OA11300A - A transformer/reactor and a method for manufacturing a transformer/reactor - Google Patents

Abstract

The present invention relates to a transformer or a reactor and a method of manufacturing a transformer/reactor. The transformer/reactor comprises a core and at least one winding, in which the core (1; 11; 21; 31) consists of at least two segments (4; 14; 24, 25; 33, 34, 36, 37), and the winding is flexible and comprises an electrically conducting core (7) surrounded by an inner semiconducting layer (8), an insulating layer (9) surrounding the inner semiconducting layer and consisting of solid material, and an outer semiconducting layer (10) surrounding the insulating layer, said layers adhering to each other.

Description

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Cil 300 Ά transformer/reactor and a method for manufacturing a transformer/reactor

The présent invention relates to a transformer/reactorcomprising a core and at least one winding.

The présent invention also relates to a method for use inthe manufacturing of a corresponding transformer/reactor.

Transformers/reactors are available in ail power ranges froma few W up to the 1000 MW range. The term "power trans-fo-rmer/reactor" generally refers to transformers/reactorshaving a rated output from a few hundred kW up to over1000 MW and a rated voltage of from 3-4 kV up to extremelyhigh transmission voltages. A conventional power transformer comprises a transformercore, hereinafter called core, of laminated oriented sheetmétal, usually ferrosilicon. The core consists of a numberof core legs joined by a yoke. A number of windings areplaced around the core legs, generally termed primary,secondary and regulating windings. In the case of powertransformers, these windings are almost always arrangedconcentrically and distributed along the core legs. Thetransformer core has a rectangular "window" through whichthe windings pass. This rectangular window is primarily aresuit of the production technique used when the core islaminated.

The use of transformer cores of varying shape is knownthrough DE 40414, US 2 446 999, GB 2 025 150, US 3 792 399,US 4 229 721, for instance. Some of these documents alsodiscloses cores made up of segments. However, none of thesedocuments pertain to high voltage power transformers, andthey would not be applicable to such transformers due to theprésent technique of oil-cooling, discussed below. 2 C11300

Conventional power transformers at the lower end of theabove-mentioned power range are sometimes provided with aircooling in order to remove the unavoidable natural losses inthe form of heat. However, most conventional power trans-formers are oil-cooled, generally by means of pressurizedoil cooling. This applies particularly to high-power trans-formers. Oil-cooled transformers hâve a number of weli knowndrawbacks. They are, for instance, large, cumbersome andheavy, thus entailing in particular considérable transportproblems, as well as the demands being extensive with regardto safety and peripheral equipment.

However, it has been proved possible to replace oil-cooledpower transformers, to a great extent, with dry transformersof a new type. This new dry transformer is provided with awinding achieved by high-voltage cable, i.e. a high-voltageinsulated electric conductor. Dry transformers can thus beused at considerably higher power rates than has previouslybeen possible. The expressions "dry transformer" and "dryreactor" thus apply to a transformer/reactor which is notoil-cooled, but preferably air-cooled.

With regard to reactors (inductors), these comprise a corewhich mostly is provided with only one winding. In otherrespects, what has been stated above concerning transformersis substantially relevant also to reactors. It should beparticularly noted that also large reactors are oil-cooled.

The object of the présent invention is to provide a trans-former or a reactor enabling some of the drawbacks inhérentin the conventionally designed power transformers/reactorsdescribed here, to be eliminated and also to provide amethod for use in manufacturing such a transformer/reactor.

The objects are achieved by means of a transformer/reactor having the features defined in claim 1, and by means of a m mq method for manufacturing such a transformer/reactor in accordance with the features defined in claim 25.

According to a first feature in claim 1, the core ccnsistsof at least two segments. The corresponding method includesthe feature of manufacturing a core including at least twosegments. The expression "segment" or "segmented core" meansthat the core of the transformer/reactor is built fromsubstantially identical segments or parts joined togetherside by side to form the core. Mâny advantages are gained with a core built from segments.First of ail, even relatively large cores can be made sub-stantially annular in shape which offers significant advan-tages which will be explained below.

Secondly, simpler winding of the core is possible since eachsegment can be wound separately. A third advantage of segmented cores is that parts of thecore can be dismantled or assembled at any time duringmanufacture.

Advantages are also obtained from the production point ofview since the core can be built in the form of modules,each comprising one or more segments. This also offersconsidérable advantages with regard to transport since thecore can be transported in segments and then assembled onthe site where it is to be used. If necessary, the windingcan also be wound on site.

According to a further feature in claim 1, the winding isflexible and comprises an electrically conducting coresurrounded by an inner semiconducting layer, an insulatinglayer surrounding the inner semiconducting layer and con-sisting of solid material, and an outer semiconducting layersurrounding the insulating layer, said layers adhering to 4 C11300 each other. According to a further feature of the method,said method comprises the step of installing a winding ontothe core which winding is defined in correspondance withclaim 1.

Thus, the windings in a transformer/reactor according to theinvention, are preferably of a type corresponding to cableshaving solid, extruded insulation, of a type now usée forpower distribution, such as XLPE-cables or cables with EPR-insulation. Such a cable comprises an inner conductor com-posed of one or more strand parts, an inner semiconductingl$_yer surrounding the conductor, a solid insulating layersurrounding this and an outer semiconducting layer surround-ing the insulating layer. Such cables are flexible, which isan important property in this context since the technologyfor the device according to the invention is based primarilyon winding Systems in which the winding is formed from cablewhich is bent (or curved) during assembly. The flexibilityof a XLPE-cable normally corresponds to a radius of curva-ture of approximately 20 cm for a cable with a diameter of30 mm, and a radius of curvature of approximately 65 cm fora cable with a diameter of 80 mm. In the présent applicationthe term "flexible" is used to indicate that the winding isflexible down to a radius of curvature in the order of fourtimes the cable diameter, preferably eight to twelve timesthe cable diameter. . .

The winding should be constructed to retain its propertieseven when it is bent and when it is subjected to thermalstress during operation. It is vital that the layers retaintheir adhesion to each other in this context. The materialproperties of the layers are décisive here, particularlytheir elasticity and relative coefficients of thermal expan-sion. In a XLPE-cable, for instance, the insulating layerconsists of cross-linked, low-density polyethylene, and thesemiconducting layers consist of polyethylene with soot andmétal particles mixed in. Changes in volume as a resuit of 5 011300 température fluctuations are completely absorbed as changesin radius in the cable and, thanks to the comparativeiyslight différence between the coefficients of thermal expan-sion in the layers in relation to the elasticity of tcesematerials, the radial expansion can take place witnout theadhesion between the layers being lost.

The material combinations stated above should be considéréeonly as examples. Other combinerions fulfilling the condi-tions specified and also the condition of being semiconduct-ing, i.e. having resistivity within the range of 10"--106ofrm-cm, e.g. 1-500 ohrn-cm, or 10-200 ohm-cm, naturally alsofall within the scope of the invention.

The insulating layer may consist, for example, of a solidthermoplastic material such as low-density polyethylene(LDPE), high-density polyethylene (HDPE), polypropylene(PP), polybutylene (PB), polymethyl pentene ("TPX"), cross-linked materials such as cross-linked polyethylene (XLPE),or rubber such as ethylene propylene rubber (EPR) or Siliconrubber.

The inner and outer semiconducting layers may be of the samebasic material but with particles of conducting materialsuch as soot or métal powder mixed in.

The mechanical properties of these materials, particularlytheir coefficients of thermal expansion, are affected rela-tively little by whether soot or métal powder is mixed in ornot - at least in the proportions required to achieve theconductivity necessary according to the invention. Theinsulating layer and the semiconducting layers thus hâvesubstantially the same coefficients of thermal expansion.

Ethylene-vinyl-acetate copolymers/nitrile rubber (EVA/NBR),butyl graft polyethylene, ethylene-butyl-acrylate copolymers 6 011300 (EBA) and ethylene-ethyl-acrylate copolymers (ΞΕΑ) may alsoconstitute suitabie polymers for the semiconductinç layers.

Even when different types of material are used as base inthe various layers, it is désirable for their coefficientsof thermal expansion to be substantially the same. This isthe case with the combination of the matériels liscec above.

The materials listed above hâve relatively good elasticity,with an E-modulus of E<500 MPa, preferably <200 MPa. Theelasticity is sufficient for any minor différences betweent&amp;e coefficients of .thermal expansion for the materials inthe layers to be absorbed in the radial direction of theelasticity so that no cracks appear, or any other damage,and so that the layers are not released from each other. Thematerial in the layers is elastic, and the adhesion between .the layers is at least of the same magnitude as in theweakest of the materials.

The conductivity of the two semiconducting layers is suffi-cient to substantially equalize the potential along eachlayer. The conductivity of the outer semiconducting layer issufficiently high to enclose the electrical field within thecable, but sufficiently low not to give rise to significantlosses due to currents induced in the longitudinal directionof the layer.

Thus, each of the two semiconducting layers essentiallyconstitutes one equipotential surface, and these layers willsubstantially enclose the electrical field between them.

There is, of course, nothing to prevent one or more addi-tional semiconducting layers being arranged in the insulat-ing layer.

Other characteristics and advantages will become apparent ·from the remaining dépendent daims. 7 011300

In addition to the above mentioned advantages obtair.ed witha winding consisting of a cable, less problems with magneticstray fields are encountered with the use of cable. This hasthe advantage that a toroidal core can be used even in high-voltage transformers, provided that the problem of arranginga sufficiently large core is solved and this is done accord-ing to the invention by using a segmented core. The impor-tant advantage follows that technology can be used which ispreviously known only from the low-voltage range and fieldof electronics.

According to a particularly advantageous feature, it isstated that the winding consists of high-voltage cable.

As another feature it is stated that the high-voltage cablepreferably has a diameter within the interval 20-250 mm anda conductor area within the interval 80-3000 mm2.

According to a particularly advantageous characteristic, thecore is substantially annular. This design has the advantageof providing a shorter magnetic path than a rectangularcore, and better flow distribution in the core. The advan-tages of an annular core with a shorter magnetic path than aconventional core include it requiring less material, itwill be less heavy and less expensive and resuit in lesspower losses and is therefore more efficient.

According to another particularly favourable characteristic,the core has a substantially toroidal shape. In a toroidalcore the coil can be distributed uniformly around the entirecore, thereby reducing the problems of undesired magneticfields. A high degree of symmetry is also favourable sincethe magnetic field diminishes more quickly with the dis-tance . C11300

According to ons embodiment the core of the trans- former /reactor has a window, which is substantially circularin shape and the annular shape of the core is circular.

Alternatively the core may comprise a window which is sub-stantially elliptical and the annular shape of the core iselliptical. The core may also be rectanguiar.

According to an advantageous embodiment the core is composedof two segments. In many cases this is naturally the sim-plest alternative, which per se constitutes an advantage.

According to another. favourable embodiment the core iscomposed of four segments, two straight segments and twosegments shaped as ring halves, the two segments shaped asring halves being joined together via the two straightsegments. This embodiment, as also the elliptical embodi-ment, has the advantage that it can be used even in crampedspaces.

That each segment comprises a plurality of plates and thatthe core is constructed as a laminated core are also statedto be advantageous features.

According to further advantageous features, the plates mayconsist of magnetically oriented Steel and the number ofsegments is sufficiently large for the magnetic orientationdirection not to be lost. Alternatively, the plates mayconsist of amorphous Steel.

According to one embodiment, adjacent segments are heldtogether by one segment having at least one protruding platewhich is fitted into a corresponding gap, between plates,arranged in the corresponding side of the nearest adjacentsegment, thereby forming an overlap joint. This results inthe advantage that no spécial attachment means are requiredto keep the segments forming the core together. Alterna- 9 01Ï300 tively, or by way of supplément, however, the trans-former/reactor may include attachment means.

According to vet ancther advantageous featurs the segmentedcore contains internai ducts which may be used for a cod-ant. According to a particular embodiment of the ccoling-ducts the core segments can be connected thereby.

Finally, the method according to the présent invention ischaracterized by the advantageous feature that the windingsof the core are wound onto the segment before the segment isas-sembled to form the core.

The invention is preferably intended for single phase trans-former .

As a summary, it should be stressed that, through the combi-nation of a winding as defined in claim 1 and a segmentedcore, it is made possible by the présent invention to pro-vide dry transformers/reactors for high voltages, with largecores of a substantially annular shape, and preferablytoroidal shape.

For a better understanding of the invention, four embodi-ments will now be described in detail, by way of example,with reference to the accompanying drawings in which:

Figure 1 shows a basic diagram in the form of a schematicview in perspective of a first embodiment of theinvention,

Figure 2 shows a schematic the invention, view of a second embodiment of Figure 3 shows a schematic invention, view of a third embodiment of the Figure 4 shows a schematic the invention, view of a fourth embodiment of 10 011300

Figure 5 shows a section through a segment of a core accord-ing to the présent invention, and

Figure 6 shows a cross-sectional view of a r.igh-voltagecable. A basic diaaram of the présent invention, also constitutinga first embodiment, is shown schematicaliy in Figure 1. Thefigure illustrâtes a transformer core 1, which could eauallywell be a reactor core, provided with a winding 2 passingthrough a substantially circular window 5. The core is builtfrom a relatively large number of segments 4, for which onlyone reference number is being used. The segments are pref-erably identical since this is an advantage from the manu-facturing point of view, but could be shaped with somedifférences if suitable. The figure shows eighteen segments,each segment consisting of a number of plates 3 which hâvebeen stacked one on top of the other in a known manner. Anexample of how these plates can be stacked on top of eachother is shown in Figure 5, illustrating a section through acore segment. The plates are normally glued together. Bystacking the plates on top of each other a so called lami-nated core is obtained. Different joining methods may beused, of which only one possible method is being illustratedin Figure 5. Another possible method is known as step lap,for instance.

The individual plates illustrated in the embodiment inFigure 1 hâve a shape corresponding to a parallel trapezium.This means that the "annular" shape of the core is in fact apolygon. However, with a relatively large number of seg-ments, as in this case, an annular shape, or toroidal as isthe case with the cross section of the core, is approximatedwith a polygonal shape.

It should be emphasized that the terms "annular, circularwindow and toroidal", which comprises a circular crosssection, and ail of which refer to the core, these terms 011300 refer in this context not only to a geometricai'ly perfectring, torus or circle, but should also be considered asincluding the approximate équivalents to these géométriefigures due to the fact that the core, becauss of the seg-ments, may hâve a through-section both in transverse andlongitudinal direction that is in fact a polygon.

Figure 2 illustrâtes a second embodiment of the invention inthe form of a core 11 with segments 14, seen from above.According to the embodiment in Figure 2, the segments hâve ashape similar to a pie piece with a truncated tip so thatthey may be combined to an approximate ring, preferably witha toroidal shape. Each plate 3 in Figure 5 is thus eut tofit the pie piece shape shown in Figure 2. In this case thecore 11 is composed of eight segments 14. The segments inthis core are built from plates of magnetically orientedSteel, as illustrated by arrows in the Figure. When magneti-cally oriented steel is used it is important that the numberof segments is sufficient for the magnetic direction oforientation not to be lost. Here too, the core has a circu-lar window 15 through which the winding or windings areintended to pass. A third embodiment of the core is shown in Figure 3. Thesegmented core 21 consists of only two segments in the formof two ring halves 23, 24 which hâve been combined to a corewith a substantially circular window 25.

The fourth embodiment is illustrated in Figure 4, from whichis seen that the core 31 preferably comprises four segments:two straight segments 36, 37 and two segments 33, 34 in theform of half rings. The two segments 33, 34 in the form ofring halves are connected via the two straight segments 36,37. The core has a window 35.

The segments can be held together or combined in variousways to form the annular core. It is thus feasible to con- 12

θ 11 3 CO figure the segments with some plates protruding outside theactual side of the segment, i.e. the side facing an adjacentsegment, and which are inserted into corresponding gaps,between plates, arranged in the corresponding side of thenearest adjacent segment, and vice versa, so that plates inadjacent segments overlap. A joint is thus obtained betweenthe plates in two adjacent segments, which is formed in anéquivalent way to the example of joints formed inside asegment which is illustrated in Figure 5. Alternatively,spécial attachment means may be used, such as clamps, yokes,screws of the like.

One advantage of a segmented core is that it may includeinternai ducts for a codant. These ducts may consist ofinterspaces 17, which hâve been provided between the platesduring lamination. Alternatively, tubes for a coolant may beinstalled in the segments during lamination of the plates.Another alternative is to subsequently drill ducts throughthe segments. It would also be possible for the segments tobe held toget'ner by the internai cooling ducts, in such away that adjacent segments are being held together by atleast one segment being provided with a cooling duct termi-nating in a protruding pipe end shaped to be fitted to acorresponding pipe end terminating the cooling duct in anadjacent segment.

Figure 6, finally, shows a section through a high-voltagecable 6 particularly suitable -for use in the invention. Thehigh-voltage cable 6 comprises a number of strands 7 made ofcopper (Cu), for instance, and having circular cross sec-tion. These strands are arranged in the middle of the high-voltage cable. Surrounding the strands 7 is a first semi-conducting layer 8. Surrounding this first semi-conductinglayer 8 is an insulating layer 9, e.g. XLPE insulation.Surrounding the insulating layer 9 is a second semi-conducting layer 10 provided. The cable illustrated differsfrom conventional high-voltage cable in that the outer,

I 13 011300 mechanically protective sheath and the matai screen thatnormally surround such cables are eliminated. Thus theconcept "high-voltage cable" in the présent application doesnot necessarily include the metallic screen or the sheaththat normally surround such cables for power distribution.

The embodiments illustrated and described above shall beconsidered only as examples and the invention shall not belimited thereto, but can be varied within the scope of theinventive concept as defined in the appended daims. Thus,the window in the cores of three of the examples illustratedhas been shown only with substantially circular form, butmay of course also be elliptical or some other shape. Simi-larly, the annular shape of the core may be ellipticalinstead of circular. This may be préférable, for instance,when the available space is limited widthwise. Furthermore,there is naturally nothing to prevent a segmented core beingmade rectangular, with a rectangular window.

The number of segments may also vary greatly depending onmany different considérations with regard to manufacturingtechnique, winding technique, transport distance, etc. Theplates may also be made of Steel other than magneticallyoriented Steel, e.g. amorphous Steel.

Finally, it should be mentioned that the invention is natu-rally also applicable to a three-phase transformer/reactorby combining three cores constructed in accordance with theinvention.

Claims (26)

1. A cransformer/reactor comprisinç a core and at leastone winding, in which the core (1; 11; 21; 31) Consista ofat least two segments (4; 14; 24, 25; 33, 34, .36, 37), andthe winding is flexible and comprises an electrically con-ducting core (7) surrounded by an inner semiconducting layer(8), an insulating layer (9) surrounding the inner semicon-ducting layer and consisting of solid matériel, and an outersemiconducting layer (10) surrounding the insulating layer,said layers adhering to each other.

2. A transformer/reactor as claimed in claim 1, characterized in that said layers (8, 9, 10) con-sist of materials having such elasticity and with such arelation between the coefficients of thermal expansion ofthe materials that the changes in volume in the layerscaused by température fluctuations during operation can beabsorbed by the elasticity of the materials, the layers thusretaining their adhesion to each other upon the températurefluctuations that occur during operation.

3. A transformer/reactor as claimed in claim 2,characterized in that the materials in said layers(8, 9, 10) hâve high elasticity, preferably with an E-modulus less than 500 MPa, most preferably less than 200 MPa.

4. A transformer/reactor as claimed in claim 3,characterized in that the coefficients of thermalexpansion for the materials in said layers (8, 9, 10) are ofsubstantially the same magnitude.

5. A transformer/reactor as claimed in claim 4,characterized in that the adhesion between thelayers (8, 9, 10) is of at least the same magnitude as inthe weakest of the materials. 15 011300

6. A cransformer/reactor as claimed in ciaim 1' or· daim2, character ized in that each of the semiconduct-ing layers (8, 10) essentially constituées one equipotential 5 surface.

7. A transformer/reactor as claimed in any of daims1-6, c h a r a c t e r i z e ci in that the windings of thestator (4) consist of high-voltage cable (6). 10

8. A transformer/reactor as claimed in claim 7,éharacter i zed in that the high-voltage cable (6)has a diameter within the interval 20-250 mm and a conductorarea within the interval 80-3000 mm^. 15

9. A transformer/reactor as claimed in any o'f the pre-ceding daims, characterized in that the core (1;11; 21) is substantially annular. 20

10. A transformer/reactor as claimed in claim 9, char- acterized in that the core (1; 11; 21) comprises awindow (5; 15; 25) having substantially circular shape andthat the annular shape of the core is circular. 25

11. A transformer/reactor as claimed in claim 9, char- acterized in that the core comprises a window whichis substantially elliptical and that the annular shape ofthe core is elliptical. 30

12. A transformer/reactor as claimed.in any of daims 1-8, characterized in that in that the core (31)comprises four segments, two straight segments (36, 37) andtwo segments (33, 34) shaped as ring halves, the two seg-ments shaped as ring halves being joined together via the 35 two straight segments. 16 011300

13. A transformer/reactor as ciaimed in any of the pre-ceding daims, c h a r a c t e r i z e d in that the çore (1;11; 21) has a substantially toroidal form.

14. A transformer/reactor as ciaimed in any of daims1-8, characterized in that the core has a rectangular form.

14 0113Û0 CLAIMS

15. A transformer/reactor as ciaimed in any of daims1-11 or 13-14, c h a r a c t e r i z e d in that the core (21)consists of two segments (23, 24).

16. A transformer/reactor as ciaimed in any of the pre-ceding daims, characterized in that each segmentcomprises a plurality of plates (3) and in that the core isconstructed as a laminated core.

17. A transformer/reactor as ciaimed in daim 16,characterized in that the plates (3) consist ofmagnetically oriented steel.

18. A transformer/reactor as ciaimed in daim 17,characterized in that the number of segments issufficiently large for the magnetic orientation directionnot to be lost.·

19. A transformer/reactor as ciaimed in daim 16,characterized in that the plates (3) consist ofamorphous Steel.

20. A transformer/reactor as ciaimed in any of daims16-19, characterized in that the adjacent segmentsare held together by one segment having at least one pro-truding plate which is fitted into a corresponding gapbetween plates arranged in the corresponding side of thenearest adjacent segment, thereby forming an overlap joint. 17 011όϋϋ

21. A transformer/reactor as claimed in any of thepre-ceding daims, characterized in chat it comprisesattachment devices, preferably clamps or screws in order tojoin the segments.

22. A transformer/reactor as claimed in any of the pre-ceding daims, characterized in that the segmented.core contains internai ducts (17) for a codant.

23. A transformer/reactor as claimed in claim 22,characterized in that adjacent segments are heldtbgether by at least one segment being provided with acooling duct (17) terminating in a protruding tube enddesigned to be fitted, to a corresponding tube end terminat-ing the cooling duct in an adjacent segment.

24. A transformer/reactor as claimed in âny of the pre-ceding daims, characterized in that the trans-former is a dry transformer/reactor.

25. A method for use in the manufacturing of a trans-former/reactor comprising a core and at least one winding,comprising the step of manufacturing a core including atleast two segments which are joined to form said core, andcomprising the step of installing a winding onto said core,said winding being flexible and composed of an electricallyconducting core (7) surrounded by an inner semiconductinglayer (8), an insulating layer (9) surrounding the innersemiconducting layer and consisting of solid material, andan outer semiconducting layer (10) surrounding the insulat-ing layer, said layers adhering to each other.

26. A method as claimed in claim 25, characterizedin that the windings of the core are wound onto thesegment before the segment is assembled to form the core.