Patent Description:
A conventional transformer typically comprises windings mounted on cores of ferromagnetic material. The cores are connected at their lower ends to a bottom yoke and at their upper ends to a top yoke. The yokes and the columns are typically assembled as a stack of magnetic sheets, e.g. grain-oriented or grain-not-oriented magnetic sheets. Further, it is known to use a frame for clamping and compressing the bottom yoke together and the top yoke together.

However, in certain cases of dynamic loads, for example vibrational loads caused by a seismic activity, the structure of the transformer assembly, i.e. the "transformer frame structure" may be displaced in such a way that high stress levels acting on critical parts of the assembly may arise. This may result in fatigue and crack propagation mechanisms and a failure of the transformer. Corresponding loads may occur for example in case of a transformer used in the field of nuclear power, wind power, marine and other industries.

<CIT> relates to a transformer. A laminated core, to which a through-hole is formed, is clamped by penetrating a bolt into the through-hole through caul plates from both end faces. A pan-shaped section of the bolt is fitted to a countersink section of a hole of one of the caul plates. The bolt is passed into an insulating sleeve mounted into the through-hole formed to the core, and the insulating sleeve insulates the bolt and the core from each other. Another end of the bolt is penetrated into holes of an insulating plate and the other caul plate installed outside the core, and a nut is screwed through an insulating washer and a metallic washer. The nut is rotated by specified torque by a spanner, etc., and the core is clamped with proper pressure.

<CIT> relates to a new core for larger various sizes of transformers and the like employing relatively interchangeable or standardized corner parts. Laminations of a core are bolted together by bolts extending completely through the laminations and cooperating with nuts. Each of the bots is insulated by an insulating sleeve where it passes through the laminations. An insulating spacer fits tightly against the sides of the innermost and outermost laminations to insulate them from steel straps.

<CIT> relates to an induction furnace comprising an iron core having a structure in which silicone steel sheets are laminated and tightened in the thickness direction with bolts.

There is a need to provide improved techniques for reducing the risk of a failure of a transformer. There is particularly a need for techniques that mitigate the risk of crack formations in a transformer frame structure. These objects are achieved by the independent claim. Dependent claims refer to preferred embodiments.

According to the present disclosure, a transformer core, comprising a first yoke, a second yoke, a column having a column main axis and extending between the first yoke and the second yoke, and an elongate clamping structure comprising an elongate rigid member having a rigid member main axis is provided. The column includes an elongate opening having an opening main axis which is oriented transversal with respect to the column main axis. The rigid member is positioned within the elongate opening such that the rigid member main axis is oriented parallel to the opening main axis.

The elongate clamping structure allows for effectuating a controllable stiffness of the column. In this manner the mechanical properties of the column and of the whole transformer core can be controlled or influenced. The stiffness of the column significantly influences the dynamic response of the transformer core and the dynamic response of the transformer to external loads such as vibrational or shock loads. Further, it influences the quality of a noise generated by the transformer core in response to such loads. More specifically, the elongate clamping structure allows for effectuating an enlarged stiffness of the transformer core resulting in an improved response behavior of the transformer to external loads.

The stiffness of the transformer core plays a dominant role in the dynamic response of the transformer, particularly to vibrational loads. The structural stiffness of the transformer core directly influences the mechanical resonance behavior of the transformer core and the corresponding relevant mode shapes. Generally, a low mechanical overall resonance frequency leads to large displacements of the structure during vibration, and therefore to high stress levels on critical parts of the transformer frame structure. The stiffness and compactness of the columns of the core are primary factors with respect to the lowest and dominant mechanical resonances of the core. This is due to the ratio between the stiffness and the mass of a column which is insofar unfavorable. The most critical mode of vibration is the one involving a longitudinal bending of the columns, as schematically sketched in <FIG> illustrating a side view of a transformer. The elongate clamping structure allows for achieving an increased flexural stiffness of the column and in this manner increased mechanical resonance frequencies of the transformer core resulting in reduced stress levels. Thus, a reduced risk of a failure of the transformer can be achieved.

The invention is defined in independent claim <NUM> and preferred embodiments are defined in the dependent claims.

Preferably, the column comprises a stack of sheets, e.g. grain-oriented or grain-not-oriented sheets. The elongate clamping structure is particularly suited in such a case, since it allows for reducing movements of the sheets relative to each other.

According to the invention, the elongate clamping structure comprises a first end portion and an opposing second end portion, wherein the first and second end portions are configured to compress a portion of the column which surrounds the elongate clamping structure. This allows for effectuating a force which compresses or tightens the column. In this manner, the stiffness of the column can be further improved.

According to the invention, the first end portion and/or the second end portion of the elongate clamping structure comprise(s) a fixing member having a conical shaped head portion. The conical shaped head portion helps generating a specifically effective compressing force.

Preferably, the fixing member for example comprises or is formed by a countersunk bolt.

According to the invention, the elongate opening comprises at least one conical end portion which is shaped correspondent to the conical shaped head portion of the fixing member, wherein the transformer core is preferably designed such that the conical shaped head portion of the fixing member is positioned completely within the elongate opening. Thus, an adverse effect of the fixing member on windings of a coil of the transformer which is wound about the column can be excluded or at least mitigated.

Preferably, the column comprises at least one leg plate extending parallel to the column main axis, wherein the at least one conical end portion of the elongate opening is formed in the at least one leg plate.

Preferably, the fixing member comprises a screw which is screwable connected to the rigid member. This allows for achieving a controllable compressive force acting on the column in a particularly suitable and easy to handle manner. Moreover, this facilitates assembling of the transformer core.

According to the invention, the transformer core further comprises at least one washer positioned around the conical shaped head portion of the fixing member.

According to the invention, the washer is of insulating material. This allows for mitigating the risk of damaging parts of the transformer core during assembly.

Preferably, the elongate clamping structure comprises an insulation tube which is positioned between the rigid member and an inner surface of the elongate opening and configured to effectuate an isolation between the rigid member and the column. The insulation tube may be a flexible insulation tube.

Preferably, the washer is formed as a part of the insulation tube. In other words, the insulation tube preferably extends to the outer end of the conical shaped head portion of the fixing member. Preferably, the insulation tube is designed such that it ends flush with a surrounding outer surface section of the column directly adjacent to the conical end portion of the elongate opening. Alternatively, the washer is preferably positioned between the conical shaped head portion of the fixing member and the insulation tube. Preferably, the insulation tube and the washer in any case are formed such that they do not protrude above the surrounding outer surface section of the column.

Preferably, the transformer core comprises at least one further elongate clamping structure which is constructed analog to the first mentioned elongate clamping structure. Preferably, the elongate rigid member of the at least one further elongate clamping structure is oriented parallel to the elongate rigid member of the first mentioned elongate clamping structure. The at least one further elongate clamping structure allows for effectuating a further improved stiffness and compactness of the column.

For example, at least three elongate clamping structures are preferably provided such that a row of elongate clamping structures along the column is formed, comprising a first elongate clamping structure, a second elongate clamping structure, and a third elongate clamping structure, in that order, wherein a first distance between the first and the second elongate clamping structures differs from a second distance between the second and the third elongate clamping structures. This allows for achieving a certain level of stiffness and compactness of the column by a particularly small number of elongate clamping structures.

Preferably, the row of elongate clamping structures is formed such that the distances between the elongate clamping structures increase with increasing distance form the first yoke, wherein preferably, the first yoke is a bottom yoke and the second yoke is an upper yoke.

Preferably, the transformer core further comprises at least one further column, wherein a main axis of the at least one further column and the main axis of the first mentioned column are positioned within a common plane, and wherein the opening main axis of the elongate opening is further oriented normal to said plane.

Preferably, the at least one further column is constructed analog to the first mentioned column. Preferably, the at least one further column comprises corresponding elongate openings, wherein rigid members of corresponding further elongate clamping structures are positioned within the elongate openings analogously.

According to a further aspect of the disclosure, a transformer comprising a transformer core according to the disclosure and a winding wound around the column is provided.

The subject-matter of the disclosure will be explained in more detail with reference to preferred exemplary embodiments which are illustrated in the attached drawings, in which:.

Example embodiments of the disclosure will be described with reference to the drawings in which identical or similar reference signs designate identical or similar elements. The features of embodiments may be combined with each other, unless specifically noted otherwise.

<FIG> is a schematic front view of a transformer core according to the disclosure. A transformer comprising the transformer core may be for example a distribution transformer. The transformer core comprises a first yoke <NUM>, a second yoke <NUM>, and a column <NUM>. The first yoke <NUM> is a lower or bottom yoke and the second yoke <NUM> is an upper yoke. The column <NUM> extends between the first yoke <NUM> and the second yoke <NUM>. The column <NUM> has a column main axis <NUM> which is preferably oriented at least essentially vertically. A cross-section of the column <NUM> normal to the column main axis <NUM> is preferably at least to a first approximation circular.

The transformer core further comprises at least one further column <NUM>', <NUM>", for example two further columns <NUM>', <NUM>", wherein a main axis <NUM>', <NUM>" of the at least one further column <NUM>', <NUM>" and the main axis <NUM> of the first mentioned column <NUM> are positioned within a common plane, for example a vertical plane. The at least one further column <NUM>', <NUM>" is preferably constructed analog to the first mentioned column <NUM>. The transformer comprises the transformer core according to the disclosure and windings wound around each one of the columns <NUM>, <NUM>, <NUM>".

The transformer core further comprises an elongate clamping structure <NUM>. <FIG> is a schematic cross-sectional view of the elongate clamping structure <NUM> and a surrounding portion of the column <NUM>. Note that <FIG> shows the distances between corresponding parts enlarged simply for improved recognizability. The cross-section of <FIG> is taken perpendicular to the common plane in which the main axes <NUM>, <NUM>', <NUM>" of the columns <NUM>, <NUM>', <NUM>" are positioned, i.e. in a plane perpendicular to the drawing plane of <FIG>.

The elongate clamping structure <NUM> comprises an elongate rigid member <NUM> having a rigid member main axis <NUM>. The rigid member <NUM> is preferably made of a material comprising a metal, particularly steel. The rigid member <NUM> preferably is tube shaped, for example in the form of a hollow-core bolt. For example, the rigid member <NUM> is a steel pipe in the form of a hollow-core bolt.

The column <NUM> includes an elongate opening <NUM> having an opening main axis <NUM> which is oriented transversal, for example perpendicular with respect to the column main axis <NUM>. Preferably, the opening main axis <NUM> is oriented horizontally. The rigid member <NUM> is positioned within the elongate opening <NUM> such that the rigid member main axis <NUM> is oriented parallel to the opening main axis <NUM>.

<FIG> are schematically illustrated side views of a prior art transformer core illustrating generally an effect of a load acting on the transformer core. The transformer core comprises a bottom yoke <NUM>, a column <NUM>, and an upper yoke <NUM>. <FIG> illustrates a state of the transformer core without the load acting on the transformer core, <FIG> a state with the load acting on the transformer core, as indicated by an arrow. The load leads to a deformation of the transformer core as shown in <FIG> which is generally unwanted since it involves an increased risk of a crack mechanism potentially leading to a failure of the transformer, as outlined above. According to the present disclosure, the stiffness of the column and of the transformer core can be suitably increased under use of the elongate clamping structure in such a way that a corresponding unwanted deformation of the transformer core can be prevented.

Preferably, the elongate rigid member <NUM> is designed such that it extends over at least <NUM>%, more preferably at least <NUM>% of the length of the elongate opening <NUM>. Preferably, both ends of the elongate rigid member <NUM> do not protrude beyond an outer surface of the column <NUM>.

The elongate clamping structure <NUM> comprises a first end portion <NUM> and an opposing second end portion <NUM>. The first and second end portions <NUM>, <NUM> are configured to compress a portion of the column <NUM> which surrounds the elongate clamping structure <NUM>. To this end the first end portion <NUM> and/or the second end portion <NUM> of the elongate clamping structure <NUM> comprises a fixing member <NUM>, <NUM>' having a conical shaped head portion <NUM>, <NUM>'. Preferably, the fixing member <NUM>, <NUM>' is a countersunk bolt. Preferably, the fixing member <NUM>, <NUM>' comprises or is formed of a screw which is screwable connected to the rigid member <NUM>, for example screwed into the rigid member <NUM> having the form of a hollow-core bolt. This enables easy connecting the elongate clamping structure <NUM> to the column <NUM> during assembly of the transformer core.

The elongate opening <NUM> comprises at least one conical end portion <NUM>, <NUM>' which is shaped correspondent to the conical shaped head portion <NUM>, <NUM>' of the fixing member <NUM>, <NUM>'. This allows for an easy and fine adjustment of a compressive force acting on the column <NUM> that is generated by the fixing members <NUM> in combination with the elongate rigid member <NUM>.

The column <NUM> preferably comprises at least one leg plate <NUM> extending parallel to the column main axis <NUM>, wherein the at least one conical end portion <NUM>, <NUM>' is formed in the at least one leg plate <NUM>. For example, the leg plate <NUM> forms an outer surface of the column <NUM>. In particular, the column <NUM> comprises a stack of sheets, wherein an inner surface of the at least one leg plate <NUM> abuts the stack of sheets. For example, as sketched in <FIG>, the column <NUM> comprises two leg plates <NUM> disposed on opposing sides of the column <NUM> with respect to the column main axis <NUM>.

The transformer core is preferably designed such that the conical shaped head portion <NUM>, <NUM>' of the fixing member <NUM>, <NUM>' is positioned completely within the elongate opening <NUM>. Accordingly, an interference of a winding of a coil wound around the column <NUM> with the elongate clamping structure <NUM> can be precluded or at least mitigated.

The elongate clamping structure <NUM> preferably further comprises an insulation tube <NUM> which is positioned between the rigid member <NUM> and an inner surface of the elongate opening <NUM> and configured to effectuate an isolation between the rigid member <NUM> and the column <NUM>. For example, the insulation tube <NUM> is made from a material comprising Polytetrafluorethylene (PTFE). Preferably, the insulation tube <NUM> is a PTFE-tube.

The transformer core further comprises at least one washer positioned around the conical shaped head portion <NUM>, <NUM>' of the fixing member <NUM>, <NUM>'. According to the invention, the at least one washer is arranged between the head portion <NUM>, <NUM>' of the fixing member <NUM>, <NUM>' and the conical end portion <NUM>, <NUM>' of the elongate opening <NUM>.

The at least one washer may be associated with the insulation tube <NUM> or formed as a part of the insulation tube <NUM>. Alternatively, as sketched in the cross-sectional view of <FIG>, the at least one washer - here indicated by reference sign <NUM> - may be an insulating washer arranged between the head portion <NUM>, <NUM>' and a surface of the conical end portion <NUM>, <NUM>' of the elongate opening <NUM>. This reduces the risk of damaging the insulation tube <NUM> during assembly, for example during tightening the fixing members <NUM>, <NUM>'.

The transformer core preferably further comprises at least one further elongate clamping structure <NUM>', <NUM>", <NUM>‴ as exemplarily sketched in <FIG>, which is constructed analog to the first mentioned elongate clamping structure <NUM>. The elongate rigid member of the at least one further elongate clamping structure <NUM>', <NUM>", <NUM>‴ is preferably oriented parallel to the elongate rigid member <NUM> of the first mentioned elongate clamping structure <NUM>.

For example, the transformer core comprises at least three elongate clamping structures <NUM>, <NUM>', <NUM>", <NUM>‴ such that a row of elongate clamping structures along the column <NUM> is formed, comprising a first elongate clamping structure <NUM>, a second elongate clamping structure <NUM>', and a third elongate clamping structure <NUM>", in that order. A first distance d<NUM> between the first and the second elongate clamping structures <NUM>, <NUM>' differs from a second distance d<NUM> between the second and the third elongate clamping structures <NUM>', <NUM>". According to the illustrated embodiment the row of elongate clamping structures <NUM>, <NUM>', <NUM>", <NUM>‴ is formed such that the distances d<NUM>, d<NUM>, d<NUM> between the elongate clamping structures <NUM>, <NUM>', <NUM>", <NUM>‴ increase with increasing distance form the first yoke <NUM>. For example, the distances di may be chosen such that di+<NUM> = di xi, where i = <NUM>, <NUM>, <NUM>,. with <NUM>,<NUM> ≤ xi ≤ <NUM>,<NUM> and xi+<NUM> > xi.

Claim 1:
Transformer core, comprising:
a first yoke (<NUM>),
a second yoke (<NUM>),
a column (<NUM>) having a column main axis (<NUM>) and extending between the first yoke (<NUM>) and the second yoke (<NUM>), and
an elongate clamping structure (<NUM>) comprising an elongate rigid member (<NUM>) having a rigid member main axis (<NUM>),
wherein the column (<NUM>) includes an elongate opening (<NUM>) having an opening main axis (<NUM>) which is oriented transversal with respect to the column main axis (<NUM>),
wherein the rigid member (<NUM>) is positioned within the elongate opening (<NUM>) such that the rigid member main axis (<NUM>) is oriented parallel to the opening main axis (<NUM>),
wherein the elongate clamping structure (<NUM>) comprises a first end portion (<NUM>) and an opposing second end portion (<NUM>), wherein the first and second end portions (<NUM>, <NUM>) are configured to compress a portion of the column (<NUM>) which surrounds the clamping structure (<NUM>),
wherein the first end portion (<NUM>) and/or the second end portion (<NUM>) of the elongate clamping structure (<NUM>) comprises a fixing member (<NUM>, <NUM>') having a conical shaped head portion (<NUM>, <NUM>'),
wherein the elongate opening (<NUM>) comprises at least one conical end portion (<NUM>, <NUM>') which is shaped correspondent to the conical shaped head portion (<NUM>, <NUM>') of the fixing member (<NUM>, <NUM>'), and
characterized in that
the transformer core further comprises at least one washer (<NUM>) positioned around the conical shaped head portion (<NUM>, <NUM>') of the fixing member (<NUM>, <NUM>') and arranged between the conical shaped head portion (<NUM>, <NUM>') and the at least one conical end portion (<NUM>, <NUM>') of the elongate opening (<NUM>), wherein the at least one washer is made from an electrically insulating material.