Transcript Chapter7_Cable - UniMAP Portal

POWER SYSTEM COMMISSIONING AND
MAINTENANCE PRACTICE
DET 310
CHAPTER 7
UNDERGROUND CABLES
7.0 INTRODUCTION
A considerable amount of transmission and distribution of
electrical energy, especially in densely populated urban areas
is carried out by means of underground cable.
The underground cable are rugged in construction and
provide greater service reliability, increased safety, better
appearance and trouble free service under a variety of
environmental conditions.
7.1 Applications Of Underground Cables
Underground cables are necessary for supply connection in
the electrical plants, in generating stations, transmission
system and distribution systems, utilization plants and so on.
List of example of underground cable application for
connecting one apparatus with the others for the following:
- Supply power to the individual machine apparatus in electrical plants
- Connection between switchgear and individual load, group load
- Connection between auxiliary transformer and switchgear
- Subtransmission line between receiving substation and distribution
substation
7.2 Underground Distribution System Vs Overhead Line
 Safety
 Reliability of supply
 Interference / Disturbance
 Maintenance
 Environment impact
 Economics
7.3 Cable Constructions
 A cable
•
•
•

consists of three main components:Conductor
Insulation
Sheath
External protection is provided by the sheath against
mechanical damage, chemical reaction, moisture an so on.
7.3 Cable Constructions
6
• Conductor
– An element design to transmit
electricity
– A single core has one
conductor while a three-core
has 3 conductors.
– A cable may be has single
core, 3 core or multiple
conductor
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7.3 Cable Constructions
7
• Insulation
– Is a material that reduces
or
prevents
the
transmission of electricity
– Each conductor is covered
by insulation
– Insulation is phase to
ground and phase to phase
XLPE
PAPER
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7.4 Cable Constructions
8
• Sheath
– Cable protective covering
– Metallic
or
nonmetallic
protective covering over the
conductor / insulation /
shield
– External
protection
is
provided by the sheath
against mechanical damage,
chemical reaction, moisture
an so on.
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7.5 Types of Underground
Cables
9
•The identification of the cable are
based on the several items :
•
•
•
•
Insulation
Voltage System
Cable Sizing And Core
Technical Specification Characteristics Of The
Cable
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7.5 Types of Underground
Cables
10
•Usually the operating voltage decides
the types of insulation and cable placed
in various categories depending upon
the voltage for which they are
designed.
–Low Voltage Cable (LV)  11kV
–High Voltage Cable (HV)  11 kV
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7.6 High Voltage Cable
Categories
11
•Paper Insulation
–3 core belted 11kV PILC cable
–Single core screened 11 kV PILC cable
•Polymer Insulation
–3 core XLPE 11 kV cable
–Single core XLPE 11 kV cable
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7.7 High Voltage Cable
Categories
12
• A = Conductor
(Aluminum)
• B = Strand Screen
(carbon black paper )
• C = Insulation (Paper)
• D = Insulation Screen
(carbon black paper)
• E = Sheath (copper
lead)
• F = Jacket
Example of Single core
screened 11 kV PILC cable
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7.7 High Voltage Cable
Categories
13
• A = Conductor
(Aluminum)
• B = Strand Screen
(extruded
semiconducting)
• C = Insulation (XLPE)
• D = Insulation Screen
(extruded
semiconducting)
• E = Shield (copper
Example of Single core XLPE 11
tape)
kV cable
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• F = Jacket
7.8 Why XLPE Cable ?
14
– Excellent Electrical & Physical
Properties
– Capable Of Carrying Large
Current At High Temperature
• Normal ~ 90oc
• Emergency ~ 130oc
• Short Circuit Conditions
~250oc
– Easy To Install – XLPE Easier
To Joint
– No Need For Metallic Sheath
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7.9 Cable Accessories
15
• A cable network must be capable of
supplying
electric
power
without
interruption.
• If a failure does occur, it is usually the
junction points on the network that are at
fault, rarely the cable.
• So it pays to choose cable accessories
with care.
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7.9 Cable Accessories
16
• Cable accessories can be divided into 3
major categories:– Joint/Splice
Joint/Splice
– Termination
– Connector
Termination
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Connector
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7.10 Examples of Cable
Joint/Splice
17
Examples
LV/MV/HV
Joint:1.LV
Joint
Heat
Of
Cable
4
Shrink
2.LV
Heat
(Branch Joint)
Shrink
3.MV
Joint
Shrink
Heat
1
2
5
4.MV
Heat
Shrink
Joint (Transition Joint
Paper To Polymeric)
5. HV
Heat Shrink
Joint Up To 72KV
6.HV
Heat
Shrink
Joint Up To 170kv
3
6
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7.10 Cable Accessories –
Termination
18
• Cable termination is
one of the important
components
in
the
electrical
power
system.
• A failure of it can
cause
a
long
interruption,
costly
repair and loss of
revenue.
• A cable termination is a
way of preparing the
end of a cable to
provide
adequate
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electrical
and Underground Cable
mechanical properties.
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7.11 Examples of Cable
Connector
Examples Of
LV/MV/HV Cable
Connector:1. MV Heat Shrink
Straight Bushing Boot
2.MV Heat Shrink
Right Angle Bushing
Boot
3.MV Push On
connector with surge
arrestor
4.MV Push On
Connector separable 3
Core
5. MV Push On
Connector 1 Core
1
2
3
5
4
6.MV Push On
Straight Bushing Boot
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7.12.0
(CABLE FAULT) INTRODUCTION
7.12.1 CAUSES OF CABLE FAULT (CONTINUE)
􀂃 Cable faults are undesirable causes because:1. Power supply is interrupted
2. Locating fault in a long underground cable is difficult
and time consuming
3. Repairing faulty cable is difficult and time consuming,
aging is a natural process
􀂃 Cable insulation gets deterioration with time.
􀂃 Preventive Maintenance, periodic monitoring is
necessary to prevent failure.
7.12.2.1 MECHANICAL
7.12.2 CAUSES OF UNDERGROUND CABLE FAILURE
Major factors that cause failure of a cable are:• Damaged accidentally by external mechanical means
• Damage caused as a results of mishandling the cable
during layout.
•
Poor workmanship in cable jointing.
•
Natural causes due to aging
of cable.
•
Damaged caused by movement
of soil and erosion
7.12.2.2 MISHANDLING
Mishandling of cable may be occurred during installation
Some of the examples are:
1. Excessive pull
2. Sharp bend.
3. Accident crush.
7.12.2.3 Poor workmanship During Cable Jointing
The cable are jointed together with poor
workmanship can lead to cable fault after a period of
time.
7.12.2.4
NATURAL CAUSES DUE TO AGING OF CABLE
CONTINUE-
CONTINUE-
CONTINUE-
CONTINUE-
CONTINUE-
7.13 TYPES OF CABLE FAULT
GENERAL:
• Series (open circuit) Fault
- Failure of continuity (conductor (s) or cable)
• Shunt (short circuit) fault
- failure of insulation
7.13 TYPES OF FAULT (CONTINUE)-
7.13 TYPES OF FAULT (CONTINUE)-
7.13.1 SERIES AND SHUNT FAULT
Are subsided into the following categories:
 Low Resistance Fault
R f 10Z o
Where Zo= cable surge impedance
=10 – 100 ohm
Usually happens in series fault.
High Resistance Fault
Where
R f 10Zo
7.13.2 INTERMITTENT OR FLASH FAULT
-Usually not apparent to insulation resistance measuring
instrument.
-Does not manifest itself at lower voltages or a surge
-Breakdown will appear under application of high voltage
dc or DC pressure test.
7.14 FAULT LOCATION PROCEDURE
The proper sequence of cable fault location are as
follows:
a) Analysis of fault
b) Pre-location
c) Pin Pointing
d) Confirmation and re-test
7.14 FAULT LOCATION PROCEDURE (cont-)
7.14 FAULT LOCATION PROCEDURE (cont-)
7.14 FAULT LOCATION PROCEDURE (cont-)
7.14 FAULT LOCATION PROCEDURE (cont-)
7.14 FAULT LOCATION PROCEDURE (cont-)
7.14 FAULT LOCATION PROCEDURE (cont-)
7.14 FAULT LOCATION PROCEDURE (cont-)
 Continuity Test
- With the cable conductor shorted or looped at the remote
end, perform continuity test on the cable.
- Measure and record the results in ohm.
- Three measurements are to be carried out between
R-Y, Y-B, B-R.
- The test will determine whether any of cable is open
circuited.
- The resistance per-conductor per km is provided in
Table VI, VII, VIII and IX (refer appendix A)
7.14 FAULT LOCATION PROCEDURE (cont-)
 if the continuity of the cable is sound, insulation resistanc
from one end are sufficient.
 If continuity is broken, IR test should be carried out at
both ends of the cable
7.14.1 BURNING A FAULT
-The continuity and IR test may indicate that burning of
fault by means of HT pressure test set is required.
-
7.14.1 BURNING A FAULT (continue-)
-Burning a fault is achieve by passing current from a DC
HT test set through the fault.
-Other conductors not under test should be earthed.
-HT is applied for about 5 to 10 minutes to burn the fault.
- HT test is used to determine which fault location
equipment is suitable to be used.
-HT is the last resort often used because it sometimes
produce ambiguous and unpredictable results.
-Therefore, fault location equipment should be attempted
first.