TECHNICAL FIELD OF THE INVENTION AND PRIOR ART
The present invention relates to a method for manufacturing
a post insulator.
The invention relates to such post insulators of any size
used for separating two electrical potentials, normally a high electrical potential
from ground. They may be used as so called station post insulators in switchgears
in converter stations of plants for transmitting electric power, such as for separating
valves in a converter in a station of a HVDC (High Voltage Direct Current) plant
with respect to ground. Another possible use is for carry overhead electrical high
Typical sizes for such a post insulator are lengths, i.e.
heights, of 6-8 m and diameters of 25-40 cm, but any other size is possible.
The voltage, i.e. potential difference, in question may
for instance be 800 kV, although quite different voltages are possible. The voltage
may be an alternating voltage or a direct voltage.
The invention relates to such post insulators comprising
a tube of an insulating stiff material, which is filled by a core of an insulating
material, such as foamed plastic. The tube may have another cross-section then circular,
such as square, although a circular cross-section is most frequent. The tube may
also have a varying cross-section, such as being conical. The invention is especially
directed to so called composite insulators, i.e. which have a tube of a composite
It is in a post insulator of this type important that no
shortcircuits between electrical potentials separated thereby occur, and this is
the reason why the inner volume of the tube is filled by a core of an insulating
US 2004/0251385 A1
shows how a post insulator of this type may be filled with foamed plastic
for preventing shortcircuits from appearing.
However, in post insulators of this type already known
there is a not negligible risk of occurrence of shortcircuits through the post insulator
as a consequence of damp penetrating into the interior of the post. The reason for
this is that it is difficult to fill the entire inner volume of the tube by said
core and keep the total filling over the time. Furthermore, cracks may also be created
in the core. Thus, damp may be introduced into spaces formed between the core and
the tube and inside the core and cause a shortcircuit through the post insulator.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a method
for manufacturing a post insulator of the type described above as well as a post
insulator reducing the risks of shortcircuits.
This object is according to the invention obtained by providing
a method for manufacturing a post insulator, which comprises the steps:
- introducing a core of an insulating material into a tube of an insulating stiff
material so as to occupy substantially the entire inner volume of the tube while
leaving a small circumferential space separating the core and the inner walls of
- closing the tube at the two ends thereof,
- introducing an adhesive into the tube through a first opening in one of said
ends while establishing a second opening at the opposite end of the tube for allowing
air to escape from the interior of the tube as said adhesive is introduced,
- closing said second opening when no more air is coming out thereof,
- continue the introduction of adhesive into the tube under overpressure until
the pressure to be applied for introducing more adhesive into the tube exceeds a
- closing said first opening, and
- curing the adhesive while maintaining an overpressure in the interior of the
By deliberately producing the space separating the core
and the inner walls of the tube and filling this space with an adhesive while creating
an overpressure inside the tube it is ensured that the inner volume of the tube
will be completely filled also after curing of the adhesive. A reliable adherence
between the core and the tube is ensured thanks to the curing of the adhesive under
overpressure. This means that compensation for possible shrinkage of the material
is obtained, since compressive stress will remain in the adhesive joint also after
the curing. Thus, a homogeneous unit with no risk of introduction of damp is obtained.
According to an embodiment of the invention it is a core
of a light, elastically compressible material that is introduced into the tube.
This means that the overpressure of the adhesive will result in a compression of
the core, so that when the adhesive shrinks during curing the overpressure is maintained
by the "backspring"-expansion of the core taking place. The core is for that sake
preferably made of foamed plastic, such as hard foam, for instance PVC-foam, or
a similar material. "Hard" is here to be interpreted to not exclude elasticity of
According to another embodiment of the invention said core
is introduced into a tube of an elastic material and having a thickness making it
expanding by the introduction of adhesive thereinto under an overpressure reaching
said predetermined level. This means that the tube will be elastically deformed
by the introduction of adhesive under overpressure, and when the adhesive shrinks
during curing thereof the overpressure is maintained by a "backspring" action of
the tube. A suitable, stiff material for the tube is a fibre composite, such as
glass fibre epoxy.
According to another embodiment of the invention a thin
cord-like member is wound substantially helically around the core with a large pitch
angle before the core is introduced into said tube for obtaining said space between
the core and the inner walls of the tube by said cord-like member acting as a spacer.
This way of winding said cord-like member around the core ensures a circumferential
space separating the core and the inner walls of the tube without any risk that
any part of the core will bear against an inner wall of the tube and thereby preventing
adhesive from being introduced between the core and the tube at that place and connecting
them by a adhesive joint. The space is then preferably obtained by said cord-like
member being cross-wound around said core, so that once the core is introduced into
said tube said cord-like member will bear against the inner walls of the tube by
cross-over points thereof.
A suitable material for said cord-like member is glass
fibre, but any insulating material having the ability to form a spacer element may
According to another embodiment of the invention said core
is introduced into the tube in the form of a plurality of elongated sections each
having a cross-section substantially corresponding to the cross-section of the inner
volume of the tube. This makes if easier to handle the core, especially when the
tube has a considerable length, and it also prevents a possible crack in the core
to propagate through the entire core.
According to another embodiment of the invention spacers
are introduced between subsequent such core sections for obtaining a distance therebetween
to be filled by adhesive. This means that adhesive having an overpressure will also
fill these spaces between adjacent core sections binding them to each other, which
results in a compressive stress in the adhesive joint connecting adjacent core sections.
Spaces in the form of a thin net are preferable introduced between subsequent said
core sections. This net may be of the same material as said cord-like member wound
around the core.
According to another embodiment of the invention it is
a two-component adhesive, such as an epoxy adhesive or a vinyl ester adhesive, that
is introduced into the tube. However, other adhesives than two-component ones are
According to another embodiment of the invention said predetermined
level of the pressure corresponds to an overpressure exceeding 1 bar, preferably
exceeding 3 bars. It has been found that an overpressure in this range will result
in the advantages mentioned above.
According to another embodiment of the invention said tube
is kept inclined with said first opening on a lower level than the second opening
at least during the first step of introducing a adhesive into the tube with said
second opening open, and the longitudinal extension of the tube is making an angle
with a horizontal exceeding 30°, preferably being about 45°. The adhesive
has in this way to work against the gravitation when introduced into the tube, so
that it will efficiently fill every empty space inside the tube while pressing air
out of the tube through said second opening.
The invention also relates to a post insulator, which comprises
a tube of an insulating stiff material occupied by a core of an insulating material,
which is characterized in that said core occupies substantially the entire volume
of the tube while leaving a small circumferential space separating the core and
the inner walls of the tube, and said circumferential space is filled by a adhesive
applying a pressure on the tube and the core after curing. The advantages of such
a post insulator appear from the above discussion of the method according to the
According to an embodiment of the invention said core is
made of a plurality of elongated core sections each having a cross section substantially
corresponding to the cross-section of the inner volume of the tube and mutually
separated by a space filled by adhesive applying a pressure upon adjacent core sections
tending to press them apart.
According to yet another embodiment of the invention said
core is made of foamed plastic.
Further advantages as well as advantageous features appear
from the following description.
BRIEF DESCRIPTION OF THE DRAWINGS
With reference to the appended drawings below follows a
specific description of an embodiment of the invention cited as an example.
In the drawings:
DETAILED DESCRIPTION OF EN EMBODIMENT OF THE INVENTION
- Fig 1
- is a partially sectioned view illustrating a post insulator according to the
- Fig 2
- is an enlarged sectioned view illustrating how adjacent core sections and the
inner wall of the tube of the post insulator according to Fig 1 are arranged,
- Fig 3
- is a schematic view illustrating a step of the method for manufacturing the
post insulator according to the present invention, and
- Fig 4
- is a perspective view of a post insulator according to the invention standing
on the ground.
Fig 1 shows schematically a post insulator according to
the present invention. This is made of a tube 1 of a fibre composite, such as glass
fibre epoxy, which here has a length of approximately six metres and an inner diameter
of 31 cm and an outer diameter of 33 cm. The tube 1 has an outer profile of rings
2 of silicon rubber. The tube is at each end thereof provided with flanges 3, 4
of aluminium adhesived to the ends of the tube. Each end of the tube is provided
with a flange cover 5, 6 having an outer diameter of about 46 cm and enclosing the
inner volume of the tube.
The inner volume of the tube is occupied by a core 7 of
an insulating material, such as foamed plastic.
The further structure of the post insulator will now be
described while simultaneously describing the method for manufacturing the post
insulator and making reference to all the figures. In this manufacturing process
one of the flange covers, such as the flange cover 6, is initially not in place
enabling introduction of the core into the inner volume of the tube. The core is
made of a number of sections 7a, 7b, 7c having each a length of approximately 1
m. These sections have a cross-section with a diameter slightly smaller than the
inner diameter of the tube, such as having a diameter being 2 mm less than the inner
diameter of the tube. A thin cord-like member 8 of for example glass fibre is wound
substantially helically around each core section before introducing the core section
into the tube. This is done with a large pitch angle resulting in a pitch of for
example 20 cm. The cord-like member may then be cross-wound around the core, so
that once the core is introduced into the tube the cord-like member will bear against
the inner walls of the tube by cross-over points 9 thereof. Thus, the cord-like
member 8 forms a spacer ensuring that a small circumferential space will separate
the core and the inner walls 10 of the tube. A further spacer 11 in the form of
a thin net is applied on the end of each core section for obtaining a space between
subsequent said core sections as shown in Fig 2.
When the core sections are in place the flange cover 6
is attached to the flange 4 by bolts and a device 21 for feeding adhesive into the
interior of the tube is connected to a first opening 12 in said flange cover. The
tube is then inclined with respect to a horizontal while making an angle therewith
of approximately 45°. The flange cover 5 has a second opening 13 to the interior
of the tube. Adhesive, such as a two-component adhesive, is now introduced into
the tube through said first opening 12 while allowing air to escape from the interior
of the tube through said second opening 13 on a higher level as said adhesive is
introduced. It is shown in Fig 3 how the two openings are eccentrically arranged
in the respective flange cover, so that in the position according to Fig 3 the first
opening 12 is arranged close to the lowest point of the flange cover 6, while the
second opening 13 is located close to the highest point of the flange cover 5.
Air present in the spaces between the core sections and
the tube wall as well as between core sections will in this way be pressed out of
the tube through the second opening 13 when these spaces are filled with adhesive.
The second opening will then be closed when no more air, but only adhesive is coming
out of this opening.
The introduction of adhesive into the tube is then continued
under overpressure until the pressure to be applied for introducing more adhesive
into the tube exceeds a predetermined level, which may correspond to an overpressure
of 3.5 bars. The connection between the device 21 and the first opening 12 is then
removed and this first opening 12 closed by screwing a plug into an internal thread
of this opening. The adhesive will then cure while maintaining an overpressure in
the interior of the tube.
By the introduction of the adhesive into the tube with
an overpressure a full compensation for possible shrinkage of the materials of the
core and the tube is obtained, since both the core sections and the tube will be
elastically deformed by the overpressure. When the adhesive then shrinks during
curing thereof the overpressure is maintained by the "springback"-expansion taking
place by the core sections and the tube. Thus, compressive stresses will result
in the adhesive joint, which are favourable for the strength thereof.
Thus, an homogenous unit with an excellent bounding of
the materials to each other is obtained.
Furthermore, this method results in a complete encapsulation
of the core, so that the material of the core may be selected so that a cost efficient
product is obtained.
The invention is of course not in any way restricted to
the embodiment described above, but many possibilities to modifications thereof
will be apparent to a person with ordinary skill in the art without departing from
the basic idea of the invention as defined in the appended claims.