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B1-210
CIGRE 2012
Condition and Remaining Life of Taped Transmission Cables through NonInvasive and Invasive Testing Based on In-service Cables and Cable Samples
Removed from Service
NS, NIRMAL SINGH, SKS, SANDEEP K. SINGH, RR, ROMMY REYES
Detroit Edison Company, United States
TZ, TIEBIN ZHAO, Electric Power Research Institute, United States
RG, REZA GHAFURIAN, Consolidated Edison Company of New York, United States
SUMMARY
The taped (laminar dielectric) transmission cables rated 69 kV through 345 kV constitute over 80% of
the US underground transmission. While a very large proportion of such cables have been removed
elsewhere in the world, the US continues to rely on taped transmission cables that are the mainstay of
its large cities, with limited additions. The US taped cable system is above 15% of the world’s total
installed transmission length. Over 60% of the US taped cables are approaching and/or have exceeded
the 40-year design life, placing increasing focus on condition and life assessment.
Of the non-invasive tests that have been considered for various types of taped cable systems, dissolved
gas analysis (DGA) and fluid testing are being increasingly applied. Almost all US cable users are
regularly utilizing this dual approach. While the former has proved quite effective as a non-invasive
test, as demonstrated by the opening of several in-service cable systems, the latter has been less useful.
This paper summarises DGA results on over 50,000 field and laboratory-investigated samples,
including unique features and some shortcomings of DGA for cables.
The life of fluid-impregnated paper system is primarily governed by temperature and time. In the early
1930s, it was established that under thermal degradation, paper loses mechanical properties in order:
folding, tear, stretch, burst and tensile strength. However, the majority of earlier studies have been
performed from the standpoint of transformers, capacitors and self-contained cables utilizing glass
tubes with headspace, and small models, with some impact of decomposition products. The degree of
polymerization (DP) was added much later. The availability of published data on the electrical,
physical and chemical properties of paper tapes removed from in-service taped transmission cables is
limited. As a part of invasive testing, this paper summarizes these properties on paper tapes from 34
cable pieces representative of over 60 investigated pieces of vintages ranging from about 20 to over 60
years, removed in the US and a few other countries to assess aging. The wet-tensile strength, copper
number and furfural content of paper have been included in cable life studies.
These findings demonstrate that the cable life offered by the fluid-paper insulation far exceeds the
design life under usual operating conditions. The occasional failures are precipitated by factors other
than paper thermal aging, such as dig-ins, fluid pressure loss, thermo-mechanical behaviours (TMB),
singhn@dteenergy.com
cable movement due to sloping profiles and excessive road traffic and vibrations, installation
irregularities, manufacturing irregularities (e.g., soft spot), operating condition changes overtime (e.g.,
soil thermal condition changes), and lead sheath fatigue for self-contained cables. The intrusive and
non-intrusive tests discussed in the paper address fluid and paper property changes caused by many of
these factors.
KEYWORDS: Dissolved gas analysis, field data, paper testing, cable condition and life
Introduction: Extruded transmission cables have been gaining increasing acceptance worldwide,
with the present installed lengths exceeding 15,000 circuit-miles (24,000 circuit-km). However,
enormous quantities of taped transmission cables are still in service, particularly in the United States,
where the installed taped cables constitute above 15% of the world’s underground transmission
system. Other countries with such cables are Japan, France, Mexico and a few in Northern Europe.
The taped cables are comprised of three types, namely, HPFF (high-pressure fluid-filled), HPGF
(high-pressure gas-filled) and SCFF (self-contained fluid-filled). The vast majority of world’s
submarine dc cables are also based on impregnated-paper, including some ac ones. Over 60% of taped
cable systems in the world are approaching or have exceeded the assumed 40-year life, and decisions
have to be made regarding the continued use of such assets, including replacements. This places a
sharp focus on diagnostic methods for condition and life assessment of taped cables and accessories,
because such decisions represent considerable utility investment – tens of billions of dollars in the US.
This paper covers both non-invasive and invasive tests for condition and life assessment. The former
deals with the fluid removed from the cable system and the latter with paper samples made available
due to failures, re-routing and additions. Both approaches date back to the 1930s and have been
significantly improved and refined over time with the introduction of new methods and sophisticated
instrumentation. It should be added that the application of non-invasive tests relating to the dielectric
fluid and invasive tests relating to paper was first applied to oil-filled transformers, and later extended
to cables.
Non-Invasive Testing: Non-invasive testing includes DGA and testing of fluid removed from the
cable system. Of the many tests, DGA is deemed to be the most effective and economical for taped
cable systems. Its appeal is further enhanced by relative simplicity and easy to understand findings.
Utility engineers can relate to the DGA results and the massive transformer experience promotes
similar efforts. The application of DGA has well progressed since the early 1980s, and almost all US
transmission cable users employ DGA regularly.
DGA relates to the analysis of various gases – lower and higher hydrocarbons, hydrogen and carbon
oxides – that are generated under electrical and thermal stresses experienced by an operating cable.
The type, distribution and concentration of such gases are governed by the specific nature of the
electrical, thermal and mechanical problems faced by the cable, giving cues to the condition of the inservice cable. While individual gas concentrations are important, one has to look at the total picture to
make proper interpretation, as most gases are interrelated - but acetylene is the single, most important
gas. The cable DGA should not be confused with transformer DGA, and it varies amongst different
types of cables and accessories.
The success of DGA depends on sampling, analysis and interpretation. The collected sample should
faithfully represent what is within the cable and no gases should escape or add to the sample during
handling, transportation and analysis. Gases with low solubility such as hydrogen and carbon
monoxide tend to escape. The high concentration of nitrogen from a pumping plant frequently leads to
bubble formation, as the pressure is significantly reduced in the sample that is invariably taken in a
glass syringe. Such bubbles present difficulties in the analysis, further compounded by the escape of
low solubility gases into these bubbles. The relatively high viscosity of dielectric fluids associated
with HPFF cables (100 SUS to 850 SUS@40°C) further adds difficulties in syringe sampling, all the
more in winter months. The handling of the collected sample involving the extraction of gases and
subsequent transferring to chemical instrumentation for gas quantification, can lead to errors. While
1
DGA interpretation can be a challenge in some circumstances, errors are often made in sampling. The
operating history of the cable should be factored in the interpretation. Unlike transformers, a cable not
only offers many sampling points but also the sampling environment is not as easy or pristine.
To overcome some of these sampling difficulties, a previous sampling-cum-analysis method termed
EPOSS (EPRI Pressurized Oil Sampling System) - described in a previous 1996 CIGRE paper- was
developed. The DGA data presented in this paper is by a method named EDOSS (EPRI Disposable Oil
Sampling System), which represents a radical modification of EPOSS. The disposable nature of its
inexpensive light, one-step, sampling and analysis vial, and low sample volume requirement (only 5/6
cc) are noteworthy.
DGA offers several unique features: high chemical measurement accuracy with due procedures and
high sensitivity to electrical/thermal stresses; low detection limits with available equipment;
accumulative process in that gases tend to stay in place, once formed; satisfactory location of a general
problem through fluid movement; potential for rough life assessment through carbon oxides; on-line
sampling for HPFF/HPGF cables, but not terminations and SCFF cables – one US utility uses a
metallic-mesh mat connected to the pipe as a sampling platform. The short-comings include: the upper
limits of various gases cannot be well established; DGA is able to identify and locate a problem in the
general vicinity but cannot define its precise location with respect to dielectric media involved nor
when the problem will precipitate failure; depending on the strategic location, some problem site(s)
may be severely detrimental or innocuous. Nevertheless, both situations will lead to various gas
concentrations posing concern. However, looking at the gas pattern as a whole and not only the
individual gases along with trending is valuable toward problem resolution. For instance, the
inordinately high hydrogen sometimes observed in HPFF cables, without other vital gases, is
invariably generated in the pipe fluid phase, mattering only when repairs at reduced (50 psi or 345
kPa) are needed due to its low solubility.
DGA Results for HPFF, SCFF and HPGF Cables: The data generated by EDOSS is discussed
for each type of cables and its accessories. The HPFF and HPGF data covers only the US, while the
results on SCFF cable systems also include non-US sources, with comments below Table I and Table
II.
Table I: DGA Results for HPFF Cable Systems, 110 to 345 kV
Gases
Methane
Ethane
Ethylene
Acetylene
Propane
Propylene
Iso-butane
nButane
t2Butene
1Butene
Isobutylene
Hydrogen
C Monoxide
C Dioxide
1
2
2
1
0.0
14
3
1
8
0
1
2
183
22
130
2
159,898
60,626
407
146
22,236
35
6,755
3,174
340
68
29,588
975
26
116
3
278
41
348
1,673
24
112
58
16
15
41
16
3,121
61
363
4
27
17
18
0.6
16
11
3
13
5
9
8
318
40
326
5
169,700
20,017
20,063
1,314
8,993
22,330
9,577
768
1,059
1,886
123,930
74,970
2,078
1,234
6
0
1.0
0
0
3.3
0.7
0
0.8
0
0
0
230
0
52
7
45
8.9
1.5
0
4.8
2.2
0
0.6
0.7
0
3.7
1,299,600
106
190
8
7.1
4.8
0.5
0
3.0
1.7
0
0.4
0
0
4.2
46,644
0
80
9
7,035
5,351
4.0
9.5
1,715
16
251
541
38
30
1,335
326
50
268
10
3,374
3,025
3.8
1.0
1,079
14
189
408
74
56
1,112
183
65
374
Column 1: Operation with no concern, 138 kV, 53-year old cable; 2&3: 138 kV terminations opened
& re-built; 4: After repairs data of 3; 5: Revealed damage as shown in Figure 1 for 16-year, 115 kV
GIS termination; 6, 7&8: Minimal gases at a splice in a 138 kV HPFF cable, showing abnormally high
hydrogen in the bubble (7) and much lower concentration in the associated liquid phase of the same
syringe (8), 55-year, 138 kV cable in operation; 9&10: Reduction in acetylene with drainage from 9.5
2
to 1 ppm for a 35-year, 345 kV cable, saturated gases came with the original fluid, this cable was
removed due to a failure experienced at another independent cable at the plant.
Besides losing properties as a function of temperature and time, paper evolves carbon dioxide and
carbon monoxide. In the EPRI Waltz Mill investigations [1], the increasing carbon oxides
concentrations were measured right close to the breakdown and termination of long-term tests. Unlike
carbon oxides, the rest of the gases remained essentially the same. These carbon oxides concentrations
provide some rough guidance of paper aging for HPFF cables. The values measured in the field for
over 50,000 data points also show that this approach, a by-product of DGA, is useful. A carbon
dioxide concentration higher than a few thousand ppm is seldom observed for HPFF cables, with
carbon monoxide concentration rarely more than 750 ppm. All this supports lack of paper aging.
Table II: DGA Results for HPFF, HPGF and SCFF Cable Systems, 120 to 230 kV
Gases
Methane
Ethane
Ethylene
Acetylene
Propane
Propylene
Iso-butane
nButane
t2Butene
1Butene
Isobutylene
Hydrogen
C Monoxide
C Dioxide
11
9.4
9.3
6.4
4.6
20
6.1
5.7
1.4
0.3
0.6
28
78,008
30
150
12
9.7
9.5
5.9
2.3
20
6.1
5.9
1.5
0.6
0.4
28
121,898
32
166
13
8.4
6.4
5.0
0.9
10
5.0
5.6
1.3
0.3
0.6
29
140,822
30
171
14
0.0
5.2
0.7
0.0
24
1.0
0.7
4.3
0.0
0.0
14
110,471
0
81
15
0.0
6.6
0.9
0.0
34
1.3
0.9
5.2
0.0
0.0
23
20,229
0
136
16
2.1
2.3
0.6
0
3.1
1.4
0
0
17
59
8.2
44
60
5.4
18
1.5
0.3
18
6.7
4.8
2.6
0
5.4
19
0.9
1.2
19
65
6.1
8.4
0
4.0
9.0
1.1
2.1
20
0.43
0.31
0.12
0
0.33
0.17
0.09
0.15
0
0
18
183
7.2
219
11
172
0.9
0
0
40
11
4,544
198
422
0.60
56.4
32.1
17
Columns 11, 12&13: Increasing hydrogen variation within riser with limited drainage, 25-year, 240
kV HPFF in operation; 14&15: Hydrogen reduction in a 35-year 138 kV HPFF cable riser with
drainage (also see Figure 2), cable continues to operate; 16&17: 47-year, 138 kV, SCFF splices, only
one phase showed high acetylene, cable in operation, DGA follow-up pending; 18: Satisfactory gas
profile for a 38-year, 230 kV SCFF cable; 19: Satisfactory data for a 50-year, 138 kV, SCFF cable in
operation; 20: Satisfactory gas analysis for a 48-year, 120 kV, HPGF cable in operation.
Figure 1: Damage Extending to Cuter
and Tapes, 138 kV GIS Termination
Figure 2: Hydrogen Profile in the
Riser/Cable Pipe with Fluid Drainage
Fluid Testing: Along with DGA, some fluid tests such as breakdown, dissipation factor (DF) and
moisture are generally performed on fluid taken from taped cable systems, with many users limiting
only to DGA. Based on our field experience, these fluid quality tests are not very useful and should be
performed only after 5 to 7 years of operation.
Invasive Evaluation of Removed In-Service Cable Pieces: As true for the application of DGA
to cables, paper testing was first employed in transformers. As early as the 1930s, V. M. Montsinger
3
[2] measured the mechanical properties of transformer paper in the form of tapes as a function of
thermal stresses in glass tubes containing mineral oil and discovered that the mechanical degradation
occurs in the following order: folding, tear, stretch, burst and tensile, contributing to the introduction
of the so-called 10°C rule. This pioneering work was later refined and adopted for cables, adding other
properties in the following decades appropriate for high voltage cables such as radial impregnated tape
dissipation factor and degree of polymerization (DP) by others – the objective always was to utilize
these paper properties to assess the degradation of paper for the estimation of cable life, keeping in
mind that paper aging is a thermal phenomena. Again, the application of DP to transformers preceded
that for cables and it is recognized to be an important aging marker for both products. Drs. P.
Gazzana-Priaroggia and G. L. Palandri [3], and later Drs. W. G. Lawson, N. A. Simons and P. S Gale
[4] made valuable contributions to such efforts, although the focus of the studies was solely on SCFF
cables, and not HPFF cables amounting to over 7000 circuit-miles (11,000 circuit-km) in the world.
The authors stated that a10% retention of any of the paper property can serve as an end of life
criterion, however, this magnitude is far too low and the paper will be in highly degraded state at 10%
reduction. Moreover, most properties show wide dispersion in measurements.
The invasive investigations included dissection of cable pieces removed from service, as well as the
radial testing of paper tapes in both dry and impregnated states. The age of cables ranged from about
20 to over 60 years. The papers in the investigations were removed from small cable pieces that
became available due to failures, additions or re-routing. A few stored cables of different vintages
were also included to offer ready comparison. The operating history details and some failures were not
known in some cases. The express intent of the users was to get a better understanding of the aging of
the taped dielectric – often away from the failure, if there was one - as many of such cables were
approaching or had exceeded 40 years of service.
Dissection of Cable Pieces: The cable construction was first defined for number of tapes, tape
thicknesses, widths, reversals and shielding utilizing a 2 to 3 in (50 to 76 mm), leaving about 3-ft (0.9
m) cable length for unwrapping. This information was necessary to determine the number and radial
location of the tape so that tapes, reflecting the entire insulation wall thickness, can be properly
selected and tagged for condition and remaining life assessment. Depending on the thickness of the
tape and its radial location, one or more tapes were taken from each reversal for evaluation in
impregnated and dry states, according to the type of paper testing. During the careful unwrapping of
the cable piece, tape by tape, particular attention was paid to visual appearance, butt-gap widths, softspots, degree of impregnation, and presence of irregularities, such as broken tapes, wrinkles, creases
and indentations.
Visual Inspection and Butt-gap Measurements: The typical butt-gap width should be 4-5% of
the tape width, and is closely kept. Theoretically, the edge of the adjacent tapes should be just butting,
but the taping machines could not maintain such a precise action in practice. Since the barrier provided
by the paper is much stronger than the fluid contained in butt-gaps, wider gaps are tantamount to
lessened paper insulation, resulting in loss of dielectric strength. Wider than normal butt-gaps can be a
contributing factor to cable failure through formation of soft spots and some reduction in dielectric
strength. While 138 kV cables may be able to tolerate non-uniformity in butt-gaps, higher voltages
will be much less forgiving.
The butt-gap uniformity and indents, as shown in Table III and Figure 3 for a 62-year, 138 kV HPFF
cable are not desirable, but the cable is still operating. Spiral wax for a 1953, 115 kV HPFF cable is
shown in Figure 4. Waxing is rare in HPFF cables. The failure of this cable after 56-years of operation
at a heavy traffic crossing is attributed to the cable movement and vibrations and not waxing. The only
available DGA was performed after the failure at two manholes (75 and 1000 ft or 23 m and 305 m on
either side of the failure). The low carbon monoxide levels (11 to 35 ppm) and other gases measured
after the failure did not show any signs of distress or aging, the blanketing of fluid shipment by dry
carbon dioxide, as opposed to the later practice of nitrogen, negated any diagnostic benefits of carbon
dioxide.
4
Table III: Butt-gap Distribution for a 138 kV HPFF Cable, 1949
Tape No.
4
9
13
15
19
21
28
32
34
36
47
54
71
93
Butt-Gap
Width (mils)
176 196
23 94
239
232
210
200229
97
241
206
229
216
202259
148
95
Butt-Gap
Width (mm)
4.5 5.0
0.6 2.4
6.1
5.9
5.3
5.15.8
2.5
6.1
5.2
5.8
5.5
5.16.6
3.8
2.4
Figure 3: 138 kV HPFF Cable, 1949,
Tape 93 (95 mil or 2.4 mm butt-gap)
Figure 4: 115 kV HPFF Cable, 1953, Tape 5
Radial Paper Testing: A series of electrical, mechanical and chemical tests were performed on
selected paper tapes removed from cable pieces of varying vintages. The %DF at 80°C and moisture
content of the paper tapes were determined in the impregnated states as soon as they were removed to
avoid moisture contamination. The rest, namely, degree of polymerization, dry- and wet-tensile
strength and folding were performed after the removal of the impregnating fluid.
The results relating to cable age, DP next to the conductor, average and range of DP, DF, moisture,
and only the average of dry-tensile strength are shown in Table IV for 34 cables. The age of the 69 to
345 kV cables from which paper tapes were removed varied from stored to over 60 years. Of the 34
cases, only 12 are known to have failed and are operating satisfactorily after the repairs. While cables
at 230 kV and above were of the HPFF type, those below this voltage included HPGF and SCFF ones.
In addition, other paper properties such as wet-tensile and folding endurance were always performed,
including limited measurements on copper number of paper and furfural content of the fluid for paper
assessment and DGA for some of the 34 cables. For want of space, these additional properties could
not be included in Table IV, but will be briefly addressed. The intent is to make available substantial
aged paper data, which is available to a limited extent, for radial DP and other more familiar properties
toward the understanding of aging.
Degree of Polymerization has been gaining increasing acceptance as an indicator of paper aging, as a
result of several earlier investigations on transformers and SCFF cables, and the later comprehensive
EPRI Waltz Mill studies [1] in the 1990s for HPFF cables involving the load-cycling of six 100-ft (30
m) 138 kV and six 100 ft (30 m) 345 kV at 125°C (135°C for 345 kV towards the final two years)
over 3.5 years. It was shown that DP next to the conductor offers the best approach for life assessment
and its minimum value for end of useful life is about 350 ( 200 accepted for transformers). Unlike
other mechanical properties that are related to the bulk properties of paper, DP shows the least
dispersion in measurements, being a fundamental measurement of the paper structure, hence its
importance. A review of the DP data presented in Table IV shows that the DP value is essentially the
same throughout the insulation wall for cables of all vintages, demonstrating lack of temperature
gradient and minimal aging - all DP values of tapes next to the conductor far exceed 350. The
observed minor DP variations for the virgin and in-service cables are due to the source of the pulp and
the paper manufacturing process.
The DF values of paper tapes for 230 kV and 345 kV cables, where distilled water-washed pulp is
utilized to reduce the polar salts, were mostly lower than those of the 138 kV cable paper tapes
without the benefit of such a manufacturing process. The comparatively high DF values of cables
below 230 kV are also partially due to age, with the exception of 120 kV HPFF cables (Set No. 7 and
5
8), where high quality dense paper is used as shown by high Dry TS (maximum tensile 24,272 and
24,274 psi). The DF values observed in bulk of the cable were on the low side and acceptable for aged
cables, but the outer and inner tapes picked up extra moisture due to handling, transportation and
storage. The same situation applies for moisture.
Table IV: Radial Distribution of Paper Properties, 69 to 345 kV Taped Cables, including Stored
Cables
Set No.
Cable
Vintage
1*
2
3
4**
5**
6
7**
8**
9*
10
11
12
13
14
15
16
17
18
19
20*
21**
22
23**
24
25**
26**
27**
28**
29***
30
31**
32****
33****
34****
69 kV, 48
69 kV, 49
69 kV, 53
115 kV, 53
115 kV, 53
115 kV, 73
120 kV, 71
120 kV, 72
138 kV, 91
138 kV, 49
138 kV, 49
138 kV, 49
138 kV, 52
138 kV, 55
138 kV, 58
138 kV, 59
138 kV, 60
138 kV, 69
138 kV, 74
230 kV, 77
230 kV, 70
230 kV, 72
230 kV, 81
230 kV, 83
345 kV, 71
345 kV, 71
345 kV, 73
345 kV, 73
345 kV, 73
345 kV, 78
345 kV, 78
345 kV, 89
345 kV, 90
345 kV, 90
Degree of Polymerization (DP)
Next to
Conductor Ave.
Range
1270
1287 1242-1372
1143
1151 1112-1198
807
796
759-824
770
779
757-809
768
785
766-812
764
776
757-809
841
873
727-985
828
842
807-870
897
905
897-915
806
815
789-854
806
848
795-917
799
819
769-865
744
770
744-798
882
950
840-1076
815
784
729-829
833
889
831-996
765
765
751-780
1146
1187 1146-1209
1144
1146 1089-1194
739
731
713-753
755
767
740-810
751
751
729-777
712
715
701-726
729
756
729-778
764
746
722-771
755
748
723-778
742
748
732-768
738
745
732-759
866
798
671-866
712
710
695-733
1459
1642 1459-1794
824
817
789-848
824
817
798-838
825
818
789-834
Moisture (ppm)
Ave.
5228
170
656
1184
1011
296
1016
261
969
1145
1013
960
838
1110
553
421
846
578
1167
665
295
733
624
690
1321
1203
503
894
349
688
281
438
438
445
Range
2430-9070
100-260
487-994
905-1678
878-1248
212-489
459-1790
141-436
715-1612
847-1660
784-1290
745-1710
712-1215
660-1750
440-792
306-569
742-1015
302-1164
582-2290
583-727
241-380
559-1028
462-1060
425-1415
899-2312
904-1804
401-648
684-1302
224-490
463-843
145-454
348-810
347-897
378-924
% DF @ 1000C
Ave.
1.440
0.275
0.729
0.834
1.530
0.250
0.360
0.235
0.273
1.117
0.993
0.875
0.579
1.052
0.390
0.328
0.515
0.305
0.367
0.248
0.234
0.283
0.363
0.245
0.819
0.813
0.293
0.364
0.221
0.578
0.228
0.141
0.176
0.184
Range
0.35-7.72
0.26-0.28
0.45-1.72
0.68-1.22
1.12-2.35
0.24-0.26
0.24-0.61
0.20-0.27
0.24-0.31
0.65-3.34
0.71-2.30
0.66-1.65
0.33-0.94
0.78-1.09
0.29-0.81
0.25-0.89
0.47-0.62
0.27-0.43
0.31-0.59
0.21-0.27
0.21-0.28
0.22-0.42
0.29-0.72
0.19-0.39
0.39-0.82
0.36-2.65
0.20-0.64
0.29-0.61
0.19-0.32
0.24-1.83
0.18-0.37
0.11-0.35
0.13-0.29
0.14-0.28
Average
Dry TS
(psi)
11,149
9,218
15,069
18,734
18,807
11,334
21,228
18,363
11,603
12,483
12,512
12,216
18,644
19,640
20,014
14,942
15,140
13,943
11,690
16,263
14,988
13983
17,841
14,220
13,331
13,658
16,903
16,398
12,585
14,269
12,157
2,817
2,817
2,805
*Stored cables; **Failure on one phase; *** TMB splice failure; ****Paper Polypropylene Paper
(PPP) (DP & Dry TS were performed on the delaminated paper and moisture and DF were performed
on the PPP Composite);
Row 1–SCFF; 2-6 and 9-34 – HPFF; 7-8 – HPGF. (1 psi = 6.89 kPa)
According to Waltz Mill algorithm based on Arrhenius model, the life estimation for a 32-year 138 kV
HPFF cable (Set No. 18, Table IV) is shown in Table V. This demonstrates that taped cables are
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characterized by exceedingly long life and many 138 kV HPFF cables have exceeded 60 years of
service under the current loading conditions, which are well below 85C.
Table V: Radial Distribution of Degree of Polymerization (DP) and Remaining Life
Tape No.
Radial Position
DP
Outermost Tape
Retained
Property
1.00
1
0.53
1208.6
1208.6
34
0.353
1209.0
1.00
51
0.262
1204.0
0.99
73
0.144
1168.0
0.97
99
0.005
1146.0
0.95
Equivalent years at 85C conductor temperature (100% load factor)
1.4
Remaining life at 85C conductor temperature (100% load factor), in years
Predicted years of service at current operating conditions (100% load factor)
204
673
Copper Number, Wet-Tensile Strength and Furfural Content: It is a number expressing the
amount of copper reduced from the cupric to the cuprous state by a given amount of cellulosic
material. A lower number signifies permanence of paper and lack of aging - thus it is reverse of DP. A
value of 0.6 to 0.75 is excellent. For all the 34 cables, it was found to be about 0.625. Wet-tensile
strength, a measurement of the tensile strength of water-soaked, fluid-free tapes, indicates the degree
of the inter-fiber-bonding. Unlike traditional properties that decrease with thermal aging, this property
initially increases to a peak and then decreases as the aging progresses. This peak is singularly
governed by temperature and time and requires a strong temperature gradient across the insulation. A
cable, whose tapes have experienced some aging, does exhibit this peak within the insulation wall. The
essentially constant values observed over the wall for all cables further support lack of aging. The
furfural content, which is a measure of very advanced state of paper aging, is too low (ppb) to be a
useful indicator for taped cables and this indeed was the case.
Conclusions and Recommendations: DGA is being increasingly recognized to be an effective and
economical diagnostic test to assess the condition of laminar dielectric cable systems. Its appeal is
further enhanced by its relative simplicity and easy-to-appreciate findings. The fluid quality tests
should be performed judiciously, as they offer less value. The fluid-impregnated paper system shows
minimal aging under the usual thermal conditions experienced by taped transmission cables, thus
holding great potential of exceedingly long life. The failures occasionally encountered in taped are due
to many factors other than the thermal aging of the dielectric. As taped cable samples become
available with or without failure, it is recommended to dissect the cable and evaluate paper properties
to get a better understanding of its life.
BIBLIOGRAPHY
[1]
[2]
[3]
[4]
N. Singh, et al, “Cable Life Evaluation and Management” (EPRI Final Report, TR-111712,
December, 1998)
V. M. Montsinger, Discussion in, “Temperature Limits Set by Oil and Cellulosic Insulation by
C. F. Hill” (AIEE, 1939, Vol. 58, page 488)
P. Gazzana-Priaroggia, et al, “The Influence of Aging on the Characteristics of Oil-Filled Cable
Dielectric” (IEE, 3348S, Nov. 1990, pages 467-490)
W. G. Lawson, et al, “Thermal Degradation of Cellulose Paper Insulation” (IEEE Electrical
Insulation Transaction, June 1996, pages 14-18)
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