1.
INTRODUCTION
Electrical insulators are used to prevent the loss of
electric charge or current from conductors
in electric power transmission lines. Electrical insulators are electrically
insulating components in various electric circuits and electrical installations.
Electrical insulators are used as a barrier layer used in a circuit, an
insulating sheathing of a current-carrying conductor or a printed-circuit board
for electronics. An electrical insulator is also an insulator as used in power
engineering for routing current-carrying lines or keeping them apart. Power
transmission and distribution systems include various insulating components
that must maintain structural integrity to perform correctly in often extreme
environmental and operational conditions. Overhead 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 these drawbacks. The
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. Since 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 insulators. In 1980, Furukawa Electric was
engaged in the development of inter-phase spacers to divvent 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 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 worlds largest. Thus 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 transmission lines. Recently
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.
2. 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 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 2.1 shows the structure of a composite insulator.
Figure 2.1 Structure of composite
insulator.
2.2
Designing of Composite Insulators
An
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". 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 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. 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 be chemically bonded, so as to
divvent 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.
The
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 comdivssion
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 cracking of
the housing.
(2) To be free of the penetration of moisture into the
interfaces 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 core
material.
(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 25kA short-circuit current arc for
0.34 sec.
(6) Not to produce a corona discharge
when dry and under service voltage, and not to generate harmful noise
(insulator string).
• 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 comdivssive load equivalent to a bending
moment of 117 Nm 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 oscillation
of the cable.
Table-1 shows the characteristics of
an insulator designed to satisfy these specifications.
Mechanical Characteristics
Product
number
Overall
length (mm)
Number
of sheds (lrg/sml)
Sheds
diameter (lrg/sml) (mm)
Effective
length (mm)
Surface
leakage distance (mm)
Weight
(kg)
Tensile
strength (kN)
Bending
breakdown strength (MPa)
Electrical Characteristics
Power
frequency withstand voltage (kV)
Lightning
impulse withstand voltage (kV)
Switching
impulse withstand voltage(kV)
|
H154-120-1880CC
1884
26/25
117/83
1640
5400
10
120
294
440
835
635
|
table1. Specification of composite
insulators
Table-1
Characteristics of insulator
3. ELECTRICAL DESIGN CRITERIA
3.1
Dry Arcing Distance (Strike Distance):-
It is the shortest distance through the surrounding
medium between terminal electrodes. In the figure given below red line shows
the dry arcing distance.
Figure 3.1 Dry arcing distance
3.2 Leakage Distance
The sum of the shortest distances measured along the insulating surfaces between
the conductive parts, as arranged for dry flashover test. In the given figure
the distance covered by red line shows the leakage- distance.
The design
engineer can find general guidance on what leakage distance is provided by a
properly designed shed shape. These recommendations have been devised for
porcelain and glass insulators but were not meant to be used for composite
insulators.
Figure 3.2 Leakage
distance
4. LEAKAGE CURRENT CONTROL AND
FLASHOVER RESISTANCE
·
Due to
chemical nature of polymer, the surface of insulator is hydrophobic (non
wetting).
·
Water on the
surface of insulators stays in form of droplets and does not form continuous
film. So the leakage current along the insulator surface is strongly
suppressed.
·
The efficient
suppression of leakage current means the risk of flashover is reduced compared
to porcelain insulators.
The following curve shows the
current comparison in ceramic and polymer insulators during the “salt fog”
test.
Figure 4.1
Leakage current
5. FACTOR
AFFECTING THE PERFORMANCE OF
COMPOSITE
INSULATORS
5.1
MATERIAL AND MANUFACTURING METHOD
·
Polymer base
and compound quality.
·
Formulation
and design.
·
Core quality
and end fitting gap attachment method.
·
Manufacturing
method and quality control.
·
Handling,
storage and delivery damage.
·
Damage during
installation.
5.2
ENVIRONMENTAL CONDITINS
·
Ultraviolet
radiations.
·
Wind and
ozone.
·
Temperature
and pressure.
·
Humidity,
rain, fog and snow.
·
Organic and
inorganic pollutions (fertilizers, dust, acid, salt etc).
5.3
POWER SYSTEM OPRATION AND DESIGN
·
Electric field
stress (continuous and transient).
·
Control stress
ring.
·
Leakage
distance.
·
Proximity of
other lines.
·
Mechanical
stress traction, compression, torsion and vibration.
6. PREDICTING SERVICE LIFE
The
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. Mechanical
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
comdivssion. 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 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 phenomenon, 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
composite insulators.
7. ADVANTAGES
Due to many
advantages the use of composite insulators has grown steadily. The polymeric
products are demonstrating their capabilities in diverse environments and are
now routinely used to prevent contamination flashover. The advantage of
composite insulators over ceramic insulator is given below:-
7.1 LIGHT WEIGHT
The density of
polymer materials is lower than other materials. It makes construction and
erection easier and faster. The reduced weight permits the use of lighter and
less costly structures and mounting arrangements. The smaller size and weight
result in lower shipping cost.
Table-2
Comparison of weight
7.2 COMPLEX
GEOMETORY
The polymers
insulators are typically molded therefore it may have a higher creep age
distance per unit length than porcelain. Weathershed profiles can be made more
complex and alternating diameter weathersheds are supplied, which improve the a.c.
wet flashover by avoiding bridging of all sheds simultaneously during heavy
wetting conditions.
7.3 POLLUTION PERFORMANCE
The hydrophobic
properties on the composite insulator have a better electrical performance in
contaminated condition. Water on the surface of hydrophobic materials forms water
bead, so the conductive contamination dissolved within the water beads is
discontinuous. This condition results in lower leakage current flow and the
probability of dry band formation, which in turn requires a higher impressed
voltage to cause flashover. The higher resistance of silicone rubber helps to limit
the arcing and minimizes the flashover. Another advantage of the composite
insulator is that it contributes to reduce the maintenance costs, such, no need
washing, and no need for application of silicone coatings and reduce the
inspections.
7.4 HOLLOW CORE HOUSING FAILURE MODE
The physical
properties of the polymer material mean that it will not shatter like
porcelain. With the initiation of an internal fault, the expected failure mode
is rupturing or bursting of the hollow structure with venting of the internal
pressure, leading to an external flashover and dissipation of the fault energy
outside of the housing.
7.5 PROCESSING
The processing time
for polymer insulator is shorter than for porcelain.
7.6 NO HAZARD CONDITION
In the event of
any fault the characteristics of a composite insulator exclude the occurrence
of hazardous condition to personnel and surrounding equipment.
7.7 EARTHQUAKE RESISTANT
Equipment using
hollow core composite insulators can withstand seismic acceleration stresses up
to 1 g (whether it is 0.5g in case of ceramics insulators) without damage due
their lower weight, high damping factor and high strength design
characteristics.
7.8
ECONOMICAL BENEFITS
1. Lower costs of manufacturing,
shipping, loading/unloading work and installation (due to lesser weight and
dimensions
2. No breakage during transportation,
handling, loading /unloading assembly works (even so must to be handling
carefully
3. Possible application in
hard-reach-areas Swampy areas arm highlands costs tam necessity at all)
insulators cleaners
4. Low costs of repair and replacement
of insulators (due to increased reliability and shock resistance as well as
easier assembly)
8. USES OF
COMPOSITE INSULATORS
The composite insulators are used
at following places:-
·
Distribution
and transmission insulators.
·
Surge arresters.
·
Line surge
arresters.
·
Bushings.
·
Circuit
breakers.
·
Instrument
transformers.
·
Capacitors.
8.1
DISTRIBUTION AND TRANSMISSION INSULATORS
Environmental demands on high-volt age transmission lines
have increaser constantly in recent years both in qualitative and quantitative
respects. For example, today it is of prime importance when planning an
overhead line to pay attention to the achievement of a pleasing and environmentally
tolerable towel configuration A large power company in Western Switzerland has
reached this goal in an exemplary manner with its new 400 kV lines in this case
the wide use of silicone composite insulators brought positive results (figure
8.1). The composite insulator with a connection length of 30 m can be
manufactured in a single piece and is almost 1.5 m shorter than the previously
used porcelain insulator strings, each with three long rod insulators type LG
85/22/1470.
Shorter insulators allow the use of shorter cross arms
without the risk of flashover due to reduced clearances to the tower as a
result of the swinging of the conductors. This has the further effect of reducing
the torsional loads in the cross towers. It requires 37% narrower way-leave,
which translates into 50% lower right-of-way cost.
Figure
8.1 Transmission and distribution insulators
8.2 OUTDOOR SUBSTATION INSULATORS
Switchyards are the nerve centers of every power grid and so
the users' expect and demand a correspondingly high degree an operational safety.
It is therefore not surprising that with the growing faith in composite
insulators - particularly due to the good experience made in their application
in overhead lines world-wide - great interest has developed in recent years in
their applies bon in outdoor substations. Today, if the customer so desires, it
is possible to design complete substations in silicone composite technology.
Figure
8.2 Outdoor substation with composite insulators
In
the above figure 8.2 ‘a’ shows voltage controlled bushings for transformers,
‘b’ shows surge arrester, ‘c’ shows live-tank circuit breaker, ‘d’ shows
current transformer, ‘e’ shows voltage transformer, ‘f’ shows voltage
controlled bushing for power transformer, and ‘g’ shows cable termination.
8.3 SURGE ARRESTERS
For the obvious reason of the danger explosion due to
overloading, surge arresters were one of the first electrical devil that were
built with silicone insulator she The advances in ZnO technology in arrester
design, which replaced the spark-gap arresters, eased the realization of
porcelain-free arresters. Today, ZnO arresters are manufactured either by
applying the silicone shed directly onto the active part, which is sometimes
done for voltage levels up to 36kV, or by using a fiberglass reinforced, silicone
coated composite tube as an insulating housing for the arrester, which is
possible up to the highest system voltages. Figure 8.3 shows ZnO surge
arresters.
Figure
8.3 ZnO Surge arresters
8.4
BUSHINGS
Increasingly,
the design of high-voltage bushings is being influenced by higher demands on
operational safety, damage-risk minimization (to persons and property) and not
least, by a greatly increased public environmental consciousness. The
consideration of these factors led to a new conception these important
components on the basis composite technology. By using superior materials, as
well as having their manufacture well under control, it has been possible to
satisfy the above-mentioned demands o the bushings. Fig.8.4 shows 420 kV and 22
kV transformer bushings and GIS Bushings for 123 kV in composite technology.
Figure
8.4 Power transformer bushing and GIS bushing
8.5 CIRCUIT BREAKERS
For
the various reasons already mentioned above there is also art increase in the
use of hot low composite insulators in high voltage circuit breakers, including
their associated control capacitors, and also recently in high voltage load disconnecting
switches. The possibility of fitting an optical fiber cable into the composite
tube for the transmission of measuring and a control signal, particularly in
circuit breakers, is regarded as an additional advantage. Figure 8.5 shows SF6
circuit breaker.
Figure
8.5 SF6 circuit breaker
8.6 INTERPHASE SPACERS
Interphase spacers are fitted mainly the points on
overhead lines at which either for reasons of design or due to external
influences, there is a danger that required distance between the conductors of two
phases will not be maintained a situation which would lead to a short circuit and
hence an interruption in service. As early as 1990, a CIGRE questionnaire brought to light that around the world, 32
power utilities had around 13000 interphase spacers in operation a practically
all voltage levels. Some of them had been in active service for many years (up
to 20 years at the time of the questionnaire). Almost a third of the interphase
spacers registered in the above report are installed in Switzerland.
As in any industrialized country, it
is becoming increasingly difficult to obtain rights of way for routes for new
lines. A possible solution to reduce the seriousness of this problem is to increase
the power transmission capacity of existing lines, such as by installing a second
circuit. In the case in question the cross arms of the concrete poles were
originally designed to guarantee the required air clearance between the
conductors at mid spam for one 12 kV circuit. So appropriate spacer made of
silicone composite insulator were designed, and installed between conductors
approximately 40m interval in order to maintain the required conductor
separation. This solution is only possible by using the silicone composite
insulators, which is very light compared with porcelain insulators, and thus do
not add sever bending stress on conductors during dynamic load (ice shedding). Figure 8.6 shows Silicone composite insulators
as interphase spacers.
Figure 8.6 Interphase spacers
8.7 INSTRUMENT TRANSFORMER
For a few years now, hollow
composite insulators have been finding use as housing for the outdoor versions
of current and volt age transformers and this in locations which put particular
demands on the mechanical strength and elasticity of the dev ice's cases such
as when there is a risk of explosion or where high mechanical stresses like
earthquake, vandalism or high short circuit forces are likely. Rare faults
leading to a CT’s or VT’s explosion and hence causing considerable risk of
injury or damage to persons and property have in Switzerland recently led to an
increase in the application of Current Transformers and Voltage Transformers
using composite technology. Figure 8.7 shows current transformers with hollow
composite Insulators.
Figure
8.7 Current transformer (CT)
9. 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 divvent galloping. They
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.
10. REFERENCES
1. Composite
insulators- Hollow insulators for use in indoor and outdoor electrical
equipment Definitions, test method and acceptance criteria. IEC 1462. Ed. 1,
Draft November 1996.
2. IEC
1109: 1992. “Composite insulators for ac overhead lines with nominal voltage
greater than 1000V- Annexure C”.
3. IEEE Std 987-2001TM.
(Revision of IEEE Std 987-1985TM). IEEE Guide for Application of
Composite insulators.
4. IEEE/PES 2010 Transmission and Distribution
Conference and Exposition New Orleans, Louisiana April 20, 2010.
