1. DET 310 UNDERGROUND CABLES
POWER SYSTEM COMMISSIONING AND MAINTENANCE PRACTICE
DET 310
CHAPTER 7
UNDERGROUND CABLES

2. 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.

3. 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

4. 7.2 Underground Distribution System Vs Overhead Line
Safety
Reliability of supply
Interference / Disturbance
Maintenance
Environment impact
Economics

5. mechanical damage, chemical reaction, moisture an so on.
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.

6. 7.3 Cable Constructions Conductor6
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
ETE503 Underground Cable
4/19/2017

7. 7.3 Cable Constructions 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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8. 7.4 Cable Constructions Sheath Cable protective covering8
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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9. 7.5 Types of Underground Cables9
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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10. 7.5 Types of Underground Cables10
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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11. 7.6 High Voltage Cable Categories11
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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12. 7.7 High Voltage Cable Categories12
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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13. 7.7 High Voltage Cable Categories13
A = Conductor (Aluminum)
B = Strand Screen
(extruded semiconducting)
C = Insulation (XLPE)
D = Insulation Screen
E = Shield (copper tape)
F = Jacket
Example of Single core XLPE 11 kV cable
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14. 7.8 Why XLPE Cable ? Excellent Electrical & Physical Properties14
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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15. 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.
ETE503 Underground Cable
4/19/2017

16. 7.9 Cable Accessories 16
Cable accessories can be divided into 3 major categories:-
Joint/Splice
Termination
Connector
Joint/Splice
Termination
Connector
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17. 7.10 Examples of Cable Joint/Splice17
Examples Of LV/MV/HV Cable Joint:-
1.LV Heat Shrink Joint
2.LV Heat Shrink (Branch Joint)
3.MV Heat Shrink Joint
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 1 2 3 4 5 6
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18. 7.10 Cable Accessories –Termination18
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 electrical and mechanical properties.
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19. 7.11 Examples of Cable Connector19
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
6.MV Push On Straight Bushing Boot 1 2 3 4 5 6
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20. 7.12.0 (CABLE FAULT) INTRODUCTION

21. 7.12.1 CAUSES OF CABLE FAULT (CONTINUE)
􀂃 Cable faults are undesirable causes because:-
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.

22. MECHANICAL

23. 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

24. MISHANDLING
Mishandling of cable may be occurred during installation
Some of the examples are:
Excessive pull
Sharp bend.
Accident crush.

25. 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.

26. 7.12.2.4 NATURAL CAUSES DUE TO AGING OF CABLE

27. CONTINUE-

28. CONTINUE-

29. CONTINUE-

30. CONTINUE-

31. CONTINUE-

32. 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

33. 7.13 TYPES OF FAULT (CONTINUE)-

34. 7.13 TYPES OF FAULT (CONTINUE)-

35. SERIES AND SHUNT FAULT
Are subsided into the following categories:
Low Resistance Fault
Where Zo= cable surge impedance
=10 – 100 ohm
Usually happens in series fault.
High Resistance Fault
Where

36. 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.

37. 7.14 FAULT LOCATION PROCEDURE
The proper sequence of cable fault location are as follows:
Analysis of fault
Pre-location
Pin Pointing
Confirmation and re-test

38. 7.14 FAULT LOCATION PROCEDURE (cont-)

39. 7.14 FAULT LOCATION PROCEDURE (cont-)

40. 7.14 FAULT LOCATION PROCEDURE (cont-)

41. 7.14 FAULT LOCATION PROCEDURE (cont-)

42. 7.14 FAULT LOCATION PROCEDURE (cont-)

43. 7.14 FAULT LOCATION PROCEDURE (cont-)

44. 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)

45. 7.14 FAULT LOCATION PROCEDURE (cont-)
if the continuity of the cable is sound, insulation resistance
from one end are sufficient.
If continuity is broken, IR test should be carried out at
both ends of the cable
BURNING A FAULT
The continuity and IR test may indicate that burning of
fault by means of HT pressure test set is required. -

46. 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.