18EE406: Transmission & Distribution

UNIT-3-(Part-A)
Underground Cables.

Dr. Nagesh Prabhu

Prof. & Head, Dept. of E&E
NMAM Institute of Technology, Nitte.

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DEPARTMENT OF ELECTRICAL AND ELECTRONICS
 UNIT-3 (Part-A)
 Underground cables: Types of cables, Constructional features,
material used, insulation resistance, thermal rating of cables,
charging current, Grading of cables, capacitance grading, Inter
sheath grading. Limitations of cables.

04 Hours

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DEPARTMENT OF ELECTRICAL AND ELECTRONICS
 UNIT-3 (Part-A)Objectives
 After reading UNIT-3 (Part-A) , you should be able to:
1. Understand the need for underground cables
2. Provide constructional features of cables
3. Understand the classifications of cables
4. Appreciate the requirement of Grading the cables (capacitance grading and
inter-sheath grading )
5. Understand the advantages and disadvantages of UG cables

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DEPARTMENT OF ELECTRICAL AND ELECTRONICS
 Under Ground (UG) Cables: Introduction
 Electric power can be transmitted or distributed either by overhead
system or by underground cables.
 The underground cables have several advantages such as
• less liable to damage through storms or lightning,
• Low maintenance cost,
• less chances of faults,
• Smaller voltage drop and better general appearance.
 However, their major drawback is that they have
• greater installation cost and
• introduce insulation problems at high voltages compared with
the equivalent overhead system.

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DEPARTMENT OF ELECTRICAL AND ELECTRONICS
 Application of UG Cables
 Underground cables are employed where it is impracticable to use
overhead lines. Such locations may be thickly populated areas
where municipal authorities prohibit overhead lines for reasons
of safety, or around plants and substations or where maintenance
conditions do not permit the use of overhead construction.
 The chief use of underground cables for many years has been for
distribution of electric power in congested urban areas at
comparatively low or moderate voltages.
 Recent improvements in the design and manufacture have led to the
development of cables suitable for use at high voltages.
 Presently it possible to employ underground cables for transmission
of electric power for short or moderate distances.

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DEPARTMENT OF ELECTRICAL AND ELECTRONICS
 Requirements of a Cable
 An underground cable essentially consists of one or more
conductors covered with suitable insulation and surrounded by a
protecting cover
 In general, a cable must fulfil the following necessary requirements :
i. 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.
ii. The conductor size should be such that the cable carries the desired load current
without overheating and causes voltage drop within permissible limits.
iii. 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.
iv. The cable must be provided with suitable mechanical protection so that it may
withstand the rough use in laying it.
v. The materials used in the manufacture of cables should be such that there is
complete chemical and physical stability throughout

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DEPARTMENT OF ELECTRICAL AND ELECTRONICS
 Constructional features of Cables

 Fig. 11.1 shows the general construction of a 3-conductor cable. The various
parts are :
 (i) 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 Fig. 11.1 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.

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DEPARTMENT OF ELECTRICAL AND ELECTRONICS
 Construction of Cables: contd..
ii. Insulation. Each core or conductor is provided with a suitable thickness of
insulation, which 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.
iii. 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 as shown in Fig. 11.1
iv. 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.
v. 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.
vi. 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.
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DEPARTMENT OF ELECTRICAL AND ELECTRONICS
 Properties of Insulating Materials for Cables
 In general, the insulating materials used in cables should have the
following properties :
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.
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.
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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DEPARTMENT OF ELECTRICAL AND ELECTRONICS
 Various insulation materials
1. Rubber.
2. Vulcanized India Rubber (V.I.R.).
3. Impregnated paper.
4. Varnished cambric./empire tape
5. Polyvinyl chloride (PVC).

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DEPARTMENT OF ELECTRICAL AND ELECTRONICS
 Classification of Cables
 Cables can be divided into the following groups :
o Low-tension (L.T.) cables — upto 1000 V
o High-tension (H.T.) cables — upto 11,000 V
o (Super-tension (S.T.) cables — from 22 kV to 33 kV
o Extra high-tension (E.H.T.) cables — from 33 kV to 66 kV
o Extra super voltage cables — beyond 132 kV
 The following types of cables are generally used for 3-phase service
o 1. Belted cables — upto 11 kV
o 2. Screened cables — from 22 kV to 66 kV. Two principal types of screened cables are
o H-type cables
o S.L. type cables.
o 3. Pressure cables — beyond 66 kV.:
o (i.) Oil filled cables
o (ii) Gas pressure Cables

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DEPARTMENT OF ELECTRICAL AND ELECTRONICS
 single-core Low Tension (LT) cable
 Fig. 11.2 shows the constructional details of a single-core
low tension cable.
 The cable has ordinary construction because the stresses
developed in the cable for low voltages (upto 6600 V) are
generally small.
• It consists of one circular core of tinned stranded copper
(or aluminium) 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.
• Single-core cables are not usually armoured in order to
Single-core low tension cable
avoid excessive sheath losses.
• The principal advantages of single-core cables are simple
construction and availability of larger copper section.

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DEPARTMENT OF ELECTRICAL AND ELECTRONICS
 Cables for 3 Phase Service
 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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DEPARTMENT OF ELECTRICAL AND ELECTRONICS
 Belted cables

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DEPARTMENT OF ELECTRICAL AND ELECTRONICS
 Screened cables

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DEPARTMENT OF ELECTRICAL AND ELECTRONICS
 Screened cables: contd..

Screened cables are also called solid cables as the dielectric used is solid

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DEPARTMENT OF ELECTRICAL AND ELECTRONICS
 Pressure cables
 For voltages beyond 66 kV, solid type cables are unreliable because
there is a danger of breakdown of insulation due to the presence of
voids.
 When the operating voltages are greater than 66 kV, pressure cables
are used.
 In such cables, voids ( air pockets in insulation) are eliminated by
increasing the pressure of compound dielectric and for this reason
they are called pressure cables.
 Two types of pressure cables viz oil-filled cables and gas pressure
cables are commonly used.

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DEPARTMENT OF ELECTRICAL AND ELECTRONICS
 Pressure cable: Oil filled

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DEPARTMENT OF ELECTRICAL AND ELECTRONICS
 Pressure cable: Oil filled
 Fig. 11.7 shows the constructional details of a single core sheath channel oil-filled cable.
 In this type of cable, the conductor is solid similar to that of solid cable and is paper
insulated. However, oil ducts are provided in the metallic sheath as shown.
 In the 3-core oil-filler cable shown in Fig. 11.8. The oil ducts are located in the filler
spaces. These channels are composed of perforated metal-ribbon tubing and are at earth
potential.

 The oil-filled cables have three principal advantages.

• Firstly, formation of voids and ionization 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. However, their major disadvantages are the high initial cost and
complicated system of laying.

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DEPARTMENT OF ELECTRICAL AND ELECTRONICS
 Gas Pressure cable

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 Insulation Resistance of a Single-Core Cable

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DEPARTMENT OF ELECTRICAL AND ELECTRONICS
 Numerical: Insulation Resistance of Cables

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DEPARTMENT OF ELECTRICAL AND ELECTRONICS
 Numerical: Insulation Resistance of Cables

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DEPARTMENT OF ELECTRICAL AND ELECTRONICS
 Numerical: Insulation Resistance of Cables

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DEPARTMENT OF ELECTRICAL AND ELECTRONICS
 Capacitance of a Single-Core Cable

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DEPARTMENT OF ELECTRICAL AND ELECTRONICS
 Dielectric Stress in a Single-Core Cable

Q = Charge density / m

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DEPARTMENT OF ELECTRICAL AND ELECTRONICS
 Dielectric Stress in a Single-Core Cable: contd..

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DEPARTMENT OF ELECTRICAL AND ELECTRONICS
 Grading of Cables
 The process of achieving uniform electrostatic stress in the
dielectric of cables is known as grading of cables.

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DEPARTMENT OF ELECTRICAL AND ELECTRONICS
 Capacitance Grading
 The process of achieving uniformity in the dielectric stress by
using layers of different dielectrics is known as capacitance
grading

Therefore, for same g we have

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DEPARTMENT OF ELECTRICAL AND ELECTRONICS
 Capacitance Grading: contd..

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DEPARTMENT OF ELECTRICAL AND ELECTRONICS
 Capacitance Grading: contd..

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DEPARTMENT OF ELECTRICAL AND ELECTRONICS
 Intersheath Grading
o In this method of cable grading, a homogeneous
dielectric is used
o The dielectric is divided into various layers by placing
metallic intersheaths between the core and lead sheath.
o The intersheaths are held at suitable potentials which
are in between the core potential and earth potential.
o Let V1 be the voltage between core and intersheath 1,
o V2 be the voltage between intersheath 1 and 2 and
o V3 be the voltage between intersheath 2 and outer
lead sheath.
o As there is a definite potential difference between the
inner and outer layers of each inter sheath, therefore,
each sheath can be treated like a homogeneous single
core cable.
o This arrangement improves voltage distribution in the
dielectric of the cable and consequently more uniform
potential gradient is obtained

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DEPARTMENT OF ELECTRICAL AND ELECTRONICS
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DEPARTMENT OF ELECTRICAL AND ELECTRONICS
 Disadvantages of Intersheath grading
 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.

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DEPARTMENT OF ELECTRICAL AND ELECTRONICS
 Intersheath grading : Additional Derivation

𝑑1 𝑑2 𝐷
Assume , = = =𝛼
𝑑 𝑑1 𝑑2

𝑟1 𝑟2 𝑅
Therefore, = = =𝛼
𝑟 𝑟1 𝑟2

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DEPARTMENT OF ELECTRICAL AND ELECTRONICS
 Intersheath grading : Additional Derivation
𝑉2 𝑉3
𝑉1 = and 𝑉2 = and 𝑉3 = 𝑉1 𝛼 2 -----------(1)
α α
1
(𝑉2 + 𝑉3 ) = (1 + ) 𝑉3 ---------------- --------(2)
α
α
and 𝑉3 = ( ) (𝑉2 + 𝑉3 ) ------------------------(3)
1+α
Comparing (1) and (3), we get
1
𝑉1 = ( ) (𝑉2 + 𝑉3 ) -------------------------(4)
α (1+α)
1
𝑉1 + (𝑉2 + 𝑉3 ) = {1+( )} (𝑉2 + 𝑉3 )
α (1+α)
1 1+α+𝛼2
V= {1+( )} (𝑉2 + 𝑉3 ) ; V= { } (𝑉2 + 𝑉3 ) -------(5)
α (1+α) α+𝛼 2
Comparing (4) and (5) we get V= {1 + α + 𝛼 2 } 𝑉1
𝑉1 𝑉 1
Now with intersheath, 𝑔𝑚𝑎𝑥 = = { }---------(6)
𝑟 ln(α) 𝑟 ln(α) (1+ α+𝛼 2 )
𝑉
Without inter sheath 𝑔 = 𝑅 -------------(7)
𝑟 ln( )
𝑟
𝑔𝑚𝑎𝑥 3
Now, from (6) and (7) we get , =
𝑔 (1+ α+𝛼 2 )
Since 𝛼>1, g max< g and therefore, 𝑔𝑚𝑎𝑥 with inter sheath is less than g

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DEPARTMENT OF ELECTRICAL AND ELECTRONICS
 Capacitance of 3-Core Cables

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DEPARTMENT OF ELECTRICAL AND ELECTRONICS
 Capacitance of 3-Core Cables: Contd..

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DEPARTMENT OF ELECTRICAL AND ELECTRONICS
Capacitance of 3-Core Cables: Contd.. (Charging current)

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DEPARTMENT OF ELECTRICAL AND ELECTRONICS
 Measurements of Ce and Cc

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DEPARTMENT OF ELECTRICAL AND ELECTRONICS
 Numerical : Charging Current

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DEPARTMENT OF ELECTRICAL AND ELECTRONICS
 Numerical : Charging Current

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DEPARTMENT OF ELECTRICAL AND ELECTRONICS
 Numerical : Charging Current

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DEPARTMENT OF ELECTRICAL AND ELECTRONICS
 Over head lines v/s UG cables
 Electric energy can be transmitted from one place to another
through either the overhead lines or the underground cables.
1. The inductance is more predominant in case of overhead lines whereas
capacitance is in case of underground cables.
2. The large charging current on very high voltage cables limits the use of
cable for long length transmission. Where a long distance transmission is
required, overhead transmission lines are used.
3. The conductor in the overhead line is less expensive than the underground
cable.
4. The size of the conductor for the same power transmission is smaller in
case of overhead lines than the cables because of the better heat dissipation
in overhead lines.
5. The insulation cost is more in case of cables than the overhead lines.
6. The erection cost of an overhead line is much less than the underground
cable

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DEPARTMENT OF ELECTRICAL AND ELECTRONICS
 Over head lines v/s UG cables: contd..
 There are certain situations where underground cables are preferred
over the high cost. They are:
1. Underground cables give greater safety to the public, less interference with
amenities and better outlook to the city.
2. For power station and substation, connections or a link in overhead lines.
3. For submarine crossings.

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DEPARTMENT OF ELECTRICAL AND ELECTRONICS
 Advantages of Underground Cables over Overhead Lines
Advantages
1. The underground cable is not affected by lightning, thunderstorms and
other severe weather conditions. So, there are no interruptions in service
2. Accidents caused by the breaking of the conductors will be reduced.
3. The maintenance cost of underground system is very low because of less
chance of fault occurrence.
4. Because of less spacing between the conductors in an underground
system, inductance is very low as compared to overhead lines; therefore,
the voltage drop is less.
5. In an underground system, surge effect is smoothened down as the
sheath absorbs surge energy.
6. In an underground system, there is no interference to communication
lines.
7. The underground system of distribution or transmission is more
aesthetic because no wiring is visible.
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DEPARTMENT OF ELECTRICAL AND ELECTRONICS
 Disadvantages of Underground Cables over Overhead Lines
Disadvantages:
 The large charging current on very high voltage cables limits the use of cable for
long length transmission. Where a long distance transmission is required,
overhead transmission lines are used
 Underground cables are more expensive than overhead lines due to the high cost
of trenching, conduits, cables, manholes and other special equipment.
 As transmission voltage increases, the cost of the cable increases.
 In this system, expansion for new loads is not possible. It can be met by
installation of new lines only.
 Joining of underground cables is difficult, so tapping of loads and service mains
is not conveniently possible in an underground system.
 The cables have high capacitance due to less spacing between the conductors so it
draws high charging current.
 Though there is only a rare chance of faults occurring in an underground system,
it is very difficult to locate the fault point and its repair is difficult and expensive.

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DEPARTMENT OF ELECTRICAL AND ELECTRONICS
 Conclusions

 Constructional features of cables are explained

 classifications and types of cables are studied
 Requirement of pressure cables for higher voltage
applications is justified.
 Requirement of Grading the cables (capacitance
grading and inter-sheath grading ) in reducing the size
of cable is understood
 Studied the advantages and disadvantages of UG
cables

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DEPARTMENT OF ELECTRICAL AND ELECTRONICS
 Any Queries

?
you are welcome
prabhunagesh@nitte.edu.in

Thank You
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DEPARTMENT OF ELECTRICAL AND ELECTRONICS