1. DET 310 UNDERGROUND CABLES
POWER SYSTEM COMMISSIONING AND MAINTENANCE PRACTICE
DET 310
CHAPTER 6
UNDERGROUND CABLES

2. 6.0 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. 6.1 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. 6.2 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.
6.3 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. 6.3 Cable Construction (continue)-
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
4/12/2019

7. 6.3 Cable Construction (continue)-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
ETE503 Underground Cable
4/12/2019

8. 6.3 Cable Construction (continue)-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.
ETE503 Underground Cable
4/12/2019

9. 6.4 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
ETE503 Underground Cable 9

10. 6.4 Types of Underground Cables (continue)-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
ETE503 Underground Cable 10

11. 6.4 Types of Underground Cables (continue)-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
ETE503 Underground Cable 11

12. 6.4 Types of Underground Cables (continue)-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

13. 6.5 Types of Underground Cables (continue)-
A = Conductor (Aluminum)
B = Strand Screen
(extruded semiconducting)
C = Insulation (XLPE)
D = Insulation Screen
E = Shield (copper tape)
F = Jacket 13
Example of Single core XLPE 11 kV cable

14. 6.5 Types of Underground Cables (continue)-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
ETE503 Underground Cable 14

15. 6.6 (CABLE FAULT) INTRODUCTION

16. 6.6 Cable Fault Introduction (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,

17. 6.6.2 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

18. MECHANICAL

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

20. 6.6.4 Poor workmanship During Cable Jointing
The cable are jointed together with poor workmanship can lead to cable fault after a period of time.

21. 6.6.5 NATURAL CAUSES DUE TO AGING OF CABLE

22. CONTINUE-

23. CONTINUE-

24. 6.6 TYPES OF CABLE FAULT
GENERAL:
Series (open circuit) Fault
- Failure of continuity (conductor (s) or cable)
Shunt (short circuit) fault
- failure of insulation

25. 6.6 TYPES OF FAULT (CONTINUE)-

26. 6.6 TYPES OF FAULT (CONTINUE)-

27. 6.6.1 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

28. 6.6.2 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.

29. 6.7 FAULT LOCATION PROCEDURE
The proper sequence of cable fault location are as follows:
Analysis of fault
Pre-location
Pin Pointing
Confirmation and re-test

30. 6.7.1 Analysis of Cable Fault
To analyze a cable fault is to determine and confirm the nature or characteristics of the fault.
Objective: to select which of the cable fault locator equipment of method is best suited to locate the particular fault.
Analysis of cable fault normally consists of insulation resistance test and continuity test carried out using 1000V or 5000 V insulation tester

31. 6.7.1 Insulation Resistance test
With all earth connections removed from the cable conductor terminal, using IR tester of 1000/2500/5000V range, measure and record the IR in M-ohms, K-ohms or ohms between conductors and between conductors and earth. Six measurement are to be taken R-E, Y-E, B-E, R-Y, Y-B, B-R. An IR of 100M-ohm indicates a healthy cable.

32. Continuity Test
With the cable conductor looped or shorted at the tail end/remote end, using IR tester selected at continuity range, perform continuity test.
This test will determine whether any of the cable is open circuited.

33. 6.7.2 Continuity test (cont-)
- 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)

34. (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
6.8 BURNING A FAULT
The continuity and IR test may indicate that burning of
fault by means of HT pressure test set is required. -

35. 6.8 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.

36. 6.9 Pre- Location of fault
Pre-location is the application of a test at the terminals of a given cable to give an indication of the distance to the fault from the test point.
Whilst the measurement should be accurate as condition will allow, the primary purpose of pre-location is to give an indication, as quick as possible, of the vicinity in which to commence the final pin-pointing tests.

37. 6.9 Pre- Location of fault (cont)-
Generally there are four pre-location methods which are practised and the are as follows:
Bridge or loop Method
Pulse Echo/ Time Domain Reflectometry (TDR) method
Impulse Current Method
Arc Reflection and secondary Impulse method.

38. 6.9.1 Bridge method
Direct reading fault localizer
Conditions required:
No break in continuity, if possible one sound core available.
The fault resistance of cable should not be more then 20 k-ohm
Applied voltage not to exceed 600 V (DC)

39. Case 1: Single conductor to earth with sound core available
Fault distance from front end, X= n/100 x L
Where n = localiser reading in %
L= Cable route length

40. Case2: Conductor to conductor with sound core available.
Fault distance from front end, X= n/100 x L
Where n = localiser reading in %
L= Cable route length

41. Case 3: Three phase fault
By Open loop test and close loop test
(i) Rf2/Rf1 > 5 and (ii) Rf1 < 20 k-ohm
Open loop test

42. Case 3: Three phase fault
Close loop test

43. Open loop and close loop test (cont)
Fault distance from front end
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length

44. High Tension Bridge
Conditions required:
No break in continuity, if possible one sound core available.
The fault resistance of cable should be more then 20 k-ohm
Applied voltage not to exceed 20kV (DC)

45. High Tension Bridge
Case 1Single Conductor to earth with sound core available
Fault Distance from test end , X = n/100 x 2L

46. High Tension Bridge
Case 2:Conductor to conductor with sound core available
Fault Distance from test end , X = n/100 x 2L

47. High Tension Bridge
Case 3:Three phase Fault with no sound core available
Rf1/Rf2> Rf1>20 kohm
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length

48. 6.9.1.2 Universal Bridge to measure capacitance of Cable
Requirement:
Open Circuit Fault
Fault resistance of cable should be more then 100 ohm
Earth all the conductors not under test

49. 6.9.1.2 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available

50. 6.9.1.2 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
Fault Distance from front end/test end = (CA/CAB) x100%

51. 6.9.1.2 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available

52. 6.9.1.2 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
Fault Distance from test end/front end = ((CA/(CA+CB))x100%

53. 6.9.1.3 Universal Bridge to measure Inductance of Cable
Requirement:
Open Circuit Fault
Fault resistance of cable should be less then 30 ohm
All conductors be left free from earth

54. 6.9.2 Pulse Echo (PE)/ Time Domain Reflectometry (TDR)
A simple method which works on travelling wave principles. It is applicable to all series and shunt faults with Rf <=Zo/10 and Rf<=10 Zo respectively. For power cables typical Zo=50 ohm
Travelling wave principles
A pulse injected into a cable is reflected back to the source by any change in characteristics impedance (Zo) of the cable
The waveform generated can be monitored using an oscilloscope as shown in Figure below

55. Continue-
The distance to the fault Lf (m) is given by
Lf = T x Vp/2

56. Continue-
Waveform interpretion
Low Resistance Shunt Fault (5 ohm<Rf<=500 Ohm)
-Reflection: Negative
-Amplitude depends on fault resistance
Series Fault (<=5 ohm)
Reflection : Positive
Amplitude depends on fault resistance

57. Continue-
Waveform interpretion
Shunt Fault
Series Fault

58. Continue
Example:
Determine the velocity of propagation, Vp/2 for a cable length of 2500 meters, t1= S and t2 = S. Calculate the fault distances.

59. Continue
Solutions:
Velocity of propagation = Vp/2 = 2500/31.64 = 79 m/S
Distance of Fault from test end = Vp/2 x t2
= 79 x 17.45 =

60. 6.9.3 Impulse Current Method
Working on Travelling wave principles, it is applicable to all types of fault
6.9.4 Arc Reflection or Secondary Impulse method
Basically PE and TDR associated with fault treatment. It is also applicable to faults of all nature, but with easily interpreted breakdown waveforms

61. 6.10 PIN POINTING
Pin Pointing is the application of a test that positively confirms the exact position of the fault.
Before the commencement of pin pointing, the prelocated fault distance should mark on the cable route which is measured by means of a trumeter
Pin pointing is normally carried out by the shock wave discharge method. The fault can be detected by the use of semisphone.

64. 6.11 Confirmation and Re-Test
After the pin-pointed position of the fault has been marked and exposed, check for physical sign of fault. If there is the fault is confirm.
After confirmation, the fault should be cut away, IR and continuity test should be carried out on the remaining cable sections to determine the soundeness of these cable sections.
The insulation resistance test are again carried out after jointing followed by pressure test before supply can be restored.

65. Flowchart for Fault location Procedure

66. Example:
Find a fault distance from end test which has a test data as below:
Bridge Balance Reading =60%; Length of Cable 140 meters
Solutions:
Fault Distance= = 60/100 x 140 = 84 meter

67. For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2.
Example: Cable 11kV Al; Length 150 m; size = 16mm2
From Table 1= Resistance for 1km =2.26 ohm
For 150 m = 150/1000 x 2.26 = ohm
From Table 1: Calculate the equivalent resistance for all the combined section.
Example: for Al = ohm, for copper = ohm
Equivalent Resistance = =0.489 ohm

68. Continue:
Calculate equivalent resistance from obtained bridge reading.
Example : Bridge Reading = 60 %
Equivalent resistance = 0.6 x ohm = ohm.
Determine which section of cable the fault occurs
Total resistance at Al section = ohm
As equivalent resistance at Al section is higher then (Equivalent resistance from bridge test), the fault is located at Aluminium section of cable.

69. Table 1: Impedance data for 6350/11000 V Cable Aluminium
Table II: Impedance data V Cable Copper

70. TABLE VI – 600/1000V (ALUMINIUM)

71. TABLE VII – 3800/6600V (ALUMINIUM)

72. TABLE VIII- 6350/11000 (ALUMINIUM)

73. TABLE V

74. 6.10 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation. Site tests are performed by applying a predetermined high voltage to the insulation. DC site test voltages, regardless of insulation type, are used to ensure that the cable, cable joints and termination are correctly made and installed.

75. High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70% of permissible factory test DC voltages (15 minutes). = 17.5kV

76. Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached. In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured.