AU4938999A

AU4938999A - A wind power plant

Info

Publication number: AU4938999A
Application number: AU49389/99A
Authority: AU
Inventors: Gunnar Kylander; Mats Leijon
Original assignee: ABB AB
Current assignee: ABB AB
Priority date: 1999-05-28
Filing date: 1999-05-28
Publication date: 2000-12-18
Anticipated expiration: 2019-05-28
Also published as: TR200103404T2; TW436581B; MXPA01011954A; BR9917306A; JP2003501993A; EE200100628A; NO20015811L; RU2221165C2; CN1352819A; AU759548B2; NO20015811D0; WO2000074198A1; CA2375125A1; EP1198872A1; AR024115A1

Description

WO 00/74198 PCT/SE99/00943 5 A WIND POWER PLANT 10 FIELD OF THE INVENTION This invention relates to a wind power plant comprising at least one wind power station, which comprises a wind turbine, an 15 electric generator driven by this wind turbine and a rectifier, and an electric direct voltage connection between the rectifier ar ranged at the wind power station and an inverter, the alternating voltage side of which is connected to a transmission or distribu tion network, the inverter being arranged on the network side of 20 the plant. The invention is preferably intended to be used in such cases where the connection between the generator and the transmis sion or distribution network includes a cable intended to be 25 submerged into water. Consequently, expressed in other words, it primarily relates to such applications where one or several wind turbines with associated generators are intended to be placed in seas or lakes, wherein the cable connection extends to the transmission or distribution network placed on land. Even 30 though the advantages of the invention in the following primarily will be dealt with in connection with location of the wind turbines in seas or lakes, the invention can, however, also imply advan tages in cases where the wind turbines and the generators are located on land and the connection, which in that case not nec 35 essarily has to consist of a cable but instead can be realized in the form of aerial lines or cables, connects several such wind WO 00/74198 PCT/SE99/00943 2 turbines/generators with the transmission or distribution net work. BACKGROUND OF THE INVENTION AND PRIOR ART 5 When locating wind power at sea it is required, in order to get economy in the project, that large groups of wind power stations are being located within a limited area. Sea based wind power requires relatively large wind power stations (3MW and above) 10 and a suitable total system power of 50-100 MW is expected. So far the planning of such wind parks has presupposed that the electrical power transmission is effected by traditional alternat ing current transmission in three-phase alternating voltage sea cable systems. In that case, the generator is almost always a 15 three-phase asynchronous generator. It is true that there are examples where synchronous generators have been used di rectly connected to the network, but this has as a rule resulted in that a complicated mechanical spring suspension has had to be installed between the generator and the engine house in or 20 der to dampen power variations caused by the varying character of the wind load. This depends on the fact that the rotor dynam ics of a synchronous generator mechanically behaves like a spring against a stiff alternating voltage network, whereas an asynchronous generator behaves like a damper. A conventional 25 asynchronous generator of 3 MW could presumably be made for about 3-6 kV and be connected in series with a transformer which steps up the voltage to, let us say 24 kV, in a first step. In a wind power park with 30-40 wind power stations there would then be provided a central transformer which further steps up 30 the voltage to 130 kV. The advantage with such a system is that it is cheap and does not require any complicated sub-systems. The disadvantage partly lies in the difficulties to technically transmit power over long distances in a high-voltage alternating voltage cable. This depends on the fact that the cable produces 35 capacitive reactive power, which increases with the length. The current through the conductor and in the cable shield then in- WO 00/74198 3 PCT/SE99/00943 creases so much that the cable cannot be realized for long dis tances. The other disadvantage lies in that the varying wind load causes voltage variations on the transmission line, which could affect the power consumers that are connected nearby. This ap 5 plies in particular if the network is "weak", i.e. has a low short circuiting power. Due to the abovementioned technical problems with long cable transmission distances, one might be forced to connect the wind park to a "weak" network. According to certain guiding principles, the voltage variation may not be more than 10 4%. Different countries have different regulations and as a rule the regulations are mitigated in case of a lower voltage level on the transmission line. Voltage variations could also have to be treated differently depending on time intervals. Rapid voltage variations causes "flicker", i.e. light variations in glow lamps, 15 which is regulated in rules. A solution to the problem above with long cable distances is to transmit the power with high-voltage direct voltage. The cable can then be drawn right up to a strong network. Another advan 20 tage is that DC-transmissions have lower losses than AC-trans missions. From a technical point of view the cable distance can then be of unlimited length. A HVDC-link consists of a rectifier station, a transmission line (cable or aerial line), an inverter station and filters for removing overtones generated during the 25 conversion. In an older variant of HVDC-links thyristors are used for rectification and inversion. Thyristors can be switched on but not switched off; the commutation takes place at the zero crossing of the voltage, which is determined by the alternating voltage, and the converters are therefore called line commu 30 tating. A disadvantage with this technique is that the converters consume reactive power and cause current overtones, which are sent out in the network. In a more modern direct voltage solu tion, IGBT:s are used instead of thyristors in the converters. An IGBT (Insulated Gate Bipolar Transistor) can be switched on as 35 well as switched off and furthermore has a high switch fre quency. This implies that the converters can be produced ac- WO 00/74198 PCT/SE99/00943 cording to a completely different principle, so called self-com mutating converters. To sum up, the advantages with self-com mutating converters are that they can deliver as well as con sume reactive power, which makes possible an active compen 5 sation of the voltage level on the network side if the network is weak. Consequently, this makes this type of converter superior to the older technique in the way that it can be connected to a network being situated closer to the wind power. The high switch frequency also leads to a reduction of the problem with over 10 tones as compared with the older generation of HVDC. A disad vantage is, however, that the losses in the converter station are higher as well as the price. A self-commutating converter is characterized in that the voltage is built up by a rapid pulse pattern, which is generated by the converter. The voltage differ 15 ence between the pulse pattern and the sinusoidal network volt age will lie above the inductances on the network side. There are two types of self-commutating inverters; a voltage stiff, VSI (Voltage Source Inverter) and a current stiff, CSI (Current Source Inverter), with somewhat different characteristics. VSI, 20 which has at least one capacitor on the DC-side, has the best power regulation. There have been built some experimental wind power stations using technique resembling the HVDC-concept, but for a com 25 pletely different reason, namely for achieving a variable rota tional speed of individual wind power stations. The generator of the wind power station is then disconnected from the network via a DC-link on low voltage, typically the 400 V or 660 V level. A variable rotational speed on the turbine gives energy gains at 30 the same time as it as a rule results in that the variations of ro tational speed can be used for eliminating the rapid power pul sations, which cause "flicker". However, it is of course not pos sible to eliminate the slow power changes, which are inherent in the nature of the wind load. The moment of inertia of the turbine 35 could be said to function as an intermediate storage for kinetic energy. In such a system a synchronous generator is not to any WO 00/74198 PCT/SE99/00943 5 disadvantage, but rather to an advantage, since the asynchro nous generator requires a more expensive and more compli cated rectifier. If it is desired to have a direct driven generator and consequently eliminate the need of a gear unit between the 5 turbine and the generator, the generator must be synchronous since it will be provided with so many poles. In other words, a direct driven generator requires a DC-intermediate link. In the concept it is also possible to actively regulate the moment by changing the trigger angle, if a controlled rectifier is used. In 10 most concepts having a variable rotational speed, an external active rotational speed control is furthermore provided by so called pitch control, which implies that the blade angle is changed on the turbine. A disadvantage with a variable rota tional speed according to the related concepts is the price of the 15 required power electronics and furthermore that the mainte nance of such power electronics out at sea will be difficult and costly. In WO 97/45908 a technical solution is suggested, which com 20 bines the good characteristics of a variable rotational speed system with the advantages of a HVDC-link of older model. By parallel connecting the wind power stations already in the DC intermediate link (see Fig 3 in said document) a number of N low-voltage inverters and one high-voltage rectifier are elimi 25 nated. According to this suggestion a rectifier with choke is to be used on the wind power turbine side and a central inverter with associated choke is to be used on the network side. The system seems at first hand to be intended for line commutated, or in any case current stiff, rectifiers and inverters, since the 30 chokes in the direct voltage link makes this current stiff. This has an advantage, namely that the DC-voltage after the rectifier can be varied within a large range. This is necessary in case of operation with variable rotational speed, since the generator in the wind power station at low rotational speeds only can deliver 35 a low output voltage. However, a disadvantage with a current stiff inverter is that it cannot regulate the reactive power through WO 00/74198 6 PCT/SE99/00943 the network as effectively as a voltage stiff inverter. Further more, it appears that the inverter in a direct current manner is to be connected in series with the parallel connected rectifiers in the wind power stations. This implies that the same direct cur 5 rent is output from the wind park as is input to the inverter on land. Furthermore, it appears that the voltage is presupposed to be on 6-10 kV, which is the typical voltage for conventional gen erators. This implies that the DC-voltage will be about 12 kV, which is an unrealistic low DC-voltage for transmitting a total 10 power of 50 MW. The losses in the cable will be very large. For a wind park of the size 50-100 MW it would instead be neces sary to transmit the power on a voltage level of about 100 kV. It is true that this would be possible if a transformer would be con nected to each generator and a sufficient number of valves 15 would be series connected in all rectifiers. However, if it is pos sible to avoid the transformer in the wind power station this would be a great advantage. Furthermore, to series connect the number of valves required for rectifying N output voltages for N wind power stations to 100 kV DC-voltage is associated with big 20 problems. PURPOSE OF THE INVENTION The purpose of the invention is to achieve, with a more simple 25 and cheap system for variable rotational speed, the same good power transmission from a sea-based wind park to the land based network as offered by a modern HVDC-system, with the possibility to eliminate the necessity of transformers and con trolled power electronics in the wind power stations. This is very 30 valuable since all maintenance carried out at sea is costly and difficult to perform. A further purpose of the invention is to be able to have such a high voltage on the DC-transmission that low losses are obtained also for a large wind park, for instance on 50-100 MW. 35 WO 00/74198 7 PCT/SE99/00943 SUMMARY OF THE INVENTION The purpose of the invention is achieved primarily through the features defined in the characterizing part of the subsequent 5 claim 1. The unsolved problem of prior technique that the DC voltage will be too low is consequently solved by connecting the DC/DC-converter out at sea with its low-voltage side electrically connected to the rectifier and its high-voltage side electrically connected to the inverter. Such a DC/DC-converter functions in 10 about the same way as a transformer for DC; it steps up the di rect voltage a factor n:1 and steps down the direct current as 1:n, where n is the conversion. This implies that the inverter and the rectifier are no longer connected in series. 15 According to a preferred embodiment, the rectifier is formed as a passive diode rectifier in series with a local step-up direct voltage converter. This is a more simple system than a line commutated rectifier and is considered to operate better at high voltages. The local step-up direct voltage converter suitably 20 consists of a choke, a series connected IGBT-valve and a series connected diode. This can also be the basic design of a DC/DC converter. Furthermore, it is preferred that the inverter is constituted by a 25 voltage stiff, self-commutated system, the characteristics of which are superior to a line commutated system from a power regulating point of view. Such a system is characterized, in an embodiment of the invention, in that at least one capacitor is connected in parallel over the inverter on the DC-link and that 30 inductances are connected in series with each phase on the network side. In a preferred embodiment, the valves are consti tuted by series connected IGBT:s. With the generator technology of today concerning wind power 35 stations, it is possible to produce a generator which can handle 10 kV, but higher voltages than that would be desirable. Fur- WO 00/74198 8 PCT/SE99/00943 thermore, the conventional insulation technology for stator windings is sensitive to temperature variations, humidity and salt, which a wind turbine generator is exposed to. 5 According to a particularly preferred embodiment of the inven tion, a solid insulation is used for at least one winding in the generator, which insulation preferably is performed according to the subsequent claim 14. The winding has more specifically the character of a high-voltage cable. A generator manufactured in 10 this way, creates the prerequisites of achieving considerably higher voltages than conventional generators. Up to 400 kV can be achieved. Furthermore, such an insulation system in the winding implies insensibility to salt, humidity and temperature variations. The high output voltage implies that transformers can 15 be completely excluded, which implies avoidance of the already mentioned disadvantages such as increase in costs, reduction in effectivity, risks of fire and risks for the environment. The latter are due to the fact that conventional transformers contains oil. 20 A generator having such a winding formed by a cable can be produced by threading the cable in slots performed for this pur pose in the stator, whereupon the flexibility of the winding cable implies that the threading work can be easily performed. 25 The two semiconducting layers of the insulation system have a potential compensating function and consequently reduce the risk of surface glow. The inner semiconducting layer is to be in electrically conducting contact with the electrical conductor, or a 30 part thereof, located inwardly of the layer, in order to obtain the same potential as this. The inner layer is intimately fastened to the solid insulation located outwardly thereof and this also ap plies to the fastening of the outer semiconducting layer to the solid insulation. The outer semiconducting layer tends to contain 35 the electrical field within the solid insulation.

WO 00/74198 9 PCT/SE99/00943 In order to guarantee a maintained adherence between the semiconducting layers and the solid insulation also during tem perature variations, the semiconducting layers and the solid in sulation have essentially the same thermal coefficient of expan 5 sion. The outer semiconducting layer in the insulation system is con nected to ground potential or otherwise a relatively low poten tial. 10 In order to achieve a generator capable of very high voltage, the generator has a number of features which have already been mentioned above and which distinctly differ from conventional technology. Further features are defined in the dependent 15 claims and are discussed in the following: Features which have been mentioned above and other essential characteristics of the generator and consequently of the wind power plant according to an embodiment of the invention com 20 prise the following: - The winding in the magnetic circuit is produced of a cable having one or several permanently insulated electrical con ductors with a semiconducting layer at the conductor and 25 outwardly of the solid insulation. Typical cables of this kind are cables having an insulation of cross-linked polyethylene or ethylene-propene, which for the purpose here in question are further developed concerning stands of the electrical con ductor and also the character of the insulation system. 30 - Cables having a circular cross section are preferred, but ca bles having another cross section can also be used, for in stance in order to achieve a better packing density.

WO 00/74198 1 PCT/SE99/00943 - Such a cable makes it possible to design a laminated core of the magnetic circuit in a new and optimal way as concerns slots and teeth. 5 - Advantageously, the winding is produced with a stepwise increasing insulation or the best utilization of the laminated core. - Advantageously, the winding is produced as a concentric ca 10 ble winding, which makes it possible to reduce the number of coil end crossings. - The shape of the slots is adapted to the cross section of the winding cable so that the slots are in the form of a number of 15 cylindrical openings extending axially and/or radially out wardly of each other and having constrictions running be tween the layers of the stator winding. - The shape of the slots is adapted to the cable cross section 20 in question and to the stepwise changing thickness of the in sulation of the winding. The stepwise insulation makes it pos sible for the magnetic core to have an essentially constant tooth width independent of the radial extension. 25 - The abovementioned further development concerning the cores implies that the winding conductor consisting of a num ber of layers brought together, i.e. insulated strands, does not necessarily have to be correctly transposed, and non-in sulated and/or insulated from each other. 30 - The abovementioned further development concerning the outer semiconducting layer implies that the external semi conducting layer is cut off at suitable points along the length of the cable and each cut-off partial length is directly con 35 nected to ground potential.

WO 00/74198 11 PCT/SE99/00943 The use of a cable of the type described above makes it possi ble that the hole length of the outer semiconducting layer of the cable, as well as other parts of the plant, can be kept at ground potential. An important advantage is that the electrical field is 5 close to zero in the coil end region outwardly of the outer semi conducting layer. With ground potential on the outer semicon ducting layer the electric field does not have to be controlled. This implies that there will occur no field concentrations neither in the core, nor in the coil end regions or in the transition sec 10 tion between them. The mixture of insulated and/or non-insulated strands packed to gether, or transposed strands, results in low eddy current losses. The cable can have an outer diameter in the order of 10 15 40 mm and a conductor area in the order of 10-200 mm 2 . According to a further embodiment, a transformer with variable transmission is arranged on the high voltage side of the inverter. 20 Further advantages and features of the invention will appear in the following and from the dependent claims. BRIEF DESCRIPTION OF THE DRAWINGS 25 With reference to the subsequent drawings, a closer description of embodiments of the invention given as examples will follow below. In the drawings: Fig 1 is a schematic axial end view of a sector of the stator in an 30 electric generator in the wind power plant according to the in vention. Fig 2 is an end view, partly cut, of a cable used in the stator winding according to Fig 1, 35 WO 00/74198 PCT/SE99/00943 12 Fig 3 is a schematic view, partly in section, of an embodiment of a wind power generator according to the invention, Fig 4 is a schematic view showing the embodiment of the wind 5 power plant according to the invention, and Fig 5 is a schematic perspective view showing an embodiment of a transformer with variable transformation. 10 DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS With the aid of Figs 1-3 the design of the generator 1 preferred in an embodiment of the invention is first explained. Fig 1 shows a schematic axial view through a sector of the stator 2. The rotor 15 of the generator is denoted as 3. The stator 2 is in a conven tional way formed of a laminated core. Fig 1 shows a sector of the generator corresponding to a pole pitch. From a yoke sec tion of the core, located furthest out in radial direction, a number of teeth 5 extend radially inwards towards the rotor 3 and these 20 teeth are separated by a slot 6, in which the stator winding is arranged. Cables 7 forming this stator winding are high-voltage cables which can be of essentially the same type as those used for power distribution, i.e. PEX-cables (PEX = cross-linked poly ethylene). A difference is that the external mechanically pro 25 tecting PVC-layer and the metal shield normally surrounding such a power distribution cable have been eliminated so that the cable for the present invention only comprises the electrical conductor and at least one semiconducting layer on each side of an insulating layer. The cables 7 are schematically illustrated in 30 Fig 1, wherein only the electrically conducting central part of each cable section or coil side is shown. It appears that each slot 6 has a varying cross section with alternating broad parts 8 and narrow parts 9. The broad parts 8 are essentially circular and surround the cable, waist sections between the broad parts 35 forming the narrow parts 9. The waist sections serve to radially fix the position of each cable. The cross section of the slot 6 be- WO 00/74198 13 PCT/SE99/00943 comes narrower radially inwards. This depends on that the volt age in the cable sections are lower the closer they are situated to the radially innermost part of the stator 1. Thinner cables can therefore be used inwards, whereas thicker cables are required 5 further out. In the illustrated example, cables with three different dimensions and arranged in three correspondingly dimensioned sections 10, 11, 12 of the slot 6 are used. A winding 13 for aux iliary power is arranged furthest out in the slot 6. 10 Fig 2 shows a stepwise cut end view of a high-voltage cable for use in the generator. The high-voltage cable 7 comprises one or several electrical conductors 14, each of which comprises a number of strands 15, which together give a circular cross sec tion. The conductors can for instance be of copper. These con 15 ductors 14 are arranged in the middle of the high-voltage cable 7 and in the shown embodiment each of the conductors is sur rounded by a partial insulation 16. It is however possible to omit the partial insulation 16 on one of the conductors 14. In the shown embodiment the conductors 14 are surrounded by a first 20 semiconducting layer 17. Around this first semiconducting layer 17 there is an insulation layer 18, e.g. of PEX-insulation, which in its turn is surrounded by a second semiconducting layer 19. Consequently, the concept "high-voltage cable" does not, in this application, have to comprise any metal shield or any external 25 protective layer of the type normally surrounding a power distri bution cable. In Fig 3 a wind power station is shown with a magnetic circuit of the type described with reference to Figs 1 and 2. The generator 30 1 is driven by a wind turbine 20 via a shaft 21. Even though the generator 1 can be direct driven by the turbine 20, i.e. that the rotor of the generator is coupled fixed in rotation to the shaft of the turbine 20, there can be a gearing 22 between the turbine 20 and the generator 1. This can for instance be constituted by a 35 single-step planetary gearing, the purpose of which is to change up the rotational speed of the generator in relation to the rota- WO 00/74198 PCT/SE99/00943 tional speed of the turbine. The stator 2 of the generator carries the stator windings 23, which are built up of the cable 7 de scribed above. The cable 7 can be unsheathed and pass on into a sheathed cable 24 via a cable joint 25. 5 In Fig 4, which in a schematic form broadly illustrates the wind power plant, two wind power stations 29 connected in parallel are illustrated, each having a generator 1. The number of wind power stations can of course be larger than two. Furthermore, a 10 rectifier 27 is comprised in each wind power station 26. The parallel connection of the wind power stations takes place at the point indicated with 28. An electric direct voltage connection is present between the 15 rectifiers 27 arranged at the wind power stations 26 and an inverter 30, the alternating voltage side of which is connected to a transmission or distribution network. The inverter 30 is ar ranged on the network side of the plant. This normally implies that the inverter 30 is located on land relatively close to the 20 transmission or distribution network 31. However, the wind power stations 26 including the generators and the rectifiers 27 are located at sea on suitable foundations. The direct voltage connection 29 comprises a section denoted as 32 in Fig 4, which section in practice can be very long. Along this section there is, 25 consequently, a connection part 33, which is critical in regard of losses. In the preferred embodiment of the invention, this con nection part 33 is considered to be formed of an underwater ca ble, namely in the case that the wind power stations 26 are situ ated out at sea or in a lake. However, the connection part 33 30 can also be formed of one or several aerial lines or cables. The plant comprises a DC/DC-converter 34 having a low-voltage side electrically connected to the rectifiers 27 and a high-volt age side electrically connected to the inverter 30. The DC/DC 35 converter 34 is arranged on the wind power station side of the plant. Expressed in other words, this implies that the previously WO 00/74198 15 PCT/SE99/00943 discussed connection part 33 is situated between the DC/DC converter 34 and the inverter 30. In practice, the converter 34 is considered to be placed on one of the foundations that are car rying one of the wind power stations 26 or alternatively there 5 can be a particular foundation for the converter 34. Independent of which type of foundation the converter 34 is placed on, the foundation in question is also provided with bus-bars in order to parallel connect the occurring wind power stations. 10 The converter 34 is arranged in such a way that it operates as a direct voltage increaser, i.e. that the direct voltage in the con nection part 33 between the converter 34 and the inverter 30 is intended to be, through the converter, higher and suitably sub stantially higher than the voltage on the input side of the con 15 verter 34. It is preferred that the inverter 30 is a voltage stiff self-commu tated inverter. A capacitor 35 is parallel connected over the DC link of the inverter 30. 20 The inverter 30 suitably has network inductances 36 connected in series with each phase on its network side. It is preferred that the inverter comprises series connected IGBT:s. 25 According to a preferred embodiment, the generators are syn chronous generators with permanent magnetized rotors. With advantage, the rectifiers 27 are passive rectifiers. This eliminates the necessity of active power control electronics out 30 at sea. As passive rectifiers, diode rectifiers are preferred. These diode rectifiers 27 are in series with a local step-up direct voltage converter 37. In a preferred embodiment, each separate converter 37 comprises a choke, a series connected IGBT-valve 39 and a series connected diode 40. The converter 34 could be 35 formed like such a step-up direct voltage converter.

WO 00/74198 16 PCT/SE99/00943 In Fig 5, a preferred embodiment according to the invention of a transformer with variable transmission is illustrated. The ad vantage with this transformer is that its windings are provided with a solid insulation in a similar manner as already described 5 with respect to the generator with reference to Figs 1 and 2. Consequently, the transformer windings are correspondingly built up with an insulation system comprising at least two semi conducting layers 17, 19, each of which constitutes essentially equipotential surfaces, and the solid insulation 18 is situated 10 between these semiconducting ,layers. Consequently, in the transformer according to Fig 5 the windings will also have the character of flexible cables. On the whole, all the features of the winding cable according to Fig 2 related to above in connection with the generator apply, with the exception that the outer semi 15 conducting layer 19, in the transformer phase, does not have to be cut up in parts along the length of the cable in order to ground these parts each by itself. The advantage of such a transformer with a solid insulation resides in a substantial im provement in effectivity in that the electrical field essentially will 20 be kept inside the outer semiconducting layer, and furthermore the important advantage is achieved that the inflammable and ecologically harmful oil occurring with conventional transformers is eliminated. 25 In Fig 5, the transformer is illustrated in a principle form for one of the phases in question. Men skilled in the art will of course realize that, in the case of a multi-phase embodiment, cores having more limbs than two and associated yoke can entail that all the phase windings are placed on one and the same core. 30 However, it is of course also possible to use a separate core for each phase in a transformer of this type. Consequently, a transformer core consisting of a yoke and two limbs is illustrated in Fig 5, a main winding 43 being applied 35 around one of the limbs and a control winding 44 being arranged around the other limb. The main winding can either be consti- WO 00/74198 PCT/SE99/00943 tuted of a primary winding or a secondary winding. Conse quently, the control winding 44 is used for varying the transfor mation of the transformer. The control winding 44 is arranged in the form of winding turns wound onto a drum 45, which drum is 5 rotatable about the core limb in question. The drum 45 is driven by means of a suitable, not shown motor, e.g. via belt driving. Consequently, the control winding 44 is functioning as a variable coil. The number of winding turns on the control winding drum 45 is varied by means of a rotatable storage drum 46 for the 10 winding 44. The winding drum 46 is also motor-driven in a suit able way. In Fig 5 it is illustrated how an end section 47 of the control winding is grounded. This end section 47 is stationary and is in electrically conducting connection with the control winding 44 on the drum 45 via a slipring contact device of a kind 15 known per se. There is a winding section 48 also in connection with the storage drum 46, which winding section is stationary and which is intended to be connected to the electrical equip ment in question. In order to electrically connect the winding section 48 with the control winding section received on the 20 winding drum, a corresponding slipring contact device is pro vided. From the description above, it appears that the transmission of the transformer can be varied rapidly and to a desired degree by 25 rotating the drums 45 and 46 so that a desired number of control winding turns are present on the drum 45. A prerequisite in this connection is that the control winding 44 is formed of the previ ously described, flexible high-voltage cable having solid insula tion. 30 The invention is of course not only limited to the described em bodiments. Several detail modifications are consequently possi ble and realized by men skilled in the technical field as soon as the basic inventional idea has been presented. Such detail 35 modifications and equivalent embodiments are included within the scope of the subsequent claims.

Claims

1. A wind power plant comprising at least one wind power sta tion (26), which includes a wind turbine (20), an electric genera 5 tor (1) driven by this wind turbine and a rectifier (27), and an electric direct voltage connection (29) between the rectifier (27) arranged at the wind power station and an inverter (30), the al ternating voltage side of which is connected to a transmission or distribution network (31), the inverter being arranged on the 10 , network side of the plant, characterized in that it comprises a DC/DC-converter (34) having a low voltage side electrically con nected to the rectifier (27) and a high voltage side electrically connected to the inverter (30), and that the DC/DC-converter (34) is arranged on the wind power station side of the plant. 15

2. A device according to claim 1, characterized in that the inverter (30) is a voltage stiff self-commutated inverter.

3. A plant according to claim 1 or 2, characterized in that a ca 20 pacitor (35) is parallel connected over the DC-link of the inverter (30).

4. A plant according to any of claims 1-3, characterized in that the inverter (30) on its network side has network inductances 25 (36) connected in series with each phase.

5. A plant according to any of the preceding claims, character ized in that the inverter (30) comprises series connected IGBT:s. 30

6. A plant according to any of the preceding claims, character ized in that the generator (1) is a synchronous generator with permanent magnetized rotor. 35

7. A plant according to claim 6, characterized in that generator (1) is direct driven by the wind turbine without gear unit. WO 00/74198 19 PCT/SE99/00943

8. A plant according to any of the preceding claims, character ized in that the rectifier (8) is a passive diode rectifier. 5

9. A plant according to claim 7 or 8, characterized in that a step-up direct voltage converter (37) is arranged in series with the passive rectifier (27) on the low-voltage side of the DC/DC converter (34).

10 10. A plant according to claim 9, characterized in that the step up direct voltage converter (37) comprises a choke (38), at least one series connected IGBT-valve (39) and at least one series connected diode (40) series connected. 15

11. A plant according to any of the preceding claims, charac terized in that several wind power stations (26), each compris ing a wind turbine (20), a generator (1) and a rectifier (27), are parallel connected on the low-voltage side of the DC/DC-con verter (34). 20

12. A plant according to claim 11 and any of claims 9 and 10, characterized in that each wind power station (26) comprises a local step-up direct voltage converter (37). 25

13. A plant according to any of the preceding claims, wherein the generator (1) comprises at least one winding (7), charac terized in that the winding is provided with a solid insulation (18). 30

14. A plant according to claim 13, characterized in that the winding comprises an insulation system comprising at least two semiconducting layers (17, 19), each of which essentially con stitutes equipotential surfaces, and that the solid insulation (18) is located between these semiconducting layers. 35 WO 00/74198 20 PCT/SE99/00943

15. A plant according to claim 14, characterized in that at least one of the semiconducting layers (17, 19) has essentially the same thermal coefficient of expansion as the solid insulation (18). 5

16. A plant according to any of claims 13-15, characterized in that the winding is formed of a high-voltage cable (7).

17. A plant according to any of claims 14-16, characterized in 10 that the innermost (17) of the semiconducting layers has essen tially the same potential as an electric conductor (14) located inwardly of this layer.

18. A plant according to claim 17, characterized in that the in 15 ner one (17) of the semiconducting layers is in electrically con ducting contact with the conductor (14) or a part thereof.

19. A plant according to any of claims 14-18, characterized in that the outer one (19) of the semiconducting layers is con 20 nected to a potential being fixed in advance.

20. A plant according to claim 19, characterized in that the fixed potential is ground potential or otherwise a relatively low potential. 25

21. A plant according to any of the preceding claims, charac terized in that the direct voltage connection (30) comprises a cable (33) intended to be submerged into water or one or sev eral aerial lines or cables. 30

22. A plant according to any of the preceding claims, charac terized in that a transformer (41) with variable transformation is arranged on the network side of the inverter (30). 35

23. A plant according to claim 22, characterized in that the transformer with variable transformation comprises at least one WO 00/74198 PCT/SE99/00943 core (41) and a control winding (44) around the core, and that the transformer comprises means for transmission of a variable part of the control winding to or from at least one storage means (46). 5

24. A plant according to claim 23, characterized in that the control winding is arranged on a rotatable control winding drum (45). 10

25. A device according to any of claims 23-24, characterized in that the storage means (46) comprises a rotatable storage drum.

26. A plant according to any of claims 22-25, characterized in that the winding(-s) (43, 44) of the transformer is (are) formed of 15 a flexible cable having a solid insulation.

27. A plant according to claim 26, characterized in that the in sulation is included in an insulation system, which besides the solid insulation comprises at least two semiconducting layers, 20 each of which constitutes essentially equipotential surfaces, the solid insulation being located between these semiconducting layers.