IEEE Transactions on Power Delivery, Vol. 11, No. 4, October 1996
1789
AN ASSESSMENT OF THE RELIABILITY OB IN-SERVICE GAPPED
SILICON-CARBIDEDISTRIBUTION SURGE ARRESTERS
M. Darveniza, Fellow IEEE, D.R. Mercer, R.M. Watson
University of Queensland
Australia, 4072
Abstract - Although gapped silicon carbide arresters are no
longer purchased by electricity authorities, they still form the
majority of the very large number of distribution arresters in
service in Australia and many other countries. Most of the
arresters of this type are now over ten years old and many are
much older. So the question must be asked - what is to be
done with this ageing and outdated class of arresters?
Extensive Australian studies in the 1960's had revealed that
internal degradation resulting from inadequate seals was the
predominant cause of failure of gapped silicon carbide
arresters. This paper describes the results of a recent
investigation. Electrical testing showed that after about 10
years of service, there is a marked upturn in the number of
arresters with unsatisfactory insulation resistance, and after
about 13 years of service, a marked upturn in the number of
arresters with reduced power frequency sparkover level.
Inspecition of the internal components of dismantled arresters
confirmed that the likelihood of significant degradation
increased markedly with years of service, and was evident in
almost 75% of arresters with 13 years or more of service.
The authors therefore recommend that all gapped silicon
carbide arresters with 13 or more years of service be
progressively replaced by modern metal oxide arresters.
I. INTRODUCTION
An earlier extensive study [ l ] of distribution surge arresters
and transformers was motivated by excessive failure rates in
service. That study revealed that the predominant cause of
failure of gapped silicon carbide surge arresters was internal
degradlation due to moisture ingress resulting from inadequate
seals. The condition of the arresters was assessed by a
combination of power frequency voltage withstand test,
impulse voltage sparkover test, measurement of insulation
96 W M 013-3 PWRD A paper recommended and approved by the IEEE
Surge Protective Devices Committee of the IEEE Power Engineering Society
for presentation at the 1996 IEEE/PES Winter Meeting, January 21-25, 1996,
Baltimore, MD. Manuscript submitted August 1, 1995; made available for
printing November 13, 1995.
resistance, and dismantling and inspection of internal
components. Since then, manufacturers have improved the
design and testing of arrester seals, and in-service arrester
failure rates reported by electricity authorities are now
generally lower than those which motivated the earlier
investigation. Subsequent investigations carried out in Norrh
America [2, 31 also showed that the dominant cause of
degradation and failure of distribution arresters was moisture
ingress. The Ontario-Hydro work [2] revealed that nearly
86% of all arrester failures could be associated with moisture
ingress, while the Detroit Edison study [3] (primarily
intended to determine the magnitude of lightning currents
discharged through distribution arresters) showed that
moisture was present in 10% of about 3000 arresters
examined after about 12 years of service life. These and
other studies suggested that some distribution arrester failures
(about 5%) could be attributed to very severe lightning
surges, eg. strokes of very high current magnitude or long
continuing current durations, and/or multiple-stroke flashes
with high multiplicity of stroke currents and power-follow
currents. Indeed, recent laboratory studies hlave demonstrated
that multiple lightning impulse currents can cause damage to
distribution lightning arresters, which is not evident in
standard single-impulse tests [ 5 ] . The North American
studies also showed that some distribution arrester failures
were caused by external contamination on the housing, and
such failures were more likely for arresters without non-linear
resistance grading of the internal gaps.
Because of the advent of metal-oxide varistor technology,
trial installations of gapless metal oxide arresters, began in
the late 1970's and early 1980's, and within a few years most
arrester installations were of the metal-oxide type. However,
gapped silicon carbide distribution arresters still form the
majority of distribution arresters in service, and represeni a
significant capital investment. With a view to (1) determining
the condition of current gapped silicon carbide distribution
surge arresters in service on Australian distribution systems,
(2) assessing the causes of their degradation and failure, and
(3) assessing their average remanent useful life, almost four
hundred such arresters were withdrawn after about 10 years'
service in Australian distribution systems, and were subjected
0885-8977/96/$05.00
0 1996 IEEE
1790
to laboratory tests and internal inspection. This paper reports
the results of the study, compares them with the findings
from earlier work, and comments on the average useful length
of service of such arresters.
II. METHODOLOGY OF THE INVESTIGATION
The Surge Arresters - Each of the participating electricity
utilities was requested to recover from its distribution system
about 50 surge arresters which had been in service for up to
10 years, and supply them for testing. Eight Australian
electricity authorities submitted a total of 379 arresters, of
which 365 were suitable for inclusion i n the project. Voltage
ratings ranged from 9 kV to 24kV, and about 80% had a
current rating of 5 kA, while the remainder were rated at 10
kA. There were seven manufacturer's names on the submitted
arresters and Table 1 provides information about arrester
numbers, years of manufacture and probable length of
service. The overall median year of manufacture was 1983,
suggesting that the median length of service of the tested
arresters was about 9 years.
Service Comments - Some of the participating utilities
provided comments about the service performance of their
arresters. One utility quoted an annual failure rate of over
1% of their distribution arresters and commented that the cost
of locating and replacing failed arresters was about six times
the purchase cost of the arresters. Other utilities, particularly
those in the drier areas of Australian prone to high risk of
bushfires, made specific mention of the fires caused by
arrester failures. One utility reported that arrester failures
seemed to be disproportionally high on overhead line underground cable junctions. Yet another utility experienced
a considerable number of arresters which failed by external
flashover during early morning periods of heavy dew presumably associated with external contamination. The
flashover problem was associated with a particular make of
arrester which had a relatively short-length porcelain housing
and which was supported by a metal bracket fitted directly on
a transformer tank.
Laboratory Tests - The arresters were tested for insulation
resistance, power frequency voltage withstand, power
TABLE I
INFORMATION ON 365 RECOVERED ARRESTERS
~~~
~
~
Manufacturer
No. of
Arresters
Range. of
Years*
Median
Year*
Median
Service**
1
2
3
4
5
6
7
4
20
200
1
11
22
107
1970-1973
1965-1975
1972-1991
1991
1957-1969
1979-1983
1978-1989
1970
1970
1986
1991
1964
1980
1981
22
22
6
1
28
12
11
* year(s) of manufacture
median length of service (in years) assumes the amesters were
installed in the year of manufacture and were recovered in 1992
**
frequency sparkover, and impulse voltage sparkover. In
addition, tests were carried out to determine the condition of
seals on about 40% of the arresters, and these and some
others were dismantled to allow inspection of internal
components. The first group of tests were designed to follow
the general pattern of the earlier investigation [ 11, in which
the intention was to identify significantly degraded arresters
in a cost-effective way, rather than to test for strict
compliance with the relevant Australian Standard [ 6 ] .
Insulation Resistance Measurement - The insulation resistance
of each arrester was measured with a 2500V insulation tester,
using the guard ring technique to exclude the effect of
leakage across the outside surface of the arrester. The highest
reading provided by the tester was 2000 MQ.. For arresters
without resistance-graded gaps, the insulation resistance was
deemed satisfactory if >2000MCl, and unsatisfactory if <
2000MQ.
Power Frequency (PF) Tests - The Standard requires that,
after two preliminary sparkovers, 5 to 10 applications of
power frequency voltage shall be made, and the voltage
raised to sparkover each time. An interval of 10 seconds shall
be allowed between applications, and the lowest sparkover
level shall be not less than a stated value related to the
voltage rating of the arrester. Arrester manufacturers publish
power frequency sparkover levels, usually average values, at
levels a few kV above the requirement of the Standard. In the
earlier work [l],a 1-minute power frequency withstand test
was substituted for the sparkover measurement, so that batch
testing could be employed in dealing with large numbers of
recovered arresters. The voltage level chosen for the
withstand test was the minimum sparkover level specified in
the Standard. For this investigation, the power frequency
withstand test was again adopted, but in addition, a power
frequency sparkover measurement was carried out, following
the procedure specified in the Standard, and the average of
the last three sparkover voltages was noted.
Lightning Impulse Sparkover ( L B O ) Measurement - The
Standard calls for groups of five impulses of each polarity to
be applied, and states that 100% operation shall be achieved
at a prospective amplitude not exceeding a stated value
related to the voltage rating of the arrester. If an arrester fails
to sparkover once during the 10 applications, a further 10
impulses shall be applied, and if 100% operation is achieved,
the arrester is deemed satisfactory. For this investigation,
impulses of negative polarity were applied in groups of 10
from a portable impulse generator producing voltage impulses
of the standard 1.2I5Ops waveform, and prospective
amplitudes up to 8OkV. The prospective amplitudes of the
applied impulses commenced at a value approximately 5 %
above the levels specified in the Standard. lf the arrester
failed to spark over on one of the 10 impulses, a further 10
were applied at the same voltage level. If 100% operation
was not then achieved, the voltage level was increased by
5kV, and this process repeated until 100% operation was
achieved.
1791
Seal Test - The seal was tested by placing the arrester in a
chambier which could be sealed and connected to a vacuum
pump. Times from pump start were noted as the pressure
reached 10, 5, 2, 1.5, and 1 Torr respectively. Free volume
within the chamber was minimised by packing with steel
pieces. For each class of arrester, pass/failure criteria for
assessing the condition of the seals were selected from the
time-pressure data for 91 arresters which had satisfactory
insulation resistance and which obviously had sound seals.
Altogether, 154 arresters, representing 42% of the arresters
included in the project, were given seal tests.
Inspecition - All of the arresters which underwent seal tests
were subsequently opened and inspected internally. Some
others, mainly those with very good or very poor results in
the electrical tests, were also opened and inspected. All
indications of departure from presumed original perfection
within the arrester were recorded. However, in assessing the
results of the inspection, minor arc marks on gap electrodes
and block faces, signifying the passage of normal discharge
currents, were regarded as acceptable. Any other recorded
evidence of change was treated as deterioration, and
summarised as moisture-related damage or as electrical
damagle, according to the presence or absence of indications
of moisture ingress or block damage such as surface
flashover, puncture or rupture.
No.
Is10
IR
Seal
Inspection
2NT
lP, 1F
5 5 ~ 49F
262
262P
262P
207P
16F
36
36F
36P
20P
66
III.
PF
66P
66F
66P
RESULTS AND DISCUSSION
1NI, 27F, 21P
4P
4F
160NT
159N1, 1F
40F
3N1, 21F, 16P
7P
SF, 2P
7NT
3N1, 4F
8F
8F
1P
1F
7NT
3N1, 2F, 2P
13F
1NI, 11F, 1P
34NT
33N1, 1F
27F
2N1, 8F, 17P
SP
2F, 3P
1NT
1P
Fig. 1 shows the results of electrical tests on the 365 arresters
investigated, and the results of seal tests and inspections
where applicable. The results for each arrester are arranged
in a horizontal strip across the diagram. Thus, the first block
represents 262 arresters, all of which passed the power
frequency and impulse tests, Fifty-five (21%) of these
arresters had less than satisfactory insulation resistance (IR).
Of this group, 2 were opened and inspected without
undergoing the seal test, and 1 failed the inspection. The
remaining 53 were seal-tested, and 49 (92%) failed the test.
One of the 49 was not inspected, and of the remainder, 27
(56%) failed, while 21 (44%) passed. The 4 arresters which
passed the seal test all failed the inspection. Of the 207
which had satisfactory IR, 160 were not seal-tested, and 159
of these were not inspected. The one which was inspected
was found to have significant deterioration. Forty (85%) of
the 47 which were seal-tested failed the test, a slightly lower
percentage than among those with unsatisfactory IR (92%).
Three of the 40 were not inspected, and of the 37 inspected,
21 (57%) failed and 16 (43%) passed. The percentage of
arresters which failed the seal test and were subsequently
found 1.0 have significant internal deterioration was almost the
same among those with unsatisfactory IR (59%) as among
those with satisfactory IR (60%), but moisture-related
deterioration was found in 94% of the former, compared with
74% of the latter. The median length of service for the 262
arresters was 9 years, and there was a slight difference in this
regard between those with unsatisfactory IR (10 years), and
those with satisfactory IR (8 years).
1
1P
1F
1P
-
PF : power frequency tests
LIS0 : lightning impulse sparkover
IR : insulation resistance
NT : not tested
NI : not inspected
P : passed
F : failed
FIG. 1 Test Results for 365 Arresters
The second block in Fig.1 represents 36 arresters which failed
one or both power frequency tests, and passed the impulse
test. Sixteen (44%) had unsatisfactory IR, so the occurrence
of unsatisfactory IR was much higher in this block than in
the first block (21%). Of the 16 with unsatisfactory IR, 9
were seal-tested, and 8 (89%) failed the test. All of the 20
arresters with satisfactory IR were seal-tested, and 13 (6546)
failed, again a lower percentage than aimong those with
unsatisfactory IR. As in the first block, bhe percentage of
arresters which failed the seal test and subsequently revealed
significant deterioration on inspection was not greatly
different for unsatisfactory IR (100%) and satisfactory IR
(92%), though the actual percentages were much higher.
Similarly, moisture-related deterioration occurred in a higher
percentage (75%) of the arresters with unsatisfactory IR than
1792
among those with satisfactory IR (45%). However, it will be
noted that these two percentages are much lower than the
corresponding figures for the first block, indicating a higher
percentage of arresters with damage due to excessive
discharge current, without evidence of moisture ingress. The
median length of service of the 36 arresters (20 years) is
much greater than that for the 262 arresters which passed the
power frequency tests, and again the arresters with
unsatisfactory IR had a slightly greater median length of
service (21 years) than those with satisfactory IR (18 years).
The third block on Fig.1 represents 66 arresters which passed
the power frequency tests but failed the impulse sparkover
test. All of these arresters had satisfactory IR, and their
median length of service was 7 years, appreciably less than
the preceding groups. The failure rate on seal test was also
lower at 84%, and the incidence of significant visible
deterioration of internal components, in those arresters which
failed the seal test and were inspected, was much lower, at
32%, and the proportion with moisture-related deterioration
was 75%, as in the second block.
The fourth block on Fig. 1 represents one arrester with 19
years of service, which failed power frequency and impulse
tests, but this does not merit further discussion.
The lack of association between failure on impulse sparkover
test and failure on other electrical tests is quite striking, and
was also encountered in the earlier investigation [ 11. The rate
of occurrence of failure on impulse sparkover test is greater
than i n the earlier investigation, but this is due to the much
lower tolerance allowed. The earlier investigation was dealing
with a population of arresters which dated back to the early
years of silicon carbide arresters, in the 1930's, and it was
considered necessary to permit a tolerance of some 25%
above the maximum impulse sparkover voltages permitted by
the Standards current in the 1960's. For the present
investigation, a tolerance of 5 % above the levels set by the
current Standard was adopted.
In the mature phase of the earlier investigation, for several
years arresters were recovered from service when they were
suspected of failing to perform satisfactorily, or when the
associated plant items were being changed to larger size. Of
1702 such arresters which were less than 15 years old, 84%
passed all electrical tests, 5 % failed the insulation resistance
test, 3% failed the power frequency voltage test, 5% failed
the impulse voltage sparkover test, and 3% failed more than
one test. In the current investigation, if the 25% tolerance on
impulse sparkover is applied to 301 arresters less than 15
years old, the corresponding figures are: 77% passed all tests,
17% failed the insulation resistance test, 2% failed power
frequency voltage test, 2% failed the impulse voltage
sparkover test, and 1% failed more than one test,
Comparison of the IR test results of the two investigations is
of limited value, as many of the arresters in the current
investigation hadresistance-graded gaps (manufacturers 6 and
7),and that feature was rarely present in the earlier work.
With that qualification, comparison of the electrical test
results of the two investigations suggests that the performance
of distribution lightning arresters has changed little in the
intervening 25 years.
All the arresters in the third block had impulse sparkover
levels which exceeded the level allowed by the Standard by
more than 15%. In earlier investigations, eg. [l], it was
noted that high impulse sparkover was sometimes
accompanied by high internal pressure, and that arresters
subjected to multipulse operating duty tests sometimes
exhibited raised impulse sparkover voltage afterwards [SI.
Evidence of raised internal pressure was observed in some
instances i n the recent investigation. The median year of
manufacture of the arresters with raised impulse sparkover
voltage was 1985, and the majority were manufactured from
1980 to 1988, mainly by one manufacturer, as discussed later
in relation to Table 3. However, the earlier investigation
showed that raised impulse sparkover level was not a
significant factor in relation to failure rates of distribution
transformers in service.
The apparent significance of length of service which emerges
from Fig. 1 is dealt with more directly in Table 2 and Fig. 2.
There is clearly an upturn in the incidence of unsatisfactory
IR after about 8 to 10 years of service, and this is followed
by a more marked upturn in failure on power frequency tests
at about 13 to 15 years of service. The incidence of visible
deterioration of internal components, as revealed by
inspection, begins to appear significant after only a few years
of service, and rises steadily thereafter. Because of the small
numbers involved, the further subdivision of fault rates into
"moisture-related'' and "purely electrical" fault rates is
somewhat less reliable, but is indicated in different ways at
the bottom of Table 2 and in Fig. 2.
The failure rates of the tested arresters are subdivided in
Table 3 by manufaturer. As is evident, most of the arresters
were from manufacturers 2 (ZO), 3 (ZOO), 6 (22) and 7 (107).
There are clear differences in the results for these four
manufacturers' arresters.
Some of the differences are
attributable to different ages of the arresters, eg. in respect of
TABLE 2
FAILURE RATES / LENGTH OF SERVICE
0 to 6
Years
(1992-1986)
7 to 12
Years
(1985-1980)
More than
13 Years
(1979-1957)
Total
119
157
70
Low IR
Failied PF test(s)
Failed LISO.
No. seal-tested
Failed seal test
No. inspected
Failed iiispection
Moisture-related
Electrical
7 (6%)
4 (3%)
24 (20%)
39
34 (87%)
39
10 (26%)
9 (23%)
1 (3%)
40 (25%)
5 (3%)
35 (22%)
74
64 (86%)
73
37 (51%)
33 (45%)
4 (6%)
21 (30%)
23 (33%)
4 (6%)
36
34 (94%)
43
38 (88%)
32 (74%)
6 (14%)
1793
TABLE 3
FAILURE RATES / MANUFACTURER
Manufacturer
1
2
3
4 5
Total
4
20
200
1 11 22
107
Median length
of service (yrs)
Low IR
22
3
Failed PI? test@)
3
11
44
(41%)
5
0
No. seal-tested
Failed seal test
3
2
6
8
(4 %)
15
(8%)
62
(31%)
64
1 28 12
1 1 7
(32%)
0 5 0
Failed LIS0
22
7
(35%)
9
(45%)
0
No. inspected
Failed inspection
3
3
Moisture-related
2
Electrical
1
12
12
(100%)
13
13
(100%)
10
(77%)
3
(23%)
59
6
0
(1
5 10
5 10
(100%)
7 10
7 1
(10%)
5 1
(100%)
2 0
0 3
1
0
(92%)
1
64
27
0
(42%)
0
17
(63%)
0
10
(37%)
Comparisons were made of test results for arresters from
electricity authorities in different climates, but with somewhiit
similar median lengths of service. These showed that,
although there appeared to be a small effect on seal failure
rates for service periods of about 7 years, for longer periods
of service the effects due to length of service were much
greater than any effect due to climate. This suggests that
replacement of arresters on the basis of age would be a
cost-effective strategy, and that the age criterion need not be
different for different areas.
7
(5%)
1
It is of interest to attempt comparisons between the above
data for Australian arresters and those for North American
arresters reported in [21 and [3, 41 from studies conducted
about fifteen years ago. The Ontario-Hydro results [2] were
for failed arresters, the Detroit Edison resulls [3] were for inservice arresters withdrawn and inspected (primarily to
determine the magnitude of lightning discharge currents) and
for failed arresters inspected to identify the cause of failure
[4]. In all of the studies, moisture ingress was the dominant
cause of arrester degradation or failure.
The current
Australian investigation is i n accord with this - 74 (about
48% of the 155) arresters opened for inspection (Table 2)
showed evidence of moisture-related degradation (internal
moisture and/or clear moisture effects such as verdigris or
severe corrosion was evident in about one-third of these, ie.
about 16% of the 155). It would be fair to suggest that these
percentages are biassed upwards, because the inspected
arresters were mainly opened because they had already
exhibited unsatisfactory electrical and/or seal test results. [f
the moisture-related degradation and the moisture-evident
numbers are expressed as a percentage of the whole sample
of arresters included in the project (365), then the percentages
of 20% and 7% agree well with the two Detroit-Edison
studies [4, 31. Block damage such as surface flashover and
block puncture or rupture was also evident in small but
58
48
(83%)
62
45
(73%)
40
(89%)
5
(11%)
insulation resistance, arresters of make 3 had a low IR failure
rate (4%) compared to 32 to 41% for makes 2, 6 and 7, but
then the median age of make 3 arresters was 6 years
compared to 11 to 22 for the others. The same type of
comment can be made (but less clearly so) in respect of the
results of power frequency tests. Reference has already been
made to the fact that most of the arresters with high lightning
impulse sparkover were of one make (3, with a failure rate of
31%). The seal failure rates of all arresters subjected to seal
tests were uniformly high among nearly all the makes. The
inspection results of the four dominant makes were mostly
similar - high percentages (63 to 100%)showing evidence of
moisture ingress and significant percentages (1 1 to 3 1%)
exhibiting electrical damage.
A
-
119 arresters, 0-6 years;
- 157 arresters; 7-12 years;
C - 70 arresters, more than 13 years
B
100% I
75%
50%
25%
0Yo
50 Hz Test@)
IS0 Test
E l A
IR Test
m
Seal Test
B
FIG. 2 Test Failure Rate vs Years Service
Inspection
m C
1794
significant percentages of North American and Australian
arresters. The current investigation's percentage is 5.7% of
the 365 arresters (13.5% of the inspected arresters) - figures
which are comparable to the 5.9% for the Ontario-Hydro
failed arresters [2] but low compared to the 40% for the
Detroit-Edison failed arresters [4]. It is of interest to note
that both types of block damage (surface flashover and
puncture or rupture) were also caused by multipulse operating
duty cycle tests conducted on similar arresters in the
laboratory [ 5 ] . Finally, both the Ontario-Hydro and the
Detroit-Edison studies reported some occurrences of housing
flashovers attributable to external contamination. In the
current Australian investigation, 9 of the 365 arresters showed
external flashover marks - 8 were associated with one
particular make of arrester with a relatively small length of
porcelain between the line terminal and the (earthed)
mounting bracket .
VI. REFERENCES
[ 1J M. Darveniza and D.R. Mercer, "Service Performance of
Distribution Lightning Arresters and Transformers",
Elect. Eng. Trans. Inst. Eng. Aust., vol. EE2, Sept. 1966,
pp 97-112.
[2] M.V. Lat and J. Kortschinski, "Distribution Arrester
Research", IEEE Trans. Power Apparatus and Systems,
vol. PAS-100, no. 7, July 1981, pp 3496-3505.
[3] G.L. Gaibrois, "Lightning Current Magnitude Through
Distribution Arresters", ibid, vol. PAS-100, no. 3, March
1981, pp 964-970.
[4] G.L. Gaibrois, "Small-Block Distribution Arresters Win
In Field Performance", Transmission and Distribution,
vol. 32, no. 9, Sept. 1980, pp 36-40.
IV. CONCLUSIONS
(I) In a significant proportion of the modern silicon carbide
distribution arresters examined in this investigation,
deterioration of internal components begins after a few
years in service, and the proportion of arresters so
affected increases steadily with increasing length of
service.
(2) After about IO years of service, there is a marked
upturn in the number of arresters with unsatisfactory IR.
(3) After about 13 years of service, there is also a very
marked upturn in the number of arresters with reduced
power frequency sparkover level.
(4) Evidence of moisture ingress has been found, but much
less frequently than in the earlier work reported in [I],
suggesting that arrester seals have improved, but not
sufficiently to completely overcome the problem of
moisture ingress.
( 5 ) In about one-eighth of significantly deteriorated arresters,
inspection of internal components reveals damage by
high currents andlor multipulse currents, without
indications of moisture ingress.
(6) No single test gives a comprehensive evaluation of the
condition of a recovered gapped silicon carbide arrester.
One of the conclusions reported in the recent work on
the effect of multipulse lightning strokes on arresters was
that conventional tests were not particularly sensitive to
some forms of deterioration within arresters.
(7) Most gapped silicon carbide distribution lightning
arresters of age in excess of 13 to 15 years have
characteristics which render them no longer suitable for
service on a distribution system.
Recommendation - It is recommended that all silicon carbide
distribution arresters older than 13 years be progressively
replaced by modern metal oxide arresters.
V. ACKNOWLEDGEMENT
The authors grateful acknowledge the contributions of the 8
Australian electricity utilities who participated in this project.
[ 5 ] M. Darveniza and D.R. Mercer, "Laboratory Studies of
the Effects of Multipulse Lightning Currents on
Distribution Surge Arresters", IEEE Trans. Power
Delivery, vol. 8, no. 3, July 1993, pp 1035-1044.
[6] A.S. 1307-1986, "Surge Arresters, Part 1 - Silicon
Carbide Type for A.C. Systems", Standards Australia,
North Sydney, Australia 2060.
Mat Darveniza (F1979) and Ron Watson work in the High
Voltage Laboratory in the Department of Electrical and
Computer Engineering at the University of Queensland.
Doug Mercer is a Honorary Research Consultant of the
University and was formerly with the Queensland Electricity
Supply Industry.
1'795
Discussion
further study of the tabulated results discloses the following:
From 160 arresters that were inspected following the leak test, 60%
failed and 40% passed the visual examination. But of the 17 thlat
The data presented in the paper very well supports the passed the leak test, 70.6% failed the visual examination. And of
130 that failed the leak test and were visually examined, 42% were
conclusions that allowing for different tolerances specified at
judged as passing the visual examination. Clearly the effectiveness
various times, the basic electrical characteristics of gapped of the leak test method used should be reconsidered.
silicon carbide distribution class arresters have changed little
from those manufactured in the 1930s up to early 1990s.
Arresters are usually factory sealed at atmospheric pressure with
Insulation Resistance : This test appears to be the main in- controlled gasket compression As the arresters age in service, the
service test done by most Utilities. Looking at the results of elastomeric gaskets may assume some permanent deformation,
the IR (Megger) tests done on the healthiest batch of relatively unable to assume their original thickness should the mechanical
young arresters (median age 9 years), it can be noted that it loading be relaxed Subjecting the arresters to a vacuum causes the
end sealing hardware to be loaded in an unintended manner gave wrong results 40% of the time when it indicated a failure, causing loss of some of the compressive force Perhaps some of
and inlerestingly wrong again 55% of the time when it these arresters became "leakers" during the lea6 test, such as the 2 1
indicated a pass! This is about a precise as flipping a coin!
of line 2 and 16 of line 5 of the first block , and the 17 of line 2
With older arresters, it appears 100% correct when it indicates of the third block A sparkover test before and after the vacuum
a failure, but unfortunately 35-80%0 wrong when it indicates a test may indicate if the arrester gasket opened and then resealled
pass! It appears that when testing older arresters IR can pick after the seal test Did the authors consider such procedure to
validate the vacuum leak test method used?
up the stronger fail signal, but it not a good pass predictor.
Lightniing Impulse Sparkover : This test has predicted failure
Measurement of intemal ioni~ationor radio mflueiice voltage (RI'V)
correctly only 35% of time. It also appears to predict pass
is used by many as a repeatable and inexpensive test to detect gap
results correctly only 10-40% of the time. This test is degradation Did the authors consider this method as an alternate
definitely the least reliable predictor.
to impulse sparkover?
Power Freqnency Sparkover : Manufacturers consider this
the better predictive test. When this test indicated a failure it The described power frequency test includes a one minute voltage
was about 90% valid. However if a pass was predicted, it was withstand test near the minnnum sparkover voltage lebel Tlirs
correct about 40-65% of the time. Hence only the "pass" voltage is several times the normally expecred system Iine-toresults from a PF test must be treated with some caution, ground voltage, which can overstress any non-linear resistors used
mainly because this test is not able to pick up damage to S i c for gap grading Did the authors notice any evidence of gap
grading resistor damage caused by this withstand test7
blocks <andis not always sensitive to moisture ingress.
Sealing Reliability : In 1987 AS 1307.2, written for the new Answers to the above questions may serve to reinforce the correct
gapless Metal Oxide types, established a more rigorous sealing conclusions reached by the authors
type test that was in effect applied to gapped silicon carbide
types still being made at the time. Unfortunately, the data
shows 89% of the 100 relatively young distribution class Manuscript received February 15, 1996.
arresters tested were unable to demonstrate gas tightness. We
may conclude that compliance with the 1987 sealing type test
has no1 in itself been sufficient to guarantee any worthwhile
degree of seal reliability. However, it must be said that not all
porcelain insulated designs on the market, particularly the
Lambert Pierrat, Senior Member, IEEE (General Technical
station class types, have unreliable seals.
Division,
Electricit6 de France, 37 rue Diderot, 38040 Grenolble
Modern gapless metal oxide polymeric distribution arresters
tend to have designs which eliminate intemal air volume. Cedex, France). Fabrice Perrot, Non-member (EA Technology
They are expected to have much better long term sealing Ltd, Technical Business & Services, Capenhurst, Chester CH1
performance and far more stable electrical characteristics.
6ES, U.K.). The data collected and analysed by the authors
present a valuabie piece of information. Effectively, the field
experience for gapped Sic surge arresters can also be used for
other network components, particularly for gapless ZnO surge
arresters. This discussion concerns firstly the arresters failure
modes which are partly connected to their engineering technology
John B Posey (Westfield Center, Of3) As the authors have stated,
and secondly the estimation of the residual failure risk of
multiple tests should improve the diagnosis However, comparison
components in service.
of the iesults of leak testing and visual examination shows little
correlation suggesting a reexamination of the leak test procedure
1) - Arrester failure modes : The authors determined that the
may be appropriate
predominent cause of the arresters failure wits due to inadequate
seal performance leading to moisture ingress and therefore
Since the text describing results of testing and examining the third
deterioration of the arrester active components. Damage thought
block of 66 arresters does not agree with the tabulated values.
David lioby (ABB Power Transmission, Australia 2170) :
1796
to be the result of high current and/or multiple currents was also
reported.
a) - Could the authors comment on whether ferro-resonance overvoltages could have been involved in some cases of arrester
failure due to excessive current absorption; especially when one
of the utilities mentionned disproportionally high arrester failure
rate at overhead line / underground cable junctions?
b) - The authors have conducted 4 types of electrical test, a seal
test and a destructive examination. The seal test was conducted
under vacuum. The authors postulate that arresters with
satisfactory insulation resistance must necessarily have sound
seals. Surely this is not quite the case, as the results of the
investigation showed that a high percentage of the arresters which
failed the seal test did not necessarily failed the insulation
resistance test. Should a test involving the immersion of the
arrester in a liquid for a temperature cycle be more appropriate
and closer to field conditions in order to determine the
mechanical performance of the seal under mechanical stress?
c) - The authors report the striking lack of association between
failure on the impulse sparkover test and failure on other
electrical tests. We are qestionning the relevance of the impulse
sparkover test due to the known erratic behaviour of spark gap
systems under impulse conditions after a few years in service.
Can the authors comment on this issue?
d) - We believe that most of the arresters tested in this study were
of the porcelain housing type. Did the authors test any polymeric
housed gapped Sic arresters and if yes could they comment on
them?
2) - Estimation of the residual failure risk : The authors
recommend that all gapped Sic distribution arresters older than
13 years be progressively replaced by modern metal oxide
arresters. It is possible to analyse the failure risk of the whole of
the arresters by considering firstly the age distribution of the
arresters and secondly their failure distribution as a function of
age. The age distribution is approximately exponential over an
average value of the order of 2 p 7 years (see data on Table 1).
The estimation of the rate of failure in service is based upon the
data shown in tables 2 & 3 and figure 2. The results of the
arrester examination show that the increase of the failure rate is
linear; which correspond approximately to a Weibull distribution.
(shape parameter P=2 and characteristic life duration : 8 p 7
years). Consequently, the Mean Time To Failure (MTTF) of the
arresters would be in the order of &/2.z0.e0=43 years. This
value could appear to be high, but one must consider its
important dispersion associated (coefficient of variation cv > 1).
The residual risk of failure of the arresters is approximately given
by a distribution of Weibull (shape parameter P < 1; characteristic
life duration = 43years). For 13 years old components (value
recommended by the authors) the failure risk would be of the
order of 30%. If the dubious components were not taken out of
service, the rate of failure of the arresters would decrease with
time. However, this would correspond to a progressive extinction
of the number of arresters after all their failure: obviously, this
catastrophic hypothesis is not realistic and we therefore agree
with the recommendations formulated by the authors. The
removal of 15 years old arresters from service should stabilise the
arresters failure rate to acceptable levels. We thank the authors
for their contribution and their answers to our questions and
comments.
Manuscript received February 26, 1996.
.Darveniza, D.R. Mercer:
The authors are grateful for the widespread interest shown in
our paper by Utilities in many countries and for the
contributions from the discussers.
David Roby’s points highlight the fact that an assessment of
the condition of anything (be it a person by a doctor, or an item
of electrical equipment by a test engineer) requires the use of
multiple diagnostic tools (tests). It is always unreliable to
make an assessment from the results of any one diagnostic test.
It is always best to use the results from as many tests a s i s
practicable.
John Posey (and Lambert Pierrat and Fabrice Perrot)
questioned the value of the impulse sparkover test. We have
sympathy for this, but included it in our set of diagnostic tools
precisely because the impulse sparkover voltage is a direct
measure of a gapped arrester’s capacity to limit lightning
overvoltage and so to protect equipment. In our earlier studies,
we did use partial discharge (PD) tests (which are equivalent to
RIV tests) and found them to be sensitive to the presence of
both internal moisture and of damaged internal components.
We did not persist with PD tests because we believed that our
other diagnostic tools (particularly IR and PF tests) were also
adequate in identifying arresters with those deficiencies.
All the discussers commented on the deleterious influence of
intemal moisture on arresters and noted the importance of an
adequate seal test as a diagnostic tool. We agree that
subjecting a distribution arrester to a full (external) vacuum is a
harsh test of the seal. But we do not believe it to be
unrealistically harsh, as it has been our experience that most
new arresters pass such a test (of course we did encounter a
(very) small number of new arresters with faulty seals and
believe that these either escaped notice during factory tests or
were physically damaged later). There are of course other
ways for testing the seals and we make no claim that our
method is the “best” - the matter of importance is that an
adequate seal test must be included in the set of diagnostic
tools.
Some other specific questions were also asked by the
discussers; to Mr Povey - we did not observe damage to
grading resistors attributable to the PF tests; to Messrs Pierrat
and Perrot - the only gapped S i c arresters available to us had
1'797
porcelain housings, and yes we think ferro-resonance could be
the cause of arrester failures at overhead linehnderground
cable junctions (though we have not made a specific study of
this).
Finally., we thank Messrs Pierrat and Perrot for adopting a
more systematic statistical approach to the problem of arrester
reliability. It has been our experience that Weibull statistics
provide a formidable tool for analysing the failure risk of
system components as they age and approach the end of their
useful life. Colleagues at our University have successful used
Weibull statistics in a number of such applications, but we did
not do so for distribution arresters. We are grateful that Messrs
Pierrat and Perrot have enhanced the value of our results by
illustrating the applicability of the Weibull distribution to the
analysis of the failure risk of aged arresters.
Finally, we would comment that the next phase of our work
will address the reliability of polymeric - lhoused metal oxide
arresters. This work has begun and is being carried out in two
strands (i) identification of failure modes by laboratory tests,
and (ii) examination of arresters which have failed or have
become degraded in service. The latter requires (and has)
active collaboration by electricity utilities in Australia. We
would also like to hear from utilities in overseas countries with
failure experiences of polymer-housed metal oxide distribution
arresters.
Manuscript received April 1, 1996.