Abstract:
A transformer and a method of manufacturing a transformer are disclosed. The transformer can include a transformer core with at least three core limbs which are arranged in parallel with respect to one another and perpendicular to corner points of an area spanned by a polygon, and wherein axial end regions of each of the at least three core limbs transition into a respective yoke segment arranged transversely with respect to the axial end regions. Main windings can be arranged around each of the at least three core limbs in a hollow-cylindrical winding region. A magnetic cross section of a respective core limb can be greater than a magnetic cross section of the respective yoke segment. Additional windings can be electrically connected to a respective main winding and can be arranged around each of the respective yoke segments.

Description:
RELATED APPLICATION(S) 
     This application claims priority under 35 U.S.C. §119 to European Patent Application No. 13004986.9 filed in Europe on Oct. 18, 2013, the entire content of which is hereby incorporated by reference in its entirety. 
     FIELD 
     The disclosure relates to a transformer, including a transformer core with at least three core limbs which are arranged in parallel with respect to one another and perpendicular to the corner points of a basic area spanned by a polygon, the respective two axial end regions of which core limbs transition on both sides into a respective yoke segment arranged transversely with respect thereto and in each case a main winding arranged around a respective core limb in a hollow-cylindrical winding region, wherein the magnetic cross section of a respective core limb can be greater than the magnetic cross section of a respective yoke segment. 
     BACKGROUND INFORMATION 
     Known transformers can be used in electrical energy distribution systems in order to couple power supply units with different voltage levels to one another. Transformers can be designed as dry-type transformers with a voltage level close to a consumer or generator and can have nominal voltages, for example, in the range from, for example, 1 kV to 6 kV on the low-voltage side and nominal voltages in the range from, for example, 10 kV to 30 kV on the high-voltage side, wherein corresponding nominal powers lie in the range from, for example, 0.5 MVA to 10 MVA. However, transformers such as this can be used in the field of wind power installations, where the nominal power of a transformer is directed towards the power of an associated wind power installation. 
     Due to the high nominal currents in the low-voltage range, which can be, for example, some 100 A, the low-voltage windings can be designed in a manner wound from a ribbon conductor, wherein the width of a ribbon conductor corresponds to at least the complete axial length of a respective transformer winding. Depending on the embodiment and specifications on the transformer, the number of low voltage-side turns can be, for example, in the region of ten turns, for example, in applications for wind power installations, where the voltage generated on the generator side can be correspondingly low and can be set to a higher operating level by the transformer. 
     For the purposes of regulation, a known procedure can be to provide the high voltage-side winding(s) of a transformer with a plurality of taps, which can be selected by a respective on-load tap changer, for example, with the result that the transformation ratio of the transformer can thus be changed within a regulation range. Increased regulability can be needed in applications for wind power installations in order to help ensure the transformer is adapted to the boundary conditions resulting from different wind conditions. 
     The active part of a transformer can have a closed iron circuit and at least one high-voltage and low-voltage winding with an integer number of closed turns around the respective core limb. The induced voltage per closed conductor loop can be dependent on the mains frequency, flux density and core cross section. 
     An on-load tap changer arranged on the high-voltage side can be complex to make due to the high voltage demand and that regulation of the voltage can take place minimally in that voltage graduation, which can correspond to the induced voltage of a complete turn. In the case of voltage regulation, the minimum regulation stage can be limited to the voltage difference between two turns. For example, in the case of the low voltage-side ribbon windings disclosed above because, owing to the relatively low total number of turns, for example, in the region of ten, fine regulation around the nominal transformation may not be possible. 
     In accordance with an exemplary embodiment, the disclosure can provide a transformer which can enable the voltage on the low-voltage side to be regulated in smaller voltage steps, and wherein a corresponding on-load tap changer can be made simpler due to the then lower voltage demand. 
     SUMMARY 
     A transformer is disclosed, comprising: a transformer core with at least three core limbs which are arranged in parallel with respect to one another and perpendicular to corner points of an area spanned by a polygon, and wherein axial end regions of each of the at least three core limbs transition into a respective yoke segment arranged transversely with respect to the axial end regions; main windings arranged around each of the at least three core limbs in a hollow-cylindrical winding region, and wherein a magnetic cross section of a respective core limb is greater than a magnetic cross section of the respective yoke segment; and additional windings which are electrically connected to a respective main winding and which are arranged around each of the respective yoke segments. 
     A method of manufacturing a transformer is disclosed, the method comprising: providing a transformer core with at least three core limbs which are arranged in parallel with respect to one another and perpendicular to corner points of an area spanned by a polygon, and wherein axial end regions of each of the at least three core limbs transition into a respective yoke segment arranged transversely with respect to the axial end regions; winding each of the at least three core limbs with a main winding arranged around a respective core limb in a hollow-cylindrical winding region, and wherein a magnetic cross section of a respective core limb is greater than a magnetic cross section of the respective yoke segment; and electrically connecting additional windings to a respective main winding, the additional windings being arranged around each of the respective yoke segments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure is explained below with reference to the exemplary embodiments shown in the drawings. In the drawings: 
         FIG. 1  shows an exemplary first transformer; 
         FIG. 2  shows an exemplary second transformer; 
         FIG. 3  shows an exemplary third transformer; 
         FIG. 4  shows an exemplary wiring of main and additional winding; and 
         FIG. 5  shows a cross section through an exemplary yoke segment. 
     
    
    
     DETAILED DESCRIPTION 
     In accordance with an exemplary embodiment, a transformer is disclosed which can include an additional winding that can be electrically connected to a respective main winding and which can be arranged around a respectively assigned yoke segment. 
     In accordance with an exemplary embodiment, a transformer is disclosed which can reduce the induced voltage of one or more complete turns which can be provided for the purposes of regulation in that the turns do not surround the magnetic cross section of a complete core limb, but rather can enclose the cross section of a respective yoke. For example, in the case of a transformer with a polygonal layout, the cross section of a yoke can be smaller, for example, than the cross section of a complete core limb. Thus, the magnetic flux enclosed by a turn and hence the respectively induced voltage can also be lower in the case of the arrangement on a yoke than in the case of the arrangement on a core limb. 
     In accordance with an exemplary embodiment, by virtue of a corresponding series connection of the main winding, and for example, arranging a selectable number of additional windings around a respective yoke of the transformer core, a finer graduation of the regulation region of the transformer can be achieved. 
     According to an exemplary embodiment of a transformer according to the disclosure, the transformer core can have the basic area of an equilateral triangle. Such shapes can be known, for example, by the names “hexaformer” or “delta core”. A delta core can have, for example, three frame-like segments which can be wound in an annular manner and which, when joined to one another according to a triangular layout, can result in a triangular transformer core. In accordance with an exemplary embodiment, the symmetrical shape of such a transformer core for three-phase transformers can favour a symmetrical operating behaviour, for example. 
     In accordance with an exemplary embodiment, it can also be seen on a delta core why the cross section of a yoke can be smaller than the cross section of a core limb. The surrounding cross section of a frame-like wound core segment can be identical along its circumference. The cross section of a core limb can be formed from the sum of the cross sections of two sides, which border one another, of adjacent core segments, while a yoke can have the surrounding cross section of a single frame segment, wherein a total of three core limbs and six yokes with in each case half the magnetic cross-sectional area can be formed, 
     According to an exemplary embodiment of the transformer according to the disclosure, the magnetic cross-sectional area of a respective yoke segment can be half the magnetic cross-sectional area of a respective core limb, 
     According to an exemplary embodiment of the transformer according to the disclosure, the main and/or the additional windings can be formed by a respective flat ribbon conductor. Flat ribbon conductors can be suitable for accommodating high currents, which can be advantageous in the case of a respective low voltage-side winding. In addition, flat ribbon conductors can have a correspondingly high fill factor. 
     According to an exemplary embodiment of the transformer according to the disclosure, at least one winding of a respective additional winding can be guided through a gap in the yoke segment. In accordance with an exemplary embodiment, as a result of this, the induced voltage in the corresponding turn of the additional winding can be reduced further because now only a part of the magnetic cross-sectional area of the yoke is surrounded by the turn and the induced voltage therefore turns out to be correspondingly lower. Thus, in accordance with an exemplary embodiment, a refinement of the graduation sections about the nominal transformation can be enabled in a simple manner, for example a regulation region of, for example, +/−15% in steps of in each case, for example, 1.5%. 
     In accordance with an exemplary embodiment, a yoke can be suitable to be divided into two cross-sectional areas by a gap at least along a section of the axial extent of the yoke because an increase, caused thereby, in the geometric cross section (in the case of a magnetic cross section which remains the same) does not lead to an increase in the installation size of the transformer. If a gap were arranged in the limb region of a transformer, an effective reduction in the magnetic cross section would result at least in the gap region owing to the limited space supply within a coil surrounding the respective limb in the case of a geometric cross section, which can remain the same. 
     In accordance with an exemplary embodiment, a correspondingly higher number of gaps can be provided and hence also of cross-sectional areas. In accordance with an exemplary embodiment, any subdivision can occur in order to meet the specifications on regulability, for example ⅓ or ¼. Specific voltage stages can also be realized, however, by a plurality of turns being laid around a part of the yoke, for example, 3 turns around ¼ of the yoke cross section or 4 turns around ⅕ of the yoke cross section. If the windings are laid separately around a part of the yoke cross section, the wiring can be used by the selection of the winding direction or the polarity of the (additional) turn in order to enable the turn to work in an either additive or subtractive manner. Hence, the number of (additional) turns for the voltage regulability can be reduced 
     According to an exemplary embodiment of the transformer according to the disclosure, the respective additional winding can be provided with a plurality of taps accessing different turns of the respective additional winding. By appropriate selection of the taps, a desired regulation region can be realized. The additional winding optionally can have a fine graduation region, which can be distinguished by taps from turns, which can be guided through the respective gap in a yoke, of the additional winding. In accordance with an exemplary embodiment, the turns can have in each case an induced voltage, which can be lower than the induced voltage of a turn, which completely surrounds the respective yoke. Furthermore, a coarse graduation region can be provided which can be distinguished by taps from turns, which in each case completely surround the yoke. In accordance with an exemplary embodiment, by appropriate series connection of the coarse and fine graduation regions, fine graduation over a wide range can be achieved. 
     According to an exemplary embodiment of the transformer according to the disclosure, switching means can be provided in order to selectively connect the main winding to one of the taps of the additional winding, with the result that the number of active turns of the electrically connected main and additional winding can thus be matched. If appropriate, separate switching means can be provided for coarse and fine graduation regions of the additional winding. 
     According to an exemplary embodiment of the transformer, the switching means can include an on-load tap changer and/or power electronic components. On-load tap changers have proven to be successful as standard components for the optional selection and wiring of taps of a transformer winding. According to specifications, power electronic components such as thyristors or IGBTs can also be provided. 
       FIG. 1  shows an exemplary first transformer  10  in a sectional view. A triangular transformer core (delta core) can be formed from three frame-like wound core segments of which in each case the lower yoke segment is provided with the reference numeral  12 ,  14 ,  16 . Respective core limbs  18 ,  20  can be formed on the contact surfaces of adjacently arranged core segments, which core limbs have twice the cross section of a respective yoke  12 ,  14 ,  16 , Respective main windings  22  can be arranged around the core limbs  18 ,  20 , which main windings can be electrically connected to additional windings  24 , which can be arranged in each case around the yoke segments  12 ,  14 ,  16 . Voltage regulation of a respective main winding  22  can be refined by the less graduated voltage induction in the additional winding  24 . 
       FIG. 2  shows an exemplary second transformer  30  in a plan view. The three limbs of a triangular transformer core  50  can be surrounded in a hollow cylindrical manner by mutually interlaced first low voltage-side primary main windings  32 ,  38 ,  44 , which can be arranged radially on the inside and by high voltage-side secondary main windings  34 ,  40 ,  46 , which can be arranged radially on the outside. In accordance with an exemplary embodiment, the primary main windings arranged on the low voltage side can be in each case electrically interconnected with additional windings  36 ,  42 ,  48  arranged around the respective upper yokes, which additional windings each have a plurality of turns with taps which can be connected to a respective on-load tap changer to adapt the transformation ratio, which is not shown in the figure. 
       FIG. 3  shows a similar transformer  60  to that in  FIG. 2  but in a side view. In accordance with an exemplary embodiment, a respective core limb can be distinguished by a dashed rectangle with the reference numeral  62  and a respective yoke segment can be distinguished by a dashed rectangle with the reference numeral  64 , wherein both differ from one another in their magnetic cross section by a factor of two. Respective first  66  and second  68  main windings can be arranged around the core limbs  62 , which main windings can be electrically connected to additional windings  70 , which can be arranged around respective yoke segments  64 . 
     In accordance with an exemplary embodiment, the additional windings  70  can configured such that that they do not encompass the total cross section of a respective yoke; rather they can be guided through a gap in the yoke, with the result that a lower voltage can be induced per turn than if the entire yoke cross section were surrounded. As a result of this, finer voltage regulation can be enabled. The so-called hexaformer core shown here, which can be made from laminated cores, which can be arranged in each case in a bunched and wound manner, lends itself in particular to this purpose because a gap can be provided, for example, in relatively simple manner between adjacent laminated cores in the yoke regions. 
       FIG. 4  shows an exemplary wiring  80  of a main  82  and additional winding  84 . These can be respectively electrically connected in series, wherein the additional winding  84  can have a plurality of taps  86 , which can optionally be tapped by means of a switching means  88 , and on-load tap changer. Electrical connections  90  and  92 , respectively, can be provided at the start and end of the series circuit. 
       FIG. 5  shows a cross section through an exemplary yoke segment  100  the magnetic total cross section of which is formed by a first cross-sectional area  102  and a second cross-sectional area  104 , wherein a gap  106  can be provided therebetween, through which gap two turns  108 ,  110  of an additional winding can be guided. In accordance with an exemplary embodiment, the additional winding can be connectable via taps  112 ,  114 . 
     Thus, it will be appreciated by those skilled in the art that the present invention can be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restricted. The scope of the invention is indicated by the appended claims rather than the foregoing description and all changes that come within the meaning and range and equivalence thereof are intended to be embraced therein. 
     LIST OF REFERENCE SIGNS 
     
         
           10  exemplary first transformer 
           12  first yoke segment of the first transformer 
           14  second yoke segment of the first transformer 
           16  third yoke segment of the first transformer 
           18  cross section of the first core limb of the first transformer 
           20  cross section of the second core limb of the first transformer 
           22  first main winding of the first transformer 
           24  first additional winding of the first transformer 
           30  exemplary second transformer 
           32  first primary main winding of the second transformer 
           34  first secondary main winding of the second transformer 
           36  first additional winding of the second transformer 
           38  second primary main winding of the second transformer 
           40  second secondary main winding of the second transformer 
           42  second additional winding of the second transformer 
           44  third primary main winding of the second transformer 
           46  third secondary main winding of the second transformer 
           48  third additional winding of the second transformer 
           50  transformer core 
           60  exemplary third transformer 
           62  first core limb of the third transformer 
           64  first yoke segment of the third transformer 
           66  first primary main winding of the third transformer 
           68  first secondary main winding of the third transformer 
           70  first additional winding of the third transformer 
           80  exemplary wiring of main and additional winding 
           82  exemplary main winding 
           84  exemplary additional winding 
           86  taps 
           88  switching means 
           90  first connection 
           92  second connection 
           100  cross section through the exemplary yoke segment 
           102  first cross-sectional area 
           104  second cross-sectional area 
           106  gap 
           108  first turn of additional winding 
           110  second turn of additional winding 
           112  first tap 
           114  second tap