Development of Composite Insulators for Overhead Lines
Tomsk Polytechnic University
Electrical Systems and
Development of Composite Insulators for Overhead Lines
Design of composite
power transmission lines require both cables to conduct the electricity and
insulators to isolate the cables from 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 disadvantages of 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
1930s and ’40s saw the appearance of the first insulators to replace inorganic
materials with organic, but these suffered problems of weather resistance, and
their characteristics 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 was advanced, 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.
these materials were new, however, there were many technical difficulties that
had to be remedied, such as adhesion between materials and penetration of
moisture, and the end-fittings, which transmit the load, had to be 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 withstand voltage of
pollution, and this had led to an explosive increase in the use of composite
1980, Furukawa Electric was engaged in the development of inter-phase spacers
to prevent galloping in power transmission 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 66-kV duty, and in 1994 their use was extended to
275-kV service with a unit 7 m in length—the world’s largest.
as composite insulators have established a track record 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
Furukawa Electric developed composite insulators for suspension and delivered,
for the first time in Japan, 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.
DESIGN OF COMPOSITE INSULATORS
Structure of Composite Insulators
1. Structure of composite insulator.
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 the elements.
For this reason we at Furukawa Electric have adopted silicone rubber, which has
superior electrical characteristics and weather resistance, for use in the
housing. Figure 1 shows the structure of a composite insulator.
Designing Composite Insulators
important feature of the composite insulators developed here is that the design
of the shed configuration is extremely free, owing to the use of silicone
rubber for the housing. Based on past experience, IEC 60815 "Guide for the
selection of insulators in respect of polluted conditions" was adopted.
Electrical and mechanical characteristics were designed to satisfy the
requirements set forth in IEC 61109 "Composite insulators for a.c.
overhead lines with a nominal voltage greater than 1000 V: Definitions, test
methods and acceptance criteria".
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 great or greater surface leakage
distances. The design value for leakage distance was referenced to the value
per unit electrical stress as determined in IEC 60815, adjusted upward or downward
according to customer requirements.
breakdown strength was determined by applying a safety factor to the long-term
degradation in tensile breakdown strength.
rubber and FRP of the housing were required not only to have sufficient
mechanical adhesion but to be chemically bonded, so as to prevent penetration
of water at the interface. And because in general a large number of interfaces
may result in electrical weak points, Furukawa Electric has adopted a composite
insulator design in which the sheds and the shank are molded as a unit,
resulting in higher reliability.
end-fittings comprise three elements, and have the greatest effect on insulator
reliability. Specifically the penetration of moisture at this point raises the
danger of brittle fracturing of the FRP and the electrical field becomes
stronger. 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.
design requirements for composite insulators for 154-kV service are set forth
To have satisfactory electrical characteristics in outdoor use, and to be free
of degradation and cracking of the housing.
To be free of the penetration of moisture into the interfaces of the
end-fitting during long-term outdoor use.
To possess long-term tensile withstand load characteristics.
be free of voids and other defects in the core material.
To be non-igniting and non-flammable when exposed to flame for short periods.
Electrical performance (insulator alone)
To have a power-frequency wet withstand voltage of 365 kV or greater.
To have a lightning impulse withstand voltage of 830 kV or greater.
To have a switching impulse withstand voltage of 625 kV or greater.
To have a withstand voltage of 161 kV or greater when polluted with an
equivalent salt deposition density of 0.03 mg/cm2.
To have satisfactory arc withstand characteristics when exposed to a 25kA
short-circuit current arc for 0.34 sec.
Not to produce a corona discharge when dry and under service voltage, and not
to generate harmful noise (insulator string).
Mechanical performance (insulator alone)
To have a tensile breakdown load of 120 kN or greater.
To have a bending breakdown stress of 294 MPa or greater.
To show no insulator abnormality with respect to torsional force producing a
twist in the cable of 180°.
To be for practical purposes free of harmful defects with respect to repetitive
strain caused by oscillation of the cable.
1 shows the characteristics of an insulator
to satisfy these specifications.
PREDICTING SERVICE LIFE
service life of a composite insulator involves both electrical and mechanical
aspects. Electrical aging involves damage from erosion or tracking due to the
thermal or chemical effects of discharge occurring when the insulation material
is polluted or wet, and may even result in flashover.
aging includes long-term drop in the strength of the core material or in the
holding force of the end-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.
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 of electrical aging. To estimate service life from the electrical aspect,
actual-scale composite insulators were exposed to electrical stress, and were
subjected to an exposure test under a natural environment. A test chamber
simulating environmental stress was also constructed, and accelerated tests were
carried out according to international standards (IEC 61109 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 henomenon, an
investigation was made based on the characteristics of leakage current obtained
during a typhoon into the effect of rapid pollution on electrical aging in
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.
have as yet, however, been infrequently used as suspension 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 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 of organic insulation
material, outdoor loading exposure tests and indoor accelerated aging tests are
continuing, and based on the additional results that will become available, work
will continue to improve characteristics and rationalize production processes
in an effort to reduce costs and improve reliability.
ethylene propylene rubber
ethylene propylene diene methylene
a core of fiber-reinforced plastic
to bear the tensile load
penetration of moisture
AC railway service
malleable cast iron
absence of adequate data
electrical weak points
To possess long-term
Sri Sundhar, Al Bernstorf, Waymon Goch, Don Linson, Lisa
Polymer insulating materials and insulators for high
outdoor applications, IEEE International symposium on
Composite insulators for a.c. overhead lines with a nominal voltage
than 1000V: Definitions, test methods and acceptance
Guide for the selection of insulators in respect of polluted conditions,
Kimata, L. Kalocsai, A. Bognar: Monitoring system for evaluation
leakage current on composite insulators, 4th
Conference on Properties and Applications of
Materials, No.5125, 1994.
Nakauchi et al.: Natural environment exposure tests and accelerated
tests of silicone rubber insulators, High-voltage
IEEJ, HV-97-41, 1997. (in Japanese)
Nakauchi et al.: Studies on pollution of silicone rubber insulators,
Symposium, IEEJ, HV-98-73, 1998. (in
Nakauchi et al.: Comparison between loading exposure tests and
aging tests of silicone rubber insulators, Proceedings
Electric Energy Workshop, No. 431,1997. (in Japanese)