AERIAL BUNCHED CABLES AND TYPICAL FAILURES

AERIAL BUNCHED CABLES AND TYPICAL FAILURES

AERIAL BUNCHED CABLES
AND
TYPICAL FAILURES
S.RAMAPRASATH
JOINT DIRECTOR ( Retd )
CENTRAL POWER RESEARCH INSTITUTE
BANGALORE
AERIAL BUNCHED CABLES
AND
TYPICAL FAILURES
• Introduction
• Salient features of ABC
• Design of ABC :
LVABC & MVABC
• Typical failures of ABC
• Condition monitoring of ABC
• Conclusion
INTRODUCTION
Distribution of Electrical Power:
1. Over Head Lines
2. Under Ground Power cables
3. Insulated Conductors:
Covered Conductors
Aerial Bunched Cables
Salient features of Aerial Bunched Cables ( ABC )
1. Aesthetic Look
2. Installation - trees
3. Customer safety
4. Insulators and cross arms
5. Minimum maintenance
6. Lesser clearances
7. Short circuit of conductors
8. Road cutting avoided
9. Third party damage
10. Anchored on to buildings
Challenge : Fault location
DESIGN OF AERIAL BUNCHED CABLES
LV ABC: 3 Phase conductors + 1 Neutral / Messenger conductor
+ 1 Lighting conductor
Phase conductors : Conductor IS 8130 / IEC 60228
Insulation : PE or XLPE
Neutral conductor: Aluminium alloy – IS 398 ( Part 4 )
Cross-sectional area based on phase conductor size
Current in neutral – Un-balanced load- Insulated neutral
Lighting conductor: 16 Sq mm Insulated Aluminium conductor
IS : 14255: 1995
Design of MVABC:
Conductor : IS 8130 and Insulation : XLPE
Each phase is insulated & screened -outer sheath PVC or PE
Three phase conductor + One insulated neutral conductor +
One lighting conductor ( Optional )
TYPICAL FAILURES OF ABC
1. Crack on the outer surface of ABC
2. Copper metallic screen damage
3. Burnt outer sheath
4. Burnt outer sheath due to animal bites
5. Dislodged straight through joint
Crack on the outer surface of ABC
Outer sheath – Exposed to UV rays
LVABC: Crack of insulationLVABC:
insulation-moisture enters the
insulation-- oxidation of conductor
insulation
Increase in Conductor Resistance :
I2R losses
MVABC : Outer sheath PVC / PE Crack of
• Density
Cu
+ H2O
outer sheath
Increase in resistance of metallic screen: I2R losses
In case of fault – Fault current damages the copper
screen-- Total damage of MVABC
screen
Requirement of Copper Metallic Screen
Cross sectional area of Screen:
A= x dm x b
d m : Mean diametre of the metallic screen (mm)
b : Nominal thickness of the metallic screen (mm)
Calculation for 11 kV 70 Sq mm MVABC
Table 1
Thickness of
metallic screen
( mm)
Cross sectional area of
metallic screen
( Sq mm)
S.C rating of
metallic screen
( Amps)
0.02
1.11
143
0.035
1.93
250
0.05
2.76
356
0.1
5.52
712
Calculations for a 11 kV MVABC for different
conductor sizes
( Copper screen thickness 0.05 mm)
Table 2
Phase
conductor
size
( Sq mm)
CSA of
copper
screen
( Sq mm)
Short circuit
current of
copper
screen
( Amps)
Short circuit
current of
conductor
( kA)
Percentage
I SC current
of screen
to I SC
current of
conductor
(%)
70
2.76
356
6.58
5.4
95
3.01
388
8.93
4.3
150
3.45
445
14.1
3.2
240
4.03
520
22.56
2.3
Failure of copper metallic screen
Importance of Cross sectional area of metallic
screen
Electrical installation in ships
IEC 60092- 352 : 2005-09
Part 352: Choice and Installations of Electric Cables
Table 2 : Sizes of Earth Continuity Conductors and Equipment
Earthing Connectors
For Fixed Installations :
Conductor ≤ 16 Sq mm, the minimum CSA of earth conductor
equal to CSA of phase conductor
Conductor > 16 Sq mm, the minimum CSA of earth conductor
equal to 50 % CSA of phase conductor
Part 350 : General Construction and test equipments
TYPICAL CROSS SECTIONAL AREAS OF METALLIC SCREENS (POWER
CABLES)
Sl No
Voltage
rating
Conductor
(CSA)
Sq.mm
Cross sectional
area of metallic
screen (sq.mm)
Pb Cu
Al
Cal
Isc
Calculated Isc (kA)
Of metallic screen
Isc (kA)
(kA)
of
condu
Pb
Cu
screen
Al Total
cond/
Metallic
.
1
66
240
(Cu)
356
203
---
34.3
8.5
26.2
--
34.7
3
34.3/
34.73
2
33
3x300
(Al)
--
9
728
28.2
--
1.16
33.5
34.7
28.2/
34.7
3x300
(Al)
--
9.7
28.2/9.7
3x300
(Al)
--
39.2
6
28.2/
39.26
3x400
(Al)
--
40
37.6/40
3x150
(Al)
--
6.02
14.1/
6.02
3
4
5
6
33
33
33
11
GI-W
GI-W
9
185
28.2
--
1.16
GI-S
GI -S
9
829
28.2
--
GI -W
15
829
37.6
--
1.16
-
38.1
1.94
38.1
105
GI -S
GI-W
GI W
GI -W
9
8.54
14.1
--
1.16
4.86
GI
Strip
Resistance of metallic screens
Cable rating
Metallic Resistance
3 x 400 Sq mm 33kV XLPE
Insulated
Cu strip screen + GI Round wire
armour
Cu screen :
1.3 mΩ / m
GIWA
:
0.35 mΩ / m
Both in parallel: 0.33 mΩ / m
3 x 150 Sq mm 11 kV XLPE
Insulated Cu strip screen + GI
strip armour
Cu Screen :
3.0 mΩ / m
GI Strip :
1.5 mΩ / m
Both in parallel: 1.1 mΩ / m
1 x 630 Sq mm 132 kV XLPE
Insulated Pb sheathed + Cu wire
screen + Al wire Armour
Al W Armour :
Cu Wire Screen:
Pb sheath :
All in parallel :
50 µΩ / m
200 µΩ / m
350 µΩ / m
40 µΩ / m
Burnt outer sheath and Damage of outer sheath
due to animal bites
Outer sheath : PE or PVC
PE sheath: Smell fruity – Rodents ( Squirrels )
Exposure of outer sheath : UV Rays & Animal bites
Copper screen + Water : Increase in metallic resistance –
higher I2R heating and when the fault current flows
damages the MVABC
Dislodged straight through joint
Failure of a phase conductor – Joint made
Joint loads the ABC and pulls the outer jacket of the joint :
Enhances the Tensile stress of the joint – pulling out of the
outer tubing of the joint
Crimping of ferrule : I2R over heating of the joint
Failure of Termination & earthing of ABC at ends
•
•
•
•
Improper selection and crimping of lugs
Dimensions of lugs and ferrules:
Concentration of stresses at termius point
Fault in the cable: ISC to the cable ends
Condition Monitoring of ABC
Tests to check the healthiness of ABC:
1. Conductor Resistance
2. Metallic Screen Resistance
3. Insulation resistance
4. Capacitance
Conductor Resistance
ABC circuit isolated at both ends
One end shorted with a cable- Proper Bolts
At the other end between the phases –
Conductor Resistance measured
Digital Micrometer- Heavy duty dc current source
Calculated value : ___ Ω / km @ 20oC
For eg 240 Sq mm Aluminium conductor :
0.125 Ω / km @ 20oC
Measured value : IS 8130 / IEC 60228
Checks the crimping of ferrules and lugs:
Quality of workman ship of jointer
Insulation Resistance
Megger – Insulation Resistance
Insulation Resistance : Calculate Volume Resistivity (  )
 = 2 x π x R x L / 2.3 log10 ( D / d )
Where
Ω-cm
 : Volume Resistivity in Ω-cm
R : Insulation Resistance in Ω
L : Length of the cable in cms
D : Diametre of the insulation below the insulation
screen in mm
d : Diametre over the conductor screen in mm
d
Higher the  : Lower the tan 
D
IR
1/ L
Table
Volume
Core Resistivity (  )
Ω-cm
( 1014 )
Red
1756
Yellow
4.2
Blue
2.3
A typical value of Volume Resistivity
tan 
( 10-3 )
0.47
27.27
66.47
 and VLF tan 
Cable : 3 X 300 Sq mm 11 kV XLPE Cable of Length 1.9 kMs
Tan  : 66.47 x 10
70
70
60
40
30
20
60
tan d ( 10-3 ) )
50
-3
50
40
Tan  : 27.27 x 10-3
30
20
Tan  : 0.47 x 10-3
10
10
0
1756
1756
4.2
4.2
 ( 1014 )
2.3
2.3
Tan  verses Volume Resistivity
Importance of I R
Insulation Resistance :
MVABC:
More realistic
Measured at ends of the cable
Measured between conductor and copper metallic
screen
LVABC:
Measured between phase conductors and between
phase conductor and neutral conductor
Metallic Screen Resistance
Identical to conductor resistance
measurement
Checks the integrity of the copper screen
ISC : Copper screen – nearest ground
Oxidation of copper screen : R
Measured value of copper screen compared
with GTP
Indirectly checks the CSA of copper screen
Capacitance Measurement
Cable : Construction is tubular
Cylindrical Electrodes
Capacitance : Cross sectional area and voltage rating
( Thickness of insulation)
C = 0.024 x €
r
/ log10 ( D/ d )
µF / km
where € r : Permittivity of XLPE insulation ( = 2.3 )
11 kV MVABC
Conductor
Capacitance
CSA ( Sq mm) ( µF / km)
70
95
0.25
0.28
150
240
0.34
0.4
240 Sq mm MVABC
Cable
Voltage
Rating (kV)
Capacitance
(µF / km)
11
0.40
22
0.27
33
0.2
CONCLUSION
Design of Aerial Bunched Cable:
Every component – Quality check
Finished Cable
Installation of ABC : Visual Inspection
Integrity of Metallic Copper Screen: I sc
Effectiveness of Earthing
Failure analysis
Condition Monitoring Tests
Reliability of Aerial Bunched Cable