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Case Study of VLF / Tan Delta & Partial Discharge
Acceptance Testing of New 23kV Cables
Subcommittee F
Fall ICC Meeting Seattle
November 14, 2012
Timothy P Hayden PE
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Overview
Testing Equipment
History of VLF Testing & Procedures
Project Overview
Test Results
Failed Splice Analysis
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Testing Equipment
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Test Equipment
Second Generation
Sinusoidal Waveform
0.1 Hz maximum
60 kV DC / 43kV AC
Bluetooth communication
for Tan Delta
Partial Discharge Add On
Only 1 PD set
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History of Testing Procedures
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Introduction of VLF Testing
Major cable outage in Massachusetts in July 2003
PSC ordered review of the cause of the outage
Root cause overuse of DC testing methods on 1970s &
1980s vintage XLPE insulation
VLF testing with Tan Delta analysis introduced
Test sets purchased
New Electric Operating Procedure (EOP) developed
The new procedure required use of VLF equipment whenavailable
The procedure also limited use of DC testing equipment
Both voltage level and test time
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First Electr ic Operating Procedure 2004
First procedure used test voltage levels from 400.2
Many existing cables tested experienced a failure
New equipment (VLF) blamed for causing cable failures
Damaging the insulation
Management became reluctant to test any in service cable
Explained that the test was finding existing weak spots
Management was not convinced
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Withstand (Maintenance) Test Voltage Levels
Second procedure (2007) modified test voltage levels
No change to acceptance test levels
Withstand (or Maintenance) test broken into 2 criteria
Maximum level remained per IEEE 400.2
Minimum level based upon electric system configuration
Effectively Grounded 125% of normal L-G voltage
Delta or ineffectively grounded 100% of normal L-L voltage
Single point grounded, grounding bank, resistance grounded
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Current Testing Procedures
All new cables shall be acceptance tested
Significant length
Not a replaced section in an existing cable
When the entire circuit is new
VLF (sinusoidal) with Tan Delta analysis
0.1 Hz standard
Test voltages from IEEE 400.2
Test duration = 60 minutes
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Project Overview
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Project Overview
New 23kV Sub-transmission Cable
1000kcMil copper, 260 mils EPR, JCN (18#14 round wire),
LLDPE jacket
2 cables per phase due to loading
Spliced to existing cable after river crossing
1000kcMil copper, 260 mils XLPE, Jacketed, drain wire shield,
LLDPE jacket
New cable from supply sub across river ~ 2200
Old Cable from far side of river to next substation ~ 2500
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Plan View Not to Scale
New Manholes
New Manholes
River Crossing
Existing Manhole
& Cable
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Circuit 2339 Cable Installation
New cable was installed from the source sub
Across the river
To an existing manhole # 597
Total distance of ~ 2200
Cold shrink termination in substation
Heat Shrink End Seal in MH 597
Acceptance Tested from Substation with Tan Delta Analysis
31kV ultimate voltage, 60 minute duration
Cable sets known as 2339X and 2339Y
Therefore 6 different phases tested
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VLF / Tan Delta Test Results / Analysis
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Circuit 2339Y Tan Delta Results
New CableOnly
Suspect
moisture on
terminations
at the start
of A phase
test
Suitable for
service
TD 6.5
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Circuit 2339X Tan Delta Results
New CableOnly
Crewreportedtest set
tripped outon B phase
Crewassumedproblem
with test set B phase not
suitable forservice
TD 11
TD 46
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Circuit 2339
The test crew did not report the problem with B phase of the2339X cable to the cable crew or project management
In fact, they reported the cable as being ready for service
Accustomed to DC testing if the set doesnt trip out, cable
is OK
However, they did report the set as tripping out at least one
time
Heat shrink end seals removed and splices to old cable installed
in MH 597 Old cable length to substation ~ 2500
Total cable length now ~ 4700
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Circuit 2339 Y Old and New Cable
Old & NewCable ~ 50%
of each
5 minute test
Max 23.0kV
Acceptance
test TD was
6.5 (slide 21)
Acceptablefor service
TD 8 - 10
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Tan Delta
Tan Delta
Ratio of IRto IC
IR (leakage current) should be
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Circuit 2339 X Tan Delta Analysis
Tan Delta expected to decrease
Length of cable more than doubled
Test voltage 75% of acceptance
Tan Delta decreased by 50%
Still much higher than other phases
Original IRstill there, IC~ doubled, TD ~ halved
Test Type Length Test Voltage Phase Tan Delta
Accept 2200 31.0
A 11
B 46
C 11
Pickup 4700 23.0A 12B 24
C 12
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Circuit 2339 X Retest of B Phase Only
Retest ofcompletecable
Results still
notacceptable
TD of 17.2
SD @ 23kVwas 0.28%
Still higherthan A and Cphases
TD 17.2
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2339 X Action Plan
Project manager wanted to put cable into service
The other 2 sub-transmission cables yet to be completed
Project in danger of running behind schedule
Ultimately convinced him that this cable was not suitable for
service in current condition
If this cable were to fail while the second circuit was out of
service for this project, only a single circuit would remain
Chance of forced outages high if that were to occur
Need to remove the splice between the new cable and the
existing cable and re-test
Partial discharge test available
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Partial Discharge Test Results
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2339 X Partial Discharge Test
Splice removed between the new cable and the old cable
Cable prepped and an air termination installed
Isolated in the manhole
Partial Discharge Test performed
Using VLF at 0.1 Hz
Tested at 25.0 and 28.0 kV
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2339 X Partial Discharge Test 25.0 kV
PD test
25kV
225 feetfrom
substation
termination
Location225 feet
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2339 X Partial Discharge Test 28.0 kV
Location250 feet
PD test
25kV
250 feetfrom
substation
termination
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2339 X Partial Discharge Test
The conduit plan showed the first manhole (592) to be 150conduit feet from the riser
Due to station and manhole conditions, actual cable feet from
the manhole to the termination is closer to 200 225 feet
It become logical to suspect the splice in this manhole as beingthe problem
All cable tested in the factory
Relatively short pull length
Low chance of problem being in the cable
Splice in this manhole (592) was removed and replaced
B phase acceptance test repeated
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Circuit 2339 X B Phase after Repair
Splicereplaced
60 minute
acceptance
test
31.0kV
A phase TD
was 11.2
C phase TDwas 11.6
Acceptable
for service
TD 11.5
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Failure Analysis of Removed Splice
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Failure Analysis
Splice Body removed from cable
Left side cable Manufacturer A, Right side Manufacturer B
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Failure Analysis
Unable to determine the exact splice body alignment on thecable
Since splice body was removed prior to receiving the splice
Alignment is critical insulation interface must be fully
engaged with cable insulation However, marks remained where the splice body was positioned
Alignment was OK
There was visible tracking on the interface of side A
There were matching tracks on that side of the splice body
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Failure Analysis
Cutbacks were correct per template
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Failure Analysis
Failure believed to be due to contamination on interface Dirt from cable jacket where splice body was parked?
Improper cleaning?
Other source?
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Questions?