insulation Resistance
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Insulation Resistance of a Single-Core Cable
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Insulation Capacitance
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Capacitance of a Single-Core Cable
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Capacitance of a Single-Core Cable
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Capacitance of a Three-Core Cable
The capacitance of a cable system is much more important
than that of overhead line because in cables
(i) Conductors are nearer to each other and to the
earthed sheath
(ii) They are separated by a dielectric of permittivity
much greater than that of air.
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Capacitance of a Three-Core Cable
Figure shows a system of
capacitances in a 3-core belted cable
used for 3-phase system.
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Capacitance of a Three-Core Cable
Potential difference exists
between pairs of conductors and
between each conductor and the
sheath, electrostatic fields are set
up in the cable as shown in Figure.
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Capacitance of a Three-Core Cable
These electrostatic fields
give rise to core-core
capacitances Cc and conductor-
earth capacitances Ce as shown
in Figure
48
Capacitance of a Three-Core Cable
The three Cc are delta
connected whereas the three Ce are
star connected, the sheath forming
the star point (See Figure)
49
Capacitance of a Three-Core Cable
They lay of a belted cable makes it reasonable to
assume equality of each Cc and each Ce.
The three delta connected capacitances Cc can be
converted into equivalent star connected
capacitances.
It can be easily shown that equivalent star
capacitance Ceq is equal to three times the delta
capacitance Cc i.e. Ceq = 3Cc. 50
Capacitance of a Three-Core Cable
The system of capacitances reduces into the
equivalent circuit shown. Therefore, the whole cable is
equivalent to three star-connected capacitors each of
capacitance
51
Capacitance of a Three-Core Cable
Each of capacitance
If Vph is the phase voltage, then charging current IC
is given by
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Capacitance of a Three-Core Cable
MEASUREMENTS OF Ce AND Cc
The core-core capacitance Cc and core-earth
capacitance Ce can be obtained from the experiential formulas
for belted cables, their values can also be determined by
measurements.
1) In the first measurement, the three cores are bunched
together and the capacitance is measured between the
bunched cores and the sheath. The bunching eliminates all
the three capacitors Cc, leaving the three capacitors Ce in
parallel. Therefore, if C1 is the measured capacitance, this
test yields :
Knowing the value of C1, the value of Ce can be determined. 53
Capacitance of a Three-Core Cable
MEASUREMENTS OF Ce AND Cc
2) In the second measurement, two cores are bunched with
the sheath and capacitance is measured between them and
the third core.
This test yields 2Cc + Ce. If C2 is the measured
capacitance, then,
As the value of Ce is known from first test and C2 is
found experimentally, therefore, value of Cc can be
determined.
54
Capacitance of a Three-Core Cable
MEASUREMENTS OF Ce AND Cc
It may be noted here that if value of CN (= Ce + 3Cc) is
desired, it can be found directly by another test.
In this test, the capacitance between two cores or lines
is measured with the third core free or connected to the
sheath. This eliminates one of the capacitors Ce so that if
C3 is the measured capacitance, then,
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Grading of Cables
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Grading of Cables
The process of achieving
uniform electrostatic stress in the
dielectric of cables is known as
grading of cables.
` The following are the two
main methods of grading of
cables :
(i) Capacitance grading
(ii) Intersheath grading
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Grading of Cables
(i) Capacitance Grading
The process of achieving
uniformity in the dielectric stress
by using layers of different
dielectrics is known as
capacitance grading.
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Grading of Cables
(i) Capacitance Grading
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Grading of Cables
(i) Capacitance Grading
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Grading of Cables
(i) Capacitance Grading
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Grading of Cables
(ii) Intersheath Grading
In this method of cable
grading, a homogeneous
dielectric is used, but it is divided
into various layers by placing
metallic intersheaths between the
core and lead sheath.
The intersheaths are held
at suitable potentials which are
in-between the core potential and
earth potential.
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Grading of Cables
(ii) Intersheath Grading
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Grading of Cables
(ii) Intersheath Grading
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Grading of Cables
(ii) Intersheath Grading
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Grading of Cables
A single-core lead sheathed cable is graded by using three dielectrics of
relative permittivity 5, 4 and 3 respectively. The conductor diameter is 2 cm and
overall diameter is 8 cm. If the three dielectrics are worked at the same
maximum stress of 40 kV/cm, find the safe working voltage of the cable.
What will be the value of safe working voltage for an ungraded cable,
assuming the same conductor and overall diameter and the maximum dielectric
stress ?
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Grading of Cables
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Grading of Cables
A single core cable of conductor diameter 2 cm and lead
sheath of diameter 5.3 cm is to be used on a 66 kV, 3-phase
system. Two intersheaths of diameter 3·1 cm and 4·2 cm are
introduced between the core and lead sheath. If the maximum
stress in the layers is the same, find the voltages on the
intersheaths.
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Grading of Cables
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