Conventional power transformers have, as disclosed e.g.

in the book "Elektriska Maskiner" by Fredrik Gustavson, Page 3.6 - 3.12, Kungliga Tekniska Högskolan, 1996, usually been cooled and insulated by oil. However, a number of problems are inherent in such oil-filled power transformers. An outer housing is required for the transformer with a transformer core with windings, oil for insulation and cooling, and mechanical bracing means of various types. The mechanical demands placed on this housing are considerable and the manufacturing and assembly processes are extremely time-consuming.

Finally, the external dimensions of the housing are large, thus entailing transport problems. Oil-cooling, particularly pressurized oil-cooling, also requires ac- cess to oil pumps, external cooling elements and expan- sion vessels, etc. The insulating material must also be extremely pure and freely from conducting particles.

The moisture content in both the oil and other insula- ting material must also be far below that of the at- mosphere. In normal production the moisture content in separate processes is reduced to values below 1 % for paper and other cellulose materials and a few micro- parts in the oil. The whole insulation system must be carefully dried at the end of the manufacturing pro-

cess. This high degree of purity and low moisture con- tent must be maintained during transport and operation of the transformer.

Through e.g. JP 4 179 107, JP 6 196 343, and JP 7 057 951 a winding in the form of prefabricated drums is previously known. However, such a winding has not been used with high-voltage cables.

A conductor is known through US 5,036,165, in which the insulation is provided with an inner and an outer layer of semiconducting pyrolized glassfiber. It is also known to provide conductors in a dynamo-electric ma- chine with such an insulation, as described in US 5,066,881 for instance, where a semiconducting pyrol- ized glass fiber layer is in contact with the two paral- lel rods forming the conductor, and the insulation in the stator slots is surrounded by an outer layer of semiconducting pyrolized glassfiber. The pyrolized glass fiber material is described as suitable since it retains its resistivity even after the impregnation treatment.

The object of the present invention is to solve the above problems and further improve such machines by simplifying manufacture, facilitating transport and re- ducing manufacturing and assembly costs. This object is achieved in that the machine according to the inven- tion is given the features defined in the characteri- zing portion of claim 1.

The invention is primarily intended for use, and its advantages are particularly apparent, with a high- voltage cable of the type built up of a core having a

plurality of strands, an inner semi-conducting layer surrounding the core, an insulating layer surrounding the inner semi-conducting layer and an outer semi- conducting layer surrounding the insulating layer. Mo- re particularly it relates to such a cable with a dia- meter in the range of 20-200 mm and a conducting area in the range of 80-3000 mm2. Such applications of the invention thus constitute preferred embodiments.

In the arrangement according to the invention the win- dings are preferably of a type corresponding to cables with solid, extruded insulation, such as those used nowadays for power distribution, e.g. XLPE-cables or cables with EPR-insulation. Such a cable comprises an inner conductor composed of one or more strand parts, an inner semiconducting layer surrounding the conduc- tor, a solid insulating layer surrounding this and an outer semiconducting layer surrounding the insulating layer. Such cables are flexible, which is an important property in this context since the technology for the device according to the invention is based primarily on winding systems in which the winding is formed from cable which is bent during assembly. The flexibility of a XLPE-cable normally corresponds to a radius of curvature of approximately 20 cm for a cable 30 mm in diameter, and a radius of curvature of approximately 65 cm for a cable 80 mm in diameter. In the present application the term "flexible" is used to indicate that the winding is flexible down to a radius of curva- ture in the order of four times the cable diameter, preferably eight to twelve times the cable diameter.

The winding should be constructed to retain its proper- ties even when it is bent and when it is subjected to

layers retain their adhesion to each other in this con- text. The material properties of the layers are deci- sive here, particularly their elasticity and relative coefficients of thermal expansion. In a XLPE-cable, for instance, the insulating layer consists of cross- linked, low-density polyethylene, and the semiconduc- ting layers consist of polyethylene with soot and metal particles mixed in. Changes in volume as a result of temperature fluctuations are completely absorbed as changes in radius in the cable and, thanks to the com- paratively slight difference between the coefficients of thermal expansion in the layers in relation to the elasticity of these materials, radial expansion can ta- ke place without the adhesion between the layers being lost.

The material combinations stated above should be consi- dered only as examples. Other combinations fulfilling the conditions specified and also the condition of being semiconducting, i.e. having resistivity within the range of 10-1-10 ohm-cm, e.g. 1-500 ohm-cm, or 10-200 ohm-cm, naturally also fall within the scope of the invention.

The insulating layer may consist, for example, of a so- lid thermoplastic material such as low-density polyet- hylene (LDPE), high-density polyethylene (HDPE), po- lypropylene (PP), polybutylene (PB), polymethyl pentene (PMP), cross-linked materials such as cross-linked po- lyethylene (XLPE), or rubber such as ethylene propylene rubber (EPR) or silicon rubber.

The inner and outer semiconducting layers may be of the same basic material but with particles of conducting material such as soot or metal powder mixed in.

The mechanical properties of these materials, particu- larly their coefficients of thermal expansion, are af- fected relatively little by whether soot or metal powder is mixed in or not - at least in the proportions required to achieve the conductivity necessary accor- ding to the invention. The insulating layer and the semiconducting layers thus have substantially the same coefficients of thermal expansion.

Ethylene-vinyl-acetate copolymers/nitrile rubber, butyl graft polyethylene, ethylene-butyl-acrylate-copolymers and ethylene-ethyl-acrylate copolymers may also consti- tute suitable polymers for the semiconducting layers.

Even when different types of material are used as base in the various layers, it is desirable for their coef- ficients of thermal expansion to be substantially the same. This is the case with combination of the materi- als listed above.

The materials listed above have relatively good elasti- city, with an E-modulus of Etc500 MPa, preferably <200 MPa.

The elasticity is sufficient for any minor differences between the coefficients of thermal expansion for the materials in the layers to be absorbed in the radial direction of the elasticity so that no cracks appear, or any other damage, and so that the layers are not re- leased from each other. The material in the layers is elastic, and the adhesion between the layers is at le-

ast of the same magnitude as the weakest of the materi- als.

The conductivity of the two semiconducting layers is sufficient to substantially equalize the potential along each layer. The conductivity of the outer semi- conducting layer is sufficiently great to enclose the electrical field in the cable, but sufficiently small not to give rise to significant losses due to currents induced in the longitudinal direction of the layer.

Thus, each of the two semiconducting layers essentially constitutes one equipotential surface and the winding, with these layers, will substantially enclose the electrical field within it.

There is, of course, nothing to prevent one or more ad- ditional semiconducting layers being arranged in the insulating layer.

The invention will now be described in more detail with reference to the accompanying drawings in which Figure 1 shows a schematic section through one phase of a power transformer according to the in- vention and Figure 2 shows a cross section through a winding cable used in the transformer according to the in- vention.

Figure 1 shows a part of a power transformer in sec- tion, having a transformer core 11, a low-voltage win- ding 12 and a high-voltage winding 13. According to the invention the windings are wound onto prefabricated drums 14 and 15. These drums are completely wound at

the factory and then transported to the site where the transformer is to be used, where they are a mounted on respective phases of the core (only one phase of the transformer is shown in Figure 1).

In the example shown in Figure 1 the high-voltage win- ding 13 is divided into two drums 15 for manufacturing and transport reasons. When the winding is divided in- to several drums the cables in the individual windings are connected by a cable joint 16 on site.

Figure 2 shows a section through a power cable 1 for use in a dry power transformer according to the present invention. The cable 1 comprises a number of strands 2 consisting of a conductor made of copper, for instance, having circular cross section. This conductor is ar- ranged in the middle of the cable 1. Around the cable is a first semi-conducting layer 3. Around the first semi-conducting layer 3 is an insulating layer 4, e.g.

XLPE insulation. Around the insulating layer 4 is a second semi-conducting layer 5. In this case, therefo- re the cable does not include the outer sheath that normally surrounds such cables for power distribution.

The cable may be of the size stated in the introduc- tion.

Tubes or ducts for cooling air are arranged between the winding cables to cool the winding in the transformer according to the present invention. These tubes or ducts are suitably arranged in the drums 14 and 15 at manufacture before the transformer is transported to where it is to be used.

Thanks to the invention a dry power transformer is achieved which is simpler to manufacture than conven- tional transformers. The transformer need not be transported as a unit from factory to site, and both transport and assembly become less expensive.

The invention is of course not limited to a power transformer but is also applicable to other electrical machines with stationary parts, such as inductive reac- tors.