MXPA99006786A - Method and device in manufacturing a transformer/reactor - Google Patents

Abstract

Gas or water-cooled transformer/reactor and a method for manufacturing the same having at least one gas or water-cooled transformer/reactor provided with winding, the winding comprising a high-voltage cable (111) and at least one additional member (9, 11, 12, 13, 14, 50, 55, 60, 65, 70, 75) is wound into the winding (2, 3, 4) arranged around the legs (22, 23, 24) of the transformer/reactor.

Description

METHOD AND DEVICE IN THE MANUFACTURE OF A TRANSFORMER / REACTOR TECHNICAL FIELD The present invention relates to a power transformer wound with cable, cooled by gas or preferably water and to a process for manufacturing such power transformer wound with cable in the voltage range up to 400 kV.

ANTECEDENTS OF THE TECHNIQUE: Modern power transformers are usually oil cooled. The core consists of a number of outer core wires joined by cylinder heads, and the windings (primary, secondary, control) are immersed in a closed container filled with oil. The heat generated in the coils and the core is eliminated by the oil circulating internally through the coils and core, which transfers the heat to the surrounding air via the walls of the container. The oil circulation can be either forced, the oil is pumped around, or it can be natural, produced by temperature differences in the oil. The circulating oil is cooled externally by arrangements for air cooling or water cooling. The cooling Ref. 030682 By external air it can be either forced or through natural convection. In addition to its role as a heat carrier, the oil also has an insulation function in high-voltage oil-cooled transformers.

Dry transformers are usually cooled by air. These are usually cooled through natural convection, since today dry transformers are used in low power loads. The present technology relates to axial cooling ducts produced by means of a folded winding as described in GB 1,147,049, axial ducts for cooling windings embedded in molding resin as described in EP 83107410.9, and the use of cross current fans at maximum loads as described in SE 7303919-0.

The cooling requirement is greater for a wired winding power transformer. Forced convection is necessary to satisfy the cooling requirement in all windings. Natural convection is not enough to cool the cable windings. A short transport route for heat to the refrigerant is important and efficiently transferred to the refrigerant. Therefore it is important that all windings are in direct contact with sufficient quantities of refrigerant.

From DE-A1-2854520, for example, a resin-encased coil with a highly flexible bent cable is known, particularly a switching coil for a rectifier assembly, provided with winding cooling ducts. However, it is not only a matter of taking into consideration a transformer / reactor provided with high voltage cable with its particular electrical / magnetic problems.

According to US Pat. No. 5,036,165, a conductor having insulation provided with an inner layer and an outer layer of semiconductive pyrolyzed glass fiber is known. It is also known to provide conductors in a dynamoelectric machine with such insulation, for example as described in US 5 066 881, where a layer of semiconducting pyrolyzed fiberglass is in contact with both the parallel rods forming the conductor and the insulation in the stator opening it is surrounded by an outer layer of semiconducting pyrolyzed fiberglass. The pyrolyzed glass fiber material is described as being appropriate, since it maintains its resistivity, even after the impregnation treatment.

OBJECTIVE OF THE INVENTION The object of the invention is to provide a transformer / reactor with a winding method, by means of which additional members are included in the winding in one or more of the spaces formed between each winding turn. The additional members are selected as required and can be cooling tubes for gas or liquid, empty tubes which can be used as desired, grounding arrangements, stabilization compounds, mechanical stabilizers, noise suppression members or transducers of various types.

Another object of the invention is to provide a transformer of the type described in the introduction, which will allow the cooling by gas and preferably water of a power transformer wound with cable. The invention relates to cooling each turn in the windings, the refrigerant is correctly distributed to meet the different cooling requirements of the windings.

The invention also relates to eliminating the use of oil cooling in power transformers and thus achieving internal cooling, which gives result in lower weight and higher filling factor, and therefore lower costs.

BRIEF DESCRIPTION OF THE INVENTION The objects mentioned above are achieved by the method and device according to the invention having the characteristics defined in the amended claims.

The present invention relates to a transformer or reactor comprising a transformer core wound with cable, arranged so that the winding is provided with a cooling duct between each turn of cable. The cooling duct is also arranged to transport water to cool all the turns of the winding in the transformer.

In the device according to the invention, the windings are preferably of a type corresponding to cables with solid extruded insulation currently used for the distribution of power, for example, XLPE cables or cables with EPR insulation. Such a cable comprises an internal conductor composed of one or more parts of wire, an inner semiconductor layer surrounding the conductor, a layer of solid insulation surrounding the inner semiconductor layer, and one. cap external semiconductor that surrounds the insulation layer. Such cables are flexible, which is an essential property in this context, since the technology for the device according to the invention is based mainly on a winding system, in which the winding is carried out with conductors, which They bend during assembly. An XLPE cable usually has a flexibility corresponding to a radius of curvature of approximately 20 centimeters for a cable of 30 millimeters in diameter and a radius of curvature of approximately 65 centimeters for a cable of 80 millimeters in diameter. In this application, the term "flexible" in this way refers to a flexible winding down to a radius of curvature in the order of four times the diameter of the cable, preferably 8-12 times the diameter of the cable.

The winding should be constructed so that it can maintain its properties, even when bending and when subjected to thermal stress during operation. It is extremely important in this context that the layers maintain their adhesion to each other. The material properties of the layers, particularly their elasticity and their relative thermal expansion coefficients are decisive here. In an XLPE cable, for example, the insulation layer is made of degraded low density polyethylene and the semiconductor layer is made of polyethylene with soot and mixed metal particles. The fluctuations in the volume as a result of temperature fluctuations are completely absorbed as changes in radius in the cable and, thanks to the slight difference comparatively in the coefficients of thermal expansion in relation to the elasticity of these materials, the radial expansion 'will be able to present without losing the adhesion of the layers to each other.

The material combinations established above should only be considered as examples. Other combinations meet the specified conditions and also with the condition of being semiconductor, that is, they have resistivity within the range of 10"1 - 106 ohm-cm, for example 1 - 500 ohm-cm, or 10 - 200 ohm-cm , naturally they also fall within the scope of the invention.

The insulation 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 pentane (PMP). ), degraded materials, such as degraded 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 conductive material, such as soot or mixed metal powder.

The mechanical properties of these materials, particularly their coefficients of thermal expansion, are relatively little affected by whether the soot or metal powder is mixed or not - at least in the proportions required to achieve the necessary conductivity according to the invention. The insulation layer and the semiconductor layers in this way have substantially the same coefficients of thermal expansion.

Ethylene-vinyl acetate / nitrile rubber copolymers, butylimp polyethylene, ethylene-butyl-acrylate copolymers and ethylene-ethyl-acrylate copolymers can also constitute suitable polymers for the semiconductor layers.

Even when different types of material are used as a base in the different layers, it is desirable for their coefficients of thermal expansion to be substantially the same. This is the case with the 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 difference between the coefficients of thermal expansion for the materials in the layers that are absorbed in the radial direction of the elasticity, so that no breakage or other damage appears and so that the layers are not released between yes. The material 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 semiconductor layers is sufficient to substantially equalize the potential along each layer. The conductivity of the outer semiconductive layer is sufficiently large to contain the electric field in the cable, but small enough not to cause significant losses due to currents induced in the longitudinal direction of the layer.

In this way, each of the two semiconductor layers essentially constitutes an equipotential surface, and these layers will substantially enclose the electric field therebetween.

Of course, there is nothing to prevent one or more additional semiconductor layers that are arranged in the insulation layer.

The cooling tubes according to the invention consist of electrically insulating material, for example, degraded polyethylene in the form of circular "XLPE tubes", which are altered with the cables, so that the heat is transferred from the cables Up to the cooling tubes mainly through heat conduction. The use of material for the polymer tube avoids problems with induced voltages and parasitic (local) currents in the tubes. Besides which, the polymer tubes are considerably more flexible than the metal tubes. None of the necessary polymer tubes have a circular cross section, but they may have a quadratic or some other cross section, which means that they occupy the entire space between the four adjacent wires. To allow each cooling tube or sleeve to come in and out in each cable layer, each cable layer is wound separately and connected and connected in series afterwards. The space between the cables and the cooling tubes is also filled with a thermally conductive compound.

The transformer core according to the invention can be cooled with either gas or liquid, for example with air from a fan and / or with water circulating in the cooling blocks provided with cooling ducts. Another possible coolant is helium.

The invention also relates to a method of manufacturing a transformer / reactor wound with cable, and to a transformer / reactor manufactured according to the method, wherein additional members consisting of transducers of various typesCooling tubes, grounding devices, stabilization compounds, mechanical stabilizers, noise suppression members or empty tubes can be wound on the winding coil when the cable is wound around the outer wires of the transformer / reactor. The empty tubes can be provided with control or measurement windings, additional magnetic material, extra windings, etc. During the winding of these empty tubes, they can be provided with drag wires.

BRIEF DESCRIPTION OF THE DRAWINGS: The invention will now be described in greater detail with reference to the accompanying drawings.

Figure 1 shows, schematically and partially in section, a three-phase power transformer according to the invention.

Figure 2 schematically shows a section through an iron core coil comprising four modes 2a, 2b, 2c, 2d, according to the present invention.

Figure 3 schematically shows a section through an iron core coil comprising additional embodiments 3a, 3b, 3c, 3d, 3e, 3f, according to the present invention.

DESCRIPTION OF THE INVENTION: Figure 1 shows a power transformer 1 provided with three winding coils 2, 3, 4, each comprising a low voltage winding 6 and a high voltage winding 5. Winding coils 2, 3, 4 are wound around of the outer wires 22, 23, 24, respectively, of an iron core, wherein the outer wires are joined on each side of the coils by an upper and a lower stock 7, 8 in the iron core. The outer wires 22, 23, 24 and the yoke 7, 8 thus form the total iron core of the transformer 1.

Figure 2 shows a cross-sectional view of part of a power transformer wound with high voltage cables 111 for use as a transformer winding according to the present invention. The high voltage cable 111 comprises a number of copper wires (Cu) 112, for example, having a circular cross section. These wires 112 are arranged in the middle of the high voltage cable 111. Around the wires 112 is a first semiconductor layer 113.

Around the first semiconductor layer 113 is an insulation layer 114, for example, an XLPE insulation. Around the insulation layer 114 is a second semiconductor layer 115. In this way the concept "high cable" "voltage" in the present application does not include the outer cover that normally surrounds such cables for power distribution.The high voltage cable 111 is wound around an outer wire 24, which is attached to the other outer wires in the transformer by a stock 7.

When the high voltage cable is wound with the straight cable, as shown in the embodiment according to Figure 2, a space 8 is formed between the cables, this space is defined by the cylindrical cover surfaces, ie the second semiconductor layer 115, of the four adjacent cables 111. Space 8 is provided with cooling ducts for the liquid phase refrigerant, suitably water, which ducts can be designed in any of four ways in the Figure.

The first embodiment of the transformer, designated 2a, is provided with cylindrical cooling tubes 9 of degraded polyethylene (XLPE tubes), is surrounded by a spacing agent 10 which acts as a thermally conductive compound, which completely fills the space between the cooling tube 9 and the cylindrical cover surfaces of each cable 111.

The second embodiment of the transformer, designated 2b, is provided with quadratic cooling tubes 11, also made of degraded polyethylene (XLPE tubes) and surrounded by a spacing agent 10 which acts as a thermally conductive compound. The spacing agent 10 completely fills the space between the cooling tube 11 and each cable 111. The quadratic shape of the cooling tube 11 allows for greater utilization of the space 8 for cooling purposes.

The third embodiment of the transformer, designated 2c, is provided with concave quadratic cooling tubes 12, the sides having the same curved shape as the cylindrical cables. This shape further minimizes the remaining space between the cooling tube 12 and the cable compared to the second embodiment. In this embodiment, also the cooling tubes 12 are made of degraded polyethylene (XLPE tubes), likewise they are surrounded by a spacing agent 10 which acts as a thermally conductive compound, which completely fills the space 8 between the tube cooling 12 and each cable 111.

In all three embodiments described above, the refrigerant in the cooling water form flows into respective cooling tubes 9, 11, 12. However, a gaseous refrigerant, such as helium is also possible. Other types of liquid refrigerants in the tubes are also possible.

The fourth embodiment of the transformer, designated 2d, is provided with cylindrical XLPE tubes 13 which move in pairs within a quadratic insertion tube 14 of degraded polyethylene (XLPE tubes), the XLPE tubes 13 are surrounded by an agent of spacing 10 inside the insertion tube 14, the insertion tube 14 is also surrounded by a spacing agent 10, which completely fills the space 8 between the cooling tube 9 and each cable 111. The spacing agent 10 acts as a thermally conductive compound both inside and outside the insertion tube 14.

The thermally conductive compound in the form of a spacing agent in all the described embodiments consists of a silicon rubber for curing of one or two components filled with a heat conducting filler material, such as aluminum oxide. In the uncured state, such rheological properties of the material, which is liquid at high cutting rates (pumpable) and which is in the form of a paste at rest, are determined.

In a first method according to the invention, the compound is first sprayed onto the cables, after which the cooling tube 9, 11, 12 or the insertion tube 14 is placed in the opening formed between the winding turns of the cable. The fresh compound is sprayed onto the cooling tube 9, 11, 12 or the insert 14 and another round of cable is wound, and so on. The winding drum rotates during the winding, but may still remain without unwinding of the spacing agent 10.

In a second process according to the invention, a silicon rubber compound for curing is melted or extruded as a spacing agent around the cooling tube 9, 11, 12 or the insert 14. In the cured state, the Composite has such consistency (similar to the pattern clay) that it is molded to fill the remaining space between the wires during winding.

According to Figure 2, furthermore, the iron core is provided with a stock 7 and an outer wire 24, the stock is provided with a longitudinal cooling channel 15.

The cooling requirement is different for the windings and the flow of liquid in the different cooling tubes in this way is also different. A greater flow It is usually required in the tubes located near the low voltage winding in the tubes located near the high voltage winding. To achieve the correct flow distribution, the tubes can have different diameters or be connected in different series and parallel combinations.

Polymer cooling tubes can be made of many materials, such as polyethylene, polypropene, polybutene, polyvinylidene fluoride, polytetrafluoroethylene or filled and reinforced elastomers. Among these, polyethylene with high density, HDPE, is preferred, since its thermal conductivity increases with the increased density. If the polyethylene degrades, which can be achieved by division with peroxide, degradation with silane or degradation by radiation, its ability to withstand pressure at increased temperature increases and at the same time the risk of stress corrosion disappears.

The tubes must be embedded, since the thermal resistance between the tube and the cables will otherwise be too high. To increase the heat transfer between the tube and the winding, the space is filled with a degradable molding compound. This can consist of a polymer, which has low viscosity and can thus be filled with a high percentage of heat conduction filler material before being injected into space, where it is converted to a non-liquid compound by a chemical reaction. Examples of suitable compounds are acyl, epoxy, unsaturated polyester, polyurethane and silicon, the latter being preferred since it is non-toxic. The heat conduction filler material may also comprise the oxides of aluminum, magnesium, iron or zinc, boron or aluminum nitrides, silicon carbide. A mixture of for example aluminum oxide and silicon, ie polydimethyl siloxane with vinyl groups, which react with polydimethyl hydrogen siloxane in the presence of a platinum catalyst, is forced to under pressure in the space between the XLPE tube and the winding, after which the curing is effected by the hydrogen atoms that are added to the vinyl groups.

Figure 3 shows an illustration corresponding to Figure 2, but in which the cooling tubes are combined and replaced by other types of members. The figure indicates that other members are suitable to be wound together with the high voltage cable 111. The high voltage cable is in the same way as those which have been described as embodiments under Figure 2. Also the transformer core / reactor is similar to that shown in Figure 2.

Figure 3a shows a further member in the form of an empty tubular member 50 arranged to be wound together with the high voltage cable 111. This tube 50 is also surrounded by a spacer 10, which in this case can also act as a thermally conductive compound, except that other properties suitable for the additional member may be given. The tube 50 is intended to allow the insertion of several components in the winding, such as extra windings for control and measurement. In other embodiments, the magnetic material can be inserted into the tubes to alter the electrical and / or magnetic properties of the transformer / reactor. It is also possible to "sew" on extra windings of the same type as the high voltage cable described above. In such a method of insertion of a winding, the tube is lubricated with an appropriate agent, for example, foamed water. To facilitate the insertion of several components into the tube 50, this is provided with one or more pull wires 51.

Another embodiment is illustrated as 3b in the Figure, the additional member is arranged as a grounding member 55. This is elliptical in the Figure, but may of course have a different cross-sectional shape.

According to yet another embodiment illustrated in Figure 3c, the additional member is in the form of a compound of stabilization 60, which is stiffer than the surrounding spacer 10 and has a defined shape, even at room temperature during storage.

Figure 3d shows a modality with an additional member in the form of a mechanical stabilizer 65, which can be produced from a number of parts in the form of an approximate arc or as a wire, which can be rolled up.

Figure 3e shows an embodiment with an additional member in the form of a noise suppression member 70, which is star-shaped to absorb mechanical vibrations.

Figure 3f shows an embodiment with an additional member in the form of an electrical transducer 75, which is wound on the winding. The transducer is also provided with conductors, not shown, for connection equipment up to calculation, evaluation and control.

The spacer 10 completely fills the space 8 between each additional member and the surrounding high voltage cables 111 in all of the embodiments described above.

The invention is not limited to the examples shown. Various modifications are feasible within the scope of the invention. The wires need to be symmetrically placed as shown in Figures 2-3, in which case the space between the adjacent windings will have a different appearance and the additional members therefore must be adapted to the shape of the space.

It is noted that in relation to this date, the best method known by the applicant to carry out the aforementioned invention is the conventional one for the manufacture of the objects to which it refers.

Having described the invention as above, the content of the following is claimed as property.

Claims (37)

1. A method of manufacturing a transformer / reactor cooled by gas or water having at least one winding, characterized in that the winding comprises a high voltage cable having an insulated electrical conductor comprising at least one charged conductor, also comprising a first layer with arranged semiconducting properties surrounding the charged conductor, a layer of fixed solid insulation that surrounds the first layer, and a second layer with arranged semiconductive properties surrounding the insulation layer and the transformer / reactor is designed with at least one additional member, which is wound in the winding when the high voltage cable is wound around the outer wires of the transformer / reactor.

2. A method according to claim 1, characterized in that a) a first cable layer is wound, b) the additional members are wound or applied, c) an additional layer of cable is wound, after which steps b) and c) are repeated until the coil is completely coiled.

3. A method according to claim 1, characterized in that the high-voltage cable is wound at the same time, since the additional members are wound or applied.

4. A method of conformance to any of claims 2-3, characterized in that the spacers are placed in the winding before the additional members are wound or applied.

5. A method according to claim 4, characterized in that the spacers are also placed in the winding after the additional members have been wound or applied.

6. A method of conformance to any of claims 1-5, characterized in that the cooling tube is wound as an additional member in the winding.

7. A method of conformance to any of claims 1-5, characterized in that a single tubular member, a grounding member, a stabilizing compound, a mechanical stabilizer, a noise suppression member or an electrical transducer is wound as a additional member in the winding.

8. A method according to claim 6, characterized in that the water flows through the cooling tube.

9. A method according to claim 8, characterized in that a) a thermally conductive compound is sprayed between the spaces formed between the turns of the winding after the first layer of cable has been wound. b) the additional thermally conductive compound is sprayed onto the cooling tube or insulation tube after the cooling tube or insulation tube has been wound in the spaces formed between the winding turns.

10. A method according to claim 8, characterized in that a) a cooling tube or an insulation tube, which has been extruded or embedded in a thermally conductive compound, is wound on the cable after the first layer of cable has been wound, b) the thermally conductive composite is formed to be adjacent to the cables when the second layer of cable is wound again.

11. A transformer / reactor cooled by gas or water having at least one winding wound around the outer wires of the transformer / reactor, characterized in that the winding is carried out using an insulated electrical conductor comprising at least one charged conductor, also comprising a first layer with arranged semiconductive properties surrounding the charged conductor, a solid insulation layer arranged around the first layer, and a second layer with arranged semiconductive properties surrounding the insulation layer and the device comprises at least one additional member wound on the winding.

12. A transformer / reactor according to claim 11, characterized in that the high voltage cable is cylindrical in shape.

13. A transformer / reactor according to any of claims 11-12, characterized in that at least one additional member is placed in the space formed between each cable during the winding process.

14. A transformer / reactor according to any of claims 11-12, characterized in that the additional member consists of at least one cooling tube placed in a space formed between each cable during the winding process, the space remaining between the tubes of Cooling and the cables are filled with a thermally conductive compound.

15. A transformer / reactor according to claim 14, characterized in that the cooling tube with a circular cross section is placed in the space.

16. A transformer / reactor according to claim 14, characterized in that the cooling tube with a quadratic cross-section is placed in the space.

17. A transformer / reactor according to claim 14, characterized in that a quadratic cooling tube with concave sides is placed in the space.

18. A transformer / reactor according to claim 14, characterized in that the cooling tube is surrounded by an insertion tube containing filler material.

19. A transformer / reactor according to any of claims 11-12, characterized in that the additional member consists of a single tubular member, a grounding member, a stabilization compound, a mechanical stabilizer, a noise suppression member or An electrical transducer in a space between each cable, the space that remains between the additional member and the cables is filled with a thermally conductive compound.

20. A transformer / reactor according to any of claims 14-19, characterized in that the thermally conductive compound has a low viscosity at a high cutting ratio and is in the form of a paste at rest.

21. A transformer / reactor according to claim 20, characterized in that the thermally conductive compound consists of a silicon rubber for curing one or two components provided with a heat conducting filler material.

22. A transformer / reactor according to any of claims 14-21, characterized in that the filling material consists of either aluminum oxide, boron nitride or silicon carbide.

23. A transformer / reactor according to any of claims 14-18 or 20-22, characterized in that the cooling tube is made of dielectric material, such as polyethylene, polypropene, polybutene, polyvinylidene fluoride, polytetrafluoroethylene or filled and reinforced elastomers .

24. A transformer / reactor according to claim 14-18 or 20-23, characterized in that the cooling tube is made of high density polyethylene (HDPE).

25. A transformer / reactor according to any of claims 14-18 or 20-24, characterized in that the cooling tube is made of degraded polyethylene (XLPE).

26. A transformer / reactor according to any of claims 11-25, characterized in that the high voltage cable is of a type comprising a conductor with a plurality of wire parts, a semiconductor layer surrounding the conductor, an insulation layer surrounding the inner semiconductor layer, and an outer semiconductor layer surrounding the insulation layer.

27. A transformer / reactor according to claim 26, characterized in that the high voltage cable has a diameter in the range of 20-250 millimeters and a conduction area in the range of 40-3000 mm2.

28. A transformer / reactor according to any of claims 11-27, characterized in that the insulated conductor or high voltage cable is flexible.

29. A transformer / reactor according to claim 28, characterized in that the layers are arranged to adhere to each other, even when the insulated conductor or high voltage cable is bent.

30. A transformer / reactor according to any of claims 11-29, characterized in that at least two adjacent layers of the winding have coefficients of thermal expansion of substantially the same magnitude.

31. A transformer / reactor according to any of claims 14-30, characterized in that all the refrigerant, in the form of gas or liquid, designed to cool the transformer / reactor, is arranged to flow through the tube cooling.

32. A transformer / reactor according to any of claims 11-31, characterized in that the winding is flexible and comprises an electrically conductive core surrounded by an internal semiconductor layer, an insulating layer of solid material surrounding the inner semiconductor layer, and a external semiconductor layer surrounding the insulation layer, whose layers adhere to each other.

33. A transformer / reactor according to any of claims 11-32, characterized in that the layers are of materials having such elasticity and such a relationship between their coefficients of thermal expansion that fluctuations in volume in the layers caused by temperature fluctuations during the operation they can be absorbed by the elasticity of the materials, so that the layers maintain their adhesion to each other in the temperature fluctuations that occur during the operation.

34. A transformer / reactor according to any of claims 11-33, characterized in that the materials in the layers have high elasticity, preferably with an E modulus of < 500 MPa, more preferably < 200 MPa.

35. A transformer / reactor according to any of claims 11-34, characterized in that the coefficients of thermal expansion of the materials in the layers are of substantially the same magnitude.

36. A transformer / reactor according to any of claims 11-35, characterized in that the adhesion between the layers is at least the same order of magnitude as in the weakest of the materials.

37. A transformer / reactor according to any of claims 11-36, characterized in that each semiconductor layer essentially constitutes an equipotential surface.