Presentation is loading. Please wait.

Presentation is loading. Please wait.

DET 310 UNDERGROUND CABLES

Similar presentations


Presentation on theme: "DET 310 UNDERGROUND CABLES"— Presentation transcript:

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 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 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.

62

63

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.


Download ppt "DET 310 UNDERGROUND CABLES"

Similar presentations


Ads by Google