TRANSMISSION AND DISTRIBUTION MODULE 4 UNIT 2
Syllabus: Module 4 Unit 2: Underground Cable: Types of cables, constructional features,
insulation resistance, thermal rating, charging current, grading of cables – capacitance and
inter-sheath. Dielectric loss. Comparison between ac and DC cables. Limitations of cables.
Specification of power cables.
1. With a neat diagram, show the various parts of a high voltage single core UG cable
and explain each part.
(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 figure below 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 underground cable.
(ii) 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.
(iii) Metallic sheath: In order to protect the underground cables from moisture, gases or other
damaging liquids (acids or alkalis) in the soil and atmosphere, a metallic sheath of lead or
aluminium is provided over the insulation as shown in Figure above.
(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. Armouring may not be done in the
case of some underground cables.
(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.
2. How underground cables are classified and explain different types of cables.
The classification of Underground cables can be done on the basis of several criteria. Various
aspects are taken into account while classification and these include:
Number of conductors in the cable
Voltage rating of the cable
Construction of cable
Type and thickness of insulation used
Installation and Laying of the cables
DEPT. OF EEE/SIT/MYSURU 1
, TRANSMISSION AND DISTRIBUTION MODULE 4 UNIT 2
Classification based upon number of conductors in the cable
Single core cable
Three core cable
Classification based upon voltage rating of the cable
Low tension cables: These have a maximum voltage handling capacity of 1000 V (1
kV)
High tension cables: These have a maximum voltage handling capacity of 11 kV.
Super tension cables: These have a maximum voltage handling capacity of 33 kV.
Extra high tension cables: These have a maximum voltage handling capacity of 66 kV.
Extra super voltage cables: These are used for applications with voltage requirement
above 132 kV.
Classification based upon construction of the cable
1. Belted cable
The cores are not circular in shape. The core is insulated from each other by use of impregnated
paper. The three cores are grouped together and belted with the help of a paper belt. The gaps
are filled with fibrous material like jute. This gives circular cross-sectional shape to the cable.
The belt is covered with lead sheath which protects cable from moisture and also gives
mechanical strength. The lead sheath is finally covered by jute like fibrous compounded
material.
Hence under high voltage cases, the cumulative effect of tangential electrical stresses is to from
space inside the cable due to leakage currents. Such air spaces formed inside the insulation is
called void formation. This void formation is dangerous because under high voltage, spaces
are ionized which deteriorates the insulation which may lead to the breakdown of the insulation.
Hence the belted cables are not used for the high voltage levels. Another disadvantage of the
belted cable is large diameter of paper belt. Due to this, wrinkles are formed and gaps may be
developed if the cable is blended. To overcome all thesedifficulties, the screened type cables
are used.
DEPT. OF EEE/SIT/MYSURU 2
Syllabus: Module 4 Unit 2: Underground Cable: Types of cables, constructional features,
insulation resistance, thermal rating, charging current, grading of cables – capacitance and
inter-sheath. Dielectric loss. Comparison between ac and DC cables. Limitations of cables.
Specification of power cables.
1. With a neat diagram, show the various parts of a high voltage single core UG cable
and explain each part.
(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 figure below 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 underground cable.
(ii) 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.
(iii) Metallic sheath: In order to protect the underground cables from moisture, gases or other
damaging liquids (acids or alkalis) in the soil and atmosphere, a metallic sheath of lead or
aluminium is provided over the insulation as shown in Figure above.
(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. Armouring may not be done in the
case of some underground cables.
(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.
2. How underground cables are classified and explain different types of cables.
The classification of Underground cables can be done on the basis of several criteria. Various
aspects are taken into account while classification and these include:
Number of conductors in the cable
Voltage rating of the cable
Construction of cable
Type and thickness of insulation used
Installation and Laying of the cables
DEPT. OF EEE/SIT/MYSURU 1
, TRANSMISSION AND DISTRIBUTION MODULE 4 UNIT 2
Classification based upon number of conductors in the cable
Single core cable
Three core cable
Classification based upon voltage rating of the cable
Low tension cables: These have a maximum voltage handling capacity of 1000 V (1
kV)
High tension cables: These have a maximum voltage handling capacity of 11 kV.
Super tension cables: These have a maximum voltage handling capacity of 33 kV.
Extra high tension cables: These have a maximum voltage handling capacity of 66 kV.
Extra super voltage cables: These are used for applications with voltage requirement
above 132 kV.
Classification based upon construction of the cable
1. Belted cable
The cores are not circular in shape. The core is insulated from each other by use of impregnated
paper. The three cores are grouped together and belted with the help of a paper belt. The gaps
are filled with fibrous material like jute. This gives circular cross-sectional shape to the cable.
The belt is covered with lead sheath which protects cable from moisture and also gives
mechanical strength. The lead sheath is finally covered by jute like fibrous compounded
material.
Hence under high voltage cases, the cumulative effect of tangential electrical stresses is to from
space inside the cable due to leakage currents. Such air spaces formed inside the insulation is
called void formation. This void formation is dangerous because under high voltage, spaces
are ionized which deteriorates the insulation which may lead to the breakdown of the insulation.
Hence the belted cables are not used for the high voltage levels. Another disadvantage of the
belted cable is large diameter of paper belt. Due to this, wrinkles are formed and gaps may be
developed if the cable is blended. To overcome all thesedifficulties, the screened type cables
are used.
DEPT. OF EEE/SIT/MYSURU 2
