Development of Composite Insulators for Overhead Lines
by Satoshi Kobayashi *, Yutaka Matsuzaki *, Hiroshi Masuya *2,
Yoshihiro Arashitani *3 and Ryuzo Kimata *The United States is leading a world-wide trend toward replacing ceramic andglass insulators with composite insulators that are lighter in weight, as well as
being superior in pollution withstand voltage characteristics and resistance to impact (to countervandalism). In the early 1990s, Furukawa Electric developed composite insulators for use as
66/77-kV inter-phase spacers, and subsequently has extended applications to include 275-kV
inter-phase spacer insulators and 154-kV and 66/77-kV class transmission line suspension insulators. The IEC, an international standards organization, has tested this kind of composite insulators for electrical and mechanical performance and has confirmed their reliability, but the use oforganic material as the insulating material has aroused reservations about long-term aging.
Accordingly work has been going forward on outdoor long-term loading cycle exposure tests andindoor accelerated aging tests.

ABSTRACT1.

INTRODUCTION

Overhead power transmission lines require both cables to
conduct the electricity and insulators to isolate the cablesfrom the steel towers by which they are supported. The
insulators have conventionally been made of ceramics or
glass. These materials have outstanding insulating properties and weather resistance, but have the disadvantagesof being heavy, easily fractured, and subject to degradation of their withstand voltage properties when polluted.
There was therefore a desire to develop insulators of a
new structure using new materials that would overcome
these drawbacks.
The 1930s and '40s saw the appearance of the first
insulators to replace inorganic materials with organic, butthese suffered problems of weather resistance, and theircharacteristics were unsatisfactory for outdoor use. In the
1950s epoxy resin insulators were developed, but they
were heavy, suffered from UV degradation and tracking,and were never put into actual service. By the mid-1970s
a number of new insulating materials had been developed, and the concept of a composite structure wasadvanced, with an insulator housing made of ethylene
propylene rubber (EPR), ethylene propylene diene methylene (EPDM) linkage, polytetrofluoro ethylene (PTFE), silicone rubber (SR) or the like, and a core of fiber-reinforced
plastic (FRP) to bear the tensile load.
Since these materials were new, however, there weremany technical difficulties that had to be remedied, suchas adhesion between materials and penetration of mois*

R & D Dept., Bare Wire & Cable Div.
Design Sec. Conductor Accessory Dept., Bare Wire & Cable
Div.
*3 2nd Development Sec., Hiratsuka Research Lab., R&D Div.
*2

ture, and the end-fittings, which transmit the load, had tobe improved. Since the 1980s, greater use has been
made of silicone rubber due to its weather resistance,
which is virtually permanent, and its hydrophobic properties, which allow improvement in the maximum withstandvoltage of pollution, and this had led to an explosive
increase in the use of composite insulators.1)
In 1980, Furukawa Electric was engaged in the development of inter-phase spacers to prevent galloping in powertransmission lines, and at that time developed composite
insulators that had the required light weight and flexibility.
In 1991 the first composite insulators having a silicone
rubber housing were used as inter-phase spacers for 66kV duty, and in 1994 their use was extended to 275-kV
service with a unit 7 m in length--the world's largest.
Thus as composite insulators have established a trackrecord in phase spacer applications and their advantages
have been recognized, greater consideration has been
given to using them as suspension insulators with a view
to cutting transportation costs, simplifying construction
work and reducing the cost of insulators in order to lower
the costs of laying and maintaining power transmission
lines.
Recently Furukawa Electric developed composite insulators for suspension and delivered, for the first time inJapan, 154-kV tension insulators and V-type suspension
insulator strings. Subsequently they were also used on a
trial basis as tension-suspension devices in 77-kV applications. Work is also under way on the development of composite insulators for 1500-V DC and 30-kV AC railway service.
This paper presents an overview of the process ofdeveloping composite insulators for suspension and tension, and of service life prediction.

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Table 1

1884
1640

Main specifications of composite insulators
H154-120-1880CC

Product number

End-fitting

Figure 1

2.

Silicone rubber

FRP rod

Structure of composite insulator

DESIGN OF COMPOSITE INSULATORS

2.1 Structure of Composite Insulators
Typically a composite insulator comprises a core material,
end-fitting, and a rubber insulating housing. The core is of
FRP to distribute the tensile load. The reinforcing fibers
used in FRP are glass (E or ECR) and epoxy resin is used
for the matrix. The portions of the end-fitting that transmit
tension to the cable and towers are of forged steel, malleable cast iron, aluminum, etc. The rubber housing provides electrical insulation and protects the FRP from theelements. For this reason we at Furukawa Electric haveadopted silicone rubber, which has superior electricalcharacteristics and weather resistance, for use in the
housing. Figure 1 shows the structure of a composite
insulator.
2.2 Designing Composite Insulators
An important feature of the composite insulators developed here is that the design of the shed configuration isextremely free, owing to the use of silicone rubber for the
housing. Based on past experience, IEC 60815 "Guide forthe selection of insulators in respect of polluted conditions" was adopted. 2) Electrical and mechanical characteristics were designed to satisfy the requirements set forthin IEC 61109 "Composite insulators for a.c. overheadlines with a nominal voltage greater than 1000 V:
Definitions, test methods and acceptance criteria".
With regard to pollution design, it has been suggested
that because of the hydrophobic properties of silicone rubber, composite insulators can be designed more compactly than in the past, but because of the absence of adequate data it was decided in principle to provide as greator greater surface leakage distances. The design value forleakage distance was referenced to the value per unit
electrical stress as determined in IEC 60815, adjusted
upward or downward according to customer requirements.
Tensile breakdown strength was determined by applying a safety factor to the long-term degradation in tensile
breakdown strength.
The rubber and FRP of the housing were required not
only to have sufficient mechanical adhesion but to bechemically bonded, so as to prevent penetration of waterat the interface. And because in general a large number ofinterfaces may result in electrical weak points, Furukawa
Electric has adopted a composite insulator design inwhich the sheds and the shank are molded as a unit,
resulting in higher reliability.
The end-fittings comprise three elements, and have thegreatest effect on insulator reliability. Specifically the pen-

130

Overall length (mm)1884

Number of sheds (lrg/sml)

26/25

Shed diameter (lrg/sml) (mm)117/83

Effective length (mm)1640

Surface leakage distance (mm)5400

Weight (kg)10Electrical characteristics
Power-frequency withstand voltage (kV)440Lightning impulse withstand voltage (kV)835

Switching impulse withstand voltage (kV)635

Mechanical characteristics
Tensile strength (kN)120

Bending breakdown strength (MPa)294

etration of moisture at this point raises the danger of brittle
fracturing of the FRP and the electrical field becomesstronger. For this reason the hardware is of field relaxing
structure and the silicone rubber of the housing is extended to the end-fitting to form a hermetic seal. The end-fitting is connected to the FRP core by a compression
method that maintains long-term mechanical characteristics.
The design requirements for composite insulators for
154-kV service are set forth below.
• Overall performance(1) To have satisfactory electrical characteristics in outdoor use, and to be free of degradation and crackingof the housing.
(2) To be free of the penetration of moisture into theinterfaces of the end-fitting during long-term outdoor
use.
(3) To possess long-term tensile withstand load characteristics.
(4) To be free of voids and other defects in the corematerial.
(5) To be non-igniting and non-flammable when
exposed to flame for short periods.
• Electrical performance (insulator alone)(1) To have a power-frequency wet withstand voltage of
365 kV or greater.
(2) To have a lightning impulse withstand voltage of 830
kV or greater.
(3) To have a switching impulse withstand voltage of
625 kV or greater.
(4) To have a withstand voltage of 161 kV or greater
when polluted with an equivalent salt deposition density of 0.03 mg/cm2.
(5) To have satisfactory arc withstand characteristics
when exposed to a 25-kA short-circuit current arc for0.34 sec.
(6) Not to produce a corona discharge when dry andunder service voltage, and not to generate harmful
noise (insulator string).

Furukawa Review, No. 19. 2000

Table 2
1

Test Items and applicable test standard

Overall performance
1.1 UV durability (ASTM G53)
1.2 Ozone durability (JIS K 6301)
1.3 Composite insulator durability (IEC 61109) (paragraph reference)(1) Test of interface and connection of the end-fitting (5.1)(2) Load-time test of core (5.2)(3) Tracking and erosion tests of the housing (5.3)(4) Core material tests (5.4)(5) Flammability test (refer to IEC 60707) (5.5)
(6)

(7)
2

Mechanical load-time test and test of the tightness at the interface
between end-fitting and insulator housing (6.4)

Photo 1

Composite insulator under test

Adhesion strength test between end-fitting and insulator housing
seal (7.4)
Electrical performance of the insulator

2.1 Power-frequency wet withstand voltage (IEC 60383) (6.2)
2.2 Lightning impulse withstand voltage (IEC 60383) (6.1)
2.3 Switching impulse wet withstand voltage (IEC 60383) (6.3)
2.4 Maximum withstand voltage of pollution (JEC 170)

Figure 2

Composite insulator string for tension tower

2.5 Arc-withstand characteristics (IEC SC36B (Secretariat) 116)
2.6 Corona characteristics (RIV) (refer to IEC 60437 (reference test) 6.5)
2.7 TV interference test (V-string insulator only)
3

Mechanical performance of the insulator
3.1 Tensile breakdown strength (IEC 61109, JIS C 3801)
3.2 Tensile withstand load (IEC 61109)
3.3 Bending characteristics (JIS C 3801)
3.4 Bending breakdown strength (JIS C 3801)
3.5 Proof test for breakage of one of two strings (reference test)
3.6 Vibration fatigue characteristics (reference test)(1) Longitudinal vibration fatigue test (reference test)(2) Transverse vibration fatigue test (reference test)
3.7 Torsional withstand load (reference test)
3.8 Repetitive torsional strength (reference test)
3.9 Swing characteristics (V-string insulator only)• Mechanical performance (insulator alone)(1) To have a tensile breakdown load of 120 kN or
greater.
(2) To have a bending breakdown stress of 294 MPa or
greater.
(3) To show no abnormality at any point after being subjected to a compressive load equivalent to a bending
moment of 117 N.m for 1 min.
(4) To show no insulator abnormality with respect to torsional force producing a twist in the cable of 180°.
(5) To be for practical purposes free of harmful defects
with respect to repetitive strain caused by oscillationof the cable.
Table 1 shows the characteristics of an insulator
designed to satisfy these specifications.

2.3 Testing to Confirm Performance
In order for the composite insulators to be used as insulator strings, tests were also conducted to confirm their performance as V-string suspension devices or tension
devices. These test items are shown in Table 2. Photo 1
shows an insulator under test, and Figure 2 shows a composite tension insulator string.
2.4 Representative Test Results
The main tests for confirming the performance of the composite insulators are (from Table 2):
1.3 Composite insulator durability, (1) Test of interface
and connection of the end-fitting;
2.5 Power arc-withstand characteristics;
3.3 Bending characteristics; and3.6 Vibration fatigue characteristics: (1) Longitudinal
vibration-fatigue test
These will be dealt with in turn below.
2.4.1 Test of Interface and Connection of the End-fitting
The interface adhesion between the housing rubber andthe FRP core in the vicinity of the end-fitting is considered
one of the most crucial aspects of composite insulator
manufacture, and as a means of verifying its suitability,
the following procedures set forth in the design test in IEC
61109 were implemented:
1) nitial dry power-frequency flashover voltage test2) Sudden load release test (30% of SML* -20 to -25°C)
3) Temperature-mechanical test (50% of SML* +50 to-35°C)
4) Water-immersion test (42 hr, boiling water, NaCl
0.1% weight)
5) Verification testsa) Visual inspection (for cracking or other abnormality)b) Steep-front impulse voltage test131c) Dry power-frequency voltage tests• Flashover test (to check for drop from initial conditions)• 80% of FOV for 30 min (to check for temperaturerise in insulator shank)* SML = specific mechanical load
The following test results were obtained:a) Visual inspection: no cracking or separation of the
housing rubber was observed.
b) Steep-front impulse voltage test: The insulators tested were separated into two sections and 1000 kV/ µs(positive- and negative-polarity) was applied 25 times
to each. All underwent external flashover, but nopuncture of the housing rubber or core occurred.
c) Dry power-frequency voltage tests: Table 3 shows
the results of five repetitions of average flashover voltage, but all of the test specimens held 90% or more of
initial flashover voltage. Next 80% of average initial
flashover voltage (328.9 kV x 0.8 = 263 kV) wasapplied to the lower end-fitting for 30 min and the temperature rise was measured immediately. All test
specimens satisfied the standards, with a temperaturerise of less than 20 K.

3 shows the setup for the power arc withstand test, inwhich the angle α of the insulator from the horizontal was
6°. The test showed that the arc moved between thehorns, and fusing of the horns was observed. There wassome discoloration of the insulator portion (housing rubber) but it was confirmed that the sheds were not chippedor damaged, and there was no fusing of the end-fitting.
Photo 2 shows a composite insulator after application of
the power arc. To investigate residual strength after the
test, the tensile breakdown load was determined by applying SML for 1 min and then applying load until breakdown
occurred.
Table 4 shows the results of the switching impulse withstand voltage test. As can be seen, there was no dropfrom the initial value.
2.4.3 Bending Characteristics
Since FRP is used in the core of the composite insulator,
it is necessary that permissible bending load is notexceeded either during laying or in later service. Bending

2.4.2 Power Arc-withstand Characteristics
Since the housing is made of organic material, the extent
of damage to the insulator due to power arcing and thevoltage withstand characteristics after testing were confirmed by the method described below.
A copper wire fuse was attached to the end-fitting on
both ends of an insulator fitted with arcing horns, a shortcircuit current of 25 kA was applied for 0.34 s, and thecondition of the housing rubber and of arc movement wasinvestigated. The arcing horn was then removed and a
wet impulse withstand voltage test was carried out. Figure
Table 3

Initial (kV)After boiling (kV)Change (%)

Sample #1

326.4 kV

334.2 kV

+2.38%

Sample #2

327.6 kV

335.2 kV

+2.32%

Sample #3

332.6 kV

326.4 kV

-1.90%

Average

328.9 kV

331.9 kV

+0.93%

Figure 3

132

Results of flashover voltage test (adjusted to standard atmospheric conditions)Setup for power arc test of composite insulator

Photo 2
Table 4Composite insulator after application of power arc
Results of switching impulse withstand voltage test

Withstand voltage (kV)Photo 3

InitialAfter power arc test

635

667

Setup for bending breakdown test

Furukawa Review, No. 19. 2000

Photo 5

Photo 4Composite insulator installed on working transmission line

characteristics tests were therefore carried out.
The tests were performed in accordance with Section
7.2.2 of JIS C 3801 "Insulator Test Methods". One end of
the unit to be tested was held stationary and a bending
stress of 294 MPa (30 kgf/mm2) was applied for 1 min.
Since no abnormality was observed, bending moment wasincreased until breakdown, which occurred at 539 MPa(55 kgf/mm2). Photo 3 shows the setup for the bending
breakdown test.
2.4.4 Longitudinal Vibration-fatigue Tests
In consideration of the galloping oscillation, sleet jumping
and aeolian vibration that occur in actual service, longitudinal vibration fatigue tests were carried out. A horizontal
tensile testing machine was used to apply a repetitive fluctuating tension of 32±12 kN.
The tests showed that after 2 million repetitions of themechanical load, there was no pullout of the core or fracture of the end-fitting, or separation between the silicone
rubber and the hardware, thereby demonstrating thatmechanical reliability was sufficiently high.
2.4.5 Summary of Test Results
The results of some of the test items covered in Table 2
were described above. It was also confirmed that there
were no problems with respect to any of the remaining
items.
Photo 4 shows a composite insulator installed on aworking transmission line.

3.

PREDICTING SERVICE LIFE

The service life of a composite insulator involves bothelectrical and mechanical aspects. Electrical aging
involves damage from erosion or tracking due to the thermal or chemical effects of discharge occurring when theinsulation material is polluted or wet, and may even result

Facility for loading exposure testing in Okinawain flashover.
Mechanical aging includes long-term drop in thestrength of the core material or in the holding force of theend-fittings, as well as brittle fractures of the core material,and can on occasion result in breakage of the insulator
string. A drop in core strength or holding force of end-fitting can be countered by adopting an appropriate safety
factor and using a reliable method of compression. Brittle
fractures, on the other hand, occur mostly near the interface between the insulation material and the end-fitting,and provided this area has been properly manufactured,
the probability of their occurrence will be lower than that ofelectrical aging.
To estimate service life from the electrical aspect, actual-scale composite insulators were exposed to electricalstress, and were subjected to an exposure test under anatural environment. A test chamber simulating environmental stress was also constructed, and accelerated tests
were carried out according to international standards (IEC61109 Annex C). Further, by comparing leakage current
waveform and cumulative charge, which may be characterized as electrical aging, evaluation of composite insulator service life was carried out.
Furthermore, since in Japan, a drop in insulation performance due to rapid pollution during typhoons is a familiar
phenomenon, an investigation was made based on thecharacteristics of leakage current obtained during atyphoon into the effect of rapid pollution on electrical aging
in composite insulators. 4)~7)
3.1 Okinawa Loading Exposure Testing Facility
Japan's position surrounded by the ocean means that salt
pollution is a common problem, necessitating an understanding of aging and decreased insulation performancedue to surface discharge when polluted or wet.
Accordingly a facility for loading exposure testing wasconstructed in Okinawa, the prefecture where contamination is most severe and typhoons most frequent.
The facility is sited on the Pacific Ocean coast at
Nakagusuku Bay. It is exposed on the east to winds off
the ocean and is about 300 m from the shoreline in anarea of severe salt pollution. Photo 5 shows the testing
facility.

133

To determine the effect of unit electrical stress (V/mm),
which is the most important design parameter for pollution
resistance, it was decided to use as test specimens composite insulators of the same material and structure but
different surface leakage distances, together with ceramic
insulators as a reference. 5)~7)
In an effort to determine the electrical behavior of insulators, the test facility monitors weather information (airtemperature, humidity, wind direction and velocity, andprecipitation) and carries out continuous measurement ofleakage current and equivalent salt deposition density(regularly and during passage of typhoons). These results
are currently being tabulated, and will be presented in a
subsequent report.
3.2 Accelerated Aging Tests
The method designated by IEC 61109 for accelerated
aging tests of the housing material of composite insulators
specifies evaluation of short specimens that satisfy unit
electrical stress levels. In this case, however, to examine
the effect both of the material and of the structure, the procedures used in the loading exposure tests being carriedout in Okinawa were followed for both the test specimens
and the voltage (77 kV AC (to ground). To serve as a suitable site for these tests, a large fog chamber was built
measuring 4.4 m square by 3.3 m in height. 7)
In order to be able to investigate the temperaturechange, humidification, precipitation, salt exposure andUV irradiation set forth in IEC 61109, at the same time as
performing accelerated aging tests on the adhesion of theend-fitting and terminal portion of the housing rubber, thisTable 5

Accelerated Aging Test Cycle

4.

Humidification
Heating
Salt spray
Artificial rain
UV irradiation
Loading weight
Voltage application(hours)0

Photo 6134

facility is provided with equipment for applying a steadyload of 20 kN.
Table 5 shows the test cycle of IEC 61109 Annex C.
This method of acceleration provides a test that promotes electrical degradation by simulating, reproducing ina short period of time, and then repeating the stresscaused by changes in weather under electrical stress.
Photo 6 shows the setup for the accelerated aging test.
Evaluation of the test results was carried out by continuous measurement of leakage current, regular measurements of the hydrophobicity of insulator surfaces, andanalysis of surface conditions after the completion of thetests using scanning electron microscopy (SEM) and Xray photoelectron spectrometry (XPS).
As Photo 7 shows, hydrophobicity was adequately
maintained after the specified period of 5000 hr, despite aminor decline from the initial conditions.
Visual observation of insulators after test completion
revealed a certain degree of gray discoloration, but SEM
results showed no difference from the initial conditions.
XPS observation of coupling energy also showed nochange from initial rubber coupling, thus confirming thatno aging occurred.
Thus we see that in actual-dimension accelerated aging
tests conducted in accordance with IEC 61109 Annex C,
the composite insulators showed no great leakage current
and no tracking or erosion of the insulator surface. Testsof withstand voltage characteristics conducted separately
also showed satisfactory results. It was accordingly concluded that these insulators would satisfactorily stand up
to use at the ordinary level of pollution prevailing in Japan.

2

4

6

8

10 12 14 16 18 20 22 24

Setup for accelerated aging test

CONCLUSION

Composite insulators are light in weight and have demonstrated outstanding levels of pollution withstand voltage
characteristics and impact resistance, and have been
widely used as inter-phase spacers to prevent galloping.
They have as yet, however, been infrequently used assuspension insulators. The composite insulators for suspension use that were developed in this work have been
proven, in a series of performance tests, to be free of

Photo 7

Surface of composite insulator after 5000 hr testingaccording to IEC 61109, Annex C

Furukawa Review, No. 19. 2000

problems with regard to commercial service, and in 1997
were adopted for the first time in Japan for use as V-suspension and insulators for a 154-kV transmission line.
To investigate long-term degradation due to the use oforganic insulation material, outdoor loading exposure tests
and indoor accelerated aging tests are continuing, andbased on the additional results that will become available,
work will continue to improve characteristics and rationalize production processes in an effort to reduce costs andimprove reliability.

REFERENCES

1) Sri Sundhar, Al Bernstorf, Waymon Goch, Don Linson, Lisa
Huntsman: Polymer insulating materials and insulators for highvoltage outdoor applications, IEEE International symposium on
EI, 1992.
2) Composite insulators for a.c. overhead lines with a nominal voltage greater than 1000V: Definitions, test methods and acceptance criteria, IEC61109, 1992-03.
3) Guide for the selection of insulators in respect of polluted conditions, 1986.
4) R. Kimata, L. Kalocsai, A. Bognar: Monitoring system for evaluation of leakage current on composite insulators, 4thInternational Conference on Properties and Applications ofDielectric Materials, No.5125, 1994.
5) Nakauchi et al.: Natural environment exposure tests and accelerated aging tests of silicone rubber insulators, High-voltage
Symposium, IEEJ, HV-97-41, 1997. (in Japanese)6) Nakauchi et al.: Studies on pollution of silicone rubber insulators, High-voltage Symposium, IEEJ, HV-98-73, 1998. (inJapanese)7) Nakauchi et al.: Comparison between loading exposure tests andaccelerated aging tests of silicone rubber insulators, Proceedingsof Electric Energy Workshop, No. 431,1997. (in Japanese)
Manuscript received on July 12, 1999.

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