Underground cables

Underground Cables
Ms. Poonam Sarawgi
Assistant Professor
EE Department

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.
3/16/2016 4Ms. Poonam Sarawgi

Advantages
The underground cables have several advantages such as,
– Better general appearance
– Less liable to damage through storms or lighting
– Low maintenance cost
– Less chances of faults
– Small voltage drops

The major drawback is that:
• They have greater installation cost.
• Introduce insulation problems at high voltages compared with equivalent overhead
system.
•Difficulty to identify the fault location.
Disadvantage

Overhead vs. Underground

NATIONAL TRENDS
• Municipalities have passed laws requiring
new distribution facilities to be placed
underground.
– For aesthetic reasons
– To increase property values (5-10%
according to some studies)
• Cost covered by developers and
ultimately paid by property owners.
• Provide better protection from storm
damage and improve reliability of power
supply.

Underground Construction and Storm
Protection
• Underground construction can improve the
reliability of the electric power system by
minimizing damage to the system from:
– High winds
– Ice and snow storms
– Falling trees
3/16/2016
10
Ms. Poonam Sarawgi

Underground Construction is NOT
Immune from All Storm Damage
• Flooding
• Hurricane Damage
• Earthquake
Damage
• Lightning Damage
• Rodent and Human
Damage (dig up)

Power System Components
• Transmission System
– 33,000 Volts and Above
– Less than 2% of all outages are
due to transmission system
outages.
Sub-transmission System
 33,000 Volts
 Sometimes 66,000 Volts

Power System Components
Distribution System
 11,000 Volts and below
 Most outages occur here

Comparison between Underground & Overhead System

Comparison Between Underground & Overhead System Cont..

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.
• 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
Core
Belted
paper
Lead
sheath
Bedding
Single wire
armoring
Overall Serving

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.
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.

Properties of Insulating Material
The insulating materials used in cables should have the following properties
 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.

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
• 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.

• 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.
• 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.

Classification Of Cables
A. Based on Voltage Handle Capability
• 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
B. Based on Construction
Single Core
Two Core
Three Core
Four Core

Single-core Low Tension Cable
A cable may have one or more than one core depending
upon the type of service for which it is intended. It may be
(i) single-core
(ii) two-core
(iii) three-core
(iv) four-core etc.
Fig. 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.

Cables For 3-phase Service
To deliver 3-phase power either three-core cable or three
single core cables may be used.
For voltages up to 66 kV, 3-core cable (i.e., multi-core
construction) is preferred due to economic reasons. For
voltages beyond 66 kV, 3-core-cables become too large
and bulky 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.

• Solid Type Cables
1. 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 33KV.
 High voltages beyond 33KV, 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.

2. 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
a. 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.

b. 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.

Above cables are referred to as solid type cables because solid insulation is used
and no gas or oil circulates in the cable sheath. 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.
(c) In practice, voids are always present in the insulation of a cable. 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 ionisation causing permanent damage to the
paper insulation.
Limitations of solid type 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.

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
Oil Filled Cables

Pipe Type Cable
Sheath Channel Oil Filled 3-Core Oil filler Cable3/16/2016 30Ms. Poonam Sarawgi

Advantages of Oil Filled Cables
Oil filled cables have the following advantages over solid 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

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.

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
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
Disadvantages
• Localization of fault is difficult
• It can be costlier in congested areas where
excavation is expensive and inconvenient.
• The maintenance cost is high.

• In this method, 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.
• 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
Draw in 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
Solid System

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.
373/16/2016 Ms. Poonam Sarawgi

• 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.
• 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
Grading of Cables
Since the stresses are maximum at surface of the conductor or inner most part of the
dielectric.

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

• 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 3-core 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.

• 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.

• 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.

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 sub-transmission voltages, 69,000
Volts and 35,000 Volts.

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.

Underground cables

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Similar to Underground cables

Similar to Underground cables (20)

Recently uploaded

Recently uploaded (20)

Underground cables