BACKGROUND OF THE INVENTION WO 97/45921 describes a rotating electric machine including an induction winding containing an, electric conductor enclosed within an insulation system comprising a first semiconducting layer, which is provided with a surrounding solid insulation layer and a second semiconducting layer that encases the solid insulation layer. The use of such an induction winding, in the stator winding of a rotating electric machine for example, allows the voltage of the rotating electric machine to be increased to such a level that it can be connected directly to a power network without the need of an intermediate transformer. Consequently this leads to savings in both economic terms and with regards to space requirements for installations comprising a rotating electric machine as a transformer constitutes an extra cost and reduces the total efficiency of the system. Such electric machines generally operate at voltages in the range 36kV up to 800kV or higher.

The induction windings of such rotating electric machines require a relatively thick insulation system due to the high voltage. The insulation system provides the desirable electric insulation but, because electric insulation materials are generally also good thermal insulators, it also unavoidably provides good thermal insulation. A thick insulation system around the machine's induction winding hinders heat from being dissipated from the induction winding's electric conductor. During the rotating machine's use, heat is generated inside the machine's magnetic core and induction winding. The rotating machine's components have to be cooled to ensure their optimum performance and prolonged lifetime. The thick insulation system also entails an increased radius of curvature of the induction winding, which in turn results in an increased size of the winding overhang.

SUMMARY OF THE INVENTION It is an object of this invention to provide an efficient insulation system surrounding an electric conductor, which is designed to provide electric insulation and facilitate cooling of said electric conductor.

This and other objects of the invention are achieved by utilizing an electric cable that incorporates an electric conductor surrounded by an insulation system comprising compressed gas. This may be achieved by enclosing at least part of said electric conductor in a casing and then pressurizing said casing by filling it with gas.

In a preferred embodiment of the invention said compressed gas is at least one of the following : nitrogen, SF6 or air, i. e. air having a pressure greater than 1 bar. In another preferred embodiment of the invention said compressed gas has a pressure in the range of 5-10 bar. In yet another preferred embodiment of the invention said compressed gas has a

pressure that provides the desired electric insulation. Dry air at atmospheric pressure has a breakdown voltage of 2.5-3.0 kV/mm however when air is compressed to a pressure of about 10 bar it's breakdown voltage increases to about 20 kV/mm, thus providing better electric insulation than cross-linked polyethylene, for example, which has a breakdown voltage of about 10 kV/mm. The breakdown strength of SF6 is about three times that of air. By adjusting the pressure of the gas used, the level of electric insulation it provides can be adapted to meet requirements.

In a preferred embodiment of the invention said electric cable further comprises a semiconducting layer surrounding said conductor, said compressed gas surrounding the semiconducting layer. The semiconducting layer provides an equipotential surface around the electric conductor, which spreads the electric field out uniformly over the insulation system. In this way the risk of breakdown of the insulation, due to local concentrations in the electric field, are eliminated.

In another preferred embodiment of the invention said insulation system further comprises a solid insulation layer surrounding said semiconducting layer, said solid insulation layer being surrounded by compressed gas. The solid insulation layer comprises for example a thermoplastic such as low/high-density polyethylene, polypropylene, polybutylene, Teflon polyvinylchloride or mica, cross-linked material such as cross-linked polyethylene or rubber for example ethylene-propylene rubber or silicone rubber. The semiconducting layer preferably comprises the same or a similar material as the solid insulation layer but contains conducting material for example carbon black.

In yet another preferred embodiment of the invention, said electric cable comprises an outer enclosing layer that encloses its insulation system. In another preferred embodiment said outer enclosing layer is semiconducting

and can be arranged to constitute a substantially equipotential surface surrounding said insulation system by connecting it to a predetermined potential such as ground potential. In a further embodiment of the invention said outer enclosing layer is applied to the inner surface of the enclosure in which at least part of the electric cable is located in order to eliminate local concentrations in the electric field around non-uniformities or protrusions on the surface of said enclosure.

The present invention concerns applications in which at least part of an electric cable is subjected to a magnetic field that induces eddy currents inside said electric conductor.

The present invention also relates to an induction winding comprising said electric cable. When said electric cable is used as an induction winding, the semiconducting layer and the solid insulation layer provide protection and mechanical reinforcement that may be advantageous or even necessary to limit damage to the electric conductor as it is being wound into position, through the slots of a stator in a generator for example. In a further embodiment of the invention said electric cable comprises additional reinforcement means, such as a thin layer or coating, applied around said solid insulation layer.

The present invention additionally relates to both a static and a rotating electric machine comprising at least one induction winding according to the invention and the use of said electric machines for electric energy generation, transmission, distribution, conversion or consumption.

In a preferred embodiment of the invention, said induction winding is held in place, substantially in the centre of the slot through which it is wound, by supporting means. Said supporting means comprise for example a fibre of insulating material wound around said induction winding or spacers

positioned for example between the induction winding and the walls of the slot through which it is wound. Said supporting means have the resilience to withstand the electric cable's tendency to vibrate when subjected to currents having a certain frequency and its changes in dimension due to thermal expansion.

In a preferred embodiment of the invention said rotating electric machine can be connected directly to a transmission or distribution network having a high voltage (higher than 10kV), via coupling elements, without requiring a step-up transformer that decreases the total efficiency of the system, consequently leading to savings in both economic terms and with regards to space requirements.

Using the example of utilizing an induction winding comprising an electric conductor, a semiconducting layer and a solid insulation layer in the stator of a generator, said generator is enclosed within a casing to enable said electric conductor to be surrounded by compressed gas. Depending on the gas pressure, a gap (1-30 mm) between the stator slot walls and the induction winding can have a breakdown voltage equal to or much higher than the solid insulation layer. This compressed gas-filled gap provides good electric insulation, which means that a much thinner solid insulation layer can be used in said induction winding. This leads to a much thinner cable having a much smaller radius of curvature, giving rise to a decreased size of winding overhang.

Another very important advantage of said induction winding is that a thinner solid insulation layer provides less thermal insulation. The compressed gas surrounding said solid insulation layer dissipates heat transferred from the induction winding's electric conductor through the thin solid insulation layer to the surroundings more readily via conduction, radiation and convection.

Heat is transferred from the generator's components to the surrounding

compressed gas enclosed in the casing and then from the casing to the surroundings. Having the more effective cooling system according to the present invention enables the generator to be run at a much higher power output.

According to preferred embodiments of the invention said electric conductor has a circular cross-section and is located in an enclosure having a circular cross-section or an elliptical cross-section to optimize the electric field distribution. According to another embodiment of the invention, said electric conductor has a rectangular cross-section and is located in an enclosure having a rectangular cross-section.

The above and other objects, features and advantages of the present invention will become more apparent from the following description and the appended claims, taken in conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING A greater understanding of the present invention may be obtained by reference to the accompanying drawing, when considered in conjunction with the subsequent description of the preferred embodiments, in which figures 1a-f depict electric cables according to preferred embodiments of the present invention figure 2 illustrates a 3-phase power transformer with a laminated core comprising an induction winding according to the present invention, figure 3 depicts schematically an axial end-view of a sector of the stator in an electric machine according to the present

invention, and figure 4 shows a schematic diagram of a generator according to a preferred embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Figure 1a shows an electric cable 10 incorporating an electric conductor 11, made up of circular strands for example, surrounded by an insulation system comprising a semiconducting layer 12 and compressed gas 13 that provides electric insulation and facilitates cooling of said electric conductor.

The compressed gas comprises at least one of the following : air, nitrogen or SF6 for example. Said electric cable is located at the centre of an enclosure whose walls 14 are shown in figure 1a. Said enclosure can be a stator slot, a rotor slot or part of the pressurized casing around said electric cable.

Figure 1 b shows an electric cable 10 incorporating an electric conductor 11 surrounded by an insulation system comprising a semiconducting layer 12 surrounding said electric conductor, said semiconducting layer being surrounded by a thin layer of solid insulation 15, comprising cross-linked polyethylene for example, said solid insulation layer surrounded by compressed gas 13. In another embodiment of the invention said electric cable comprises an outer enclosing layer 16 located between said electric cable 10 and for example, the walls 14 of the enclosure in which the electric cable is located.

Fig 1c shows an electric cable 10 supported substantially at the centre of an enclosure by supporting means 17 comprising spacers. Said spacers are manufactured either together with the enclosure or together with said electric cable 10 being designed so as not to impede or complicate the

insertion of the electric cable into said enclosure. The spacers comprise electric insulation material and support said electric cable as the pressurized casing in which it is located is pressurized and while the electric cable is in use.

Figure ld shows an electric cable 10 according to the present invention held in place in an enclosure 14 by supporting means 18 comprising a fibre of insulating material that is wound around the cable's solid insulation layer 15.

Figures 1e and 1f show an electric cable 10 supported substantially at the centre of an enclosure by supporting means 17 comprising spacers having a long discharge distance.

Figure 2 illustrates a three-phase power transformer comprising an induction winding 20 according to the present invention and a laminated core. The core comprises three legs 21,22,23 and two yokes 24,25.

Induction windings according to the present invention are concentrically wound around the core's legs. Three such concentric induction windings 26,27,28 are shown. The inner induction winding 26 is a primary induction winding and the other two 27,28 represent secondary induction windings.

The induction windings 20 comprise an electric conductor surrounded by a semiconducting layer. Spacers 29,30 comprising electrically insulating material are placed between the induction windings and function to facilitate cooling and to mechanically support the induction windings. The whole transformer is enclosed within a pressurized casing.

In another embodiment of the invention the induction windings comprise an outer enclosing layer comprising conducting material which functions as the pressurized casing enclosing said induction winding, The spacers 29,

30 then comprise electrically conducting material and function as part of the grounding system for the pressurized casing.

Figure 3 depicts schematically an axial end-view of a sector of the stator 31 of a generator according to the present invention. The sector shows a segment 32 of the stator and a segment 33 of the rotor with a rotor pole 18.

From a yoke portion 32 of the core situated radially outermost, a number of teeth 34 extend radially inwards towards the generators rotor 33. The teeth are separated by slots 35 in which the stator's induction winding is arranged. Only the electric conductor of the induction winding 11 has been shown for clarity. Compressed gas 13 fills the stator slots 35. Each slot 35 has varying cross-section with alternating wider parts 36 and narrower parts 37. The wider parts 36 are substantially circular and surround the induction winding lead-throughs. The narrower parts serve to radially position each induction winding lead-through. The cross-section of the slot 35 as a whole becomes slightly narrower in the direction radially inwards.

This is because the voltage in the induction winding lead-throughs is lower the closer they are situated to the radially inner part of the stator. Narrower cable lead-throughs can therefore be used here, whereas increasingly wider cable lead-throughs are required further out. In the embodiment shown, induction windings of three different dimensions are used, arranged in three correspondingly dimensioned sections 38,39,40 of the slots 35.

Figure 4a shows a generator enclosed in a pressurized casing 43 filled with gas 13. The generator comprises a stator 41 that incorporates an induction winding 20, and a rotor 42. The rotor is driven by a turbine 44 via a shaft 45 that enters the pressurized casing via an inlet 46. The inlet is sealed with ferrofluid for example. The induction winding comprises an electric conductor 11 surrounded by a semiconducting layer 12 and an outer enclosing layer 16. Compressed gas fills the gap between the

semiconducting layer 12 and the outer enclosing layer 16. The generator is connected directly to a power grid for example via a rectifier.

Figure 4b shows an induction winding 20 having a circular cross-section in an enclosure 14 having an elliptical cross-section and an induction winding 20 having a rectangular cross-section in an enclosure 14 having a rectangular cross-section.

While only certain preferred features of the present invention have been illustrated and described, many modifications and changes wilf be apparent to those skilled in the art. It is therefore to be understood that all such modifications and changes of the present invention fall within the scope of the claims.