Case Studies on Power Cables
Transcript of Case Studies on Power Cables
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Prof. Charles Q. SuThe Petroleum Institute
Case Studies on Power Cables
Case study - 1
Quality Management of Distribution Cables
Professor Charles Q. Su( PhD, Fellow IET, SM IEEE, CIGRE A2 )
Prof. Charles Q. SuThe Petroleum Institute
About the workshop instructor- Prof. Charles Q. Su
Industrial experience
• 1970-1973 Operations engineer
• 1974-1978 HV testing engineer
• 2002-2006 Chief Technologist, Singapore Power (SPPG) Ltd
Research & teaching experiences
• 1985 Research Associate, University of Western Australia
• 1990-1991 Lecturer at University of NSW, Australia
• 1992-2001 Senior Lecturer & Associate Professor, Monash University
• 2007-now Professor, Chair of Research Committee (EE)
Petroleum Institute, UAE
Membership of professional organisations
Fellow of IET, Senior Member IEEE (91), member of CIGRE SC A2 (Transformer)
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Prof. Charles Q. SuThe Petroleum Institute
Ron E James & Q. Su “Condition Assessment of HV Insulation in Power System
Equipment” - IET Power and Energy Series No.53, April 2008
Prof. Charles Q. SuThe Petroleum Institute
Some important issues in distribution cable
management
Causes of distribution cable failure:
1. Damages (road digging, land movement etc)
2. Manufacture defects (material of quality control problems)
3. Poor workmanship (cable joints and terminals)
4. Insulation ageing (water seepage, water treeing etc)
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Prof. Charles Q. SuThe Petroleum Institute
Condition assessment of distribution cables
Criterion of condition assessment:
1. The total failure rate.
2. Frequency of a type of failure – warrantees root cause analysis.
3. The consequences of failures.
4. Costs of repair or replacement.
Prof. Charles Q. SuThe Petroleum Institute
Cable insulation ageing in the life span
Bathtub curve - Determined from the failure rate change (a number of the same insulation samples)
TwTs
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Random or slowly increa sing fa ilure ra te
Bur n-in
period
Use ful life period We arout
period
Ope rating
life
Failure
rate
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Prof. Charles Q. SuThe Petroleum Institute
Background
There are over 3,700 km of 6.6kV cables in a utility. The average failure rate from 2000 - 2004 was 30 cables per year.
Serious consequences:
1. In-service failures of 6.6kV cables cause local blackout.
2. Due to the time of failure (e.g. at mid-night) and the possible bad weather conditions at failure (e.g. thunder storm), restoration of power supply is difficult and could take many hours.
There was an urgent need to reduce the in-service failures.
AGE PROFILE OF DISTRIBUTION CABLES (2004)
0.35% (13.109 km)
9% (323.093 km)
14% (541.152 km)
25% (954.976 km)
36% (1,349.877 km)
15% (583.758 km)
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6.6kV
Cable Length
: 3,768.647 km
22kV
Cable Length
: 4,948.379 km
21 – 25 yrs
16 – 20 yrs
11 – 15 yrs
6 – 10 yrs
< = 5 yrs
6% (288.551 km)
13% (668.678 km)
25% (1,218.271 km)
34% (1,694.035 km)
22% (1,078.844 km)
> 30 yrs
26 – 30 yrs
21 – 25 yrs
16 – 20 yrs
11 – 15 yrs
6 – 10 yrs
< = 5 yrs
20 40 60 80 100 (%)
0.07% (2.682 km)
19% beyond the
age of 15
27% beyond the
age of 15
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Prof. Charles Q. SuThe Petroleum Institute
How to reduce the in-service failures of 6.6kV cables?
A. Cable replacements according to their designed life time?
B. HV tests on all cables to flash out incipient faults? (using DC, AC or VLF)
C. Replace the type of cable joints of high failure rate?
Which one would you prefer if you are the asset manager?
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10
20
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50
60
70
80
Cable (XLPE) Cable (PILC) Joint
Ye
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Categories of 6.6kV Cable Failures( 2000-2005 )
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25%
45
28%
76
47%
* Total 160 failures
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Prof. Charles Q. SuThe Petroleum Institute
0.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35
Ye
ar
Age Profile of Failed 6.6kV XLPE Cables( 2000-2005 )
Average age is around 20
Implication: XLPE cable insulation is generally reliable within 15 years.
Prof. Charles Q. SuThe Petroleum Institute
Age Profile of Failed 6.6kV Cable Joints( 2000-2005 )
Average 28 years
Implication: Cable joint can fail at any time due to mainly poor workmanship, as
well as bad quality of materials and insulation ageing
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EXISTING MAINTENANCE TESTS
for 6.6kV cables
Megger measurement - Resistance
- Comparison between phases
Polarization index (R10/R1)
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Effectiveness of Megger Test
• Detect the leakage caused by terminal
contamination (surface crapping resistance)
• Water seepage to the joint
• Insulation deterioration (ageing), especially
paper/oil cables
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Megger Test Alone Is Not Conclusive
• For example:
– Water tree contamination (before electric tree
is established)
– Bubbles and unbridged cracks in XLPE or
epoxy insulation
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Failures of cables with high megger readings
Case (1)
• For example, cable A:
– Megger readings 1G/1G/1G on 16 Nov 2005
– The circuit failed on 17 Nov 2005 at 6:14am
• Also, in this utility a number of 6.6kV cables of
high megger reading failed in the past.
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Case (2)
A 66kV XLPE cable under a bus stop failed;
It was found that the failure was due to an
early damage caused by sinking an earthing
rod;
Lost about 1/3 of the XLPE insulation …
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A close look
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A surprise …
It was found that the bus stop was built five years
before the failure.
So, after the bad damage, the “poor” cable survived
five more years before its insulation broke down;
More surprisingly ….
Its insulation resistance was measured three times
during the five years, always giving high megger
readings!
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Why megger tests could not detect the
incipient fault (damage)?
XLPE insulation has a very large volume
resistivity of 1016 Ω.cm.
The damage did not bridge the insulation.
Water trees do not affect insulation
resistance before electric treeing is
established across the electrodes.
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How to use megger tests effectively?
Comparison of megger readings between phases;
Trend analysis;
Stability of insulation resistance reading under dc high voltage.
Add polarisation index measurement in the analysis (PI = R10min / R1min)
Prof. Charles Q. Su
ACTION PLAN – VLF tests on selected cables
• Selection criteria
• Cables with seram joints (more frequent failures)
• First leg feeders (important)
• Megger readings –VLF test is carried out if :
1. M < 50 M or
2. 50 M < M < 200 M and K > 1.5 or
3. 200 M < M < 1000 M and K > 5
Where M is the minimum megger reading for the three phases and K the ration between the maximum and minimum phases.
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Prof. Charles Q. Su
VLF tests voltage and time duration
• For cables less than 10 years old, 2Uo for 15 minutes
• For cables older than 10 years, 1.7 Uo for 20 minutes
• These test voltages and time are in line with the new IEEE Standard on VLF tests IEEE Std 400.2TM – 2004. The IEEE/EPRI/CEA and other world engineering bodies recommended test level for MV extruded cables is two to three times line to ground voltage for 15-60 minutes.
Initial VLF Test
Flowchart
Note: M is the minimum megger
reading of the three phases
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Incipient Faults Averted by VLF Tests (May 2003 – Dec 2005)
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5
10
15
20
25
30
35
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45
50
2003 2004 2005
Nu
mb
er
of
failu
res
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VLF Tests on 6.6kV Cables(May 2003 to Dec 2005)
Total Circuits Tested Failed During VLF Tests
(incipient faults averted)
540 97
100% 18%
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Statistics of 6.6kV Cable Failures between 2000-2005( Total 160 cable and joint failures )
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5
10
15
20
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35
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1999 2000 2001 2002 2003 2004 2005 2006
Year
Nu
mb
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of
failu
res
Before 2004, the cable failure rate was around 30 per year.
In 2005, it dropped to 12, about 1/3 yoy
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Why Cable Still Fails after Passing VLF Test?
Total Circuits
Tested
Failed During
VLF Tests
Failed in Service
after VLF Tests
540 97 20
100% 18% 3.7%
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Profile of the 20 Cables Failed After VLF Tests
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4
6
8
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12
14
16
18
XLPE PILC Joint
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2 1
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Age Profile of XLPE Cables Failed After VLF Test(Average 19.5 Years)
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5
10
15
20
25
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
Ye
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Why Cable Fails After VLF Test? ...
The possible causes:
• More than one defects.
• Bad water tree contamination.
• VLF test time is too short.
Electrical Tree Grow During VLF Tests(In case of two large water trees)
Cable sheath
Conductor
XLPE
insulation
1. Star VLF test at 1.7Uo which may initiate electrical treeing on some large water trees.
No electrical treeing is triggered on small water trees.
2. The electrical trees start to grow until the largest one bridges across the insulation and
causes flashover.
No electrical tree is initiated on small water trees and
defects
Electrical trees are initiated at large water trees
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Initial and Modified VLF Test Criteria
Note: M is the minimum megger reading of the three phases
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Electrical Tree Growth under Different Voltages(IEEE Standard)
Voltage Tree Growing Speed( mm/hour )
50Hz 1.7
0.1Hz
Cos-rectangular
7.8
0.1Hz Sine 12.3
Implication: The VLF test time should be sufficiently long.
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6.6kV Cable Failures in 2005
0.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
Ye
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5 PILC
Ave Age = 30
4 XLPE
Ave Age = 22
3 Joints
Ave Age = 6
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HV Oscillating Wave Tests
• Energise cable by DC voltage source.
• After the voltage reaches a certain level, discharge through an inductor to ground.
• A damped oscillating voltage is established which may last for a few mini-seconds.
• Detect partial discharges and dielectric dissipation factor during OW tests.
• Locate PDs using PD mapping techniques.
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PD mapping under OW tests.
• Some defects, especially those in cable joints, could be detected by PD mapping
• Detector sensitivity is not better than 100pC at site in noisy environment
• Not suitable for the detection of defects in XLPE insulation (very low PD level, normally <50pC)
• Cannot detect water tree if no electrical tree is triggered
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PD MAPPING TEST RESULTS
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PD MAPPING TEST RESULTS
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PD MAPPING TEST RESULTS
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Prof. Charles Q. Su
• Selection consideration:
• Recommended by standards
• From utilities’ experiences
TEST VOLTAGES
FOR 6.6KV AND 22KV XLPE CABLES
Prof. Charles Q. Su
Test Standards for 6.6kV and 22kV XLPE Cables
IEC Standard 60502-2: 2005
“Power cables with extruded insulation and their accessories for
rated voltages from 6 kV up to 30 kV”
European Standard CELENEC HD 620 S1 and HD 621 S1
IEEE Standard 400.2-2004
“IEEE guide for field testing and evaluation of the insulation of
shielded power cable systems using VLF”
EPRI report RP 3392-01/CEA 200-D-780A (1996)
“Trial guide for high voltage 0.1Hz tests on power cable systems
in the field”
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Prof. Charles Q. Su
IEC RECOMMENDED ELECTRICAL TESTS( For new 22kV cables after installation )
AC 50/60Hz 1.7 Uo for 5 minutes
or
24 hours under system voltage *
AC 50/60Hz test voltage and time are determined by agreement between the purchaser and the contractor
Other test methods (VLF, OW etc) are under consideration
* IEC Standard 60502-2: 2005
Power cables with extruded insulation and their accessories for rated voltages from 6 kV up to 30 kV
EUROPEAN STANDARD(for PE and XLPE cables from 6kV to 36kV)
Frequency Test voltage (rms)
Test time
0.1 Hz 3 x Uo 60 minutes
50 Hz 2 x Uo 60 minutes
• European Standard for cable after laying test CENELEC HD 620 S1 AND 621 S1
• 15 European countries signed the harmonization document 620 S1 and 621 S1 in 1996
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VLF test voltage and duration
adopted by some utilities in North America
Age of cable Test voltage (RMS)
6.6kV 22kV
Newly installed 12kV (3.1Uo) 38kV (3.0Uo)
1~10 years old 9.5kV(2.5Uo) 32kV (2.5Uo)
10~30 years old 6.5kV(1.7Uo) 22kV (1.7Uo)
Note: 1. Test duration is always 15 minutes. 2. The data is from HV Inc, America.
IEEE Standard 400.2-2004“IEEE guide for field testing and evaluation of the insulation of
shielded power cable systems using VLF”
System Voltage
rms in kV
Acceptance Test
rms or (peak)
Maintenance Test
rms or (peak)
5 10 (14) - 3.5Uo 7 (10) – 2.4Uo
8 13 (18) – 2.8Uo 10 (14) – 2.2Uo
15 20 (28) – 2.3Uo 16 (22) – 1.85Uo
25 31 (44) – 2.15Uo 23 (33) – 1.6Uo
35 44 (62) – 2.2Uo 33 (47) – 1.6Uo
Test duration : 60 minutes
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Prof. Charles Q. Su
RECOMMENDATIONS(for 6.6kV and 22kV cable after laying tests)
• Insulation resistance test at 5kV
• - Purpose: detect poor workmanship and joint/terminal insulation leakage
• VLF tests at 2Uo RMS for 60 minutes
• - Purpose: “flush-out” insulation defects. If failed during VLF test, after repair the cable should be VLF tested again regardless of the insulation resistance.
• If necessary*, oscillating wave and PD mapping tests could be carried out at the following peak voltages: 1 Uo, 1.5 Uo and 2 Uo.
• - Purpose: detect and locate defective joints and insulation weakness
• * Criteria of PD level to be determined
Prof. Charles Q. Su
VLF and OW PD mapping tests flow chart
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Prof. Charles Q. Su
NEW DIAGNOSTIC TESTS( maintenance tests )
DC component in AC leakage current
- water tree detection
Propagation characteristic spectroscopy
- LV pulse attenuation versus frequency
- For insulation ageing detection
- Can apply to in-service cables
AC superposition test (101Hz)
- Detect the 1 Hz component
- Detect water tree
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SUGGESTIONS
• Apply VLF tests to old PILC cables (age>20)
• For XLPE cables– if 200M<M<1000M and the ratio between the highest and
lowest phases is <5, don’t do VLF test.
– If M>1000M, don’t do VLF test.
– If age > 15, don’t do VLF test.
• PD mapping may be used on important cable circuits to detect joint defects
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CONCLUSIONS
• VLF Test has been successful in reducing 6.6kV cable failures and should be used according to the total insulation condition of the cable and joint assets.
• Review the test procedure and failures every two years.
• Some defects, especially those in cable joints, could be detected by PD mapping, during either VLF or OW tests.
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Case study - 2
Failure Analysis of a 230kV/200MVA
Transformer-Cable Termination
Professor Charles Q. Su( PhD, Fellow IET, SM IEEE, CIGRE A2 )
Training Course for Continuous Education
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Background
• A new installation of transformer and cable
termination
• The failure of yellow phase terminal occurred only 10
days after commissioning
• The failure caused an explosion and fire
• The transformer/cable terminal box was destroyed
• The transformer was significantly damaged
• About a quarter of the city was blackout
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Case study - 3
Three 230kV Cables Failed After Only 3 Years
Operation - Caused by a Design Problem
Professor Charles Q. Su( PhD, Fellow IET, SM IEEE, CIGRE A2 )
Training Course for Continuous Education
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63 Prof. Charles Q. Su
Background
• 230kV 2000 mm2 XLPE cable, circuit length 7.2 km
• Installed in the middle of 2000 by a consortium of
three manufacturers
• Loading was around 40% of rating
• F1 failed on 12 September 2003
Only three years new
Serious impacts to customers due to voltage dips
Investigators of OEM insisted that the cable was damaged
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Background – cont…
• On 6 June 2004, another cable F3 failed
• Again, serious impacts to customers
• In June 2004, off-line PD measurement was carried
out on feed 2 (F2)
• Large partial discharges (>100pC) were detected and located
• A 10m long cable was cut and sectionised
• Burnt damages to water swellable tape and semicon screen
were found.
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Questions
• Are the failures due to mechanical damage?
• Are they isolated failures?
• If not due to cable damages, what are the
possible root causes?
• How to prevent the recurrence of the type of
failures
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Case study - 4
230kV Cable Joint Failures Due to
Poor Workmanship
Professor Charles Q. Su( PhD, Fellow IET, SM IEEE, CIGRE A2 )
Training Course for Continuous Education
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67 Prof. Charles Q. Su
Background
Failures of two 230kV XLPE cable joints during HV accommissioning tests. The cable and joints were madeby different manufacturers. The cable joints wererubber pre-moulded joints.
• Cable Joint A: circuit I, Red phase, joint bay 5/6: PDs were detected under 1.1Uo, PD inception voltage 120kV (0.9 Uo).
• In Red phase, circuit II, joint bay 2/3: The joint failed at 27kV (0.2 Uo) during HV ac tests.
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Wrong position – the gripping shield
is shifted out of the semi-conductive
electrode, as shown by the
corresponding mark left on the
internal wall of the rubber moulding
The mark on the internal wall of
EPR rubber moulding