TECHNICAL FIELD OF THE INVENTION The present invention generally relates to a high-voltage rotating electric machine, and an electric energy conversion system for high voltage comprising such a rotating electric machine. More particularly, the invention relates to the cooling of such machines and systems.

BACKGROUND OF THE INVENTION Rotating electric machines have conventionally been designed for voltages in the range from 10 kV up to a maximum of 30-35 kV, and are normally built with a stator core provided with slots in which the stator winding is arranged.

In most conventional rotating electric machines, the stator slots as well as the conductors have a rectangular or trapezoidal cross-section. So-called stator teeth are formed between the slots. Each winding phase comprises a number of coil groups connected in series and each coil group comprises a number of coils connected in series. A coil comprises a number of conductors that are brought together. The different parts of the coil are designated coil side for the part placed in the stator core, and end winding for the part located outside of the stator core. Between each conductor there is a thin insulation such as epoxy/glass fiber. The coil itself is electrically insulated from the slot by coil insulation, i. e. an insulation intended to withstand the rated voltage of the machine relative to ground. Various plastic materials, varnish and glass fiber materials may be used as insulating material. Usually, so-called mica tape is used. The insulation is applied to the coil by winding several layers of the tape around the coil. The insulation is impregnated and the coil side is thereafter painted with a graphite-based paint to improve the contact with the surrounding stator, which is connected to ground potential. The stator core may be constructed of laminated, normal or oriented steel, or other materials

such as amorphous or powder based materials. There are also machines in which the power windings are placed in the rotor, and the field windings in the stator.

In the case of a generator, the generator must be connected to the transmission or distribution network, hereinafter referred to as the power network, via a transformer. The transformer steps-up the voltage of the generator to the level of the power network-normally exceeding 130 kV.

The above type of conventional electric machine is normally provided with a cooling system for forced cooling of the machine.

Conventionally, two different types of air-cooled systems exist: radial cooling where the air is forced through radial ducts in the machine, and axial cooling where the air is blown into the pole gaps by axial fans.

Gas cooling of both the stator and the rotor is frequently used for cooling large alternating current machines. It is usual for the gas to be transported radially through the stator in cooling ducts, which are formed by radially placed spacers. The spacers, separating the laminated core of the stator into units of approximately 30 mm in axial length, are normally 6 mm high and 2 mm thick straight rectangular steel elements.

The circulation of the gas may be arranged according to different principles. A hydro-generator, for example, is a multi-pole generator, which is characterized by a large stator diameter and salient poles. The rotor in a hydro-generator may be designed with radial cooling ducts so that the air or other gas is transported radially within both the rotor and the stator. It is also usual for the gas to be pressed axially into the air gaps by fans on both ends of the machine, after which the gas turns 90° and then departs radially through the stator ducts. A turbo-generator having few poles, i. e. 2 or 4 poles, is characterized by a substantially cylindrically formed rotor, and the rotor

conductors are frequently cooled by gas transported within axial ducts connected to the conductors. The heated gas is released into the air gap via radial ducts. The stator in a turbo-generator is normally divided into different cooling chambers in which the direction of the gas flow may change so that cold air may be forced down into the air gap in some chambers, and warm air may escape from the air gap in other chambers. So-called reversed cooling is applied to some turbo machines, which means that the rotor fans suck gas from the air gap instead of pressing gas into the air gap. This is advantageous as the stator is cooled in this way by cool air instead of warm rotor air. The rotor fan blades are then placed on top of the rotor retaining rings instead of being mounted axially behind the rings.

The cool air may consist of the surrounding air, but at powers exceeding 1 MW, closed cooling systems with heat exchangers is used.

Hydrogen cooling is normally used in turbo-generators and large synchronous condensers up to approximately 400 MW. This cooling method works in the same way as air cooling with heat exchangers, but instead of air as cooling medium hydrogen is used. Hydrogen has better cooling capacity than air, but difficulties arise at seals and in monitoring leakage.

For turbo-generators in the power range of 500 to 1000 MW it is also known to use water cooling of the stator winding as well as the rotor winding. In this case, the cooling ducts are made as tubes placed inside conductors in the windings. A problem in large machines is that the cooling tend become non- uniform, leading to temperature variations in the machine.

A recent progress within the field of high-voltage rotating electric machines is Powerformerz of Asea Brown Boveri AB which is a rotating electric machine that is based on solidly insulated high-voltage cables as described for example in WO 97/45919. By using high-voltage insulated-electric conductors in the stator winding, with solid insulation similar to that used in cables for

transmitting electric power, the voltage of the machine can be increased to such levels that it can be connected directly to the power network without intermediate transformers. Such electric machines generally operate at voltages exceeding 10-15 kV, and typically operate in the voltage range from 36 kV up to 800 kV or even higher. The concept requires the stator slots in which the cables are placed to be deeper than with conventional technology (thicker insulation due to higher voltage, and more turns in the winding).

With regard to cooling, the above type of electric machine with high-voltage cables in the stator winding allows forced cooling to be made at earth potential.

WO 97/45914 describes an arrangement for cooling the stator teeth of a rotating electric machine with solidly insulated high-voltage cables in the stator winding. The arrangement comprises axially-running tubes, electrically insulated, which are drawn through axial apertures in the stator teeth. The tubes are permanently glued in the apertures to ensure good cooling when a coolant is circulated in the tubes.

WO 99/17429 also describes cooling of a rotating electric machine with solidly insulated high-voltage cables in the stator winding. The stator is normally cooled by water running in stator ducts, the rotor is cooled by gas driven by means of conventional rotor fans, and a thermally insulating cylinder is arranged in the air gap between the rotor and the stator to protect the stator from being heated by the heated gas from the rotor.

SUMMARY OF THE INVENTION The present invention relates to a new principle for cooling high-voltage rotating electric machines.

It is a general object of the present invention to provide efficient cooling of rotating electric machines for high voltages, from 15 kV up to the voltage level of power networks.

It is another object of the invention to provide efficient cooling of an electric energy conversion system comprising such a high-voltage rotating electric machine.

In particular, it is an object to provide efficient cooling of rotating electric machines having a winding based on solidly insulated high-voltage cable.

These and other objects are met by the invention as defined by the accompanying patent claims.

The present invention is mainly concerned with the type of rotating electric machine that comprises at least one magnetic core and at least one electrical winding, the winding comprising a conductor, an inner semiconductive layer surrounding the conductor, a solid insulation layer surrounding the inner semiconductive layer and an outer semiconductive layer surrounding the insulation layer thus enclosing the electric field.

Briefly, the invention involves direct contact cooling of the magnetic core or cores and of at least part of the solidly insulated winding or windings by the same kinetic-energy carrying medium that drives or is driven by the rotating electric machine as the medium flows into contact with and past the core and the winding. This generally means that the rotating electric machine is arranged at least partly within the flow of the medium that drives or is driven by the rotating electric machine, and that the rotating electric machine and the whole energy conversion system is constructed as an open system allowing the medium to flow into contact with the magnetic core and the solidly insulated winding to enable direct contact cooling thereof.

The cooling principle according to the invention eliminates the need for forced cooling and separately driven cooling circuits.

Examples of energy carrying media are: streaming water driving a rotating electric machine in a hydro-generator plant, water being pumped by an impeller axially connected to a motor-driven rotating electric machine and flowing air driving a wind-power plant.

The winding is preferably made of high-voltage cable with solid extruded insulation similar to that used in cables for power distribution, such as an XLPE (cross-linked polyethylene) cable. Such a solidly insulated high-voltage cable generally comprises a conductor, an inner semiconductive layer surrounding the conductor, a solid insulation layer surrounding the inner semiconductive layer and an outer semiconductive layer surrounding the insulation layer.

Thanks to the solid insulation, which may be reinforced by coating the outer layer of the cable by a suitable polymer system, the electrical winding can operate in direct contact with the energy carrying medium.

In addition, by using such a high-voltage cable the voltage of the rotating electric machine can be increased to such levels that it can be connected directly to the power network without intermediate step-up transformers. Such rotating electric machines generally operate at voltages exceeding 10-15 kV, and typically operate in the voltage range from 36 kV up to 800 kV or even higher. In general, the power rating exceeds 1 MW, preferably exceeding 5 MW.

For corrosion control, the complete magnetic circuit is preferably coated by a suitable polymer, which acts as a water and moisture barrier.

Advantageously, the part of the winding that is placed in the end winding region outside of the magnetic core is cooled in direct contact with the medium, whereas the part of the winding that is placed within the magnetic core primarily is cooled by the cooling of the core. It should though be understood that small amounts of the medium may flow into the slots in which the winding is placed to give a direct cooling effect of the winding.

In general, the invention not only relates to cooling of the magnetic core and the electrical winding, but also to direct contact cooling of electrical connections, and power electronics connected to the rotating electric machine. These components are preferably also coated by a polymer for protection against water or moisture.

For additional cooling, the magnetic core or cores may be provided with one or more throughholes through which the energy carrying medium may flow.

In a rotating electric machine, the stator teeth as well as a yoke portion of the stator core are preferably provided with axially running throughholes to obtain efficient cooling of the stator core as well as indirect cooling of the stator winding, which is wound in stator slots between the stator teeth. In the same way, the rotor core may have axially running throughholes.

Preferred examples of electric energy conversion systems that may be cooled according to the invention are a hydro-generator plant submersed into streaming water, a high-voltage pumping system submersed into water, a high-voltage compressor and a multi-pole wind-power plant having a power rating of 3 MW and higher.

According to a second aspect of the invention, the rotating part, also referred to as the spinning member of the rotating machine, is constructed as an integrated combination of an electromagnetic element, such as an electromagnetic rotor, and a mechanical energy converter, such as a turbine.

For example, this principle makes it possible to integrate the hydro-turbine of a hydro-power plant into the rotor of the rotating electric machine.

The invention offers the following advantages: -Efficient cooling in a simple manner; -Eliminates the need for forced cooling; -Cooling circuits and heat exchangers are not necessary, rendering the overall manufacturing of the rotating electric machine easier; and -Savings in power plant space. In particular, the volume of the rock chamber for a hydro-power plant may be substantially reduced.

Other advantages offered by the present invention will be appreciated upon reading of the below description of the embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS The invention, together with further objects and advantages thereof, will be best understood by reference to the following description taken together with the accompanying drawings, in which: Fig. 1 is a simplified view, partially in section, of relevant parts of a hydro-power plant arrangement according to the invention; Fig. 2 is a simplified circuit diagram for the hydro-power plant according to the invention; Fig. 3 is a partially stripped view of a cable used by the invention; Fig. 4 is an axial end view of a sector of a magnetic circuit according to the invention; and

Fig. 5 is a simplified view, partially in section, of relevant parts of a wind-power plant according to the invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION Throughout the drawings, the same reference characters will be used for corresponding or similar elements.

By rotating electric machine is not only meant the traditional synchronous rotating machine but also double-fed machines, applications of asynchronous converter cascades, outer pole machines, synchronous flux machines, alternating current machinery as well as combinations thereof.

By electric energy conversion is generally meant the conversion that takes place in for example an electric power plant where mechanical energy is converted into electric energy and/or electric energy is converted into mechanical energy. Accordingly, hydro-generator plants, wind power plants, pump plants are all examples of electric energy conversion systems. Other terms sometimes used for electric energy conversion are electromechanical energy conversion and electromagnetic energy conversion.

Fig. 1 is a simplified schematic view of a hydro-power plant according to the invention. The hydro-power plant is based on a rotating electric machine 10, which basically comprises a stator 1A having a stator winding 1B, and a rotor 2A having a rotor winding 2B. The windings 1B, 2B are preferably made of solidly insulated high-voltage cable, as will be described in more detail below with reference to Fig. 3. A hydro-turbine 3 is integrated into the rotor 2A and the rotor/turbine 2A, 3 rotates around a shaft 4. The rotating electric machine 10 is submersed into streaming water 11, and connected to the walls 12 of the water tunnel of the power plant by means of an open frame structure (not shown) including conventional thrust bearings in connection with the rotor shaft 4. The substantially open architecture of the hydro-power plant

illustrated in Fig. 1 enables direct contact cooling of the stator core 1A, the rotor core 2A and the solidly insulated high voltage cable in the electrical windings 1B, 2B by the streaming water 11 as the water flows into contact with and past the cores and the windings.

As can be seen from Fig. 1, the magnetic cores of the stator and the rotor and the end windings are cooled in direct contact with the streaming water.

Some water will flow into the slots in which the windings are placed to contribute to the cooling of the coil side of the windings. However, the part of the windings that is placed within the magnetic cores is primarily cooled by the cooling of the cores.

The complete magnetic circuit, including the stator core, the rotor core and the windings, is preferably coated by a suitable polymer. In water, high- density polyethylene (HDPE), polypropylene and aliphatic polyketones are examples of suitable polymers.

Although not explicitly illustrated in Fig. 1, it should be understood that it is possible to cool electrical connections to the rotating machine, power electronics, reactors and other auxiliary equipment, in direct contact with the streaming water as well. All such components are preferably coated by a polymer for protection against water or moisture.

Preferably, each one of the stator core 1A and the rotor core 2A are provided with axially running throughholes (shown in Fig. 4) in which the water 11 may flow to provide additional cooling of the cores.

The high-voltage cable of the stator winding 1B normally changes from an unscreened cable 7 to a screened cable 9 at the cable splicing 8. As indicated in Fig. 2, the screened cable 9 is connected more or less directly to the power network 18 via a circuit breaker 17. The cable 9 may also be provided with

surge arresters 16 and other conventional auxiliary equipment for connection to the power network 18.

It is apparent that there are various ways of realizing an open architecture of the rotating electric machine and the overall electric energy conversion system such that the streaming water is allowed to flow into contact with the magnetic circuit in which the electrical losses are generated. The architecture may be more or less open, ranging from an almost completely open system with a substantially casing-free rotating electric machine to a partially open system which has a flow path for the streaming water in contact with the magnetic core and the winding.

The power plant of Fig. 1 is preferably designed for operation either as a generator to generate electric voltage for the power network as described above, or as a pump plant to be driven from the power network. In the latter case, the rotating electric machine of the power plant operates as a motor driving a pump impeller.

Conventionally, the rotating electric generator of a hydro-power plant is situated in a generator hall, which normally is in the form of a rock chamber. The hydro-turbine is located in the streaming water and connected to the generator in the rock chamber by a common shaft.

It is evident that the cooling principle according to the invention eliminates the need for the large rock chamber required by conventional hydro-power plants. In addition, the integration of the hydro-turbine into the rotor allows for a more compact hydro-generator arrangement.

Typically, the hydro-power plant according to the invention is a multi-pole system which operates with voltages in the range from 15 kV up to 800 kV or even higher, and has a power rating exceeding 1 MW. Preferably, the hydro-

power plant has more than 8 poles, an operating voltage exceeding 36 kV and a power rating substantially higher than 5 MW.

Fig. 3 shows a step-wise stripped end view of a high-voltage cable for use in a rotating electric machine according to the invention. The cable 20 comprises at least one conductor 21 which preferably is made up of a number of strands that together give the conductor a substantially circular cross section. Around the conductor 21 there is an inner semiconductive layer 22, which in turn is surrounded by solid insulation 23. The solid insulation layer 23 is surrounded by an outer semiconductive layer 24.

In the field of power engineering, there is generally no clear distinction between an electric wire and an electric cable. Usually though the term"wire"refers to a single, solid metallic conductor, with or without insulation, whereas the term "cable"refers to a stranded conductor or to an assembly of insulated conductors. Therefore, the term"cable"is used throughout the disclosure.

When used as an electrical winding, the cable should be bendable to enable assembly of the winding. Furthermore, the cable must retain its properties even when it is bent and when it is subjected to thermal or mechanical stress during operation. It is important that the layers retain their adhesion to each other in this respect. The material properties of the layers are decisive here, particularly their elasticity and relative coefficients of thermal expansion. The coefficients of thermal expansion should be harmonized to eliminate the risk of defects, cracks or the like at thermal movement in the winding.

For example, the insulation layer 23 may consist of a solid thermoplastic material such as low-density polyethylene (LDPE), high-density polyethylene (HDPE), polypropene (PP), polybutylene (PB), polymethyl pentane (PMP), cross- linked materials such as cross-linked polyethylene (XLPE), or rubber such as ethylene propylene rubber (EPR) or silicon rubber.

The inner and outer semiconductive layers 22,24 may be of the same basic material as the insulation layer 23 but with particles of conductive material such as soot (for example Carbon Black) or metal powder mixed therein. The conductivity of the semiconductive layers 22,24 is sufficient to substantially equalize the potential along the layers, respectively. The conductivity of the outer layer 24 is sufficiently large to contain the electrical field in the cable, but sufficiently small to not give rise to significant losses due to currents induced in the longitudinal direction of the layer. The inner semiconductive layer is arranged to be at substantially the same potential as the conductor, and the outer semiconductive layer is arranged in such a way that it essentially constitutes an equipotential surface surrounding the conductor. The outer semiconductive layer is connected to a chosen potential. According to the invention, the outer semiconductive layer is preferably held at earth potential by the conductivity of the surrounding water and/or by means of low-ohmic connections. For additional information on earthing of the winding, reference is made to WO 97/45929 and WO 99/17428.

In a so-called XLPE cable, also referred to as a PEX cable, the insulation layer consist of cross-linked, low-density polyethylene, and the semiconductive layers consist of polyethylene with both soot and metal particles mixed therein.

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

For more details on high-voltage cables intended as windings in rotating electric machines, reference is made to WO 97/45919.

Preferably, as can be seen from Fig. 3, the outer mechanically-protective sheath and the metal shield normally surrounding power distribution cables are eliminated. Instead, the outer semiconductive layer may be reinforced by

coating it with a suitable polymer system for additional protection against water or moisture. Such a polymer system may be produced from many materials, for example high-density polyethylene or more water tight though somewhat stiffer materials such as polypropylene and aliphatic polyketones. It should though be noted that the outer semiconductive layer is earthed before it is coated by the polymer.

Fig. 4 is an axial end view of a sector of a magnetic circuit according to the invention. The illustrated sector 30 shows a segment 31 of the stator and a segment 35 of the rotor with a rotor pole 34. From the radially outermost end of the stator, a number of teeth 32 extend radially inwards towards the rotor.

Between the teeth 32 there are a corresponding number of slots 33 in which the cable 20 is wound to form the stator winding of the rotating electric machine. The use of cables 20 permits deep slots, and the slots generally have a form resembling that of a bicycle chain. For additional cooling of the stator core and the cable-based winding, the stator core is advantageously provided with one or more substantially axially running throughholes 36 through which the streaming water may flow. To achieve efficient cooling, a number of throughholes 36 are preferably arranged in each stator tooth, the throughholes 36 of a stator tooth being radially aligned. Also, the outer yoke portion of the stator may be provided with a number of throughholes.

In the same way, the rotor and possibly also the rotor pole may be provided with axial throughholes 36.

Fig. 5 is a simplified view, partially in section, of relevant parts of a wind-power plant according to the invention. The overall architecture of the wind-power plant of Fig. 5 is similar to the hydro-power plant of Fig. 1, except for the horizontal set-up and the turbine not being integrated in the rotor. The wind- power plant is based on a rotating electric machine, which basically comprises a stator 1A having a stator winding 1B, and a rotor 2A having a rotor winding 2B. In the same manner as for the hydro-power plant described in connection

with Fig. 1, the windings 1B, 2B are made of solidly insulated high-voltage cable. A wind-turbine 13 is connected to the rotor 2A via a common a shaft 4.

The substantially open architecture shown in Fig. 5 enables exposure of the rotating electric machine with its magnetic cores and electrical windings to the flowing air 11 that drives the wind-turbine 13. In this way, the rotating electric machine is cooled in direct contact with the flowing air as the air flows into contact with and past the cores and the windings. The rotating electric machine is upheld by means of an open frame structure (not shown) including conventional thrust bearings in connection with the rotor shaft 4.

For sea-based wind-power plants, the air 11 that drives the power plant normally has relatively high amounts of sea salt. For corrosion control, the magnetic cores, electrical connections and power electronics connected to the electric machine are preferably coated by a polymer. In air, the number of suitable polymers that can be used is much larger. For example, the polymer coating may consist of any of the materials used as insulation in the high- voltage cable described above. With regard to the cable-based winding 1A, 1B, the outer semiconductive layer itself may provide sufficient protection against moisture and salty air. Of course, there is nothing that prevents the outer semiconductive layer from being reinforced by yet another layer consisting of a polymer.

The wind-power plant according to the invention is a multi-pole system, which typically operates with voltages exceeding 15 kV, and has a power rating exceeding 1 MW. Preferably, the wind-power plant has an operating voltage exceeding 36 kV and a power rating of 3 MW or higher. Normally the wind- power plant has more than 30 poles, and preferably more than 50 poles.

In all other aspects, the wind-power plant of Fig. 5 is similar to the power plant of Fig. 1, including axially running throughholes for the flowing air, and direct connection of the rotating electric machine to the power network.

The embodiments described above are merely given as examples, and it should be understood that the present invention is not limited thereto. For example it should be understood that in a rotating electric machine, the magnetic circuit may be located in either the stator or in the rotor of the machine, or in both.

Further modifications, changes and improvements which retain the basic underlying principles disclosed and claimed herein are within the scope of the invention.