Patent Description:
Transformers, as any other industrial products, must comply with various requirements on noise levels. It is known to people skilled in the art that the acoustic power P emitted from a vibrating structure acted upon by forces F can be expressed <MAT> in which Φ represents the collection of mode shapes associated with the mechanical properties of the structure, and the operator BFΦ implicitly depends on the geometry of the structure, the frequency, and also materials properties of the acoustic and structural media in question. Furthermore, H denotes the Hermitian transpose of the vector, and T denotes a regular vector transposition. The quantity ΦTF is here to be interpreted as the scalar or dot product of the two vectors, indicating that when these two vectors are orthogonal, the resulting acoustic power goes to zero. This orthogonality is in this invention proposed to be brought about by promoting asymmetric winding resonance modes which are acted upon by the inherently symmetric force distributions. Regardless of the actual proximity of the frequency of the mode to the double the network frequency, the resulting acoustic power is reduced.

In more detail, the equation of motion for a mechanical assemly, in this context typically a winding with supporting structures or a set of such windings, is in numerical approaches generally expressed <MAT> in which u is the displacement vector, M, C, K, are the system mass, damping, stiffness, matrices, respectively, and F the force vector.

Based on the above system matrices and introducing in a well-known manner the system mode shapes Φ and modal coordinates z, <MAT> it is equally well known that the frequency domain modal displacement zn at frequency ω is given by <MAT> such that the modal displacement component umn - arbitrary location m in the winding, mode n - can be expressed <MAT>.

Here, the parameter ξn denotes the damping ratio (fraction of critical damping), and for further clarity the quantity um is expressed as a summation over the system modes according to <MAT>.

Further studying the fraction in this expression, the classical approaches to mitigate noise and vibrations can readily be discussed. Obviously, when the driving frequency ω is close to a resonance frequency ωn , or a narrow set of such frequencies, the structural responses xm might grow beyong permissible levels, and the commonplace methods to alleviate this effect are.

<CIT> discloses a method of damping where piezoelectric transducers/actuators are arranged on a tank wall of a transformer. They are aligned with areas of significant deflection of the tank wall at natural frequencies. Vibrations of the wall are measured and analysed, whereafter the piezoelectric actuators are controlled to absorb the vibrations and consequently reduce the noise levels. However, in the transformer noise context it is difficult to add damping to the extent vibration levels are significantly reduced.

Furthermore, the second commonplace method of changing the resonance frequencies might lead to resonance phenomena controlled by the new resonances which will inevitably appear close to the exciting frequency ω. In fact, in the transformer noise context, it is important to also pay close attention to winding dynamics during short-circuit events, in that here the mechanical frequency content during a few cycles of the network frequency (usually, but not limited to, <NUM> or <NUM>) varies between the network frequency and two times the same. The latter being the steady state driving frequency ω implicitly assumed in the above theory background. In oherwords, shifting resonances generally has to be executed with great care for ensuring the integrity of the transformer system as a whole. <CIT> discloses an example where two windings are configured to have different resonance frequencies and are arranged to compensate each other.

According to its abstract, <CIT> relates to a stationary induction apparatus having a plurality of windings concentrically disposed around a leg portion of an iron core. In the apparatus, the distribution of ampere-turns in the axial direction within an inner one of the windings is reduced at substantially equally spaced intervals to vary the vibration mode of the winding so as to prevent the winding from resonating at the power source frequency and its harmonics and to reduce the electromagnetic force developed in the axial direction of the winding due to a short-circuit current flowing through the winding in the event of a short-circuit occurring in a system to which this apparatus is connected and due to leakage flux appearing in the apparatus.

According to its abstract, <CIT> relates to a stationary induction apparatus in which the number of natural vibrations of a winding formed by winding an insulated conductor by a plurality of turns about a leg portion of an iron core is shifted from the power source frequency and the frequency which is double the power source frequency so as to prevent the winding from resonating with the latter frequencies and to reduce the electromagnetic force generated in the axial direction of the winding due to a short-circuit current which may flow into the apparatus winding and due to the leakage flux occurring in the apparatus, thereby to obtain a mechanically strong winding structure.

According to its abstract, <CIT> relates to a low cost stationary inductor without deterioration of short-circuit mechanical strength of winding improving the long-term reliability of the same stationary inductor. An installation interval of spacers among the coils is changed depending on circumferential direction distribution of an axial direction generating force of a winding. At the internal diameter side winding, the installation interval of the spacer is set wider at the area near the position Pin within the window of iron core having a smaller axial direction generating force in comparison with that at the position near the position Pout at the outside of window of the iron core. Namely, the spacers among coils are arranged to satisfy the relationship θin>θout when the spacer installation interval at the position in the window of iron core is defined as θin and spacer installation interval at the area nearest to the tank at the outside of window of the iron core as θout. An interval between the positions Pin and Pout is determined so that the surface pressure of spacer in each position in the circumference direction becomes equal or becomes the value within the predetermined deviation depending on the distribution of generated force.

Finally, the electromagnetic force distributions acting on the winding conductors should be considered as givens with few design degrees of freedom for controlling noise.

Therefore, an object of the disclosure is to provide an improved winding for a transformer. More specifically, an object of the disclosure is to provide a winding having appropriately low noise emissions and which is cost-effective to build and assemble. Another object of the disclosure is to provide a transformer comprising such a winding and a transformer arrangement comprising such a transformer in a transformer tank.

According to a first aspect of the disclosure the object is achieved by a winding according to claim <NUM>.

For the sake of clarity, the present disclosure does not make any further reference to the controlling of resonances ωn for noise minimization, or any of the other classical approaches discussed in the background section above.

A vibration mode of the winding describes the deformation that the winding would show when vibrating at the natural frequency during excitation under load. The set of vibration modes thus indicates how the winding behaves under a dynamical load, such as when excited by an oscillating electromagnetic field generated by the alternating current at the predetermined frequency. The vibration modes determine the acoustic power of the winding, e.g. how much air/oil is displaced during vibration, and consequently how efficiently noise is generated by the winding at the mechanical main frequency. The acoustic power of the winding in turn affects the acoustic power of a transformer in which the winding is comprised.

The predetermined frequency may for instance be <NUM> or <NUM>. At these frequencies, the corresponding main frequencies of vibration, at which the winding is operating, thus become <NUM> or <NUM>, respectively.

The at least one main contributing vibration mode is, as outlined above, the vibration mode contributing to the highest acoustic power, when the winding is excited by the load at the main frequency. The acoustic power generated by the winding, and consequently noise generation, may thus be reduced when the winding is adapted such that the dot products ϕnTF of a winding approach zero. By way of example, the mode shapes in a structure may be modified by adapting the mass and/or the elasticity of the structure. However, it is also envisaged that other characteristics of the winding may have an impact on the mode shapes. In the present disclosure case, the structure is exemplified by a winding, a transformer and/or a transformer tank.

Generally, the object is achieved by focusing on the nominator of the governing fraction given in the background section above, in that the dot products ϕnTF are optimized to approach zero, regardless of the properties of the mechanisms being represented by the terms forming the denominator. Thus, the structural vibrations can be controlled for low noise performace.

The term winding is herein used to denote a single winding of a phase winding of a transformer, such as an inner winding or an outer winding, a low voltage winding or a high voltage winding, etc..

By the provision of a winding as disclosed herein, the vibration modes may be changed by modifying the elasticity, i.e. stiffness, of the winding. Providing winding portions of different winding portion stiffnesses is a convenient and cost-effective way of modifying the main contributing mode shape, from a symmetric mode shape to an asymmetric mode shape, as discussed hereinabove.

Optionally, the first winding portion has a first winding portion stiffness, as seen along the coil axis. The second winding portion has a second winding portion stiffness, as seen along the coil axis. The first winding portion stiffness is different from the second winding portion stiffness.

Optionally, the winding winding is provided with a plurality of spacers between the coil turns. The first winding portion is provided with a first spacer distribution and the second winding portion is provided with a second spacer distribution. The first spacer distribution being different from said second spacer distribution.

The symmetric force distribution of the electromagnetic load may excite large vibrations along the coil axis (first axis) of the at least one winding. Therefore, arranging the different winding portions, with different stiffnesses, along the coil axis is an efficient way of affecting the vibration mode shapes of the winding and to reduce noise of the winding at the main mechanical frequency. As non-limiting examples, stiffness of a winding may be modified by arranging the winding portions with different spacers, CTC cables and/or different stiffness distributions.

Optionally, the first type of spacers has a first modulus of elasticity and the second type of spacers has a second modulus of elasticity, said first modulus of elasticity being different from said second modulus of elasticity.

The spacers are conventionally distributed along the axial length of the winding, between the coil turns, so as to separate and electrically insulate the turns of the coil from each other. When the coil turns vibrate, the elasticity of the spacers affect the elasticity of the winding, and in turn, the transformer as a whole. Thereby, the mode shape of the at least one main contributing mode, or the symmetric mode, of the winding may be modified by providing spacers of different modulus of elasticity in different winding portions. The modulus of elasticity may for instance be selected by selecting appropriate materials for the spacers. The modulus of elasticity of selectable/applicable materials range between <NUM> GPa - <NUM> GPa, or higher.

Apart from adapting the stiffness through the modulus of elasticity of the spacer materials, the spacers may have a structural shape to provide an increased, or a reduced, stiffness as compared to conventional spacers. Consequently, the first type and the second type of spacers might conceivably be of the same material but be provided with different shapes in order to provide at least the first and the second winding portions with different stiffnesses. However, the modification of the stiffness by the structural design of the spacers does not offer many degrees of freedom due to design requirements on windings and transformers.

Optionally, the first spacer distribution comprises spacers arranged at a first distance between each other in a direction around the coil axis and the second spacer distribution comprises spacers arranged at a second distance between each other in a direction around the coil axis, said first distance being different from said second distance.

The spacers are conventionally equidistantly distributed along the coil turns. By decreasing the distance between the spacers in, for instance, the first winding portion as compared to the second winding portion, the stiffness of the first winding portion is increased as compared to the second winding portion. Here also, the degrees of freedom are limited due to design requirements on windings and transformers. A reduced distance between the spacers reduces the cooling efficiency of the electrically insulating liquid in which the winding (transformer) is immersed in a transformer tank.

Optionally, the first winding portion is located at a different axial position as seen along the coil axis in relation to the second winding portion.

The winding may have the first winding portion and the second winding portion in different positions along an axial length of the coil axis. The winding may, for instance, be divided into axial sections corresponding to the winding portions. The first winding portion may also have a different axial length as compared to the second winding portion. As disclosed hereinabove, the provision of a first winding portion whose mass or stiffness differs from the second winding portion modifies the main contributing mode, or the symmetric mode, of the transformer so as to reduce vibrations and noise at the main frequency. Arranging the first winding portion and the second winding portion in different positions along an axial length of the coil axis is a way of breaking the structural symmetry of the winding.

By a sector of the winding is herein meant a winding portion delimited by a circumferential arc length around the coil axis and an axial length along the coil axis of the winding. The arc length is determined by a central angle α at the coil axis, between two radii extending between the coil axis and the coil turns of the winding portion. The winding may, for instance, be divided into sectors corresponding to the winding portions. The first winding portion may also have a different arc length as compared to the second winding portion. As disclosed hereinabove, the provision of a first winding portion whose mass and/or stiffness differs from the second winding portion modifies the vibration mode shape of the at least one main contributing mode, or the symmetric mode, of the transformer so as to modify the vibration mode shape towards an asymmetric mode and to reduce vibrations and noise at the main frequency.

According to a third aspect of the present disclosure there is provided a transformer comprising at least one winding according to the first aspect of the disclosure.

When the transformer comprises at least one winding according to the present disclosure, the acoustic power of each winding may either reduce the acoustic power of the transformer as a whole, such as when at least one of three windings is in accordance with the present disclosure.

According to a fourth aspect of the disclosure there is provided a transformer arrangement comprising the transformer in accordance with the third aspect, wherein the transformer is enclosed in a transformer tank.

The transformer may be immersed in an electrically insulating medium, such as oil, in the transformer tank. By the provision of at least one winding according to the disclosure, the at least one main contributing mode, or the symmetric mode, of the transformer may be modified to reduce vibration and noise of the transformer. Consequently, such a transformer in a transformer tank will cause the transformer tank walls to generate less noise.

Further objects and advantages of, and features of the disclosure will be apparent from the following description of one or more embodiments, with reference to the appended drawings, where:.

The present disclosure is developed in more detail below referring to the appended drawings which show examples of embodiments. The disclosure should not be viewed as limited to the described examples of embodiments; instead, it is defined by the appended patent claims.

<FIG> show side view cross-sections of an exemplary prior art windings <NUM>' in a transformer <NUM>' under different vibration modes. The prior art winding <NUM>' has a first extension along a first axis z, a second extension along a second axis x and a third extension along a third axis y (not shown). The first, second and third axes are perpendicular to each other. The prior art winding <NUM>' is further exemplified as comprised in a transformer having three identical windings <NUM>' being located at a distance from each other as seen along said second axis (x). The transformer <NUM>' may have a phase winding for each phase of the transformer. Each phase winding may comprise a winding <NUM>', such as an inner winding and an outer winding, which may be a low voltage winding and a high voltage winding, respectively.

Each winding has a first end and an opposite second end along the first axis (z). The first and second ends are respectively provided with a first pressplate <NUM>' and a second pressplate <NUM>', between which two pressplates the winding <NUM>' is clamped. When the transformer <NUM>' is in operation, electromagnetic forces and the clamping of the windings <NUM>' between the pressplates generate load noise, which is a significant part of the total noise of transformers <NUM>, radiated by the windings <NUM>', especially for large units.

Symmetric movements (piston-like displacements) of a transformer tank <NUM>', in which the transformer <NUM>' may be enclosed, radiate significant noise to the far field as compared to asymmetric movement because symmetric vibrations displace more air outside the transformer tank <NUM>' and thereby radiate sound more efficiently than asymmetric movements. Windings <NUM>' under load usually vibrate at <NUM> or <NUM> mechanical main frequency (i.e. usually <NUM> or <NUM> predetermined electrical operating (excitation) frequency multiplied by two).

<FIG> illustrate the movement of the pressplates <NUM>', <NUM>' by arrows M of the transformer <NUM>'. For the sake of clarity, the arrows are only shown for one phase winding <NUM>'. In practice, for the prior art transformer <NUM>', all phase windings <NUM>' exhibit the same vibration pattern, albeit at a <NUM>° phase shift in relation to each other, for e.g. a three-phase transformer <NUM>' such as shown in <FIG>.

<FIG> shows how acoustic power of the transformer <NUM>', as a result of vibrations of the windings <NUM>', varies with frequency. The horizontal axis displays the mechanical vibration frequency. The curve represents a superposition of vibration modes of the structure of the transformer <NUM>' as a result of vibrations of the windings <NUM>'. The modes of interest of the transformer <NUM>' may be identified at the peak amplitudes, where the acoustic power is largest.

<FIG> illustrate symmetric and asymmetric vibration modes, respectively and further explain the sound producing properties thereof. <FIG> conceptually shows a symmetric mode acting on the pressplate <NUM>' of a winding <NUM>' of the prior art transformer <NUM>'. It can be seen that a certain volume of a surrounding medium, ΔV (positive or negative), such as oil or air, is displaced as the pressplate <NUM>' vibrates. This displacement radiates noise to the audible far field, which may be perceived as disturbing noise. In contrast, the asymmetric vibration mode shown in <FIG> moves one part of the pressplate <NUM>' up as another part is moved down, theoretically resulting in a net volume displacement, ΔV, equal to zero. Such an asymmetric vibration mode radiates noise to the near field, which is not audible at a distance. In other words, it is not perceived as disturbing noise. A centre plane P is shown in <FIG>. The arrows M in <FIG> illustrate how every portion of the winding <NUM>', located on opposite sides of the centre plane P, are displaced in the same direction at the same time for displacements in directions parallel to the centre plane P. In <FIG> the asymmetric vibration mode results in opposing directions on opposite sides of the centre plane P.

<FIG> shows a side view cross-section of an exemplary winding <NUM>, according to the present disclosure, comprised in a transformer <NUM>. The transformer <NUM> may have a phase winding for each phase of the transformer. Each phase winding may comprise at least one winding <NUM>, such as an inner winding <NUM> and an outer winding <NUM>, which may be a low voltage winding and a high voltage winding, respectively. The illustrated exemplary transformer comprises three phase windings, each comprising windings <NUM> according to the present disclosure. For the sake of simplicity, and since the effect of the present invention may be achieved by modification of a single winding <NUM> comprised in a phase winding, the term winding <NUM> is hereafter used to denote a single winding of a phase winding of a transformer <NUM>. Each winding <NUM> has coil turns <NUM> (<FIG>) around a coil axis (z). The transformer <NUM> is adapted to transform voltage at a predetermined frequency, when the transformer <NUM> is operating. The winding <NUM> is excited by a mechanical load having a main frequency corresponding to the predetermined frequency multiplied by two and having vibration modes. The combination of load and vibration modes results in vibration of the winding <NUM>. The winding <NUM> further has a set of vibration modes, each vibration mode having a vibration mode frequency, where a at least one main contributing vibration mode of the set of vibration modes is the vibration mode which results in the largest acoustic power, of the vibration modes, when the winding <NUM> is excited by the load. The winding <NUM> comprises a plurality of winding portions <NUM>. The plurality of winding portions <NUM> comprises at least a first winding portion 116a and a second winding portion 116b. The first winding portion 116a has a first winding portion stiffness and the second winding portion 116b has a second winding portion stiffness. A stiffness difference between said first winding portion stiffness and said second winding portion stiffness is such that the acoustic power is minimized at said main frequency.

<FIG> shows a magnified detail of the coil turns <NUM> of a winding <NUM>. The winding <NUM> is provided with a plurality of spacers <NUM> between the coil turns <NUM>. The spacers are conventionally distributed along the axial length of the winding <NUM>, between the coil turns, so as to separate and electrically isolate the turns of the coil from each other.

The winding <NUM> further has a first extension along a first axis z. The coil axis is parallel to the first axis z. The winding <NUM> has a second extension along a second axis x and a third extension along a third axis y (see <FIG>). The first, second and third axes are perpendicular to each other and the centres of the illustrated windings <NUM> are located at a distance from each other as seen along the second axis x. The winding <NUM> comprises a first centre plane A which extends along the second axis x and third axis y and splits the winding <NUM> in half, as seen in along the first axis z. The winding <NUM> comprises a second centre plane B (see <FIG>) which extends along the second axis x and first axis z and splits the winding <NUM> in half, as seen in along the third axis y. The winding <NUM> comprises a third centre plane C which extends along the third axis y and first axis z and splits said winding <NUM> in half, as seen in along the second axis x.

Each winding <NUM> may have a first end and an opposite second end along the coil axis, i.e. parallel with the first axis z. The first and second ends are respectively provided with a first pressplate <NUM> and a second pressplate <NUM>, between which two pressplates the winding <NUM> is clamped.

A symmetric mode of mechanical vibration of said winding <NUM> results in that every portion of said winding <NUM>, located on opposite sides of one of said centre planes A, B, C, are displaced in the same direction at the same time for displacements in directions parallel to the centre plane concerned. An asymmetric mode of mechanical vibration of said transformer <NUM> results in that every portion of said transformer <NUM>, located on opposite sides of one of said centre planes A, B, C, are displaced in the opposite direction at the same time for displacements in directions parallel to the centre plane concerned.

A mode spectrum may be used to study a structure's vibration amplitude in response to different frequencies. Devices and methods for creating a mode spectrum are known to a person skilled in the art. A transformer tank wall can for instance be caused to vibrate by means of a pulse hammer and the vibrations of the tank wall can be measured by acceleration sensors or by piezoelectric force transducers that are distributed over the surface of the tank wall. The measured signals can be forwarded to a computer system which performs a modal analysis and numerically determines the dynamic characteristics of the tank wall therefrom.

As discussed in conjunction with <FIG>, the noise generating mechanism of a winding <NUM>, is controlled by a nearly symmetric winding axial force distribution,. The winding <NUM> of the present disclosure seeks to break this match by introducing an asymmetric vibration shape such that the dot products ϕnTF tend towards zero. The force distribution for a winding <NUM> is a given due to the structure. The shape and design of the core, the coil turns <NUM> and/or pressplates are presets to obtain the required electrical performance of the transformer <NUM>. Other properties on which winding <NUM> vibrations depend may, however, be modified without affecting performance. Such a property is mechanical stiffness. Another property is the mass of the windings <NUM>. Possibilities to modify the mass are limited due to design requirements placed on windings and transformers. For this purpose, the transformer <NUM> according to the present disclosure, has at least one of its windings <NUM> provided with the plurality of winding portions <NUM> having different winding portion stiffnesses.

In the exemplary embodiment of <FIG>, which is a top-side cross-sectional view of the windings <NUM> of the embodiment of of <FIG>. Each phase winding is shown to have an inner winding <NUM> and an outer winding <NUM>. The inner winding may be a low voltage winding and the outer winding may be a high voltage winding, or vice versa. Each winding <NUM> may have different winding portions <NUM>.

According to the present disclosure, a winding <NUM> comprises at least two winding portions <NUM>. Thus, any number of winding portions <NUM> greater than two is also within the scope of the disclosure.

A winding portion <NUM> herein means a part of the coil turns of a winding <NUM>. A winding portion may be a part of a winding, such as an axially elongated section of a winding, limited in length along the first axis z (not shown). A winding portion may also/alternatively be a sector of a winding, limited by a centre angle α to a circumferential sector arc length of the winding.

The introduction of a stiffness difference between the winding portions <NUM> breaks the symmetric mode of mechanical vibration and instead introduces an asymmetric mode of vibration in the winding <NUM> comprising the differing winding portions. As a result, the symmetric mode of mechanical vibration of the winding <NUM> and the transformer <NUM> as a whole is broken.

In a transformer <NUM>, such as shown in <FIG>, comprising at least one winding <NUM> according to the present disclosure, and in a transformer arrangement <NUM>, such as shown in <FIG>, comprising the transformer <NUM> having at least one winding <NUM> according to the present disclosure, enclosed in a transformer tank <NUM>, the symmetric mode of mechanical vibration of a winding <NUM>, and consequently of the transformer <NUM> and of the transformer tank <NUM>, is broken by the introduction of the first winding portion 116a having a first winding portion stiffness, as seen along the coil axis z.

The second winding portion 116b may further have a second winding portion stiffness, as seen along the coil axis z. As before, the first winding portion stiffness is different from said second winding portion stiffness.

The first winding portion 116a is provided with a first spacer distribution and the second winding portion 116b is provided with a second spacer distribution. The first spacer distribution is different from said second spacer distribution. Choice of materials for the spacers <NUM> is a factor that may be used to break the symmetric mode of mechanical vibration. When the coil turns <NUM> vibrate, the elasticity provided by the spacers <NUM> affects the stiffness of the winding <NUM> and the transformer <NUM> as a whole, and thereby affects the modes of vibration of the winding <NUM> and the transformer <NUM>. It should be noted that the detail of <FIG> only shows a part of one spacer distribution.

The first spacer distribution may comprise a first type of spacers and the second spacer distribution may comprise a second type of spacers. The first type of spacers is different from said second type of spacers. The first type of spacers may for instance have a first modulus of elasticity and the second type of spacers may have a second modulus of elasticity. The first modulus of elasticity is different from said second modulus of elasticity by at least <NUM> Gpa, or more preferably by at least <NUM> Gpa, such as at least <NUM> Gpa.

The mode shape of the main contributing mode, or the symmetric mode, of the winding <NUM> may thus be modified by providing spacers <NUM> of different modulus of elasticity. The modulus of elasticity may for instance be selected by selecting appropriate materials for the spacers <NUM>. The modulus of elasticity of selectable/applicable materials range between <NUM> Gpa - <NUM> Gpa, or higher.

Alternatively, the first spacer distribution may comprise spacers arranged at a first distance between each other in a direction around the coil axis and the second spacer distribution may comprise spacers arranged at a second distance between each other in a direction around the coil axis. The first distance is different from said second distance. By decreasing the distance between the spacers in, for instance, the first winding portion as compared to the second winding portion, the stiffness of the first winding portion may be increased as compared to the second winding portion. This would mean a greater number of spacers per unit length of the coil turns <NUM> in the first winding portion as compared to the second winding portion.

Optionally, the first type of spacers are structurally shaped to have a first stiffness as seen along the coil axis and the second type of spacers are shaped to have a second stiffness as seen along the coil axis, said first stiffness being different from said second stiffness. The spacers <NUM> may have structural shapes to provide an increased, or a reduced, stiffness as compared to conventional spacers. Consequently, the first type and the second type of spacers may be of the same material but may be provided with different shapes in order to provide at least the first and the second winding portions with different stiffnesses. As an example, hollow spacers <NUM> may provide a reduced stiffness as compared to solid spacers <NUM>.

<FIG> illustrates an exemplary configuration of a winding according to the present disclosure, wherein the first winding portion 116a is located at a different axial position as seen along the coil axis in relation to the second winding portion 116b. In addition a third winding portion 116c and a fourth winding portion 116d have also been provided at different axial positions along the coil axis. It should be noted that if the winding <NUM> comprises an inner and an outer winding, both windings, or only one of the inner and outer winding, may comprise winding portions located at different axial positions as seen along the coil axis in relation to each other. Also, a transformer <NUM> according to the present disclosure comprises at least one winding <NUM> according to the present disclosure. In other words, the transformer <NUM> may have one or more windings <NUM> provided with a plurality of winding portions <NUM>. In the example illustrated in <FIG>, all three windings <NUM> have an identical configuration of winding portions according to the present disclosure. A different transformer <NUM>, still according to the present disclosure, may have one winding <NUM> comprising a plurality of winding portions, whereas the other two windings are conventional windings.

By way of example, an optimization study used different types of spacers <NUM> to assign a different modulus of elasticity to different configurations of winding portions, i.e. different numbers of winding portions <NUM>, and different axial positions of the winding portions <NUM> in relation to each other, along the coil axis. <FIG> shows simulation results of the study for five different winding configurations, where the number, N, of winding portions <NUM> were varied from one winding portion to five winding portions along the coil axis. The curves show acoustic power radiated by a transformer arrangement <NUM> having a transformer tank <NUM> comprising a transformer <NUM>, which in turn comprises three identical windings <NUM> according to the present disclosure. It can be seen that, in the illustrated example, N = <NUM> yields the lowest acoustic radiation of <NUM> dB from the transformer tank <NUM> at the main frequency of <NUM>. In comparison, at N = <NUM>, i.e. in which the stiffness or mass of the winding(s) is evenly distributed along the coil axis, similar to a conventional winding, the acoustic power is <NUM> dB at the main frequency of <NUM>.

<FIG> shows another exemplary configuration of windings <NUM> according to the present disclosure. Herein, the first winding portion 116a is located in a different sector of the winding <NUM> than the second winding portion 116b. As an example, the inner winding comprises the first winding portion 116a and the outer winding comprises the second winding portion 116b. All the three windings <NUM> of the illustrated transformer <NUM> are illustrated as identical in this example, but as described hereinabove, the windings <NUM> may have different configurations of winding portions <NUM>, in relation to each other.

An arc length of a winding portion sector is determined by a centre angle α at the coil axis, between two radii r extending between the coil axis and the coil turns of the winding portion. The first winding portion 116a may have a different arc length as compared to the second winding portion 116b. Arranging the first winding portion 116a and the second winding portion 116b in different sectors of the winding <NUM> is another way of breaking the structural symmetry of the winding <NUM>. In the illustrated examples the first winding portion 116a is defined by the central angle α<NUM> and the radii r<NUM>. The second winding portion 116b is defined by the central angle α<NUM> and the radii r<NUM>. The winding portions <NUM> may also have an axial length along the coil axis. In the example of <FIG>, the axial lengths of the winding portions are equal to the length of the winding (not shown).

In another exemplary optimization study, shown in <FIG>, winding portions <NUM>, located in different sectors of the winding <NUM>, were each assigned with spacers <NUM> having a certain modulus of elasticity. Simulation results of the study for three different winding configurations, where the number, N, of winding portions <NUM> were studied at one, two or four winding portions <NUM>. The curves show acoustic power radiated by a transformer arrangement <NUM> having a transformer tank <NUM> comprising a transformer <NUM>, which in turn comprises three identical windings <NUM> according to the present disclosure. It can be seen that, in the illustrated example, N = <NUM> yields the lowest acoustic radiation of <NUM> dB from the transformer tank <NUM> at the main frequency of <NUM>. In comparison, at N = <NUM>, i.e. in which the stiffness or mass of the winding(s) is evenly distributed along the coil axis, similar to a conventional winding, the acoustic power is <NUM> dB at the main frequency of <NUM>.

It follows from the above examples, that different winding portions <NUM> may be located in different axial sections along the coil axis and at the same time be located in different sectors. Worded differently, the examples of <FIG> and <FIG> may be combined, for instance such that the first winding portion 116a and the second winding portion 116b of <FIG> have limited extensions along the coil axis and are located at different axial positions as seen along the coil axis.

Claim 1:
A winding (<NUM>) for a phase winding of a transformer (<NUM>), said winding (<NUM>) having coil turns (<NUM>) around a coil axis (z), said winding (<NUM>) being adapted to transform voltage in a transformer (<NUM>) at a predetermined frequency, when said transformer (<NUM>) is operating, said winding (<NUM>) is excited by a mechanical load having a main frequency corresponding to said predetermined frequency multiplied by two and having vibration modes, wherein the combination of load and vibration modes results in a vibration of said winding (<NUM>), said winding (<NUM>) having a set of vibration modes, each vibration mode having a vibration mode frequency, wherein at least one main contributing vibration mode of the set of vibration modes is the vibration mode resulting in the largest acoustic power, of said vibration modes, when the winding (<NUM>) is excited by said load,
wherein the winding (<NUM>) comprises a plurality of winding portions (<NUM>), said plurality of winding portions (<NUM>) comprising at least a first winding portion (116a) and a second winding portion (116b), wherein said first winding portion (116a) has a first winding portion stiffness and said second winding portion (116b) has a second winding portion stiffness,
wherein a stiffness difference between said first winding portion stiffness and said second winding portion stiffness is such that the acoustic power is minimized at said main frequency, and
characterized in that the first winding portion (116a) is located in a different sector of the winding (<NUM>) than the second winding portion (116b) so as to break a structural symmetry of the winding (<NUM>), and wherein the first winding portion (116a) is located in a sector of the winding (<NUM>) delimited by a first centre angle (α<NUM>) and wherein the second winding portion (116b) is delimited by a second centre angle (α<NUM>), wherein a respective sector is delimited by a circumferential arc length around the coil axis (z), which arc length is determined by the respective first and second centre angles (α<NUM>, α<NUM>) between two radii extending between the coil axis (z) and the coil turns (<NUM>) of the respective first and second winding portions (116a, 116b).