TECHNICAL FIELD

The present invention generally relates to improved bushings foil design. BACKGROUND

A bushing is a lead-trough structure via which a conductor can pass. Bushings are commonly used in transformers and other high voltage equipment. To obtain a well defined electrical field distribution inside and along a bushing, aluminum foils are wrapped in between paper insulation on the inside of the bushing to capacitively control the electrical field such that electrical field stress can be reduced and breakdown is avoided .

When the aluminum foil has been wrapped one turn around the inside of the bushing, i.e. when the foil has been wrapped for a full 360°, one end of the foil is applied another 10 to 100 mm to form an overlap with the other end of the foil with a paper insulation layer in between. This is illustrated in Fig. 2, where three pieces of aluminum foil has been wrapped inside the bushing with paper insulation layers arranged between each piece of aluminum foil.

The aluminum foil overlap area in the bushing has been

identified as a problem since it effectively forms an electric resonance circuit that is excited by very fast transients (VFTs) which are caused by factors such as e.g. switching operations, faults and disturbances. These transients cause high overvoltages between the foils and occasionally lead to breakdowns in the bushings. In oil-impregnated paper bushings, this problem has been solved by short-circuiting every

aluminum foil overlap by punching through both the two foils and the intermediate paper insulation with a sharp tool. When producing resin-impregnated paper bushings, it has not been possible to adapt this technique. The foil overlap has been used because it has been a common opinion that the edges of the foil cannot be exposed in the strong electrical field occurring during operation of the bushing.

SUMMARY OF THE INVENTION

A general object of the present invention is to solve or at least mitigate the above described problems in the art.

This object is attained in an aspect of the invention by a lead-trough device for an electrical conductor, which

structure comprises an insulating body arranged for housing the electrical conductor along a central axis of the

insulating body. Further, the lead-trough structure comprises insulating layers and conducting layers arranged on the inside of the insulating body, which insulating layers and conducting layers are concentrically wrapped around the central axis of the body and alternatingly arranged along a transaxial direction of said insulating body. At least one conducting layer is wrapped concentrically around the central axis of the body for less than 360° such that ends of the at least one conducting layer are spaced apart.

The present invention is advantageous in that a gap is created between the two ends of a conducting layer arranged inside the lead-trough structure instead of the foil overlap employed in the art .

First, the inventive conductive layer arrangement does not pick up as strong circulating current since no overlap is present .

Second, the resonance frequency of the inventive conductive layer arrangement has a much higher resonance frequency since the capacitive contribution of the overlap has disappeared. Higher frequencies are thus more attenuated and not as likely to excite with a VFT due to greater dielectric losses.

Third, insulation length between the ends of a conductive layer can be made greater with a gap than with an overlapping foil structure, where the insulation length is the distance between the two foils in the overlapping section. This reduces the risk of a bushing breakdown.

The conductive layers are formed of a material being

sufficiently low in resistivity such that a resulting electric field can be controlled.

In an embodiment of the present invention, the conducting layer is an aluminum foil, or any other appropriate metal being shaped in a sheet-like structure such that it can be wrapped concentrically around the central axis of the lead- trough device, which typically is embodied in the form of a bushing .

In another embodiment, the conductive layers are embodied in the form of a coating arranged on insulation of the insulating body. The coated insulation is subsequently wrapped

concentrically around the central axis of the lead-trough device such that the coating forms the conductive layers arranged in between the insulating layers. For example, the coating may be embodied in the form of conductive carbon-based paint which is printed onto the insulation, being for instance paper, during wrapping around the central axis.

In a further embodiment, the conducting layer is sheet-like structure of conductive material not necessarily being a metal. Many different variations are possible. Additional features and advantages will be disclosed in the following .

BRIEF DECRIPTION OF THE DRAWINGS

Embodiments of the present invention and advantages thereof will now be described by way of non-limiting examples, with reference to the accompanying drawings in which:

Fig. 1 shows a side view of a bushing in which the present invention can be applied,

Fig. 2 illustrates a cross-section of a prior art bushing, showing prior art wrapping of conducting foils,

Fig. 3 is a cross-section of the bushing in Fig. 1 taken along line 110 - 110, showing wrapping of conducting foils in accordance with an embodiment of the present invention,

Fig. 4 is a cross-section of the bushing in Fig. 1 taken along line 110 - 110, showing wrapping of conducting foils in accordance with another embodiment of the present invention, and

Fig. 5 is a cross-section of the bushing in Fig. 1 taken along line 110 - 110, showing wrapping of conducting foils in accordance with a further embodiment of the present invention.

DETAILED DESCRIPTION

Fig. 1 shows a side view of a bushing in which the present invention can be applied. The bushing 101 is comprised of an insulating body 102 formed by an outer insulating shell typically made of silicone rubber insulation, and a paper body housed by the outer shell. The paper body may be resin

impregnated. An electrical conductor 103 can be inserted into the bushing along a central axis. Conducting foils 104 are concentrically wrapped around the central axis in between paper insulation 105 on the inside of the insulating body to obtain a well defined electrical field distribution inside and along the bushing.

The bushing may further comprise a test tap 106 for accessing the inside of the bushing in order to perform capacitance and voltage measurements. The bushing is typically coupled via a flange 107 to a transformer 108 such that energy can be transferred via the electrical conductor 103.

Fig. 2 illustrates a cross-section of a prior art bushing, showing prior art wrapping of three layers of conducting foils 204. When the conducting foil has been wrapped one turn around the central axis 203 of the bushing 201, i.e. when the foil has been wrapped for a full 360°, one end of the foil is applied another 10 to 100 mm to form an overlap 211 with the other end of the foil with a paper insulation layer 205 in between. The insulating layers and conducting foils are concentrically wrapped around the central axis 203 of the bushing and alternatingly arranged along a transaxial

direction 212 of the bushing. Disadvantages of this prior art overlap have been discussed in the above.

Fig. 3 is a cross-section of the bushing of Fig. 1 taken along line 110 - 110, showing wrapping of conducting layers 304 in accordance with an embodiment of the present invention. When wrapping the conducting layers 304 around the central axis 303 of the bushing 301 in this embodiment of the invention, each layer is wrapped less than 360°, such that ends 313, 314 of each conducting layer are spaced apart. Thus, a gap 315 is created between the two ends 313, 314 of a conducting layer wrapped inside bushing. The insulating layers 305 and

conducting layers 304 are concentrically wrapped around the central axis 303 on the inside of the insulting body 302 of the bushing and alternatingly arranged along a transaxial direction of the body such that each conducting layer is arranged with an insulating layer on each side. The bushing of the present invention may optionally comprise a test tap 306 and/or a flange 307. As can be seen in Figs. 1 and 3, the conducting layers and the insulating layers may be formed like sheets extending along the length of the insulating body. As has been previously mentioned, coatings can alternatively be used to create the conducting layers, in which case the coatings are applied to the insulation of the bushing

insulting body

In Fig. 3, all conducting layers are wrapped such that a gap 315 is formed between the two ends of the respective layer. It is to be understood that not every conductive layer must be wrapped in this manner, but could be wrapped with an overlap as described in the above. However, the effects of the present invention as discussed hereinabove will be more apparent with a greater number of non-overlapping conducting layers.

Fig. 4 is a cross-section of the bushing of Fig. 1 taken along line 110 - 110, showing wrapping of conducting foils 404 in accordance with another embodiment of the present invention. When wrapping the conducting layers 404 around the central axis 403 of the bushing 401 in this embodiment of the

invention, each layer is wrapped less than 360°, such that ends 413, 414 of each conducting layer are spaced apart.

Again, a gap is created between the two ends 413, 414 of a conducting layer arranged inside bushing. However, in contrast to the embodiment shown in Fig. 3, the gaps 415 of Fig. 4 are not necessarily aligned along the same transaxial direction of the insulating body 402. It is understood that the gaps created by the conducting layers may be of varying sizes. The gaps within one and the same bushing may further mutually be of different sizes. Fig. 5 is a cross-section of the bushing of Fig. 1 taken along line 110 - 110, showing wrapping of conducting layers 504 in accordance with a further embodiment of the present invention. When arranging the conducting layers 504 around the central axis 503 of the bushing 501 in this embodiment of the

invention, some conducting layers are wrapped less than 360°, such that ends 513, 514 of these conducting layers are spaced apart creating gaps 515 (aligned or non-aligned) . However, in contrast to the embodiments of Figs. 3 or 4, some of the conducting layers arranged inside the insulating body 502 are short-circuited. In this particular illustration, two

conducting layers 516 have been short-circuited. This

embodiment is advantageous in that possible breakdown voltages occurring across the gaps 515 can be avoided.

The skilled person in the art realizes that the present invention by no means is limited to the examples described hereinabove. On the contrary, many modifications and

variations are possible within the scope of the appended claims .