Note: Descriptions are shown in the official language in which they were submitted.
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Electric high voltage device with a fiber composite casing and a method
to manufacture such device.
TECI-INICAL FIELD
The present invention relates to an electric high voltage device with a fiber
composite casing and in particular a device with a tubular fiber composite
casing. More spe-
cific, the present invention relates to an electric high voltage device with a
fiber composite
casing for use in an electrical installation where the casing is subjected to
an electrical field.
In particular the present invention relates to an electric high voltage device
with a fiber com-
posite insulant or an electrically insulating casing in an insulated
electrical high voltage in-
stallation with an insulation system comprising liquid or gaseous electrically
insulating me-
dia. The casing can also be used in a device comprising a solid or a gelled
electrically insu-
lating media.
The present invention also relates to a method for manufacturing such an elec-
tric high voltage device with a fiber composite casing.
BACKGROUND ART
A conductor in an electric device is electrically insulated from other
conductors
or any grounded casing or structural element made from an electrically
conductive material
and which are at a different electric potential. The electrical insulation is
provided by an
electrically insulating media and can comprise solid, liquid and/or gaseous
media. Electric
devices comprising insulants or casings of the hollow core type such as
breakers, bushings,
instrument transformers, etc and support insulations often have an insulation
system com-
prising liquid or gaseous media, in some cases also solid and gelled media.
Devices with an
open structure such as high voltage switchgear using air as insulating media
are shielded by a
casing and gas insulated devices using a gas, e.g. SF6-gas or air are enclosed
in a casing. To
ensure the mechanical integrity of a resin based casing they often comprises a
mechanically
reinforcing self supporting coherent structure. This supporting, load carrying
structure, which
can be achieved in various ways, are typically impregnated with and.covered by
an electri-
cally insulating resin composition. The structure typically exhibit an open
reinforcing, sup-
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porting and load carrying net like structure of fibers, bars or wires
surrounded and essentially
covered by the electrically insulating resin composition. The structure can be
an ordered net
but is often a weave of the non-woven type, such a fiber mat that has been
formed to the de-
sired shape. A typical shape is tubular. The casing can also comprise
reinforcing fibers dis-
persed in a matrix of the electrically insulating resin composition. Although
any body, struc-
ture or fiber used will be essentially covered by a layer of the electrically
insulating resin
composition only electrically insulating materials are suitable for use and an
other criteria for
choosing reinforcement will be the mechanical properties required of the
casing. Of course
should also any interactions between the reinforcement and any electric or
electromagnetic
field created at the conductor be considered when designing the casing.
Suitable materials for
the casing are electrically insulating polymeric materials, such as a
polyolefin, a polyamide, a
phenolic resin or the like. The reinforcement can comprise electrically
insulating fiber mate-
rials or yarns, wires or nets made from as glass, polymeric materials, e.g.
polyamide based
materials, polyolefin or the like.
However, in a casing essentially made from electrically insulating materials,
space charge accumulation has been noted. Especially when the dielectric is
subjected to a
DC-field. Such space charge accumulations distorts the electrical stress
distribution and per-
sist for long periods because of the high resistivity of the polymers. Space
charges in an in-
sulation body do when subjected to the forces of an electric DC-field
accumulate in a way
that a polarized pattern similar to a capacitor is formed. There are two basic
types of space
charge accumulation patterns, differing in the polarity of the space charge
accumulation in
relation to the polarity. The space charge accumulation results in a local
increase at certain
points of the actual electric field in relation to the field, which would be
contemplated when
considering the geometrical dimensions and dielectric characteristics of an
insulation. The
increase noted in the actual field might be 5 or even 10 times the
contemplated field. Thus the
design field for an insulation must include a safety factor taking account for
this considerably
higher field resulting in the use of thicker and/or more expensive materials
in the insulation.
The build up of the space charge accumulation is a slow process, therefore
this problem is
accentuated when the polarity of the field acting on the insulation after
being operated for a
long period of time at the same polarity is reversed. As a result of the
reversal a capacity field
is superimposed on the field resulting from the space charge accumulation and
the point of
maximal field stress is moved from the interface and into the insulation.
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When discussing the electrical strength of an insulation system or individual
components making up the insulator some expressions are used that needs to be
defined;
- "breakdown voltage" means in this application the voltage across the
insulation at which the
insulation loses its electrical insulation function,
-"partial discharge inception voltage" means in this application the voltage
across the insula-
tion for which a partial breakdown occurs or more accurately starts in a part
of the insulation
system, the part being one whole component or a part of a component in the
insulation sys-
tem.
As understood from the foregoing, great care should thus be taken to ensure
that
the conductor, any spacers and the casing are carefully designed such that ali
parts of the
electrical insulation system exhibits the required electrical properties.
It is known to increase the capability to withstand partial discharge and thus
re-
duce the risks for electric breakdown, i.e. increase the breakdown strength
for solid electri-
cally insulating materials which are subjected to electrical fields, by
applying a coating com-
prising a sufficient amount of a particulate filler with a non-linear
resistivity, and in particular
a field dependent resistivity. Such a coating is described in US-A-4 666 142.
However it has
been experienced in field tests that such a coating is not capable to
withstand the combined
therma.i and electrical conditions prevailing on the inside of a casing of a
gas or air insulated
high voltage direct current device. It has also been suggested, in EP-A 0 440
865, to incorpo-
rate such a particulate filler with a non-linear, field dependent resistivity
in the resin used for
the hulk of a spacer in a gas or air insulated high voltage electric device.
SL1MMARY OF THE INVENTION
It is the object/aim of the present invention to provide a gas or air
insulated
electric device comprising a mechanically load carrying and electrically
insulating casing
with a decreased tendency for space charge accumulation when subjected to an
electric field,
thereby providing an increased capability to withstand partial discharge and a
reduced risk for
electric breakdown. Further it is an object of the present invention to
provide a method for
manufacturing such a fiber composite object.
To achieve this the invention suggest a gas or air insuiated high voltage
direct
current device comprising a conductor and a casing spaced apart from each
other according to
the preamble of claim 1, which is characterized by the features of the
characterizing part of
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claim 1. Further developments of the invention are characterized by the
features of the addi-
tional claims 2 to 10. The invention also suggest a method for manufacture of
such an electric
device with a fibber composite casing comprising a self supporting
mechanically load carry-
ing structure of associated and connected fibers that is impregnated with and
essentially
coated by a resin composition according to claim 11 which is characterized by
the measures
of the characterizing part of claim 10. Further developments of the invention
are character-
ized by the features of the additional claims 12 and 14.
For an electric high voltage device according to the present invention the im-
pregnating resin composition comprises at least in the internal surface areas
of the casing a
particulate filler with a resistivity dependent of the electrical field its
subjected to. Preferably
the particulate filler exhibits a non-linear resistivity dependent of the
electrical field its sub-
jected to.
According to one embodiment the resin based composition comprises a par-
ticulate filler with an intrinsic resistivity at from 104 to 108 ohm m at 25
°C and !ow electrical
field strength. Suitable fillers have shown to be particulate chromium oxide,
Cr203, iron ox-
ide, Fez03, silicon carbide and doped zinc oxide or an metal phtalocyanine or
a mixture of at
least two of these substances. As example of usable metal phtalocyanines may
be mentioned
phtalocyanines of copper, iron, nickel, cobalt, magnesium; aluminum,
manganese, tin, chro-
mium and zinc, separately or as a mixture. Particularly preferred among others
for economi-
cal reasons is copper phtalocyanine and then in particular a-copper
phtalocyanine. Preferably
the particulate filler is added at a level of 10 % by volume or more.
The filler containing fiber composite casing is according to one embodiment
intended for use as a part in the electrical insulation system for a high
voltage electrical alter-
nating current, AC device.
According to an alternative embodiment, the fiber composite casing is intended
for use as a part in the electrical insulation system for a high voltage
electrical direct current,
DC device.
According to one preferred embodiment the fiber composite casing has a tubu-
lar form. The tubular fiber composite casing comprises a wound or pultruded
fiber structure
impregnated with a resin based composition and that at least the resin
contained in the wound
or pultruded layer adjacent to the internal face of the tube comprises the
particulate filler with
a non-linear resistivity. Alternatively the wound or pultruded fiber structure
is impregnated in
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fixll with a resin based composition comprising the particulate filler with a
non-linear resis-
tivity. Thus a tubular fiber composite casing with a field dependent
resistivity in the surface
and with a surface finish as good as for conventional fiber composite tube is
provided. Such a
tubular fiber composite casing is in particular suited for use as an insulant
for a gas or air in-
sulated electric high voltage device, such as a bushing, a reactor or switch-
gear a support in-
sulant. An electrical device comprising such fiber composite casing according
to the present
invention with a particulate filler with a field dependent resistivity exhibit
a substantially re-
duced tendency for space charge accumulation and thus an improved electrical
strength.
The present invention also provides a method for manufacture of an electric de-
vice with a composite casing comprising a self supporting, load carrying
structure of associ-
ated and connected fibers that are impregnated with and essentially coated by
a resin compo-
sition, wherein a particulate filler with a field dependent resistivity
according to the present
invention is added to at least the resin composition that is used for
impregnation of outmost
areas of the tubular casing, such that at least the inner surface of the tube
comprises said par-
ticulate filler with field dependent resistivity. According to one embodiment
also the outer
surface of the tube also comprises said particulate filler with field
dependent resistivity.
According to one embodiment of the invented method the fiber composite
structure is impregnated in full with a resin comprising a particulate filler
with a field de-
pendent resistivity.
According to an embodiment of the invented method which is especially suited
for production of an electric device with a tubular fiber composite casing,
-a tubular fiber structure is wound or puitruded from a tape, yarn or sheet
com-
prising a coherent fiber web, or non-woven structure,
- the tape, yarn or sheet ,prior to or in association with the winding or
pultrud-
ing is impregnated with a resin based composition, and
- wherein a particulate filler with a resistivity dependent of the electrical
field it
is subjected to is added at least to the resin composition which is
impregnated into the in-
nermost layer of the wound or pultruded fiber composite tube, such that at
least the inner
surface of the tubular casing comprises said particulate filler with non-
linear resistivity.
According to one other embodiment a particulate filler with a resistivity de-
pendent of the electrical field it is subjected to is added at least to the
resin composition
which is impregnated into the innermost layer and outermost of the wound or
pultruded fiber
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composite tube, such that at least the inner and outer surface of the tubular
casing comprises
said particulate filler with non-linear resistivity.
Alternatively can the tape, yarn or sheet be coated with the resin comprising
the
particulate filler with a non-linear field dependent resistivity be used for
impregnating the
fiber structure.
By the adoption of any of these alternative embodiments of the process accord-
ing to the present invention can a tubular fiber composite casing with a field
dependent resis-
tivity at least in a zone at the inner or internal surface and with a surface
finish as good as for
conventional fiber composite tube be provided. Such a tubular fiber composite
casing is in
particular suited for use as an insulant for a gas or air insulated electric
high voltage device.
And in particular for use as a hollow core type insulant for a breaker, an
instrument trans-
former, a bushing or the like or to be used as a support insulant. A fiber
composite casing
comprising a particulate filler with a field dependent resistivity exhibits a
substantially re-
duced tendency for space charge accumulation and thus an improved electrical
strength.
BRIEF DESCRIPTION OF THE DRAWINGS
A preferred embodiment of the present invention shall be described more in
detail while referring to the drawings, where;
Figure 1 shows in a side view a simple schematic sketch of an electric device
with a casing of the hollow core type comprising a tubular fiber composite
casing according
to one embodiment of the present invention.
DESCRIPTION OF PREFERRED EMBODIMENTS, EXAMPLES.
A compact high voltage electric device with an electrically insulated casing
in
the form of a resin impregnated tubular fiber composite casing according to
one embodiment
of the present invention and as shown in figure 1 comprises a conductor 11
spaced apart and
electrically insulated from a casing 15. The casing 15 is made in an
electrically insulating
fiber composite body and comprises a structural hollow core from a fiber
composite tube.
The tube 15 is typically surrounded by a flanged exterior structure made in a
polymeric mate-
rial 16 to provide weather proofing and extend the creeping distances to
reduce or eliminate
leakage currents or flash overs along the exterior surface. This type of
hollow core insulants
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are typically used for breakers or other switch gear, instrument transformers,
bushings and as
support insulant. A liquid, gelled or gaseous insulating media surrounds the
conductor 11.
The breakdown of the insulation is fairly well understood and the
understanding of the elec-
trical stresses in the electrically insulating casing 15 is believed to depend
on the space
charge profile developed in the insulation when subjected to an electric
field. According to
the present invention the tubular fiber composite casing 15 at least in a
layer at the~inner sur-
face 150 is impregnated with a resin composition that comprises a particulate
filler with a
resistivity dependent of the electrical field it is subjected to . The
mechanically self support-
ing and load carrying fiber structure is impregnated and coated with resin
such that essen-
tially no part of the fiber structure is in contact with the electrically
insulating media, i.e. the
fiber structure is surrounded and essentially covered by the resin. The casing
15 needs a suf
ficient mechanical integrity to withstand all pressure, bending and traction
forces for which it
is subjected, it carries all load imposed on it by the weight of the conductor
and any attraction
or repulsion forces caused by a high current flowing through the conductor or
an adjacent
conductor. Suitable materials the fiber structure is fibers, yarns, wires or
nets made from as
glass, polymeric materials, e.g. polyamide based materials, polyolefin or the
like. Suitable
particulate fillers with a non-linear field dependent resistivity typically
comprises chromium
oxide, Cr203, iron oxide, Fez03, is silicon carbide and doped zinc oxide or an
metal phtalo-
cyanine or a mixture of at least two of these substances. Preferably the
particulate filler is
added at a level of 10 % by volume or more.
The tubular fiber composite casing in the embodiment shown in figure 1
comprises a wound
or pultruded fiber structure impregnated and essentially coated with a resin
based composi-
tion. At least the resin contained in the wound or pultruded layer adjacent to
the internal face
of the tube comprises the particulate filler with resistivity dependent of the
electrical field it is
subjected to. Alternatively the wound or pultruded fiber structure is
impregnated in full with
a resin based composition comprising the particulate filler with a field
dependent resistivity.
