The present invention relates to an electrical insulator, and to
methods for preparing the same.
It is known, e.g. from Italian Patent No. 1,114,909, that in composite
insulators with a ribbed covering of an organic material, the mechanical support
function is entrusted to a central cylindrical component of fiberglass-refinforced
Such component is generally manufactured as a solid cylindrical rod
(either continuously or batchwise) by the pultrusion method, or by the method of
stratification of fiberglass roving clothes, followed by a tool machining.
Another geometrical shape of the product is that of a hollow cylindrical
body, obtained by the method consisting in filament winding of continuous fibers,
impregnated with a thermosetting resin, with said continuous fibers being generally
wound according to a helical pattern.
One of the critical points of these composite insulators turns out
to be the connections, i.e. the connection with the end terminal parts destined
to transmit the stresses from the insulators both towards the support elements,
and towards the electrical conductors. The realization of these connections depends:
- -- on the geometrical shape of the end of the element of fiberglass-reinforced
- -- on the geometrical shape of the metal part destined to be coupled with it;
- -- on the method used for connecting the above said first element with the
above said second element; and
- -- on the technology used for practically accomplishing said connection.
The most common connections for composite insulators up to date used,
or anyway known, are classified according to the essential geometries of the ends
of the articles, and of the fastening methods. Therefore connections with cylindrical
ends, with conical ends, or with ends would on a metal insert, performing the
function of terminal, exist.
For the first type, the solid cylindrical rods as previously described
The connection is obtained by means of a radial compression stress,
according to the following methods:
- -- plastic deformation of a cylindrical and hollow metal terminal, inside which
the end of the rod is inserted, according to the technique of compression, as disclosed
in U.S. Patent No. 3,898,372;
- -- application of resin cones to the ends of the rod, and insertion inside
metal terminals having opposite conicity (principle of Morse cones), as described
in "IEEE Transactions on Power Apparatus and Systems", Volume PAS-102, No. 9,
September 1983, page 3, 123.
According to this method, the slipping of the cones on the rod is
partially counteracted by adhesive forces and, mainly, by means of a pre-tensioning
step, which generated strong radial stress components.
According to an alternative route, the cones of resin can be replaced
by conical metal jaws.
The drawbacks of this first type of juncture are mainly due to the
difficulty met in metering the radial compression stress sufficient to supply an
axial component which is at least equal to the rated tensile stress of the insulator,
but not so high as to endanger the strength of the end of the rod, considering
that this is a permanent stress, destined to last throughout the life of the insulator.
In the second type of juncture, with the article having conical ends,
said conical ends are coupled with metal terminals having an opposite conicity,
generally with the insertion of a filling material (either a resin or cement) capable
of transmitting the stresses, according to techniques known for a long time in
the art, and tested on "cap-and-pin" insulators and "rod" insulators of ceramic
The main differences between the various solutions derive from the
technologies used to form the end cones which, in any case, have as their outer
surface the resin-impregnated fiberglass.
The end cones can be formed either by means of the tool-machining
of a cylindrical rod, or by acting on the end of the same rod during the polymerization
of the resin, with the following geometries:
- -- tapered-end-shape (the diameter of the smallest cross-section of
the cone is smaller than the diameter of the rod);
- -- threading;
- -- elliptical-cross-section cone obtained by "squashing" the cylindrical
- -- wedge-shaped ends, by forcibly inserting a small-angle cone into
the center of the end cross seciton of the end of the cylindrical rod.
The drawbacks of this second type of couplings reside in the methodology
used for forming the cones, which requires either the removal of fiberglass in
case of tool machining, or the deformation thereof during the polymerization, with
an unavoidable weakening, in all cases, of the juncture, which thereupon becomes
the weak point of the insulator. CH-A-576690 discloses an insulator according to
the preamble of claim 1. DE-A-2 046 774 discloses a fibre-glass reinforced insulator.
SU-1148-050-A discloses shaping a fibre-glass air-blast circuit breaker cylinder.
The third type of connection, i.e., that type wherein the fiberglass
is wound on a metal terminal provided with a shoulder, is disclosed in French Patent
No. 1,390,405 and in French Patent Application No. 73/30,900, relating to line
According to these patents, an insulating tube, filled with an expanding
insulating material, is inserted inside two metal terminals provided with a shoulder
having a suitable shape.
The glass filament, impregnated with resin, is wound, in a helical
pattern, with a suitable winding angle, both the tube and the outer surfaces of
the two metal terminals being such as to permanently connect them with each other.
The whole structure is then coated with a ribbed insulating material.
The main drawbacks shown by this type of connection are the following:
- -- a larger diameter, with the strength being the same, and hence higher costs
and larger overall dimensions of the external ribbed coating, in as much as both
the tube and its filling do not transmit longitudinal stresses;
- -- possibility of partial discharges, with consequent decay in insulation,
due to the strong electrical gradient generated by the metal parts inserted in
the article, due to the possible presence of vacuoles inside the tube filling;
- -- poor protection against the penetration of moisture in correspondence of,
and along, the surfaces of the metal parts, which, among others, are electrically
separated from each other only by the cylinder of insulating material.
The present invention also provides an electrical insulator, comprising
a support structure and a covering, the insulator further comprising connections
for connecting the insulator to support elements and electrical conductors, the
support structure being made of fibre glass-reinforced resin, and comprising a
central cylindrical portion (A) and ends (B) having the shape of solids having
a surface of revolution, with axial symmetry, having diameters larger than the
diameter of the central cylindrical portion (A), characterised in that the ends
(B) are radiused with the central cylindrical portion (A) without discontinuity,
in that said central cylindrical portion (A) and said ends (B) comprise superimposed
and crossed layers of glass filaments (2) impregnated with a thermosetting resin,
wound around a cylindrical element (1) with a helical winding angle smaller than
90°, and in that the superimposed and crossed layers of glass filaments (2) are
alternated, in the vicinity of the end portions of the cylindrical element, with
further layers of filaments (3) wound with a winding angle larger than the helical
By the term "helical winding angle" as used in the disclosure and
in the claims, the acute angle is understood which is formed between the projections,
on the same longitudinal plane, of the wound filaments and of the longitudinal
axis of the body.
In the support structure of the present invention, the ends having
the shape of solids with surfaces of revolution, with axial symmetry, may be constituted
by superimposed and cross layers of glass filaments alternating, in the vicinity
of the end portions of the cylindrical element, with further layers of glass filaments
wound with a winding angle larger than the winding angle of the helical winding.
As an alternative, said ends may consist of superimposed and crossed
layers of glass filaments wound around a cylindrical element constituted by a cylinder
of insulating material having the end portions already shaped as solids with surfaces
of revolution, with axial symmetry, with diameters larger than the diameter of
A method for preparing the electrical insulator of the present invention,
provided with a support structure, comprises:
- (a) winding around a cylindrical element at least one continuous glass filament,
impregnated with a thermosetting resin, with a helical winding angle smaller than
- (b) alternating and super imposing upon the helical windings, in the vicinity
of the end portions of the central cylindrical body, other windings, with a winding
angle larger than the winding angle of the preceding (a) stop; and
- (c) polymerizing and curing the impregnating resin.
The ends of the support structure for electrical insulators, according
to the present invention, have preferably the shape of a frustrum of a cone, and
may be obtained by alternating, in correspondence to the end portions of the cylindrical
element to the superimposed and crossed layers of glass filaments, further layers
of filaments wound with an approximately right winding angle, less and less extended
in the longitudinal direction, and having their beginning more and more shifted
towards the end sections, such as stepwise and gradually to increase the winding
A further method for preparing the support structure of the present
- (a) winding at least one continuous glass filament, impregnated with a thermosetting
resin, with a helical winding angle smaller than 90°, around a cylindrical element
having its ends already shaped as solids with a surface of revolution, with axial
aymmetry, with diameters larger than the diameter of said cylindrical element;
- (b) polymerizing and curing the impregnating resin.
The cylindrical element around which the helical winding of the continuous
filament is applied, is preferably constituted by a bundle of parallel glass fibers
impregnated with a resin.
This bundle, whose thickness is of a few mm, such as up to 10 mm,
may be obtained by using the same filament used as the winding filament.
As an alternative, the cylindrical element may be constituted by
solid cylinders of a few millimeters of diameter obtained, e.g., by pultrusion,
or by hollow cylinders, to be filled with an insulating material; such cylinders
may remain inserted inside the end article, provided that they have a good mechanical
strength, optimum electrical qualities, physical properties similar to those of
the wound article, and high enough elasticity to follow the deformations thereof.
As a second alternative, the winding may be started on rigid rods,
also of a metal material, to be removed at the end of the same winding step; the
so-formed hollow may then be filled with an insulating material, or it may be left
empty, when the use of the structure as a bushing or hollow insulator is contemplated.
The helical winding angle is selected as a function of the stresses
that the support structure of the present invention must withstand; preferred is
an angle within the range of from 1 to 60°, and, more preferably, of from 5 to
30°, to endow the article with an axial tensile strength of the same order of magnitude
as that of a parallel-fiber pultruded rod having the same diameter, with a strength
of resistance to a radical component of the stress being at the same time obtained
in the article.
Such a radial resistance is very useful in case of stresses different
from pure tensile stresses, such as the stresses due to aeolian vibrations, to
sudden load detachments, to unsymmetrical loads, and so forth.
The glass filament used to prepare the support structure for insulators
of the present invention has a count preferably within the range of from 600 to
4,800 tex, and is preferably impregnated with cycloaliphatic epoxy resins, such
as the epoxy resins based on diphenylolpropane and epichlorohydrin.
Further examples of thermosetting resins which may be used are vinyl
ester resins, unsaturated polyester resins, polyurethane resins, and so forth.
Glass filament is preferred in the manufacture of the support structure
of the present invention, because, besides being endowed with well-known excellent
dielectric, chemical and physical properties, it gives the composite the optimum
elasticity of this type of articles.
The selection of the glass filament, to prepare the support structure
of the present invention, should not be considered as limitative, however, inasmuch
as filaments made from other materials having properties similar to glass may be
used. Examples of such materials are the aramidic polymers used, e.g., in the
preparation of Kevlar fibers.
The support structure for electrical insulators of the present invention
shows preferably ends in the shape of the frustrum of a cone, which are suitable
for the assemblage with metal parts having opposite conicity, with the interposition
of a bonding material, according to techniques known in the field of insulators,
without suffering from the drawbacks due to such method of formation of the cones,
as hereinabove described. In fact, the outer surface of the conical end is completely
coated, with cross-wound layers, by the glass fiber, without solution of continuity,
with a geometric precision and a uniform tension, is carefully impregnated with
resin, and is polymerized and heat-cured in a heating apparatus, thus, avoiding
interruptions in the process, forced deformations or cutting of the fibers, as
encountered in other methodologies.
Another advantage displayed by the present invention relates to the
ends of the article and the possibility of accurately radiusing them to the cylindrical
portion, avoiding reductions in strength which are caused by sharp changes in
cross section, typical of other structural solutions.
Due to the same reasons of uniformity in manufacturing, the same
cylindrical portion shows not indifferent advantages as compared to the pultruded-rod
solution, besides the advantage of withstanding stresses different from the already-described
axial tensile stress; in pultruded rods, reductions in strength are likely to
be easily found, which are due to the uneven pulling tension of the glass fibers,
which are, all together, pulled parallel to the extruder. An uneven co-operation
and distribution of stresses between the fibers may derive therefrom, with a consequent
reduction in tensile strength.
Articles made of fiberglass-reinforced resin of the present invention
may be used as such, as the mechanical support for any types of composite insulators
for substations, and for overhead electrical lines, and with any adequate types
of covering. They may have diameters within the range of from 10 to 800 mm, and
lengths within the range of from 100 mm to 6,000 mm.
They may furthermore be used with any voltage values, even larger
than 300 KV, for alternating currents or for continuous currents, for both indoor
and outdoor use.
The invention will further be described, by way of example, only
with reference to the accompanying drawing, which shows a longitudinal sectional
view, and a view thereof, of a support structure for electrical insulators according
to the invention.
Referring to the drawings, the support structure comprises a cylindrical
central body A and ends B.
The cylindrical central body A comprises, in its turn, a cylindrical
element 1, constituted by a bundle of filaments, and superimposed and crossed layers
2 which are obtained by winding the filament according to a helical pattern around
the cylindrical element 1 as described above.
The ends B, radiused to the central body without solution of continuity,
comprise the cylindrical element 1, the layers 2, and further layers 3 obtained
by winding the filaments at a nearly right angle in correspondence to the end
portions of the cylindrical element 1.