Development of Composite Insulators for Overhead Lines(Part 2)by Satoshi Kobayashi * , Yutaka Matsuzaki * , Yoshihiro Arashitani *2 and Ryuzo Kimata *Furukawa Electric's composite insulators are characterized by a hermeticallysealed structure made by integral molding with a silicone rubber sheath that covers up to the end fittings. Exposure tests on full-size insulators were carried out in Okinawa at 77
kV over a 5-year period to evaluate them under the usage conditions prevailing in Japan. No
deterioration was observed, nor was any deterioration observed in accelerated aging tests carried out on full-size insulators in accordance with IEC 61109 Annex C, demonstrating ample reliability. Service life of the composite insulators was estimated by charged exposure tests andaccelerated aging tests with leakage current as the parameter. A figure of not less than 34 years
was arrived at as the period within which slight deterioration would occur under the severe seasalt contamination occurring in Okinawa, thereby confirming a sufficiently long period of resistance.

ABSTRACT1.

INTRODUCTION

The materials used in insulators for overhead transmission lines have been ceramics and glass. Both offer good
insulation characteristics and excellent weather resistance, but they also have several disadvantages--they areheavy, have poor resistance to impact, and suffer major
deterioration in terms of the voltage withstand characteristics under contamination. To overcome these problems
insulators have been proposed and adopted having acomposite structure--a sheath of silicone rubber, which is
weather resistant and has excellent voltage withstandcharacteristics when contaminated, and a core of glass
fiber reinforced plastic (FRP) to bear the tensile force.
They have not been in general use in Japan, with the
exception of special applications such as interphase spacers or experimentally for purposes of research.
Interest in composite insulators has, however, increased
in recent years as they become more widely used overseas and social conditions favor attempts to reduce costs.
In response the Electric Technology Research Association
has established an investigative committee with a view to
conducting specialized studies, and its report 1), while recognizing the cost advantages realized by the use of composite insulators overseas and offering no objection totheir use, points out that no method has been establishedfor estimating their service life.
Since the development of Furukawa Electric's composite insulators, which are characterized by a watertight end* Engineering Dept., Bare Wire Div.
*2 Furukawa Electric Institute of Technology

fitting structure, research has brought about successive
improvements in their electrical and mechanical reliability.
Part 1 of this paper described the process of developing
composite insulators for overhead transmission lines andthe results of performance tests 2), and in Part 2 we reporton our on studies of their long-term reliability.

2.

STRUCTURE OF COMPOSITE INSULATORS

Composite insulators comprise a core material, end fittings and an outer sheath of rubber. The core is made of
FRP to provide mechanical strength against tensile force.
The end fittings transmit tension from the lines to thetower and are made of forged steel, malleable cast iron or
aluminum alloy. The outer sheath is made of silicone rubber, which offers superior electrical insulation properties
and weather resistance. The silicone rubber used in the
sheaths is particularly high in tearing strength so that it will
not break when damaged during construction or otherwise. The end fittings and the FRP are joined by compression. Since electrical fields are concentrated at the tips ofthe end fittings making corona discharge more likely, thedesign is optimized by electrical field calculation to relax
field stress. Once the end fitting has been compression
joined to the FRP, the silicone rubber sheath is applied
over the end fitting to form an integral unit. In a structurethat is unique to Furukawa Electric, surface finishing isapplied to the end fittings to improve their adhesion to therubber, and the boding length is sufficient to prevent theentry of moisture from the end fittings along the FRP interface. Figure 1 shows the structure of a typical composite
insulator.

56

End fitting

Figure 1

3.

Silicone rubber

FRP rod

Structure of composite insulator.

LONG-TERM RELIABILITY OF COMPOSITE INSULATORS

Composite insulators for overhead lines typically are used
in a severe natural environment and are constantly subjected to tensile loads and electrical stress. Thus it isessential that they have mechanical properties such thatthe joints between the end fittings and the FRP have longterm integrity, without slippage. Electrically it is essentialthat they withstand problems associated with organic
materials, involving erosion due to dry-band localized arcing and corona discharge occurring under conditions ofcontamination or humidity.
3.1 Deterioration Phenomena
The factors influencing deterioration of composite insulators may be electrical, mechanical or a combination. The
main ones are as follows:
Electrical factors include tracking, erosion, puncture ofsheds, cracking, etc., while mechanical factors include
long-term degradation of tensile strength and degradationof strength due to repetitive bending and twisting.
As combination factors we may note brittle fracture, inwhich the glass fiber of the FRP core becomes corrodedby acid so that it fractures under comparatively low levelsof strain. It is thought that brittle fracture is caused by nitricacid that is generated when there are problems at theinterface between the end fittings and the silicone rubber
outer sheath, resulting in corona or partial discharge at
those points where moisture penetrates to the FRP. In thisrespect Furukawa Electric's composite insulators have ahermetically sealed structure in which the silicone rubber
sheath is molded to cover the interface between the fittings and the FRP, to improve reliability against brittle
fracture.
3.2 Methodology for Evaluating Reliability
There are currently a large number of organizations doing
research on the phenomenon of the deterioration of theouter sheath in composite insulators. Among internationally recognized standards, IEC 61109 Annex C establishesa method for accelerated aging tests by means of environmental stress, and IEC 60587 and IEC 61302 give tests
for specific outer sheath materials.
Since long-term sheath deterioration varies with environmental stress, the ideal method of determining the reliability of composite insulators would be by prolonged
exposure to the usage environment, but much time andmoney would have to be expended before a result could
be obtained. Accordingly evaluation is accomplished by

57

concentrating and accelerating the environmental stressfactors impacting deterioration. Silicone rubber--the mostcommonly used material for the outer sheaths--is characterized by outstanding properties of weather resistance
together with a level of hydrophobicity that gives high voltage withstand characteristics under contamination. In
addition, if hydrophobicity declines it recovers again, complicating the evaluation.
For this reason it is appropriate to estimate service life
by conducting accelerated tests, while comparing usage
conditions. Based on this philosophy, Furukawa Electrichas proceeded to evaluate the reliability of composite
insulators by conducting accelerated aging tests, as
described below, at the Okinawa Charged Exposure Test
Site.

4.

RELIABILITY EVALUATION TESTS

4.1 Measuring Leakage Current
The deterioration that most generally affects composite
insulator with a silicone rubber outer sheath of suitablemechanical design is caused by flows of leakage current
on the surface in contaminated environments and by theerosion resulting from thermal and electrical factors. As
erosion proceeds the silicone rubber sheath becomes corroded exposing the FRP, and this can lead to insulation
breakdown and brittle fracture. It is thought that leakage
current is the most suitable parameter by which to evaluate this erosion deterioration.
To obtain a clear picture of erosion deterioration by
long-term reliability tests, the leakage current resulting
from dry-band localized arc discharge was classified interms of waveform characteristics using the method
described below and designated as intermittent current,
and this was distinguished from continuous current, whichis the resistance component current 3).
(1) Definition of continuous current (resistance component current): A current which continues for a period
longer than one sine-wave current cycle and of which
the duration below the threshold value at the zerocross-over is 3 msec or less (Figure 2).
(2) Definition of intermittent current (dry-band localized
arcing current): A current which continues above thethreshold level for 1 msec or more, and of which theduration below the threshold value at the zero crossover is 3 msec or more (Figure 3).
4.2 Exposure Testing 4)~6)
Since 1995 77-kV line-to-ground charged exposure tests
have been carried out at a test site located approximately
300 meters from the seacoast in Okinawa Prefecture, a
region of high temperatures, severe salt contaminationand comparatively frequent typhoons, providing continuous monitoring of the contamination and deterioration
properties of the composite insulators.

Furukawa Review, No. 21

2002

(1) Insulators tested:
In order to investigate the influence of unit charge
stress (V/mm), an important parameter in the withstand-voltage design against contamination, we madeuse of silicone rubber composite insulators having thesame structure but different values of surface leakage
distance. Porcelain insulators were also tested forcomparison. Table 1 shows the specifications of theinsulators tested.
(2) Results of observation:(a) External visual inspection: After 5 years of testing
the composite insulators showed satisfactory characteristics, and there was no observable erosion or
tracking deterioration despite the severe levels of salt
contamination at the test site.
(b) Equivalent salt deposition density: ESDD was
measured by absorbing 200 ml of distilled water into a
dust-free wipe Bencot and wiping down the surface ofthe composite insulator.

>1 cycle

Measured threshold level

Measurements at 1-month intervals over the courseof four years shows that the 50% cumulative ESDD
was not particularly high--0.015 mg/cm2--but maximum values in the aftermath of typhoons reached ashigh as 0.30 mg/cm2 This shows that the test site suffers from severe contamination under the abnormal
conditions presented by typhoons.
Figure 4 shows the results of investigations of volumetric trends in contamination of composite insulators. It will be noted that the values of ESDD measured for samples exposed for 1, 3, 6 and 32 months
show a tendency to saturate, demonstrating that
accumulation of contaminants on the surface of thesilicone rubber sheath does not occur.
(c) Hydrophobicity: Hydrophobicity was measured
every year by cutting out of the same insulator a
specimen 5 mm square, and using a contact angle
gauge, taking as the static contact angle the contact
angle obtained by dripping 4 µl of distilled water for 1
minute, then applying 8 µl of distilled water, and taking as the receding contact angle the contact angle 1
minute after 8 µl suction.
Table 2 shows the results of an investigation of thecontact angle of water droplets at the surface of theouter sheath as a measure of the hydrophobicity of
the insulators tested. For all of the test samples thestatic contact angle showed values higher than theinitial hydrophobicity. The receding contact angle, on
the other hand, although on occasion higher than theinitial value, showed results that, depending on thesample, were as much as 14 degrees lower than the<3 msec
0.1= ESDD= average ESDD

Definition of continuous current.
ESDD (mg/cm2)

Figure 2

0.01

0.001
0

5

10

15

20

25

30

35

Period of exposure (months)

Figure 4

Measured thresholdlevel>1 msec

Figure 3>3 msec

Table 2

Definition of intermittent current.
SampleTable 1

Specifications of sampleinsulators in exposure tests.

Sample Section length Effective distance Number of sheds Electrical stress

ESDD during exposure test.

Surface hydrophobicity of test samples.

Static contact angle (deg.)

Receding contact angle (deg.)

Period of exposure (months)

Period of exposure (months)

29

41

53

29

41

53

A

128.0

130.4

132.9

103.8

94.9

97.8

A

1359 mm1107 mm31

24.2 V/mm

B

112.3

123.1

114.7

79.1

88.1

74.0

B

1488 mm1237 mm35

21.5 V/mm

C

121.1

130.8

136.8

88.9

99.4

102.9

C

1618 mm

1367 mm

39

19.4 V/mm

D

112.0

105.9

111.6

82.4

70.9

68.5

D

1813 mm

1560 mm45

16.9 V/mm

Original

99.6

82.0

58

Four-year cumulative current charge at test site.
Cumulative current charge

Samplex / (x + y)Intermittent (x)Continuous (y)A

0.81 mAh

55.20 mAh

1.4%

B

2.26 mAh

62.05 mAh

3.5%

C

0.36 mAh

49.85 mAh

0.7%

D0.64 mAh

38.69 mAh

1.6%

Table 4

Specifications of sample insulators in accelerated
aging tests.

Sample Section length Effective distance Number of sheds Electrical stressA

1359 mm1107 mm31

24.2 V/mm

E1748 mm

1495 mm

43

17.6 V/mm

initial value. Furthermore the year-to-year changes inhydrophobicity were not only declines; recovery also
occurred. Examining the effects of electrical stress,
we may say that there is no correlation with hydrophobicity.
(d) Leakage current: Table 3 shows cumulative
results for leakage current charge over a period of 4
years from 1995, classified for continuous and intermittent current. Almost all of the leakage current
occurring at this exposure test site was of the continuous type, with only a few percent consisting of theintermittent current that is thought to influence sheath
deterioration in composite insulators. Furthermore,
the three typhoons that passed through the area during the test period accounted for approximately 50%of the intermittent current charge and about 95% of
the continuous current charge. Thus we can state that
virtually all instances of leakage current flow at thetest site were concentrated during typhoon periods.
Despite the reversal of continuous current charge
between the A and B samples, Table 3 reveals aloose correlation with charge stress. For intermittent
current, on the other hand, no correlation wasobserved between charge stress and electrical charge
within the range of stress examined.
4.3 Accelerated Aging Tests 4), 5)
Equipment was constructed capable of accelerated aging
tests as stipulated in IEC 61109 on samples of the same
size and at the same voltage (77 kV AC to ground) as arebeing tested at the Okinawa site, and tests were conducted 4).
(1) Test methodology:
The test conditions were as follows:• Charge voltage:
77 kV AC (voltage to ground)• Test chamber dimensions:
4.4 x 4.4 x 3.3(ht) m• Tension:
20 kN
Since these tests involve continuing the daily cycle

59

Cumulative intermittent current charge (Ah)Table 30.01

sample A-1
sample A-2
sample A-3
sample A-4
sample A-5
sample A-6
sample E-1
sample E-2

0.009
0.008
0.007
0.006
0.005
0.004
0.003
0.002
0.001
0
0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

Test period (hr)Figure 5

Transition of intermittent current.

shown in IEC 61109 Annex C for 5000 hr, a considerable
time is required. To make the conditions even more rigorous, tests without rainfall cycles were also conducted toeliminate the washing effect of rainfall in the test cycle ofAnnex C.
(2) Insulators tested:
Table 4 shows the specifications of the test samples.
The insulators used for the regular accelerated test
cycle were A samples, which had the most severe
stress at the Okinawa test site, and the E samples forhigh-contamination applications. For the test without
rainfall, only A samples were used.
(3) Test results:(a) General: It has been reported in earlier work 1), 2)
that no erosion or tracking deterioration wasobserved. In the present research we carried out atest specific to the test apparatus used, applying atension of 20 kN, as a means of verifying the watertightness of the end-fittings, which has an impact onbrittle fracture, even though no such test is provided
for in IEC 61109 Annex C. However no peeling orother problems with the end fittings occurred.
Tests were also conducted on insulators (identicalto the A samples) on which scratches 20 mm in lengthand 1 mm in depth were inscribed reaching to thecore at right angles to the length of the insulator at theend of the outer sheath and around the body, but noadvance of tracking or erosion at the scratches wasobserved.
In the test which omitted the rainfall portion of theartificial weather cycle as a means of increasing theseverity of the conditions, erosion did occur on theouter sheath of the composite insulators. This suggests that the washing action of the rainfall in IEC
61109 Annex C lightens the deterioration conditions
with respect to the insulators.
(b) Leakage current: Table 5 shows the time courseof change in the intermittent leakage current chargeduring accelerated aging tests. Major deviations canbe observed in the leakage current charge, and it canbe seen that even after the elapse of 5000 hr there

Furukawa Review, No. 21

2002

Table 5

Measurements of cumulative charge. (after 5000 hr)
Cumulative current charge

Test condition
Rainfall excluded

x / (x + y)Intermittent (x)Continuous (y)41.47 mAh

0.81 mAh

97.9%

are some insulators in which hardly any leakage current occurs. Further virtually no intermittent leakage
current occurred in the initial stages of the test,
demonstrating the sound hydrophobic properties of
the composite insulators. After the elapse of a further
period there was a tendency toward sudden flows ofintermittent leakage current, and considerable difference in onset times was seen among samples. Thisshows that because these tests were conducted using
actual insulators there were individual differences, inthat the drop in hydrophobicity in the length directionof the insulators could hardly be expected to occur
generally within the same test chamber, and may be
attributed to the nature of accelerated aging tests
using full-size insulators.
(c) Erosion in intermittent leakage current: Table 5shows the leakage current charge for samples showing erosion in the test omitting rainfall, measured after
5000 hr, and Photo 1 shows the occurrence of erosion
The process by which erosion occurred in the test
omitting rainfall was as follows: First of all, erosion
was found by visual observation at two points on thecenter portion of the insulator sheds 2550 hr after thestart of the test, at which time the intermittent current
charge was 18.98 mAh. When the test was then continued to 5000 hr the number of points of erosion
increased to five and the intermittent current charge
was 2.2 times greater, or 41.47 mAh 5). This showedthat erosion deterioration of actual insulators is notnecessarily concentrated at a single point, but occurs
in a random manner.
Furthermore, since no erosion was found during
four years of testing at the Okinawa site despite thefact that the continuous current charge accumulated
was in excess of 60 mAh (more than three times
greater than the intermittent current charge of about
19 mAh at which erosion was confirmed in the accelerated aging test), we may conclude that continuous
current charge does not contribute significantly to erosion. 2)

5.

ESTIMATING SERVICE LIFE OF COMPOSITE INSULATORS

Since erosion was confirmed in the accelerated aging test
conducted without rainfall, estimates were made of theservice life of the silicone rubber outer sheath from theresults of measurements at the Okinawa test site and of
the accelerated aging tests.

Photo 1

Occurrence of erosion.

The estimates were based on the following assumptions:• That since it was the intermittent current charge that
impacted erosion of the silicone rubber sheath, the intermittent charge current values measured at the Okinawa
test site and in accelerated aging tests would be used;• That the cumulative intermittent current charge occurring up to the onset of composite insulator erosion were
the same for both test environments (Okinawa test site
and accelerated aging test).
• Assuming that final sheath life for use as an insulator
is the condition in which the transmission voltage can not
be withstood, the limitation by the IEC 61109 standard iswhen erosion reaches the core. At the present stage,
however, to be on the safe side, slight erosion of thesheath was considered to be the first stage of erosion.
Based on the above assumptions, a comparison wasmade between the intermittent current charge at the onsetof erosion in the accelerated aging test without rainfall(18.98 mAh) and the value obtained in the exposure testat the Okinawa site for B samples, which had the highest
intermittent current charge among the insulators tested(2.26 mAh). The number of years to the onset of erosion
under the conditions at the Okinawa test site were calculated according to the equation
18.98 mAh / (2.26 mAh / 4 yr) = 34 yr(1)

In this way it was estimated that the time required forslight erosion to occur on the surface of a composite insulator under the conditions of the passage of typhoons andrapid contamination such as can occur at the Okinawa
test site was 34 years.
This service life is the result of considering the point at
which erosion would occur on the surface of a composite
insulator. In actuality if, once erosion has begun on theinsulator surface, the silicone rubber sheath were to be
attacked by corrosion that penetrated to the core, thiswould be a fatal condition as far as a composite insulator
was concerned. In general the silicone rubber sheath of
the insulator has a thickness of 3 mm or more at the stem,so that it would require an even longer time for this fatal
condition to be reached.

60

6.

SUMMARY OF TEST RESULTS

In long-term charged exposure tests carried out over the
more than five years since 1995 in the severe salt-contamination of the Okinawa test site, Furukawa Electric's
composite insulators for phase spacer applications andtransmission applications exhibited no deterioration.
Furthermore, large-scale accelerated aging testing
equipment was constructed capable of combining theaccelerated aging test conditions and tension loads specified in IEC 61109 Annex C. Full-size composite insulators
were charged at 77 kV and were subjected both to theregular accelerated aging cycle and to a test cycle withthe washing effect of rainfall omitted. In the regular accelerated aging cycle there were no problems--no penetration of salt or moisture into the ends and no deteriorationof the silicone rubber outer sheath. In addition, to simulate
damage that could occur during construction, standard
accelerated aging tests were also conducted with the silicone rubber sheath cut back to expose the FRP. Due tothe superior bonding at the interface between the outer
sheath and FRP of Furukawa Electric's insulators, there
were no electrical problems due to the entry of moistureand it was confirmed that even if damage were to occur, it
would not immediately result in breakdown or failure.
Since it is possible in the course of the accelerated
aging test without rainfall to induce erosion artificially inthe center trunk portion of an insulator, such accelerated
aging tests may be considered effective not in confirming
the quality of composite insulators, but as a means of promoting deterioration. From the accelerated aging test without rainfall and the charged exposure test in Okinawa,
estimates of the service life of composite insulators using
the cumulative intermittent current charge occurring during
discharge as the parameter affecting insulator deterioration, results were obtained that give a figure of 34 years or
more--highly satisfactory for practical purposes--even in
an area of severe salt contamination like Okinawa.
These investigations, however, are based on comparative data assuming salt contamination, a major factor ininsulator design. Further investigation would need to bedone before these results could be applied to areas withspecial conditions such as high humidity or severe industrial pollution.

7.

transmission lines had been considered problematic, but it
has been confirmed that it has ample service life.

ACKNOWLEDGMENTS
The charged exposure tests in Okinawa and the accelerated aging tests were carried out at the behest of theTokyo Electric Power Company as part of research on
evaluating the service life of organic interphase spacers.
We are grateful for the cooperation of Prof. Higashimori
and Mr. Iha of the Dept. of Electrical Engineering, Facultyof Engineering, Ryukyu University in obtaining measurements for the charged exposure tests. We would also liketo express our thanks to others too numerous to mention.

RFERENCES
1) The present situation and future perspective of organic insulatorsfor overhead transmission lines, Electrical Cooperative Research,
No. 1, Vol. 56, May 2000 (in Japanese)2) Kobayashi et al.: Development of Composite Insulators forOverhead Lines (Part 1), Furukawa Review, No. 19, April 2000
3) R. Kimata, L. Kalocsai, A. Bognar: Monitoring system for evaluation of leakage current on composite insulators, 4th International
Conference on Properties and Applications of DielectricMaterials, No.5125, 1994
4) Nakauchi et al.: Exposure tests and accelerated aging tests for silicone rubber insulators, High Voltage Research Group, IEEJ, HV97-41,1997 (in Japanese)5) Sumitani et al.: Comparison of IEC61109 Annex C tests and exposure tests for silicone rubber insulators, Proceedings of IEEJ, Vol
7-051,2000 (in Japanese)6) Nakauchi et al.: Study on the contamination of silicone rubber
insulators, High Voltage Research Group, IEEJ, HV-98-73,1998 (inJapanese)
7) A. Bognar, P. Szaplonczay, Pz. Pinter, G. Banhegyi: Investigations
on a 20 years old composite insulator, 10th ISH, 1997

Manuscript received on April 28, 2001.

CONCLUSION

Because of thoroughgoing quality control and a uniquewatertight structure for the end fittings, Furukawa
Electric's composite insulators have exhibited high reliability in electric power transmission lines. They have beenused without problems overseas for over 20 years as suspension insulators 7), and in Japan for 10 years as interphase spacers and about four years as strain insulators.
This paper has reported on two types of tests carriedout on composite insulators: charged exposure tests at atest facility in Okinawa Prefecture and accelerated aging
tests conducted indoors. The use of silicone rubber in61

Furukawa Review, No. 21

2002