Background Art The use of a DC transmission cable implies that it is not necessary to take into ac- count the capacitive load generated by the cable. In addition, it is possible to make the HVDC transmission cables longer than the AC transmission cables. These AC transmission cables must not exceed a so-called"critical length".

W097/45908 discloses a wind power park where each wind turbine is equipped with a synchronous generator. The output power of the synchronous generator is rectified by means of an AC/DC rectifier and carried through a DC transmission cable to a DC/AC inverter and a transformer so as to be transferred to the regional supply network. Means are provided for regulating the power. This known wind power site is encumbered with the draw-back that it cannot to a sufficient extent compensate for sudden, strong gusts, and it is not equipped with an HVDC output by means of a transformer with several output windings.

In addition, the AC/DC rectifier comprises controlled rectifiers, which are able to compensate for possible variations in the speed of rotation through a suitable control by means of particular control circuits. However, such control circuits are rather complicated.

Brief Description of the Invention.

The object of the invention is to provide a wind turbine plant where each wind tur- bine can tolerate sudden gusts to a far higher extent than previously known, and where each wind turbine is of a far more simple structure than hitherto known.

A wind turbine plant of the above type is according to the invention characterised in that it comprises a magnetic field controller connected to the synchronous generator, said magnetic field controller being adapted to vary the magnetic field in the synchro- nous generator in response to an output parameter of said synchronous generator in such a manner that possible variations in the speed of rotation are compensated for, whereby a possible increase of the speed of rotation causes a reduction of the magnet- ic field in the synchronous generator. As a result, passive rectifier elements suffice in the rectifier. In addition, the controllable rectifiers and the associated control circuits are avoided which should otherwise be used for compensating for possible variations in the speed of rotation.

Moreover according to the invention the magnetic field controller may be adapted to detect the current generated by the synchronous generator, a negative feedback being established by means of the magnetic field controller for regulating the current through the rotor winding.

In addition, the magnetic field controller may according to the invention be adapted to detect the voltage generated by the synchronous generator, a negative feedback being established by means of said magnetic field controller.

Moreover, the magnetic field controller may according to the invention be adapted to detect the power generated by the synchronous generator, a negative feedback being established by means of said magnetic field controller.

Furthermore, the negative feedback may according to the invention include a P, I or D regulation, optionally a combination of said regulations.

Moreover, the rotor of the synchronous generator may according to the invention be dimensioned with a relatively low inductance. As a result, the time constant of the magnetic field controller can be reduced.

Moreover, the rotor may according to the invention be adapted to rotate at a rela- tively high speed of rotation. As a result it is possible to further reduce the induc- tance of said rotor.

When the generator furthermore is multipolar, it is possible to further reduce the inductance of the rotor.

Brief Description of the Drawings The invention is explained in greater detail below with reference to the accompanying drawings, in which Fig. 1 shows a wind power plant according to the invention comprising a synchro- nous generator and an AC/DC rectifier, Fig. 2 illustrates a magnetic field controller for the synchronous generator of Fig. 1, Fig. 3 shows a transformer connected to the synchronous generator, Fig. 4 illustrates the entire plant, where the AC/DC rectifier has been shown in greater detail, and Fig. 5 shows the voltage versus the speed of rotation at various magnetizing currents

to the synchronous generator.

Best Mode for Carrying Out the Invention The wind power plant shown in Fig. 1 comprises a number of arms 1 secured to a hub communicating with a synchronous generator 3, optionally through a gear 2. The synchronous generator 3 is preferably a conventional three-phase synchronous gener- ator with an energy supply to the rotor winding not involving a collector ring. The three-phase output of the synchronous generator 3 communicates with an AC/DC rectifier 7 through a possibly three-phase transformer 5. The AC/DC rectifier 7 delivers a DC voltage to a DC transmission cable 9. The most simple embodiment of the high-voltage rectifier 7 is formed by ordinary diodes coupled in series, and accordingly it is a passive rectifier. The series coupling of the diodes is established by means of several output windings on the output of the transformer 5. In this manner the voltage is reduced to all the diodes, and the harmonic flows in the gener- ator/transformer are reduced. The three-phase transformer 5 can be designed as indicated in Fig. 3, where the primary side is connected to the generator output and comprises a star connection, and the secondary side is connected to the AC/DC rectifier 7 and can be composed of a A-connection and two combined A-star connec- tions. The voltages generated by the secondary side of the transformer 5 are trans- ferred to an AC/DC rectifier in form of a so-called B6 diode bridge, cf. Fig. 4. This B6 diode bridge comprises a total of eighteen rectifier elements, viz. six on each secondary winding, where each of the three phase conductors of each secondary winding is connected to the connection point of two rectifier elements coupled in the same direction, said three pairs of rectifier elements being coupled in parallel. The parallel coupling of the rectifier elements associated with each of the three secondary windings is subsequently coupled in series with the result that an HVDC-voltage is transmitted from the combined coupling of rectifier elements to an HVDC transmis- sion cable 9. This transmission cable 9 can be several km long, such as 10 km. The use of such a DC transmission cable 9 instead of an AC cable is advantageous in the

length being arbitrary and almost unlimited. Thus it is not a question of a critical length as in connection with an AC cable. The end of the DC transmission cable 9 can be connected to a conventional DC/AC inverter converting into a mains fre- quency and be connected to the regional supply network optionally through a three-- phase transformer. Measures have, of course, been taken to ensure that the alternat- ing voltage generated by the DC/AC inverter 7 is in phase with the regional supply network.

A demand exists, of course, for a possibility of running the wind power plant at a speed of rotation depending on the wind speed. However, an increase of the speed of rotation implies that a high voltage is generated because the voltage is proportional to the speed of rotation, cf. the curves of Fig. 5. However, the output voltage is also proportional to the magnetic field in the generator 3, which can be utilized for stabi- lizing the voltage in case the speed of rotation is changed. The latter has according to the invention been obtained by means of a magnetic field controller 4 detecting an output parameter of the generator 3, such as the current and the voltage or the prod- uct thereof. This magnetic field controller 4 regulates the current supply to the rotor windings in the generator in response to the output parameter. As a result, a negative feedback is established with the result that when the output power is increased the current supply to the rotor winding 3a is reduced, whereby the system automatically seeks equilibrium. When the speed of rotation for instance is increased to 130% relative to an ordinary speed of rotation, the magnetizing current to the rotor winding 3a is reduced to 80%, cf. Fig. 5. When, on the contrary, the speed of rotation de- creases to 80 % relative to the ordinary speed of rotation, the magnetizing current to the rotor winding 3a is increased to 130%.

Fig. 2 shows an embodiment in greater detail of the magnetic field controller 4, and it appears that in two of the three phases of the generator the currents IG1, IG2 and the voltages UG1, UG2, respectively, are detected. These parameter values are multiplied in pairs to obtain an expression of the output power Pgen. This output

power Pgen is compared with a reference value Prof, and the difference therebetween, viz. the error signal, is transferred to a regulation unit 11. In response to this error signal, the regulation unit generates a value for the magnetizing current Im which is to be transferred to the rotor winding, optionally through a PWM 12 (Pulse Width Modulator) and an amplifying power transistor 13 controlling the supply of current to the rotor winding. The negative feedback is established by the detected power Pgen being deducted from the reference power Pref. As a result a detected increased power Pgen implies that the error signal is reduced and that the power supplied to the rotor windings is reduced as well.

The power for the magnetizing of the generator 3 is typically of the magnitude 1 % of the nominal power of the generator.

A portion of the magnetizing can optionally be provided by means of permanent magnets, where an electric magnetizing is then used for controlling the speed of rotation. The speed of rotation is downwardly limited by a maximum magnetizing current due to the limited thermal properties of the rotor windings and the magnetic structure of the generator, viz. the magnetic saturation. In order to extend the lower limit of the speed of rotation it is therefore advantageous when either the generator is provided with an additional output presenting an increased nominal output voltage or the transformer is provided with an input presenting a reduced nominal voltage.

These additional inputs are only to be dimensioned for low power as the wind energy is low at a low speed of rotation.

According to a particularly advantageous embodiment, the wind turbine comprises a transformer with n output windings coupled in series with rectifiers so as to obtain an HVDC. The output windings can be coupled as a star or a A or as a combination thereof in order to obtain a sinusoidal input current.