Distribution transformers

Abstract: A distribution transformer has a wound magnetic core (50) of overall circular shape and rectangular cross-section with between two and four overall rectangular shape electric coils (20, 30) extending through the core window. The coils (20, 30) are pre-formed and assembled so that their parts (20A, 30A) which meet form a circular section solid cylinder. A mandrel (40) is then located around this cylinder (20A, 30A) and continuous non-amorphous steel strip is wound thereon to form an unannealed, uncut wound magnetic core of axial length in the range 250mm to 1m.

Description: This invention relates to electrical power distribution transformers. In particular the invention relates to such distribution transformers of the type which include a core and coil assembly having a wound magnetic core with a central window and one or more electric coils which extend through said core window. Two known transformer core and coil assemblie of the above-defined type, in which the core is of overall rectangular shape and methods of making them, will now be described. In the first such known assembly the wound core is made by winding magnetic steel strip of single width into a circular roll, and in winding each turn it is cut at approximately the same point. The circular roll is then pressed into an overall rectangular shape core having distributed gaps through one side of the rectangle where the turns were cut, and it is then annealed to fix the rectangular shape. The cut core turns are then opened up and bent out to form a U-shape, a preformed rectangular cylindrical coil is assembled on each of the two legs of the U-shape, and the cut core turns are then closed to re-form the rectangular core shape and are jointed. However well the cuts are jointed they will add significantly to the power loss of the core. Also with this method, the machinery for cutting the magnetic steel strip involves significant cost which is thereby reflected in the cost of the transformer. Furthermore the present and expected future trend is to use progressively thinner magnetic steel strip which has inherently lower power loss, but thinner strip is more difficult to handle in processes which involve cutting. Another disadvantage of this method is that the equipment and process involved in annealing the core contributes significantly to the cost of manufacturing the transformer and hence the cost of the transformer so made. In the second known assembly with a rectangular shape wound core transformer, magnetic steel strip of varying width is wound continuously without cuts on to a rectangular mandrel to form an overall rectangular shape core with an approximately circular cross-section. The core is then annealed to fix the rectangular shape. Split mandrels are then fitted over two legs of the core and a circular cylindrical coil is wound on to each mandrel. This second assembly avoids the manufacturing and power loss disadvantages associated with cutting in the above-described first assembly. However there is still the cost disadvantage of annealing the core. There are two further disadvantages of this second assembly. Firstly the only approximately circular cross-section of the core within the circular coils gives a significant reduction in space factor and hence higher power loss. Secondly, for larger size coils there is an increased level of difficulty in winding the coils leading to a practical upper limit of approximately 50KVA rated power for transformers made this way, which does not cover the full rated power range required for distribution transformers. GB-A-0499010 discloses a polyphase transformer in which the phase windings are arranged in planes which form the sides of a prism, adjacent limbs of the windings being surrounded by iron cores. In one embodiment the cross-sections of the windings are arranged so that the resultant sections surrounded by the cores are circular. The cores may be formed by metal strip wound in spiral form, or may be built up from flat plates, or may be formed from metal rings through which the windings are threaded, or may be formed from a number of single turn strips. The circular section of the windings surrounded by the core will give a high space factor. However, we believe that the state of the art concerning thickness of sheet metal strip and the techniques for using it for transformer cores at the time this document was published was such that the cores would have discontinuities with resultant power loss as has been mentioned above. Conventionally, rectangular shape wound transformer cores, whether cut or uncut, have been made with non-amorphous steel strip. More recently such transformer cores have become known which are made with amorphous steel strip. This material has much lower power loss than non-amorphous steel, but this advantage is partially offset by the higher intrinsic material cost. Also, amorphous steel has only been available with a strip width up to approximately 200mm, 213mm being the highest strip width of which we are aware, which limits the size of wound cores using a single strip width and hence the rated power of transformers using such cores so that they do not cover the full rated power range required for distribution transformers. In IEEE Transactions on Power and Apparatus Systems, Vol.PAS-103, No.11, November 1984, pages 3365 to 3372 there is published a paper by E.L. Boyd and J.D. Borst entitled "Design concepts for an amorphous metal distribution transformer". In the summary at the end of this paper it is stated that "The unique characteristics of amorphous metals present significant challenges to the transformer designer and will likely result in a radically different core-coil assembly. This paper has defined a broad range of theoretical core-coil configurations and refined these to a feasible set of solutions through qualitative analysis of amorphous metal characteristics, transformer design requirements, and transformer assembly techniques." One of the feasible theoretical core-coil configurations discussed as worth future consideration for use with amorphous metal shows an overall circular shape uncut core with rectangular cross-section and two rectangular coils extending through the window of the core (configuration IIB in Figure 3). These two rectangular coils are shown each to have a substantially semi-circular cross-section where they meet, so that the core window is substantially filled by the two coils. This configuration would avoid power loss associated with a discontinuous core, and provide a low space factor power loss. However, on page 3367, left-hand column, it is stated that "The core-coil configuration may be significantly different from presently used conventional electrical steel configuration". There is thus no indication in this paper that configuration IIB may possibly be useful for wound core transformers using conventional (non-amorphous) steel. According to the present invention there is provided an electrical power distribution transformer which includes a core and coil assembly having a wound magnetic core with a central window and electric coils which extend through said core window, wherein the core is uncut, is of overall circular shape and rectangular cross-section, and consists of continuous single width steel strip, and wherein each said coil is of overall rectangular shape, and each said coil has a cross-section which is a sector of a circle at least where said coils pass through the core window with the sector cross-sections together substantially filling the core window, characterised in that the core is unannealed and consists of a single roll or up to four stacked co-axial rolls each wound of continuous single or multiple thickness non-amorphous steel strip, in that the or each strip has a single width in the range 250mm to lm and the total axial length of the roll or co-axial rolls is in the range 250mm to 1m, and in that the number of said electric coils is in the range between two and four. In this transformer the power loss associated with the cuts in the above-described known cut transformer is avoided, and the poor space factor of the above-described known uncut core transformer is avoided. We expect that the above-described distribution transformer according to the invention may have a power rating in the range 10KVA to 2000KVA. The upper end ofthis range, which we can achieve with a single roll core having a strip width of up to 1m, is higher than can be provided with the above-described known uncut core transformers having the coils wound on to the pre-formed core, and is higher than can be provided with the above-described known transformers having a single strip of amorphous steel. For a transformer core which is required to have a given cross-section area to carry the flux necessary to induce given required voltages in the coils, the mean path length of a circular wound core of non-amorphous steel in the core-coil configuration of a transformer according to the invention is substantially reduced down to possibly half the mean path length of a rectangular wound core of non-amorphous steel in the core-coil configuration of an equivalent power rated transformer as previously known. This accordingly by comparison reduces the volume and hence the weight of the core steel. The cost of the steel used in the transformer and its power loss, which are both proportional to its weight, are therefore both reduced by comparison with such an equivalent previously known transformer. Before making and testing a transformer in accordance with the invention as defined above we had expected that the high proportion of the coils outside the core in the circular core configuration specified, compared with that proportion in the previously known rectangular core configuration, would result in high flux leakage giving the transformer an unacceptably high reactance in the range of perhaps 20 to 60%. Surprisingly, we have found that reactance of transformers according to the invention is acceptably low in the region of 4%. In a transformer according to the invention as defined above, the low weight, low cost, low loss advantages over previously known rectangular wound core transformers may be enhanced by the non-amorphous steel strip being of a high permeability, low loss, type defined as having a power loss of less than 1.00 Watts/Kg at a magnetic induction of 1.7 Tesla at 50HZ. In a transformer according to the invention as defined above, the high permeability, low loss, non-amorphous steel strip as just-described may have a thickness between 0.2mm and 0.1mm. Such a strip is too thin and possibly too brittle to be economically used to make cut transformer cores, but it can be more easily wound and so may be economically advantageously used in a transformer according to the invention. In a transformer according to the invention as defined above, the steel strip forming the wound magnetic core is preferably of single thickness for ease of manufacture. Also in a transformer according to the invention as defined above, the wound magnetic core will preferably consist of a single roll of steel strip for ease of manufacture. Most conveniently in a transformer according to the invention as defined above, two said electric coils extend through said core window in the transformer, each coil having a semi-circular cross-section where it passes through the core-window. One reason is that it may be desirable to impregnate the coils with resin to enable them to withstand short-circuit forces and this will be done for each coil before the coils are assembled together. The support structure of such as assembly will be more difficult to arrange if there are more than two resin impregnated coils. Another reason is that, in the case where the support on which each such coil is wound comprises sectioned former, then this former need only have two sections. If there are more than two coils, then for each coil the former will need to have more than two sections which will provide a groove having, for where the coil will pass through the core, the shape of a sector of a circle less than a semi-circle and will enable these sections to be removed from the coil after that coil has been wound. In a transformer according to the invention as defined above the transformer may be single phase with all the coils extending through only one core. If multi-phase transformation is required using a transformer in accordance with the invention it will be possible to provide a suitable number of discrete side-by-side single-phase transformer configurations. Alternatively, for a three-phase transformer, we consider it possible to provide a configuration according to the invention with three said overall circular, rectangular cross-section, wound cores and four said rectangular coils, with each core window having two of said coils passing through it and these two coils each having a semi-circular cross-section where they pass through this core window. Examples of transformers in accordance with the invention will now be described with reference to the accompanying drawings, in which Figure 1 shows a perspective view of a sectioned former on which a coil for a transformer is to be wound, Figure 2 shows a part elevation of the former of Figure 1, on enlarged scale, with a coil wound thereon, Figure 3 shows the coil-core configuration of a transformer with two pre-formed coils assembled together, a mandrel around a circular cylinder formed by the coils where they meet, and an incomplete magnetic core formed by winding steel strip on the mandrel, and Figure 4 shows the coil-core configuration of a three-phase transformer having three wound cores and four rectangular coils. Referring now to the drawings, Figure 1 shows a rectangular former made up of two sections 1A, 1B of any suitable material with their edges shaped so that when held together (and meeting where shown by the dotted line) they provide a semi-circular shape groove 1C. Figure 2 shows the configuration of an electrical coil for a transformer wound in the groove 1C of the former 1A, 1B. The whole groove is first lined with an insulation layer 21 and a flat insulation layer 22 is then positioned at the innermost part of the groove. Electrical conductor is then wound into the groove to form an inner primary winding 23 for the transformer which may have an input primary voltage of 33KV. A further flat insulation layer 24 is placed on the primary winding 23, and further electrical conductor is then wound into the groove 1C to fill the groove and form an outer secondary winding 25 for the transformer which may have an output secondary voltage of 400V. The windings 23 and 25, with the insulation layers 21, 22, 24 provide a pre-formed coil 20 from which the former sections 1A and 1B are then removed. The shape of the pre-formed coil 20 can then be consolidated by taping. The preformed coil 20 is then assembled together with a similar pre-formed coil 30 as shown in Figure 3 so that where they meet their semi-circular cross-sectioned parts 20A, 30A combine to form a circular section solid cylinder. A hollow circular mandrel 40 of electrically insulating material, for example epoxy resin, is then formed around the circular cylinder 20A, 30A. The mandrel 40 is then rotated to wind thereon a roll of continuous non-amorphous conventional grain orientated electrical steel strip to form an uncut, unannealed, wound magnetic core 50 which fills the space within the coils 20, 30. For each of illustration only an inner part of the core is shown in Figure 3. The mandrel 40 may be rotated for example by means of gear teeth provided at one end, or by being belt driven at one end, or by a wheel contacting the steel strip. The mandrel 40 would be left to remain in the finished transformer. The mandrel 40 located around the coil cylinder 20A, 30A could alternatively be of metal, preferably non-magnetic, with electrical insulation provided between the mandrel and the coil cylinder. There is thus provided, as shown in Figure 3, a core-coil configuration having a wound magnetic core 50 with a central window, the core being of overall circular shape and rectangular cross-section formed of non-amorphous steel strip having a single width and two electric coils which are of overall rectangular shape and extend through the core window with the coil cross-sections substantially filling the core window. The primary windings of the two coils 20, 30 may be connected in series with the secondary windings of the two coils connected in parallel to form a single-phase power distribution transformer. The width of the non-amorphous steel strip from which the core 50 is wound is in the range 250mm to 1m and it is of single thickness, although multiple thickness strip could be used. We consider it would be difficult to handle and uneconomic to wind a strip having a width greater than 1m. This strip width will enable transformers to be made having a power rating in the range 10KVA to 2000KVA. A core having this same axial length in the range of 250mm to 1m could be made up to four stacked coaxial rolls, for example two rolls each having a strip width of 500mm. As discussed in the introductory portion of this patent specification the weight, cost and power loss of the transformer may be reduced by substituting the conventional grain orientated electrical steel strip with a different non-amorphous steel strip having a power of less than 1.00 Watts/Kg at a magnetic induction of 1.7 Tesla at 50HZ, which may have a thickness between 0.2mm and 0.1mm. High permeability, low loss, non-amorphous steel strips of this type known as Hi-B, domain refined Hi-B and 6% Si-Fe are described and discussed, for example, in an article "Modern Transformer Core Materials" by M.R. Daniels published in GEC REVIEW Volume 5, NO. 3, 1990 at pages 132 to 139. It is essential that the two coils 20, 30 have a semi-circular cross-section at least in their legs where they will pass through the core window. A possible alternative to all four legs of each coil 20, 30 having a semi-circular cross-section would be for the leg opposite the core window to be of rectangular section with the two linking legs providing a transformation from semi-circular to rectangular section. As discussed in the introductory portion of this patent specification it is most convenient to have two coils 20, 30 extending through the core 50. More than two coils can be provided, each pre-formed on a former having more than two second. Each such former will provide a groove having for where the coil will pass through the coil, the shape of a sector of a circle less than a semi-circle such that when the coils are assembled together these circle sector cross-sectioned parts will combine to form a circular section solid cylinder where they meet. It will be difficult to provide a former having the number of sections required for a coil which will be one of a set of more than four coils assembled together to extend through the core window. Partly for this reason and also because, as mentioned in the introductory portion of this patent specification, it may be desirable to impregnate the coils with resin before they are assembled together, we consider the assembly of four coils together to be a practicable upper limit. The former sections 1A, 1B which are held together constitute a support which provides the groove 1C in which the coil conductors are wound. These former sections must be separated for removal of the coil. However, instead of providing former sections which are completely removed after winding the coil, it may be possible to provide a sectioned former assembly which is expanded to separate the sections for removal of the coil while still holding these sections together. A modulated insulating frame may be provided which is fitted in the sectioned former before winding the coil conductors, and this insulating frame may remain as part of the consolidated coil after its removal from the former. It may be possible that such an insulating frame can itself be the support providing the groove for winding the coil, obviating the need for a sectioned former. As discussed in the introductory portion of this patent specification, if multi-phase transformation is required it will be possible to provide a number of discrete side-by-side single-phase transformer configurations. Figure 4 shows an alternative coil-core configuration for a three-phase transformer. There are three overall circular, rectangular cross-section, wound cores 50, 51, 52 and four rectangular coils 20, 30, 21, 31. Each core window has two of the coils passing through it and these two coils each have a semi-circular cross-section where they pass through the respective core window.

Claims: An electrical power distribution transformer which includes a core and coil assembly having a wound magnetic core (50) with a central window and electric coils (20, 30) which extend through said core window, wherein the core (50) is uncut, is of overall circular shape and rectangular cross-section, and consists of continuous single width steel strip, and wherein each said coil is of overall rectangular shape, and each said coil has a cross-section which is a sector of a circle at least where said coils pass through the core window with the sector cross-sections (20A, 30A) together substantially filling the core window, characterised in that the core is unannealed and consists of a single roll or up to four stacked co-axial rolls each wound of continuous single or multiple thickness non-amorphous steel strip, in that the or each strip has a single width in the range of 250mm to 1m and the total axial length of the roll or co-axial rolls is in the range 250mm to 1m, and in that the number of said electric coils (20, 30) is in the range between two and four. A transformer as claimed in Claim 1, in which said non-amorphous steel strip has a power loss of less than 1.00 Watts/Kg at a magnetic induction of 1.7 Tesla at 50Hz. A transformer as claimed in Claim 2, in which said non-amorphous steel strip has a thickness between 0.2mm and 0. 1mm. A transformer as claimed in any preceding claim, in which the steel strip forming the wound magnetic core is of single thickness. A transformer as claimed in any preceding claim, in which the wound magnetic core consists of a single roll of steel strip. A transformer as claimed in any preceding claim, in which the transformer is single-phase with all said coils extending through only one said core. A transformer as claimed in Claim 6, in which the transformer has two said electric coils each having a semi-circular cross-section where it passes through the core window. A transformer as claimed in any of Claims 1 to 5, in which the transformer is three-phase and has three said cores and four said coils, with each core window having two of said coils passing through it and these two coils each having a semi-circular cross-section where they pass through this core window.

Inventors: ALLAN DENNIS JOSEPH, GRANT JOHN VICTOR, ALLAN, DENNIS JOSEPH, GRANT, JOHN VICTOR
Applicants: GEC ALSTHOM LTD, GEC ALSTHOM LIMITED
Application year: 1992
Main classification: H01F27
IPC codes: H01F41, H01F30, H01F27