GB2476790A

GB2476790A - Hydraulic transmission system for wind or water turbines

Info

Publication number: GB2476790A
Application number: GB0921094A
Authority: GB
Inventors: Bjorn Skaare; Dagfinn Sveen; Ake Pramstig
Original assignee: Statoil ASA
Current assignee: Equinor ASA
Priority date: 2009-12-01
Filing date: 2009-12-01
Publication date: 2011-07-13
Also published as: WO2011067561A1; GB0921094D0

Abstract

A hydraulic transmission system 1 for wind or water turbine installations comprises a hydraulic pump 4a connected via a transmission line S, 6 to a plural fixed displacement hydraulic motors 4b arranged such that they can be coupled to and drive a common load 2. The motors 4b can each be selectively coupled in and out of operation to drive the load 2 and effectively achieve differing gear ratios to optimize turbine tip speed ratios based on a controller assessment of turbine fluid speed changes, generator power and transmission system pressure. Accumulators 9, 10, safety valves or actively controlled valves may be provided to reduce torque variations and ensure hydraulic pressure lies in a range about a reference value based on detection of hydraulic pressure. In an offshore system the natural frequency of the hydraulic system may be arranged to lie outside the frequency range of the waves in a desired installation site.

Description

Hydraulic transmission system The present invention relates to the field of hydraulic transmission systems.

In particular, it relates to hydraulic transmission systems for use in a wind turbine installation, and methods of using such transmission systems.

In a conventional wind turbine, a nacelle is provided at the top of a tower.

The nacelle contains a generator and related components, and a turbine rotor is attached to the nacelle and connected to the generator by a mechanical transmission.

However, such an arrangement has several disadvantages. For example, as the transmission is mechanical, the generator has to be located at approximately the same height as the rotor, creating a top-heavy and potentially unstable installation.

Furthermore, due to the number of mechanical components in such a system, these transmission systems can be difficult and expensive to install and maintain.

Moreover, the relatively high position of the transmission and the generator makes it difficult to access these components for installation and maintenance purposes.

In light of these disadvantages, more recently, hydraulic transmission systems have been developed that allow the generator to be located at a lower level than the rotor. These systems use a hydraulic pump in the nacelle connected to a hydraulic motor at the base of the tower which drives the generator. Such systems are provided with a continuously variable gear ratio, by means of variable displacement hydraulic motors. These variable displacement hydraulic motors allow the rotor to operate at the most aerodynamically advantageous speed, whilst allowing the generator to run at a fixed speed.

The present invention relates to a hydraulic transmission system for use in a wind turbine installation comprising a hydraulic pump connected via a transmission line to a plurality of fixed displacement hydraulic motors arranged such that the hydraulic motors can be coupled to and drive a common load, whereby the hydraulic motors can each be selectively coupled in and out of operation to drive the load.

Accordingly, the system of the present invention is effectively provided with a set of discrete gear ratios which are determined by coupling given combinations of motors.

As mentioned above, continuously variable gear ratios as provided by variable displacement hydraulic motors have been considered to provide the optimal transmission arrangement for a wind turbine installation. However, the inventors of the present invention have realised that, contrary to this common assumption, due to a potentially higher inherent efficiency of fixed displacement hydraulic motors compared with variable displacement hydraulic motors, the arrangement of the invention is generally more efficient than using variable displacement hydraulic motors.

Furthermore, the present invention is also simpler in construction compared to previous systems. Apart from a system for changing the gear ratio from time to time, the hydraulic transmission system itself may be a passive system without active control.

As the present invention enables fixed generator speed and discrete variable rotor speed operation, it thereby avoids the need to use costly power electronics.

The transmission system could be used in all kinds of wind turbines, e.g. floating wind turbines or water-or land-based fixed-foundation wind turbines.

The common load may comprise a generator.

In addition to the hydraulic motors, the hydraulic pump may also be a fixed displacement hydraulic pump, thereby further improving the efficiency of the system.

Various configurations of the hydraulic motors are possible. For example, the hydraulic motors may be directly couplable to a common drive shaft, which is connectable to the common load. The motors may thus be coupled and de-coupled from the drive shaft in order to change the gear ratio of the hydraulic transmission.

The transmission system may comprise one or more controllable valves for connecting and isolating (i.e. coupling and de-coupling) one or more of the individual motors to and from the transmission system drive shaft. Preferably a valve is provided for each motor. The valves may be arranged such that they can switch the motors into and out of a free-circulating (also referred to as free-wheeling or short-circuiting) mode where the motors are not coupled to the shaft. Such a free-circulating mode may be achieved by the motors operating with retracted pistons.

However, a drawback of the free-circulating mode is that the whole transmission must be stopped during coupling and dc-coupling thus resulting in a loss of power production.

The transmission system may comprise controllable mechanical shaft couplings (or clutches) for coupling and dc-coupling the motors to and from the shaft. Preferably, one such coupling is provided for each motor. These couplings may be provided in addition to the valves described above. The valves and couplings should be coupled/dc-coupled in a certain sequence, which should be performed when the transmission system is not loaded (i.e. when the rotor and pump are stopped), thereby resulting in a loss of power production.

Preferably, instead of the controllable mechanical shaft couplings, the system comprises a freewheel or over-running clutch (i.e. a mechanical device arranged to automatically disengage the motor from the drive shaft when the drive shaft rotates faster than a driving shaft of the motor) arranged between each motor and the drive shaft, in addition to the controllable valves. Such an arrangement can allow the motors to be coupled/dc-coupled during normal operation, does not require accurate control sequencing and does not result in the loss in power production associated with the above alternatives. As a result, this coupling/dc-coupling arrangement can provide a considerable increase in power production compared to the alternatives described above.

Preferably, the hydraulic pump and/or the hydraulic motors is/are radial piston hydraulic machines. Such machines can be more robust and more efficient than the variable displacement axial piston motors that are used in other known hydraulic transmission systems. Furthermore, as these machines are more robust, downtime and the amount of maintenance work can be reduced.

Another advantage of using fixed displacement radial piston motors to drive the load is that in the case of a failure of a motor, the failed motor may be freewheeled and the shaft (which is couplable to all of the motors) can still operate at a reduced power provided by the motor(s) that has(have) not failed. This is possible since in a radial piston motor, the rollers and the cam ring are the critical components, not the shaft bearing. However, if an axial piston motor were used, this freewheeling feature would not be possible because in an axial piston motor the rear shaft bearing is the critical component and if that bearing fails, the whole shaft will fail.

The above freewheeling feature which is possible with radial piston motors may be of particular benefit in offshore locations, where access to the turbine for repair and/or maintenance may be difficult for long periods of time.

The transmission line may be a high-pressure hydraulic transmission line which is coupled to at least one hydraulic accumulator.

Hydraulic accumulators can be useful for reducing torque variations on the motors and generator of a wind turbine installation, particularly one with a rotor blade pitch control system. This may not be possible for hydraulic transmission systems with a continuously variable gear ratio, which is provided by tight, active control of the swash plate angle in variable displacement axial piston motors.

Alternatively or additionally, the hydraulic transmission line could be coupled to one or more hydraulic valves. Such valves could be safety valves arranged to limit the maximum pressure of the hydraulic fluid to a desired maximum value and/or active controlled valves arranged to control the pressure of the hydraulic fluid to lie in a certain range around a mean value. The safety and active controlled valves could be the same kind of valve, or even the same valve, but with different pressure control arrangements for different operation points (e.g. above/below rated wind speed, the rated wind speed of a wind turbine installation being the wind speed at and above which maximum generator power output, i.e. rated power, can be achieved.) Coupling one or more hydraulic valves to the transmission line can help avoid undesired pressure fluctuations, as discussed in further detail below.

Hydraulic valves may be preferred to hydraulic accumulators as they can generally be cheaper and/or easier to maintain and replace than hydraulic accumulators. Furthermore, it may be difficult to provide a hydraulic accumulator that can operate over the wide range of pressures experience in wind turbine installation hydraulic transmission systems (e.g. 20 -300 bar).

Preferably, the hydraulic motors are connected to a controller for controlling the coupling of the hydraulic motors in and out of operation. The controller may be arranged to couple the motors in and out of operation on the basis of a wind speed, a generator power and/or a pressure in the hydraulic transmission system. Thus, the controller may be connected to one or more sensors for directly or indirectly measuring or estimating a wind speed, a generator power and/or a pressure in the hydraulic transmission system. The pressure could be measured in a transmission line, in an accumulator, in a valve, in a pump and/or in a motor.

In a preferred embodiment, the controller is arranged to couple the motors in and out of operation according to changes in a wind speed in order to optimise the tip speed ratio of a rotor to which the hydraulic transmission system is connectable.

The tip speed ratio is the speed at which the tips of the rotor blades are moving divided by the wind speed. The optimum tip speed ratio is the ratio at which optimum energy extraction can be achieved for a given wind speed. By providing a controller arranged in this way, optimum or close to optimum energy extraction may be achieved. In a wind turbine installation, for example, this can be particularly advantageous when operating below rated wind speed.

The present invention also relates to a wind turbine installation comprising a hydraulic transmission system as described above, a generator and a rotor, wherein the hydraulic motors are couplable to the generator and the hydraulic pump is connected to the rotor. The hydraulic motors could be couplable to a single generator or to two or more generators.

As noted above, preferably, the hydraulic motors and the generator(s) are located at positions lower than those of the rotor and the hydraulic pump. For example, the pump may be located in a nacelle of the wind turbine installation, with the motors and generator(s) at the base of a tower of the wind turbine installation.

This arrangement enables wind turbines to be produced with a significantly lower centre of gravity than systems where all of these components are provided at rotor level. Since critical components can be located at or around ground/sea level, these components are more accessible, maintenance work can be made easier and simplified and, due to its lower centre of gravity, the system is more stable.

The hydraulic motors are preferably directly couplable to the generator(s) via a common shaft. Such an arrangement is relatively compact and simple to construct.

The present invention also relates to a method of using a wind turbine installation as described above, wherein the hydraulic motors are coupled in and out of operation in order to change a gear ratio of the hydraulic transmission.

Preferably, the hydraulic motors are coupled in and out of operation on the basis of a wind speed, a generator power and/or a pressure in the hydraulic transmission system. By using one or more of these variables to determine which motors should be coupled in or out of operation (i.e. which gear ratio should be used), the power output of the system can be optimised.

For example, the motors may be coupled in and out of operation according to changes in a wind speed in order to obtain an optimum tip speed ratio of a rotor to which the hydraulic transmission system is connectable. By coupling the motors in and out of operation in this way, energy extraction may be optimised.

The method preferably comprises measuring or estimating a wind speed, a generator power and/or a pressure in the hydraulic transmission system. Further, the method may comprise averaging the wind speed, the generator power and/or the pressure in the hydraulic transmission system over a period of time. The hydraulic motors may then be coupled in and out of operation on the basis of an average wind speed, an average generator power and/or an average pressure in the hydraulic transmission system. The period of time could be any period of time in the range from around one second to several (e.g. fifteen) minutes. By averaging the wind speed, generator power and/or a pressure in the hydraulic trarsmission system in this way, fluctuations in these variables can be accounted for.

The dynamic system of a wind turbine installation comprising a rotor and a hydraulic transmission system can, simplified and linearised, be considered as a mass-spring-damper system where the inertia of the rotor represents the mass, m, the elasticity in the (high pressure) hydraulic volume represents the elasticity, which is given by constant k, and the pressure losses in the hydraulic system represent the damping, which is given by constant d. The differential equation for such a mass-spring-damper system that is exposed to an excitation force FE (or torque in the case of a wind turbine) can then be given by: rnx+dx+kx=FE, (A) where x represents position (or rotor angle in the case of a wind turbine). The natural frequency co of the dynamic system in equation (A) can be given by: (B) The inventors of the present invention have realised that it is desirable to design hydraulic transmission systems for offshore or water-based wind turbines (especially floating wind turbines) such that the natural frequency of the hydraulic transmission as given by equation (B) is below the frequency range of waves in the intended installation site.

A further aspect of the invention therefore relates to an offshore or water-based wind turbine installation comprising a hydraulic transmission system, wherein the hydraulic transmission system is arranged such that its natural frequency is outside of a frequency range of waves in a desired installation site.

The action of waves on an offshore or water-based wind turbine installation can cause motions such as heave, pitch, and roll, which in turn cause fluctuations in the torque experienced by the rotor, which then cause fluctuations in the pressure in a hydraulic transmission. Pressure fluctuations in the hydraulic transmission can cause undesired fluctuations in generator power. By ensuring that the natural frequency of the hydraulic transmission is outside of a frequency range of waves in a desired installation site, such pressure fluctuations can be minimised or reduced. In the particular case of a floating wind turbine installation, it may also be advantageous to avoid the natural frequency in the pitch mode of motion for the installation.

As an example, the wave frequency range in the North Sea is typically 0.05 to 0.2 Hz. Therefore, in a preferred embodiment, the natural frequency of the hydraulic transmission is below 0.05 Hz or greater than 0.2 Hz. This is clearly particularly of use in the North Sea.

In order to achieve a natural frequency above the wave frequencies for a multi-megawatt wind turbine installation (around 5 MW and above), it would be necessary to have transmission lines with smaller diameters which would require higher pressures andlor higher flow rates in the transmission lines which would lead to more wear of hydraulic components and higher losses. Furthermore, a hydraulic transmission system with a lower natural frequency would smoothen out disturbances/excitations at higher frequencies leading to better operating conditions for the load or generator, as well as a smoother output power delivered to the electrical grid. Therefore, it is preferred, particularly in high power wind turbine installations, that the natural frequency of the hydraulic transmission is below a frequency range of waves in a desired installation site.

The wind turbine installation may be a floating wind turbine installation. It is believed that this aspect of the present invention is of particular use to floating wind turbine installations, where the effect of the waves will be greatest. However, the wind turbine installation could be a fixed-foundation wind turbine installation.

The natural frequency of the hydraulic transmission may be arranged to lie outside of or below a certain range by adjusting the elasticity of the hydraulic volume. The elasticity of the hydraulic volume may be adjusted by changing the hydraulic transmission fluid to a fluid with a different bulk modulus, by changing the elasticity of the hydraulic lines or pipes, and/or by adjusting the length and/or diameter of the hydraulic lines or pipes, for example. Alternatively, the inertia of the rotor, m, may be adjusted, but this is generally less desirable as it could have other less advantageous consequences, such as a reduction in aerodynamic power extraction, for example.

The inventors of the present invention have realised that, in general, in any wind turbine installation (not just water-based), undesired hydraulic pressure fluctuations may be avoided or reduced by using one or more hydraulic accumulators coupled to the transmission line. Alternatively or additionally to the use of hydraulic accumulator(s), one or more hydraulic valves may be coupled to the hydraulic transmission line.

Thus, a further aspect of the present invention relates to a hydraulic transmission system for use in a wind turbine installation comprising a hydraulic pump connected via a transmission line to at least one hydraulic motor arranged such that the at least one hydraulic motors can be coupled to and drive a load, wherein one or more hydraulic accumulators and/or hydraulic valves are coupled to the hydraulic transmission line.

Hydraulic valves may be preferred to hydraulic accumulators as they can generally be cheaper and/or easier to maintain and replace than hydraulic accumulators. Furthermore, it may be difficult to provide a hydraulic accumulator that can operate over the wide range of pressures experience in wind turbine installation hydraulic transmission systems (e.g. 20 -300 bar). Therefore, preferably, one or more hydraulic valves are coupled to the transmission line.

The one or more hydraulic valves may comprise safety valves and/or active controlled valves. The safety and active controlled valves could be the same kind of valve, or even the same valve, but with a different pressure control arrangements for different operation points (e.g. above/below rated wind speed).

The hydraulic transmission system is preferably arranged such that during operation above rated wind speed one or more safety valves can be used. The safety valves are preferably arranged to open when the pressure of the hydraulic fluid in the hydraulic transmission rises above a desired maximum pressure and to close when the pressure falls below the desired maximum pressure, during operation above rated wind speed.

Such an arrangement can provide two important effects: (i) pressure fluctuations in the hydraulic transmission line can be efficiently prevented or reduced; (ii) a limitation of the maximum pressure can also cause a limitation of the maximum generator power, such that power variations around the rated power of the wind turbine installation can be reduced.

In addition or alternatively, the hydraulic transmission system is preferably arranged such that during operation below the rated wind speed one or more active controlled valves can be used. The active controlled valves are preferably arranged to control the hydraulic pressure to lie in a certain range around a mean value of the pressure in order to avoid large pressure fluctuations during operation below rated wind speed. It is particularly desirable to avoid pressure fluctuations at the natural frequency of the transmission system in order to avoid resonance. The desired mean value of the pressure may be determined by taking an average of the pressure over a time period such as 20 to 40 seconds using various low-pass or notch filtering techniques or similar. The active controlled valves may control the hydraulic pressure to lie in a range around the mean value by opening and closing.

The volume flow q(t) through a valve may be represented by a general valve equation and the requirement that there should be no flow through the valve when the referencepressure is less than the pressure in the hydraulic transmission line: q(t) CdA(pI Pr)(Pi -0) when1 > Pref (C) 0, when p, «= Pref where Cd is the constant discharge coefficient for the valve, A is the cross section of the orifice that can be controlled as desired according to the difference between the transmission line pressure and the reference pressure, p is the density of the hydraulic transmission fluid (e.g. hydraulic oil), and p, and Prei are the pressure in the hydraulic transmission line and the desired pressure reference, respectively. 10 is the constant pressure on the low pressure side of the transmission line.

As described above, one or more active controlled valve(s) may be used during operation below the rated wind speed. The active controlled valve(s) may control the pressure around a "mean value" of the pressure in order to avoid undesired pressure fluctuations due to excitation of the resonance (or natural) frequency of the transmission line. The reference for this "mean value" of the pressure may be found by taking an average over a certain time (e.g. 20 -40 seconds), by various low-pass or notch filtering techniques or similar.

Active valve control below the rated wind speed may be implemented with a valve with the dynamic properties as given in equation (C) with a reference pressure given as: Prej = p,,, (D) where p,1 is the low pass filtered signal of the transmission line pressure p,. Use of a second order Butterworth low pass filter and introduction of the Laplace transform leads to a low pass filtered signal of the form: (E) S +.JcDS+U) where s is the Laplace transform variable, and cv is the filter frequency in rad/s which should be below the resonance frequency of the hydraulic transmission system.

As also described above, one or more safety valve(s) may be used during operation above the rated wind speed. The safety valve will typically open when the pressure rises above a desired maximum pressure.

Safety valve control above the rated wind speed may be implemented by use of a valve with the dynamic properties as given in equation (C) and a reference pressure given as: PrefPmaX' (F) where p1 is a desired maximum pressure in the hydraulic transmission line.

Preferably, the hydraulic transmission system comprises a pressure sensor arranged to detect and/or measure the hydraulic pressure of the transmission system.

Preferably, the one or more active controlled valves and/or one or more safety valves are arranged to be controlled on the basis of this detected or measured pressure.

The present invention also relates to a wind turbine installation comprising any of the hydraulic transmission systems described above.

The present invention also relates to a method of using a hydraulic transmission system in a wind turbine installation, the hydraulic transmission system comprising a hydraulic pump connected via a transmission line to at least one hydraulic motor arranged such that the at least one hydraulic motor can be coupled to and drive a load, wherein one or more hydraulic accumulators and/or one or more hydraulic valves are coupled to the hydraulic transmission line, the method comprising: detecting and/or measuring a hydraulic pressure of the hydraulic transmission system, and controlling the hydraulic valves on the basis of the detected or measured hydraulic pressure.

Preferably, the one or more hydraulic valves comprise one or more safety valves, and the method comprises the steps of: opening the one or more safety valves when a hydraulic pressure of the hydraulic transmission rises above a desired maximum pressure; and closing the one or more safety valves when the pressure -12-falls below the desired maximum pressure. Preferably, these steps are performed during operation above rated wind speed.

Alternatively, or in a addition, the one or more hydraulic valves preferably comprise one or more active controlled valves, and the method preferably comprises the steps of: opening and/or closing the active controlled valve such that the hydraulic pressure lies in a preset range around a mean hydraulic pressure.

Preferably, these steps are performed during operation below rated wind speed.

Although the aspects of the present invention have been described in relation to wind turbine installations, it will be appreciated that at least some of the hydraulic transmissions and methods described in this application could also be used with other kinds of turbines such as water turbines (e.g. wave, tidal or hydroelectric).

Accordingly, the present invention also relates to a hydraulic transmission system for use in a water turbine installation comprising a hydraulic pump connected via a transmission line to a plurality of fixed displacement hydraulic motors arranged such that the hydraulic motors can be coupled to and drive a common load, whereby the hydraulic motors can each be selectively coupled in and out of operation to drive the load.

The present invention also relates to a water turbine installation comprising such a hydraulic transmission system, and a method of using such a water turbine, wherein the hydraulic motors are coupled in and out of operation in order to change a gear ratio of the hydraulic transmission.

According to another aspect, the present invention relates to a hydraulic transmission system for use in a water turbine installation comprising a hydraulic pump connected via a transmission line to at least one hydraulic motor arranged such that the at least one hydraulic motors can be coupled to and drive a load, wherein one or more hydraulic accumulators and/or one or more hydraulic valves are coupled to the hydraulic transmission line.

The present invention also relates to a water turbine installation comprising such a hydraulic transmission system.

According to another aspect, the present invention relates to a method of using a hydraulic transmission system in a water turbine installation, the hydraulic transmission system comprising a hydraulic pump connected via a transmission line -13 -to at least one hydraulic motor arranged such that the at least one hydraulic motors can be coupled to and drive a load, wherein one or more hydraulic valves are coupled to the hydraulic transmission line, the method comprising: controlling the hydraulic valves on the basis of a detected hydraulic pressure.

According to another aspect, the present invention relates to an offshore water turbine installation comprising a hydraulic transmission system, wherein the hydraulic transmission system is arranged such that its natural frequency is outside of a frequency range of waves in a desired installation site.

In any of the above aspects of the invention relating to water turbine installations, the water turbine installation may be a wave, tidal or hydroelectric turbine installation, for example. In addition, any of these aspects may additionally comprise any of the optional or preferred features described above in relation to the aspects relating to wind turbine installations and/or hydraulic transmission systems therefor.

Preferred embodiments of the present invention will now be described by way of example only and with reference to the accompanying drawings in which: Fig. I shows a radial piston hydraulic machine which may be used in embodiments of the present invention; Fig. 2 shows a schematic diagram of a preferred embodiment of a hydraulic transmission system in a wind turbine installation; Fig. 3 shows a graph of aerodynamic power extraction as a function of wind speed; Fig. 4 shows a graph of efficiency as a function of wind speed; Fig. 5 shows a graph of generator power extraction as a function of wind speed; Fig. 6 shows a probability distribution of wind speed according to IEC-standard 61400-1 with a reference wind speed of 50 ms; Fig. 7 is a table of annual generator power production for various transmission systems; Fig. 8 illustrates a MATLAB Simulink model of a valve and the generation of the reference for the mean value of the pressure; -14-Fig. 9 shows a snapshot from a simulation of pump pressure during simulations with and without active valve control; Fig. 10 shows a snapshot from a simulation of generator power during simulations with and without active valve control; Fig. 11 shows a snapshot from a simulation of rotor speed during simulations with and without active valve control; and Fig. 12 shows a snapshot from a simulation of pump shaft torque during simulations with and without active valve control.

Fig. 2 shows a hydraulic transmission system 1 in a wind turbine installation according to an embodiment of the invention.

The hydraulic transmission system 1 comprises one (although more than one may be used) large fixed displacement radial piston pump 4a connected to the wind turbine rotor 3 on a low speed side of the hydraulic gear. The pump 4a is connected to the rotor 3 via a shaft 17. Hydraulic transmission lines 5, 6 connect the pump 4a to the motors 4b that are located around ground or sea level. The transmission lines 5, 6 comprise a high pressure transmission line 5 and a low pressure transmission line 6.

The system comprises at least two (in this case four) smaller fixed displacement radial piston motors 4b that are couplable to the same shaft 7 as the generator 2, on a high speed side of the hydraulic gear.

The hydraulic machines 4a, 4b form a discrete variable gear 8 by coupling the fixed displacement radial piston motors 4b in and out of operation, i.e. to and from the shaft 7.

Fixed displacement radial piston hydraulic machines 4 of the type shown in Fig. 1 are used as both the hydraulic pump 4a and the hydraulic motors 4b.

Generally, the machines used as the pump 4a and the motors 4b will be of differing sizes with different displacement. In this installation, a generator 2 can operate at fixed speed while a rotor 3 operates at a speed determined by discrete hydraulic gear ratios. The total displacement of the motors will typically be smaller than the displacement of the pump(s) such that the generator shaft will rotate with a higher speed than the rotor.

-15 -The hydraulic gear 8 is formed by connecting the outlet chamber of the hydraulic pump 4a with the inlet chamber of a hydraulic motor 4b. The power for a hydraulic pump 4a can be given as P = = (1) where p is the pump power, Pp is the working pressure for the pump (the difference between the pressures on the low and high pressure sides), T Dp is the pump torque, D is the pump displacement and is the pump frequency.

Similarly, the power for a hydraulic motor 4b can be expressed as PM = 1'1w,1 = DAIPMWI, (2) where M is the motor power, PM is the working pressure for the motor (the difference between the pressures on the low and high pressure sides), TM = DAIpM is the motor torque, DA,, is the motor displacement and aiM is the motor frequency.

By connecting the outlet chamber of the hydraulic pump 4a to the inlet chamber of a hydraulic motor 4b and neglecting any losses, it can be shown that PPPM (3) and = M (4) Insertion of equations (1) to (3) into equation (4) gives a hydraulic gear ratio nJ, of the form = -= -(5) w,. D1 The gear ratio can be varied by discrete amounts by coupling the fixed displacement hydraulic motors 4b to and from the shaft 7 as desired.

The transmission lines 5, 6 are connected to hydraulic accumulators 9, 10.

The hydraulic accumulators 9, 10 reduce torque variations on the motors 4b and generator 2 during operation above and below the rated wind speed of the wind turbine installation.

In an alternative embodiment, instead of or in addition to the hydraulic accumulators 9, 10, hydraulic valves (not shown) are used. The hydraulic valves consist of one or more safety valves and/or one or more active controlled valves. -16-

The active controlled valves are opened and closed to control the hydraulic pressure to lie in a predetermined range around a (reference) mean value. This type of control is performed when the wind turbine installation is operating below rated wind speed.

The safety valves are opened when the hydraulic pressure is greater than a desired maximum value and are closed when the hydraulic pressure is less than a desired maximum value. This type of control is performed when the wind turbine installation is operating above rated wind speed.

The fixed displacement motors 4b are coupled in and out of operation by a controller (not shown) according to changes in the wind speed, the generator power and/or a pressure in the hydraulic transmission system. The coupling is provided for each motor by a controllable valve and an over-running clutch. By coupling the motors in and out of operation in this way, a tip-speed ratio closer to the value for optimum energy extraction can be achieved.

This is illustrated in Fig. 3, where it is shown that, by changing the gear ratio in order to optimise the tip-speed ratio, more power can be extracted from systems where the gear ratio can be changed, compared to a system where the gear ratio is fixed and cannot be changed.

Fig. 3 shows aerodynamic power extraction as function of wind speed for a fixed speed turbine, a variable speed turbine with optimal tip-speed ratio, a turbine with hydraulic transmission with four discrete hydraulic gear ratios, and a wind turbine with hydraulic transmission with ten discrete hydraulic gear ratios. The "fixed speed turbine" line 11 represents both a turbine with mechanical transmission and a turbine with hydraulic transmission with one discrete gear ratio. The "optimal tip speed" line represents both a turbine with mechanical transmission and a turbine with hydraulic transmission with continuous variable gear ratio. The lines corresponding to turbines with hydraulic transmissions with four and ten discrete gear ratios represent embodiments of the present invention.

Fig. 3 shows that fixed speed turbines have significantly lower aerodynamic power extraction than the turbines with variable gear ratios, especially at lower wind speeds.

In the embodiments, the changing of the gear ratio is based on direct or indirect measurements or estimates of the wind speed, the generator power, and/or the pressure in the hydraulic transmission. Such measurements or estimates are averaged over a period of several minutes (e.g. 10 minutes or longer), for example.

A comparison of the expected efficiencies for the two different types of hydraulic machine (variable displacement axial and fixed displacement radial) as a function of wind speed is shown in Fig. 4. Fig. 4 shows efficiencies of the variable displacement axial piston machines 12 and the fixed displacement radial piston machines 13 used in embodiments of the present invention as function of wind speed. The data points making up the graph are determined by combining measurements from experiments and data sheets for the two types of hydraulic machine (axial and radial). The efficiency of the transmission lines is estimated to be 0.97. Fig. 4 shows that the fixed displacement radial machines 4 used in the present invention have consistently better expected efficiency than variable displacement axial machines. Furthermore, the fixed displacement radial machines 4 are more robust than variable displacement axial piston machines.

Fig. 5 shows generator power extraction as function of wind speed for a variable speed wind turbine with mechanical transmission operating at optimal tip-speed ratio, a fixed speed wind turbine with mechanical transmission, a wind turbine with hydraulic transmission with continuously variable gear ratio and operating at optimal tip-speed ratio 14, a fixed speed wind turbine with hydraulic transmission with one gear ratio 15, a wind turbine with hydraulic transmission with ten discrete hydraulic gear ratios according to an embodiment of the invention, and a wind turbine with hydraulic transmission with four discrete hydraulic gear ratios 16 according to another embodiment of the invention.

The graph in Fig. S shows that, contrary to what is commonly perceived, the embodiments of the present invention (i.e. the lines that represent hydraulic transmissions with discrete variable gear ratios) have higher generator power extraction than hydraulic transmissions with continuous variable gear ratios.

From a standard wind speed distribution as shown in Fig. 6, the expected annual energy production can be calculated for various transmission systems. As shown in Fig. 7, all of the systems with discrete variable hydraulic gear ratios (Cases -18- 5-8), which correspond to embodiments of the invention, have higher annual energy production than systems with continuous variable hydraulic gear ratios (Case 3).

As mentioned above, in an embodiment of the invention, the hydraulic transmission line is coupled to one or more active controlled valves and/or one or more safety valves.

An example of an actively controlled valve that is taken from a numerical simulation model in MATLAB Simulink is shown in Fig. 8, where the reference mean value of the pressure is found by low-pass filtering of the transmission line pressure by use of a second order Butterworth filter with a filter frequency below the natural frequency indicated in equation (B) above. In Fig. 8 "phigh" is the pressure in the transmission line on the high pressure side of the system, which is the pressure that it is desired to stabilise through use of the valve, "Transfer Fcn" stands for "transfer function, "OmegaV" is the filter frequency (in rad/s) of the second order Butterworth low pass filter that is used to generate the reference pressure signal "preP' for the active controlled valve. The filter frequency "OmegaV" is below the natural oscillation frequency of the transmission system. The skilled person will appreciate that other transfer functions, estimators or mean values could be used to generate the signal "preP'. A desired reference pressure signal would be the actual pressure without the resonant oscillations. "p2" is the high pressure acting on the control valve. "qv" and "qvalve" are the same signal, the flow from the high pressure side to the low pressure side of the valve. Typically this flow would be through a drain line additional to the transmission line on the low pressure side of the hydraulic system.

Simulation snapshots with and without use of the active controlled valve are shown in Figs. 9 to 12.

Fig. 9 shows the hydraulic pressure in the pump with active valve control (20) and without (21). The size of the fluctuations in pump pressure is clearly reduced with active valve control.

Fig. 10 shows the generator power with active valve control (20) and without (21). The size of the fluctuations in generator power is clearly reduced with active valve control. -19-

Fig. 11 shows the rotor speed with active valve control (20) and without (21). The size of the fluctuations in rotor speed is clearly reduced with active valve control.

Fig. A 2 shows the pump shaft torque with active valve control (20) and without (21). The size of the fluctuations in pump shaft torque is clearly reduced with active valve control.

The improvement in the system parameters described above is achieved at a cost of just 0.65% reduction in the generator power production in the case considered.

Although the present invention has been described particularly in relation to wind turbines, at least some of the hydraulic transmissions and methods described in this application could also be used in other kinds of turbines such as water (e.g. wave, tidal or hydroelectric) turbines.

It will also be appreciated that the term "wind turbine installation" encompasses all kinds of wind turbines, including those based on land, and fixed foundation and floating water-based wind turbines.

Claims

-20 -Claims I A hydraulic transmission system for use in a wind turbine installation comprising a hydraulic pump connected via a transmission line to a plurality of fixed displacement hydraulic motors arranged such that the hydraulic motors can be coupled to and drive a common load, whereby the hydraulic motors can each be selectively coupled in and out of operation to drive the load.
2. A hydraulic transmission system as claimed in claim 1, wherein the hydraulic motors are directly couplable to a common drive shaft, which is connectable to the common load.
3. A hydraulic transmission system as claimed in claim 1 or 2, wherein the hydraulic pump is a fixed displacement hydraulic pump.
4. A hydraulic transmission system as claimed in claim 1, 2 or 3, wherein the hydraulic pump and/or the hydraulic motors is/are radial piston hydraulic machines.
5. A hydraulic transmission system as claimed in any preceding claim, wherein at least one of the hydraulic motors is couplable to the common drive shaft by a controllable valve.
6. A hydraulic transmission system as claimed in any preceding claim, wherein at least one of the hydraulic motors is couplable to the common drive shaft by mechanical coupling.
7. A hydraulic transmission system as claimed in claim 5, wherein the at least one of the hydraulic motors that is couplable to the common drive shaft by a controllable valve is also couplable to the common drive shaft by an over-running clutch. -21 -
8. A hydraulic transmission system as claimed in any preceding claim, wherein the transmission line is a high-pressure hydraulic transmission line.
9. A hydraulic transmission system as claimed in any preceding claim, wherein the transmission line is coupled to one or more hydraulic accumulators and/or one or more hydraulic valves.
10. A hydraulic transmission system as claimed in claim 9, wherein the one or more hydraulic valves comprise one or more safety valves and/or one or more active controlled valves.
11. A hydraulic transmission system as claimed in any preceding claim, wherein the hydraulic motors are connected to a controller for coupling the hydraulic motors in and out of operation.
12. A hydraulic transmission system as claimed in claim 11, wherein the controller is arranged to couple the motors in and out of operation on the basis of a wind speed, a generator power and/or a pressure in the hydraulic transmission system.
13. A hydraulic transmission system as claimed in claim 11 or 12, wherein the controller is connected to one or more sensors for directly or indirectly measuring or estimating a wind speed, a generator power and/or a pressure in the hydraulic transmission system.
14. A hydraulic transmission system as claimed in claim 11, 12 or 13, wherein the controller is arranged to couple the motors in and out of operation according to changes in a wind speed in order to obtain an optimum tip speed ratio of a rotor to which the hydraulic transmission system is connectable.
15. A wind turbine installation comprising a hydraulic transmission system as claimed in any preceding claim, a generator and a rotor, wherein the hydraulic -22 -motors are couplable to the generator and the hydraulic pump is connected to the rotor.
16. A wind turbine installation as claimed in claim 15 wherein the hydraulic motors are directly coupi able to the generator via a common shaft.
17. A wind turbine installation as claimed in claim 15 or 16, wherein the hydraulic motors and the generator are located at positions lower than those of the rotor and the hydraulic pump.
18. A method of using a wind turbine installation as claimed in claim 15, 16 or 17, wherein the hydraulic motors are coupled in and Out of operation in order to change a gear ratio of the hydraulic transmission.
19. A method as claimed in claim 18, wherein the hydraulic motors are coupled in and out of operation on the basis of a wind speed, a generator power and/or a pressure in the hydraulic transmission system.
20. A method as claimed in claim 18 or 19, wherein the motors are coupled in and out of operation according to changes in a wind speed in order to obtain an optimum tip speed ratio of the rotor.
21. A method as claimed in claim 18, 19 or 20, comprising measuring or estimating a wind speed, a generator power and/or a pressure in the hydraulic transmission system.
22. A method as claimed in claim 21, comprising averaging the wind speed, the generator power and/or the pressure in the hydraulic transmission system over a period of time.
23. A hydraulic transmission system for use in a wind turbine installation comprising a hydraulic pump connected via a transmission line to at least one -23 -hydraulic motor arranged such that the at least one hydraulic motors can be coupled to and drive a load, wherein one or more hydraulic accumulators and/or one or more hydraulic valves are coupled to the hydraulic transmission line.
24. A hydraulic transmission system as claimed in claim 23, wherein the one or more hydraulic valves comprises one or more safety valves and/or one or more active controlled valves.
25. A hydraulic transmission system as claimed in claim 24, wherein the one or more safety valves are arranged to open when a hydraulic pressure of the hydraulic transmission rises above a desired maximum pressure and to close when the pressure falls below the desired maximum pressure.
26. A hydraulic transmission system as claimed in claim 25, wherein the one or more safety valves are arranged to open when a hydraulic pressure of the hydraulic transmission rises above a desired maximum pressure and to close when the pressure falls below the desired maximum pressure during operation above rated wind speed.
27. A hydraulic transmission system as claimed in claim 24, 25 or 26, wherein the active controlled valves are arranged to control a hydraulic pressure of the hydraulic transmission around a desired mean value.
28. A hydraulic transmission system as claimed in claim 27, wherein the active controlled valves are arranged to control a hydraulic pressure of the hydraulic transmission around a desired mean value during operation below rated wind speed.
29. A hydraulic transmission system as claimed in any of claims 23 to 28 comprising a pressure sensor for detecting a hydraulic pressure of the system.

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30. A wind turbine installation comprising a hydraulic transmission system as claimed in any of claims 23 to 29.
31. Use of a hydraulic transmission system as claimed in any of claims I to 14 or 23 to 29 in a wind turbine installation.
32. A method of using a hydraulic transmission system in a wind turbine installation, the hydraulic transmission system comprising a hydraulic pump connected via a transmission line to at least one hydraulic motor arranged such that the at least one hydraulic motors can be coupled to and drive a load, wherein one or more hydraulic valves are coupled to the hydraulic transmission line, the method comprising: controlling the hydraulic valves on the basis of a detected hydraulic pressure.
33. A method as claimed in claim 32, wherein the one or more hydraulic valves comprise one or more safety valves, the method comprising the steps of: opening the one or more safety valves when a hydraulic pressure of the hydraulic transmission rises above a desired maximum pressure; and closing the one or more safety valves when the pressure falls below the desired maximum pressure.
34. A method as claimed in claim 33, wherein both steps are performed during operation above rated wind speed.
35. A method as claimed in claim 32, 33 or 34, wherein the one or more hydraulic valves comprise one or more active controlled valves, the method comprising the steps of: opening the one or more active controlled valves when a hydraulic pressure of the hydraulic transmission rises above a mean pressure; and closing the one or more active controlled valves when the pressure falls below the mean pressure.
36. A method as claimed in claim 35, wherein both steps are performed during operation below rated wind speed.
37. An offshore or water-based wind turbine installation comprising a hydraulic transmission system, wherein the hydraulic transmission system is arranged such that its natural frequency is outside of a frequency range of waves in a desired installation site.
38. An installation as claimed in claim 37, wherein the natural frequency of the hydraulic transmission system is below a frequency range of waves in a desired installation site.
39. An installation as claimed in claim 37 or 38, wherein the natural frequency of the hydraulic transmission system is outside of the range 0.05 to 0.2 Hz.
40. An installation as claimed in claim 37, 38 or 39, wherein, in use, the wind turbine installation is a floating wind turbine installation.
41. An installation as claimed in any of claims 37 to 40, wherein the natural frequency of the hydraulic transmission system is arranged to be outside of or below a frequency range of waves in a desired installation site by adjusting the elasticity of the hydraulic volume.
42. An installation as claimed in any of claims 37 to 41, wherein the hydraulic transmission system is as claimed in any of claims Ito 14 or 23 to 29.
43. A hydraulic transmission system for use in a water turbine installation comprising a hydraulic pump connected via a transmission line to a plurality of fixed displacement hydraulic motors arranged such that the hydraulic motors can be coupled to and drive a common load, whereby the hydraulic motors can each be selectively coupled in and out of operation to drive the load.

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44. A water turbine installation comprising a hydraulic transmission system as claimed in claim 43.
45. A method of using a water turbine installation as claimed in claim 44, wherein the hydraulic motors are coupled in and out of operation in order to change a gear ratio of the hydraulic transmission.
46. A hydraulic transmission system for use in a water turbine installation comprising a hydraulic pump connected via a transmission line to at least one hydraulic motor arranged such that the at least one hydraulic motors can be coupled to and drive a load, wherein one or more hydraulic accumulators and/or one or more hydraulic valves are coupled to the hydraulic transmission line.
47. A water turbine installation comprising a hydraulic transmission system as claimed in claim 46.
48. A method of using a hydraulic transmission system in a water turbine installation, the hydraulic transmission system comprising a hydraulic pump connected via a transmission line to at least one hydraulic motor arranged such that the at least one hydraulic motors can be coupled to and drive a load, wherein one or more hydraulic valves are coupled to the hydraulic transmission line, the method comprising: controlling the hydraulic valves on the basis of a detected hydraulic pressure.
49. An offshore water turbine installation comprising a hydraulic transmission system, wherein the hydraulic transmission system is arranged such that its natural frequency is outside of a frequency range of waves in a desired installation site.

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50. A hydraulic transmission system substantially as hereinbefore described with reference to any of the figures.
51. A wind turbine installation substantially as hereinbefore described with reference to any of the figures.
52. A method of using a hydraulic transmission system of a wind or water turbine installation substantially as hereinbefore described with reference to any of the figures.