Note: Descriptions are shown in the official language in which they were submitted.
33~34
HIGH VOLTAGE INSULATORS
This invention is related to high voltage
electrical insulators intended for operation at voltages of
greater than 15 kilovolts.
High voltage transmission lines have historically
been insulated with porcelain and glass insulators. In order
to operate at the reliability level required, these
insulators are designed to operate at a low electrical stress
level. For use in clean atmospheres the stress level is
generally about 1 kV per inch. In areas where the insulator
is subjected to contamination, as along a sea coast or in an
industrial area, the stress level is ordinarily on the order
of 0.5 kV per inch or less, resulting in a large and bulky
insulator. These insulators are very heavy and still develop
high level leakage current and dry band arcing, which leads
to flashover. In practice, utilities often carry out
extensive insulator maintenance such as washing and greasing,
or coating with a room temperature curing silicone rubber, of
the insulators at regular intervals of time. In some severe
applications, utilities have gone to the use of resistance
grading of the insulator to give heated insulators to avoid
excessive leakage currents.
Other types of insulators have been proposed for
these types of uses, such as those below as described in
various patents and scientific literature.
British Patent 915,052, published January 9, 1963,
teaches an electrical insulator comprising a rod or tube of
resin-bonded glass fiber having a closely-fitting,
longitudinally continuous covering of a relatively
non-tracking elastomeric polymeric insulating material,
lZ~3~34
extending over the whole or a major part of the length of the
rod or tube.
U.S. Patent No. 3,511,698, issued May 12, 1970, to
Talcott taught weatherable insulators comprising a rigid
resin base member and a coating over the surface of said base
member of at least ten mils thickness of cured, organopoly-
siloxane elastomer which comprised a silicone elastomer stock
containing SiH groups and Si-alkenyl groups, and a platinum
catalyst.
British Patent 1,292,276, published October 11,
1972, discloses an insulator which comprises a central
support of which the outer surface comprises a non-tracking
electrically insulating material and at least one shed
installed on the central support. The sheds are heat-
shrinkable.
U.S. Patent No. 3,965,065, issued June 22, 1976 to
Elliott teaches a method of preparing an improved elastomer-
forming composition which comprises forming a mixture
comprising an organopolysiloxane which is convertible to the
solid elastic state and aluminum hydrate, and heating the
mixture at a temperature of at least 100C. for a time of at
least 30 minutes. The composition is taught to be
particularly useful for the fabrication of electrical
insulation having improved resistance to electric arcing and
tracking.
A filler system for polymers which provides high
voltage insulation is described by Penneck in U.S. Patent
No. 4,001,128, issued January 4, 1977. The filler system
utilizes a combination of alumina trihydrate and a chemically
treated silica filler.
A rod-type insulator having improved withstand
voltage characteristics under a contaminated condition is
described in U.S. Patent No. 4,174,464, issued November 13,
` ~ 3C34
--3--
1979. The design specifies that the leakage distance per
shed divided by the pitch between the adjoining sheds is
between 3.8 and 5.0 and that the leakage distance between a
given point on a lower surface of one of the adjoining sheds
and another given point on an upper surface of the other
opposing shed divided by the distance between the given
points in between 4.5 and 6Ø The lower surface of each
shed has coaxial ribs forming an undulating surface.
German Patent 2,650,363, issued September 10, 1985
was published November 17, 1977. Its U.S. equivalent is
Patent No. 4,217,466, issued August 12, 1980. It teaches
that the rod used in an insulator must be made from a
non-saponifiable resin and the screens used must be a
moisture-repellent, non-saponifiable polymer, and that there
must be an intermediate layer of material between the rod and
the screens to protect the rod.
A process for forming open-air compound insulators
is taught in U.S. Patent 4,246,696, issued January 27, 1981
to Bauer and Kuhl. A pre-fabricated glass fiber rod treated
with silane has a rubber layer extruded over it, then after
strengthening the rubber layer, prefabricated screens are
bonded on by vulcanization. Their discussion of the prior
art teaches the importance of the screens.
British Patent No. 1,601,379, published October 28,
1981, teaches that for high voltage insulators, and
particularly very high voltage insulators, the surface
creepage distance i8 at least equal to 2 and preferably 3 or
even higher than 3 times the straight line distance between a
live conductor and ground. This is particularly true when
insulators are intended for use under polluted and very
polluted conditions.
Another method of assembling an insulator with
sheds or screens is taught in U.S. Patent No. 4,505,033,
_4_ 1~93~3~
issued March 19, 1985, by Wheeler. Sheds are either molded
on or placed over a sheath of unvulcanized elastomer on a
core, then the assembly is vulcanized.
In addition to the teaching represented by the
above patent references, there has been a great deal of
scientific literature published over the years describing
insulator applications in both test applications and in
commercial applications which have been closely monitored to
determine the operation of the insulators. Eor example,
Niemi and Orbeck described their concept of the properties
they felt an insulation material should have and means for
measuring these properties in an article, "High Surface
Resistance Protective Coating For High Voltage Insulators",
presented at the IEEE Power Engineering Society Summer
Meeting in San Francisco, CA, on July 9-14, 1972. They
discuss a test in which an insulator is stressed at a level
of 6.7 kv/in for a time period of up to 8 hours. A paper by
Robert, Davis, and Dexter, presented in Russia in June of
1977 discusses the tests performed upon a silicone elastomer
suggested for use in manufacturing insulators. They conclude
that the silicone elastomer offers the best combination of
physical, electrical, and surface properties to enable it to
perform in the widest variety of environments.
High voltage polymeric insulators are normally
operated under stresses in the range of from 0.25kv/in to
0.75kv/in as is recorded in the paper by Weihe, Macy, and
Reynders, "Field Experience and Testing Of New Insulator
Types", presented at the 1980 session of the International
Conference on Large High Voltage Electric Systems. A
silicone insulator is described in "Silicone Elastomer
Insulators For High Voltage Outdoor Applications on British
Rail", a paper by Wheeler, Bradwell, Dams, and Sibbald at a
BEMA Conference in May, 1982, that is a resin bonded glass
~3~34
fiber rod covered with a layer of silicone elastomer bonded
to the rod and with end fittings adhesively bonded on each
end. It appears that the insulators are 1070 mm (42.1
inches) long between end fittings and are used at 25 kv 50
Hz, giving a stress level of 0.6 kv/in.
The initial testing of new polymer based insulators
indicated higher performance capability when compared to
porcelain; they could operate at higher stress levels. Longer
term service and test experience has shown that with aging
and contamination, the polymer insulators such as are taught
in the above references lose their initial voltage capability
and in many cases are not able to provide the same service as
porcelain and glass insulators. Current practice often
recommends that polymer insulators be used only for clean
service conditions and with designed surface stresses of 0.5
to 0.7 kv/in. Operation in the field at higher stress levels
has shown failure due to tracking and flashover under wet
conditions. There is a need for polymer insulators capable
of operating under wet, contaminated service conditions at
higher stress levels exceeding 1.0 kv/in for extended periods
of time.
A high voltage electrical insulator for use with
voltages in excess of 15 kilovolts line to ground comprises a
non-conducting, fiber reinforced polymeric support rod, a metal
support fitting at each end of the support rod, and a
continuous silicone elastomeric cover securely bonded to the
support rod and to the metal fittings. The cover is shaped
to provide at least one shed and has certain specified
geometrical ratios which control its shape. The cover is
comprised of a specified cured silicone elastomeric
composition, which has been shown to provide electrical
stability even under high electrical stress and in wet,
highly contaminated environments.
lZg3~34
--6--
Figure l shows a typical insulator of this
invention having ~ne shed as required. Also shown are
optional sheds. The dimensions used to define the shape of
the insulator are shown.
Figure 2 shows an optional shed shape.
This invention relates to an electrical insulator
for use at high voltages. The uniquely arc-resistant
silicone material used, combined with a design optimizing the
voltage stability, provides an insulator with excellent
performance under wet, contaminated service conditions.
The length of an insulator is determined by the
operating voltage and potential impulse over-voltage caused
by switching surges and lightning strikes. The surface of
the insulator is shaped to give minimum leakage currents and
to reduce the probability of flashover caused by dry-band
arcing. The design takes advantage of the surface properties
of the silicone elastomeric composition used to make the
surface of the insulator. The insulator suppresses leakage
current over its surface during conditions of use so that the
electrical arcing on the polymer surface is suppressed or
kept at a low level throughout the ~ervice life of the
insulator. Arc resistance is defined as the ability to
re~ist trac~ing and erosion when ~ubjected to a fog chamber
test in which the fog conductivity is lOOO microsiemens per
centimeter and the electrical stress is at a level of 1.5 kV per
inch of sample length.
Thi~ invention provides a high performance, high
voltage electrical insulator, for use with voltages in excess
of 15 kilovolts line to ground in an outdoor environment,
comprising
(1) a non-conducting, fiber reinforced, polymeric
support rod,
_7_ ~Z~3~3~
(2) a metal support fitting attached securely to
each end of the support rod, and
(3) a continuous, arc-resistant silicone
elastomeric cover securely bonded to the support rod and each
metal support fitting, the cover being shaped to provide at
least one shed and so that the following ratios are present,
Ds equal to or less than 1.5
Lc equal to or less than 1.7
a
D equal to or less than 3
D8
c
where D~ is shed diameter, L~ is the minimum distance between
adjacent sheds, Lc is leakage distance between the support
fittings, La is straight line distance between the support
fittings, and Dc is diameter of the cover over the support
rod, the silicone elastomeric cover comprising a cured
composition resulting from a composition comprising a mixture
of
(a) from 70 to 90 parts by weight of a dimethyl-
vinylsiloxy endblocked polydimethylsiloxane having a Williams
plasticity number of greater than 50,
(b) from 10 to 30 parts by weight of a dimethyl-
vinylsiloxy endblocked polydiorganosiloxane having about 98
mol percent dimethylsiloxane units and 2 mol percent
methylvinylsiloxane units and a Williams plasticity number of
greater than 25,
(c) from 13 to 17 parts by weight of fume silica
having a surface area of greater than 50 m2/g, and a treated
surface which prevents reaction with (1) and (2),
(d) from 1.5 to 2.5 parts by weight of a hydroxyl
endblocked polydiorganosiloxane having methyl and vinyl
lZ93~3~
--8--
radicals and having about lO weight percent vinyl radical and
about 16 weight percent hydroxyl radical, and
(e) from 90 to 220 parts by weight of aluminum
trihydrate, the mixture having been heated at a temperature
of at least 100C. for a time of at least 30 minutes.
An electrical insulator for use on high voltage
transmission lines is required to operate without
maintenance, or little maintenance for long periods of time,
many years in fact, without failing in its job of insulating
the conductor from ground. This must be done in the face of
all kinds of weather and in spite of weathers harsh effects
upon the insulator. In the development of such insulators,
the use of test methods is required in order to reduce the
time required to evaluate an insulator. Tests have been
devised in which insulators and insulator materials can be
exposed to viable conditions of dry and wet, contaminated
conditions in a relatively short period of time which expose
the material to the total stresses found in service over a
much longer period of time. Work on such tests has resulted
in a means for designing an insulator which can be tested in
a short period of time with the expectation that an insulator
which passes the required tests will also function in actual
use for the required number of years.
The insulator of this invention is a high
performance, high voltage insulator designed to operate at
line to ground voltages of greater than 15 kV, such as are
found in transmission systems. In order to take advantage of
the particular physical and electrical properties of the high
arc-resistant, low leakage current silicone elastomer used to
form the outer surface of the insulator, the insulator can be
designed to have an average surface electrical stress of
greater than 1.0 XV/inch. That is, the design voltage of the
transmission line, in kilovolts, divided by the length of the
~Z93(~34
leakage distance over the surface of the insulator between
end fittings in inches, is greater than 1.0 kV/inch. The
voltage can be as great as l.SkV/inch. This is equivalent to
60 volts per millimeter. This level of voltage stress is
considerably higher than that normally used for designing
transmission line insulators. This is the meaning of "high
performance" in this application. Furthermore, the
insulator is designed to have at least one shed present, but
makes use of a minimum number of sheds so as to make as
simple and light an insulator as is possible by taking
advantage of the unique electrical properties of the silicone
elastomer used to form the sheds.
The shape of the outer surface of the insulator
between the metal end fittings is such that the following
ratios are present,
-s equal to or less than 1.5
Ls
Lc equal to or less than 1.7
a
Ds equal to or less than 3
Dc
where Ds is shed diameter, Ls is the minimum distance between
adjacent sheds, Lc is leakage distance between the support
fittings, La i~ straight line distance between the support
fittings, and Dc is diameter of the cover over the support
rod. These dimensions are illustrated in Figure 1. The ratio of
Ds/Ls equal to or less than 1.5 means that the diameter of the
shed must be equal to or less than 1.5 times the distance between
the sheds. The ratio Lc/La equal to or less than 1.7 means that
the leakage distance over the surface of the insulator between
the support fittings is equal to or less than 1.7 times the
distance between the supporting fittings. The ratio Ds/DC equal
to or less than 3 means that the diameter of the sheds is
-10- 1.~3~3~
equal to or less than 3 times the diameter of the support rod
cover. Using the material specified for this insulator, it
has been found that the insulator sheds do not need to be of
any greater diameter. By limiting the diameter of the sheds,
the surface area of the insulator is limited. For a given
potential across the insulator, the smaller the surface area,
the smaller the leakage current will be. When dry bands are
formed on an insulator surface, arcing occurs. The intensity
of the arc is dependent on the leakage current feeding the
arc. If the leakage current is controlled at a low level,
the arc energy will be low, and the potential for damage or
flashover is low. By maintaining the low leakage currents
with a special silicone elastomer with unique surface
properties, and keeping the surface area at a minimum, the
surface leakage current of the insulator between the fittings
is maintained at a minimum.
The structural strength of the insulator is derived
from a fiber reinforced polymeric support rod, part 1 in
Figure 1. The fibers are electrically non-conducting. The
fibers are continuous from one end of the rod to the other
and are of the maximum amount able to be properly impregnated
with resin to give the maximum tensile strength. The fibers
are treated on their surface with a sizing or primer which is
compatible with the resin used to give a strong, void free
bond between the fibers and the resin. The preferred fibers
are glass. The fibers are bound together in a void free
manner with a rigid resin polymer. The preferred resins are
of the polyester or epoxy types, both of which can be
processed without solvent so that a void free cured rod can
be produced, as by the pultrusion process. To function
properly, the cured rod must be void free and have a smooth,
crack free surface. Any cracks or voids in the rod become
~3~3~
weak points when subjected to the electrical field set up
around a high voltage insulator.
A metal end fitting is securely attached to each
end of the core rod by swaging, use of metal wedges,
adhesives or combination of methods. The metal end fittings
are part 2 of Figure 1. The method of attachment of the end
fitting to the rod is well known in the art and is not a part
of this invention. The end fitting is attached to the rod so
that under test to failure, the rod maintains a load well
above the design load of the insulator.
Because the core rod can not provide the required
electrical insulation characteristics for the insulator under
outdoor conditions of wet contaminants, it is covered with a
void free coating of æilicone elastomer, part 3 in Figure 1.
The coating is molded over the core rod with the end fittings
attached to completely cover the rod and the junction to the
end fittings. The rod and end fittings are primed so that
the silicone elastomer bonds to the rod and to the fitting so
that there are no voids present in the interface between the
silicone elastomer coating and the rod or end fitting where
moisture could penetrate.
The insulator of this invention is required to have
at least one shed 80 that the surface leakage distance
between the metal support fittings is greater than the
straight line distance between them. The shed is part 4 of
Figure 1. A preferred configuration is an insulator having
two sheds, generally located near each end of the insulator.
Another preferred configuration has a multitude of sheds,
generally spaced equidistant from each other and the metal
support fittings. The preferred sheds have an upper and
lower surface which are approximately parallel. Preferably
the lower surface of the shed is planar; that is, it does not
have ribs as are commonly found on high voltage insulators.
1~93~34
(a) from 70 to 90 parts by weight of a
dimethylvinylsiloxy end~locked polydimethylsiloxane having a
Williams plasticity number of greater than 50,
(b) from 10 to 30 parts by weight Of a dimethyl-
vinylsiloxy endblocked polydiorganosiloxane having about 98
mol percent dimethylsiloxane units and 2 mol percent
methylvinylsiloxane units and a Williams plasticity number of
greater than 25,
(c) from 13 to 17 parts by weight of fume silica
having a surface area of greater than 50 m2/g, and a treated
surface which prevents reaction with (a) and (b),
(d) from 1.5 to 2.5 parts by weight of a hydroxyl
endblocked polydiorganosiloxane having methyl and vinyl
radicals and having about 10 weight percent vinyl radical and
about 16 weight percent hydroxyl radical, and
(e) from 90 to 220 parts by weight of aluminum
trihydrate, the mixture having been heated at a temperature
of at least 100C. for a time of at least 30 minutes.
A preferred silicone elastomeric coating comprising
the surface of the insulator is a specific composition found
to have the unique physical and electrical properties
required to enable one to make an electrical high voltage
insulator as is herein described. The composition includes
two different polydimethylsiloxane polymers, both having
dimethylvinylsiloxane endblocking. Polymer (a) has a
Williams plasticity number of greater than 50, with a
preferred polymer having a viscosity which gives a Williams
plasticity number of about 80. The other polymer, (b), has
vinyl radicals present as endblockers and also along the
chain in an amount sufficient to give about 2 mol percent
vinyl radicals in the polymer. The additional vinyl polymer
gives a higher crosslink density to the cured polymer.
Polymer (b) has a viscosity which gives a Williams plasticity
-
1~93~34
of greater than 25 with a preferred Williams plasticity of
about 28. The amount of polymer (a) is from 70 to 90 parts
by weight with from 80 to 90 parts preferred, and 85 parts by
weight most preferred. The amount of polymer (b) is from 10
to 30 parts by weight with from 10 to 20 parts by weight
preferred, and 15 parts by weight most preferred.
The composition Gontains from 13 to 17 parts by
weight of fume silica having a surface area of greater than
50 m2/g, and a treated surface which prevents reaction with
(a) and (b). The fume silica can be any of the different
types of fume silica normally used as reinforcement in
silicone rubber. The silica has a surface treated to prevent
reaction with the polymers (a) and (b). The reaction of
silicone polymer with silica is the known reaction which
causes the uncured mixture to crepe upon aging. The silica
can be pretreated as with hexamethyldisilazane, or it can be
treated in situ by including a hydroxy containing fluid such
as ingredient (f), hydroxyl endblocked polydimethylsiloxane
fluid having a viscosity of about 0.04 Pa-s at 25C. and
about 4 weight percent silicon-bonded hydroxyl radicals, to
treat the silica surface. The preferred amount is from 7 to
9 parts by weight. Preferred is a fume silica having about
250 m2/g surface area that is treated in situ, with 15 parts
by weight of the silica and 8 parts by weight of the in situ
treating agent (f) most preferred.
Ingredient (d) i~ felt to impart a tougher cured
product by introducing more crosslinking in a non-homogeneous
manner. It i~ present in from 1.5 to 2.5 parts by weight,
with 2 parts by weight most preferred.
An essential ingredient in the composition is
aluminum trihydrate (e), in an amount of from 90 to 220 parts
by weight. Preferred is an amount of from 180 to 220 parts,
with 200 parts by weight most preferred. The aluminum
1~3~34
hydrate is known to improve the arc resistance of silicone
elastomers. In the composition of this invention, it is
necessary that the aluminum hydrate be added to the mixture
in the preparation container before the mixture is subjected
to heating at a temperature of at least 100C. for a time of
at least 30 minutes.
The silicone surface of the insulator of this
invention is a silicone elastomeric material which is
required to have the following performance capabilities when
evaluated as 1 inch diameter rods having a length of 6 inches
between test electrodes:
In a fog chamber test, stressed at 1.5 kV/inch,
conductivity of 200 microSiemens per centimeter, and 30
cycles of 16 hours voltage exposure and 8 hours recovery,
1. greater than 50 hours to establish leakage
current of greater than 2 milliamperes,
2. greater than 100 hours to establish discharge
pulses of greater than 15 milliamperes,
3. total accumulated current during the 30 cycles
of less than 7000 coulombs, and
4. no flashover during the test.
In a fog chamber test, stressed at 1.5 kV/inch,
conductivity of 1000 microSiemens per centimeter, and 30
cycles of 16 hours voltage exposure and 8 hours recovery,
1. no tracking on the surface,
2. no flashover of the rod,
3. less than 4 percent weight loss, and
4. less than 1 percent foreign contaminants
accumulated on the surface (determined by ESCA test).
When evaluated as molded slabs having a thickness
of about 1/4 inch, the silicone elastomeric material has the
following property:
I ~{
`` lZ~3~3~
In a track resistance test, in accordance with ASTM
D 2303, run at 4.5 Kv and 8 strokes per minute,
1. greater than 1000 minutes to failure
A silicone elastomer composition meeting the above
requirements, when tested as shown, can be used as the
silicone elaætomeric cover of the insulator of this
invention.
A complete insulator, as in Figure 1 for example,
when tested in a fog chamber and stressed at 1.5 kV/inch
under a fog conductivity of 200 microsiemens per centimeter,
should suppress leakage currents to a level of less than 2
milliamperes for more than 10 hours; have no leakage pulses
exceeding 50 milliamperes during a total test period of 250
hours; have a total accumulated current of less than 50.
The silicone elastomeric material under test is
molded into 1 inch diameter test rods, using the same molding
conditions that will be used to mold the final insulator. A
suitable fog chamber and test procedure is described below in
Example 4.
The surface composition of the insulator is
determined using an analytical technique known as
X-ray-induced photoelectron spectroscopy, which is widely
known as ESCA: Electron Spectroscopy for Chemical Analysis.
In this technique, low energy electrons emitted by the
specimen are analyzed to provide composition information in
the one-to-ten atom layer region near the surface. A solid
sample in a high vacuum system is irradiated with a high flux
of X-rays. Core (inner-shell) electrons are ejected from all
atoms in the sample, and analysis of the kinetic energy of
these photo-ejected electrons provides information upon a
number of important properties. The precise location of the
measured peaks identifies not only the elements present, but
lZ~3~34
also their chemical environment. This test determines the
nature of a surface by its chemical composition.
The track resistance test referred to above is an
ASTM D 2303 test of the American Society for Testing and
Materials. The current edition is found in section 10 of the
Annual Book of ASTM Standards. The contaminant pump is
operated at a rate of 8 strokes per minute, resulting in a
flow of 0.60 ml/min. The voltage is set at 4.5 kV. The
sample is tested under the time-to-track method. The
insulator material is required to withstand greater than 1000
minutes without tracking or severe erosion.
The following examples are included for
illustrative purposes only and should not be construed as
limiting the invention, which is properly set forth in the
appended claims. All parts are parts by weight.
EXAMPLE 1
Vertical suspension insulators of an ethylene-
propylene elastomer formulation and of a silicone
formulation, with similar filler contents, were prepared and
tested under use conditions to compare their relative
resistance to wet contamination conditions under voltage
stress.
The insulators were designed for use at 115 kV.
They were tested at 66 kV, line to ground, under heavily
contaminated conditions. The insulators had a straight line
distance between the end fittings of 37.75 inches. There
were 9 sheds having a diameter of 4.5 inches and 8 sheds
having a diameter of 3.5 inches. A smaller shed was located
between each of the larger sheds. The minimum distance
between sheds was 1 inch. The upper surface of the larger
sheds had an angle of about 55 degrees to the core while the
smaller sheds had an angle of about 45 degrees. The lower
surface of each shed was approximately parallel to the upper
1293~:?39~
surface. The leakage distance between the support fittings
was 67.5 inches.
A silicone elastomer composition was prepared using
a procedure falling under the method used to make insulators
of this invention. The composition was prepared by mixing in
a dough mixer, 85 parts of dimethylvinylsiloxy endblocked
polydimethylsiloxane having a Williams plasticity number of
about 80, 15 parts of dimethylvinylsiloxy endblocked polydi-
organosiloxane having about 98 mol percent dimethylsiloxane
units and 2 mol percent methylvinylsiloxane units and a
Williams plasticity number of about 28, 8 parts of hydroxyl
endblocked polydimethylsiloxane fluid having a viscosity of
about 0.04 Pa-s at 25C. and about 4 weight percent silicon-
bonded hydroxyl radicals, 2 parts of hydroxyl endblocked
polydiorganosiloxane having methyl and vinyl radicals and
having about 10 weight percent vinyl radical and about 16
weight percent hydroxyl radical, 15 parts of fume silica
having a surface area of about 250 m2/g, and 200 parts of
aluminum hydrate, then mixing and heating to about 175C. for
about one half hour. After cooling, 100 parts of the
composition was mixed with 0.45 part of catalyst of 50
percent 2,5 bis(tert-butylperoxy)-2,5-dimethyl hexane in
powdered carrier.
An ethylene-propylene-dimer composition containing
about 95 parts by weight of polymer and about 180 parts by
weight of aluminum trihydrate filler was used to prepare
comparative insulators.
The insulators were installed in a test station
under a test voltage of 66 kV, line to ground. The
insulators were continuously stressed electrically. The test
station was located along the seacoast, where it was
subjected to periodic fog, as well as salt contamination.
Standard porcelain suspension insulators operating at 0.5
lZ~3~34
kV/inch failed by flashover in less than one year of testing.
At 1 kV/inch, they generally failed in 1 to 3 months. The
insulators were periodically inspected for damage. A record
was also maintained as to whether there was a flashover of
the insulator. The observations are summarized in Table I.
The results show that the silicone elastomer insulator
performed much better than the similar insulator made of
ethylene-propylene elastomer.
The insulators made of ethylene-propylene-dimer did
not function satisfactorily in that they flashed over many
times during the test. The silicone insulators functioned in
that they did not flash over, track, or puncture.
EXAMPLE 2
Test insulators were prepared to evaluate the
ability of a silicone elastomer insulation material of the
type claimed herein to resist the effects of arcing caused by
dry band formation during a long term, extended, tracking
wheel test.
The core of the insulator was fiberglass reinforced
cycloaliphatic epoxy rod having a diameter of 0.67 inches.
Standard metal high voltage end fittings were applied to each
end of the core rod, with a distance of 7.5 inches between
end fittings. An epoxy composition was molded over the core
to form a layer of about 0.25 inches thickness. An
integrally formed core cover and sheds were then molded over
the core, using the silicone composition of Example 1. The
core cover diameter was 1.25 inches. There were 4 sheds,
having a diameter of 3 inches and a distance of 2 inches
between each shed. The upper surface of the shed was at an
angle of about 55 degrees to the core and the lower surface
of the shed was at an angle of about 90 degrees to the core.
The total leakage path distance between the end fittings was
12.5 inches. The design ratios for this insulator were:
1~3~3~
D = 3.0 = 1.5
Ls 2.0
Lc = 12 5 = 1.7
Ds = 3 0 = 2.4
Dc 1.25
Four of these insulators were evaluated in a
tracking wheel test. In this test, four insulators were
mounted on a wheel by attaching one end of the insulator to
the wheel so that the insulators were extended radially at
right angles to each other. The wheel is rotated to position
each insulator in a given location for 15 seconds. The
travel time between positions is 15 seconds. The first
position is the dip position. A dip tank was placed under
the wheel so that the position submerged the insulator in a
salt water solution in the tank. The salt water was heated
to 30C. and contained 1 percent sodium chloride to give a
conductivity of 2000 ~ 500 microsiemens per centimeter.
The second position is the drip position. Excess
saline solution is allowed to drip off the insulator. The
dry bands are formed where the leakage current density is the
highest. Usually this occurs along the shank of the
insulator. Sparking across high current dry bands usually
leads to tracking and/or erosion of the shed material.
The third position is the energized position. An
electrical connection is made to the outer end of the
insulator applying voltage across the insulator. During
energization, sparking across dry bands concentrates along
mold release lines, joints between sheds, and at the end
seals. Heat produced from the sparking may result in
tracking and/or erosion of the material. Tracking leads to
failure along the surface of the insulator. Erosion may lead
lq
1~3~3~
to water ingress and electrical failure along the rod-shed
interface.
The fourth position is the cooling position. The
shed material heated by the arcing is allowed to cool.
Bonded portions of an insulator are thus subjected to thermal
cycles.
Experience with many materials in this test
procedure has established that 60,000 cycles is sufficient to
discriminate superior designs from inferior ones at a stress
level of approximately 0.5 kV/inch.
When the silicone insulators were tested under 20
kV test voltage (1.5 kV/inch), a flashover occurred after
about 7000 cycles due to surface contamination. The test
method requires cleaning of the insulator before the test is
continued. If the insulator shows severe tracking or erosion
it is removed from the test. The insulator was cleaned and
the test continued.
After the 80,000 cycles, the insulators were
removed and cleaned and examined. There was no material
damage evidenced by either erosion or tracking.
EXAMPLE 3
Test insulators were prepared to compare the
methods of this invention to prior methods using epoxy
compositions as the surface of the insulator.
An insulator was molded from a cycloaliphatic epoxy
composition. The core was fiberglass reinforced with a
diameter of 0.67 inches. Standard metal high voltage end
fittings were applied to each end of the core, with a
distance of 7.5 inches between them. Then the epoxy
composition was molded over the core to form 7 equally spaced
sheds having a diameter of 3.0 inches and a distance of 1.0
inch between each shed. The diameter of the cover over the
core was 1.0 inches. The upper surface of the shed was at an
1 ~93~34
angle of about 55 degrees to the core and the lower surface
was at an angle of about 90 degrees to the core. The total
leakage path distance between the end fittings was 16.25
inches. This is insulator A. A duplicate was made which is
insulator D.
An epoxy insulator having a shorter leakage path
was prepared by modifying the mold to eliminate 3 of the
sheds, leaving four e~ually spaced sheds having a diameter of
3.0 inches and a distance of 2.0 inches between each shed.
The sheds were the same size and shape as above. The total
leakage path distance between the end fittings was 12.5
inches. This is insulator B. A duplicate was made which is
insulator E. For this insulator, the design ratios were:
D = 3.0 = 1.5
LSs 2.0
Lc = 12.5 = 1.7
La 7.5
D = 3.0 = 2.4
- DCs 1.25
A silicone insulator falling under the method of
this invention was prepared by molding the composition of
Example 1, using the 4 shed mold described above to prepare
the 4 shed epoxy insulator. This is insulator C, the
duplicate insulator is insulator F.
The 6 insulators were then tested in the fog
chamber described above. The insulators were suspended in a
circle in a vertical position around the center of the fog
chamber. The top of the insulators were attached to a metal
wheel, which was suspended at its center from the high
voltage terminal. The bottom of each insulator was attached
to ground, through the measuring system used to measure the
current flow over the surface of each insulator during the
~:93~34
test. The spray nozzles used to create the fog in the
chamber were directed toward the center of the chamber. The
water had a conductivity of 200 microSiemens per centimeter.
The voltage was set at 20 kilovolts.
Table II shows the accumulated charge in coulombs
during the test by hours. The leakage current of the
silicone insulators was orders of magnitude less than that of
the epoxy insulators, thus reducing the probability of
flashovers.
EXAMPLE 4
The silicone elastomeric composition of Example 1
was evaluated in the form of 1 inch diameter rods. This is
Composition A in Tables III, IV, and V.
Comparative rods were also prepared. Composition B
was an ethylene-propylene-dimer polymer filled with 120 parts
by weiqht of aluminum trihydrate. Composition C was a
silicone composition of 100 parts by weight of base and 120
parts by weight of aluminum trihydrate filler added to the
base. The base was 100 parts by weight of polydiorgano-
siloxane gum having about 0.14 mol percent vinyl radicals and
the rest methyl radicals with dimethylvinylsiloxy endblockers
and a Williams Plasticity of about 150, 7.5 parts by weight
of hydroxyl endblocked polydimethylsiloxane fluid having a
viscosity of about 0.04 Pa~s at 25C. and about 4 weight
percent silicon-bonded hydroxyl radicals as a treating agent
for the filler, and 23 parts by weight of fume silica having
a surface area of about 250 m2/g. The base was mixed and
heated to treat the filler and remove volatile materials
before the aluminum trihydrate was mixed into the base.
The composition was molded into rods, 1 inch in
diameter. Pieces 6 inches long were fitted at each end with
carbon disc electrodes by placing a stainless steel screw
through the center of the disc into the end of the rods. The
1'~93~34
samples were then suspended in a circle in the center of the
fog chamber by connecting the top electrode to a common high
voltage bus. The fog chamber was a cubicle having 2.54 meter
sides constructed from 3.2 mm thick "Plexiglas" sheet. The fo~
was created by nozzles which are constructed according to IEC
Publication 507, 1975. The fog chamber had four nozzles
placed equidistant on a pair of stainless steel tubes having
an internal diameter of 7.94 mm and forming a ring of 2.54 m
in diameter. A corrosion-resistant feed pump supplied salt
water to the nozzles from a reservoir. The flow rate was
adjusted to 0.58 MPa (80 psig). The water was recycled
during the test. The test ~upply was a 14.4 kV/230V, 37.5
kVA distribution transformer controlled by a 10 XVA, 0 to 208
V "Variac"**. The data acquisition system used was similar to
that given in the paper "Evaluation of Polymer Systems for
Outdoor H.V. Insulators Application by Salt-fog Chamber
Testing", Reynart, Orbeck, and Seifferly, IEEE International
Symposium of Electrical Insulation Conference, 1982. The
output gave peak and average instantaneous current values on
both the positive and negative half cycles. The current was
integrated over the duration of the test to obtain the
cumulative charge. The number of leakage current pulses
between preset limits of current values was also obtained.
The 6 inch long rods were subjected to a voltage of
9 kV rms to give a stress of 1.5 kV per inch. In a first
test, the conductivity of the fog was 250 microsiemens per
centimeter, obtained by using tap water. In a second test,
the conductivity was 1000 microsiemens per centimeter, using
sodium chloride to increase the conductivity. A test cycle
consisted of 16 hours under fog and voltage stress, followed
by 8 hours with no fog or voltage. The tests were carried
out for 30 cycles. The test results are shown in Table III
for the 200 microsiemens fog.
*Trademark for poly(methyl methacrylate)
** Trademark
~ 3
1~3034
Rods evaluated using 1000 microsiemens per
centimeter for the conductivity of the fog at this
voltage level are subjected to heavy arciny from the
continuous dry band arcing that is going on. The heavy
arcing causes tracking and erosion, depending upon the
composition used. The silicone rod described here did
not track or erode significantly when tested, other
compositions having other filler loadings and types, as
well as other types of polymer eroded to failure in this
test. The test results are shown in Table IV for the
1000 microsiemens fog.
The surface composition of the rods was determined
using an analytical technique known as X-ray-induced
photoelectron spectroscopy, which is widely known as ESCA:
Electron Spectroscopy for Chemical Analysis. In this
technique, low energy electrons emitted by the specimen are
analyzed to provide composition information in the one-to-ten
atom layer region near the surface. A solid sample in a high
vacuum system is irradiated with a high flux of X-rays. Core
(inner-shell) electrons are ejected from all atoms in the
sample, and analysis of the kinetic energy of these
photo-ejected electrons provides information upon a number of
important properties. The precise location of the measured
peaks identifies not only the elements present, but also
their chemical environment. This test determines the nature
of a ~urface by itæ chemical composition. The results of a
surface test of the rods after fog exposure is given in Table
V. The silicone rods before exposure would have an analysis
of approximately 25 percent oxygen, 25 percent silicon, and
50 percent carbon.
The sodium and calcium would be expected to come
from the salt-water fog. The aluminum comes from the
aluminum trihydrate filler in the compositions. The silica
on the EPDM must come from contaminants in the water during
the test. If the amount of sodium and calcium are used as
~3~34
indicators, the test shows that the EPDM accumulates a higher
amount of contaminant on its surface during both levels of
fog contamination. This accumulation of pollutant may cause
more leakage current and dry-band activity which may lead to
flashover.
The track resistance was determined in accordance
with ASTM D 2303 test of the American Society for Testing and
Materials. The current edition is found in section 10 of the
Annual Book of ASTM Standards. Because of the extre~e water
repellency of the silicone material, the test sample was
turned over so that the contaminant solution flowed down the
face of the test sample. The contaminant pump was operated
at a rate of 8 strokes per minute, resulting in a flow of
0.60 ml/min. The voltage was set at 4.5 kV. The sample was
tested under the time-to-track method. The silicone
composition was molded into a slab having a thickness of
about 0.075 inches for this test. The test was terminated
after 4000 minutes as the sample had still not failed at that
time.
1~93~34
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