The rotating electric machines which are referred to in this context com- prise synchronous machines, which are principally used as generators for con- nection to distribution and transmission networks, commonly called power net- works. The synchronous machines are also used as motors as well as for phase compensation and voltage control, and, in that case, as mechanically open-cir- cuited machines. This technical field also comprises normal asynchronous ma- chines, double-fed machines, ac machines, asynchronous converter cascades, external pole machines and synchronous flux machines. These machines are in- tended to be used at high voltages, by which are meant here electric voltages which primarily exceed 10 kV. A typical range of operation for such a rotating ma- chine may be 36-800 kV, and preferably 72.5-800 kV.

Rotating electric machines have conventionally been designed for volt- ages within the interval 6-30 kV, and 30 kV has normally been considered to be an upper limit. In the generator case, this normally implies that a generator must be connecte to the power network via a transformer which steps up the voltage to the level of the network, which lies within the range of about 130-400 kV.

Over the years, various attempts have been made to develop special syn- chronos machines, preferably generators, for higher voltages. Examples of this are described, inter alia, in"Electrical World", October 15,1932, pages 524-525, the article"Water-and-Oil-cooled Turbogenerator TVM-300"in J. Elektrotechnika, No. 1,1970, pages 6-8, and patent publications US 4,424,244 and SU 955 369.

However, none of these attempts has been successful, nor have they resulted in any commercially available product.

In conventional types of rotating electric machines, the stator body often comprises a welded sheet-steel structure. In large machines, the stator core, also called the laminated core, is normally made of preferably 0.35-0.50 mm thick so- called electric sheets divided into stacks. The stator core is provided with radial slots for receiving the winding in radial layers at different radial distances from the air gap which is provided between the stator and a rotor. The word layer thus means layers of the winding at different radial distances from the centre axis of the stator. That part of the winding which runs back and forth once through the stator between different layers forms one winding turn, and several winding turns are normally collecte into a so-called coil. A coil thus comprises several aggregated conductors, insulated from each other, with an arc-shaped coil end outside each end surface of the stator. The coil ends from all the windings of the stator form a coil overhang at each end of the stator.

Normally, all large, conventionally constructed generators are provided with a two-layer winding and equally large coils. The fact that the coils must be equally large is due to the generators for high powers often requiring a parallel connection of the coils. The coils are stiff and prefabricated and the winding is in- stalled by inserting coils in a radial direction into the slots of the stator core. Join- ing or connection then takes place between each coil in the winding when all the coils have been placed in position in their slots. Because all the coils must have the same size, all the joints must be placed in the coil overhang. The coil over- hang will therefore contain a large number of joints. This method has the disad- vantage of being time-consuming and results in a number of joints which are sensitive to various kinds of faults and external influence.

The object of the present invention is to solve the above-mentioned prob- lems. This object is achieved by means of the method according to the preamble to claim 1, which has the characteristic features described in the characterizing portion.

Thus, the present invention relates to a method for the manufacture of a winding for a stator of a rotating electric machine for high voltage, wherein the stator comprises a core provided with slots for receiving the winding in radial lay- ers at different radial distances from the air gap which is present between the

stator and the rotor, whereby that part of the winding which runs back and forth once through the stator between various layers forms a coil, with an arc-shaped coil end projecting outside each end surface of the stator, the coil ends from all the windings of the stator forming a coil overhang at each end of the stator, the method being characterized in that the necessary joints in the winding are placed outside the coil overhang.

The method described has the essential avantage that the winding may be jointed or spliced in a very simple manner. Instead of jointing each coil inside the coil overhang, which is narrow and awkward, the winding may thus be jointed outside the coil overhang where there is ample space and easy access. One ad- vantage of the winding of the kind discussed above is that it allows series connec- tion of the coils. In case of a series connection, it is not required that the coils be equally large, and, therefore, a freer location of the necessary joints is possible, which makes the present invention possible.

Another avantage achieved with the method is that it will be possible to provide output terminals for lower voltages in the winding at optional locations, which locations are situated outside the coil end overhang.

Additional avantages and characteristic features will become clear from the dependent claims.

According to a particularly advantageous feature, the method is character- ized in that the winding comprises an insulated electric conductor and that ends of insulated electric conductors in the winding are drawn out outside the coil over- hang, where the respective ends are joined to ends of other insulated electric conductors in the winding.

According to another advantageous characteristic feature, it is stated that the end of at least one of the insulated electric conductors of the winding is drawn out to an optional extent outside the coil end region, where it forms an output ter- minal for lower voltage, for example an external power network. The output termi- nals may be varied as desired as regards location, voltage, number, etc. In princi- ple, such a long conductor may be drawn out that it may be extended to the near- est switchgear, without the need of supporting bars and the like. As an additional advantageous characteristic feature, it is thus stated that the end of at least one of

the insulated electric conductors of the winding is drawn out to an optional extent outside the coil overhang, where it is connecte to an optional apparats. Such an apparats may be a generator breaker and/or a disconnector or the above- mentioned switchgear and, in that case, it is thus a question of full voltage.

Furthermore, the method according to the invention is characterized in that the winding is achieved by threading the insulated electric conductor axially back and forth repeatedly in the slots in the stator core. In this way, many coils, i. e. turns in the winding, may be achieved without interruption and without joints, which is both time-saving and cost-effective. Further, it has the avantage that the winding is not formed until the final mounting in the stator core and no reforming is therefore required.

According to a particularly advantageous characteristic feature, the insu- lated electric conductor is provided with means for enclosing a generated electri- cal field inside the winding for at least one winding turn.

According to the invention, the windings are preferably of a type corre- sponding to cables having solid, extruded insulation, of a type now used for power distribution, such as XLPE-cables or cables with EPR-insulation. Such a cable comprises an inner conductor compose of one or more strand parts, an inner semiconducting layer surrounding the conductor, a solid insulating layer surround- ing this and an outer semiconducting layer surrounding the insulating layer. Such cables are flexible, which is an important property in this context since the tech- nology for the arrangement according to the invention is based primarily on wind- ing systems in which the winding is formed from cable which is bent during as- sembly. The flexibility of an XLPE-cable normally corresponds to a radius of cur- vature of approximately 20 cm for a cable with a diameter of 30 mm, and a radius of curvature of approximately 65 cm for a cable with a diameter of 80 mm. In the present application the term"flexible"is used to indicate that the winding is flexible down to a radius of curvature in the order of four times the cable diameter, pref- erably eight to twelve times the cable diameter.

The winding should be constructed to retain its properties even when it is bent and when it is subjected to thermal or mechanical stress during operation. It is vital that the layers retain their adhesion to each other in this context. The ma-

terial properties of the layers are decisive here, particularly their elasticity and relative coefficients of thermal expansion. In an XLPE-cable, for instance, the in- sulating layer consists of cross-linked, low-density polyethylene, and the semicon- ducting layers consist of polyethylene with soot and metal particles mixed in.

Changes in volume as a result of temperature fluctuations are completely ab- sorbed as changes in radius in the cable and, thanks to the comparatively slight difference between the coefficients of thermal expansion in the layers in relation to the elasticity of these materials, the radial expansion can take place without the adhesion between the layers being lost.

The material combinations stated above should be considered only as ex- amples. Other combinations fulfilling the conditions specified and also the condi- tion of being semiconducting, i. e. having a resistivity within the range of 10-1-106 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 solid thermoplastic material such as low-density polyethylene (LDPE), high-density polyethylene (HDPE), polypropylene (PP), polybutylene (PB), polymethyl pentene ("TPX"), cross-linked materials such as cross-linked polyethylene (XLPE), or rubber such as ethylene propylene rubber (EPR) or silicon rubber.

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

The mechanical properties of these materials, particularly their coefficients of thermal expansion, are affecte relatively little by whether soot or metal powder is mixed in or not-at least in the proportions required to achieve the conductivity necessary according 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 (EVA/NBR), butyl graft polyethylene, ethylene-butyl-acrylate copolymers (EBA) and ethylene-ethyl- acrylate copolymers (EEA) may also constitute suitable polymers for the semicon- ducting layers.

Even when different types of material are used as base in the various lay- ers, it is desirable for their coefficients of thermal expansion to be substantially the same. This is the case with the combination of the materials listed above.

The materials listed above have relatively good elasticity, with an E- modulus of E<500 MPa, preferably <200 MPa. The elasticity is sufficient for any minor differences between the coefficients of thermal expansion for the materials in the layers to be absorbe in the radial direction of the elasticity so that no cracks appear, or any other damage, and so that the layers are not released from each other. The material in the layers is elastic, and the adhesion between the layers is at least of the same magnitude as in the weakest of the materials.

The conductivity of the two semiconducting layers is sufficient to substan- tially equalize the potential along each layer. The conductivity of the outer semi- conducting layer is sufficiently high to enclose the electrical field within the cable, but sufficiently low 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 these layers will substantially enclose the electrical field between them.

There is, of course, nothing to prevent one or more additional semicon- ducting layers being arrange in the insulating layer.

By using an insulated conductor as described above as a winding in a rotating electric machine, the important avantage is achieved that the voltage of the ma- chine may be increased to such levels that it may be directly connecte to the power network without intermediate transformers. Thus, for example, the very im- portant avantage is achieved that the conventional transformer may be elimi- nated.

To continue, the winding is further characterized in that it is made with an insulated electric conductor comprising at least one current-carrying conductor, and that the field-enclosing members mentioned comprise a first layer with semi- conducting properties arrange to surround the current-carrying conductor, a solid insulating layer arrange to surround the first-mentioned layer, and a second layer with semiconducting properties arrange to surround the insulating layer.

According to a particularly advantageous characteristic feature, the insu- lated electric conductor is flexible and the three layers adhere to one another, which, among other things, has the avantage of facilitating installation and re- moval of the winding, respectively.

The high-voltage insulated electric conductor may be designed in a plural- ity of advantageous ways. As one advantageous feature it is stated that the insu- lated conductor comprises a cable, preferably a high-voltage cable. Further, the first semiconducting layer is substantially at the same potential as the curent- carrying conductor. The second semiconducting layer is preferably arrange so as to constitute a substantially equipotential surface surrounding the current-carrying conductor/conductors and the insulating layer. It is also connecte to a predeter- mined potential, preferably ground potential. According to another characteristic feature, the current-carrying conductor may comprise a number of stands, whereby only a few of the strands are uninsulated from one another.

Finally, it may be mentioned that the insulated conductor preferably has a diameter which is in the interval 20-250 mm and a conductor area which is in the interval 80-300 mm2.

The insulated conductor or high-voltage cable which is used in the present invention is, as mentioned, flexible and of the kind described in more detail in PCT applications SE97/00874 (WO 97/45919) and SE97/00875 (WO 97/45847). A further description of the insulated conductor or cable is to be found in PCT- applications SE97/00901 (WO 97/45918), SE97/00902 (WO 97/45930) and SE97/00903 (WO 97/45931).

According to a particularly advantageous feature, the winding is character- ized in that it is formed during the final mounting in the core. As already men- tioned, this facilitates the manufacture since no reforming is necessary.

The method is also characterized in that a lubricant is supplie when the winding is drawn through the stator slots. Where applicable, a bracing hose for the winding may be drawn through the stator slots, after the winding has been drawn, and the method is then characterized in that a lubricant is supplie to the slots in connection with the bracing hose being drawn. This lubricant is preferably a dry lubricant. En example of a suitable lubricant is boron nitride, preferably of a lamel-

lar structure. Examples of so-called bracing hoses are described in the patent applications SE 9700362-8, SE 9700363-6, PCT/SE97/00897 (WO 97/45935), PCT/SE97/00898 (WO 97/45936), PCT/SE97/00906 (WO 97/45938) and PCT/SE97/00907 (WO 97/45932).

Finally, the method is characterized in that the winding is attache in the stator slots by means of resilient elements, for example a bracing hose of some of the kinds stated in the above-mentioned patent applications.

Further, the insulation system of the winding comprising the first and sec- ond semiconducting layers, respectively, and the insulating layer positioned there- between, may be manufactured by extrusion. The insulation of the winding is pref- erably manufactured of a material with a high coefficient of linear expansion.

According to one characteristic feature, the winding has mutually insulated strands in the current-carrying conductor. Further, it is stated that the curent- carrying conductor of the winding has a continuous, uncontrolled transposition.

This simplifies the manufacture of the winding. The current-carrying conductor also advantageously has a circular cross section, which also has the avantage of simplifying the manufacture in that the conductor may be bent in an arbitrary di- rection.

As a further characteristic feature it is stated that the current in the cur- rent-carrying conductor of the winding is low, preferably less than 1000 A. This has the avantage of resulting in lower mechanical forces because of fault cur- rents, compare with conventional machines. It also implies that the bracing of the coil end is simplified.

Further, the method is characterized in that the winding has a continuous corona protection device, which is advantageously grounded. The corona protec- tion device comprises the second semi-conducting layer.

The present invention also relates to a stator for a rotating electric ma- chine for high voltage, comprising a stator core and a winding, which is character- ized in that the winding is manufactured in accordance with the method according to any of the claims relating to the method. The invention also relates to a rotating electric machine for high voltage comprising the stator mentioned.

In summary, thus, the present invention provides a considerably simplifie method for the manufacture of a winding, which shows the way to other improve- ments and also directly results in technical avantages as well as avantages from the point of view of cost.

To increase the understanding of the invention, it will now be described in detail, with reference to the accompanying drawings, illustrating a non-limiting embodiment, wherein Figure 1 schematically shows, in perspective, a part view of a stator end with coil ends comprising unjointed conductors, Figure 2 schematically shows, in perspective view, the stator end in Figure 1, after jointing, and Figure 3 shows an insulated electric conductor, in cross section, which is suit- able for use as a winding.

Figure 1 schematically illustrates an example of a part of a coil overhang 1 of an end surface 3 of a stator core 2 according to the present invention. The fig- ure shows that the winding is arrange in radial layers at different radial distances from the air gap present between the stator and a rotor, whereby that part of the winding which runs back and forth once through the stator between different lay- ers forms a coil, with an arc-shaped coil end 5 projecting from each end surface 3 of the stator, the coil ends from all the windings of the stator forming a coil over- hang 1 at each end of the stator.

The winding in the figure is achieved by threading a cable or an insulated electric conductor (6) of the kind described above axially back and forth repeatedly in the slots in the stator core 2, whereby a plurality of coils are being formed with- out joints. However, the length of the cable (6) is not infinie, but sooner or later the first cable comes to an end and a new cable must be used. As a result of this, the coil overhang 1 will exhibit a number of loosely hanging cable ends 8,9,15, which, for example, are to be joined with each other. These cable ends are lo- cated outside the actual coil overhang 1.

Figure 2 shows the same stator end as in Figure 1 but with the difference that the loose cable ends 8,9 have here been joined with each other by means of some suitable type of cable joint 12, preferably a prefabricated cable joint. As is

clear, also the joints are outside the coil overhang 1. The joints may possibly be attache mechanically to some type of support, which, however, is not shown in the figure.

In the example shown, the jointing has been performed only after at least a major part of the winding has been placed in position, but it is, of course, possi- ble to join the cable ends as the winding is being threaded. Usually, however, the entire winding is threaded before jointing takes place.

Figure 2 also shows an example of a winding end 15 which serves as a partial output terminal 16 for voltage or, alternatively, is optionally connecte, for example to a switchgear unit or a generator breaker.

Finally, Figure 3 shows a cross section of a cable which is particularly suited for use as a winding in the stator according to the invention. The cable 30 comprises at least one current-carrying conductor 31 surrounded by a first semi- conducting layer 32. Around this first semiconducting layer, there is arrange an insulating layer 33, and around this layer there is arrange, in its turn, a second semiconducting layer 34. The electric conductor 31 may comprise a number of strands 35. The three layers are formed such that they adhere to one another also when the cable is bent. The shown cable is flexible and this property is retained in the cable during its service life. The illustrated cable also differs from a conven- tional high-voltage cable in that the outer mechanically protecting casing and the metal screen which normally surrounds it may be eliminated.

The invention should not be considered limited to the illustrated embodi- ment, but may, of course, comprise a number of variations and modifications within the scope of the inventive concept, as it is defined in the subsequent claims. For example, the number of joints and/or output terminals may be varied where necessary and desired. Further, the winding may, for example, also be in- stalled radially.