WO2001095463A1

WO2001095463A1 - A device for direct voltage generation and a plant for generating electric power

Info

Publication number: WO2001095463A1
Application number: PCT/SE2001/001139
Authority: WO
Inventors: Peter Hessling; Gunnar Kylander
Original assignee: Abb Ab
Priority date: 2000-06-06
Filing date: 2001-05-22
Publication date: 2001-12-13
Also published as: SE0002093L; SE0002093D0; AU2001264454A1

Abstract

A device for generating direct voltage comprises a generator, a rotor having magnetic poles and a stator provided with a plurality of windings (14, 20) and surrounding the rotor as well as means for converting the alternating voltage generated in the stator windings through rotation of the rotor into direct voltage. Said rectifying means comprise members (15, 21) integrated with the generator and associated with the stator windings (14, 20) thereof in a circuit comprising the windings for rectifying of the alternating voltage generated across the individual windings through said rotation and adding direct voltages so obtained for delivering a direct voltage on the output of the generator.

Description

A DEVICE FOR DIRECT VOLTAGE GENERATION AND A PLANT FOR GENERATING ELECTRIC POWER

FIELD OF THE INVENTION AND PRIOR ART

The present invention relates to an apparatus for direct voltage generation comprising a generator, a rotor having magnetic poles and a stator provided with a plurality of windings and surrounding the rotor as well as means for converting the alternating voltage generated in the windings of the stator through the rotation of the rotor into direct voltage, as well as a plant for generating electric power.

The invention is not restricted to any particular ranges with respect to power and voltage levels.

The energy source through which the rotor of the generator receives the kinetic energy may be any, but the invention is particularly directed to apparatuses of said type having a generator receiving energy from wind power, so that it is a turbine driven by wind that drives the rotor to rotate, and the invention and the problems upon which it is based will hereinafter be discussed for exactly that energy source, although the invention is not in any way restricted to exactly wind power.

When generating direct voltage while utilizing wind power special demands are put on such an apparatus, since the input power and thereby the load on the apparatus will vary strongly, since the wind velocities vary a lot both on short and long terms. This requires a great flexibility of such apparatuses. Furthermore, the demands of robustness and reliable function of such an apparatus in particular in the case of wind power are very high, since it is often difficult to gain access to the generator and it is connected with considerable problems and thereby costs to carry out repairs or replacing components. Many new power plants are also expected to be remotely located, as for example at sea. The height of the towers may substantially complicate such works.

A possible apparatus is very schematically illustrated in Fig 1 and has a generator 1 of said type, which on the output thereof generates an alternating voltage, which is rectified in a rectifier 2 schematically indicated, such as a 6-pulse bridge. This rectifier 2 is controlled by a control arrangement 3. Moreover, there may possibly be a filter 4 with a capacitor 5, resistor 6 and an inductor 7 for filtering different disturbances of the voltage away, such as harmonics, as well as phase compensation. The rectified voltage is then led to an inverter 8, which is controlled by a control arrangement 9, for example according to a determined pulse width modulation pattern (PWM) so as to obtain an alternating voltage delivered to a three phase alternating volt- age network 10.

Another type of such an apparatus is known through WO 97/45907 of the applicant, which describes how a direct voltage on high level may be generated without using any transformer by utilizing a high voltage generator. Different ways of designing and controlling the rectifying of the alternating voltage generated in the stator windings of the generator are described there. SUMMARY OF THE INVENTION

The object of the present invention is to provide an apparatus of the type defined in the introduction, which is robust and has a high reliability and flexibility with respect to adaption to different operation situations.

This object is according to the invention obtained by providing such an apparatus, in which said rectifying means comprise members integrated with the generator and associated with the stator windings thereof in a circuit including the windings for rectifying the alternating voltage generated across the individual windings through said rotation and adding the direct voltages so obtained for delivering a direct voltage on an output of the gen- erator.

By building in the very rectifying function in this way in a generator having a plurality of windings and carry out a separate rectifying of the alternating voltage generated in each individual stator winding or group of stator windings the apparatus gets more robust than would be the case should the rectifying of an alternating voltage generated by the generator take place outside the generator. The entirety will not at all be that dependent upon that each individual part functions, as is the case in a rec- tifying of a higher alternating voltage with one single rectifying circuit outside the generator. The improved reliability and robustness results in a reduced number of necessary maintenance and repair works. Furthermore, the individual rectifying of the different alternating voltages of the stator windings results in an improved possibility to control the rectifying function and adapting it to operation conditions varying strongly, such as for wind power plants. Another advantage is that the total induced voltage is distributed on a plurality of stator windings and thereby gets distributed on the physical rectifying members. The voltage over each rectifying part will therefore be fully controllable and restricted to what the magnetic induction may cause, not to what possible asymmetric voltage dividing components may create. A known problem of voltage sharing of rectifying components connected in series is hereby avoided. This voltage sharing, division or distribution increases the robustness and the reliability of the apparatus.

According to another preferred embodiment of the invention the apparatus has an asymmetry with respect to said stator windings in the sense that at least one stator winding is differently designed than other stator windings and/or at least the rectifying members associated with one stator winding are different to the construction and/or the control than the rectifying members associated with other stator windings. "Asymmetry" is here to be given a very broad interpretation and is intended to cover all types of deviations from symmetry for stator winding and rectifying for the different groups, except for evenly distributed electric phase angle for the induced voltages and their correspondence with opposite signs. Exactly through the . fact that each winding/group of windings contributes to only one direct voltage no electric symmetry in the form of for example three-phase is needed. To the contrary, there are apparent advantages of an asymmetric distribution of the different phases and/or rectifying components and/or rectifying circuits according to this embodiment. The fact that primarily enables this asymmetry is that the inducing alternating voltage is immediately rectified in rectifying members integrated with the generator, as well as the robust voltage sharing obtained by the presence of a plurality of windings, in which this robustness gets the better the more windings existing, in which these are preferably more than three. Thus, the generator is constructed only for functioning with the intended rectifying configuration. However, the symmetry from the generator's point of view has of course to be maintained for maintaining mechanical balance. If the level of the direct voltage only has to be regulated within a narrow interval, it gets for in- stance possible to provide for example all windings except one with passive rectifying members, while this single winding has controllable rectifying members so as to obtain a regulation desired. It would then also be well possible to make the windings with different size when suited, i.e. provide them with different numbers of winding turns, so that said one regulating winding could be smaller than the other stator windings. These different asymmetries enable achieving of a desired regulation in an advantageous way from the cost and efficiency point of view.

According to a preferred embodiment of the invention at least one rectifying member associated with one stator winding is of passive type with one or more passive rectifying parts, in which a passive rectifying part may be a rectifying diode. According to another preferred embodiment of the invention at least one rectifying member associated with one stator winding is of active type and comprises one or more controllable rectifying parts, which for example may be semiconductor devices of turn-off type, such as GTO-s or IGBT-s. It is of course a great advantage to have the regulation possibility built in through the active rectifying member, since it gets possible to cancel variations of the wind power arriving through the rotor through regulation in this way, but a mixing of the two types of rectifying members may be particularly advantageous, so that passive rectifying members are arranged for certain stator windings, while active ones are arranged for others. A substantial saving of costs with respect to the case of arranging active rectifying members for all stator windings and a simplifying of the regulation is hereby obtained. Furthermore, the regulating range of the active rectifying members may be utilized considerably better, since each such active rectifying member has to achieve a regulation over a wider voltage range. A great advantage is also that the losses upon rectifying gets lower, since the passive rectifying members have normally lower losses than the active ones, i.e. a diode has usually a lower loss than controllable valves with GTO-s or IGBT-s. The regulation is advantageously primarily carried out so as to eliminate rapid power fluctuation and/or for regulating the number of revolutions. Regulation of the number of revolutions may be desired to increase the efficiency of the turbine. There may be a regulating winding for regulating the number of revolutions and another one (probably smaller) for regulating rapid power fluctuations.

The majority of the stator windings are preferably associated with rectifying members of passive type, and more exactly it is advantageous that at least 80% of the stator windings are associated with rectifying members of passive type to obtain just said advantages.

The rapid power variations called ripples and which are caused by the commutation of a rectifying member with a low pulse number may be reduced through a plurality of windings with dif- ferent phase positions, without substantially increasing the total number of components. The smoothing is made directly at the generation, which results in no possibility for fluctuations to propagate in the system. The need of large smoothing components, such as reactors and capacitors, is hereby reduced. A side effect is that also stresses on the power plant and the generator decreases by reduction of the moment pulsations on the generator axle and the construction upon direct smoothing in the generation point.

According to another preferred embodiment of the invention the apparatus comprises for at least one stator winding members for phase compensation of the alternating voltage generated in the individual winding through said rotation. The possibility of the occurrence of any drawbacks or negative properties of the volt- age outside the generator is by this further reduced. If such a phase compensation member is divided on a great number of individual windings both an improved possibility to appropriate phase compensation on varying need thereof and a high reliability and robustness are obtained through the fact that a fail- ure of a phase compensation member enables continued operation of the apparatus without any requirement of an immediate replacement of this phase compensation member, such as a capacitor, and when the replacement takes place this may happen without any interruption of the operation. It may then be advantageous if the phase compensation member comprises members adapted to switch in one or more capacitors in a controllable way in series or in parallel with the stator winding in question so that the phase compensation may be adapted to the operation conditions prevailing.

According to another preferred embodiment of the invention filters are adapted to select that part of the variations of voltage and/or power which is desired to be cancelled by regulation. By means of filters the most undesired part of the disturbances may be cancelled out. How the filter is selected depends upon the character of the disturbance and what is causing most problems in the following part. Only the magnitude of the disturbance filtered out is then reduced, so that the regulation may be made over a smaller range.

According to another preferred embodiment of the invention the number of stator windings is higher than three, preferably higher than 10. By arranging a very high number of stator windings the advantages mentioned above with respect to robustness, reliability and adaptability to operation conditions varying strongly are increased. Thus, the entirety gets even less dependent upon that a certain small part functions.

According to another preferred embodiment of the invention at least one stator winding has another number of winding turns and so generates an alternating voltage of another level upon rotation of the rotor than another of the stator windings. It is hereby possible to obtain the most natural voltage distribution among the power components, such as capacitors and diodes, connected to the generator. It may then be advantageous to as- sociate smaller windings with active rectifying members so as to provide a control possibility while utilizing a greater part of the regulation range of the semiconductor device in question. This also means that if one of the rectifying members of such a smaller winding drops out this is not that important, since it may be compensated in the other windings.

According to another preferred embodiment of the invention the stator windings considering the distribution of the magnetic poles of the rotor, are placed in different phase positions for generating alternating voltages having a mutual phase shift therein. This phase shift may then be made arbitrary, so that an additional parameter is obtained for an optimum overall rectifying of the alternating voltage generated by the rotor in the stator windings.

According to yet another preferred embodiment of the invention the apparatus comprises an arrangement adapted to control said rectifying means so that the power delivered by the generator is kept substantially constant on variations of the medium flow arriving to the rotor. Electric disturbances downstream of the generator in lines and networks may by this be reduced and the operation of for example an inverter may be simplified. This is in an advantageous way obtained by an adaption of the angle of turning on and/or turning off of semiconductor devices included in said rectifying members, i.e. it is adjusted, so that the power delivered is kept substantially constant.

According to another preferred embodiment of the invention the apparatus comprises an arrangement adapted to control semiconductor devices included in the rectifying members so as to adjust the input power to the rectifying members for obtaining a maximum power on the output of the generator. This is preferably also done through an adjustment of the angle of turning on and/or turning off of the semiconductor devices, and it is of course advantageous to be able to ensure that a maximum power may all the time be drained on the output of the generator. According to another preferred embodiment of the invention the apparatus comprises an arrangement adapted to control controllable semiconductor devices included in at least one of said rectifying members depending upon electric conditions prevail- ing in the apparatus so as to consider a more or less predictable development of these conditions at the control. It is possible with a certain probability/accuracy to predict and influence the future power development in the generator on the basis of known facts through such a forward coupling. This is particularly well possible for wind power plants, in which certain changes are repeated periodically as a consequence of tower and dead passages of the propeller blades.

According to another preferred embodiment of the invention the generator is of the type with cable winding with a cable having an insulation system surrounding an inner electric conductor and comprising at least two semiconducting layers constituting equi- potential surfaces and a solid insulation arranged therebetween. By using such a generator with a cable winding, which is known for example through the WO-publication mentioned above, the division of the overall stator winding into different windings of the generator gets very simple. The electric field is controlled through the cable insulation, so that no further electrical problems may occur.

The invention also relates to an apparatus of the type defined in the introduction, in which the number of stator windings is higher than six. The advantages of a high number of stator windings of a generator in an apparatus of this type with respect to primarily the robustness appear clearly from the description above. According to a particularly preferred embodiment of the invention the number of stator windings is higher than 12, higher than 20, higher than 40 or higher than 100.

The invention also relates to a plant for generating electric power, which comprises at least two apparatuses according to the invention with an arrangement adapted to control said rectifying means so that the direct voltage on said output of the generator is kept substantially constant upon changes of the medium flow arriving to the rotor, and these two apparatuses are connected in parallel and connected to one and the same input of a direct voltage intermediate link. By in this way ensuring that the generators deliver a direct voltage and this direct voltage is equally high a great number of generators may be connected to the same direct voltage intermediate link and by that one and the same inverter be used for converting the direct voltage into alternating voltage and feeding power to an alternating voltage transmission network. This also means that the alternating voltage network will only feel properties of the inverter assumed to keep a constant power out during a short period of time, and no voltage variations as a consequence of the wind generators will occur, since said generators all the time deliver a constant direct voltage and thereby constant power. It will also be comparatively simple to add new and remove old generators when required, for example when a wind power park is expanded.

Further advantages as well as advantageous features of the invention appear from the following description and the other dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

With reference to the appended drawings, below follows a description of preferred embodiments of the invention cited as ex- amples.

In the drawings:

Fig 1 is a very schematic view of a possible plant for generating electric power, Fig 2 is a view corresponding to Fig 1 , which very schematically illustrates the principle of the present invention,

Fig 3 is a simplified view of an apparatus according to a first preferred embodiment of the invention,

Fig 4 is a graph illustrating the development of possible power disturbances over time,

Fig 5 is a view corresponding to Fig 3 of an apparatus according to a second preferred embodiment of the invention,

Fig 6 is a detail view of a possible phase compensation member illustrated in Figs 3 and 5,

Fig 7 is a very simplified view of an apparatus according to a third preferred embodiment of the invention,

Fig 8 is a simplified view of a plant for generating electric power according to the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

The very basic principle of the invention is very schematically illustrated in Fig 2, in which an apparatus according to the invention is illustrated through a box 1 1 , and it appears that the simplification with respect to the prior art according to Fig 1 is considerable, since rectifying, phase compensation and filtering have been "lifted" into the generator, i.e. these different functions have been associated directly with the stator windings of the generator.

It is illustrated in Fig 3 how an apparatus according to the in- vention may be constructed, and the different configurations of the stator windings of the generator included in the apparatus arranged in the series connection between the output terminals 12, 13 is very schematically shown here. The apparatus has smaller windings 14 associated with rectifying members 15 of the type full wave bridge with two branches 16, 17 connected in parallel and having each at least two rectifying parts 18 connected in series, here in the form of controllable parts, more exactly in the form of thyristors of turn-off type (GTO-s) or IGBT-s. Furthermore, an arrangement 19 is arranged for controlling the semiconductor devices 18 in a desired way. Furthermore, some stator windings 20 of the generator are arranged in a so called 6-pulse bridge 21 with three branches 22-24 connected in parallel and each having at least two rectifying parts connected in series, here in the form of passive parts, namely diodes 25, and a stator winding 20 is for each branch connected to a mid- point between the diodes and the three stator windings are at opposite ends connected to a common point 26. This rectifying circuit may easily be generalized for an arbitrary number of phases, with four phases 8 pulses per period are obtained, with n phases 2n pulses are generated out from the bridge. The stator windings 20 are larger than the stator windings 14, so that the IGBT-s or GTO-s used in the rectifying member 15 may be utilized to an optimum with respect to the regulation range. The costs for the rectifying members simultaneously get lower if more passive semiconductor devices may be used. A rectifying of the alternating voltage generated in each stator winding through the rotation of the rotor will through this configuration take place locally at the stator winding in question, and through the series connection of the rectifying members 15 and 21 the direct voltages so formed will be added and an overall direct voltage be applied on the output terminals 12, 13 of the generator.

It is also shown how phase compensation members 43 in the form of a capacitor 27 connected in parallel with the winding 14, 20 and a switching device 28 connected in series therewith may be arranged directly at an individual stator winding. By switching in and switching out the capacitor 27 through a control arrangement 43 a desired phase compensation may take place directly inside the very machine, i.e. in connection with the generator, so that no drawbacks are allowed to occur outside the apparatus 1 1 . The dashed symbols for the windings on the rectifying members 15, 21 are there for indicating that it may be a question of a winding with phase compensation members 43.

It is also possible to filtrate the power measured through for ex- ample low pass, band pass or high pass filters before regulation. From a possible disturbance or noise illustrated in Fig 4 a high pass filter may filter out the high frequency variations 29 considered to be a problem for then being cancelled by regulation, while the low frequency power variations having a frequency of about 0.05 Hz may stay after the regulation, since they are considered to be less disturbing. The power development across a winding is here shown versus the time t in seconds.

An apparatus of this type for generating a direct voltage is very useful for energy sources, such as wind power, which are neither completely controllable nor stable. It may typically be used in wind power plants where the wind changes all the time and very rapidly. Disturbances from the tower and from other adjacent power stations also exist, which it is necessary to compen- sate. A rapid electronic control illustrated is required for achieving this, since a mechanical control or regulation of for example the rotor blades would be far too slow.

An apparatus according to another preferred embodiment of the invention is illustrated in Fig 5, in which it is shown that different types of rectifying members may be mixed in many different ways. Besides this, it is as already mentioned possible to have different numbers of winding turns of the different stator windings and to arrange them distributed with respect to the angle arbitrarily around the rotor so as to obtain particular phase positions of the alternating voltages generated therein. The rectify- ing member 30 is of a similar type as the rectifying member 15 in the embodiment according to Fig 3, but the controllable semiconductor devices are here replaced by passive semiconductor devices in the form of diodes 31 . Also there is no phase compensation in connection with the winding 14. The rectifying member 32 is of the same type as the rectifying member 15 according to Fig 3, but no phase compensation is present here. The rectifying member 33 differs from the rectifying member 15 in Fig 2 by the fact that the semiconductor devices are passive diodes 31 . It is easily understood that further combinations are possible. The number of stator windings may be arbitrary and is preferably more than three, and advantageously also more than 10, while the number well may be as high as 50 or 100.

An alternative to the phase compensation member 34 associated with the stator winding 14 is shown in Fig 6. It is shown how an optional number of capacitors 27 may be switched in through the control arrangement 35 for a suitable phase compensation.

It is schematically illustrated in Fig 7 how an apparatus of the type shown in Figs 3 and 5 advantageously is controlled through a determined control algorithm illustrated through the box 36, which receives information from the generator 1 about different parameters thereof, such as alternating voltage across the different phase windings and the like. The regulating algorithm influences the different controllable (turn on and/or turn off) semiconductor devices of the apparatus, which are here summarized through the box 37. More exactly, the angle of turning on (firing and/or turning off) of the semiconductor devices is controlled so that the direct voltage out on the output of the apparatus and thereby across the capacitor 38 is kept substantially constant. The regulation is simultaneously made so that the input power to the generator is adjusted so that a maximum power comes out on the output thereof. By controlling the rectifying of the generator in this way, so that the voltage out thereof is substantially constant, it is possible to interconnect several apparatuses according to the invention to a common direct voltage intermediate link 39 in the way shown i Fig 8. It is by this possible to utilize one single inverter 40 for an amount of different wind power plants or stations so as to feed electric power out through this on an alternating voltage transmission network 41 . Voltage fluctuations are eliminated also directly at the generation source without propagating to the alter- nating voltage network 41 . Forward coupling loops are used so as to control the direct voltage out from the respective apparatus and into the direct voltage intermediate link 39, i.e. the current and the voltage further on are predicted by looking at the history. This control type will be successful thanks to the char- acter of the disturbances. These are namely in the wind power case substantially of two types: 1 . Varying wind. However, the great moment of inertia of the turbine results in slow variations of the rotation speed. 2. The blade passage close to the tower. This leads to a nearly harmonic amplitude modulation of the voltage with a frequency of mn Hz, in which m is the number of revolutions per second and n the number of turbine blades. 3. The effect of shadow, so-called dead position, from other wind power stations. The effect out from the station/the stations gets also in this case more or less periodically varying. All distur- bances of the types mentioned above result in a high short time correlation of the voltage, which means that it is possible to comparatively precisely predict the future behaviour of the signal.

The invention is of course not in any way restricted to the preferred embodiments described above, but many possibilities to modifications thereof will be apparent to a man with skill in the art without departing from the basic idea of the invention as defined in the appended claims. It would for example be possible to build in completely different combinations of rectifying members and other equipment together with the stator windings of the generator, but it would also be possible to have the same type of rectifying members for all stator windings, should that be desired.

Tap changers could be arranged for switching in and switching out a plurality of windings forming the regulating winding by means of power electronic. The windings could be of binary type with respect to voltage generation according to 1 , 2, 4...2^N. This is very schematically illustrated by 44 in Fig 3.

"Adding of direct voltages so obtained" in the claims is to be put equal to an addition of the direct voltages across the different windings, i.e. a series connection of the windings with associated rectifying members.

Claims

1 . An apparatus for direct voltage generation comprising a generator (1 ), a rotor having magnetic poles and a stator provided with a plurality of windings and surrounding the rotor as well as means for converting the alternating voltage generated in the windings of the stator through the rotation of the rotor into direct voltage, characterized in that said rectifying means comprise members (1 5, 21 , 30, 32-33) integrated with the generator and associated with the stator windings (14, 20) thereof in a circuit including the windings for rectifying the alternating voltage generated across the individual windings through said rotation and adding the direct voltages so obtained for delivering a direct voltage on an output of the generator.

2. An apparatus according to claim 1 , characterized in that it has an asymmetry with respect to said stator windings in the sense that at least one stator winding (14) differently designed than other stator windings (20) and/or at least the rectifying members (21 ) associated with one stator winding are different to the construction and/or the control than the rectifying members (15) associated with other stator windings.

3. An apparatus according to claim 1 or 2, characterized in that at least one rectifying member (21 , 30, 33) associated with one stator winding is of passive type with one or more passive rectifying parts (25, 31 ).

4. An apparatus according to any of claims 1 -3, characterized in that at least one rectifying member (15, 32) associated with one stator winding is of active type and comprises one or more controllable rectifying parts (18).

5. An apparatus according to claim 3, characterized in that said passive rectifying parts are diodes.

6. An apparatus according to claim 4, characterized in that said controllable rectifying parts are semiconductor devices of turn- off type, such as GTO-s (Gate Turn-off Thyristor) or IGBT-s (Insulated Gate Bipolar Transistor).

7. An apparatus according to any of the preceding claims, characterized in that it comprises for at least one stator winding members (27, 28, 34) for phase compensation of the alternating voltage generated in the individual winding through said rota- tion.

8. An apparatus according to claim 7, characterized in that the phase compensation member comprises at least one capacitor (27).

9. An apparatus according to claim 8, characterized in that the phase compensation member comprises members (35) adapted to controllably switch in one or more capacitors (27) in parallel with the stator winding in question.

10. An apparatus according to any of the preceding claims, characterized in that at least one rectifying member associated with one stator winding is of a different type and/or construction than one rectifying member associated with another stator winding.

1 1. An apparatus according to claims 3 and 4, characterized in that a majority of the stator windings are associated with rectifying members of passive type.

12. An apparatus according to claim 11 , characterized in that at least 80% of the stator windings are associated with rectifying members of passive type.

13. An apparatus according to any of the preceding claims, characterized in that it comprises at least one rectifying mem- ber (21 ) formed by a so-called 6-pulse bridge with three branches (22-24) connected in parallel having each at least two rectifying parts connected in series and for each branch having a stator winding (20) connected to a mid point between said rec- tifying parts.

14. An apparatus according to any of the preceding claims, characterized in that at least one rectifying member (15) is formed by a so-called full wave bridge with two branches (16, 17) connected in parallel each having at least two rectifying parts (18) connected in series and a stator winding (14) connected between midpoints of the respective branch between the rectifying parts of the branch connected in series.

15. An apparatus according to claims 13 and 14, characterized in that at least one said full wave bridge is of active type with controllable rectifying parts and at least one said 6-pulse bridge is of passive type with passive rectifying parts.

16. An apparatus according to any of the preceding claims, characterized in that it has filters adapted to select that part of the variations of voltage and/or power which is desired to be cancelled by regulation.

17. An apparatus according to claim 16, characterized in that said filters are high-pass filters adapted to remove disturbances with a frequency above a predetermined level.

18. An apparatus according to claim 17, characterized in that the high-pass filter has said predetermined level at 2 Hz.

19. An apparatus according to any of the preceding claims, characterized in that the number of stator windings (14, 20) is higher than three.

20. An apparatus according to claim 19, characterized in that the number of stator windings (14, 20) is higher than six, higher than 12, higher than 20, higher than 40 or higher than 100.

21 . An apparatus according to any of the preceding claims, characterized in that at least one stator winding has another number of winding turns and generates by that an alternating voltage of another level upon rotation of the rotor than another of the stator windings.

22. An apparatus according to any of the preceding claims, characterized in that the stator windings (14, 20), considering the distribution of the magnetic poles of the rotor, are placed in different phase positions for generating alternating voltages having a mutual phase shift therein.

23. An apparatus according to any of the preceding claims, characterized in that it comprises a regulating stator winding constituted by a plurality of small windings and a tap changer (44) adapted to switch in or out these small windings for voltage regulation.

24. An apparatus according to any of the preceding claims, characterized in that the generator is of the type with cable winding with a cable having an insulation system surrounding an inner electric conductor and comprising at least two semiconducting layers constituting equipotential surfaces and a solid insulation arranged therebetween.

25. An apparatus according to any of the preceding claims, characterized in that it comprises an arrangement (36) adapted to control said rectifying means so that the power on said output of the generator (1 ) is kept substantially constant at variations of the medium flow arriving to the rotor.

26. An apparatus according to claims 4 and 25, characterized in that the arrangement (36) is arranged to adapt the angle for turning on and/or turning off semiconductor devices included in said rectifying members so as to keep said power substantially constant.

27. An apparatus according to any of the preceding claims, characterized in that it comprises an arrangement (36) adapted to control semiconductor devices included in the rectifying members so as to adjust the input power to the rectifying members for obtaining a maximum power on the output of the generator.

28. An apparatus according to any of the preceding claims, characterized in that it comprises an arrangement (36) adapted to control controllable semiconductor devices included in at least one of said rectifying members in dependence of electric conditions prevailing in the apparatus for considering a predictable development of these conditions at said control.

29. An apparatus according to any of the preceding claims, characterized in that the generator is included in a wind power plant having a turbine driven by wind adapted to drive the rotor to rotate.

30. An apparatus according to any of the preceding claims, characterized in that it is adapted to generate a direct voltage on the output of the generator exceeding 1 kV, preferably exceeding 10 kV.

31. A plant for generating electric power, characterized in that it comprises at least two apparatuses according to claim 25 connected in parallel and connected to one and the same input of a direct voltage intermediate link (39).

32. A plant according to claim 31 , characterized in that it comprises an inverter (40) connected to the direct voltage intermediate link (39) and in common to said apparatuses, said inverter being adapted to convert direct voltage into alternating voltage for delivering to an alternating voltage network (41 ).

33. An apparatus for generating direct voltage comprising a generator (1 ), a rotor having magnetic poles and a stator provided with a plurality of windings and surrounding the rotor as well as means for converting the alternating voltage generated in the windings of the stator through the rotation of the rotor into direct voltage, characterized in that the number of stator windings is higher than six.

34. An apparatus according to claim 33, characterized in that the number of stator windings is higher than 12, higher than 20, higher than 40 or higher than 100.