The document summarizes the use of very low frequency partial discharge (VLF PD) testing for cable diagnostics. It discusses the principles of VLF testing, provides examples of test results on branched cable circuits, and highlights limitations and developments in the technology. Examples show how test results are mapped and used to identify partial discharge zones, recommend repairs or retesting, and predict cable condition. The document demonstrates how VLF PD testing can be used for predictive maintenance of cable networks.

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Willem Boone, KEMA Diagnostic Services, Chalfont, PA
ICC Fall 2000 Educational Program on PD Cable Diagnostics
November 1st 2000, St. Petersburg, Florida
Very Low Frequency Partial
Discharge Detection;
an Experienced Diagnostic tool for
Distribution Cables

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 Purpose of Diagnostic Testing
 Principles of VLF testing Method
 Test Results
 Limitations and Innovative
Developments
 Cost/Benefit Evaluations
 Future Developments
 Conclusions

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MAINTENANCE OPTIONS
 Reactive maintenance,
“wait-and-see”
 Active maintenance,
“predictive maintenance”

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REACTIVE MAINTENANCE
 To repair or replace if something is broken
 Short term view
(“no-health insurance”, “no-fire insurance”)
 Short term benefit

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ACTIVE MAINTENANCE
 To avoid service failures
 To specify network quality
 To target replacement money
 To improve customer satisfaction
 To avoid customer claims

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TO TRANSFER “MAINTENANCE” INTO
“PREDICTIVE MAINTENANCE” IN ORDER TO
REDUCE COSTS

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HOW TO TRANSFER MAINTENANCE
INTO PREDICTIVE MAINTENANCE
Maintenance + diagnostic testing =
predictive maintenance

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PURPOSE OF DIAGNOSTIC TESTING
 To avoid failures in service
 To reduce costs
 To target replacement money

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Principles of present
VLF diagnostic testing of cables

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TRADITIONAL ONE-SIDED CABLE
NETWORK DIAGNOSTICS
1
defect
1
time

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MEASUREMENT SETUP
SCOPE PC PRINTER
0.1 Hz
HV- supply
CABLE

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CHARACTERISTICS VLF PDD
 Sine wave
 Non-destructive
 Off-lin

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SYSTEM PARAMETERS
 Maximum voltage 41kV, RMS
 0,1 Hz, sine wave
 Discharge-free
 Maximum cable length to be tested 15,000
ft
 All type of cable or accessory, however
defect should generate P.D.
 Non destructive test (maximum test voltage
2x phase-neutral voltage)

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KEMA VLF PD TESTING
APPLICATIONS
 PILC
 All kinds of accessories
 Both point to point and branched cable
circuits
 Very long cable lengths

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ADVANTAGES OF 0,1 Hz POWER
SUPPLY OVER 60 Hz VOLTAGE
 Longer lengths of cable to be tested
 Compact voltage source

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TEST RESULTS
 Test diagrams
 Detected defects
 Data base

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BURNED PAPER

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IMPLODED JOINT

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CRACKED HOUSING

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RECOMMENDATIONS
 Replace/repair asap (<3 month)
 Inspect asap
 Retest (<2 year)
 No action (retest < 5 year)

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VLF PD - DATABASE
 A database application for storage and
consulting measured data of MV-
powercables
 Data are used to diagnose cable system
including splices and termination
 Actual condition of cable and/or
accessory can be predicted from the
measured data stored

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Limitations of present VLF
cable diagnostic testing
 Only straight cable connections
 Only cable lengths up to 14,000 ft

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Principle of multi-terminal cable
circuit diagnostics:
 Two-sided
 Branched

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TWO-SIDED SYNCHRONISED
CABLE NETWORK DIAGNOSTICS
1
defect
1
time
2
2

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BRANCHED CABLE NETWORK
SYNCHRONISED DIAGNOSTICS
1 2
3
defect
1
2
3
time

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Trials in te USA to Test
BCC-Diagnostics
 September 1998
 April 1999

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Improvements After First Trial
 Coupling to the Tested Cable
 Master/Slave Unit
 Digital Filter

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Improvements After the
Second Trial
 Software to Analyze Data
 Remote Switching of Phases

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KEMA - Diagnostic Services
Branched Cable Testing

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Introduction
 General Test Procedure
 Example Cable Map
 Cable Tested Procedure
 Results
 Examples Taken From Actual Tests, In
The USA, During The Past Year

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General Test Procedure
 Besides the 0.1 Hz voltage source used for the PTP
testing, additional equipment is required such as a
measuring node at each of the branches to be
monitored. A node consist of:
 GPS (Global Position System) Interface
 Master-Slave Computer System
 Communication Interface
 Figure 1 is a schematic of a three-branched test set-
up with one master node (#1) and two slave nodes
(#2 and 3).

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General Test Procedure
Figure 1
Three-Node BCC Synchronized Test Set-Up

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Sample Cable Map Example #1
 5 Ended Distribution Cable
 4 Ends Have Cable Access
 All Underground
 Required 2 tests
Test #1 location
Test #2 location
Test Cap
Test #1
location
Substation
#
Test #2
location

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Cable Tested Procedure Example #1
Test #1
Location A
Test #2
Location B
Test
Cap
Test #1
Location B
Test #2
Location A
644975
108498
078627
191638
122472
055384
055238
04886
080570
125765
6964741
540504
124605
138843
1210186
44975
139202
114843
648505
828507
868510
988507
037505
826503
740503
505501
425500
245504
702252
Test #2
Test #1

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Results Example #1
Test #1
Location B
Test #2
Location B
Test
Cap
Test #1
Location B
Substation
Test #2
Location B
644975
108498
078627
191638
122472
055384
055238
04886
080570
125765
6964741
540504
124605
138843
1210186
44975
139202
114843
648505
828507
868510
988507
037505
826503
740503
505501
425500
245504
702252
8 kV – 4,300 pC
12 kV – 7,700 pC
Retest or replace the
section in 1 year
8 kV – 2,100 pC
12 kV – 3,500 pC
Inspect the splice
8 kV – 2,100 pC
12 kV – 3,300 pC
Inspect the splice.
Retest the section in
1 year
8 kV – 2,100 pC
12 kV – 3,300 pC
Inspect the splice
8 kV – 3,300 pC
12 kV – 6,000 pC
Inspect the splice
8 kV – no PD
12 kV – 3,000 pC
Retest the section
in 1 year
8 kV – 5,200 pC
12 kV – did not increase
Inspect the splice
8 kV – 2,500 pC
12 kV – did not increase
Retest the section in 1 year
Results are shown on the mapping
diagram for test #2. All distances
are shown from Location A to
Location B.
Results are shown on the mapping
diagram for test #1. All distances
are shown from Location A to
Location B.
PD Zone

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Included In Test Report
 Maps As Shown
 Text Describing “Zones”
 Summary Table

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Sample Cable Map #2
 Multiple Ended Distribution Cable
 4 Ends Have Cable Access
 All Underground
 Required 3 Tests

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Sample Cable Map/Test Procedure
Test location
#2
High School
Test location
#3
Hospital
Test
location
#1.
Substation.
Test location
#4
Substation
- Section 1
- Section 2
- Section 3

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Results Example #2
 The network was virtually divided into
three Sections for the reporting purposes.
 Test results and Mapping diagrams are
presented individually for each Section.

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Results Example #2 / Section #1
16 kV – 3,800 pC
24 kV – not increased
Inspect the splice
To
High School
Test Location
Test location
Substation
16 kV – 8,500 pC
24 kV – not increased
Replace the section
16 kV – 5,200 pC
24 kV – not increased
Replace the section
16 kV – 1,700 pC
24 kV – not increased
Retest in one year
132
129
133
225
130
128
116
148
147
127
72
256
255
150
149
253
254
131
117
257
16 kV – 6,300 pC
24 kV – not increased
Replace the section 16 kV – 3,500 pC
24 kV – not increased
Retest in one year
To
Hospital
and
Substation

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Results Example #2 / Section #2
16 kV – 2,800 pC
24 kV – not increased
Retest in one year
16 kV – 3,300 pC
24 kV – 4,000 pC
Retest in one year
124
16 kV – 8,500 pC
24 kV – not increased
Inspect the splice
Test location
High School
Test location
Hospital
16 kV – 3,300 pC
24 kV – 4,000 pC
Inspect the splice
16 kV – 2,700 pC
24 kV – not increased
Retest in one year
16 kV – 4,000 pC
24 kV – not increased
Retest in one year
249
246
250
247
223
126
305
125
222
251
119
306
220
248
221
No name 3
253
No name 1
No name 2
252
336
To Byram
Substation
To Substation
16 kV – 1,500 pC
24 kV – 2,000 pC
Not critical
Short branch
to VLT 222

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Results Example #2 / Section #3
124
16 kV – 4,300 pC
24 kV – not increased
Inspect the splice
Branch to VLT
214
16 kV – 800 pC
24 kV – not increased
Not critical 245 137
Test location
Substation
To Hospital
Test Location
16 kV – 4,300 pC
24 kV – not increased
Retest in one year
16 kV – 4,300 pC
24 kV – not increased
Inspect the splice in manhole 138
Test the branch originated in
manhole 138 with the BCC nodes
set at the network transformers in
order to locate possible PD
sources and measure PD
magnitudes in a more accurate
way
235
230 232 233
228
227
231
236
229
226
234
238
244
243
242
219
241
240
239
237
118
123
136
138
No name 314
Branch to
Greenwich
Ave. Branch to
Pickwick Pl.
To
Substation
and High
School
16 kV – 5,300 pC
24 kV – not increased
Inspect the splice
16 kV – 5,300 pC
24 kV – not increased
Inspect the splice

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Included In Test Report
 Maps As Shown
 Text Describing “Zones”
 Summary Table

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Sample Cable Map #3
 Multiple Ended Distribution Cable
 2 Ends Had Cable Access
 All Other Connections Required
Utility To Cut Cable
 All Underground
 Required 4 Tests
 Three Recording Nodes Were
Used On Some Tests Sessions

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Sample Cable Map
Test location
Saratoga St.
MH 1399/2
Test location
Chestnut Park Apt.
Building A
Test location
Bridge St.
MH 227/10
Test location
Chestnut St.
MH 330/16
Test location
Columbus Ave.
MH 1900/12
Test location
Power Plant
Branched
Network

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Cable Tested Example #3 / Test #1
Test location
Columbus Ave.
MH 1900/12
Test location
Power Plant
Branched
Network

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Test location
Saratoga St.
MH 1399/2
Test location
Chestnut St.
MH 330/16
Test location
Power Plant
Branched
Network
Cable Tested Example #3 / Test #2

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Cable Tested Example #3 / Test #3
Test location
Chestnut Park Apt.
Building A
Test location
Bridge St.
MH 227/10
Test location
Chestnut St.
MH 330/16
Branched
Network

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Cable Tested Example #3 / Test #4
Test location
Chestnut Park Apt.
Building A
Test location
Bridge St.
MH 227/10
Branched
Network

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Results Example #3
 The network was virtually divided into
three Sections for the reporting purposes.
 Test results and Mapping diagrams are
presented individually for each Section.

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Results Split Into 3 Sections
Test location
Saratoga St.
MH 1399/2
Test location
Chestnut Park Apt.
Building A
Test location
Bridge St.
MH 227/10
Test location
Chestnut St.
MH 330/16
Test location
Columbus Ave.
MH 1900/12
Test location
Power Plant
Branched
Network
- Section 1
- Section 2
- Section 3

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Results Example #3 / Section #1
Test location
Power Plant
Test location
Saratoga St.
MH 1399/2
Test location
Columbus Ave.
MH 1900/12
1900/30
1900/29
1900/31
1900/24
1900/28
1900/27
1900/26
1900/25
1900B/24 1900B/23
1900B/22
1900B/21
1900B/20
947/1
1119/2
977/26
1399/2
1900B/19
1900/17
1900/16
1900/18
1900/13
1900/14
1900/15
8 kV – 23,000 pC
12 kV – not increased
Replace the section
Scattered PD’s with no
separable zones
8 kV – 16,000 pC
12 kV – 20,000 pC
Retest after all other
replacements
8 kV – 23,000 pC
12 kV – not increased
Replace the section
8 kV – 7,000 pC
12 kV – 36,000 pC
Replace the section
8 kV – 13,000 pC
12 kV – 20,000 pC
Replace the section
8 kV – 7,000 pC
12 kV – 28,000 pC
Replace the section

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Results Example #3 / Section #2
Test location
Chestnut St.
MH 330/16
330/15
330/14
966/5
492B/16J
492B/15
492B/14
492/14
1523/6
1523/5
1523/4
1523/3
1523/2
1523/1
977B/58
977/58 977/57
977/56
977/55
977/54
977/53
977/52
977/51
977/50
977/49
977/48
977/47
977/46
977/45
977/43
977B/43
391B/4
391B/3
391B/2
237/1
1900B/31
8 kV – 12,500 pC
12 kV – 36,000 pC
Replace the section
8 kV – 11,000 pC
12 kV – not increased
Inspect the splices in
manholes 977/52 and
977/53
8 kV – 4,000 pC
12 kV – 12,000 pC only for the
manhole 1523/1, the rest of the
zone not increased
Replace splice in manhole 1523/1
Inspect the splices in manholes
977/58, 977B/58, 1523/2, and
1523/2.
8 kV – 12,000 pC
12 kV – not increased
Inspect the splices in
manholes 1523/5 and
1523/6
8 kV – 23,000 pC
12 kV – not increased
Replace the section
8 kV – 7,000 pC
12 kV – not increased
Inspect the splice

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Results Example #3 / Section #3
Test location
Chestnut Park Apt.
Building A
Test location
Bridge St.
MH 227/10
492B/14 492B/12
492B/13 492B/11
492B/10
492B/9 492B/8 492B/7
492B/6
492B/6M
277B/12M
277B/12
277B/11
492B/5
492B/4 492B/3
492B/2
492B/E1
492B/1
277B/10
8 kV – 18,000 pC
12 kV – not increased
Replace the section.
8 kV – 6,000 pC
12 kV – 12,000 pC only for
the “Y” splice in manhole
492B/E1, not increased in the
termination.
Replace the “Y” splice in
manhole 492B/E1.
Inspect the termination.
8 kV – 6,000 pC
12 kV – not increased.
Inspect the “Y” splice in
manhole 492B/2.
8 kV – 13,500 pC
12 kV – not increased
Replace the section.

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Included In Test Report
 Maps As Shown
 Text Describing “Zones”
 Summary Table

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COST/BENEFIT EVALUATIONS

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DIAGNOSTIC TESTING
Cost vs Benefit evaluation
factors to be considered
 Cost of testing
 Cost of repair of detected problem area(s)

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WAITING FOR FAILURE
 Cost of failure locating
 Cost of repair of failure
 Cost of other damage initiated by failure
 Cost of loss of revenue
 Loss of customer satisfaction

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DIAGNOSTIC TESTING OF
DISTRIBUTION CABLE SYSTEMS
Utility tie to industrial customer in the Netherlands
Cable circuit 18.000 ft of 10 kV PILC cable
installed in 1996
Problem three failures in three months
Proposed solutions
 Replace 18,000 ft of cable
 Perform or diagnostic testing

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DIAGNOSTIC TESTING OF
DISTRIBUTION CABLE SYSTEMS
Cost (in NLG)
To replace the cable 1.800.000 NLG
Cost of diagnostic testing 8.000 NLG
Cost of replacing 2,500 ft
of cable 240.000 NLG
Saving by diagnostic testing 1.550.000
NLG
Percent savings: 86%

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DIAGNOSTIC TESTING OF
DISTRIBUTION CABLE SYSTEMS
Industrial customer in the Netherlands
Cable circuit 35.000 ft of 10 kV PILC cable
with 28 splices
Problem Six splice failures in three months
Proposed solutions
 Replace all 28 splices
or
 Diagnostic testing

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DIAGNOSTIC TESTING OF
DISTRIBUTION CABLE SYSTEMS
Cost (in NLG)
Replace another 22 splices 110.000 NLG
Diagnostic testing 12.000 NLG
Replace 9 splices 45.000 NLG
Total cost of repair 57.000 NLG
Savings: 110.000 - 57.000 = 53.000 NLG
Percent savings: 48%
Added benefit:
Cable and 6 replaced splices tested OK

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DIAGNOSTIC TESTING OF
DISTRIBUTION CABLE SYSTEMS
Preventive/predivtive maintenance diagnostic testing
example from the Netherlands
The cable: 10 kV PILC, 105 circuits,
total of 500,000 ft
Cost of testing 420.000 NLG
Repair of 29 problems 145.000 NLG
Total cost of testing and repair 565.000 NLG

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DIAGNOSTIC TESTING OF
DISTRIBUTION CABLE SYSTEMS
Wait for failure
Assume all 29 problem would fail in 1996
Fault locating cost (500 NLG ea.) 14.500 NLG
Repair cost (8.500 NLG ea.) 246.500
NLG
Customer damage cost
(20.000 NLG ea.) 580.000 NLG
Total cost 841.000 NLG
Total savings performing diagnostic testing
841.000 - 565.000= 276.000 NLG
Percent savings: 33%

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DIAGNOSTIC TESTING OF
DISTRIBUTION CABLE SYSTEMS
Preliminary US cost benefit analysis
Based on data compiled by US utility, the cost
benefit ratio ranges from 1.5 to 1.9 based on actual
utility cost data
The assumptions include:
 The circuits selected for testing have shown poor
performance
 The major discharge sites will fail within three years
 The minor sites will fail within 20 years

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New Developments
 Multi terminal testing for long lenghts of
cable and for branched circuits
 Non-PDD testing for watertree aged cables
 On-line testing using monitoring and expert
systems

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Conclusions
 VLF PDD is an experienced diagnostic
method for distribution cable testing
 Multi terminal testing is necessary for long
lenghts of cable or branched cable circuits
 Non-PDD field testing method has to be
selected for detecting watertree aged
cables
 On-line testing has to be used in
combination with monitoring/expert
systems