POWER TRANSFORMER/INDUCTOR

Abstract: The present invention relates to a power transformer/inductor comprising at least one winding. The windings are designed by means of a high voltage cable (10), comprising an electric conductor, and around the conductor there is arranged a first semiconducting layer (14), around the first semiconducting layer (14) there is arranged an insulating layer (16) and around the insulating layer (16) there is arranged a second semiconducting layer (18). The second semiconducting layer (18) is directly earthed at both ends of each winding (221, 222) and furthermore at least at two points per turn of every winding (221, 222), whereby one or several points are indirectly earthed.

Description: POWER TRANSFORMER/INDUCTOR Technical field The present invention relates to a power transformer/inductor . In all transmission and distribution of electric energy transformers are used for enabling exchange between two or more electric systems normally having different voltage levels. Transformers are available for powers from the VA region to the 1000 VA region. The voltage range has a spectrum of up to the highest transmission voltages used today. Electromagnetic induction is used for energy transmission between electric systems. Inductors are also an essential component in the transmission of electric energy in for example phase compensation and filtering. The transformer/inductor related to the present invention belongs to the so-called power transformers/inductors having rated outputs from several hundred kVA to in excess of 1000 MVA and rated voltages of from 3-4 kV to very high trans is- sion voltages. Background Art Generally speaking the main object of a power transformer is to enable the exchange of electric energy, between two or more electric systems of mostly differing voltages with the same frequency. Conventional power transformers/inductors are e.g. described in the book "Elektriska Maskiner" by Fredrik Gustavson, page 3-6 - 3-12, published by The Royal Institute of Technology, Sweden, 1996. A conventional power transformer/inductor comprises a transformer core, referred to below as core, formed of laminated commonly oriented sheet, normally of silicon iron. The core is composed of a number of core legs connected by yokes. A number of windings are provided around the core legs normally referred to as primary, secondary and regulating winding. In power transformers these windings are practically always arranged in concentric configuration and distributed along the length of the core leg. Other types of core structures occasionally occur in e.g. so-called shell transformers or in ring-core transformers. Examples related to core constructions are discussed in DE 40414. The core may consist of conventional magnetizable ma- terials such as said oriented sheet and other magnetizable materials such as ferrites, amorphous material, wire strands or metal tape. The magnetizable core is, as known, not necessary in inductors. The above-mentioned windings constitute one or several coils connected in series, the coils of which having a number of turns connected in series. The turns of a single coil normally make up a geometric, continuous unit which is physically separated from the remaining coils. A conductor is known through US 5 036 165, in which the insulation is provided with an inner and an outer layer of semiconducting pyrolized glassfiber. It is also known to provide conductors in a dynamo-electric machine with such an insulation, as described in US 5 066 881 for instance, where a semiconducting pyrolized glassfiber layer is in contact with the two parallel rods forming the conductor, and the insulation in the stator slots is surrounded by an outer layer of semiconducting pyrolized glassfiber. The pyrolized glassfiber material is described as suitable since it retains its resistivity even after the impregnation treatment. The insulation system, partly on the inside of a coil / winding and partly between coils/windings and remaining metal parts, is normally in the form of a solid- or varnish based insulation and the insulation system on the outside is in the form of a solid cellulose insulation, fluid insulation , and possibly also an insulation in the form of gas. Windings with insulation and possible bulky parts represent in this way large volumes that will be subjected to high electric field strengths occurring in and around the active electric magnetic parts belonging to transformers. A detailed knowledge of the properties of insulation material is required in order to predetermine the dielectric field strengths which arise and to attain a dimensioning such that there is a minimal risk of electrical discharge. It is important to achieve a surrounding environment which does not change or reduce the insulation properties. Today' s predominant outer insulation system for conventional high voltage power transformers/inductors consists of cellulose material as the solid insulation and transformer oil as the fluid insulation. Transformer oil is based on so-called mineral oil. Conventional insulation systems are e.g. described in the book "Elektriska Maskiner" by Fredrik Gustavson, page 3-9 - 3-11, published by The Royal Institute of Technology, Sweden, 1996. Additionally, a conventional insulation system is rela- tively complicated to construct and special measures need to be taken during manufacture in order to utilize good insulation properties of the insulation system. The system must have a low moisture content and the solid phase in the insulation system needs to be well impregnated with the sur- rounding oil so that there is minimal risk of gas pockets. During manufacture a special drying process is carried out on the complete core with windings before it is lowered into the tank. After lowering the core and sealing the tank, the tank is emptied of all air by a special vacuum treatment before being filled with oil. This process is relatively time-consuming seen from the entire manufacturing process in addition to the extensive utilization of resources in the workshop. The tank surrounding the transformer must be constructed in such a way that it is able to withstand full vacuum since the process requires that all the gas be pumped out to almost absolute vacuum which involves extra material consumption and manufacturing time. Furthermore the installation requires vacuum treatment to be repeated each time the transformer is opened for inspection. Summary of the invention According to the present invention the power transformer/ inductor comprises at least one winding in most cases arranged around a magnetizable core which may be of different geometries. The term "windings" will be referred to below in order to simplify the following specification. The windings are composed of a high voltage cable with solid insulation. The cables have at least one centrally situated electric conductor. Around the conductor there is arranged a first semiconducting layer, around the semiconducting layer there is arranged a solid insulating layer and around the solid insulating layer there is arranged a second external semiconducting layer. The use of such a cable implies that those regions of a transformer/inductor which are subjected to high electric stress are confined to the solid insulation of the cable. Remaining parts of the transformer/inductor, with respect to high voltage, are only subjected to very moderate electric field strengths. Furthermore, the use of such a cable eliminates several problem areas described under the background of the invention. Consequently a tank is not needed for insulation means and coolant. The insulation as a whole also becomes substantially simple. The time of construction is considerably shorter compared to that of a conventional power transformer/inductor. The windings may be manufactured separately and the power transformer/inductor may be assembled on site. However, the use of such a cable presents new problems which must be solved. The semiconducting outer layer must be directly earthed at or in the vicinity of both ends of the cable so that the electric stress which arises, both during normal operating voltage and during transient progress, will primarily load only the solid insulation of the cable. The semi-conducting layer and these direct earthings form together a closed circuit in which a current is induced during operation. The resistivity of the layer must be large enough so that resistive losses arising in the layer are negligible. Besides this magnetic induced current a capacitive current is to flow into the layer through both directly earthed ends of the cable. If the resistivity of the layer is too high, the capacitive current will become so limited that the potential in parts of the layer, during a period of alternating stress, may differ to such an extent from earth poten- tial that regions of the power transformer/inductor other than the solid insulation of the windings will be subjected to electric stress. By directly earthing several points of the semiconducting layer, preferably one point per turn of the winding, the whole outer layer will remain at earth po- tential and the elimination of the above-mentioned problems is ensured if the conductivity of the layer is high enough. This one point earthing per turn of the outer screen is performed in such a way that the earth points rest on a genera- trix to a winding and that points along the axial length of the winding are electrically directly connected to a conducting earth track which is connected thereafter to the common earth potential. In extreme cases the windings may be subjected to such rapid transient overvoltage that parts of the outer semiconducting layer carry such a potential that areas of the power transformer other than the insulation of the cable are subjected to undesirable electric stress. In order to prevent such a situation, a number of non-linear elements, e.g. spark gaps, phanotrons, Zener-diodes or varistors are connected in between the outer semiconducting layer and earth per turn of the winding.Also by connecting a capacitor in between the outer semiconducting layer and earth a non-desirable elec- trie stress may be prevented from arising. A capacitor reduces the voltage even at 50 Hz. This earthing principle will be referred to below as "indirect earthing". In the power transformer/inductor in accordance with the present invention, the second semiconducting layer is directly earthed at both ends of each winding and is indirectly earthed at at least one point between both the ends. The individually earthed earthing tracks are connected to earth via either, l.a non-linear element, e.g. a spark gap or a phanotron, 2. a non-linear element parallel to a capacitor, 3. a capacitor or a combination of all three alternatives. In a power transformer/inductor according to the invention the windings are preferably composed of cables having solid, extruded insulation, of a type now used for power distribu- tion, such as XLPE-cables or cables with EPR-insulation. Such cables are flexible, which is an important property in this context since the technology for the device according to the invention is based primarily on winding systems in which the winding is formed from cable which is bent during assembly. The flexibility of a XLPE-cable normally corresponds to a radius of curvature of approximately 20 cm for a cable 30 mm in diameter, and a radius of curvature of approximately 65 cm for a cable 80 mm in diameter. In the present application the term "flexible" is used to indicate that the winding is flexible down to a radius of curvature in the order of four times the cable diameter, preferably eight to twelve times the cable diameter. Windings in the present invention are constructed to retain their properties even when they are bent and when they are subjected to thermal stress during operation. It is vital that the layers of the cable retain their adhesion to each other in this context. The material properties of the layers are decisive here, particularly their elasticity and relative coefficients of thermal expansion. In a XLPE-cable, for instance, the insulating layer consists of cross-linked, low-density polyethylene, and the semiconducting layers con- sist of polyethylene with soot and metal particles mixed in. Changes in volume as a result of temperature fluctuations are completely absorbed as changes in radius in the cable and, thanks to the comparatively slight difference between the coefficients of thermal expansion in the layers in rela- tion to the elasticity of these materials, the radial expansion can take place without the adhesion between the layers being lost. The material combinations stated above should be considered only as examples. Other combinations fulfilling the conditions specified and also the condition of being semiconducting, i.e. having resistivity within the range of 10 ~ 1-10^ ohm-cm, e.g. 1-500 ohm-cm, or 10-200 ohm-cm, naturally also fall within the scope of the invention. The insulating layer may consist, for example, of a solid thermoplastic material such as low-density polyethylene (LDPE) , high-density polyethylene (HDPE) , polypropylene (PP) , polybutylene (PB), polymethyl pentene (PMP), cross- linked materials such as cross-linked polyethylene (XLPE) , or rubber such as ethylene propylene rubber (EPR) or silicon rubber. The inner and outer semiconducting layers may be of the same basic material but with particles of conducting material such as soot or metal powder mixed in. The mechanical properties of these materials, particularly their coefficients of thermal expansion, are affected relatively little by whether soot or metal powder is mixed in or not - at least in the proportions required to achieve the conductivity necessary according to the invention. The insu- lating layer and the semiconducting layers thus have substantially the same coefficients of thermal expansion. Ethylene-vinyl-acetate copolymers/nitrile rubber, butyl graft polyethylene, ethylene-butyl-acrylate-copolymers and ethylene-ethyl-acrylate copolymers may also constitute suitable polymers for the semiconducting layers. Even when different types of material are used as base in the various layers, it is desirable for their coefficients of thermal expansion to be substantially the same. This is the case with combination of the materials listed above. The materials listed above have relatively good elasticity, with an E-modulus of E < 500 MPa, preferably < 200 MPa . The elasticity is sufficient for any minor differences between the coefficients of thermal expansion for the materials in the layers to be absorbed in the radial direction of the elasticity so that no cracks or other damage appear and so that the layers are not released from each other. The mate- rial in the layers is elastic, and the adhesion between the layers is at least of the same magnitude as the weakest of the materials. The conductivity of the two semiconducting layers is sufficient to substantially equalize the potential along each layer. The conductivity of the outer semiconducting layer is sufficiently large to contain the electrical field in the cable, but sufficiently small not to give rise to significant losses due to currents induced in the longitudinal direction of the layer. Thus, each of the two semiconducting layers essentially con- stitutes one equipotential surface, and these layers will substantially enclose the electrical field between them. There is, of course, nothing to prevent one or more additional semiconducting layers being arranged in the insulat- ing layer. The above indicated and other advantageous embodiments of the present invention are stated in the dependent claims. The invention will now be described in more detail in the following description of preferred embodiments with particular reference to the accompanying drawings. Brief description of the drawings Figure 1 shows a cross-sectional view of a high voltage cable; Figure 2 shows a perspective view of windings with three in- direct earthing points per winding turn according to a first embodiment of the present invention; Figure 3 shows a perspective view of windings with one direct earthing point and two indirect earthing points per winding turn according to a second embodiment of the present invention; Figure 4 shows a perspective view of windings with one di- rect earthing point and two indirect earthing points per winding turn according to a third embodiment of the present invention; and Figures 5 shows a perspective view of windings with one di- rect earthing point and two indirect earthing points per winding turn according to a fourth embodiment of the present invention . Detailed description of the embodiments of the invention Figure 1 shows a cross-sectional view of a high voltage cable 10 which is used traditionally for the transmission of electric energy. The shown high voltage cable may for example be a standard XLPE cable 145 kV but without mantle and screen. The high voltage cable 10 comprises an electric conductor, which may comprise one or several strands 12 with circular cross-section of for example copper (Cu) . These strands 12 are arranged in the center of the high voltage cable 10. Around the strands 12 there is arranged a first semiconducting layer 14. Around the first semiconducting layer 14 there is arranged a first insulating layer 16, for example XLPE insulation. Around the first insulating 16 there is arranged a second semiconducting layer 18. The high voltage cable 10, shown in Figure 1 is manufactured with a conductor area of between 80 and 3000 mm 2 and with an outer cable diameter of between 20 and 250 mm. Figure 2 shows a perspective view of windings with three indirect earthing points per winding turn according to a first embodiment of the present invention. Figure 2 shows a core leg designated by the numeral 20 within a power transformer or inductor. Two windings 22ι and 22 2 are arranged around the core leg 20 which are formed from the high-voltage cable (10) shown in Figure 1. With the aim of fixing windings 22 x and 22 2 there are, in this case six radially arranged spacer members 24ι, 24 2 , 24 3 , 24 , 24 5 , 24 6 , per winding turn. As shown in Figure 2 the outer semiconducting layer is earthed at both ends 26ι, 26 2 ; 28ι, 28 2 of each winding 22 x , 22 2 . Spacer members 24 x , 24 3 , 24 5 , which are emphasised in black, are utilised to achieve, in this case, three indirect earthing points per winding turn. The spacer member 24ι is di- rectly connected to a first earthing element 30ι, spacer member 24 3 is directly connected to a second earthing element 30 2 and spacer member 24 5 is directly connected to a third earthing element 30 3 at the periphery of the winding 22 2 and along the axial length of the winding 22 2 . Earthing elements 30ι, 30 2 , 30 3 may for example be in the form of earthing tracks 30ι - 30 3 . As shown in Figure 2 the earthing points rest on a generatrix to a winding. Each and every one of the earthing elements 30ι - 30 3 is directly earthed in that they are connected to earth via their own capacitor 32ι, 32 2 , 32 3 . By earthing indirectly in this way any non-desirable electric stress may be prevented from arising. Figure 3 shows a perspective view of windings with one direct earthing point and two indirect earthing points per winding turn according to a second embodiment of the present invention. In Figures 2 and 3 the same parts are designated by the same numerals in order to make the Figures more clear. Also in this case the two windings 22 x and 22 2 , formed from the high-voltage cable 10 shown in Figure 1, are ar- ranged around the core leg 20. Windings 22ι, 22 2 are fixed by means of six spacer members 24ι, 24 2 , 24 3 , 24 4 , 24 5 , 24 6 per winding turn. At both ends 26ι, 26 2 ; 28ι, 28 2 of each winding 22ι, 22 2 the second semiconducting layer (compare with Figure 1) is earthed in accordance with Figure 2. Spacer members 24ι, 24 3 , 24 5 , which are marked in black , are used in order to achieve in this case one direct and two indirect earthing points per winding turn. In the same way as shown in Figure 2 spacer member 24ι is directly connected to a first earth- ing element 30ι, spacer member 24 3 is directly connected to a second earthing element 30 2 and spacer member 24s is directly connected to a third earthing element 30 3 . As shown in Figure 3 earthing element 30ι is directly connected to earth 36, while earthing elements 30 2 , 30 3 are indirectly earthed. Earthing element 3O 3 is indirectly earthed in that it is connected in series to earth via a capacitor 32. Earthing element 3O2 is indirectly earthed in that it is connected in series to earth via a spark gap 34. The spark gap is an example of a non-linear element , i.e. an element with a non- linear voltage current characteristic. Figure 4 shows a perspective view of windings with one direct earthing point and two indirect earthing points per winding turn according to a third embodiment of the present invention. In Figures 2 - 4 the same parts are designated by the same numerals in order to make the Figures more clear. Figure 4 shows windings 22 x , 22 2 , a core leg 20, spacer members 24 x , 24 2 , 24 3 , 24 4 , 24 5 , 24 6 and earthing elements 30ι, 30 2 , 30 3 arranged in the same way as shown in Figure 3 and will therefore not be described in further detail here. Earthing element 30 x is directly connected to earth, while earthing elements 30 2 , 30 3 are indirectly earthed. Earthing elements 302, 30 3 are indirectly earthed in that they are connected in series via their own capacitor. Figure 5 shows a perspective view of windings with one direct earthing point and two indirect earthing points per winding turn according to a fourth embodiment of the present invention. In Figures 2 - 5 the same parts are designated the same numerals in order to make the Figures more clear. Figure 5 shows windings 22χ, 22 2 , core leg 20, spacer members 24ι, 24 2 , 243, 24 4 , 24s, 26δ, end earthing points 26ι, 26 2 ; 26ι, 282 and earthing elements 30 , 30 2 , 30 3 arranged in the same way as shown in Figures 3 and 4 and will therefore not be described in further detail here. Earthing element 30ι is directly connected to earth 36, while earthing elements 30 2 , 30 3 are indirectly earthed. The earthing element 3O2 is indirectly earthed in that it is connected in series to earth via a discharge gap. Earthing element 30 3 is indirectly earthed in that it is connected in series to earth via a circuit, comprising a spark gap 38 connected parallel to a capacitor 40. Only the spark gap in the above shown embodiments of the present invention is shown by way of example. The power transformer/inductor in the above shown Figures comprises a magnetizable core. It should however be under- stood that a power transformer / inductor may be built without a magnetizable core. The invention is not limited to the shown embodiments because several variations are possible within the frame of the attached patent claims.

Claims: CLAIMS 1. A power transformer/inductor comprising at least one winding, characterized in that the winding / windings are composed of a high-voltage cable (10), comprising an electric conductor, and around the conductor there is arranged a first semiconducting layer (14), around the first semiconducting layer (14) there is arranged an insulating layer (16) and around the insulating layer (16) there is arranged a second semiconducting layer (18), whereby the second semiconducting layer (18) is directly earthed at both ends of each winding (22ι, 22 2 ) and that at least one point between both the ends is indirectly earthed. 2. A power transformer/inductor according to claim 1, characterized in that the high-voltage cable (10) is manufactured with a conductor area of between 80 and 3000 mm 2 and with an outer cable diameter of between 20 and 250 mm. 3. A power transformer/inductor according to any one of claims 1 - 2, characterized in that the direct earthing (36) is performed by means of galvanic connection to earth. 4. A power transformer/inductor according to any one of claims 1 - 3, characterized in that indirect earthing is performed by means of a capacitor (32; 32ι - 323) inserted between earth and the second semiconducting layer (18) . 5. A power transformer/inductor according to any one of claims 1 - 3, characterized in that indirect earthing is performed by means of an element (34) with non- linear voltage-current characteristic inserted between the second semiconducting layer (18) and earth. 6. A power transformer/inductor according to any one of claims 1 - 3, characterized in that the indirect earthing is performed by means of a circuit inserted between the second semiconducting layer (18) and earth, the circuit comprising an element with non-linear voltage-current characteristic in parallel to a capacitor (40). 7. A power transformer/inductor according to claim 6 characterized in that indirect earthing is performed by means a combination of alternatives according to claims 4 - 6. 8. A power transformer/inductor according to any one of claims 1 - 7, characterized in that the element with non-linear voltage-current characteristic constitutes a spark gap (36), a gas-filled diode, a Zener-diode or a varistor . 9. A power transformer/inductor according to any one of claims 1 - 8, characterized in that the power transformer / inductor comprises a magnetizable core. 10. A power transformer/inductor according to any one of claims 1 - 8, characterized in that the power transformer / inductor is built without a magnetizable core. 11. A power transformer/inductor according to claim 1, characterized in that the winding / windings are flexible (a) and in that said layers adhere to each other. 12. A power transformer/inductor according to claim 11, characterized in that said layers are of a material with such an elasticity and with such a relation between the coefficients of thermal expansion of the material that during operation changes in volume, due to temperature variations, are able to be absorbed by the elasticity of the material such that the layers retain their adherence to each other during the temperature variations that appear during operation. 13. A power transformer/inductor according to claim 12, ch racterized in that the materials in the said layers have a high elasticity, preferably with an E-module less than 500 MPa and most preferably less than 200 MPa. 14. A power transformer/inductor according to claim 12, characterized in that the coefficients of thermal expansion in the materials of the said layers are substantially equal. 15. A power transformer/inductor according to claim 12, characterized in that the adherence between layers is at least of the same rating as in the weakest of the materials . 16. A power transformer/inductor according to claim 11, or 12, characterized in that each semiconducting layer constitutes substantially an equipotential surface.

Inventors: FROMM UDO, HOERNFELDT SVEN, HOLMBERG PAER, KYLANDER GUNNAR, LEIJON MATS, MING LI, FROMM, UDO, HOERNFELDT, SVEN, HOLMBERG, PAER, KYLANDER, GUNNAR, LEIJON, MATS, MING, LI
Applicants: ASEA BROWN BOVERI, FROMM UDO, HOERNFELDT SVEN, HOLMBERG PAER, KYLANDER GUNNAR, LEIJON MATS, MING LI, ASEA BROWN BOVERI AB, FROMM, UDO, HOERNFELDT, SVEN, HOLMBERG, PAER, KYLANDER, GUNNAR, LEIJON, MATS, MING, LI
Application year: 1998
Main classification: H01F27
IPC codes: H01F27