WO1999033074A2

WO1999033074A2 - Switch gear station

Info

Publication number: WO1999033074A2
Application number: PCT/SE1998/002148
Authority: WO
Inventors: Mats Leijon; Lars Walfridsson
Original assignee: Abb Ab
Priority date: 1997-11-28
Filing date: 1998-11-27
Publication date: 1999-07-01
Also published as: SE512105C2; CN1279813A; WO1999033074A3; SE9704455D0; NO20002704D0; NO20002704L; SE9704455L; JP2001527373A; AU1515599A; CA2311748A1; EP1034547A2

Abstract

A switch gear station comprises at least one switch gear and at least one transformer/reactor comprising at least one winding (1) including at least one electric conductor (2). The conductor has an insulation system comprising an insulation (4) formed by a solid insulation material and, inwardly of the insulation, an inner layer (3) having an electric conductivity which is lower than the conductivity of the electric conductor but sufficient to cause the inner layer to operate for equalisation as concerns potential and, accordingly, equalisation as concerns the electric field exteriorly of the inner layer.

Description

Switch gear station

FIELD OF THE INVENTION AND PRIOR ART

This invention is related to a switch gear station comprising at least one switch gear and at least one transfor- mer/reactor comprising at least one winding including at least one electric conductor.

For all transmission and distribution of electrical energy, transformers are used and their task is to allow exchange of electrical energy between two or more electric systems and for this, electromagnetic induction is utilized in a well- known manner. The transformers primarily intended with the present invention belong to the so-called power transformers with a rated power of from a few hundred kVA up to more than 1000 MVA with a rated voltage of from 3-4 kV and up to very high transmission voltages, 400 kV to 800 kV or higher.

Although the following description mainly refers to power transformers, the present invention is also applicable to re- actors, both single-phase and multi-phase reactors. As regards insulation and cooling there are, in principle, the same embodiments as for transformers. Thus, air-insulated and oil-insulated, self-cooled, pressure-oil cooled, etc., reactors are available. Although reactors have one winding (per phase) and may be designed both with and without a magnetic core, the description of the background art is to a large extent relevant also to reactors.

The winding may in some embodiments be air-wound but com- prises as a rule a magnetic core of laminated, normal or oriented, sheet or other, for example amorphous or powder-based, material, or any other action for the purpose of allowing an alternating flux, and a winding. The circuit often comprises some kind of cooling system etc.

To be able to place a power transformer/reactor according to the invention in its proper context and hence be able to describe the new approach which the invention means as well as the advantages afforded by the invention in relation to the prior art, a relatively complete description of a power transformer as it is currently designed will first be given below as well as of the limitations and problems which exist when it comes to calculation, design, insulation, grounding, manufacture, use, testing, transport, etc., of these transformers .

A conventional power transformer comprises a transformer core, in the following referred to as a core, often of laminated oriented sheet, usually of silicon iron. The core comprises a number of core limbs, connected by yokes which to- gether form one or more core windows. Transformers with such a core are often referred to as core transformers. Around the core limbs there are a number of windings which are normally referred to as primary, secondary and control windings. As far as power transformers are concerned, these windings are practically always concentrically arranged and distributed along the length of the core limbs. The core transformer normally has circular coils as well as a tapering core limb section in order to fill up the coils as closely as possible.

Also other types of core designs are known, for example those which are included in so-called shell-type transformers. These are often designed with rectangular coils and a rectangular core limb section.

Conventional power transformers, in the lower part of the above-mentioned power range, are sometimes designed with air cooling to carry away the unavoidable inherent losses. For protection against contact, and possibly for reducing the external magnetic field of the transformer, it is then often provided with an outer casing provided with ventilating open- ings .

Most of the conventional power transformers, however, are oil-cooled. One of the reasons therefor is that the oil has the additional very important function as insulating medium. An oil-cooled and oil-insulated power transformer is therefore surrounded by an external tank on which, as will be clear from the description below, very high demands are placed.

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

The windings of the transformer are formed from one or several series-connected coils built up of a number of series- connected turns. In addition, the coils are provided with a special device to allow switching with the aid of screw joints or more often with the aid of a special changeover switch which is operable in the vicinity of the tank. In the event that changeover can take place for a transformer under voltage, the changeover switch is referred to as an on-load tap changer whereas otherwise it is referred to as a de-energized tap changer.

Regarding oil-cooled and oil -insulated power transformers in the upper power range, the breaking elements of the on-load tap changers are placed in special oil-filled containers with direct connection to the transformer tank. The breaking elements are operated purely mechanically via a motor-driven rotating shaft and are arranged so as to obtain a fast movement during the switching when the contact is open and a slower movement when the contact is to be closed. The on-load tap changers as such, however, are placed in the actual transformer tank. During the operation, arcing and sparking arise. This leads to degradation of the oil in the containers. To obtain less arcs and hence also less formation of soot and less wear on the contacts, the on-load tap changers are normally connected to the high-voltage side of the transformer. This is due to the fact that the currents which need to be broken and connected, respectively, are smaller on the high- voltage side than if the on-load tap changers were to be con- nected to the low-voltage side. Failure statistics of conventional oil-filled power transformers show that it is often the on-load tap changers which give rise to faults.

In the lower power range of oil-cooled and oil-insulated power transformers, both the on-load tap changers and their breaking elements are placed inside the tank. This means that the above-mentioned problems with degradation of the oil because of arcs during operation, etc., affect the whole oil system.

From the point of view of applied or induced voltage, it can broadly be said that a voltage which is stationary across a winding is distributed equally onto each turn of the winding, that is, the turn voltage is equal on all the turns.

From the point of view of electric potential, however, the situation is completely different. One end of a winding is normally connected to ground. This means, however, that the electric potential of each turn increases linearly from prac- tically zero in the turn which is nearest the ground potential up to a potential in the turns which are at the other end of the winding which correspond to the applied voltage. This potential distribution determines the composition of the insulation system since it is necessary to have sufficient insulation both between adjacent turns of the winding and between each turn and ground. The turns in an individual coil are normally brought together into a geometrically coherent unit, physically delimited from the other coils. The distance between the coils is also de- termined by the dielectric stress which may be allowed to occur between the coils. This thus means that a certain given insulation distance is also required between the coils. According to the above, sufficient insulation distances are also required to the other electrically conducting objects which are within the electric field from the electric potential locally occurring in the coils.

It is thus clear from the above description that for the individual coils, the voltage difference internally between physically adjacent conductor elements is relatively low whereas the voltage difference externally in relation to other metal objects - the other coils being included - may be relatively high. The voltage difference is determined by the voltage induced by magnetic induction as well as by the ca- pacitively distributed voltages that may arise from a connected external electric system on the external connections of the transformer. The voltage types that may enter externally comprise, in addition to operating voltage, lightning overvoltages and switching overvoltages .

In the current leads of the coils, additional losses arise as a result of the magnetic leakage field around the conductor. To keep these losses as low as possible, especially for power transformers in the upper power range, the conductors are normally divided into a number of conductor elements, often referred to as strands, which are parallel -connected during operation. These strands must be transposed according to such a pattern that the induced voltage in each strand becomes as identical as possible and so that the difference in induced voltage between each pair of strands becomes as small as pos- sible for internally circulating current components to be kept down at a reasonable level from the loss point of view.

When designing transformers according to the prior art, the general aim is to have as large a quantity of conductor material as possible within a given area limited by the so-called transformer window, generally described as having as high a fill factor as possible. The available space shall comprise, in addition to the conductor material, also the insulating material associated with the coils, partly internally between the coils and partly to other metallic components including the magnetic core.

The insulation system, partly within a coil/winding and partly between coils/windings and other metal parts, is normally designed as a solid cellulose- or varnish-based insulation nearest the individual conductor element, and outside of this as solid cellulose and liquid, possibly also gaseous, insulation. Windings with insulation and possible bracing parts in this way represent large volumes that will be sub^¬ jected to high electric field strengths, which arise in and around the active electromagnetic parts of the transformer. To be able to predetermine the dielectric stresses, which arise and achieve a dimensioning with a minimum risk of breakdown, good knowledge of the properties of insulating materials is required. It is also important to achieve such a surrounding environment that it does not change or reduce the insulating properties.

The currently predominant insulation system for high-voltage power transformers comprises cellulose material as the solid insulation and transformer oil as the liquid insulation. The transformer oil is based on so-called mineral oil.

The transformer oil has a dual function since, in addition to the insulating function, it actively contributes to cooling of the core, the winding, etc., by removal of the loss heat of the transformer. Oil cooling requires an external cooling element, an expansion coupling, etc.

The electric connection between the external connections of the transformer and the immediately connected coils/wmdmgs is referred to as a bushing aiming at a conductive connection through the tank which, m the case of oil-filled power transformers, surrounds the actual transformer. The bushing is often a separate component fixed to the tank and is designed to withstand the insulation requirements being made, both on the outside and the inside of the tank, while at the same time it should withstand the current loads occurring and the ensuing current forces. It should be pointed out that the same requirements for the insulation system as described above regarding the windings also apply to the necessary in^¬ ternal connections between the coils, between bushings and coils, different types of changeover switches and the bushings as such.

All the metallic components inside a power transformer are normally connected to a given ground potential with the exception of the current-carrying conductors. In this way, the risk of an unwanted, and difflcult-to-control , potential m- crease as a result of capacitive voltage distribution between current leads at high potential and ground is avoided. Such an unwanted potential increase may give rise to partial discharges, so-called corona. Corona may be revealed during the normal acceptance tests, which partially occur, compared with rated data, at increased voltage and frequency Corona may give rise to damage during normal operation.

The individual coils in a transformer must have such a mechanical dimensioning that they may withstand any stresses oc- curring as a consequence of currents arising and the resultant current forces during a short-circuit process. Normally, the coils are designed such that the forces arising are absorbed within each individual coil, which in turn may mean that the coil cannot be dimensioned optimally for its normal function during normal operation.

Within a narrow voltage and power range of oil-filled power transformers, the windings are designed as so-called sheet windings. This means that the individual conductors mentioned above are replaced by thin sheets. Sheet-wound power trans- formers are manufactured for voltages of up to 20-30 kV and powers of up to 20-30 MW.

The insulation system of power transformers within the upper power range requires, in addition to a relatively complicated design, also special manufacturing measures to utilise the properties of the insulation system in the best way. For a good insulation to be obtained, the insulation system shall have a low moisture content, the solid part of the insulation shall be well impregnated with the surrounding oil and the risk of remaining "gas" pockets in the solid part must be minimal. To ensure this, a special drying and impregnating process is carried out on a complete core with windings before it is lowered into a tank. After this drying and impregnating process, the transformer is lowered into the tank, which is then sealed. Before filling of oil, the tank with the immersed transformer must be emptied of all air. This is done in connection with a special vacuum treatment. When this has been carried out, filling of oil takes place.

To be able to obtain the promised service life, etc., pumping out to almost absolute vacuum is required in connection with the vacuum treatment. This thus presupposes that the tank which surrounds the transformer is designed for full vacuum, which entails a considerable consumption of material and manufacturing time. If electric discharges occur in an oil-filled power transformer, or if a local considerable increase of the temperature in any part of the transformer occurs, the oil is disintegrated and gaseous products are dissolved in the oil. The transformers are therefore normally provided with monitoring devices for detection of gas dissolved in the oil .

For weight reasons large power transformers are transported without oil. In-situ installation of the transformer at a customer requires, in turn, renewed vacuum treatment. In addition, this is a process which, furthermore, has to be repeated each time the tank is opened for some action or inspection.

It is obvious that these processes are very time-consuming and cost demanding and constitute a considerable part of the total time for manufacture and repair while at the same time requiring access to extensive resources.

The insulating material in conventional power transformers constitutes a large part of the total volume of the transformer. For a power transformer in the upper power range, oil quantities in the order of magnitude of hundreds of cubic metres of transformer oil may occur. The oil which exhibits a certain similarity to diesel oil is thinly fluid and exhibits a relatively low flash point. It is thus obvious that oil together with the cellulose constitutes a non-negligible fire hazard in the case of unintentional heating, for example at an internal flashover and a resultant oil spillage.

It is also obvious that, especially in oil-filled power transformers, there is a very large transport problem. Such a power transformer in the upper power range may have a total weight of up to 30-40 tons. It is realised that the external design of the transformer must sometimes be adapted to the current transport profile, that is, for possible passage of bridges, tunnels, etc.

Here follows a short summary of the prior art with respect to oil -filled power transformers and what may be described as limitation and problem areas therefor:

An oil-filled power transformer

- comprises an outer tank which is to house a transformer comprising a transformer core with coils, oil for insulation and cooling, mechanical bracing devices of various kinds, etc. Very large mechanical demands are placed on the tank, since, without oil but with a transformer, it shall be capa- ble of being vacuum-treated to practically full vacuum. The tank requires very extensive manufacturing and testing processes and the large external dimensions of the tank also normally entail considerable transport problems;

- comprises for larger power ranges a so-called pressure-oil cooling. This cooling method requires the provision of an oil pump, an external cooling element, an expansion vessel and an expansion coupling, etc.;

- comprises an electrical connection between the external connections of the transformer and the immediately connected coils/windings in the form of a bushing fixed to the tank. The bushing is designed to withstand any insulation requirements made, both regarding the outside and the inside of the tank;

- comprises coils/windings whose conductors are divided into a number of conductor elements, strands, which have to be transposed in such a way that the voltage induced in each strand becomes as identical as possible and such that the difference in induced voltage between each pair of strands becomes as small as possible;

comprises an insulation system, partly within a coil/winding and partly between coils/windings and other metal parts which is designed as a solid cellulose- or varnish-based insulation nearest the individual conductor element and, outside of this, solid cellulose and a liquid, possibly also gaseous, insulation. In addition, it is extremely important that the insulation system exhibits a very low moisture content;

- comprises as an integrated part an on-load tap changer, surrounded by oil and normally connected to the high-voltage winding of the transformer for voltage control;

- comprises oil which may entail a non-negligible fire hazard in connection with internal partial discharges, so-called corona, sparking in on-load tap changers and other fault condi- ions;

- comprises normally a monitoring device for monitoring gas dissolved in the oil, which occurs in case of electrical discharges therein or in case of local increases of the tempera- ture;

- comprises oil which, in the event of damage or accident, may result in oil spillage leading to extensive environmental damage .

The presentation above shows, accordingly, that the switch gear stations of today leave much to be desired as a consequence of the design of the transformers/reactors. It is also pointed out that the switch gear station as concerns the de- sign of the switch gear thereof is very bulky and, accordingly, costly in at least some embodiments. It is pointed out that the term switch gear station here refers to a station, which is intended for collection and/or distribution of electrical energy and comprises, for this task, required equipment for a.o. switching and supervising.

SUMMARY OF THE INVENTION

The object of the present invention is primarily to provide a switch gear station, in which at • least one or some of the disadvantages discussed above and impairing prior art has/have been eliminated.

The primary object is obtained by means of a device of the kind defined in the enclosed claims, and then first of all in claim 1.

In a wide sense, it is established that the design according to the invention reduces the occurring losses such that the device, accordingly, may operate with a higher efficiency as a consequence of the fact that the invention makes it possible to substantially enclose the electric field occurring due to said electric conductor in the insulation system. The reduction of losses results, in turn, in a lower temperature in the device, which reduces the need for cooling and allows possibly occurring cooling devices to be designed in a more simple way than without the invention.

The conductor/insulation system according to the invention may be realised as a flexible cable, which means substantial advantages with respect to production and mounting as compared to the prefabricated, rigid windings which have been conventional up to now. The insulation system used according to the invention results in absence of gaseous and liquid in- sulation materials. As to the aspect of the invention as a power transformer/ reactor, the invention, first of all, eliminates the need for oil filling of the power transformers and the problems and disadvantages associated thereto.

The design of the winding so that it comprises, along at least a part of its length, an insulation formed by a solid insulating material, inwardly of this insulation an inner layer and outwardly of the insulation an outer layer with these layers made of a semiconducting material makes it possible to enclose the electric field in the entire device within the winding. The term "solid insulating material" used herein means that the winding is to lack liquid or gaseous insulation, for instance in the form of oil. Instead the in- sulation is intended to be formed by a polymeric material. Also the inner and outer layers are formed by a polymeric material, though a semiconducting such.

The inner layer and the solid insulation are rigidly con- nected to each other over substantially the entire interface. Also the outer layer and the solid insulation are rigidly connected to each other over substantially the entire inter^¬ face therebetween. The inner layer operates equalising with respect to potential and, accordingly, equalising with re- spect to the electric field outwardly of the inner layer as a consequence of the semiconducting properties thereof . The outer layer is also intended to be made of a semiconducting material and it has at least an electrical conductivity being higher than that of the insulation so as to cause the outer layer, by connection to earth or otherwise a relatively low potential, to function equalising with regard to potential and to substantially enclose the electric field resulting due to said electric conductor inwardly of the outer layer. On the other hand, the outer layer should have a resistivety which is sufficient to minimise electric losses in said outer layer. The rigid interconnection between the insulating material and the inner and outer semiconducting layers should be uniform over substantially the entire interface such that no cavi- ties, pores or similar occur. With the high voltage levels contemplated according to the invention, the electric and thermal loads which may arise will impose extreme demands on the insulation material. It is known that so-called partial discharges, PD, generally constitute a serious problem for the insulating material in high-voltage installations. If cavities, pores or the like arise at high electric voltages, internal corona discharges may arise, whereby the insulating material is gradually degraded and the result could be electric breakdown through the insulation. This may lead to seri- ous breakdown of the electromagnetic device. Thus, the insulation should be homogenous.

The inner layer inwardly of the insulation should have an electric conductivity which is lower than that of the elec- trie conductor but sufficient for the inner layer to function equalising with regard to potential and, accordingly, equalising with respect to the electrical field externally of the inner layer. This in combination with the rigid interconnection of the inner layer and the electric insulation over sub- stantially the entire interface, i.e. the absence of cavities etc, means a substantially uniform electrical field externally of the inner layer and a minimum of risk for PD.

It is preferred that the inner layer and the solid electrical insulation are formed by materials having substantially equal thermal coefficients of expansion. The same is preferred as far as the outer layer and the solid insulation are concerned. This means that the inner and outer layers and the solid electrical insulation will form an insulation system which on temperature changes expands and contracts uniformly as a monolithic part without those temperature changes giving rise to any destruction or disintegration in the interfaces. Thus, intimacy in the contact surface between the inner and outer layers and the solid insulation is ensured and conditions are created to maintain this intimacy during prolonged operation periods.

Furthermore, it is pointed out that it is essential that the materials in the inner and outer layers and in the solid insulation has a high elasticity so that the materials may en- dure the strains occurring when the cable is bent and when the cable during operation is subjected to thermal strains. A good adhesion between the solid insulation and the inner and outer layers and a high elasticity of these layers and the solid insulation respectively are particularly important in case the materials in the layers and the solid insulation would not have substantially equal thermal coefficients of expansion. Furthermore, it is preferable that the materials in the inner and outer layers and in the solid insulation have substantially equal elasticity (E-modulus) , which will counteract occurrence of shear stresses in the boarder zone between the layers and the solid insulation. It is preferred that the materials in the inner and outer layers and in the solid insulation have an E-modulus which is less than 500 MPA, preferably less than 200 MPA. In order to be able to form windings by means of the cable, it is essential that the flexibility thereof is good. It is preferred that the cable should be capable of being subjected to bending, without negative influence on the function, with a radius of curvature ■ which is 20 times the cable diameter or less, suitably 15 times the cable diameter or less. It is preferred that the cable should be possible to bend to a radius of curvature of four or five times the cable diameter or even less.

The electric load on the insulation system decreases as a consequence of the fact that the inner and the outer layers of semiconducting material around the insulation will tend to form substantially equipotential surfaces and in this way the electric field in the insulation properly will be distributed relatively uniformly over the thickness of the insulation.

It is known that high voltage cables for transmission of electrical energy may be constructed of conductors with an insulation of a solid insulation material with inner and outer layers of semiconducting material . In transmission of electrical energy, it has since long been realised that the insulation should be free from defects. However, in high voltage cables for transmission, the electric potential does not change along the length of the cable but the potential is basically at the same level. However, also in high voltage cables for transmission purposes, instantaneous potential differences may occur due to transient occurrences, such as lightning. According to the present invention a flexible cable according to the enclosed claims is used as a winding in the electromagnetic device.

An additional improvement may be achieved by constructing the electric conductor in the winding from smaller, so-called strands, at least some of which are insulated from each other. By making these strands to have a relatively small cross section, preferably approximately circular, the ag- netic field across the strands will exhibit a constant geometry in relation to the field and the occurrence of eddy currents is minimised.

According to the invention, the winding is thus preferably made in the form of a cable comprising the electric conductor and the previously described insulation system, the inner layer of which extends about the conductor. Outside of this inner semiconducting layer is the main insulation in the form of a solid insulation material. The outer semiconducting layer shall according to the invention exhibit such electric properties that a potential equalisation along the conductor is ensured. The outer layer may, however, not exhibit such conductivity properties that an induced current will flow along the surface, which could cause losses which in turn may create an unwanted thermal load. For the inner and outer layers the resistance statements (at 20°C) defined in the enclosed claims 5 and 6 are valid. With respect to the inner semiconducting layer, it must have a sufficient electric conductivity to ensure potential equalisation for the electrical field but at the same time this layer must have such a resistivety that the enclosing of the electric field is ensured.

It is important that the inner layer equalises irregularities in the surface of the conductor and forms an equipotential surface with a high surface finish at the interface with the solid insulation. The inner layer may be formed with a varying thickness but to ensure an even surface with respect to the conductor and the solid insulation, the thickness is suitably between 0.5 and 1 mm.

Such a flexible winding cable which is used according to the invention in the electromagnetic ^■ device thereof is an im- provement of a XLPE (cross-linked polyethylene) cable used per se for transmission purposes or a cable with EP (ethyl - ene-propylene) rubber insulation. The improvement comprises, inter alia, a new design both as regards the strands of the conductors and in that the cable, at least in some embodi- ments, has no outer casing for mechanical protection of the cable. However, it is possible according to the invention to arrange a conducting metal shield and an outer mantle externally of the outer semiconducting layer. The metal shield will then have the character of an outer mechanical and elec- trie protection, for instance to lightning. It is preferred that the inner semiconducting layer will lie on the potential of the electric conductor. For this purpose at least one of the strands of the electric conductor will be uninsulated and arranged so that a good electric contact is obtained to the inner semiconducting layer. Alternatively, different strands may be alternatingly brought into electric contact with the inner semiconducting layer. Manufacturing transformer or reactor windings of a flexible cable according to the above entails drastic differences as regards the electric field distribution between conventional power transformers/reactors and a power transformer/reactor according to the invention. The decisive advantage with a cable-formed winding according to the invention is that the electric field is enclosed in the winding and that there is thus no electric field outside the outer semiconducting layer. The electric field caused by the current-carrying conductor occurs only in the solid mam insulation. Both from the design point of view and the manufacturing point of view this means considerable advantages :

- the windings of the transformer may be formed without hav- mg to consider any electric field distribution and the transposition of strands, mentioned under the background arc, is omitted;

- the core design of the transformer may be formed without having to consider any electric field distribution;

- no oil is needed for electrical insulation of the winding, that is, the medium surrounding the winding may be air;

- no special connections are required for electric connection between the outer connections of the transformer and the immediately connected coils/windings, since the electric connection, contrary to conventional plants, is integrated with the winding; - the manufacturing and testing technology which is needed for a power transformer according to the invention is considerably simpler than for a conventional power transformer/reactor since the impregnation, drying and vacuum treatments described under the description of the background art are not needed.

Above it has already been described that the outer semiconducting layer of the winding cable is intended to be con- nected to ground potential. The purpose is that the layer should be kept substantially on ground potential along the entire length of the winding cable. It is possible to divide the outer semiconducting layer by cutting the same into a number of parts distributed along the length of the winding cable, each individual layer part being connectable directly to ground potential. In this way a better uniformity along the length of the winding cable is achieved.

Above it has been mentioned that the solid insulation and the inner and outer layers may be achieved by, for instance, extrusion. Other techniques are, however, also well possible, for instance formation of these inner and outer layers and the insulation respectively by means of spraying of the material in question.

It is preferred that the winding cable is designed with a circular cross section. However, also other cross sections may be used in cases where it is desired to achieve a better packing density.

BRIEF DESCRIPTION OF THE DRAWINGS

With reference to the enclosed drawings, a more specific description of embodiment examples of the invention will follow hereinafter.

Claims

In the drawings :Fig 1 is a view showing the electric field distribution around a winding of a conventional power trans- 5 former/reactor;Fig 2 is a partly cut view showing the parts comprised m the modified standard cable in question;10 Fig 3 is a perspective view showing an embodiment of a power transformer according to the invention;Fig 4 is a plan view of a switch gear;15 Fig 5 is a side view of the switch gear according to Fig 4;Fig 6 is a cut side view illustrating an oil filled power transformer according to prior art; 20Fig 7 is a side view illustrating a power transformer according to the invention;Fig 8 is a plan view illustrating a switch gear msula-25 tion in a building;Fig 9 is a section through the building;Fig 10 is a plan view similar to the one Fig 8 but i - 30 lustratmg an alternative possibility for a transformer designed according to the invention;Fig 11 is a vertical section through the building;35 Fig 12 is a plan view of a building containing a switch gear station conceived to be gas insulated; Figs 13 and 14 are a front view and a side view respectively illustrating switching equipment comprised a switch gear;Fig 15 is a perspective and diagrammatical view illustrating how a plurality of box like metal casings have been co-ordinated for formation of a switch gear station;Fig 16 is a partly cut view illustrating electric conductors being covered by means of an electrically insulating layer; andFig 17 is a diagrammatical view illustrating m section how windings are disposed m the core window of a transformer.DESCRIPTION OF PREFERRED EMBODIMENTSAn important condition for being able to manufacture a magnetic circuit in accordance with the invention, is to use for the winding a conductor cable with a solid electrical insulation with an inner semiconducting layer between the msula- tion and one or more electrical conductors located inwardly thereof and with an outer semiconducting layer located outwardly of the insulation. Such cables are available as standard cables for other power engineering fields of use, namely power transmission. To be able to describe an embodiment, initially a short description of a standard cable will De made. The inner current-carrying conductor comprises a number of strands. Around the strands there is a semiconducting inner layer or casing. Around this semiconducting inner layer, there is an insulating layer of solid insulation. The solid insulation is formed by a polymeric material with low electrical losses and a high breakthrough strength. As concrete examples polyethylene (PE) and then particularly cross-linked polyethylene (XLPE) and ethylene-propylene (EP) may be mentioned. Around the outer semiconducting layer a metal shield and an outer insulation casing may be provided. The semicon- ducting layers consist of a polymeric material, for example ethylene-copolymer, with an electrically conducting constituent, e. g. conductive soot or carbon black. Such a cable will be referred to hereunder as a power cable.A preferred embodiment of a cable intended for a winding appears from Fig 1. The cable 1 is described in the figure as comprising a current-carrying conductor 2 which comprises transposed both non- insulated and insulated strands. Electro- mechanically transposed, solidly insulated strands are also possible. These strands may be stranded/transposed m a plurality of layers. Around the conductor there is an inner semiconducting layer 3 which, m turn, is surrounded by a homogenous layer 4 of a solid insulation material. The insulation 4 is entirely without insulation material of liquid or gaseous type. This layer 4 is surrounded by an outer semiconducting layer 5. The cable used as a winding the preferred embodiment may be provided with metal shield and external sheath but must not be so. To avoid induced currents and losses associated therewith in the outer semiconducting layer 5, this is cut off, preferably in the coil end, that is, m the transitions from the sheet stack to the end windings. The cut-off is carried out such that the outer semiconducting layer 5 will be divided into several parts distributed along the cable and being electrically entirely or partly separated from each other. Each cut-off part is then connected to ground, whereby the outer semiconducting layer 5 will be maintained at, or near, ground potential m the whole cable length. This means that, around the solidly insulated winding at the coil ends, the contactable surfaces, and the surfaces which are dirty after some time of use, only have negligible potentials to ground, and they also cause negligible electric fields .In an alternative embodiment, the cable which is used as a winding may be a conventional power cable as the one described above. The grounding of the outer semiconducting layer then takes place by stripping the metal shield and the sheath of the cable at suitable locations.Figure 1 shows a simplified and fundamental view of the electric field distribution around a winding of a conventional power transformer/reactor, where 17 is a winding and 18 a core and 19 illustrates equipotential lines, that is, lines where the electric field has the same magnitude. The lower part of the winding is assumed to be at ground potentialThe potential distribution determines the composition of the insulation system since it is necessary to have sufficient insulation both between adjacent turns of the winding and be- tween each turn and ground. The figure thus shows that the upper part of the winding is subjected to the highest insulation loads. The design and location of a winding relative to the core are this way determined substantially by the electric field distribution the core window.The cable which can be used in the windings contained the dry power transformers/reactors according to the invention have been described with assistance of Fig 2. The cable may, as stated before, be provided with other, additional outer layers for special purposes, for instance to prevent excessive electric strains on other areas of the transformer/reactor. From the point of view of geometrical dimension, the cables in question will as a rule have a conductor area which is between 2 and 3000 mm^ and an outer cable dia- meter which is between 20 and 250 mm The windings of a power transformer/reactor manufactured from the cable described m the disclosure of the invention may be used both for single-phase, three-phase and polyphase transformers/reactors independently of how the core is shaped. One embodiment is illustrated in Figure 3 which shows a three- phase laminated core transformer. The core comprises, conventional manner, three core limbs 20, 21 and 22 and the retaining yokes 23 and 24. In the embodiment shown, both the core limbs and the yokes have a tapering cross section.Concentrically around the core limbs, the windings formed with the cable are disposed. As is clear, the embodiment shown Figure 3 has three concentric winding turns 25, 26 and 27. The innermost winding turn 25 may represent the pri- mary winding and the other two winding turns 26 and 27 may represent secondary windings. In order not to overload the figure with too many details, the connections of the windings are not shown. Otherwise the figure shows that, the embodiment shown, spacing bars 28 and 29 with several different functions are disposed at certain points around the windings. The spacing bars may be formed of insulating material intended to provide a certain space between the concentric winding turns for cooling, bracing, etc. They may also be formed of electrically conducting material m order to form part of the grounding system of the windings.A conventional air insulated switch gear station is illustrated in Fig 4 and 5. It comprises a high voltage switch gear 42 (which is the one which is air insulated) , two trans- formers 43 and a medium voltage switch gear 44, which is built into a building 45. 7 denotes simply a fence around the area question.The transformers 43 are of such a type which is oil msu- lated. As already has been discussed above, oil insulation of a transformer involves a considerable risk for fire and the environment. Besides, the transformers become more heavy and bulky than what is possible according to the present invention in case the winding is formed by means of flexible cables having a solid insulation. More specifically, oil filled transformers require concrete walls 46 around the transformers. Furthermore, there must be, under the transformers, a comparatively costly oil collection pit case there would be a leakage.Fig 6 illustrates more clearly that the transformer requires a costly foundation 47, on which the transformer 43 and the previously mentioned firewalls are placed. As appears from Fig 6 there is also under the transformer a collection space for oil in case transformer leakage would occur. Oil flows down into the space 48 through a distinguishing layer, which for instance consists of a basket filled with stones.Fig 7 illustrates a transformer 49 according to the invention. Thus, this transformer has its windings achieved by means of the flexible cable described. As a consequence thereof, the transformer is not filled with oil and therefore it does not need any oil collection space under itself. The transformer according to the invention becomes considerably lighter and is also more easy to connect by means of output cables 50 since these cables do not have to pass through any sealing devices etc for oil. Considerable advantages are thus obtained by replacing the conventional oil filled transformer with the transformer according to the invention a switch gear station of the kind illustrated in Fig 4 and 5.Figs 8 and 9 illustrate in plan view and side view a switch gear station built into a building generally denoted 51. The building 51 comprises a first section 52 housing the medium voltage switch gear and a control unit, a second section 53 housing the high voltage switch gear and a third section 54 located therebetween and housing the transformer part of the station, here in the form of two transformers 55. The different sections are separated from each other by means of fireproof walls 56. Also the two transformers are separated by means of a fireproof wall .The switch gear station according to Figs 8 and 9 intended to be built into a building involves substantial advantages relative to the station illustrated with the aid of Figs 4 and 5. First of all an air insulated station as the one m Figs 4 and 5 is negative from the point of view of appearance. Such stations are not well suited for location near populated areas. On the contrary, the station illustrated Figs 8 and 9 may very well be located close to built-up areas as a consequence of the total encasing of the station without cause to anticipate disturbances. Even if the switch gear station according to Figs 8 and 9 is enclosed, it is conceived to be air insulated. Such a station could per se also have its devices encased in hermetical containers with the components surrounded by SF6-gas. Such gas causes the re- sistance to break through to increase considerably, for what reason the requirements for a safety distance between components at different voltage level may be decreased so that a more compact building mode is made possible. However, such encasing is very costly and a rigorous supervision is also required with regard to the risk of leakage.Use of convention oil filled transformers m the station according to Figs 8 and 9 involves the practical disadvantages already described with regard to oil leakage and fire risk Furthermore, limitations are practice involved as to the location of the transformers. Besides it is established that it per se would be possible to provide conventional o l filled transformers with cable bushings to the adjacent high voltage and medium voltage switch gears but such cable bushings become complicated and expensive since the oils of the cable termination and the transformer may not be mixed with each other. In oil filled transformers one therefore often selects conventional porcelain bushings on the transformer and in wall bushings as connections to the high voltage switch gear. Pressure relieving channels must pass under the ceiling m the transformer space since outlets in the roof of the high voltage switch gear part involve a higher risk of water leakage. This combination with porcelain bushings on the transformer and the oil container under the transformer lead to a large height of the transformer part the building, which clearly appears from Fig 9. Since minimal distances are desirable between the transformers and the high voltage and medium voltage switch gears respectively, the transformers are placed between the switch gears . When more than two power transformers stand a row the problem arises that the intermediate transformer can not be lifted with a reasonable size of a mobile crane It is, namely, normal procedure to place and withdraw respectively the transformers into/out of the building through the roof thereof. By replacing the conventional oil filled transformers Figs 8 and 9 with the transformer according to the invention illustrated Fig 7 the advantages already expressed are obtained, including more easy handling with respect to lifting.In addition the transformer according to the invention involves a larger amount of freedom with respect to the design of the switch gear station, which is explained with the aid of Figs 10 and 11. Again, the high voltage switch gear is denoted 53 whereas the medium voltage switch gear is denoted 52. The transformer section is denoted 54. As is apparent, the transformer section is here placed at one side of tbe building so that accordingly the medium voltage section is located between the high voltage section and the transformer section. This involves very easy access to the transformers 49 case any of them would have to be replaced or transported away for service. The reason why the transformers 49 according to Fig 10 may be placed in a section at one side of the building without appreciable disadvantages is due to the circumstance that the windings in the transformer are formed by means of flexible cables 50 and provide freedom to have comparatively large distances between the transformers and in this case the high voltage switch gear without disadvantages worth mentioning; the cables may simply be drawn in the most suitable manner between the transformers into the high voltage switch gear.When comparing Figs 8 and 9 with Figs 10 and 11 it can be seen that resorting to a transformer designed in accordance with the invention with cable technology means that the building volume may be considerably decreased. In particular, the intermediate medium voltage section can be made lower, which simplifies pressure relieving of the high voltage switch gear. The lower ceiling height is a consequence of the fact that a transformer with cable technology as a rule becomes smaller and lighter and the connections to the switch gears become more convenient by means of the cables.Fig 12 illustrates in plan view a switch gear station conceived to be of a gas insulated type. The gas may for in¬ stance be SF6. As a consequence of the improved insulation by means of gas, such gas insulated switch gears may be constructed extremely compact. Transformers occupy a large part of these stations. For this reason transformers according to the present invention fit extremely well into these switch gear stations in view of their compact design and the absence of need for extra space requiring measures caused by oil. InFig 12, 55 denotes a section for the medium voltage switch gear and control and power systems. The section 56 denotes a relay and control room. The gas insulated high voltage switch gear is placed in the section 57. On both sides of this sec- tion there are sections 58 for two transformers. If these transformers instead of being conventionally oil filled are constructed according to the invention substantially smaller space and investments in the form of buildings, oil spillage spaces etc are required.Figs 13 and 14 illustrate parts of a switching equipment for a switch gear station intended to function with air lines. More specifically, two or more switching elements, for instance two circuit breakers 59 and two disconnectors 60, comprised in the switch gear station are intended to be arranged on a single common column-like carrier 61, which is intended to have one single point of support against the underlayer and a height which is less than the horizontal extent of the switching equipment located thereabove . As appears from Fig 14, the disconnectors 60 may for instance have the character of pantograph constructions.The arrangement of several switching elements on one and the same carrier in the manner just described involves occupation of a considerably reduced land area as compared to the prior art according to which one always have carried switching elements of the kind in question with individual carrying structures. By combining such compact carrying structures with transformers likewise designed in a compact manner in accordance with the present invention one obtains an optimisation of the switch gear station in its entirety.Fig 15 illustrates diagrammatically a switch gear station, a basic idea of which is that electric devices belonging to the switch gear station are provided in air insulated and grounded metal casings 62. Each such casing has basically the character of a box and individual casings are carried out such that a switch gear station in its entirety may be built up by a combination of a larger number of such casings placed side by side and above each other as indicated in Fig 15. More specifically, three layers of casings stacked above each other are shown. The principle to enclose the switch gear equipment in such grounded metal casings involves a more compact building mode due to the reduced need for insulation distance. To this compactness, the use of transformers accordance with the cable technology in accordance with the m- vention contributes. The compactness is further increased in case insulation by means of another gas than air for instance SF6 is resorted to.Fig 16 illustrates a further embodiment directed towards re- ducmg the insulation distances and, accordingly, creating and increasing compactness switch gear stations. More specifically, it is illustrated Fig 16 how an electric conductor 63 m a branch point 75 is branched-off to a further conductor 64. 65 denotes a high voltage device. To improve insulation of the conductors 63, 64 these are coated with an electrically insulating coating 65. This coating should preferably have such a thickness and be of such a material that it may withstand electric puncture by means of voltage which is at least 30% of the total rated voltage applied on a gap between two conductors or between a conductor and another conducting part, for instance ground the switch gear Thus, a considerable increase of the insulation is achieved with a consequent possibility to arrange the switch gear equipment more closely in the switch gear.Fig 16 also illustrates that means in the form of shields 66 may be present to prevent propagation of creep charges. These shields may either be integrated with the insulating coating 65 or placed m contact with the coating.Fig 17 illustrates diagrammatically a transformer, the core of which is denoted 71. Windings present the core are shown to be cut . The windings are formed by means of flexible cables of the kind previously described. The transformer com- prises at least one high voltage winding and at least one low voltage winding. Two layers of high voltage windings are de- voltage winding. Two layers of high voltage windings are denoted 72. Between these layers there is in the example one layer of a low voltage winding. This mixing of high voltage and low voltage windings is possible in the transformer ac- cording to the invention as a consequence of the fact that the flexible cable forming the windings has an insulation system ensuring the electric field around the conductor of the cable is substantially enclosed in the cable and thus can not operate disturbing on adjacent winding turns. As a conse- quence of this considerably smaller losses occur in the transformer according to the invention. Furthermore, the windings may easily be arranged such that current induced forces at least partially balance each other.It is per se known to cast the windings of transformers and reactors in resins. However, this causes problems due to the low thermal conductivity of resins. According to the present invention it is proposed to embed the winding/windings of the transformer/reactor in a substantially inorganic material . The embedding proper occurs as a rule by casting. The material in question is suitably concrete. This material has the advantage that it is non-expensive and easy to handle. Besides, it has a higher thermal conductivity than materials previously used for this purpose and, furthermore, a higher specific heat capacity. This is important with regard to overload conditions. According to a particularly preferred embodiment, a constituent comprising a material increasing the heat conductivity of the concrete is mixed into the same. The material in question may for instance be an arbitrary metal powder, for instance aluminium.POSSIBLE MODIFICATIONSIt is evident that the invention is not only restricted to the embodiments presented hereinabove. Thus, men skilled within this art will realise that numerous detail modifica- tions are possible when knowledge about the basic inventive concept has been obtained without for this reason deviating from the inventive concept as it is defined in the enclosed claims. As an example, it is pointed out that the invention is not restricted to the specific selections of materials exemplified above. Functionally equivalent materials may accordingly be used instead. With respect to the manufacturing of the insulation system according to the invention, it is pointed out that also other techniques than extrusion and spraying are possible as long as intimacy between the different layers is achieved. Furthermore, it is pointed out that a larger number of equipotential layers could be arranged. For example, one or more equipotential layers of semiconducting material could be provided in the insulation between the lay- ers designated " inner" and " outer" above. Although some specific switch gear stations have been illustrated above it is emphasised that the present invention should not necessarily be considered as restricted to any of the embodiments now described. On the contrary, the ideas according to the inven- tion are generally applicable to switch gear stations. Claims

1. A switch gear station comprising at least one switch gear (42, 44, 52, 53) and at least one transformer/reactor (49) comprising at least one winding (1) including at least one electric conductor (2), characterIzed in that the conductor (1) has an insulation system comprising an insulation (4) formed by a solid insulation material and, interiorly of the insulation, an inner layer (3) having an electric conductiv- lty which is lower than the conductivity of the electric conductor but sufficient for the inner layer to operate for equalisation of potential and accordingly for equalisation with respect to the electric field externally of the inner layer.

2. A station according to claim 1, characterized that the insulation system comprises, externally of the insulation, an outer layer (5) which has an electric conductivity which is higher than that of the insulation to make the outer layer capable, by connection to earth or otherwise a relatively low potential, of operating to equalise potential and substantially enclose the electric field arising as a consequence of said electric conductor (2) inwardly of the outer layer (5) .

3. A station according claim 1 or 2 , characterized m that said at least one conductor (2) forms at least one induction turn.

4. A station according to any preceding claim, characterizeα in that the inner and/or outer layer (3, 5) comprises a semiconducting material.

5. A station according to any preceding claim, characterized m that the inner layer (3) and/or the outer layer (5) has a resistivety in the range 10^"6 Ωcm-100 kΩcm, suitably 10^"3- 1000 Ωcm, preferably 1-500 Ωcm, and particular 10-200 Ω cm.

6. A station according to any preceding claim, characterized that the inner layer (3) and/or the outer layer (5) has a resistance which per length meter of the conductor/insulation system is in the range 50 μΩ - 5 MΩ.

7. A station according to any preceding claim, characterized in that the solid insulation (4) , the inner layer (3) and/or the outer layer (5) are formed by polymeric materials.

8. A station according to any preceding claim, characterized m that the inner layer (3) , and/or the outer layer (5) and the solid insulation (4) are rigidly connected to each other over substantially the entire interface to maintain adhesion between the respective layers and the solid insulation on temperature changes and bending of the conductor and its insulation system.

9 A station according to any preceding claim, characterized that the inner layer (3) , the outer layer (5) and the solid insulation (4) are formed by materials presenting substantially equal thermal coefficients of expansion.

10. A station according to any preceding claim, characterized in that the inner layer (3) and the outer layer (5) have been provided by extrusion simultaneously with extrusion of the solid insulation (4) .

11 A station according to any preceding claim, characterized m that the conductor (2) and its insulation system constitutes a winding formed by means of a flexible cable (1) •

12. A station according to any of claims 2-11, characterized in that the inner layer (3) is in electric contact with the at least one electric conductor (2) .

13. A station according to claim 12, characterized in that said at least one electric conductor (2) comprises a number of strands and that at least one strand of the electric conductor (2) is at least in part uninsulated and arranged in electric contact with the inner layer (3) .

14. A station according to any preceding claim, characterized in that the inner and outer layers (3, 5) and the insulation (4) are of materials having such an elasticity that the layers maintain their adhesion to the solid insulation despite the temperature variations occurring during operation.

15. A station according to claim 14, characterized in that the materials in the layers and the solid insulation have an E-modulus which is less than 500 MPA, preferably less than 200 MPA.

16. A station according to any of claims 14 and 15, characterized in that the adhesion between the layers and the in- sulation is of at least the same order as in the weakest of the materials.

17. A station according to any preceding claim, character^¬ ized in that the conductor (2) and its insulation system is designed for high voltage, suitably in excess of 10 kV, in particular in excess of 36 kV and preferably more than 72,5 kV.

18. A station according to any preceding claim, characteri- zed in that the outer layer (5) is divided into a number of parts, which are separately connected to ground or otherwise a low potential .

19. A station according to claim 18, characterized in that it comprises a magnetic core.

20. A station according to any of claims 1-18, characterized in that it is air-wound, thus being formed without a magnetic core.

21. A station according to any preceding claim, comprising at least two galvanically separated windings, characterized in that the windings (25-27) are concentrically wound.

22. A station according to any preceding claim, characterized in that the switch gear (42) is designed for outdoor use in air insulated state.

23. A station according to any of claims 1-21, characterized in that switch gear (44, 52, 53, 55, 57) is arranged within a building.

24. A station according to claim 23, characterized in that also the transformer (49, 55) is arranged within a building.

25. A station according to claim 2 or 24, characterized in that the switch gear comprises at least one high voltage switch gear (53) , at least one transformer (49, 55) and at least one medium voltage switch gear (52) .

26. A station according to claim 25, characterized in that these switch gears and the transformer are arranged in a common building having different sections.

27. A station according to claim 25, characterized in that the medium voltage switch gear (53) is arranged in a section between sections (53, 54), in which the high voltage switch gear and the transformer are arranged.

28. A station according to any preceding claim, character- ized in that the transformer is of a " dry" type, i.e. not requiring any oil for insulation and/or cooling.

29. A station according to any preceding claim, characterized in that the entire or parts of the equipment of the switch gear station is/are enclosed in a gas increasing the sparking resistance.

30. A station according to any preceding claim for use in power lines, characterized in that two or more switch ele- ments, in particular at least one breaker (59) and at least one disconnector (60) , included in the station are arranged on a common column like carrier (61) .

31. A station according to any preceding claim comprising a plurality of electric devices, characterized in that at least one of the electric devices is arranged in an air insulated and grounded metal casing (62) .

32. A station according to claim 31, characterized in that the electric devices are arranged divided in a plurality of casings (62), which are arranged adjacent each other.

33. A station according to any preceding claim comprising conductors (63, 64), conductor connection points (75) and high voltage equipment (76) connected to the conductors, characterized in that the conductors (63, 64) are covered by means of electrically insulating layers (65) capable of resisting electric puncture of a voltage which is at least 30% of the total nominal voltage applied on a gap between two conductors or between a conductor and another ^'conducting part, for instance earth.

34. A station according to claim 33, characterized in that the conductors (63, 64) comprise means in the form of e.g. shields (66) to prevent propagation of creep charges.

35. A station according to any preceding claim, the transformer having at least one low voltage winding and at least one high voltage winding, characterized in that the low and high voltage windings are intermixed with each other.

36. A station according to claim 35, characterized in that the windings (72, 73) are arranged such that current induced forces at least partially balance each other.

37. A station according to any preceding claim, characterized in that the winding/windings of the transformer/reactor is/are imbedded in a substantially inorganic material .

38. A station according to claim 37, characterized in that the inorganic material is concrete.

39. A station according to claim 38, characterized in that the concrete contains constituents promoting heat conductivity.