Note: Descriptions are shown in the official language in which they were submitted.
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INSULATED ELECTRICAL CONDUCTOR FOR HIGH-VOLTAGE WINDINGS
The present invention relates to an insulated electrical
conductor. More specifically, the invention relates to an
insulated conductor, for use in high-voltage windings, having
an outer layer of (at least semi-) conductive material which
is contacted for grounding purposes. The conductor is
intended to be used in large motors, generators and
transformers at voltages in excess of 10 kV, in particular in
excess of 36 kV, and preferably more than 72.5 kV up to very
high transmission voltages, such as 400 kV to 800 kV or
higher. In addition, the invention relates to a method of
establishing electrical contact with (semiconductive)
polymeric material.
A particular conductor which can be used in the invention
is shown in cross section in Figure 1. The conductor 10
comprises strands 12, for example of copper, the majority of
which are insulated, surrounded by a first conductive layer
14. An insulating layer 16, for example of cross-linked
polyethylene (XLPE) surrounds the first conductive layer 14
and is in turn surrounded by a second conductive layer 18.
Whilst the layers 14, 18 are described as "conductive"
they are in practice formed from a base polymer mixed with
carbon black or metallic particles and have a volume
resistivity of between 1 and 105 S2~cm, preferably between 10
and 500 S2~cm. Suitable base polymers for the layers 14, 18
(and for the insulating layer 16) include ethylene vinyl
acetate copolymer/nitrile rubber, butyl grafted polythene,
ethylene butyl acrylate Copolymer, ethylene ethyl acrylate
copolymer, ethylene propene rubber, polyethylenes of low
density, poly butylene, poly methyl pentene and ethylene
acrylate copolymer.
The first conductive layer 14 is rigidly connected to the
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insulating layer 16 over the entire interface therebetween.
Similarly, the second conductive layer 18 is rigidly connected
to the 'insulating layer 16 over the entire interface
therebetween. The layers 14 - 16 form a solid insulation
system and are conveniently extruded together around the
strands 12.
Whilst the conductivity of the first conductive layer 14
is lower than that of the electrically conductive strands 12,
it is still sufficient to equalise the potential over its
surface. Accordingly, the electric field is distributed
uniformly around the circumference of the insulating layer 16
and the risk of localised field enhancement and partial
discharge is minimized.
The potential at the second conductive layer 18, which
should be zero or ground, is equalized at this value by the
conductivity of the layer. At the same time, the conductive
layer 18 has sufficient resistivity to enclose the electric
field. In view of this resistivity, it is desirable to
connect the conductive polymeric layer to ground at intervals
therealong.
A problem experienced in making electrical contact with
polymeric layers is that they expand in use, due to their high
thermal expansion coefficient, and also creep under mechanical
loading.
It is an object of the invention to maintain the second
conductive layer substantially at ground by providing a
suitable contacting device.
Accordingly, the present invention provides an electrical
conductor for high voltage windings, comprising central
conductive means and an outer semiconductive layer,
characterised in that at least one electrically conductive
contacting device penetrates into the outer semiconductive
layer.
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In a preferred embodiment, the central conductive means
comprises one or more strands of wire, which are surrounded
in turn by an inner layer of lower conductivity than the wire,
then by an electrically insulating layer, and then by the
outer layer which preferably has a higher conductivity than
the insulating layer.
In an embodiment of the invention, the contacting device
is metallic and comprises a substantially planar member having
a plurality of barbs which penetrate into the semiconductive
layer. The barbs may have re-entrant portions for engaging
the semiconductive layer. The barbs may be of shape-memory
metal.
A plurality of contacting devices may be provided at
different points on the surface of the conductor. The
contacting devices may be secured on the conductor surface by
at least one resilient band, and for example several bands may
each secure a plurality of the devices.
A single contacting device may penetrate into the outer
semiconductive layer of a plurality of conductors or of a
plurality of turns of a wound conductor. Biassing means, for
example, one helical spring for each turn, is preferably used
to urge said single contacting device into engagement with the
turns.
One or more grounding wires may be connected, for example
soldered, to the or each contacting device.
The present invention also provides a method of
establishing electrical contact with semiconductive polymeric
material, comprising causing an electrically conductive
contacting device to penetrate into the material.
One embodiment of the method comprises placing a
substantially planar contacting device on the surface of the
polymeric material and punching out portions of the device
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such that said portions penetrate into the material. These
steps are not necessarily carried out in the order stated
above.
An alternative embodiment comprises accelerating the
contacting device towards the semiconductive polymeric
material, for example by firing it from a gun means. The
method may include a preliminary step of heating the polymeric
material, at least in the region to be contacted.
If the polymeric material is a semiconductive outer layer
of an electrical conductor, a plurality of contacting devices
may be connected thereto and the method may include connecting
at least one grounding wire to the or each contacting device,
for example by soldering.
The present invention is particularly convenient for
rapidly connecting a large number of reliable and durable
contacting devices to the outer semiconductive polymeric
layer.
Embodiments of the invention will now be described in
more detail, by way of example only, with reference to the
accompanying drawings, in which:-
Figure 1 is a transverse section through a conductor
according to the invention, but not showing the contacting
device;
Figure 2 is a schematic perspective view showing a first
embodiment of contact device mounted on the conductor of
Figure 1;
Figure 3 shows a tool for use with the embodiment of
Figure 2;
Figure 4 is a schematic section of a second embodiment
of contact device mounted on a plurality of turns;
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Figure 5 is a schematic section of a third embodiment of
contacting device mounted on the conductor;
Figure 6 is a diagram showing the distribution of barbs
on the contacting device of Figure 5;
Figure 7 is a side view of a fourth embodiment of
contacting device;
Figure 8 is a schematic section showing the contacting
device of Figure 7 mounted on the outer polymeric layer; and
Figure 9 is a diagram showing the distribution of barbs
on the contacting device of Figure 7.
Figure 2 shows two contacting devices 20 mounted to the
outer polymeric layer 18 of the conductor 10. The contacting
devices 20 are preferably of silver or silver-clad copper and
each comprises a substantially planar member.
Prior to connecting the devices 20, the surface of the
semiconductive polymeric layer 18 can be degreased, roughened
and/or sprayed with a silver spray. Each contacting device
is then placed on the surface of the layer. A tool 30, shown
in Figure 3, is then used to punch out portions of the planar
member. The punched-out portions penetrate into the polymeric
material, increasing the contact surface, and additionally
causing the silver spray to penetrate the material. The
contacting devices 20 then have a ~~grater~~ surface.
Alternatively, prior to applying the planar member to the
layer 18, this member can be shaped using the tool to punch
out the portions or barbs and the thusly formed contacting
device can be pressed or fired into the layer 18.
A strip spring or watch spring 22 is used to secure the
contacting devices 20 to the conductor 10, and a grounding
lead 24 is soldered to each contacting device 20.
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Figure 4 shows how a single contacting device 40 having
a ~~grater~~ surface can be used to rapidly contact a number of
turns 42 of the conductor 10. The contacting device 40 is
laid across the turns 42, which abut each other. Springs 44
(one spring for each turn 42) urge the contacting device 40
to penetrate into the outer layer. The springs 44 are placed
against a support 46 which may, for example, form part of the
housing of a machine or transformer.
Figure 5 shows a third embodiment of contacting device
50, comprising a planar member 52 having re-entrant barbs 54
which operate in the manner of fish-hooks. The barbs 54 are
preferably resilient or of shape-memory metal. In the latter
case they can unfold after insertion into the polymeric layer
18. Figure 6 schematically shows the distribution of the
barbs 54 on the contacting device 50.
Figures 7 to 9 show a fourth embodiment of contacting
device 60 having two rows of diverging barbs 62. The barbs
are advantageously of shape-memory metal and are bent so as
to diverge further when they penetrate into the semiconductive
polymeric layer 18, as shown in Figure 8.
Prior to connecting any of the contacting devices of the
invention to the conductor, the latter can be heated locally
to soften the polymeric layer and facilitate insertion of the
barbs. The devices 50, 60 can be shot into the polymeric
layer using a tool similar to a nail gun.
The contacting devices of the invention provide a large
number of uniformly distributed points of contact on the
external semiconductive layer and adhere well thereto. This
guarantees a low and uniform current density, if earth current
flows. Electrical contact is established easily and rapidly
and remains stable over a long period of time.
The conductor of the invention may alternatively be a
superconductor in which the central conductive means comprises
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superconductive material.
The electrical insulation of an electrical conductor
according to the invention is intended to be able to handle
very high voltages, e.g. up to 800 kV or higher, and the
consequent electric and thermal loads which may arise at these
voltages. By way of example, electrical conductors according
to the invention may comprise windings of power transformers
having rated powers from a few hundred kVA up to more than
1000 MVA and with rated voltages from 3 - 4 kV up to very high
transmission voltages of from 400 - 800 kV or more. At high
operating voltages, partial discharges, or PD, constitute a
serious problem for known insulation systems. If cavities or
pores are present in the insulation, internal corona discharge
may arise whereby the insulating material is gradually
degraded eventually leading to breakdown of the insulation.
The electric load on the electrical insulation in use of an
electrical conductor according to the present invention is
reduced by ensuring that the inner layer of (semi)conductive
material of the insulation system is at substantially the same
electric potential as conductors of the central electrically
conductive means which it surrounds and the (semi)conductive
outer layer is at a controlled, e.g. earth, potential. Thus
the electric field in the electrically insulating layer
between these inner and outer layers is distributed
substantially uniformly over the thickness of the intermediate
layer. By having materials with similar thermal properties
and with few defects in these layers of the insulation system,
the possibility of PD is reduced at given operating voltages.
The electrical conductor can thus be designed to withstand
very high operating voltages, typically up to 800 kV or
higher.
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