1. UNIT – IV
UNDER GROUND CABLES
1

2. Under Ground cables-Requirements
• 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. 2

3. Under Ground cables-Requirements
• 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.
3

4. Construction of UG Cables
4

5. Construction of UG Cables
Cores or Conductors.
• A cable may have one or more than one core
(conductor) depending upon the type of service
for which it is intended.
• For instance, the 3-conductor cable shown in
Figure is used for 3-phase service. The conductors
are made of tinned copper or aluminium and are
usually stranded in order to provide flexibility to
the cable.
5

6. Construction of UG Cables
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.
6

7. Construction of UG Cables
Metallic sheath
• In order to protect the cable from moisture, gases
or other damaging liquids (acids or alkalies) in the
soil and atmosphere, a metallic sheath of lead or
aluminium is provided over the insulation.
Bedding
• Over the metallic sheath is applied a layer of
bedding which consists of a fibrous material like
jute or hessian tape.
• The purpose of bedding is to protect the metallic
sheath against corrosion and from mechanical
injury due to armouring.
7

8. Construction of UG Cables
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.
8

9. Construction of UG Cables
Serving
• In order to protect armouring from atmospheric
conditions, a layer of fibrous material (like jute)
similar to bedding is provided over the armouring.
This is known as serving.
9

10. 10

11. Advantages of UG Cables
• It ensures non-interrupted continuity of supply.
The possible supply interruptions due to lightning,
storms and other weather conditions are
eliminated because of UG cables.
• It requires less maintenance
• The accidents caused due to breakage of overhead
line conductors are eliminated due to use of
underground cables.
• The voltage drop on UG cables is less.
• The life of UG cables is long compared to OH lines.
11

12. Advantages of UG Cables
• The beauty of cities and towns get maintained due
to UG network of cables.
• The overhead lines use bare conductors which is
unsafe in thickly populated areas. Hence from
safety point of view, the UG cables are more
advantageous.
• The only drawbacks of the UG cables are the
extremely high initial cost and insulation problems
at high voltages.
12

13. Insulating materials for UG Cables
• The satisfactory operation of a cable depends to a
great extent upon the characteristics of insulation
used.
• Therefore, the proper choice of insulating
material for cables is of considerable importance.
13

14. Insulating materials for UG Cables
In general, the insulating materials used in cables
should have the following properties:
• High insulation resistance to avoid leakage
current.
• High dielectric strength to avoid electrical
breakdown of the cable.
• High mechanical strength to withstand the
mechanical handling of cables.
• Non-inflammable.
14

15. Insulating materials for UG Cables
• Non-hygroscopic i.e., it should not absorb
moisture from air or soil. The moisture tends to
decrease the insulation resistance and hastens
the breakdown of the cable. In case the insulating
material is hygroscopic, it must be enclosed in a
waterproof covering like lead sheath.
• Low cost so as to make the underground system a
viable proposition.
• Unaffected by acids and alkalies to avoid any
chemical action.
15

16. Insulating materials for UG Cables
The principal insulating materials used in cables are
• Rubber,
• Vulcanized India rubber,
• Impregnated paper,
• Varnished cambric and
• Polyvinyl chloride.
16

17. Classification of UG Cables
Cables for underground service may be classified in two
ways according to
• the type of insulating material used in their
manufacture
• the voltage for which they are manufactured.
However, the latter method of classification is generally
preferred, according to which cables can be divided into
the following groups :
• Low-tension (L.T.) cables — upto 1000 V
• High-tension (H.T.) cables — upto 11,000 V
• Super-tension (S.T.) cables — from 22 kV to 33 kV
• Extra high-tension (E.H.T.) cables - from 33 kV to 66 kV
• Extra super voltage cables — beyond 132 kV
17

18. Classification of UG Cables
• The cable has ordinary construction because the stresses
developed in the cable for low voltages (upto 6600 V) are
generally small.
18

19. Cables for Three phase Service
• For voltages upto 66 kV, 3-core cable (i.e., multi-
core construction) 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.
19

20. Belted cables
• These cables are used for voltages upto 11kV but in
extraordinary cases, their use may be extended upto
22kV. Figure shows the constructional details of a 3-core
belted cable.
• The belted type construction is suitable only for low and
medium voltages.(beyond 22 kV screened cables are
used)
20

21. Screened cables
(i) H-type cables:
• This type of cable was first designed by
H. Hochstadter and hence the name.
21

22. Screened cables
Advantages of H-type cable:
• 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.
22

23. Screened cables
(i) S.L. (Separate lead) -type cables:
23

24. Screened cables
Advantages of S.L. 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.
• Drawbacks of S.L. 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
24

25. Limitations of solid type cables
• The voltage limit for solid type cables is 66 kV due to
the following 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.
The moisture reduces the dielectric strength of
insulation and may eventually cause the breakdown
of the cable.
25

26. Limitations of solid type cables
• (c) In practice, voids are always present in the
insulation of a cable. Modern techniques of
manufacturing have resulted in void free cables.
• However, 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.
• The voids nearest to the conductor are the first to
break down, the chemical and thermal effects of
ionization causing permanent damage to the paper
insulation.
26

27. Pressure Cables
(i) Oil filled cables:
Single core conductor channel
Single core sheath channel
Three core filler space channel
27

28. Oil filled cables
• The range being from 66 kV upto 230 kV
• Advantages:
• Firstly, formation of voids and ionisation are
avoided. Secondly, allowable temperature range
and dielectric strength are increased. Thirdly, if
there is leakage, the defect in the lead sheath is at
once indicated and the possibility of earth faults is
decreased.
• Disadvantages:
• the high initial cost and complicated system of
laying.
28

29. Pressure Cables
(i) Gas filled cables:
• designed by Hochstadter, Vogal and Bowden
29

30. Gas filled Cables
• The triangular section reduces the weight and
gives low thermal resistance but the main reason
for triangular shape is that the lead sheath acts as
a pressure membrane.
• The cable is laid in a gas-tight steel pipe. The pipe
is filled with dry nitrogen gas at 12 to 15
atmospheres.
• Moreover, maintenance cost is small and the
nitrogen gas helps in quenching any flame.
• However, it has the disadvantage that the overall
cost is very high.
30

31. Insulation Resistance of a single core cable
• The cable conductor is provided with a suitable
thickness of insulating material in order to
prevent leakage current.
• The path for leakage current is radial through the
insulation.
• The opposition offered by insulation to leakage
current is known as insulation resistance of the
cable.
• For satisfactory operation, the insulation
resistance of the cable should be very high.
31

32. Insulation Resistance of a single core cable
• Consider a single-core cable of conductor radius r1
and internal sheath radius r2 as shown in Figure. Let l
be the length of the cable and ρ be the resistivity 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 2π x l.
32

33. Insulation Resistance of a single core cable
33

34. Capacitance of a single core cable
• A single-core cable can be considered to be
equivalent to two long co-axial cylinders. The
conductor (or core) of the cable is the inner
cylinder while the outer cylinder is represented by
lead sheath which is at earth potential.
• Consider a single core cable with conductor
diameter d and inner sheath diameter D.
34

35. Capacitance of a single core cable
• Let the charge per metre axial length of the cable
be Q coulombs and ε be the permittivity of the
insulation material between core and lead sheath.
• Obviously ε = ε0 εr where εr is the relative
permittivity of the insulation.
• Consider a cylinder of radius x metres and axial
length 1 metre. The surface area of this cylinder is
= 2 π x × 1 = 2 π x m2
35

36. Capacitance of a single core cable
36

37. Capacitance of a single core cable
37

38. Dielectric stress in a single core cable
• Under operating conditions, the insulation of a
cable is subjected to electrostatic forces. This is
known as dielectric stress. The dielectric stress at
any point in a cable is infact the potential gradient
(or electric intensity) at that point.
38

39. Dielectric stress in a single core cable
• Consider a single core cable with core diameter d
and internal sheath diameter D. The electric
intensity at a point x metres from the centre of
the cable is
39

40. Dielectric stress in a single core cable
• potential difference V between conductor and
sheath is
40

41. Dielectric stress in a single core cable
41

42. Dielectric stress in a single core cable
• It is clear that dielectric stress is maximum at the
conductor surface and its value goes on
decreasing as we move away from the conductor.
It may be noted that maximum stress is an
important consideration in the design of a cable.
• For instance, if a cable is to be operated at such a
voltage that maximum stress is 5 kV/mm, then the
insulation used must have a dielectric strength of
at least 5 kV/mm, otherwise breakdown of the
cable will become inevitable.
42

43. Most economical conductor size in a cable
• It has already been shown that maximum stress in
a cable occurs at the surface of the conductor.
• For safe working of the cable, dielectric strength
of the insulation should be more than the
maximum stress. Rewriting the expression for
maximum stress, we get,
43

44. Most economical conductor size in a cable
• The values of working voltage V and internal
sheath diameter D have to be kept fixed at certain
values due to design considerations.
• This leaves conductor diameter d to be the only
variable in the above expression. For given values
of V and D, the most economical conductor
diameter will be one for which gmax has a
minimum value. The value of gmax will be
minimum when d loge D/d is maximum i.e.
44

45. Most economical conductor size in a cable
45

46. Most economical conductor size in a cable
• For low and medium voltage cables, the value of
conductor diameter arrived at by this method
(i.e., d = 2V/gmax) is often too small from the point
of view of current density.
• Therefore, the conductor diameter of such cables
is determined from the consideration of safe
current density.
• For high voltage cables, designs based on this
theory give a very high value of d, much too large
from the point of view of current carrying capacity
and it is, therefore, advantageous to increase the
conductor diameter to this value.
46

47. Most economical conductor size in a cable
There are three ways of doing this without using
excessive copper :
• Using aluminium instead of copper because for
the same current, diameter of aluminium will be
more than that of copper.
• Using copper wires stranded round a central core
of hemp.
• Using a central lead tube instead of hemp.
47

48. Grading of cables
• The process of achieving uniform electrostatic
stress in the dielectric of cables is known as
grading of cables.
• The unequal stress distribution in a cable is
undesirable for two reasons.
• Firstly, insulation of greater thickness is required
which increases the cable size.
• Secondly, it may lead to the breakdown of
insulation.
48

49. Grading of cables
• In order to overcome above disadvantages, it is
necessary to have a uniform stress distribution in
cables.
• This can be achieved by distributing the stress in
such a way that its value is increased in the outer
layers of dielectric. This is known as grading of
cables.
• The following are the two main methods of
grading of cables :
• (i) Capacitance grading
• (ii) Intersheath grading
49

50. Capacitance Grading
• The process of achieving uniformity in the dielectric stress
by using layers of different dielectrics is known as
capacitance grading.
50

51. Capacitance Grading
• Relative permittivity εr of any layer is inversely
proportional to its distance from the centre.
• However, ideal grading requires the use of an
infinite number of dielectrics which is an
impossible task.
• In practice, two or three dielectrics are used in the
decreasing order of permittivity ; the dielectric of
highest permittivity being used near the core.
51

52. Capacitance Grading
• There are three dielectrics of outer diameter d1,
d2 and D and of relative permittivity ε1, ε2 and ε3
respectively.
• If the permittivities are such that ε1 > ε2 > ε3 and
the three dielectrics are worked at the same
maximum stress, then,
52

53. Capacitance Grading
• Potential difference across the inner layer is
53

54. Capacitance Grading
• If the cable had homogeneous dielectric, then, for
the same values of d, D and gmax, the permissible
potential difference between core and earthed
sheath would have been
• Obviously, V > V′ i.e., for given dimensions of the
cable, a graded cable can be worked at a greater
potential than non-graded cable. Alternatively, for
the same safe potential, the size of graded cable
will be less than that of non-graded cable.
54

55. Capacitance Grading
The following points may be noted :
• (i) As the permissible values of gmax are peak values,
therefore, all the voltages in above expressions should be
taken as peak values and not the r.m.s. values.
• (ii) If the maximum stress in the three dielectrics is not
the same, then,
• The principal disadvantage of this method is that there
are a few high grade dielectrics of reasonable cost whose
permittivities vary over the required range.
55

56. 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.
• This arrangement improves voltage distribution in
the dielectric of the cable and consequently more
uniform potential gradient is obtained.
56

57. Intersheath Grading
57

58. Intersheath Grading
• Maximum stress between core and intersheath 1 is
• Since the dielectric is homogeneous, the maximum stress
in each layer is the same i.e.,
58

59. Intersheath Grading
• As the cable behaves like three capacitors in
series, therefore, all the potentials are in phase
i.e. Voltage between conductor and earthed lead
sheath is V = V1 + V2 + V3
• Intersheath grading has three principal
disadvantages. Firstly, there are complications in
fixing the sheath potentials. Secondly, the
intersheaths are likely to be damaged during
transportation and installation which might result
in local concentrations of potential gradient.
Thirdly, there are considerable losses in the
intersheaths due to charging currents. For these
reasons, intersheath grading is rarely used.
59

60. Difficulties in Grading
• In the intersheath grading, the intersheath has to be
thin. Hence there is possibility of damage to it while
laying the cable. Similarly intersheath has to carry the
charging current which can cause overheating of
cable.
• In capacitance grading, to have uniform distribution
of stress it is necessary to select the dielectrics of
proper permittivities. But practically it is difficult to
get the proper values of permittivities. Similarly the
permittivity of dielectric changes with the time which
can cause uneven distribution of stress. Such uneven
distribution may lead to breakdown at the normal
operating voltage.
60

61. Capacitance of Three Core cables
• 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.
61

62. Capacitance of Three Core cables
62

63. Capacitance of Three Core cables
63

64. Measurement of Ce and Cc
The following two measurements are required :
• In the first measurement, the three cores are bunched
together (i.e. commoned) 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. 64

65. Measurement of Ce and Cc
• 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.
65

66. Measurement 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,
66

67. Power factor and Heating of cables
• For a single core cable the insulation resistance is
given by
• Here ρ is the specific resistance and l is the length
of the cable. Assuming ρ to remain constant, the
impressed voltage V will send a current (in phase
with V) through the insulation.
• Capacitive current drawn from the supply,
• Where, C is the cabale capacitance. The charging
current (IC)leads V by
67

68. Power factor and Heating of cables
• The equivalent circuit with its phasor diagram is
shown in figure. The resultant current I leads V by
ϕ (= ) where cos ϕ gives the power factor of
the cable. 68

69. Power factor and Heating of cables
• But
• The cable power factor thus can be expressed as,
• Dielectric losses are causes by dielectric absorption or
polarization. It is small for voltages upto 33 kV, but for
higher voltages has an increasingly important effect on
current rating. The loss in dielectric is due to loss in the
equivalent leakage resistance.
69

70. Power factor and Heating of cables
70

71. Heating of Cables
Under working condition, the temperature of the
cables increases due to the following factors
• The heat produced within the cables
• The dissipation of heat up to the periphery of the
cables
• The heat dissipation to the surrounding medium
• The current carried by the cables.
• The various load conditions like continuous,
distributed, intermittent etc.
71

72. Heating of Cables
The heat is produced with in the UG cables due to
the following losses
• Copper loss which is also called I2R loss
(temperature, length, area of cross section)
• Dielectric loss (W=V2ῳCtanδ)
• Sheath loss (sheath eddy current, sheath circuit
current)
72

73. Heat flow in Cables
• Heat flowing radially out from core to ground
through dielectric, sheath, bedding, armouring
and serving.
73

74. Thermal characteristics of Cable
• The current in an electric circuit is given by
I= potential difference/resistance = V/R
Similarly heat flow H is given by
H= temperature difference/Thermal resistance = θ/S
• Thermal resistance S is defined as the resistance which
allows the heat flow of 1 watt when a temperature
difference of 1oC is maintained. It is given by
S= kl/A thermal ohms
Where k= thermal resistivity of the material
l= length of the path of heat flow
A= area of section through which heat flow
74

75. D.C. Cables
When a cable is used for d.c. transmission then its
voltage and current carrying capacity increases.
This is because,
• In d.c. system, there is no charging current hence
the corresponding copper losses are absent.
• In d.c. system the dielectric hysteresis loss is
absent. The only loss present is copper loss due
to leakage current. Hence for the same
temperature rise of the insulation, d.c. cables can
carry much more current.
75

76. D.C. Cables
• The skin effect is absent in d.c. hence
corresponding power loss is absent.
• In d.c. system, no voltage gets induced in the
sheath hence sheath losses due to the induced
current is zero.
• Due to all these reasons, the current carrying
capacity of d.c. cable is higher than a.c. cables.
Also the d.c. breakdown stress of the dielectric is
more than the a.c. breakdown stress hence the
voltage rating of d.c. cables is much higher
compared to a.c. cables.
76