Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Tins invention relates to composite plastic insulators particularly
for high voltage power lines, and having a glass fire reinforced plastic rod,
plastic sheds arranged on the rod, and fittings on the ends of the God.
Composite insulators of this general type are known, for example, from
Nazi 2650363 and satisfy specific electrical requiremeues. The rod must be
electrically resistant to puncture discharge. In addition, the sheds must be
secured to the rod (also called a trunk) so that no discharge penetration can
take puce in the junction area with the rod. m e sheds themselves must be
sufficiently thick that they too do jot puncture. Finally, the sheds must be
weather-, ultraviolet-, and ozone-rssistant and display great resistance to flyer.
Besides the electrical strength, the glass fire reinforced plastic
rod must show considerable mechanical strength. Such mechanical strength
depends on the composition of the material, the type and positioning of the
fires, and the bond between the fires and the plastic.
It is known chat the electrical and mechanical strength of the glass
reinforced rod of a composite insulator can fall considerably after an extended
period of use, particularly in exposed high voltage power lines as a result of
environmental influences or weathering. Attempts have been made to shroud the
rods with the sheds in a manner such that atmospheric effects do penetrate to
the rod itself. However, to date this has not been achieved satisfactorily
and the possibility of insulator breakdown is always present.
Another proposal (DEMOS 2650363) treats the problem of water attack
on the glass fire reinforced plastic rod as being primarily responsible for
the strength reduction. Glass fires asp thus employed which, although of
conventional composition, are of particularly low alkali content. Such fires
by their very nature ensure a low level of water volubility and if alkali is
leached from glass fires it can initiate and accelerate hydrolysis ox the
bonding resin. In addition to the use of low alkali content glass fires
a non-saponifiable bonding resin resistant to water attack is also employed.
Despite the measures described above, breakdown still occurs,
particularly in exposed power line insulators of this type. Initially such
failures appeared inexplicable and occurred even after a relatively short
period of use under relatively slight mechanical load. The fractures
displayed by the insulators which failed in use, differed visually and quite
distinctly from those which occurred, for example, during fracture tests
conducted in laboratories and also in long term strength tests measured in
years in exposed test areas. A plastic rod reinforced with continuous glass
fires laid parallel to the axis breaks under mechanical load by separation of
the bonding resin from the glass fires which then tear. The rod thus
splinters longitudinally. Fractures which occur on site however are oriented
almost perpendicularly to the longitudinal axis of the rod and the fracture
surfaces are smooth.
Surprisingly, tests reveal that the smooth fractures perpendicular to
the longitudinal axis of the rod result from the effects of aqueous nitric
acid. It has been known for a considerable time that nitric acid is formed
from atmospheric nitrogen by electrical discharge in the presence of air and
water. Clearly, this occurs during electric discharge activity on the surface
of the insulator sheds in the presence of dirt and moisture. It appears that
such nitric acid diffuses through the surface of the sheds or is conveyed
through gaps and cracks between individual sheds and reaches the glass fire
reinforced plastic rod to cause the smooth transverse fracture. This would
explain the absence of occurrence of transverse fractures in laboratory tests
and why such fractures are not described in the literature covering glass
fire reinforced plastics.
It is an object of the present invention to prevent transverse
fractures which occur on site in effffl~ffn~* insulators of the type described
above.
Here described is a my insulator ox i)]flStiC, consisting ox a
glass fire reinforced plastic rod, of plastic sheds arranged on the rod, an-l
fittings on the rod ends, in which glass ~ibres arrange axially parallel in
the plastic rod consist of aluminum-silicate glass tilclt has a low boron
content or else preferably, is free of boron. In the corltext of this
disclosure, glass having a low boron content is understood to be a glass that
contains boron or a boron compound, calculated as B20~ at one percent by
weight maximum.
Ryan free glasses are lasses which contain boron or boron
compounds, calculated as B203, at less than 0.01 percentage parts by weight
lower boron contents can be measured only by trace analysis and are negligible
for purposes of this disclosure.
Particularly suitable are glass fires of glass having the following
composition yin percentage parts by weight):
Sue 55-80
Aye 20-30
Moo 5-15
Coo 0-10
Noah 0-1
Preferred are glass fires of a glass having the following composition.
Sue 60-80
Aye 20-30
Moo 5-15
Coo 0-2
Noah
Glass fires having no Coo are particularly resistant and are thus
preferred.
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The glass Flares should have a thickness of 5 to I Jim laid up in
continuous lengths parallel to the axis of the rod.
Additionally in the glass fires here described, it is advantageous
that the alkali oxide content of the glass be less than I part by weight.
Preferably the glass fires should be free of alkali. Water attack can thus
be prevented and the resistance to electrical puncture, can be increased.
To increase the resistance of the rod to electrical breakdown tune
bonding resin surrounding the glass fires can be a resin resistant to attack
by water. It is preferred that the bonding resin have no molecules which can
be hydrolyzed. An epoxy resin of the glycidyl ether type is suitable.
So that the insulator is economical to produce, prefabricated
components for the sheds may be used. Insulators of any desired length can be
produced. The glass fire reinforced plastic rod can be made for example, in
a continuous drawing process. The surface of the rod, and of the sheds, can
be treated in a known manner with adhesive agents and joined, for example, by
integral casting, vulcanizing, cementing, or the like.
The radial joints between the sheds no longer play a dominant role,
since the glass fire reinforced plastic rod is resistant to nitric acid as
well as to water.
In considering the transverse fracture problem, two conceivable
solutions to the problem outlined above emerged. Firstly, attempts could be
made to ensure that no aqueous nitric acid reached the plastic rod. This
possibility now appears, for all practical purposes, to be impracticable,
since the nitric acid diffuses through plastic and over the long term it was
impossible to ensure that gaps or breaks did not occur in the sheds, between
them or between the sheds and the rod end fittings. Secondly, a solution
could be to use materials for the insulator rod resistant to nitric acid.
Tests in which rod material of commercially available compound
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insulators was stored in nitric acid, revealed that neither the glass fires
nor the plastic used as the rod material were attacked by aqueous nitric acid,
and thus one was led to conclude that selection of the rod material alone
would not lead to a solution. It is most surprising therefore that,
replacement of the normally used glass flare types by boron free glass fires
essentially eliminated the danger of transverse fracture. The precise reasons
for this phenomenon have not yet been determined.
Aqueous nitric acid attack on glass containing boron can be accepted
as a cause for the transverse fractures found in insulators. Thus if glass
lo with a boron content is simultaneously subjected to tension and nitric acid it
is possible that minuscule cracks will occur in the surface ox the individual
glass fires, and that these cracks will propagate as helixes around the
fire. These minuscule cracks are responsible, at least in laboratory tests,
for transverse fracture of the rod. Clearly, what is involved is not chemical
attack, in the sense of swelling or dissolution) but more a type of tension
crack corrosion that does not occur in boron-free fires or which occurs only
at higher degrees of elongation or higher acid concentrations.
It is, however, known that, glass fiber having a low alkali content
and free of boron or boron compounds render mechanically loaded insulated
components sufficiently strong for high voltage switching equipment which
contains sulfuric hexafluoride gas (European Patent 0 02~ 281, published I May
1983)~ Knowledge of is effect on the fires of a so called R-type glass does rut
immediately lead the man skilled in the art to the solution of the problems
herein, since the decomposition products of SF6 do not occur in the areas of
use of exposed power line compound insulators.
The selection of glass fires having a low or no boron content did
not suggest itself since f1bres of glass containing boron, the so-called
E glasses, are usually used for such electrical components on account ox their
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extremely good electrical resistance. E type glass is not attacked by
aqueous nitric acid, so that its exchange for other glass fires could not
reasonably be considered as warranted. This obstacle must be overcome in
order to solve the problem outlined herein and to which the present invention
provides a solution.
ore particularly in accordance with the invention there is provided
s composite insulator for outdoor high volts lines, comprising:
a lass fiber reinforced plastic rod having ~etsl fittings at the
end thereof, the glass fibers brine disposed parallel to the axis of the rod,
and consisting essentially of a Cougher glass hazing little or no boron
content, and consisting essentially of from about 55 to about 80 White Sue,
from stout 20 to about 30 wit% Aye, from about 5 to about 15 wit% Moo and
from Nero to about 1 wit% Noah, whereby the composite insulator it resistant
to in situ occurring transverse fractures The boron content based on B203
is at most one percent part by weight and when free of boron contain less than
0.01 % parts by weight of B203. The gloss fires preferably have an
alkaline oxide content of less than one percent part by weight, but more
preferably are alkaline free 80nding resin surrounding the glass fires is
preferably resistant to attack by water contains no hydrolyzable molecules and
may be an epoxy resin of the glyc;dyl ether type. The lass fires may be
continuous and a thickness of 5 to 40 us
Specific embodiments of the invention will now be described with
reference to the accompanying drawings in which;
Fig. 1 is a schematic representation of a test set up used to examine
resistance to transverse fracture;
Fig. 2 is a graph of the test results,
Fix. 3 is an insulator in partial section, embodying the invention.
The test piece shown in Fig. 1 consists of a g1BSS fire reinforced
plastic rod 1 with end fittings 2, to which a tensile force Z can be applied.
AL ICY
Carried on the rod is an acid reservoir I, or example, a modified
polyethylene bottle Inounted on the rod and sealed with insulating tape.
Fig. 2 shows graphs of the relationship between Lye tensile force Z
and the fracture time for test pieces of Fig. 1 plotted as Log Z against
Log T. Curve 4 indicates the tensile force/time until fracture of the rod of
a glass fire reinforced plastic rod not exposed to the effects of acid.
Curve 5 shows the tensile force/time to fracture relationship with the acid
reservoir 3 filled with a lo HN03 solution (approximately 6.5% nitric acid
by weight of a glass ire reinforced plastic rod, with fires of boron
content, calculated as B203 between 2 and 6% by weight. The fracture/time
behavior of a rod, with glass fires containing no boron is indicated by
curve 6. The differences in fracture time between known glass fire
reinforced plastic rods (glass containing boron) and the new rods, with boron
free glass, is clearly demonstrated.
The use of boron glass in electrical technology, and particularly
glass fire reinforced plastic rods in compound insulators, is conventional
(see DEMOS 27 46 870, page 10). The so-called "E type glasses" are used in
electrical technology (the "E" standing for "electrical"). All commercially
available glass fires obtainable with the E-type glass designation contain
various quantities of boron. Curve 5 in Fig. 2 for typical commercial glass
fires can display certain deviations from that illustrated and this can be
traced back to the varying boron content of E-type glasses. In comparison to
the behavior of the new rods here described (Fig. 2, line 6) this variation
among such E-type glasses is neglible.
Fig. 3 shows a compound insulator embodying the invention, which
consists of a glass fire reinforced plastic rod 7 made of epoxy resin of the
glycidyl ether type, and continuous parallel glass fires of boron content,
calculated as B203, of less than 0.01~, and of an alkali content,
calculated as Noah, less than lo (all parts by weight).
The insulator also consists of a shed string made of individual
prefabricated sheds 8, installed on the rod and securely bonded to it if. both
the mechanical and electrical sense. In addition, there are metallic
connector fittings 9, secured to the ends of thecompositeinsulator. The
connection between the rod 7 and the fittings 9 can be made using known
technology such as by pressing or keying.
Hoover, dependent on the type of material used for the sheds, it may
be advantageous to prefabricate the string and install it in a separate
lo production stage. Other materials used for the shed string can comprise
casting, pressing 9 extruding or injection molding of the shed bodies in a
single or multiparty mold as the most economical solution.
Silicone elastomers can be used for the shed coverings Sue sxhhaMe Ann
proven to be good insulator materials. Silicone elastomers of various
consistencies containing fillers determined by the area of Sue, such as quartz
flower or aluminum oxide hydrate, and Rich kitten pigments and polymerizing
agents can be easily processed. For specific areas of application, elastomers
based on ethylene-propylene can be suitable as materials for the shed
elements. Other materials such as cycloaliphatic epoxy resins or
polytetrafluoroethylene can also be used for insulators embodying the
invention.
