HK1022041A - Transformer/reactor - Google Patents

Description

Transformer/reactor

no marking

The present invention relates to power transformers/reactors.

All transmission and distribution systems for electric energy use transformers, the task of which is to exchange electric energy between two or more electric systems. For over 100 years, transformers have been conventional electrical products, both in theory and practice. This is clear from the german patent specification DE40414 in 1885. Transformers are available in the full power range from 1VA to 1000 MVA. With respect to the voltage range, there are families that reach the highest transmission voltage currently in use.

In relation to the basic mode of operation, transformers belong to a relatively easy to understand class of electrical products. The transfer of energy between electrical systems utilizes electromagnetic induction. A number of books and papers describe more or less theoretically, and in practice, the theory, calculation, manufacture, use, operational life, etc. of transformers. In addition, there is a large body of patent literature concerning subsequent examples of modifications of different parts of the transformer, such as windings, cores, boxes, accessories, cooling systems, etc.

The invention relates to a so-called power transformer having a rated power range of several hundred KVA to more than 1000MVA and a rated voltage range of very high transmission voltage of 3-4KV to 400KV to 800KV or more.

The inventive concept on the basis of the present invention is also suitable for reactors. The following description of the background art relates primarily to power transformers. As is known, reactors can be designed as single-phase and three-phase reactors. With regard to insulation and cooling, there is in principle the same embodiment as a transformer. Thus, a reactor of air insulation and oil insulation, self cooling, oil cooling, and the like can be obtained. Although reactors have one winding (per phase) and may be designed with or without a ferromagnetic core, the description of the background art is largely related to reactors.

In order to place the power transformer/reactor according to the invention in a suitable positional relationship and thus to be able to describe the new method according to the invention in addition to the advantages obtained with respect to the invention of the prior art, a relatively complete description of the power transformer according to the current design will first be given below, in addition to the disadvantages and problems of these transformers in performing calculations, designing, insulating, grounding, manufacturing, use, testing, transport etc.

With regard to the above, there is a comprehensive literature describing transformers in general, and power transformers in particular. For example, reference may be made to:

The J & P Transformer Book,A Practical Technology of thePower Transformer,by A.C.Franklin and D.P.Franklin,Published by Butterworths,edition 11,1990.

with regard to the internal electrical insulation of the windings, reference may be made to:

Transformerboard,Die Verwendung von Transfomerboard inGrossleistungstransformatoren by H.P.Moser,Published byH.Weidman AG,CH-8640 Rapperswil.

from a purely general point of view, the main task of a power transformer is to enable the exchange of electrical energy between two or more electrical systems, usually of the same frequency at different voltages.

Conventional power transformers comprise a transformer core, hereinafter referred to as core, often in the form of directional laminated sheets, typically of silicon steel. The core comprises a number of core legs connected by yokes which together form one or more core windows. Transformers having such cores are often referred to as core transformers. Around the core leg are several windings commonly referred to as primary, secondary and control windings. In the case of power transformers, the windings are virtually always distributed concentrically along the length of the core leg. Core transformers typically have a circular coil and a core leg cross-section that tapers conically in order to fill the window as efficiently as possible.

In addition to core-type transformers, there are also so-called shell-type transformers. These are often designed with rectangular coil and rectangular core leg cross sections.

Conventional power transformers at the lower end of the above power range are sometimes designed to be air cooled to dissipate the inherent lossy heat. To prevent contact and to reduce the external magnetic field of the transformer as much as possible, housings with vents are often provided.

But most conventional transformers are oil cooled. One of the reasons is that the oil has an additional important function as an insulating medium. The oil cooled and oil insulated power transformer is thus surrounded by an outer casing, for which high demands will be made clear from the following description.

A water cooling of the oil is usually provided.

The following part of the description will mostly refer to oil-filled power transformers.

The transformer winding is formed by one or several coils connected in series, constituting several turns connected in series. In addition, the coil is equipped with specific means allowing switching between coil taps (taps). Such devices may be designed to tap a tap by means of a threaded engagement or more often by means of a special switch operable in the vicinity of the tank. In the case of under-voltage transformers where such a transition can occur, the change-over switch is referred to as a loaded tap changer, otherwise as a de-energized tap changer.

For oil-cooled oil-insulated power transformers in the higher power range, the contacts with the load tap changer are directly connected to the transformer tank and placed in a special oil-filled container. The contacts are purely mechanically operated via a motor driven rotating shaft and are configured to obtain a fast movement when the contacts are open and a slower movement when the contacts are closed during the transition. Such a load tap changer is placed in the actual transformer tank. During operation arcing and sparking occur. This leads to degradation of the oil in the container. To obtain less arcing and thus less putty formation and less contact wear, a load tap changer is usually connected to the high voltage side of the transformer. This is because the current to be switched on and off respectively is much smaller on the high voltage side than when the loaded tap changer is connected to the low voltage side. Fault statistics for conventional oil-filled power transformers show that it is often the load tap switch that fails.

In the lower power range of oil-cooled oil-insulated power transformers, the load tap changers and their contacts are placed inside the tank. This means that the above-mentioned problems regarding the degradation of the oil due to arcing during operation affect the whole oil system.

In the case of an applied or induced voltage, it can be assumed that the voltage, which is constant across the winding, is distributed equally over the individual turns of the winding, i.e. the turn voltage is equal over all turns.

But from a potential point of view the situation is completely different. One end of the winding is typically grounded. But this means that the potential of each turn increases linearly from a substantially zero value for the turn closest to ground potential to the potential of the turn at the other end of the winding corresponding to the applied voltage.

This potential distribution determines the composition of the insulation system, since there must be sufficient insulation between adjacent turns of the winding and between each turn and ground.

The turns in the individual coils are typically gathered into geometrically coherent units that are physically demarcated from other coils. The dielectric stress that can be allowed to occur between the coils also determines the distance between the coils. This means that a given insulation distance is also required between the coils. As mentioned above, sufficient insulation distances are also required for other electrically conductive objects located in the electric field of the potentials occurring locally in the coil.

It can thus be seen from the above description that for an individual coil, the internal voltage difference between the actually adjacent conductor units is relatively low, while the external voltage difference with respect to other metal objects (including other coils) can be relatively high. The voltage difference is determined by the voltage induced by the magnetic induction and the capacitively distributed voltage generated in the external electrical system connected to the external connection of the transformer. In addition to the operating voltage, the types of voltages accessible from the outside include lighting overvoltages and switching overvoltages.

In the current conductor of the coil, additional losses arise from the magnetic leakage field around the conductor. To keep these losses as low as possible, especially for power transformers in the higher power range, the conductor is usually divided into several conductor elements, often called strands, which are connected in parallel during operation. The exchange of these strands must be based on the pattern that the voltages induced in the individual strands become as equal as possible, so that the difference in induced voltages between the individual pairs of strands becomes as small as possible when the internally circulating current components are suppressed to a reasonable level from a loss point of view.

When designing transformers according to the prior art, it is generally an aim to have as large a quantity of conductor material as possible in a given area defined by a so-called transformer window, usually described as having as large a fill factor as possible. In addition to the conductor material, the available space also includes insulating material in connection with the coils, partly inside between the coils and partly other metal parts comprising the magnetic core.

The insulation system, partly within the coil/winding and partly between the coil/winding and other metal parts, is usually designed as solid fibre material or as painted insulation closest to the individual conductor units, and outside it as solid fibre material and liquid insulation, possibly also gas insulation. In this way the winding with the insulating and possibly supporting parts has a large volume which will withstand the high electric field strengths generated in and around the active electromagnetic parts of the transformer. Knowledge of the properties of the insulating material is required in order to predetermine the resulting dielectric stress and to achieve dimensions with a minimum risk of breakdown. It is also important to achieve an ambient environment that does not alter or degrade the insulating properties.

The currently more popular insulation systems for high voltage power transformers comprise a fibrous material as solid insulation and transformer oil as liquid insulation. Transformer oils are based on so-called mineral oils.

The transformer oil has a dual function, since it cools the magnetic core, windings, etc. by removing the heat losses of the transformer, in addition to the insulating function. Oil cooling requires an oil pump, an external cooling unit, an expansion vessel, and the like.

In the case of oil-filled power transformers, the electrical connection between the external connections of the transformer and the directly connected coils/windings is called a bushing, the purpose of which is to make an electrically conductive connection through the wall of the tank surrounding the actual transformer. The bushing is often a separate element fixed to the wall of the tank, designed to withstand the insulation requirements made inside and outside the tank, while it should withstand the current load and current forces generated.

It should be noted that the same requirements as described above with respect to the insulation system of the windings also apply to the necessary internal connections between the coils, between the bushings and the coils, between different types of switches and bushings.

All metal elements within an electrical transformer, except the current carrying conductor, are typically connected to a given ground potential. In this way, the risk of an undesired, difficult to control potential increase as a result of capacitive voltage distribution between the high-potential current line and ground is avoided. This undesirable increase in potential can lead to partial discharges, so-called corona discharges, which sometimes occur at increased voltages and frequencies in conventional acceptance tests compared with nominal data. Corona discharge can lead to damage during operation.

The mechanical dimensions of the individual coils in the transformer are such that they withstand any stresses resulting from the currents and current forces generated during the current short-circuiting. In general, the design of the coils is such that the forces generated are absorbed in the individual coils, which in turn means that the coils cannot be optimally dimensioned for their normal function during normal operation.

In oil-filled power transformers of a narrow voltage and power range, the windings are designed as so-called spiral windings. This implies that the individual conductors described above are replaced by foils. Spiral wound power transformers are manufactured for voltages up to 20-30KV and power up to 20-30 MW.

In addition to a relatively complex design, the insulation system of a power transformer in the higher power range requires specific manufacturing measures in order to utilize the properties of the insulation system in the best possible way. In order to obtain good insulation to be obtained, the insulation system should have low humidity, the solid parts of the insulation should be well impregnated with surrounding oil, and the risk of residual "air" pockets in the solid parts must be minimized. To ensure this, a specific drying and impregnation process is carried out before the entire core with the windings is loaded into the box. After the drying and impregnation process, the transformer is loaded into a box and then sealed. The tank containing the flooded transformer must be evacuated of all air prior to filling with oil. This is done by means of a special vacuum treatment. After this is done, the tank is filled with oil.

To obtain a predetermined life, etc., an almost absolute vacuum is required during the vacuum treatment. This presupposes that the tank surrounding the transformer is designed to be a full vacuum, which causes a considerable consumption of material and manufacturing time.

If an electrical discharge occurs in an oil-filled power transformer, or if a locally large increase in the temperature of any part of the transformer occurs, the oil breaks down and the gas products dissolve in the oil. Therefore transformers are usually equipped with monitoring devices to detect the gas dissolved in the oil.

For weight reasons, high power transformers are transported oil-free. The transformer is installed at the customer's site and requires re-vacuuming. In addition, this is a process that has to be repeated each time for repair or inspection.

It is clear that these processes are time consuming and expensive, become a significant portion of the time used for manufacture and repair, and require the use of significant resources.

The insulating material in a conventional power transformer constitutes a large part of the overall volume of the transformer. For power transformers in the higher power range, the amount of oil used is typically in the order of tens of cubic meters of transformer oil. Oils that exhibit certain diesel-like properties are dilute liquids and exhibit relatively low flash points. It is therefore clear that the oil and the fibre material together constitute a fire risk that is not negligible in the event of unintentional heating, such as an internal flashover and the resulting oil spill.

There are clearly significant transportation problems, particularly with oil-filled power transformers. Such power transformers in the high power range can have a total oil volume of up to tens of cubic meters, and can weigh several hundred tons. It should be recognized that the transformer exterior design must sometimes be adapted to the current transportation conditions, i.e. any passage of bridges, tunnels, etc.

The following is a brief overview of the prior art regarding oil-filled power transformers, followed by a description of its limitations and scope of problems.

Oil-filled conventional power transformer

-comprising an outer casing containing a transformer comprising a transformer core with windings, insulating cooling oil, various mechanical support means, etc. There is a great mechanical requirement regarding the tank, since it is capable of performing a vacuum treatment of practically full vacuum when there is no oil but a transformer. The boxes require a cumbersome manufacturing and testing process and the large outer dimensions of the boxes also often require transportation considerations.

-generally includes so-called oil pressure cooling. This cooling method requires an oil pump, an external cooling unit, an expansion vessel, an expansion coupling (expansion coupling), and the like;

-electrical connections between coils/windings connected outside the transformer and directly connected in the form of bushings fixed to the tank wall. The bushing is designed to withstand any insulation requirements created by the inside and outside of the tank.

-comprises coils/windings, the conductors of which are divided into conductor elements, strands, which are switched in such a way that the voltages induced in the strands become as equal as possible, so that the difference in induced voltage between each pair of strands becomes as small as possible;

-comprises an insulation system, partly inside the coil/winding, partly between the coil/winding and other metal parts, designed as a solid fibre-based material or as a lacquer-based insulation closest to the respective conductor unit, and outside it, as an insulation of the solid fibre material and a liquid (possibly also a gas). In addition, it is of great importance that the insulation system exhibits very low humidity;

-comprising as an integrated component a load tap changer surrounded by oil, usually connected to the transformer high voltage winding for high voltage control;

-comprises oil causing a non negligible fire risk in connection with internal partial discharges, so called corona discharges, and electrical sparks in load tap changers and other fault conditions.

-comprises monitoring means, usually for monitoring a gas dissolved in the oil, in case of electric discharges occurring therein or in case of local increases in temperature;

-oil that, in the event of damage or accident, can cause spillage of oil that causes a significant environmental hazard.

The object of the present invention is to provide a transformer design in the power range already described in the introduction, namely a so-called power transformer with a rated power range from a few hundred KVA to over 1000MVA and with a rated voltage range from 3-4KV to, for example, a transmission voltage of 400KV to 800KV or more, which does not give rise to the various disadvantages, limitations and problems already apparent from the above description of the prior art in connection with prior art oil-filled power transformers. The present invention is based on the recognition that the electric field in the winding or in the winding in a transformer/reactor can be maintained throughout the device by designing the winding to comprise a solid insulation surrounded by outer and inner equipotential semiconducting layers, with the electric conductor located in the inner layer. According to the invention, the electrical conductor is arranged in such a way that it is in electrically conductive contact with the inner semiconducting layer that no harmful potential difference is generated in the boundary layer between the innermost part of the solid insulation and the inner semiconducting layer that runs around in the direction of the length of the conductor. The power transformer according to the invention presents significant advantages with respect to conventional oil-filled power transformers. This advantage will be described in detail below. The invention also provides a conceptual design suitable for reactors with and without a ferromagnetic core, as described in the preamble of the description.

The essential difference between a conventional oil-filled power transformer/reactor and a power transformer/reactor according to the invention is that the winding/windings comprise a solid insulation which is surrounded by an outer and an inner potential layer and inside which at least one electrical conductor is mounted which is designed to be semi-conducting. What constitutes a definition of the semiconducting concept will be explained below. According to a preferred example, the winding/windings are designed as a flexible cable.

At the high levels required by the power transformer/reactor according to the invention, which is connected to a high voltage network with a very high operating voltage, the electrical and thermal load it generates will be extremely demanding on the insulation material. It is known that so-called partial discharges and PDs often constitute a serious problem for insulating materials in high-voltage devices. If cavities, holes, etc. are created in the insulating layer, an internal corona discharge can occur at high voltages, whereby the insulating material will gradually degrade, eventually leading to an electrical breakdown of the insulating material. It will be appreciated that this may lead to severe breakdown of e.g. a power transformer.

The invention is based in particular on the recognition that semiconducting potential layers exhibit thermal properties with similar coefficients of thermal expansion and that these layers are fixed to the solid insulation. The semiconducting layers according to the invention are preferably integrated with solid insulation to ensure good contact between these layers and adjacent insulating layers exhibiting similar thermal properties, irrespective of temperature variations on the wire under different loads. In the presence of a temperature difference, the insulating component with the semiconducting layer will constitute a unitary component, without the occurrence of defects caused by the different temperature expansion of the insulating and surrounding layers. Since the semiconducting component surrounding the insulating layer will constitute an equipotential surface and thus the electric field within the insulating component will be approximately evenly distributed over the thickness of the insulating layer, which will as a result reduce the electrical load on the material.

According to the invention, it must be ensured that the insulation is not broken down by the phenomenon described above. This is achieved by using several insulating Layers which are manufactured in such a way that the risk of cavities and holes is minimized, for example extruded Layers (extruded Layers) of suitable thermoplastic materials such as cross-linked PE (polyethylene), XLPE and EPR (ethylene propylene rubber) as insulating Layers. The insulating material is therefore a low loss material with a high breakdown strength, which exhibits shrinkage when under a tape load.

This will result in a reduced electrical load on the material, since the semiconducting components surrounding the insulating layer will constitute an equipotential surface and thus the electric field within the insulating component will be approximately evenly distributed over the thickness of the insulating layer.

In connection with transmission cables for high voltage and power transmission, it is known per se to presuppose that the insulation is free of defects for the design of conductors with extruded insulation. In these transmission cables, the potential is in principle at the same level along the entire length of the cable, which provides high electrical stress in the insulating material. The transmission cable is equipped with an inner and an outer semiconducting layer for equipotential purposes.

The invention is therefore based on the recognition that: i.e. by designing the winding according to the characteristic features described in the claims with respect to the solid insulating layer and with respect to the surrounding equipotential layer, it is possible to obtain a transformer/reactor in which the electric field is maintained inside the winding. Additional improvements can be obtained by constructing the conductors with smaller insulating members, i.e., strands. By making these strands small and circular, the magnetic field passing through the strands will exhibit a constant geometry with respect to the field and the occurrence of eddy currents will be minimized.

According to the invention, the winding/windings are preferably made in the form of a cable comprising at least one conductor comprising a number of conductor bundles and having an inner semiconducting layer surrounding the conductor bundles. Outside this inner semiconducting layer is the main insulating layer of the cable in solid insulation form, and around this solid insulating layer is the outer semiconducting layer. The cable may have additional outer layers in a sense.

According to the invention, the outer semiconducting layer should exhibit electrical properties such that an equipotential along the conductor is ensured. However, the semiconducting layer must not exhibit such conductivity characteristics that induced currents cause undesirable thermal loads. And the electrical properties of the layer are such as to ensure that an equipotential surface is obtained. The resistivity p of the semiconducting layer should exhibit a minimum value p_min=1 Ω cm and maximum value ρ_max=100k Ω cm, and the electrical resistance R of the semiconductive layer per unit length in the axial direction of the cable should exhibit a minimum value R_min=50 Ω/m and maximum value R_max=50MΩ/m。

In order to function in forming an equipotential, the inner semiconducting layer must have sufficient conductivity to thereby homogenize the electric field outside the inner layer. In this respect, it is important that the layer has such properties that it renders any unevenness on the conductor surface uniform, and that it results in the formation of equipotential surfaces with a high surface finish (finish) on the boundary layer with solid insulation. Thus, the layer may have a varying thickness, but a suitable thickness is between 0.5 and 1mm, with respect to the conductor and the solid insulation to ensure a uniform surface. But the conductivity of the layer must not be so great that it generates an induced voltage. Thus, for the inner semiconducting layer, ρ_min=10^-6Ωcm,R_min=50 μ Ω/m, corresponding ρ_max=100KΩcm,R_max=5MΩ/m。

Such cables used according to the invention are thermoplastic cables and/or modifications of crosslinked thermoplastic such as XLPE or cables with Ethylene Propylene (EP) rubber insulation or other rubbers such as silicone. The improvements include, among other things, new designs both in relation to the conductor bundle and in that the cable does not have a housing for mechanically protecting the cable.

From an insulation point of view, the windings comprising such cables will necessarily cause very different conditions than those applied in conventional transformers/reactors due to the distribution of the electric field. In order to take advantage of the advantages obtained by using the above-described cable, there are other possible embodiments for the grounding of the transformer/reactor according to the invention, besides the grounding applied to conventional oil-filled power transformers.

For the winding of a power transformer/reactor according to the invention it is necessary and essential that at least one strand of the conductor is uninsulated and arranged to obtain a good electrical contact with the inner semiconducting layer. Thus, the inner layer will remain permanently at the potential of the conductor. In addition, different conductor bundles can be selected for making electrical contact with the inner semiconducting layer.

As for the remaining wire bundles, all or some of them may be insulated by painting.

According to the invention, the termination of the high-voltage and low-voltage windings may be of the joint type (when connected to a cable system) or of the cable termination type (when the connection is to a switchgear or to an upward transmission line). These parts also comprise solid insulation materials and therefore meet the same PD requirements as the whole insulation system.

According to the invention, the transformer/reactor may have external, meaning gas or liquid cooling at ground potential, or internal, meaning gas or liquid cooling within the windings.

With regard to the electric field distribution between the conventional power transformer/reactor and the power transformer/reactor according to the present invention, a great difference in the electric field distribution is necessarily caused when manufacturing the transformer or reactor winding according to the above-described cable. A decisive advantage with a winding formed of a cable according to the invention is that the electric field is sealed inside the winding, so that no electric field is present outside the outer semiconducting layer. The electric field from the current carrying conductor is only present in the solid main insulating layer. This has great advantages, both from a design or manufacturing point of view:

-it is possible to form the windings of the transformer without taking into account any electric field distribution, the exchange (transposition) of the wire bundles mentioned in the background can be omitted;

-forming a core design of the transformer without any consideration of the electric field distribution;

-oil for electrical insulation of the windings is not required, i.e. the medium surrounding the windings may be air;

-oil used for cooling of the windings is not required. Cooling can be done at ground potential and gas or liquid can be used as a cooling medium;

-electrical connection between the transformer external connection and the directly connected coil/winding does not require special connections, since the electrical connection is integrated with the winding compared to conventional devices.

-no conventional transformer/reactor bushings are needed. The field conversion from radial to axial field outside the transformer/reactor can instead be achieved similar to the termination method of a conventional cable;

manufacturing and testing techniques for power transformers according to the invention are much simpler than conventional power transformers/reactors, since the impregnation, drying and vacuum treatment described in the background description are no longer necessary. This provides for a relatively short production time;

by adopting the insulating technique according to the invention, great possibilities are provided for developing the magnetic circuit of the transformer given in the prior art.

The invention will now be described with reference to the accompanying drawings.

Figure 1 shows the electric field distribution around a conventional power transformer/reactor winding.

Fig. 2 shows an example of a winding according to the invention in the form of a cable in a power transformer/reactor.

Fig. 3 shows an example of a power transformer according to the invention.

Fig. 1 shows a simplified basic view around the electric field distribution of a conventional power transformer/reactor, where 1 is the winding, 2 is the magnetic core and 3 is the equipotential lines, i.e. the lines with the same electric field magnitude. The lower part of the winding is assumed to be at ground potential.

The potential distribution determines the composition of the insulation system, since there must be sufficient insulation between adjacent turns of the winding and between each turn and ground. The figure shows that the upper part of the winding is subjected to the highest dielectric stress. So that the design and mounting of the winding with respect to the core is substantially determined by the electric field distribution in the core window.

Figure 2 shows an example of a cable that can be used for a winding of a power transformer/reactor according to the invention. The cable comprises a conductor 4 consisting of a number of conductor bundles 5(strands) and an inner semiconducting layer 6 arranged around the conductor bundles. Outside the inner semiconducting layer is a main insulating layer 7 of the cable in the form of a solid insulating layer and around this solid insulating layer is an outer semiconducting layer 8. As mentioned before, the cable may be provided with further additional layers for specific purposes, e.g. to prevent too high electrical stresses on other areas of the transformer/reactor. From a geometrical point of view, the cables in question will have 30 and 3000mm²And outer cable diameters of 20 and 250 mm.

The power transformer/reactor winding made from the cable described in the summary of the invention can be used for single phase, three phase and multi-phase transformer/reactors, regardless of the shape of the core. An example is shown in fig. 3, which shows a three-phase laminated core transformer. The core comprises in a conventional manner three core legs 9,10 and 11, and fixed yokes 12 and 13. In the example shown, the core legs and the yoke taper to a conical cross-section.

A winding formed by a cable is concentrically mounted around the core leg. It is apparent that the example shown in fig. 3 has three concentric winding turns 14,15 and 16. The innermost winding turn 14 may represent the primary winding. The other two winding turns 15 and 16 may represent secondary windings. The connections of the windings are not shown in order not to complicate the drawing with too much detail. Further examples are shown in which shims 17 and 18 having several different functions are mounted at determined points around the winding. The spacers may be made of an insulating material to provide a defined space between the concentric turns for cooling, support, etc. They may also be made of an electrically conductive material so as to form part of the grounding system of the winding.

Claims (25)

1. A power transformer/reactor comprising at least one winding, characterized by: the winding/windings comprise one or more current carrying conductors, a first layer (6) having semiconducting properties is/are mounted around each conductor (4), a solid insulating member (7) is mounted around the first layer, and a second layer (8) having semiconducting properties is/are mounted around the insulating member.

2. A power transformer/reactor according to claim 1, characterized in that: the first layer (6) is at substantially the same potential as the conductor.

3. A power transformer/reactor according to one or more of the preceding claims, characterized in that: the second layer (8) is mounted such that it essentially constitutes an equipotential surface surrounding the conductor/conductors.

4. A power transformer/reactor according to one or more of the preceding claims, characterized in that: the second layer (8) is connected to ground potential.

5. A power transformer/reactor according to one or more of the preceding claims, characterized in that: the semiconducting layers (6,8) and the insulating member (7) have substantially the same coefficient of thermal expansion, so that defects, cracks, etc. do not occur in the boundary layer between the semiconducting layers and the insulating member when there is thermal movement in the winding.

6. A power transformer/reactor according to one or more of the preceding claims, characterized in that: each semiconducting layer (6,8) is fixed to an adjacent solid insulation part (7) along substantially the entire adjacent surface.

7. A power transformer/reactor according to one or more of the preceding claims, characterized in that: the winding/windings are designed in the form of a flexible cable.

8. A power transformer/reactor according to claim 7, characterized in that: the conductor area of the cable is between 30 and 3000mm²And the outer cable diameter is between 20 and 250 mm.

9. A power transformer/reactor according to one or more of the preceding claims, characterized in that: the solid insulating member (7) is formed of a polymeric material.

10. A power transformer/reactor according to one or more of the preceding claims, characterized in that: the first layer (6) and/or the second layer (8) are formed from a polymeric material.

11. A power transformer/reactor according to one or more of the preceding claims, characterized in that: the solid insulating member (7) is obtained by extrusion.

12. A power transformer/reactor according to one or more of the preceding claims, characterized in that: the current-carrying conductor (4) comprises a plurality of conductor bundles which are insulated from each other, except for a few conductor bundles which are not insulated by contact in order to be fixed with the first semiconducting layer (6).

13. A power transformer/reactor according to one or more of the preceding claims, characterized in that: the wire bundle of at least one conductor (4) is uninsulated and mounted in electrical contact with the inner semiconducting layer.

14. A power transformer/reactor according to one or more of the preceding claims, characterized in that: a power transformer/reactor includes a magnetic core made of a magnetic material.

15. A power transformer/reactor according to one or more of the preceding claims, characterized in that: a power transformer/reactor includes a ferromagnetic core formed of a core leg and a yoke.

16. A power transformer/reactor according to claims 1-13, characterized in that: air-wound (air-wound) of power transformers/reactors.

17. A power transformer/reactor comprising at least two electrically separated windings according to any of the preceding claims, characterized in that: the windings are concentrically wound.

18. A power transformer/reactor according to one or more of the preceding claims, characterized in that: the power transformer/reactor is connected to two or more levels.

19. A power transformer/reactor according to one or more of the preceding claims, characterized in that: the terminals of the high and/or low voltage windings are joined to the power cable and/or made similar to the power cable end connector.

20. A power transformer/reactor according to one or more of the preceding claims, characterized in that: substantially all of the electrically insulating parts in the transformer/reactor are sealed between the conductor (4) and the second layer (8) of the winding and the insulating parts are in the form of solid insulating parts.

21. A power transformer/reactor according to one or more of the preceding claims, characterized in that: the windings are designed for high voltages, suitably in excess of 10KV, especially in excess of 36KV, preferably in excess of 72.5KV, and to very high transmission voltages, for example 400KV to 800KV or more.

22. A power transformer/reactor according to one or more of the preceding claims, characterized in that: the transformer/reactor is designed for a power range exceeding 0.5MVA, preferably exceeding 30 MVA.

23. Cooling of a power transformer/reactor according to one or more of the preceding claims, characterized in that: the power transformer/reactor is cooled with a liquid and/or gas at ground potential.

24. A method of electric field control in a power transformer/reactor comprising a magnetic field generating circuit having at least one winding with at least one electrical conductor and insulation present on the outside thereof, characterized by: the insulation is formed of a solid insulating material and the exterior of the insulation is provided with an outer layer which is connected to earth or a relatively low potential and has a conductivity which is higher than the conductivity of the insulation but lower than the conductivity of the electrical conductor in order to function in forming an equipotential and to enclose the electric field substantially in the winding inside the outer layer.

25. A method of producing a power transformer/reactor according to one or more of the preceding claims, characterized in that: the winding is made of flexible cable and the cable can be wound on site to form the winding/windings of a power transformer/reactor.