KR20010032376A - A power transformer - Google Patents

Abstract

Having at least one electric winding with electrically conductive means 13-15, cooling means for cooling them to improve conductivity of the electrically conductive means, and electrically insulating means 20-22 surrounding them. As a power transformer (1), said electrically insulating means comprise an inner layer (20) of semiconductive material in electrical contact with said electrically conductive means, an outer layer (21) of semiconductive material having a controlled electrical potential along its length, and And an intermediate layer 22 of electrically insulating material between the inner and outer layers 20, 21.

Description

Power Transformer {A POWER TRANSFORMER}

Technical field

The present invention relates to a power transformer having one or more electrical windings having electrical conductive means, cooling means for cooling them to improve the electrical conductivity of the electrical conductive means, and electrical insulating means surrounding them. In particular, the conductive means has a superconductivity, and the present invention relates to a superconducting power transformer having an output power from several hundred kVA to 1000 MVA, and a voltage from 3-4 kV to a very high transmission voltage such as 400 kV to 800 kV. will be. Although the present invention mainly relates to a core type transformer, the present invention may also be related to a shell type transformer and another type of transformer such as a coreless transformer such as an air core.

Background of the Invention

Type II superconductors (for example, niobium titanium and NbTi) have a property of gradually changing from a superconducting state to a steady state of resistance when an external magnetic field increases. Rather than moving directly to a steady state, these materials are brought into a second state, called a vortex or mixed state, in which some magnetic induction (B) penetrates the material in the form of flux lines and direct current flows. When some loss occurs. As the applied magnetic field increases, more and more magnetic induction penetrates the material until the superconductor saturates and becomes normal in a given field called H _c2 . The physical properties of type II superconductors can be summarized in graphs of temperature, magnetic field and current density as shown in FIG. All known superconductors related to the power portion are type II superconductors operating in a mixed state.

Thermodynamic equilibrium in a superconductor is achieved when magnetic induction is uniformly distributed, i.e., when a current passes through the superconductor. As the magnetic induction changes and current flows, undesirable measurable energy losses occur in power applications. Substances whose magnetic fields change and reach equilibrium are known as ″ reversible ″ or ″ soft ″. Substances whose magnetic fields do not change (referred to as ″ pinned ″ magnetic fields) are called ″ irreversible ″ or ″ hard ″ type II superconductors. do.

When a current passes through a superconductor in a magnetic field, the Lorentz force (F; product of current density (J) and magnetic induction (B)) attempts to push the flux lines outward. As the current density and / or the magnetic induction increases, the Lorentz force is increased until the pinning force is exceeded and the flux lines begin to move, thereby consuming energy. The point at which the flux lines start to move is called the critical current density J _c , which is dependent on magnetic induction and temperature as shown in FIG. 1. Typically, T _c is expressed as a threshold temperature or transition temperature for a zero applied magnetic field and zero current density. Similarly, it is common to express H _c as the critical magnetic field and zero current density for zero temperature. However, Jc typically represents a critical current density under actual operating conditions such as 77 K in a magnetic field of 1T.

Conventional ″ low temperature ″ type II superconductors operating at temperatures near absolute zero have been known for many years. However, a useful application for these superconductors has been problematic because they require the use of expensive helium liquids to keep these superconductors below 4K. In particular, in recent years, high-temperature Type II superconductors (hereinafter referred to as ″ HTS ″) having a transition temperature or critical temperature of up to 135K (or 164K with pressure), that is, at or above the boiling point of liquid nitrogen at 77K have been developed. Since the discovery of HTS, the design of superconducting power transformers has become possible.

Several advantages of superconducting power transformers over conventional oil filled transformers are reduced size and weight, reduced ohmic losses, removal of transformer oil, and reduced manufacturing costs of superconducting power transformers. For the benefits of such superconducting power transformers over known superconducting power transformers and conventional oil filled transformers, see ″ Transforming Transformers ″ by Sam P Mehta, NIcola Aversa, and Michael S Walker in the July 1997 issue of IEEE Spectrum. Is disclosed.

A known superconducting power transformer is disclosed in EP-A-0740315. In this known power transformer, the first coil and the second coil are composed of HTS embedded in epoxy or plastic material. These coils are contained in coolant, typically liquid nitrogen, which also acts as a dielectric insulator. If the electrical stress caused by the electric field of the superconductor exceeds the dielectric strength of this liquid nitrogen, a discharge occurs, especially when bubbles are formed in the liquid nitrogen, where the HTS windings are partially embedded in the epoxy or plastic material in which the HTS windings are embedded. Discharge may occur. For example, when the temperature is reduced to cryogenic temperature for superconductivity, these materials shrink. Depending on their composition ratio, the materials are exported at different rates, thereby increasing the possibility of void formation between materials such as conductors and epoxy materials. In order to prevent discharge, the conductor and the epoxy material should have the same coefficient of thermal expansion, but this is only possible if the materials included are the same.

Summary of the Invention

It is an object of the present invention to provide an improved power transformer having a cooling winding of a superconductor, such as a high temperature superconductor, electrically insulated such that corona discharge is unlikely to occur.

The power transformer according to the present invention includes an inner layer of semiconductor material in which electrical insulating means is in electrical contact with the electrically conductive means, an outer layer of semiconductive material at a controlled electrical potential along its length, and between the inner and outer layers. And an intermediate layer of electrical insulating material.

In this specification, the term " semiconductive material " refers to a material that has a significantly lower conductivity than an electrical conductor but does not have as low a conductivity as an electrical insulator. Here, the semiconducting material should have a volume resistivity of 1 to 10 ⁵ Ωcm, more preferably 10 to 500 Ωcm, most preferably 10 to 100 Ωcm, typically 20 Ωcm.

It is substantially convenient for the electrical insulation to be of a single type with layers joined by mechanical contact, or more preferably by extrusion, for example. These layers are conveniently formed of a plastic material which is elastic at least at ambient operating temperature. This allows the cable to form the winding in the form of a flexible and desirable winding. By using only a few manufacturable materials for these layers, it is possible to reduce similar thermal properties, thermal loads and electrical loads in the insulator. In particular, the interlayer insulating film, the semiconductive inner layer and the outer layer should have at least substantially the same coefficient of thermal expansion (α) so that defects due to different thermal expansions do not occur when these layers are heated or cooled.

Conventional interlayer dielectrics include low or high density polyethylene (LDPE or HDPE), polypropylene (PP), polybutylene (PB), solid thermoplastic materials such as polymethylpentine (PMP), ethylene (ethyl) acrylate air Coalescing, crosslinking materials such as crosslinked polyethylene (XLPE), or rubber insulators such as ethylene propylene rubber (EPR) or silicone rubber. The semiconductive inner layer and outer layer may comprise a material similar to the intermediate layer. Typically, special insulating materials such as EPR have been found to have similar mechanical properties when they contain some or no carbon particles. The intermediate layer may be subdivided into one or more additional intermediate layers of semiconductive material.

The screens of the semiconductive inner and outer layers form substantially the same potential surface on the inside and outside of the interlayer insulating layer so that in the case of the concentric semiconductive insulating layer the electric field is substantially radiant and confined within the interlayer. . In particular, the semiconductive inner layer is arranged to be in electrical contact with the conductive means it encloses and to have the same potential as the conductive means. The semiconductive outer layer is designed to act as a screen to prevent losses caused by induced voltages. The induced voltage in the outer layer can be reduced by increasing the resistance of the outer layer. This resistance is increased by reducing the thickness of the outer layer, but this thickness cannot be reduced below a predetermined minimum thickness. In addition, this resistance may be increased by selecting a material for the layer with a higher resistance. On the other hand, if the resistance of the semiconductive outer layer is too large, the potential between adjacent spaced points becomes too large, increasing the risk of corona discharge. Thus, the semiconductive outer layer is a compromise between low resistance conductors and high induced voltage losses, but is easily connected to a control potential, typically an earth or ground potential. Therefore, the resistivity (ρ _s ) of the semiconductive outer layer must be within the range ρ _min <ρ _s <ρ _max , where ρ _min is due to the allowable power loss due to eddy current losses and the voltage induced by the magnetic flux. It is determined by the resistance loss, and ρ _max is determined by the condition of no corona or glow discharge.

If the semiconductive outer layer is grounded or spaced at regular intervals along its length and connected to some other control potential, no protective plate and outer metal plate are needed to surround the semiconductive outer layer. Thus, the diameter of the cable is further reduced, making it possible to provide more turns for a given winding size.

In most practical applications, the conductive means is superconducting. However, the present invention is not limited to the conductive means having superconductivity, but is intended to include all the conductive means whose electrical conductivity is remarkably improved at low temperatures such as, for example, 200 K or less. In the case of the preferred superconducting conductive means, the conductive means may comprise low temperature semiconductors, but most preferably it is provided with an HTS material such as a HTS wire or tape wound spirally in the inner tube. A simple HTS tape is a silver-sheathed BSCCO-211 or BSCCO-2223 (where the number represents the number of atoms of each element in the [Bi, Pb] ₂ Sr ₂ Ca ₂ Cu ₃ O _x molecule). This HTS tape is hereinafter referred to as ″ BSCCO tape (s) ″. BSCCO tapes are made by casing fine filaments of oxide superconductors in a silver or silver oxide matrix in powder-in-tube (PIT) draw, roll, silicide and roll processes. In addition, this tape is formable by a surface coating process. In either case, the oxide is dissolved and solidified again in the final process. Other HTS tapes such as TiBaCaCuO (TBCCO-1223) and YBaCuO (YBCO-123) have been made by various surface coating or surface deposition techniques. Ideally, the HTS wire should have a current density of j _c -10 ⁵ Acm ^-2 or greater at operating temperatures from 65K, but is preferably 77K or greater. The filling factor of the HTS material in the matrix needs to be large so that the current density j _e ≥ 10 ⁴ Acm- ² j _c will not decrease significantly depending on the field applied within the Tesla range. The helical HTS tape is cooled below the critical temperature of the HTS by cooling flow, preferably liquid nitrogen, and passes through the inner support tube.

A cryostat layer may be disposed around the spirally wound HTS tape to thermally insulate the cooled HTS tape from the electrical insulating material. On the other hand, however, there may be no cryohold layer, and the entire winding assembly may be submerged in a coolant such as a liquid nitrogen bath. In the latter case, an electrically insulating material may be applied directly on the conductive means. In addition, a space may be provided between the conductive means and the insulating material surrounding the space, which may be a void space or a space filled with a compressible material such as a high compressive foam material. This space reduces the expansion / contraction force on the insulation system during cooling / heating to cryogenic temperatures. When this space is filled with compressible material, the latter becomes semiconducting to ensure electrical contact between the semiconductive inner layer and the conductive means.

Other designs of conductive means are also possible, and the invention relates to transformer windings formed of a superconducting cable of a suitable design having an enclosed electrical insulator of the type described above. For example, other types of conductive means with superconductivity may include externally cooled HTS or externally and internally cooled HTS in addition to internally cooled HTS. In the latter HTS cable, two concentric HTS conductors separated by low temperature insulation and cooled by liquid nitrogen are used for electrical conduction. The outer conductor serves as a return path, and the two HTS conductors may be formed in one layer or multiple layers of HTS tape for carrying the required current. The inner conductor may comprise an HTS tape wound on a tubular support through which liquid nitrogen passes. The outer conductor is externally cooled by liquid nitrogen and may be surrounded by a cryostat that is thermally insulated from the entire assembly.

In order to further increase the current density in the superconducting state, it is desirable to have a low external magnetic field. In a particularly preferred design this is achieved by mixing the low and high voltage windings of the transformer. In this way, the magnetic fields are at least partially canceled with each other, whereby the leakage inductance can be reduced to increase the critical current density. In conventional superconducting transformers, this was difficult because the windings had to be spaced apart from each other and dielectrically insulated from each other. In the present invention, the electric field outside the winding is negligible so that the high voltage winding and the low voltage winding can be mixed, so that the transformer design is more compact.

Brief description of the drawings

Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

1 is a graph showing changes in temperature, magnetic field and current density of type II conductors.

2 is a schematic diagram of a magnetic core and a winding of a power transformer according to the present invention.

3 is a cross-sectional view taken along the line A-A of FIG. 2.

4 is an enlarged schematic cross-sectional view of a portion of a superconducting cable to which a winding of a transformer is wound.

5A and 5B are schematic diagrams showing portions of a high voltage winding and a low voltage winding mixed with each other and wound around a transformer core.

2 and 3 show a laminated magnetic core 2 with limbs 3, 4, 5 for three different phases, and a connecting upper yoke and a lower yoke 6, 7. A three phase superconducting power transformer 1 is shown. The branches 3, 4 and 5 have windings 8, 9 and 10, respectively, wound around their respective branches. Each winding 8, 9 and 10 has three concentric winding turns separated from each other by an electrical insulator 11. For the winding 8, the innermost windings 8a and 8b represent the first winding or the high voltage winding, and the other winding 8c represents the second winding or the low voltage winding.

Each winding is formed from a superconducting cable 12 schematically shown in FIG. This superconducting cable 12 has a copper tubular support 13 on the inside thereof, on which the elongated HTS material, for example spirally wound, is formed to form a superconducting layer 14 around the tubular support 13. There is a BSCCO tape. The cryostat 15 disposed outside the superconducting layer has two spaced apart flexible corrugated metal tubes 16 and 17. The space between these tubes 16 and 17 is maintained in vacuum to thermally maintain the super insulation 18. Liquid nitrogen or other coolant passes through the tubular support 13 to cool the superconducting layer! 4 below its critical superconducting temperature T _c . The tubular support 13, the superconducting layer 14 and the cryostat 15 form together the superconducting means of the cable 12.

An electrical insulator is disposed outside the superconducting means. This electrical insulator is an integrated form comprising an inner semiconducting layer 20 in electrical contact with the superconducting layer 14, an outer semiconducting layer 21, and an insulating layer 22 sandwiched between the semiconducting layers. These layers 20 to 22 comprise a thermoplastic material which is in mechanical contact with each other or preferably connected to each other at its interface. These thermoplastics have a similar coefficient of thermal expansion and are conveniently elastic at least at room temperature. These layers 20 to 22 are extruded from one another around the inner superconducting means to provide a single structure, thereby minimizing the risk of cavities or pores in the electrical insulator. The presence of such pores and cavities in the insulator is undesirable because they cause corona discharge in the electrical insulator at high electric fields. If the semiconducting layer 20 is in contact with the tube 17, its contact surface must allow thermal movement between the contact surfaces in the event of a change in the heat increase or decrease between the inside and outside of the cable 12. .

For example, the solid insulating layer 22 may comprise crosslinked polyethylene (XLPE). However, on the one hand, this solid insulating layer is composed of other crosslinking materials, low density polyethylene (LDPE), high density polyethylene (HDPE), polypropylene (PP), or ethylene propylene rubber (EPR), ethylene-propylene-diene monomer (EPDM) Or a rubber insulator such as silicone rubber. The semiconductive material of the inner and outer layers 20 and 21 may comprise a solid insulating layer 22 and a base polymer of high electroconductive particles, such as, for example, carbon black or metallic particles embedded in the base polymer. . The volume resistivity of these semiconducting layers is, for example, about 20 Ωcm, which can be adjusted as needed by varying the proportion and type of carbon black added to the base polymer. The following shows how the resistivity can be varied using different shapes and amounts of carbon black.

Base Polymer Carbon Black Form Carbon Black Volume (%) Volume Resistivity (Ωcm)

Ethylene Vinyl EC Carbon Black -15 350-400

Acetate copolymer /

Nitriding rubber

″ P Carbon Black -37 70-10

″ Remaining conductivity -35 40-50

Carbon Black, Type I

″ Remaining conductivity -33 30-60

Carbon Black, Type II

Butyl grafted ″ -25 7-10

Polyethylene

Ethylene Butyl Acrylic Acetylene Carbon Black -35 40-50

Latex copolymer

″ P Carbon Black -38 5-10

Ethylene Propylene Residual Conductive Carbon -35 200-400

Rubber black

The outer semiconducting layer 21 is connected spaced apart to the control potential along its length. In most practical applications, this control potential is earthed or grounded, and the spacing of adjacent earth points depends on the resistivity of layer 21.

The semiconducting layer 21 serves as a grounded outer layer and an electrostatic plate for maintaining the electric field of the superconducting cable in the solid insulator between the semiconducting layers 20 and 21. The loss by induced voltage in layer 21 is reduced by increasing the resistance of layer 21. However, since layer 21 should be at least a predetermined minimum thickness, for example 0.8 mm, resistance can be increased by selecting a material of a layer having a relatively high resistivity. However, the resistivity cannot be increased so much, and increasing the voltage of the layer 21 between two adjacent ground points will increase the possibility of occurrence of corona discharge.

The thickness of the electrical insulation need not be constant along the length of the windings. The thickness needs to be thick for high voltages, but does not have to be thick for low voltages. Thus, the thickness of the electrical insulation may be stepped along its length, with the thicker insulation being located at the high voltage end of the winding. In practice, cables with different insulation thicknesses are combined to form specific windings. For example, if a winding is formed around a transformer in the form of a core, a cable with one thickness of electrical insulation is wound around the core limb and then core structure for another cable having a different thickness of electrical insulation. Is connected to the outside. This other cable is then wound around the core rim. Another cable may be connected in the longitudinal direction of the winding at the connection at a location away from the core structure and may be a winding having different thicknesses of insulation along its length.

The windings 8a, 8b and 8c do not need to be physically separated from each other, and in practice, it is preferable to mix the windings in order to reduce the leakage inductance. This is possible because the electric field outside the windings is negligible. By reducing the leakage inductance, larger critical current densities are possible. In addition, it is possible to design the transformer more compactly by the mixing arrangement of the windings, and in particular, as described above, a design having electrical insulation portions of different thicknesses is possible when the winding portion is in the form of a &quot; step &quot;.

Examples of hybrid windings are shown schematically in FIGS. 5A and 5B. 5A shows a transformer with low voltage winding layers labeled 26 and high voltage winding layers labeled 28 having a transformer ratio of 1: 2. Between the various winding layers and turns are laminated magnetic material 27 and spacers 29 for the provision of an air gap to improve the efficiency of the transformer. FIG. 5B shows an arrangement in which the low voltage winding layers 30 and the high voltage winding layers 32 and turns are regularly mixed symmetrically and in parallel.

Although the present invention mainly relates to a power transformer with conductive means having a superconductivity and having a winding cooled to such a superconducting temperature, the present invention does not exhibit superconductivity at low operating temperatures, preferably around 200K but at least at a desired low operating temperature. It is intended to include conductive means with improved electrical conductivity that may not. At cryogenic temperatures higher than this, liquid carbon dioxide can be used to cool the conductive means.

The electrical insulation means of the power transformer according to the present invention is to be able to handle ultra-high voltages and to handle electrical and thermal loads that may be generated at these voltages. For example, the power transformer according to the present invention may have a power range of several hundred KVA to 1000 MVA and a very high transmission voltage of 3-4 kV to 400-800 kV. At high operating voltages, partial discharges or PDs cause serious problems in known insulation systems. If cavities or pores are present in the insulation, an internal corona discharge is generated, which gradually reduces the insulation and eventually destroys the insulation. The electrical load on the electrical winding insulation of the power transformer according to the invention is such that the inner layer of the insulation is substantially equal to the electrical potential of the internal conducting means and the outer layer of the insulation is of a controlled, eg ground potential. By reducing it. As such, the electric field in the intermediate layer of insulating material between the inner and outer layers is evenly distributed over the entire thickness of the intermediate layer. In addition, having a material with few defects and similar thermal properties in the layers of insulating material reduces the likelihood of PD at a given operating voltage. As such, it is possible to design a power transformer that can withstand very high operating voltages, typically up to 800 kV or more.

While it is preferred that the electrical insulation means be extruded in position, it is also possible to build up a tightly wound and superimposed film layer or sheet material. In this manner, both the semiconductive layer and the electrical insulation layer can be formed. The insulation system may consist of all synthetic films with inner and outer semiconducting layers or polymer thin films of PP, PET, LDPE or HDPE with embedded conductive particles such as carbon black or metal particles.

For the superposition concept, a sufficient thin film has a smaller gap than the so-called Paschen minima, thus making liquid injection unnecessary. In addition, the dried wound multilayer thin film insulation has excellent thermal properties and can be combined with a superconducting pipe as an electrical conductor through which it can have a coolant such as liquid nitrogen.

Another example of an electrical insulation system is similar to a conventional cellulosic based cable in which a thin cellulosic based or synthetic paper or non-woven material is wound around a conductor strip. In this case, the semiconductive layers on either side of the insulating layer may be made of a nonwoven material or cellulose paper made from fibers of the insulating material. This insulating layer is made of the same base material, and other materials may be used.

Another example of an insulation system is obtained by combining a film and a fiber insulation material. An example of this insulation system is the so-called paper polypropylene laminate, PPLP, which is commercially available, but other combinations of films and fibers are possible. In these systems, various infusions such as mineral oil or liquid nitrogen may be used.

Claims (26)

Having at least one electric winding with electrically conductive means 13-15, cooling means for cooling them to improve conductivity of the electrically conductive means, and electrically insulating means 20-22 surrounding them. Power transformer (1), The electrically insulating means includes an inner layer 20 of semiconductive material in electrical contact with the electrically conductive means, an outer layer 21 of semiconductive material having a controlled electrical potential along its length, and the inner and outer layers ( Power transformer, characterized in that it has an intermediate layer (22) of electrically insulating material between 20 and 21. The method of claim 1, The semiconducting outer layer (21) is characterized in that it has a resistivity of 1 to 10 ⁵ Ωcm. The method of claim 1, The power transformer, characterized in that the outer layer (21) has a resistivity of 10 to 500 Ωcm, preferably 10 to 100 Ωcm. The method according to any one of claims 1 to 3, The resistance per axis unit of the semiconductive outer layer (21) is a power transformer, characterized in that 5 to 50000 Ωm ^-1 . The method according to any one of claims 1 to 3, The resistance per axis unit of the semiconductive outer layer (21) is a power transformer, characterized in that 500 to 25000 5000m ^-1 , preferably 2500 to 5000 Ωm ^-1 . The method of claim 1, wherein The semiconductive outer layer 21 is contacted by conductive means with the controlled electrical potential spaced apart in regions along its length, the adjacent contact regions being electrically insulated by the voltage at the midpoints between the adjacent contact regions. And a power transformer adjacent enough to generate a corona discharge within the power transformer. The method of claim 1, wherein And the control potential is a ground potential or close to the ground potential. The method of claim 1, wherein And the intermediate layer (22) is in mechanical contact with each of the inner and outer layers (21 and 21). The method according to any one of claims 1 to 7, The intermediate layer (22) is connected to the inner and outer layers (20 and 21) respectively. The method of claim 9, The strength of the connection between the intermediate layer (22) and the semiconductive outer layer (21) is of the same magnitude as the intrinsic strength of the material of the intermediate layer. The method according to claim 9 or 10, And the layers (20-22) are connected to each other by extrusion. The method of claim 11, An inner layer and an outer layer (20, 21) and an intermediate insulating layer (22) of the semiconductive material are applied over the conductive means through a multilayer forming die. The method of claim 1, wherein The inner layer 20 has a first plastic material having first electroconductive particles dispersed therein, and the outer layer 21 has a second plastic material having second electroconductive particles dispersed therein. And the intermediate layer (22) comprises a third plastic material. The method of claim 13, Each of the first plastic material, the second plastic material, and the third plastic material is ethylene butyl acrylate copolymer rubber, ethylene-propylene-diene monomer rubber (EPDM), ethylene-propylene copolymer rubber (EPR), LDPE, HDPE, A power transformer comprising PP, XLPE, EPR or silicone rubber. The method according to claim 13 or 14, And the first, second and third plastic materials have at least substantially the same coefficient of thermal expansion. The method according to claim 13 or 14 or 15, And the first, second and third plastic materials are the same material. The method of claim 1, wherein The conductive means (13-15) have superconductivity, and the cooling means are arranged to cool the conductive means below a threshold temperature. The method of claim 17, And the conductive means is an HTS material. The method of claim 18, And the HTS material comprises a spiral wound HTS tape or conductor. The method of claim 18, Wherein the HTS material has an HTS tape wound spirally on a support tube, wherein a coolant, such as liquid nitrogen, passes through to cool the HTS tape below a critical temperature of the HTS material. The method according to any one of claims 17 to 20, And the conductive means comprises a thermally insulated outer layer. The method according to any one of claims 1 to 16, And in the use of the transformer, the cooling means cools the conductive means to 200 k or less. The method of claim 1, wherein The semiconducting inner layer has a resistivity of 1 to 10 ⁵ Pacm, typically 10 to 500 Pacm, and preferably 50 to 100 Pacm. The method of claim 1, wherein A power transformer, characterized in that the low voltage and high voltage windings are mixed to reduce leakage inductance. The method of claim 1, wherein The electrical insulation means is suitably designed for high voltages above 10 kV, in particular high voltages above 36 kV, preferably from 72.5 kV up to very high transmission voltages such as 400 kV to 800 kV or more. Power transformer characterized in that. The method of claim 1, wherein The electrical insulation means is designed for a power range exceeding 0.5 MVA, preferably for a power range exceeding 30 MVA to 1000 MVA.