AU2010268928B2

AU2010268928B2 - Hydrostatic drive of a wind turbine

Info

Publication number: AU2010268928B2
Application number: AU2010268928A
Authority: AU
Inventors: Peter Meuser; Torsten Pleikis; Manfred Turk
Original assignee: Inw Beteiligungs GmbH
Current assignee: Inw Beteiligungs GmbH
Priority date: 2009-07-09
Filing date: 2010-07-08
Publication date: 2014-11-20
Anticipated expiration: 2030-07-08
Also published as: EP2452069B1; AU2010268928A1; WO2011003405A2; EP2452069A2; WO2011003405A3

Abstract

The invention relates to a hydrostatic drive for wind turbines, which gives wind turbines power plant characteristics. For this purpose, the connection between the rotor and the generator is established by a total volumetric flow conveyed by a hydraulic pump (1), which is driven by the rotor (19), wherein the total volumetric flow can be distributed to one or more closed hydraulic sub-circuits (T

Description

1 Hydrostatic drive of a wind turbine Drives with mechanical gears and gearless direct drives are now predominantly being used as drive concepts for convention al wind turbines. Apart from other serious problems which exist in respect of the said drive concepts, it is not possible or only possible to a limited extent to implement with them conventional wind turbines with genuine power plant characteristics, i.e. wind power plants. For the further expansion of the use of wind en ergy for power generation in operation in parallel with the network or in isolated operation it is precisely these genuine power plant characteristics that are indispensable. These power plant characteristics are achieved in more than 99% of the conventional power plants that are operated world wide by means of the use of synchronous generators for power generation that are directly connected to the network or to the consumers. These directly connected synchronous generators require a constant rotational speed for their drive. An energy conversion device is known from WO 2008/113699 A2 where the rotational speed of the rotor and the driving speed of a hydraulic motor are to be controllable independent of each other in order to achieve a constant rotational speed of the drive shaft of a generator thus facilitating the use of synchronous generators. This energy conversion device has the disadvantage that it does not work as described. Embodiments of the invention may provide a hydrostatic drive for wind turbines which gives them power plant characteris tics. Disclosed herein is a hydrostatic drive of a wind power plant. The hydrostatic drive comprises Hydrostatic drive of a wind power plant comprising a rotor which can be driven by the wind energy and, for the generation of a rotation, a rotor shaft connected with the rotor. The drive further comprises a hy draulic pump which can be driven via the rotor shaft for the conversion of the rotation energy of the rotor shaft into hy draulic energy. The drive further comprises a hydraulic motor combination which can be connected via hydraulic lines with the hydraulic pump and can be driven by the same. In the hy draulic motor combination a conversion of the hydrostatic en ergy into a rotation movement is accomplished and this move ment of the hydraulic motor combination comprising a common shaft or being rigidly coupled to each other is transmitted to a drive shaft of a synchronous generator for the generation of electrical energy. The hydrostatic drive in a first embodi 5800274_1 (GHMatters) P89486.AU BENB 2 ment only comprises a closed hydraulic circuit or, alterna tively, in a second embodiment, several closed hydraulic sub circuits, to which the total volumetric flow which is conveyed by the hydraulic pump is distributed. The circuit and/or each sub-circuit comprise(s) a hydraulic motor combination having an adjustable hydraulic motor and a fixed hydraulic motor, wherein the adjustable hydraulic motor and the fixed hydraulic motor of each circuit each have a flow (VP) from the hydraulic pump on the high-pressure side and a return (RP) to the hy draulic pump on the low-pressure side, with the connection from and to the hydraulic pump being made indirectly in each case via a valve set. A proportional current control valve is arranged in line with the volumetric flow in the respective high-pressure side flow (VP). On the hydraulic motor side downstream of the proportional current control valve an anti cavitation valve switched in parallel and connecting the flow (VV) on the high-pressure side and the return (RN) on the low pressure side is arranged, and the displacement of the adjust able hydraulic motors can be set by a controller such that during isolated operation the rotational speed of the drive shaft of the associated synchronous generator assumes a con stant value so that the synchronous generators of the circuits can be connected directly to a load on the output side and during operation in parallel with a network at a constant ro tational speed determined by the network a power equilibrium exists between the power collected by the rotor from the wind and the power emitted by the generators, taking account of the conversion losses in the hydrostatic drive and in the genera tors. Also disclosed herein is a method for the operation of a hy drostatic drive of a wind power plant in which a rotor is driven by wind energy and drives a rotor shaft that is coupled with the rotor which in turn drives a hydraulic pump for the conversion of the rotation of the rotor shaft into a hydraulic energy. The total volumetric flow which is conveyed by the hydraulic pump is distributed to one closed hydraulic circuit or, alternatively to several closed hydraulic sub-circuits. Each circuit comprises a hydraulic motor combination having an adjustable hydraulic motor and a fixed hydraulic motor. The adjustable hydraulic motor and the fixed hydraulic motor of each circuit each have a flow (VP) from the hydraulic pump on the high-pressure side and a return (RP) to the hydraulic pump on the low-pressure side. The connection from and to the hy draulic pump is made indirectly in each case via a valve set. A proportional current control valve is arranged in line with the volumetric flow in the respective high-pressure side flow (VP). On the hydraulic motor side downstream of the propor tional current control valve an anti-cavitation valve switched in parallel and connecting the flow (VIV) on the high-pressure side and the return (RN) on the low-pressure side is arranged. In each case an adjustable hydraulic motor and a fixed hydrau 5800274_1 (GHMatters) P89486.AU BENB 3 lic motor are arranged on a common shaft or rigidly coupled to each other and drive the drive shaft of a synchronous genera tor. The displacement of the adjustable hydraulic motors is set by a controller such that the rotational speed of the drive shaft of the associated synchronous generator assumes a constant or approximately constant value, and the synchronous generators of the circuits are connected directly to a network or to a load on the output side. In accordance with embodiments of the invention a positive connection between rotor and generators in the drive train of the wind power plant is waived. This connection is substituted by a total volumetric flow conveyed by a hydraulic pump which is driven by the rotor, wherein the total volumetric flow can be distributed to one or more closed hydraulic sub-circuits (T-,), wherein each sub-circuit (T-,) comprises a hydraulic motor combination having an adjustable hydraulic motor and a fixed hydraulic motor, wherein the hydraulic motors are ar ranged on a common shaft or rigidly coupled to each other and drive the drive shaft of a synchronous generator. The displacement of the adjustable hydraulic motors is set by a controller such that, during isolated operation, the rota tional speed of the drive shaft of the associated synchronous generator assumes a constant or approximately constant value so that the synchronous generators of the sub-circuits (T-,) can be connected directly to a load on the output side. During operation in parallel with the network the rotational speed of the synchronous generators is set by the network so that the controller then sets the drive torque of the genera tor shafts in such a way that a torque equilibrium between the rotor and the generators exist. Thus, the synchronous generators are independent in both modes of operation of the rotor rotation and can rotate with the relevant rotational speed which is forced by the network and/or which is required by the loads to be supplied. The control of the volumetric flow and of the working pressure within the closed hydraulic system which is required for this is accomplished by the adjustable hydraulic motors the dis placement of which can be controlled very quickly and accu rately by means of an electro-hydraulic adjustment of the pis ton displacement of their preferable axial pistons. The maximal total volumetric flow in each sub-circuit (T-,) of the hydraulic motor combination results from the sum total of the displacements of fixed hydraulic motor and adjustable hy draulic motor at an adjustment of 100 %, with the displacement of the fixed hydraulic motor being smaller, but maximally 5800274_1 (GHMatters) P89486.AU BENB 4 equal to the maximal displacement of the adjustable hydraulic motor. The major part, at least, however, half of the volumetric flow in the respective sub-circuit (T-,) is to be received by the adjustable hydraulic motors of the hydraulic motor combination of each sub-circuit (T-n). At a given total volumetric flow, the system pressure and thus the torque opposite the rotor and in the end the rotational speed of the rotor are directly influenced via the volumetric flow control. This facilitates the rated power control and its limitation to the rated power of the wind power plant by means of the systematic rotational speed control of the rotor. The desired rotational speed of the rotor can be precisely main tained at any point in time. Furthermore, the hydrostatic drive can reduce the speed of the rotor directly and practically without wear and tear when this is required by the operational regime. The kinetic energy of the rotor which is converted during this operation is generat ed on a short-term basis as heat energy in the hydraulic oil and can easily be dissipated on account of the high, volume conditioned thermal capacity of the entire hydraulic system. This is a reliable method that protects the plant and that is almost independent of external influences, and the use of which is not possible in this manner in conventional plant technology. This type of output control makes it also possible to do without the mechanical adjustment of the rotor blades around their longitudinal axis (pitch). Correspondingly arranged overpressure valves in the hydraulic system of the main drive make it possible to limit the oil pressure to the design-dependent maximal value. A system pres sure in excess of the opening pressure and thus an excessive torque as counter-torque for the rotor are reliably ruled out. Instead thereof, the speed of the rotor will, as a general rule, be reduced. This is connected with a reduction of its aerodynamic efficiency. Consequently, at a reduced rotational speed it takes less energy from the wind so that by means of a systematic speed reduction the plant output can be controlled and limited. In the event that the braking torque applied onto the rotor by the hydraulic pump at maximum pressure in the presence of ex treme wind gusts does not result in a reduction of the rota tional speed of the rotor, the rotor at first reacts with higher rotational speeds which can, however, not overload the plant and which are in the end countered by the security sys tem when permissible limit values are exceeded. 5800274_1 (GHMatters) P89486.AU BENB 5 The advantages which may be achieved by embodiments of the in vention are that the use of the hydrostatic drive according to the invention makes it possible to erect and operate wind power plants with genuine power plant characteristics, and that these wind power plants provide, even when there is no wind and they are consequently unable to generate electricity, a valuable system service for the network to which they are connected by the hooking-up of their synchronous generators motor-wise as phase shifters and the provision of capacitive or inductive reactive power up to the amount of their rated output. The hydrostatic drive according to the invention is to be ex plained by means of embodiments in which Fig. 1 shows a hydrostatic drive with two closed sub circuits, Fig. 2 shows a valve set and Fig. 3 shows a hydrostatic drive with a closed sub-circuit. Fig. 1 shows a hydrostatic drive with two closed sub-circuits Ti and T2. The hydrostatic drive is, in its essence, a fluid drive, in corporated into the flow of power between rotor 19 and one or several synchronous generators 5. It comprises the hydraulic pump 1, the valve sets 2, the hydraulic motors 3 and 4, the synchronous generators 5 as well as the hydraulic system com prising the high-pressure store 6, the low pressure store 7, the feed pump 8, the return filter 9, the oil heat exchanger 10, the pressure control valve 11 and the high-pressure pump 12 with a reversing valve to the high-pressure feeding 13 as well as the on/off valve 14 of the flush valve. The hydraulic motor of each sub-circuit Ti and T2 is a fixed hydraulic motor 3 and the other one is an adjustable hydraulic motor 4. The hydraulic pump 1 which is mechanically directly connected with the rotor 19 converts the mechanical energy of rotor 19 with a comparatively low loss into hydraulic flow energy and discharges the same through a total of 8 ports, of them 4 ports each for flow VP and 4 ports for return RP, to two sepa rate closed hydraulic sub-circuits Ti and T2. In both sub circuits the flow energy is fed to a hydraulic motor combina tion of the fixed hydraulic motor 3 and the adjustable hydrau lic motor 4 which convert the same again into mechanical ener 5800274_1 (GHMatters) P89486.AU BENB 6 gy for the driving of the synchronous generators 5 which are connected with the hydraulic motor combinations. The synchronous generators 5 carry out the final energy con version into electric energy. The same is fed directly into the network or supplied to the connected consumers. A slow-speed hydraulic positive-displacement pump, preferably a radial piston pump, is used as hydraulic pump 1. It is pref erably dimensioned such that its speed-torque characteristic curve conforms as exactly as possible to that of rotor 19 at an optimal total efficiency of the hydraulic pump 1. The hydraulic motor combinations comprise one fixed hydraulic motor 3 each in an inclined disk design or an inclined piston design and an adjustable hydraulic motor 4, preferably in in clined disk design. The adjustment units for the electro-hydraulic control of the displacement of the adjustable hydraulic motors 4 form a con structive unit with the same. One hydraulic motor combination 3, 4 each is mechanically con nected with one synchronous generator 5 each. For this purpose the casing of the hydraulic motor combination 3, 4 is firmly connected, preferably screwed up with the housing of the syn chronous generator 5 via a motor bearer. The shafts of the hy draulic motors are rigidly interconnected and the output shaft of the hydraulic motor combination 3, 4 and the drive shaft of synchronous generator 5 are preferably connected by a flexible coupling. The adjustable hydraulic motor 4 and the fixed hydraulic motor 3 of each sub-circuit Ti and T2 each have a flow (VIV) on the high-pressure side and a return (RN) on the low-pressure side which are indirectly connected with the hydraulic pump 1 via the valve set 2. One valve set 2 each is in in-line arrangement between the hy draulic pump 1 and each hydraulic motor combination 3, 4. This is shown in Fig. 2. On the high-pressure side a proportional current control valve 15 is arranged in the path flow VP -> VM by means of which the flow can be blocked as a whole or in part. On the side of the pump upstream of the proportional current control valve 15, a pressure control valve 16 and a safety pressure control valve 17 connect the flow on the high-pressure side with the return on the low-pressure side by one circuit in parallel each. It is the task of these two valves to ensure that a maximally permissible system pressure is not exceeded in the connected sub-circuits Ti and T2 thus practically ruling out an overload 5800274_1 (GHMatters) P89486.AU BENB 7 of the wind power plant due to an outside impact. On the motor side an anti-cavitation valve 18 connects the flow and the return. This anti-cavitation valve 18 ensures that when the connected synchronous generator 5 passes from operation as generator to the operation as motor the supply of feed oil is continued. It also ensures the intended operation as motor of the synchronous generator 5 when the same is to operate motor-wise as a phase shifter when the rotor 19 is out of operation. One or also several high-pressure stores 6, connected on the high-pressure side of at least one of the closed sub-circuits Ti and/or T2, serve as anti-shock store and ensure in the event of very rapidly arising peak loads which may start from the rotor 19 an adequate reaction of the operation of the wind power plant. Furthermore, they are responsible for an adequate setting of the natural frequency of the entire hydrostatic drive which must be done in such a way that natural resonances are reliably avoided during operation. One or several low-pressure stores 7, connected on the low pressure side of at least one of the closed sub-circuits Ti and T2, ensure that enough oil is available within the closed system for the adequate filling of high-pressure store 6 and/or of the high-pressure stores 6. Feed pump 8 ensures during operation the supply of feed oil to the closed sub-circuits Ti and T2. The amount provided by it is such that oil losses of the hydraulic pump 1 and of the hy draulic motors 3, 4 due to leakage are replenished, on the one hand. On the other hand, an excess amount of cooled feed oil from the tank is fed to the hydraulic pump 1 which is flushed out in the same amount downstream of the hydraulic motors 3, 4 when the on/off valve 14 of the flush valve is off. The flushed-out oil passes through the return filter 9 which cleans it from entrained particles, it passes through the oil heat exchanger 10 where it is cooled and returns into the tank via the pressure control valve 11. Upstream of the pressure control valve 11 a part of the cleaned and cooled oil volumetric flow is branched off and fed into the tank via the casing of the hydraulic pump 1 whereby a flushing of the casing of hydraulic pump 1 is achieved for an additional cooling. A switchable outgoing feeder of the high-pressure pump 12 which is anyhow included for the supply of other units of the wind power plant is, depending upon the position of the switching valve of the high-pressure feeding 13, blocked in the off-position or is connected either with the low-pressure 5800274_1 (GHMatters) P89486.AU BENB 8 side or with the high-pressure side of the closed sub-circuits Ti and T2. This arrangement has two functions: With completely closed proportional current control valves 15 in all valve sets 2, released mechanical rotor brake, connec tion of the high-pressure pump 12 with the low-pressure side and connection of the high-pressure side through the on/off valve 14 of the flush valve with the return filter 9, the hy draulic pump 1 becomes under the influence of the volumetric flow of high-pressure pump 12 a slow-speed hydraulic motor in the open hydraulic circuit which brings about an initial rota tional speed which is advantageous for the rapid run-up of ro tor 19. This is in particular indispensable when a wind power plant with two-blade rotor does not have a pitch system because then the independent run-up of rotor 19 would be possible, if at all, only at very high wind velocities. With completely opened proportional current control valves 15 in one or several valve sets 2, pulled mechanical rotor brake, connection of the high-pressure pump 12 with the high-pressure side and connection of the low-pressure side with the return filter 9 through the on/off valve of the flush valve 14 the associated hydraulic motor combinations 3, 4 are driven with rotor 19 at standstill under the impact of the volumetric flow of the high-pressure pump 12 in the open hydraulic circuit and provide for the wattless run-up of the associated synchronous generators 5 up to the nominal rotational speed in generator operation. They can then be synchronised with the network, thereafter they pass into motor operation and function as phase shifters. At the same time, the hydraulic motors 3, 4 pass into pump operation the feed oil supply for which is en sured by the anti-cavitation valves 18 in the valve sets 2 and run concurrently in no-load operation. A starting of the syn chronous generators as motors from standstill would otherwise not be possible. The synchronous generators 5 convert the mechanical drive en ergy of the hydraulic motors 3, 4 into electrical energy. Brushless three-phase synchronous generators with a rotating excitation system are used. The excitation of the synchronous generators 5 is appropriate ly controlled by the operations management system both during production operation and phase shifter operation. The same ap plies to the synchronisation of the synchronous generators 5 with each other, with an existing network and/or with other wind power plants or electric generators. 5800274_1 (GHMatters) P89486.AU BENB 9 The synchronous generators 5 work in production operation with a fixed rotational speed. This is done completely independent of the respective rotational speed of rotor 19 of the wind power plant. The required control of the adjustable hydraulic motors 4 in dependence upon the volumetric flow is performed by the operations management system. The synchronous genera tors 5 can also work with a variable rotational speed when this is required by the connected consumers. During operation in parallel with the network the power factor cos @ as the measure of the ratio of active power to reactive power is effected in accordance with the demand by the opera tions management system both in production operation and in phase shifter operation by the relevant setting of the excita tion voltage of the synchronous generators 5. In isolated operation or operation in local networks it is possible that not all of the entire electric energy which could be supplied by the wind power plant at a given wind ve locity can actually also be taken over. If this power demand is below the performance characteristics for the one-generator operation, it is possible to work with one synchronous genera tor 5 only up to the maximum output of the same. Rotor 19 works with an accordingly reduced rotational speed and the hy draulic pump 1 works due to the reduced volumetric flow with one closed sub-circuit Ti or T2 only. The waste heat which arises in the sub-circuits Ti and T2 is dissipated via the oil-heat exchanger. If there is no demand for heat at the site, oil-air-heat exchangers are used. Other wise the waste heat is fed via oil-water-heat exchangers into a heat circuit at the site. Vice versa, it is possible also in this manner at extremely low temperatures to attain the mini mal operating temperature of the wind power plant through the supply of heat from the said heat circuit very much faster than the wind power plant could do on its own. A second embodiment of the invention is shown in Fig. 3. It differs from the first embodiment in that it has only one closed hydraulic circuit Ti and thus also only one synchronous generator 5. The function does not differ from the first em bodiment; only the particular features based on the presence of two synchronous generators 5 are not given here. This type of embodiment is preferably to be preferred for wind power plants with a nominal output of up to about 100 kW. The embodiment with two synchronous generators 5 can be accom plished up to a nominal output of about 1,000 kW. For wind power plants with a still greater nominal output the use of more than two synchronous generators 5 is advantageous. 5800274_1 (GHMatters) P89486.AU BENB 10 In general, the number of the synchronous generators 5 is, however, optional and is, in the last analysis, determined by economic and technical requirements that are placed on the wind power plant. List of reference numerals 1 Hydraulic pump 2 Valve set 3 Hydraulic motor as fixed motor 4 Hydraulic motor as adjustable motor 5 Synchronous generator 6 High-pressure store 7 Low-pressure store 8 Feed pump 9 Return filter 10 Oil-heat exchanger 11 Pressure control valve 12 High-pressure pump 13 Switching valve of the high-pressure supply 14 On/off valve of the flush valve 15 Proportional current control valve 16 Over-pressure valve 17 Safety over-pressure valve 18 Anti-cavitation valve 19 Rotor 5800274_1 (GHMatters) P89486.AU BENB

Claims

1. Hydrostatic drive of a wind power plant comprising: a rotor which can be driven by the wind energy, for the generation of a rotation, a rotor shaft con nected with the rotor, a hydraulic pump which can be driven via the rotor shaft for the conversion of the rotation energy of the ro tor shaft into hydraulic energy, a hydraulic motor combination which can be connected via hydraulic lines with the hydraulic pump and can be driven by the same; wherein in the hydraulic motor combination a conver sion of the hydrostatic energy into a rotation movement is accomplished and this movement of the hydraulic motor com bination comprising a common shaft or being rigidly coupled to each other is transmitted to a drive shaft of a synchro nous generator for the generation of electrical energy; wherein the hydrostatic drive in a first embodiment only comprises a closed hydraulic circuit or, alternative ly, in a second embodiment, several closed hydraulic sub circuits, to which the total volumetric flow which is con veyed by the hydraulic pump is distributed; wherein the circuit and/or each sub-circuit com prise(s) a hydraulic motor combination having an adjustable hydraulic motor and a fixed hydraulic motor, wherein the adjustable hydraulic motor and the fixed hydraulic motor of each circuit each have a flow (VP) from the hydraulic pump on the high-pressure side and a return (RP) to the hydrau lic pump on the low-pressure side, with the connection from and to the hydraulic pump being made indirectly in each case via a valve set, a proportional current control valve is arranged in line with the volumetric flow in the respective high pressure side flow (VP), and on the hydraulic motor side downstream of the propor tional current control valve an anti-cavitation valve switched in parallel and connecting the flow (VIV) on the high-pressure side and the return (RN) on the low-pressure side is arranged, and the displacement of the adjustable hydraulic motors can be set by a controller such that dur ing isolated operation the rotational speed of the drive shaft of the associated synchronous generator assumes a constant value so that the synchronous generators of the circuits can be connected directly to a load on the output side and during operation in parallel with a network at a constant rotational speed determined by the network a power equilibrium exists between the power collected by the rotor from the wind and the power emitted by the generators, tak ing account of the conversion losses in the hydrostatic 5800274_1 (GHMatters) P89486.AU BENB 12 drive and in the generators.

2. Hydrostatic drive according to Claim 1 wherein the valve sets have an over-pressure valve and a safety over-pressure valve too.

3. Hydrostatic drive according to Claim 1 or 2 wherein at least one of the circuits is provided in the flow on the high-pressure side with at least one high-pressure store which serves the setting of the natural frequency of the hydrostatic drive and the control of peak loads.

4. Hydrostatic drive according to any one of Claims 1 to 3 characterised in that the hydraulic pump is a slow-speed radial piston pump for the supply of the circuits which is sized such that its ro tational speed-torque characteristics are in conformity with that of the rotor at an optimal or approximately opti mal efficiency of the radial piston pump.

5. Hydrostatic drive according to any one of Claims 1 to 4 wherein in each circuit the maximal total displacement of the hy draulic motor combination is the sum total of the displace ments of fixed hydraulic motor and adjustable hydraulic mo tor combination at an adjustment of 100 %, with the dis placement of the fixed hydraulic motor being smaller than but not more than equal to the maximal displacement of the adjustable hydraulic motor.

6. Hydrostatic drive according to any one of Claims 1 to 5 wherein the major part, at least, however, half of the volumetric flow in the respective sub-circuit can be received by the adjustable hydraulic motors of the hydraulic motor combina tion of each circuit.

7. Hydrostatic drive according to any one of Claims 1 to 6 wherein a slightly biased low-pressure store is arranged on the low-pressure side within the return (RP) in at least one circuit.

8. Method for the operation of a hydrostatic drive of a wind power plant in which a rotor is driven by wind energy and drives a rotor shaft that is coupled with the rotor which in turn drives a hydraulic pump for the conversion of the rotation of the rotor shaft into a hydraulic energy, where in the total volumetric flow which is conveyed by the hy draulic pump is distributed to one closed hydraulic circuit or, alternatively to several closed hydraulic sub-circuits, with each circuit comprising a hydraulic motor combination 5800274_1 (GHMatters) P89486.AU BENB 13 having an adjustable hydraulic motor and a fixed hydraulic motor; wherein the adjustable hydraulic motor and the fixed hydraulic motor of each circuit each have a flow (VP) from the hydraulic pump on the high-pressure side and a return (RP) to the hydraulic pump on the low-pressure side, with the connection from and to the hydraulic pump being made indirectly in each case via a valve set, a proportional current control valve is arranged in line with the volumetric flow in the respective high pressure side flow (VP), and on the hydraulic motor side downstream of the propor tional current control valve an anti-cavitation valve switched in parallel and connecting the flow (V) on the high-pressure side and the return (RN) on the low-pressure side is arranged; in each case an adjustable hydraulic motor and a fixed hydraulic motor are arranged on a common shaft or rigidly coupled to each other and drive the drive shaft of a synchronous generator, wherein the displacement of the adjustable hydraulic motors is set by a controller such that the rotational speed of the drive shaft of the associ ated synchronous generator assumes a constant or approxi mately constant value, and the synchronous generators of the circuits are connected directly to a network or to a load on the output side.

9. Method according to Claim 8 wherein an over-pressure valve which is arranged in each of the sub-circuits opens automatically when its actuation pres sure is reached thereby limiting the maximum pressure on the high-pressure side and the maximum torque directed by the hydraulic pump against the rotor wherein in the event of the failure of the over-pressure valve a safety over pressure valve automatically assumes the function of the same.

10. Method according to Claim 8 or 9 wherein a high-pressure store which is directly connected to the high-pressure side (V) of a circuit is sized such that it receives and compensates like an anti-shock store a sudden ly occurring pressure build-up, adequately shifts the natu ral frequency of the hydrostatic drive from the range of the excitation frequency by the rotating rotor due to the passing of the same through the dynamic pressure zone in front of the tower and prevents to the largest possible ex tent swings of the synchronous generators due to the cor rection of load fluctuations in the network or at the con sumers.

11. Method according to any one of Claims 8 to 10 wherein the slightly biased low-pressure store(7) which is connect 5800274_1 (GHMatters) P89486.AU BENB 14 ed to the low-pressure side of a hydraulic circuit is sized such that it provides within short and independent of the feed system the oil volume which is required for filling the high-pressure store(6).

12. Method according to any one of Claims 8 to 11 wherein dependent upon the design, only one circuit and thus a hy draulic motor combination of fixed hydraulic motor and ad justable hydraulic motor and a synchronous generator are in operation at low wind speeds, with further circuits being completely blocked by means of the associated proportional current control valves.

13. Method according to any one of Claims 8 to 12 wherein with a low load demand and independent of the present wind speed only a combination of fixed hydraulic motors and ad justable hydraulic motors and an associated synchronous generator are in operation, up to maximally half of the rated output of the wind power plant, with the hydraulic pump working only at half of its delivery capacity.

14. Method according to any one of Claims 8 to 13 wherein the proportional current control valves which are integrat ed into the closed hydrostatic circuits(Ti-n) are used for exerting a direct and immediate influence on the rotational speed of the rotor.

15. Method according to any one of Claims 8 to 14 wherein at a complete standstill of the rotor in a braked state, for instance as a protective measure during a storm or dur ing a lull, at least one synchronous generator is running as phase shifter in parallel to the network in motor opera tion, over-excited or under-excited operation and as watt less operation, with the generator run-up of the synchro nous generator, its synchronisation with the network and its automatic passing into motor operation being ensured by the inventive configuration of the hydrostatic drive.

16. Method according to any one of Claims 8 to 15 wherein the hydraulic pump can be switched to motor operation, with the rotor thereby being given a minimum rotational speed adequate for run-up that may be required in particular when the rotor blades are not adjustable or do not become ad justable by means of pitch.

17. Method according to any one of Claims 8 to 16 wherein the waste heat that is generated in the hydraulic circuits is fed by means of oil-water heat exchangers into a heat circuit and/or vice versa at extremely low temperatures the minimum operating temperature of the wind power plant is more rapidly reached in this manner. 5800274_1 (GHMatters) P89486.AU BENB