Power transformer and method for current balancing

Abstract

The present invention relates to a power transformer and to a method for current balancing in a power transformer. The power transformer comprises a magnetic core, a primary winding, and a secondary winding comprising plurality of winding sections that are electrically connected in parallel, wherein each winding section includes a coil. The power transformer further comprises at least one inductive element, each being coupled to a respective coil to compensate an inductance of a corresponding at least one winding section for current distribution balancing amongst the winding sections during operation.

Description

[0001] POWER TRANSFORMER AND METHOD FOR CURRENT BALANCING

[0002] FIELD

[0003] The present invention generally relates to a power transformer. The present invention further relates to a method for current balancing in such a power transformer. The present invention may by applicable to, for example, solid state transformers, arc furnace transformers, or green hydrogen production, where the transformers may deal with medium frequency, medium voltages and high currents.

[0004] BACKGROUND

[0005] Solid-state transformers (SSTs) have garnered significant attention recently due to their potential for high-power applications, such as green hydrogen production. These advanced transformers offer several advantages over traditional transformers, including enhanced efficiency, improved power quality, and greater flexibility in grid integration. As SSTs are increasingly employed in high-power applications, they are commonly connected to medium-voltage grids and tasked with supplying relatively low-voltage loads. This operational scenario necessitates handling high currents on the secondary side, presenting technical challenges that must be addressed to ensure reliable and efficient performance.

[0006] Handling high currents in the secondary side of SSTs requires innovative solutions, mainly when operating at medium voltages and frequencies. Techniques that have matured in other high-current applications for dealing with the high current, especially in the secondary side of a step¬ down transformer, include continuously transposed conductors (CTCs) and implementing parallel paths in the secondary winding to distribute current evenly and reduce losses.

[0007] However, SSTs face additional complexities due to the proximity and skin effects, which exacerbate high current issues. While Litz wires, for example, are a viable solution for low-voltage power electronics applications to mitigate these effects, they are impractical for medium-voltage, medium-frequency SSTs. Secondary parallel paths for the current in the secondary may also improve current sharing. These parallel paths can be along the conductor like Litz wires and CTCs with several strands and transposition to avoid extra losses. Alternatively, the parallel paths can be within the same disk in the winding. Still, these techniques may not be sufficient, especially in high current power transformer applications, and / or do not provide optimal current distribution on the secondary side.Having poor current sharing among parallel secondary windings results in extra loss and rising temperatures, especially at hotspots in the windings, which is detrimental to performance. This phenomenon is more severe in medium or high-frequency transformers for two main reasons. Firstly, the higher the frequency, the higher the proximity and skin effect. Secondly, the unbalancing occurs because the difference in the inductances of parallel disks is more significant when the frequency is much higher than 50 Hz. Component tolerances may also contribute Currently available solutions for current balancing in transformers at 50 Hz may not be feasible for medium or high frequency transformers. There is thus a need for current balancing techniques that are additionally or alternatively suitable for medium- to high frequency transformers.

[0008] SUMMARY

[0009] It is an object of the present invention to improve existing power transformers, for example high-power SSTs or arc furnace transformers, and in particular, but not limited to, medium- to high-frequency transformers. Also, it is an objective to improve applications using parallel winding paths and address the severity of the high-current problems in medium-frequency transformers and propose optimized solutions for their design and operation. A further goal of this invention is to facilitate the development of more efficient and reliable SSTs for high-power applications, contributing to the broader goal of advancing sustainable energy technologies.

[0010] The present invention aims at improving at least part of the above-identified negative effects.

[0011] This is achieved by a power transformer that comprises a magnetic core, a primary winding, and a secondary winding comprising a plurality of winding sections that are electrically connected in parallel, wherein each winding section includes a coil. In accordance with the present invention, the power transformer further comprises at least one inductive element, each being coupled to a respective coil of a parallel winding section, to compensate an inductance of a corresponding at least one winding section for current distribution balancing amongst the winding sections during operation.

[0012] Preferably, but not necessarily, the present transformer is a high current transformer. Namely, the present invention is especially advantageous when the secondary windings carry high current in operation. The secondary winding and primary winding are arbitrary, and it may also be that the secondary winding as defined in the present disclosure is the primary winding and the primary winding is actually the secondary winding. Furthermore, the coupling of the inductive element may for example be electric / metallic / conductive coupling or magnetic coupling. The term ‘connecting’ usually refers to electric / metallic / conductive coupling in this document. The inductiveelement may be any element that affects the inductance of the section in operation. A parallel winding section may comprise all elements, circuits, wires, etc. that are connected in parallel, and may be referred to as “parallel winding section”, “winding section”, “secondary winding section”, “parallel section”, or simply “section”.

[0013] Advantageously, the at least one inductive element affects the inductance of the associated winding section(s) with respect to the inductance of other winding sections without the inductive element, at least in operation. By adding an inductive element to a winding section amongst the plurality of parallel winding sections of the transformer, the inductance of the winding section can be varied or adjusted. In that way, inductance may be balanced amongst the parallel sections and thus current distribution can be improved.

[0014] The applicant has found that compensating inductances of certain individual winding sections can eliminate or at least mitigate the effects of the proximity effect on the current balance amongst the winding sections. For example, depending on the structure of the transformer and the arrangement of the winding sections, certain coils thereof may experience a higher or lower current than others due to their position around the core and / or mutual position and / or position with respect to the primary winding, resulting, for example, in different coupling coefficients. The at least one inductive element can be provided where needed to balance the current between the parallel winding sections, thereby reducing losses due to, for example, hot-spots in the transformer. Current balancing may for example be considered achieved when the current through each coil of the plurality of winding sections in the secondary winding is within a range of 0-15% from one another, preferably about 10%, or preferably within 0-10% from one another.

[0015] A coil may also be an inductive element, and each winding section of the power transformer by default comprises at least one coil with turns around the core of the transformer. However, in this document, the term ‘inductive element’ does not refer to that coil comprised in every respective winding section by default. The inductive element is, so to say, an extra inductive element, providing an extra compensation inductance next to that of the default coil, to tune the inductance of the respective winding section.

[0016] The inductive element may be anything that, when coupled to a circuit / a section, changes the inductance of the circuit / that section. For example, it may be an extra coil, a magnetic or magnetizable object of any shape, etc.

[0017] The term tuning may herein refer to configuring or setting or providing a certain compensation inductance to a circuit, specifically a winding section amongst the plurality of parallel winding sections. Tuning may happen only once, for example during assembly or integration of the transformer, though the present invention is not limited thereto. Tuning may even happen during the design of the power transformer. Tuning may also happen regularly and / or in a feedback loop and / or during maintenance for example. Tuning may be done every time parts of thetransformer are changed, replaced, removed or added. The result of tuning is that a suitable compensation inductance is provided for the respective winding section, by coupling the section to an inductive element. Preferably, the compensation inductances provided for respective secondary winding section(s) compensate for the difference in the inductance value that the winding sections would have in operation without the inductive elements. The term tuning thus refers to adjusting or configuring the at least inductive element to provide a compensation inductance to a respective winding section, compared to a situation without the inductive element, in operation.

[0018] In a preferred embodiment, the at least one inductive element may be configured to provide a compensation inductance to a respective winding section. Thus, the inductive element is coupled to the winding section and configured such that it provides a compensation inductance with respect to the inductance of the corresponding coil in that winding section, in operation.

[0019] Specifically, in a possible embodiment, the at least one inductive element may add a series inductance to the respective secondary winding section with respect to the coil of said respective secondary winding section, in operation.

[0020] In the ideal situation, all parallel coils have the same inductance and carry the same amount of current. This minimizes losses, and thus heat, in the system. By providing a compensation inductance to one or more winding sections (in operation), the inductance of the plurality winding sections during operation may be more levelled. For example, a compensation induction may be added to the secondary winding sections that have a relatively lower inductance during operation compared to other secondary winding sections amongst the plurality of secondary winding sections (for example due to proximity effects and skin effects). Thus, the inductance can be levelled and therefore the current may be balanced. Or, for example, a relatively higher compensation induction may be added to the secondary winding sections that have a relatively lower inductance during operation compared to other secondary winding sections amongst the plurality of secondary winding sections, and a relatively lower compensation inductance may be added to the secondary winding sections that have a relatively higher inductance during operation. Thus, the inductance can be levelled and therefore the current may be balanced.

[0021] In a preferred embodiment, thus, a plurality of inductive elements may be coupled to a respective plurality of parallel winding sections, to provide compensation inductance to those winding sections. In other words, a first inductive element among the at least one inductive element may be coupled to a first coil of a first winding section, and a second inductive element among the at least one inductive element may be coupled to a second coil of a second winding section.

[0022] Advantageously, by connecting not only one inductive element, to one coil / one winding section, but multiple inductive elements to multiple coils / winding sections respectively, theinductance of a plurality of winding sections can be compensated to thereby further improve the current balancing in the power transformer. Accordingly, losses can be further reduced.

[0023] In most preferred embodiments of this invention, the at least one inductive element may comprise a magnetic or magnetizable toroid surrounding part of a corresponding winding section to provide a compensation inductance with respect to a corresponding coil in that section.

[0024] Hereinafter, the magnetic or magnetizable toroid may also referred to as “magnetic toroid” or simply “toroid”. The terms magnetic and magnetizable may both refer to an object comprising any material that is magnetizable, for example having ferromagnetic properties, or any material that is permanently magnetic. Usually, this document refers to embodiments wherein the toroid comprises a material with ferromagnetic properties, which may also be referred to as a “magnetic toroid” or simply “toroid”.

[0025] A toroid can be any surface of revolution with a hole in the middle, wherein the axis of revolution passes through the hole and so does not intersect the surface. For example, the cross¬ section of the toroid may have a rectangular shape or a circle shape, in that case the toroid would be a torus. The term toroid may also be used to describe any toroidal polyhedron, preferably having a topological genus of 1. In the embodiments discussed in detail in this document, as shown by the figures, a toroid with a rectangular cross-section, more preferably a square cross-section is used.

[0026] Advantageously, magnetic toroids may in practice be readily available, easily manufacturable and cheap. Furthermore, a magnetic toroid is effective for adding compensation inductance to a circuit, for example a parallel section of the secondary winding. A toroid may thus provide a suitable inductive element for the application of this invention.

[0027] In some embodiments, the plurality of secondary winding sections may be electrically connected in parallel via connecting portions, such as busbars, each coil being electrically connected to the connecting portions via individual connection segments, such as connection wires. Busbars may be metal or at least conductive connection elements to which terminals of a plurality of secondary winding sections can be attached, for example by means of soldering, welding, or the like.

[0028] In an example, a coil in a respective winding section may comprise two terminals, and each terminal may be connected to a busbar via a conductive wire. An inductive element may be comprised in the respective winding section by coupling the inductive element to the coil and / or to one or both of the connection segments in the coil.

[0029] For example, in a preferred embodiment, each toroid of the at least one inducti ve element may surround part of a connection segment of a corresponding winding section.

[0030] Preferably, the inductive elements may comprise a first magnetic toroid coupled to a first winding section from the plurality of winding sections by surrounding part of a connection segment of the first winding section, and a second magnetic toroid coupled to a second windingsection from the plurality of winding sections by surrounding part of a connection segment of the second winding section.

[0031] By arranging the toroid such that it surrounds part of the connection segment or the coil, for example part of the conductive wire in the winding section, the toroid can provide a passive compensation inductance during operation. Advantageously, a toroid may be coupled relatively easily to the coil of the corresponding winding section and may also be configurable or tunable after coupling. The toroid can be added to a winding section of existing power transformers without the need for extra connection / coupling materials or other infrastructure in the transformer, or more generally without requiring significant structural adaptations of existing power transformers, which also is a significant advantage.

[0032] When the toroid is arranged such that it surrounds part of a connection segment, it can add a series compensation inductance to the respective winding section with respect to the corresponding coil of that winding section. As such, the inductance of an arbitrary winding section may be increased advantageously, to provide a better balance of inductance and corresponding distribution of current during operation.

[0033] Furthermore, a magnetic / magnetizable toroid connected to the winding section as such may provide a very durable configuration and reliable compensation inductance. Especially when the toroid is symmetric with respect to its central axis.

[0034] In a preferred embodiment of the present invention, the at least one inductive element, preferably the toroid, may be coupled to part of the respective coil. For example, the toroid may surround part of the corresponding coil of the respective winding section, for example the first turn of the respective coil.

[0035] Exemplary embodiments of the transformer may comprise a plurality of said magnetic or magnetizable toroids, each surrounding part of a respective parallel secondary winding section to add a series inductance to tire respective secondary winding section with respect to the coil of the respective winding section.

[0036] Advantageously, the inductance of secondary winding sections with relatively higher current during operation can be compensated and the performance of the transformer optimized by improving the current distribution.

[0037] In an embodiment, a compensation inductance provided by the at least one inductor may be fine-tuned according to the respective winding section for current distribution balancing amongst the plurality of secondary winding sections, by tuning an inductance value of the inductive element.

[0038] Namely, if the compensation inductance is tuned / chosen according to the inductance of the winding section during operation, the current distribution can be further optimized. The value ofthe compensation inductance affects / tunes the current sharing; therefore, it is advantageous to fine¬ tune this value before or during installation.

[0039] In one example, the power transformer may be provided with a plurality of said inductive elements, which inductive elements may be configured to provide mutually different compensation inductances, preferably by having mutually different inductance values, in operation.

[0040] Preferably, each coil may be connected to another inductive element and each inductive element has another inductance value which is fine-tuned according to the inductance of the corresponding winding section in operation. As a result, the inductive elements provide different compensation inductance values, and thus the inductance of the plurality of secondary winding sections may be relatively fine-tuned and levelled.

[0041] The compensation inductance may be fine-tuned in multiple ways. For fine-tuning, there are multiple possible configurations of inductive elements proposed in this document. For example, the inductance of the inductive element may be tuned. For example, other material, other shape, other size, different coating, magnetic field, etc. Alternatively, the configuration of the inductive element in the circuit may be altered / chosen such that the compensation inductance it provides is altered.

[0042] For example, the at least one inductive element may have an adjustable inductance. In this way, advantageously, the compensation inductance provided by the inductive element can also be adjusted after having been coupled to the coil of the corresponding secondary winding section.

[0043] The plurality of secondary winding sections may be arranged in parallel. In other words, they may not only be electrically coupled in parallel but they may also be spatially arranged in parallel, for example surrounding a same portion of the core, such as a portion opposite another portion around which the primary winding is provided.

[0044] In an embodiment of the power transformer in which the core of the power transformer is an airgap-less core, the at least one inductive element may be configured to tune the inductance of corresponding secondary winding sections such that a relatively higher compensation inductance is provided in outer secondary winding sections with respect to a centrally arranged secondary winding section. For example, the at least one inductive element may be provided at least for outer secondary winding sections, or may be provided such that at least for outer secondary winding sections it provides a relatively higher compensation inductance. In this embodiment, one or more central secondary winding sections may for example not have an associated inductive element for inductance compensation, as this may not be needed to achieve sufficient current distribution balancing, or one or more central secondary winding sections may have an associated inductive element for relatively lower compensation inductance, as the default inductance of these windings in operation is relatively higher.Alternatively, for a power transformer wherein the core is a cut core including an airgap, the at least one inductive element may be configured to tune the inductance of corresponding secondary winding sections such that a relatively higher compensation inductance is provided in a centrally arranged secondary winding section and / or a secondary winding section relatively close to the air-gap with respect to outer secondary winding sections / secondary winding sections further away from the air-gaps.

[0045] Secondary winding sections are usually arranged quite close to each other and adjacent to each other along the core. The outer winding sections are the winding sections relatively close to the edge or edges of the row of adjacent parallel winding sections. The inner winding sections are arranged further from these edges, relatively closer to the middle of the row of adjacent parallel winding sections. In other words, the outer winding sections have less other winding sections in their proximity, and the inner winding sections have more other parallel winding sections in their proximity. Because of the difference in proximity to other winding sections, the proximity effects and skin effects differ for the respective winding sections. As a result, the inductance of the winding sections thus mutually differs during operation, depending on the position of the respective winding section in the row of winding sections.

[0046] Accordingly, tire inductance of an inner (i.e., more central) winding section when the core comprises no airgaps may be higher than the inductance of outer (i.e., more outwardly arranged) winding sections in operation. Thus, a relatively higher compensation inductance may be provided for the outer winding sections to level the inductances of the winding sections and improve the current distribution and balancing.

[0047] In the same way, a winding section close to an air-gap in the core may have a lower inductance. Namely, the impact of the air gap on the inductance of coils consists of two facts. The first impact is that having air gaps in the core decreases the overall inductance of the winding section. The equivalent magnetic circuit, considering the air gap, will be changed. The effective reluctance of the in the coil increases and, since the inductance is inversely proportional to the reluctance, the inductance of the coil closer to the airgap will be relatively low.. This results in a relatively higher current compared to other winding sections. The second impact of having an air gap in the core is the fringing effect. The fringing effect changes the field in the areas near the gap. The impact of the fringing effect is on tire flux inside the core window / airgap. The coils nearer to the gap will be more affected by the fringing effect. The fringing flux has an element in the opposite direction to the magnetic flux around the wire of a coil / winding section and, as a consequence, weakens it. Thus, in a winding consisting of several parallel-connected coils / winding sections around a gapped magnetic core, the coil / winding section nearer to the air gap has lesser inductance and, consequently, higher current. Thus, when the core comprises a gap, a relatively higher compensation inductance is provided for the inner winding sections / winding sections closerto the airgap in the core with respect to the winding sections further from the airgap, to level the inductances of the winding sections and improve the current distribution and balancing.

[0048] In another example, a plurality of inductive elements comprising a plurality of toroids may be provided, such that the toroids surround part of a respective winding section, and the plurality of toroids have a mutually different inductances to provide mutually different compensation inductances to the respective winding sections.

[0049] As such, advantageously, this embodiment allows relative fine tuning of the inductance of the parallel secondary winding sections such that the difference in inductance between the winding sections can be optimized, i.e. minimized, i.e. levelled, and the current balancing optimized.

[0050] In an example embodiment, the respective toroids may comprise mutually different materials or material compositions for providing mutually different compensation inductances.

[0051] The composition of materials in the magnetic toroid may determine, at least in part, the compensation inductance it provides to the respective winding section in operation. Thus, this embodiment facilitates a plurality of different compensation inductances which may be tuned / employed according to the winding section and its inductance in operation.

[0052] In some embodiments, the respective toroids may comprise mutually different air-gaps for providing mutually different compensation inductances. A difference in the airgap can be for example the width of the airgap, the penetration depth of the airgap, the shape and location of the airgap. The airgap determines the inductance value of the toroid in operation. Thus, this embodiment facilitates a plurality of different compensation inductances which may be tuned / employed according to the winding section and its inductance in operation.

[0053] In some embodiments, one or more permanent magnets may be provided at or near a corresponding inductive element (e.g., toroid), to tune the compensation inductance of said inductive element. Preferably, a plurality of permanent magnets is provided at or near a plurality of corresponding toroids, to provide mutually different compensation inductances in operation.

[0054] For example, the position of the permanent magnet on the inductive element, preferably toroid, may be tuned for providing different compensation inductances. Alternatively, or additionally, the size, strength, shape, orientation, or any parameter that affects the inductance of the respective toroid in operation, of the magnet may be tuned. Preferably, the permanent magnets may be arranged at the plurality of toroids, respectively, at different radial positions with respect to the central axis of the toroid. For example, in a preferred embodiment, a permanent magnet may be arranged on a corresponding toroid at a radial position such that the toroid with permanent magnet provides the desired compensation inductance to the corresponding winding section.

[0055] The permanent magnet provides the advantageous effect that exact same toroids may be used for every winding section amongst the plurality of parallel winding sections, and the compensation inductance of the toroid according to the corresponding winding section may betuned by the corresponding permanent magnet. A further advantage is that the same permanent magnets may be used to provide different compensation inductances by providing the same magnet at a different radial position on the corresponding toroid. Thus, this embodiment provides very accurate fine-tuning of the compensation inductance provided by the inductive element, comprising a magnetic toroid and a permanent magnet, by using only one type of toroid and one type of permanent magnet. Thus, this is a very cost-efficient manner of current balancing in a high power transformer.

[0056] In an alternative preferred embodiment, an auxiliary winding may be provided around a corresponding toroid. For example, the auxiliary winding may be wound around part of the toroid. The auxiliary winding may affect the inductance of the toroid during operation, and thus also the compensation inductance it provides.

[0057] Namely, the compensation toroid here can be said to act like a transformer. The primary of this transformer consists of one turn, which is the conductor of one of the parallel discs of the power transformer. The secondary of this transformer consists of one or more turns, here we name them the auxiliary winding. The magnetizing inductance of this transformer from primary point of view is the compensation inductance, which may be tuned. If the secondary of this transformer is open circuit, the magnetizing inductance (^compensation inductance) is maximum. If the secondary of this transformer is short circuit, the magnetizing inductance is minimum. Fine tuning is achievable here by loading on the secondary side of this transformer to adjust the magnetizing inductance (i.e., compensation inductance) between minimum and maximum values as follows: Lsc < Lcomp < Loc, where Lsc is the magnetizing inductance from primary point of view when the secondary winding, i.e. auxiliary winding, is short circuited, Loc is the magnetizing inductance from primary point of view when the secondary (auxiliary) winding is open circuited. Lcomp is the value of compensation inductance.

[0058] The auxiliary winding may comprise another electronic tuning component for providing a variable or tunable inductance. For example, the auxiliary winding may comprise a potentiometer for adjusting the inductance of the corresponding toroid. Advantageously, in this embodiment, the compensation inductance provided by the toroid with its auxiliary winding in operation can be easily tuned by adjusting the resistance of the potentiometer. In another example, the auxiliary winding comprises a positive temperature coefficient (PTC) thermistor for providing a variable inductance of the corresponding toroid.

[0059] In preferred embodiments, the power transformer is a high current transformer, for example, a solid state transformer, arc furnace transformer, or green hydrogen production transformer, and / or the power transformer is configured to deal with medium frequency, and / or relatively low or medium voltages and / or high current loads.Although the present invention is not limited to any of the following, an operational frequency of the power transformer may for example be 400 Hz to 100 kHz, for example 400 Hz, 1000 Hz, 8 kHz 10 kHz, 25 kHz 40 kHz, 50 kHz,, 85 kHz, and 100 kHz. Suitable voltages for the primary winding may be in the range of 1 kV to 72 kV, for example 1 kV to 10 kV, 10 kV to 30 kV, 30 kV to 72 kV. Suitable currents for the primary winding may be in the range of 10 A to 2 kA, for example 10 A to 100 A, 100 A to 1000A, 1 kA to 2 kA. Suitable voltages for the secondary windings may be in the range of 50 V to 2 kV, for example 50 V to 100 V, 100 V to 1000 V, 1 kV to 2 kV. Suitable currents in the secondary winding may be in the range of 100 A to 100 kA, for example 100 A to 1000 A, 1 kA to 10 kA, 10kA to 100kA.

[0060] In some embodiments, the parallel secondary winding sections or coils may be arranged in corresponding (parallel) discs. The discs may be mutually electrically insulated at the coil portion and coupled at end terminals thereof. However, at the terminations, the parallel winding sections may be connected to each other (e.g., via busbars).

[0061] In some embodiments according to the present disclosure, the transformer may be provided with a shell-type construction. In other embodiments, the transformer may be provided with a core type construction. In some embodiments, the transformer may be a single-phase transformer. In other embodiments, the transformer may be a three-phase transformer, or even comprise another number of phases. Hence, although reference is made to a primary and secondary winding, the primary and secondary winding may be part of a three-phase transformer or other types of transformers.

[0062] In some embodiments, the core is airgap-less (i.e., does not comprise airgaps). In other embodiments, the core comprises an airgap.

[0063] The current technique may furthermore be combined with additional current balancing techniques, such as continually transposed conductors, varying diameters of disks, varying height of winding sections, and the like.

[0064] The present invention further relates to a method for current balancing in a power transformer comprising a magnetic core, a primary winding, and a secondary winding comprising a plurality of winding sections, that are electrically connected in parallel, wherein each winding section includes a coil. The method comprises coupling at least one inductive element to a respective at least one coil to adjust an inductance of a corresponding at least one winding section for current distribution balancing amongst the winding sections during operation.

[0065] In an embodiment, a compensation inductance may be provided by the at least one inductive element to the respective winding section.

[0066] In an embodiment, a first compensation inductance may be provided to a first winding section by coupling the first winding section to a first inductive element, and a second compensation inductance may be provided to a second winding section by coupling the secondwinding section to a second inductive element, wherein the inductive element is preferably provided as a magnetic or a magnetizable toroid.

[0067] In an embodiment, the at least one inductive element may comprise a magnetic or magnetizable toroid and a compensation inductance may be provided to the respective winding section by surrounding part of the winding section with the corresponding toroid.

[0068] The method may comprise: determining a current through each of the plurality of secondary winding sections during operation; determining an inductance compensation value of each of the at least one inductive element required for current balancing; and coupling the at least one inductive element with corresponding inductance compensation values to corresponding secondary winding sections.

[0069] In an embodiment, an inductance of the at least one inductive element is fine-tuned for the respective winding section, by tuning an inductance value of the inductive element.

[0070] The method may comprise: coupling the at least one inductive element to the corresponding at least one secondary winding section; determining a current through each of the plurality of secondary winding sections during operation; determining an inductance compensation value of each of the at least one inductive element required for current balancing; and adjusting the at least one inductive element to provide the required inductance compensation value.

[0071] The core of the transformer may be an airgap-less core, and the method may comprise providing a relatively higher compensation inductance for the coil(s) of one or more outer secondary winding sections with respect to the coil(s) of one or more centrally arranged secondary winding sections. Alternatively, the core of the transformer may be a cut core (i.e., including an airgap), and the method may comprise providing a relatively higher compensation inductance for the coil(s) of one or more centrally arranged secondary winding sections with respect to the coil(s) of one or more outer secondary winding sections.

[0072] In an embodiment, different compensation inductances are provided to respective secondary winding sections by providing a plurality of inductive elements having mutually different inductances.

[0073] The plurality of toroids may for example comprise mutually different materials for providing different compensation inductances to the respective winding sections.

[0074] In some embodiments, a permanent magnet may be provided at or near a corresponding inductive element, preferably magnetic toroid, to fine-tune the corresponding compensation inductance Furthermore, the position, such as a radial position, of the permanent magnet on the toroid may be tuned to provide the desired compensation inductance.

[0075] In some embodiments, an auxiliary winding may be provided around the toroid for fine- tuning the inductance compensation for the respective winding section. Optionally, the compensation inductance of the respective toroid is adjusted or variable by means of said auxiliarywinding. For example, a potentiometer may be coupled to the auxiliary winding for adjusting an inductance value of the corresponding toroid. Alternatively, a resistor with a positive temperature coefficient, ‘PTC’, may be coupled to the auxiliary winding for adjusting an inductance value of the corresponding toroid.

[0076] In some embodiments, the tuning of the compensation inductance provided by the respective inductive element is based on the measured current through the winding sections in operation. For example, the method of tuning the compensation inductances may be an iterative process before or upon coupling the at least one inductive element to the transformer, by repeatedly measuring currents through individual coils of individual winding sections and adjusting the inductance compensation using the at least one inductive element until current distribution balancing is achieved in a sufficient manner.

[0077] BRIEF DESCRIPTION OF THE DRAWINGS

[0078] Next, the present invention will be described with reference to the appended drawings, wherein:

[0079] FIG. 1 is a schematic drawing of a transformer with a core type construction;

[0080] FIG. 2 is a schematic drawing of a transformer with a shell type construction;

[0081] FIG. 3 is a schematic drawing of a plurality of parallel winding sections of a conventional power transformer;

[0082] FIG. 4- 7 are schematic drawings of a plurality of parallel winding sections of a power transformer in accordance with various embodiments of the present invention;

[0083] FIG. 8A - 8C illustrate an inductive element embodied as a toroid, in accordance with an embodiment of the present invention;

[0084] FIG. 9 and 10 illustrate an inductive element embodied as a toroid, in accordance with another embodiment of the present invention;

[0085] FIG. 11 and 12 are schematic drawings of a plurality of winding sections of a power transformer, in accordance with embodiments of the present invention.

[0086] DETAILED DESCRIPTION

[0087] Hereinafter, reference will be made to the appended drawings. It should be noted that identical reference signs may be used to refer to identical or similar components. Moreover, unless explicitly stated otherwise, various elements shown in the appended drawings may not be drawn to scale, and parts may be exaggerated in size or omitted for convenience of explanation.FIG. 1 and 2 are schematic drawings of a transformer 1 with a core-type construction 11 and transformer 1 with a shell-type construction 12, respectively. This invention applies to both types of power transformers. The embodiments shown in the other figures may relate to both shell type and core type transformers. The power transformer according to the present invention comprises a primary winding and a secondary winding comprising a plurality of winding sections.

[0088] In FIG. 1, in the core type construction 11, respective coils of a primary winding 10 and of a secondary winding 20 are wound around two different sides of a magnetic core 5 of the transformer. For convenience of illustration, only one secondary winding 20 (including one coil) is drawn, but a plurality of parallel-connected and parallel-arranged secondary winding sections may be included (see figures 3-7,11 and 12). As shown in FIG. 1, for example, the primary winding 10 may be provided around the right side of the core 5, and the secondary winding 20 around the left side of the core, or the other way around.

[0089] In FIG. 2, which illustrates the shell type construction 12, the primary winding 10 and secondary winding 20 are adjacently wound around the same side of the magnetic core 5. For example, the primary winding 10 may be provided above the secondary winding 20 as viewed from the perspective of figure 2, or the other way around. Although not shown in FIG. 2, it is also envisaged that part of secondary winding 20 may be provided above primary winding 10 and another part of secondary winding 20 may be provided below primary winding 10.

[0090] With reference to FIG. 3-6, the present invention relates to transformers with a plurality of secondary winding sections 201- 205that are electrically connected in parallel, wherein each secondary winding section 20 includes a coil 17. Although the embodiments included herein show five secondary winding sections, the present invention equally envisages a lower or higher number of secondary winding sections in the transformer. Furthermore, although in the figures different symbols may be used for representing the coils, all symbols may represent similar or equivalent coils comprising turns around the magnetic core.

[0091] Secondary winding sections may be referred to using reference number 20xor 20(x), wherein x indicates the respective section (figures 3-7 and 11, 12), but a winding section or multiple winding sections may also be referred to in the text simply using reference number 20.

[0092] Hereinafter, the coil 17 comprises one or more turns around the magnetic core 5 of the transformer 1. The secondary winding section 20 comprises the coil and connecting segments 18. In other words, the term ‘(secondary) winding section’ or ‘section’ refers to the collection of all coupled elements in the respective parallel part of the circuit. For example, in FIG. 3, winding section 201comprises coil 171, i.e., modeled with an inductive part 1711and a resistive part 1721, and connection segments 18. In the embodiment of FIG. 4, winding section 201comprises inductive element 301, coil 171and connection segments 18.FIG. 3 is a schematic drawing of parallel secondary winding sections in a transformer without inductive elements that are configured to compensate the inductance of the respective secondary winding sections. The secondary winding sections are connected to each other via connection portions 15 and 16. The coils 17 are modeled schematically by an inductive part 171 and a resistive part 172 in series. The resistive part 172 represents the resistance of the winding section 20, and thus part of the resistance of the coil. For simplicity, reference is made to the coil with reference number 17 or 17xwherein x indicates the corresponding parallel secondary winding section 20x. Connection portions 15 and 16 may for example be busbars or other connection portions, to which terminals of secondary winding sections 20 are coupled, for example by means of welding or soldering. The connection segments 18 connect or couple the elements comprised in a respective winding section such as the coils 17 and the inductive elements 30 to each other and to the connection portions 15 and 16.

[0093] Due to the skin effects and proximity effects, even when the parallel winding sections are identical to each other (i.e., no tolerances, exact same component values, and so forth), the current they carry during operation may be mutually different. For example, a spatial arrangement of secondary winding sections 20 relative to one another, relative to magnetic core 5, and / or relative to primary winding 10 may result in different currents through different winding sections. As a result, hotspots may for example occur in winding sections which carry more current than others, leading to higher losses in the power transformer and potentially even damage during operation.

[0094] FIG. 4 is a schematic drawing of a plurality of parallel secondary winding sections 201, 202, 203, 204, 205wherein each secondary winding section includes a coil 17 (171-175) and wherein each secondary winding section comprises an inductive element 30 (301-305), each being coupled to a respective coil to compensate an inductance of a corresponding secondary winding section 20, preferably provide a compensation inductance to the respective secondary winding section, for current distribution balancing amongst the secondary winding sections 20 during operation.

[0095] In the embodiment in FIG. 4, a plurality of inductive elements 301-305is connected in series to the respective coil in the corresponding winding section. In other embodiments, the inductive element may be coupled to the coil or the entire winding section in a different way. For example, in some embodiments as shown in FIG. 5 - 7, the inductive element, which in those examples is embodied as a toroid 31, are magnetically coupled to the coils 17 in the winding sections 20. Furthermore, in other embodiments, only some of the secondary winding sections 20 are provided with an inductive element 30, for example as shown in FIG. 6 or 7.

[0096] The inductive element 30 affects the total inductance of the corresponding winding section 20. In this way, the problem of unequal inductances and poor current distribution can be improved. In preferred embodiments, as shown in FIG. 5 and 6, the inductive element is configured to provide a compensation inductance to the corresponding winding section 20. The desiredinductance of each inductive element 30 may be determined ahead of time, for example by measuring currents through the secondary winding sections 20 during operation and selecting suitable inductance values for inductive elements 30. Alternatively, inductive elements 30 may be adjustable or tunable and may be adjusted or tuned while it is coupled to the coil of the corresponding secondary winding sections 20 during operation.

[0097] FIG. 5, 6 and 7 show preferred embodiments of the present invention, wherein toroids 31 are provided to part of or all of the plurality of parallel winding sections 20xin the secondary winding. The toroids 31 surround part of the corresponding secondary winding section 20. In the preferred embodiments shown in FIG. 5 and 6, every toroid 31 surrounds part of a respective connection segment 18 of the corresponding secondary winding section 20. The toroid may also be arranged to surround another part of the winding section, for example a turn of coil 17. The toroids are magnetic, i.e. magnetizable, for example ferromagnetic, or permanently magnetic, and will provide an inductive response when a current passes though. Thus, in this way, the toroid 311adds a series inductance to the respective secondary winding section 201with respect to the coil 171. In preferred embodiments, toroids 31 are magnetizable, for example comprising a ferromagnetic material.

[0098] In FIG. 6, only the inner winding sections 202, 203, 204are provided with corresponding inductive elements. Specifically, inner winding sections 202, 203, 204comprise toroids 312, 313, 314. The embodiment of FIG. 6 may be suitable for application in a transformer with an airgap in the middle of the magnetic core / close to the central winding section(s) 203(and 202, 204).

[0099] Typically, the airgap is located at or near a middle of the magnetic core 5, which, as discussed above, affects the proximity effect of secondary winding sections 20 near' the middle (i.e. the airgap) with respect to outer secondary winding sections 20 (i.e. relatively further from the airgap). In preferred embodiments wherein the core comprises an airgap, the airgap is located in the middle of the core, and in further preferred embodiments furthermore, the middle section of the parallel sections is located closest to the airgap in the core. In those embodiments, the inductance of central (inner) parallel section is lesser than the outer sections since the gap is (in this embodiment, but not necessarily) placed in de middle of the leg of the core, near to the central (inner) sections. The lesser the inductance, the higher the current. In this situation, inner section thus carry more current and adding a compensation inductance is thus advantageous. Thus, the configuration of FIG. 6 may be especially suitable for application in a power transformer with an airgap in the core, where it is typically observed that secondary winding sections arranged further from the center / the airgap may carry higher currents, for providing a compensation inductance to the outer winding sections.

[0100] In FIG. 7, only outer winding sections 201and 205are provided with corresponding inductive elements, specifically outer winding sections 20!and 205comprise toroids 311and 31'. The embodiment shown in FIG. 7 may be especially suitable for application in a power transformerwith a gap-less core (i.e. with no air-gap) where it is typically observed that secondary winding sections arranged further from the center may carry higher currents, for providing a compensation inductance to the outer winding sections.

[0101] Of course, it is also envisaged that combinations of secondary winding sections 20 with inductive element 30 and secondary winding sections 20 without inductive element other than those shown in FIG. 4 - 7 are employed. For example, depending on the required compensation for current balancing, only one or only some of the secondary winding sections 20 may be provided with an inductive element 30, and these do not necessarily need to be central or outer secondary winding sections.

[0102] In FIG. 4 - 7, the inductive elements 30 (e.g., toroids 31) are illustrated identically.

[0103] However, in practice, different compensation inductances may be required for different secondary winding sections, to provide optimal tuning of the inductance of the respective winding section in operation and thus better current distribution between the winding sections. Accordingly, the at least one inductive element may provide mutually different inductance compensation.

[0104] For example, toroids with mutually different airgaps and / or comprising different materials may be provided to respective winding sections, thereby providing mutually different compensation inductances to the corresponding winding sections.

[0105] Alternatively, a permanent magnet may be provided at or near the inductive element, for example toroid, to fine tune its added series inductance with respect to the coil in the corresponding winding section. The advantage of a toroid in combination with a permanent magnet is that the same toroids and the same permanent magnets may be used to provide a plurality of mutually different compensation inductances to the respective winding sections. Furthermore, magnetic toroids and permanent magnets are cheap and easy to attain.

[0106] Figures 8 A, 8B and 8C show a toroid 31 provided with a permanent magnet 35. Figure 8 A shows a schematic drawing in perspective of a magnetic toroid 31 with a permanent magnet 35. Toroid 31 may be symmetric with respect to its central axis as illustrated in FIG. 8A, and has a rectangular or square cross-section in this example. Magnet 35 is a permanent magnet which may have any conventional shape. In the shown embodiment, permanent magnet 35 is also symmetric with respect to a central axis and has the shape of a disk. Permanent magnet 35 may be attached to toroid 31 by means of an adhesive, solder, welding, or the like.

[0107] By tuning / adju sting the position of permanent magnet 35 on toroid 31, its inductive response and therefore the compensation inductance it provides to the coupled secondary winding section, in operation, changes. Specifically, the inductive response of the toroid 31 can be altered by adjusting the radial position of the permanent magnet 35 with respect to the central axis of the toroid 31.In the preferred embodiment according to FIG. 8A - 8C and FIG. 11 (described further below), the plurality of toroids 31 ‘-315may have mutually different inductances to provide mutually different compensation inductances to the respective winding sections during operation. Advantageously, the permanent magnet 35 may be arranged on the toroid at a position chosen such that the toroid (with permanent magnet) provides the desired compensation inductance. The desired position, size, and / or magnetic force of the permanent magnet 35 on the toroid may be found iteratively upon assembly, integration and / or testing of the transformer, or may be based on earlier testing or simulations. The permanent magnet, once the desired position has been determined, may be fixedly mounted with respect to the central axis of the toroid. For example, permanent magnet 35 may for example be adhered to toroid 31 at a desired radial position with respect to the central axis of toroid 31.

[0108] Additionally or alternatively, as shown in FIG. 9 and 10, mutually different compensation inductances may be provided by providing an auxiliary winding 40 around the toroid 31, the auxiliary winding optionally being coupled to a tuning element that facilitates adjustable or variable inductance of the toroid. Preferably, the auxiliary winding forms a closed circuit comprising the tuning element.

[0109] In the embodiment of FIG. 9, auxiliary w'inding 40 comprises a potentiometer 50. The potentiometer has an adjustable resistance. As a result, the inductance compensation of the toroid can be made adjustable. Therefore, the embodiment of FIG. 9 provides a way to couple a parallel secondary winding section 20 to an inductive element with an adjustable inductance during operation.

[0110] In the embodiment of FIG. 10, auxiliary winding 40 comprises a positive temperature coefficient (PTC) thermistor. The PTC has a variable resistance, which varies as a function of its temperature. As a result, the inductance compensation of the corresponding toroid 31 can be made variable.

[0111] In a PTC thermistor, the resistivity rises as temperature becomes higher. If we assume that the compensation toroid behaves like a transformer, of which the core is formed by the toroid, the primary winding is formed by the conductor of one parallel section of the power transformer (single turn), and the secondary w'inding is formed by the auxiliary winding. The PTC thermistor is connected as the load of to the auxiliary w'inding (secondary of this transformer). First, we consider the situation that the temperature is low. In this case, the resistivity of PTC is low and the lower the resistivity of the load, the lesser the magnetizing inductance from the primary point of view (i.e., compensation inductance). Second, we consider the operation scenario, or the situation wherein the temperature is high. The temperature of a current carrying conductor rises because of the ohmic loss (Loss =

[0112]

[0113] * I2). By putting the PTC in a suitable position inside or near the winding / winding section (e.g., hotspot), the temperature rise of the winding / section increases the resistivity of thePTC. The higher the resistivity of PTC, the higher magnetizing inductance from primary point of view (i.e., compensation inductance). The higher the compensation inductance, the lesser the current. Then the temperature of the hotspot decreases and this cycle repeats until balancing in the currents is achieved.

[0114] Using the PTC in this way provides a feedback loop for current balancing which does not need any control component but a PTC.

[0115] In a way, a PTC or other sensor can measure the temperature and this value can be used as a feedback parameter for balancing.

[0116] FIG. 11 and 12 show embodiments of the parallel secondary winding sections 20 provided with inductive elements, specifically toroids 31, that provide mutually different compensation inductances to the corresponding secondary winding section in operation. In FIG. 11, some toroids 31 are provided with permanent magnets 352- 354to fine-tune the inductance compensation they provide relative to one another and to remaining toroids that do not necessarily comprise such magnets. In FIG. 12, some toroids 312- 314are provided with auxiliary windings coupled to potentiometers or thermistors with a positive temperature coefficient, and some toroids 31’ and 315are provided with permanent magnets.

[0117] As illustrated using FIG. 11 and 12, various combinations of embodiments described above are envisaged within the scope of the present invention. In any case, at least one secondary winding section is provided with an inductive element 30 (e.g., toroid 31). The inductive elements 30 may be mutually different to thereby provide different compensation values. Not all inductive elements 30 need to be embodied as toroids, and not all toroids (when multiple are included) need to be tuned in the same manner. Some may include the permanent magnet 35 as shown in FIG. 8A-8C and 11, some may include an auxiliary winding 40 coupled to a potentiometer 50, some may include an auxiliary winding 40 coupled to a PTC resistor 55, some may be formed of mutually different materials, and / or some may include an airgap of mutually different widths.

[0118] In the above description, the present invention has been explained using detailed embodiments thereof. However, the present invention is not limited to any of these embodiments. As will be appreciated by the skilled person, various modifications can be implemented without deviating from the scope of the present invention as defined by the appended claims.

Claims

CLAIMS1. Power transformer comprising a magnetic core, a primary winding, and a secondary winding comprising a plurality of winding sections that are electrically connected in parallel, wherein each winding section includes a coil,characterized in thatthe power transformer further comprises at least one inductive element, each being coupled to a respective coil to compensate an inductance of a corresponding at least one winding section for current distribution balancing amongst the winding sections during operation.

2. Transformer according to claim 1, wherein the at least one inductive element is configured to provide a compensation inductance to a respective winding section.

3. Transformer according to any of the previous claims, wherein the at least one inductive element adds a series inductance to the respective winding section with respect to the coil of said respective winding section.

4. Transformer according to any of the previous claims, wherein a first inductive element among the at least one inductive element is coupled to a first coil of a first winding section, and wherein a second inductive element among the at least one inductive element is coupled to a second coil of a second winding section.

5. Transformer according to claims 1 or 2, wherein the at least one inductive element comprises a magnetic or magnetizable toroid surrounding part of a corresponding winding section to provide a compensation inductance to a corresponding coil.

6. Transformer according to any of the previous claims, wherein the plurality of winding sections are electrically connected in parallel by connecting portions, such as bus bars, each coil being electrically connected to the connecting portions via individual connection segments, such as connection wires.

7. Transformer according to claims 5 and 6, wherein each toroid of the at least one inductive element surrounds part of a connection segment of a corresponding winding section.

8. Transformer according to any of the previous claims, wherein the at least one inductive element, preferably the toroid as defined in claim 4, is coupled to the first turn of the respective coil.

9. Transformer according to any of the claims 4-8, comprising a plurality of said magnetic or magnetizable toroids, each surrounding part of a respective parallel winding section to add a series inductance to the respective winding section with respect to the coil of the respective winding section.

10. Transformer according to any of the previous claims, wherein a compensation inductance of the at least one inductive element is fine-tuned according to the respective winding section for current distribution balancing amongst the plurality of winding sections, by tuning an inductance value of the inductive element.

11. Transformer according to any of the previous claims, comprising a plurality of said inductive elements configured to provide mutually different compensation inductances, preferably by having mutually different inductance values, in operation.

12. Transformer according to any of the previous claims, wherein an inductance value of the at least one inductive element is variable or adjustable.

13. Transformer according to any of the previous claims, wherein the core of the power transformer is an airgap-less core, wherein the plurality of winding sections is arranged in parallel, and wherein the at least one inductive element is configured to compensate the inductance of corresponding winding sections such that a relatively higher compensation inductance is provided in outer winding sections with respect to a centrally arranged winding section.

14. Transformer according to any of the previous claims, wherein the core of the power transformer is a cut core including an airgap, wherein the plurality of winding sections is arranged in parallel, and wherein the at least one inductive element is configured to compensate the inductance of corresponding winding sections such that a relatively higher compensation inductance is provided in a centrally arranged winding section with respect to outer winding sections.

15. Transformer according to any of the previous claims, the at least one inductive element comprising a plurality of toroids surrounding part of a plurality of winding sections, respectively, the plurality of toroids having a mutually different inductances to provide mutually different compensation inductances to the respective winding sections.

16. Transformer according to claim 15, wherein the plurality of toroids comprise mutually different materials or material compositions for providing mutually different compensation inductances.

17. Transformer according to claim 15 or 16, wherein the plurality of toroids comprise mutually different air-gaps for providing mutually different compensation inductances.

18. Transformer according to any of the claim 15-17, wherein one or more permanent magnets is provided at or near a corresponding toroid to provide the mutually different compensation inductances.

19. Transformer according to claim 18, wherein the permanent magnet is arranged on the toroid at a position, such as a radial position, and / or has a size and / or magnetic force such that the toroid provides the desired compensation inductance.

20. Transformer according to claim 15-19, wherein an auxiliary winding is provided around a corresponding toroid.

21. Transformer according to claim 20, wherein the auxiliary winding is coupled to a potentiometer for adjusting the inductance of the corresponding toroid.

22. Transformer according to claim 20, wherein the auxiliary winding is coupled to a positive temperature coefficient, ‘PTC’, thermistor for providing a variable inductance of the corresponding toroid.

23. Transformer according to any of the previous claims, wherein the transformer is a high current transformer, for example, a solid state transformer, arc furnace transformer, or green hydrogen production transformer, and / or wherein the transformer is configured to deal with medium frequency, and / or relatively low or medium voltages and / or high current loads.

24. Transformer according to any of the previous claims, wherein the parallel winding sections or coils are arranged in corresponding disks.

25. Transformer according to any of the previous claims, wherein the transformer is provided as a shell type construction.

26. Transformer according to any of the previous claims, wherein the core is an airgap-less core.

27. Transformer according to any of the claims 1-25, wherein the core comprises airgaps.

28. Method for current balancing in a power transformer comprising a magnetic core, a primary winding, and a secondary winding comprising a plurality of winding sections, that are electrically connected in parallel, wherein each winding section includes a coil, characterized in that the method comprises coupling at least one inductive element to a respective at least one coil to compensate an inductance of a corresponding at least one winding section for current distribution balancing amongst the winding sections during operation.

29. Method according to claim 28, wherein the at least one inductive element is configured to provide a compensation inductance to the respective winding section.

30. Method according to claim 28 or 29, wherein a first compensation inductance is provided to a first winding section by coupling the first winding section to a first inductive element, and a second compensation inductance is provided to a second winding section by coupling the second winding section to a second inductive element, wherein the inductive is preferably provided as a magnetic or a magnetizable toroid.

31. Method according to any of the claims 28-30, the at least one inductive element comprises a magnetic or magnetizable toroid and wherein a compensation inductance is provided to the respective winding section by surrounding part of the winding section with the corresponding toroid.

32. Method according to any of the claims 28-31, wherein the method comprises:determining a current through each of the plurality of winding sections during operation;determining an inductance compensation value of each of the at least one inductive element required for current balancing; andcoupling the at least one inductive element with corresponding inductance compensation values to corresponding winding sections.

33. Method according to any of the claims 28-32, wherein an inductance of the at least one inductive element is fine-tuned for the respective winding section, by tuning an inductance value of the inductive element.

34. Method according to claim 33, wherein the method comprises:- coupling the at least one inductive element to the corresponding at least one winding section;- determining a current through each of the plurality of winding sections during operation;- determining an inductance compensation value of each of the at least one inductive element required for current balancing; and- adjusting the at least one inductive element to provide the required inductance compensation value.

35. Method according to any of the claims 28-34, wherein the core of the transformer is an airgap-less core, and wherein the method comprises providing a relatively higher compensation inductance for the coil(s) of one or more outer winding sections with respect to the coil(s) of one or more centrally arranged winding sections, orwherein the core of the transformer is a cut core, and wherein the method comprises providing a relatively higher compensation inductance for the coil(s) of one or more centrally arranged winding sections with respect to the coil(s) of one or more outer winding sections.Method according to any of the claims 28-35, wherein different compensation inductances are provided to respective winding sections by providing a plurality of inductive elements having mutually different inductances.

37. Method according to claim 36, wherein a plurality of toroids comprising mutually different materials are used for providing different compensation inductances to the respective winding sections.

38. Method according to claim 36 or 37, wherein a plurality of toroids comprise mutually different materials for providing different compensation inductances to the respective winding sections.

39. Method according to any of the claims 36, comprising providing a permanent magnet at or near a corresponding inductive element, preferably magnetic toroid, to fine-tune the corresponding compensation inductance40. Method according to claim 37, wherein the permanent magnet is arranged on the toroid at a position, such as a radial position, such that the toroid provides the desired compensation inductance.

41. Method according to any of the claims 36-40, wherein an auxiliary winding is provided around a corresponding toroid.

42. Method according to claim 41, wherein the inductance compensation of the respective toroid is varied or adjusted, preferably by either one of:providing an auxiliary winding comprising a potentiometer for adjusting the inductance of the corresponding toroid;providing an auxiliary winding comprising a positive temperature coefficient (PTC) thermistor for providing a variable inductance of the corresponding toroid.

43. Method according to any of the claims 39-42, wherein the fine-tuning of the compensation inductance provided by the respective inductive element is based on the measured current through the coils.

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