Patent Description:
Electrical bushings are used to insulate and transduce electrical power through planes with different electrical potential, such as grounded transformer housings. The structure and dimensions of such bushings depend on the respective requirements, and most types of bushings are manufactured according to specific application needs and parameter ranges.

Condenser core bushings are a common variety of electrical bushings for medium to high voltage applications. In a condenser core bushing, a core comprising several layers of intermittent conductive layers and dielectric films is arranged around a central conductor, thereby controlling the distribution of the electric field by capacitive grading.

One way to fixate the core and the flange of such bushings to each other is to use a locking compound as described in document <CIT>.

Document <CIT> relates to a flange and to a method to produce a connection for a flange.

In some applications, the bushings may be subjected to large temperature changes during operation, in particular for outdoor applications. For example, in outdoor transformer applications the highest occurring temperature is related to the hot oil filling of the transformer and the lowest temperature corresponds to the ambient temperature if the transformer is not operating under high power load. Due to different coefficients of thermal expansion of the materials used for the bushing, repeated mechanical stress within the cured locking compound and at interfaces to the core and the flange may result in premature failure of the joint and hence the bushing.

It is an object of the present invention to mitigate the effects of the relative thermal expansion of the core and the flange on the locking compound and on the core material.

This object is obtained by an electrical bushing and a method for producing an electrical bushing according to the independent claims. Developments and expediencies are subject of the further claims.

According to the invention, the electrical bushing comprises a flange with a lower part and an upper part affixed to one another and further comprises a core surrounded by the flange. The flange is affixed to the core by a locking compound disposed in a volume of a joint between the flange and the core. The volume of the joint further comprises a compressible material, the compressible material being configured to compress or expand in response to a change in the volume of the joint.

The terms "lower part of the flange" and "upper part of the flange" do not imply a limitation regarding the actual position of these elements in space. For instance, the lower part of the flange is that part that provides a mounting face for mounting the bushing to an electrical appliance, for instance a transformer housing, a switchgear or a reactor.

Thus, the joint comprises a portion of the volume of the joint that is filled with a compressible material. For example, the volume of the joint may change during operation of the bushing on account of different thermal expansion coefficients of materials for the flange and the core. For example, the compressive material may be arranged directly adjacent to the locking compound. For instance, the compressive material fills between <NUM>% and <NUM>% of the volume of the joint. The compressive material may be stable throughout the lifetime of the bushing, or it may fully or partially disintegrate or degrade after the locking compound has cured during production of the bushing.

The portion comprising the compressible material is provided in addition to the locking compound and has the advantage of higher temperature tolerance between high and low load cycles. This is an effect of the added volume held by the easily compressible material, which functions as an expansion joint when the thermal expansion or contraction of the bushing materials, such as the core, the flange or the locking compound itself would otherwise lead to compression or shear of the locking compound. This means that the bushing disclosed here can be operated in a wider temperature range of the environment and under higher loads as conventional bushings. A further advantage is the potential use of a broader range of suitable locking compounds, e.g. some high-toughness but brittle resins can now be used in place of less durable rubber-like polymers, thereby improving the mechanical properties of the bushing.

For example, the locking compound may prevent axial or radial movement of the core in relation to the other bushing components, for instance in relation to the flange.

The means for mounting the bushing to the electrical appliance include, but are not limited to bolts, rivets, clamps etc. The core can extend through the flange and into the volume of the electrical appliance. The flange may have one or more recesses, channels or grooves on the inside of the cylindrical portion of the flange that, when the core is seated in the flange, forms a joint volume between the flange and the core. The groove can be annular, or it can be interrupted into segments along the circular path. The joint volume may be made up of one or more separate volumes. The groove may include additional features that, when the joint is formed, provide means for improved form locking, such as indentations, slots etc. The flange may be provided with one or more injection channels that allow the injection of a locking compound into the joint volume. The flange may have additional gasket channels that, when a gasket is provided, seal the flange against the core and/or the lower part of the flange angainst the upper part of the flange.

The flange, for example the upper part of the flange may be configured to receive an insulator that surrounds the core. The insulator may form a closed seal with the flange. However, depending on the application of the bushing such an insulator may also be dispensed with.

According to the invention, the core has a first section with a diameter that is larger than a diameter of a second section and a diameter of a third section, wherein the first section is arranged between the second section and the third section along an axial direction of the bushing. The first section is that part of the core that has the largest diameter of the core within the flange. In axial direction, the first section is delimited by two transitions where the diameter of the core decreases.

The flange forms a seat for a first transition between the first section and the second section of the core, wherein the volume of the joint is located at a second transition between the first section and the third section of the core. The second section may be arranged closer to the mounting face of the bushing than the third section or vice versa. The transition between the first section and the second section and/or the transition between the first section and the third section may have a tapered or stepped shape when seen in a cross sectional view of the bushing. The seat of the flange may be sufficiently form locked such that the degrees of freedom for motion of the core in relation to the flange are limited in one or more directions. For instance, the seat is a conical seat, a spherical surface seat, a step-type seat or the like.

The compressible material is located on a side of the locking compound that faces away from the seat. Thus, the compressible material may be compressed as the volume of the joint decreases.

According to at least one embodiment, the seat is located at the lower part of the flange and the joint is located at least in part at the upper part of the flange or vice versa. In other words, the seat and the joint are at least in part adjacent to different parts of the flange.

According to at least one embodiment, the compressible material is compressible by at least <NUM>% or by at least <NUM>% or by at least <NUM>% with respect to its unloaded volume. For example, a compression in axial direction by at least <NUM>% of an axial extent of the compressible material may be performed elastically so that the compressible material expands as the volume of the joint increases again.

According to at least one embodiment, the compressible material comprises at least one of: an elastomer, a gel, a compressible filler, expancels. For instance, a foamed polymer or a foamed elastomer material may be used. Using these materials a high long term reliability of the joint can be obtained. In principle, any material may be used that provides sufficient compressibility and that is sufficiently mechanically stable during manufacture and operation of the bushing.

According to at least one embodiment, the joint is an annular joint spanning a circumference of the core. Thus, the joint completely surrounds the core in a plane extending parallel to the mounting face of the bushing.

According to at least one embodiment, the locking compound is an Epoxy-type resin, an Epoxy-type adhesive, a Silicone-type adhesive or a Polyurethane-type adhesive. These materials enable a reliable and mechanical stable joint between the core and the flange.

According to at least one embodiment, the bushing is a capacitance graded bushing.

Furthermore, a method for producing an electrical bushing is specified. The method is suited for the production of the bushing described above, for instance. Therefore, features described in connection with the bushing may also apply for the method and vice versa.

According to the invention, the method comprises the steps of providing a core, wherein the core has a first section with a diameter that is larger than a diameter of a second section and a diameter of a third section, wherein the first section is arranged between the second section and the third section along an axial direction of the bushing, and a flange with an upper part and a lower part, arranging the upper part and the lower part of the flange around the core, affixing the upper part and the lower part to one another, and forming a joint between the flange and the core, wherein the flange forms a seat for a first transition between the first sections and the second section and wherein a volume of the joint is located at a second transition between the first section and the third section.

According to the invention, forming the joint comprises the steps of injecting a locking compound filling a second portion of the volume of the joint, wherein a compressible material is provided in a first portion of a volume of the joint, wherein the compressible material is located on a side of the locking compound that faces away from the seat. The locking compound is cured while it is in contact with the compressible material. The method steps are performed in the above oder.

According to the invention, the upper part of the flange and the lower part of the flange are connected to each other, for instance mechanically. For example, the core is inserted into one of the upper or lower part of the flange the other part of the flange is lowered over the core before the flange parts are connected to each other. The lower part and the upper part form a seal, for instance.

Typically, the locking compound is injected through one or more injection channels by a technician, but the process can also be automated. The joint volume is filled such that the compressible material and the locking compound together fill the whole volume, for instance. In other words, the locking compound fills the remaining volume of the joint which has not been filled with the compressible material before. However, partial filling can be acceptable. After filling, the injection channel may be sealed, e.g. by plugging.

For example, the compressible material is provided as a prefabricated element that is attached to one of the flange parts, for instance before the flange parts are connected to one another. This facilitates the arrangement of the compressible material within the volume for the joint to be formed.

According to at least one embodiment, the locking compound is injected into the joint after affixing the upper part and the lower part to one another. In other words, the core is affixed to the flange in a state where the flange parts have already been connected to one another.

According to at least one embodiment, the injection is performed with hand-held equipment or mixing equipment, wherein the joint is sealed after injection.

According to at least one embodiment, the locking compound is hardened by heating it to a temperature of at least <NUM>. The temperature and time of the curing step typically are dependent on the locking compound and may be selected appropriately. Once the locking compound has hardened, the position of the core relative to the flange is fixed.

Further embodiments and developments of the bushing and the method will become apparent form the exemplary embodiments described below in association with the figures. Features illustrated or described as part of one embodiment can be used on or in conjunction with any other embodiment to yield yet a further embodiment. It is intended that the present disclosure includes such modifications and variations.

In the exemplary embodiments and figures similar or similarly acting constituent parts are provided with the same reference signs. Generally, only the differences with respect to the individual embodiments are described. Unless specified otherwise, the description of a part or aspect in one embodiment applies to a corresponding part or aspect in another embodiment as well.

The elements illustrated in the figures and their size relationships among one another are not necessarily true to scale. Rather, individual elements or layer thicknesses may be represented with an exaggerated size for the sake of better representability and/or for the sake of better understanding.

<FIG> is a schematic view of a bushing according to an embodiment of the invention. The bushing <NUM> has a core <NUM> seated in a flange <NUM>. The flange comprises a lower part <NUM> and an upper part <NUM>. The lower part <NUM> forms a mounting face <NUM> for mounting the flange <NUM> to an appliance.

The flange <NUM>, for example the lower part <NUM> and the upper part <NUM> may be made from a metal or a metal alloy, for instance from an aluminum alloy or stainless steel.

The core <NUM> is a machined resin impregnated paper condenser core, for example. Along an axial direction of the bushing <NUM>, the core comprises a first section <NUM> arranged between a second section <NUM> and a third section <NUM>, wherein a diameter of the first section <NUM> is larger than that of the second section <NUM> and the third section <NUM>. The lower part <NUM> of the flange <NUM> has a tapered section that forms a seat <NUM> that fits a first transition <NUM> between the first section <NUM> and the second section <NUM> of the core. In the exemplary embodiment shown, the first transition <NUM> is a tapered section of the core <NUM>, thereby providing means for aligning and fixing the core <NUM> within the lower part <NUM> of the flange <NUM>. However, other shapes may be used for the first transition <NUM> as well.

The upper part <NUM> of the flange <NUM> is connected to the lower part <NUM> of the flange <NUM>, for instance clamped or bolted (not explicitly shown), and sealed via O-ring gaskets 5a. The gaskets are arranged in annular axial grooves formed in at least one of the upper part <NUM> and the lower part <NUM>, for instance in the lower part as shown in <FIG>. Furthermore, the flange <NUM>, for example the upper part <NUM> is sealed against the core <NUM> by three O-ring gaskets 5b. These gaskets are located within recesses on the inner surface of the upper part <NUM> of flange <NUM>. The gaskets 5a, 5b prevent ingress of contaminants and moisture and prevent the loss of liquid locking compound during injection.

The upper part <NUM> of flange <NUM> further has a recess that, when mounted on the core <NUM>, forms the volume of joint <NUM>. The groove forming the joint volume is annular and consists of a first portion <NUM> and a second portion <NUM> adjacent to the first portion.

When seen along the axial direction, the volume of the joint <NUM> is delimited by a flange transition <NUM> on one side and by the tapered second transition <NUM> between the first section <NUM> and the third section <NUM> of the core <NUM> on the opposite side. This arrangement provides the core <NUM> to be secured in position with respect to the flange <NUM> after locking. In the embodiment shown, the flange transition <NUM> is embodied as a step forming a flange surface extending perpendicular to the axial direction. However, angles other than <NUM>° may also be used.

In radial direction, the second transition <NUM> adjoins a tapered section <NUM> of the flange resulting in a volume of the joint <NUM> that symmetrically tapers in axial direction towards the mounting face <NUM>.

The first portion <NUM> is the portion of the joint <NUM> that is filled with the compressible material <NUM>, and the second portion <NUM> is filled with the locking compound <NUM>. For instance, the compressible material comprises a foam such as a closed-cell silicone foam and the locking compound is a thermoset epoxy-type resin. However, other materials, for example those mentioned in the general part of the description may also be used.

The first portion <NUM> is arranged on that side of the second portion that faces away from the seat <NUM>.

If the core <NUM> expands more strongly than the flange <NUM> in axial direction due to different coefficients of thermal expansion, the volume of joint <NUM> decreases and the compressible material <NUM> is elastically compressed. In this case, the second transition <NUM> axially moves in a direction away from the mounting face <NUM> thereby exerting force on the locking compound <NUM> which is transferred to the flange transition <NUM> via the compressible material <NUM>. By means of this configuration, the flange transition <NUM> provides sufficient securing to make the bushing <NUM> rigid enough for its functionality, while very small movement is enabled to compensate for differential thermal expansion by means of the compressible material <NUM>. Thus, the mechanical load on the further elements of the bushing <NUM>, for example on the locking compound <NUM> and the flange <NUM> is reduced, in particular compared to the case where the volume of the joint is completely filled with a stiff locking compound.

In the exemplary embodiment of <FIG> the seat <NUM> is located at the lower part <NUM> of the flange <NUM> and the joint <NUM> is located at the upper part <NUM> of the flange <NUM>. For example, the entire volume of the joint is located in one part of the flange <NUM>, namely in the upper part <NUM>. However, the arrangement may also be inverted so that the seat <NUM> is provided by the upper part <NUM> of the flange <NUM>.

<FIG> is a schematic view of a bushing according to a further embodiment of the invention.

This further exemplary embodiment essentially corresponds to that of the previously described embodiment.

Unlike in the previously described embodiment, the volume of joint <NUM> is at least partly formed adjacent to the same part of the flange <NUM> as the seat <NUM>, namely in the lower part <NUM> of the flange <NUM>. In other words, the volume is located at an interface between the lower part <NUM> of the flange <NUM> and the upper part <NUM> of the flange. For instance, the first portion <NUM> of the volume is located adjacent to the upper part <NUM> and the second portion <NUM> is adjacent to the lower part <NUM> of the flange <NUM>. For instance, the volume of the joint <NUM> is formed by an annular groove within the upper part <NUM> and a further annular groove within the lower part <NUM>, so that when both parts are mounted around the core <NUM> each form part of the volume of joint <NUM>.

As in the previous embodiment, the lower part <NUM> of the flange <NUM> is sealed against the core by three O-ring gaskets 5a. The lower part <NUM> of the flange is bolted or clamped to the upper part <NUM> of the flange <NUM>. The upper part <NUM> of the flange <NUM> is sealed against the core by an O-ring gasket 5b. The gaskets 5a, 5b are located within recesses on the inner surface of the upper part and the lower part of the flange <NUM>.

The upper part <NUM> has an annular recess <NUM> configured to receive an end of an insulator (not shown in the Figure).

As in the previous embodiment, the first section <NUM> of the core <NUM> is delimited by a first transition <NUM> and a second transition <NUM>, wherein the first transition is located at the seat <NUM> and the second transition is located at the joint <NUM>. The diameter of the core <NUM> outside of the first section <NUM> is smaller than in the first section, so that the core can be easily inserted into the lower part <NUM> of the flange during production of the bushing <NUM>. However, the diameter of the first section <NUM> does not have to be constant throughout the entire axial extent of the first section as shown in <FIG>.

<FIG> illustrates a method for producing a bushing <NUM> which may be configured as described in connection with <FIG> and <FIG>, for instance. For better understanding, the same reference signs are used for the constituents of the bushing as in <FIG> and <FIG> even though the individual constituents are not drawn in <FIG>.

In a method step <NUM> a core <NUM> and a flange <NUM> with an upper part <NUM> and a lower part <NUM> are provided. The upper part and the lower part are separate elements of the flange configured to be affixed to one another in a subsequent step. The upper part and the lower part may be formed by means of casting, for instance.

In a method step <NUM>, the upper part <NUM> and the lower part <NUM> of the flange <NUM> are arranged around the core <NUM>. For instance, the core <NUM> is inserted into the lower part <NUM>. This can be performed by lowering the core <NUM> into the lower part <NUM> of flange <NUM> either manually or by a hoist or crane. At this stage, the core <NUM> may rest on a seat <NUM> of the lower part <NUM>. After lowering the core <NUM>, the correct seating of the core <NUM> may be inspected and an adjustment of the position of the core may be performed.

Afterwards, the upper part <NUM> may be lowered over the core <NUM> in a similar manner. Alternatively, the seat <NUM> may also be provided by the upper part <NUM> of the flange <NUM>. In this case, the core is inserted into the upper part <NUM> of the flange.

For example, a compressible material <NUM> is provided in a first portion of a volume of a joint <NUM> that is to be formed between the core <NUM> and the flange <NUM>. For instance, the compressible material <NUM> is a prefabricated annular element that rests in one of the flange parts.

In a method step <NUM>, the upper part <NUM> and the lower part <NUM> are fixed to one another, for example mechanically, for instance by clamping or bolting. A seal between the upper part <NUM> and the lower part <NUM> may be obtained by one or more gaskets between the two parts, for instance. At this stage, the core <NUM> is already held within the flange <NUM>.

In a method step <NUM>, a joint <NUM> between the flange and the core is formed. For this purpose a locking compound <NUM> is injected into a second portion <NUM> of the volume of the joint <NUM>. For example, the locking compound <NUM> fills the entire remaining volume of the joint <NUM> that has not been filled with the compressive material <NUM> before. Typically, the locking compound <NUM> is injected through one or more injection channels by a technician, but the process can also be automated. After the injection, the injection channels are sealed, e.g. by plugging.

After the injection, the locking compound <NUM> is cured while it is in direct contact with the compressible material <NUM> until the locking compound has hardened. The curing step can involve heating the bushing <NUM> or part of the bushing <NUM>, e.g. by utilizing an oven or heating mats. The temperature and time of the curing step typically are dependent on the locking compound, e.g. a thermoset polymer may require a temperature of <NUM> for a period of <NUM> hours.

Once the locking compound has hardened, the core <NUM> is aligned within the flange <NUM> and held in place in radial as well as in axial direction. However, if the volume of the joint <NUM> decreases during operation of the bushing <NUM> due to an axial thermal expansion of the core that is larger than that of the flange <NUM>, the compressive material <NUM> is elastically compressed, thereby mitigating the mechanical stress on the further elements of the bushing <NUM>, for example on the locking compound <NUM> and the flange <NUM>.

The bushing <NUM> has been described in connection with a condenser core bushing, but the inventive concept may also be used for any other kind of electrical bushing, such as a solid bushing, also known as a bulk type bushing. The skilled technician is aware that the components of the bushing may be made from a number of available materials and composites, such that the mention of one specific material must not be understood as a limitation. The core <NUM> of the bushing can be made from any suitable material or compound, such as resin impregnated paper, resin impregnated synthetic or solid epoxy.

Claim 1:
An electrical bushing (<NUM>), comprising:
- a flange (<NUM>) comprising a lower part (<NUM>) and an upper part (<NUM>) affixed to one another, and
- a core (<NUM>) surrounded by the flange,
wherein:
- the core has a first section (<NUM>) with a diameter that is larger than a diameter of a second section (<NUM>) and a diameter of a third section (<NUM>), wherein the first section is arranged between the second section and the third section along an axial direction of the bushing, wherein the flange forms a seat (<NUM>) for a first transition (<NUM>) between the first section and the second section characterized in that the flange is affixed to the core by a locking compound (<NUM>) disposed in a volume of a joint (<NUM>) between the flange and the core, and the volume of the joint further comprises a compressible material (<NUM>), the compressible material being configured to compress or expand in response to a change in the volume of the joint, and in that the volume of the joint is located at a second transition (<NUM>) between the first section and the third section; and
- the compressible material (<NUM>) is located on a side of the locking compound (<NUM>) that faces away from the seat (<NUM>).