GB2491488A

GB2491488A - Electromechanical driveline with power splitting device

Info

Publication number: GB2491488A
Application number: GB1210008.7A
Authority: GB
Inventors: Annabel Shahaj; Adam Austin; James Day
Original assignee: Romax Technology Ltd
Current assignee: Romax Technology Ltd
Priority date: 2011-06-03
Filing date: 2012-06-06
Publication date: 2012-12-05
Also published as: GB2491400A; GB201109307D0; GB201210008D0

Abstract

An electromechanical driveline for electrical power generation from a power source, such as a turbine blade, 1 includes a motor 6, a generator 9 and a power splitting device 4. The power splitting device comprises a first shaft 3 rotationally connected to the power source, a second shaft 5 rotationally connected to the motor 6, and a third shaft 10 rotationally connected to the generator. The driveline also includes a controller 12 electrically connected to the motor and a sensor 11 connected to the controller and configured to provide rotational speed information about the power source. The controller governs a supply of power to the motor according to the rotational speed information and the motor provides power to the power splitting device via the second shaft to maintain a rotational speed of the third shaft within a selected controllable speed range.

Description

Electromechanical Driveline The invention relates to an electromechanical driveline for power generation allowing decoupling of speed from the power source for speed control of the output shaft. In particular it relates to a driveline for use in wind or water turbine installations.

Simple electromechanical drivelines generally consist of a simple fixed ratio gearbox connected to a motor/generator of some design. Drivelines such as this can be found in various applications including power stations, industrial drives, electric vehicles and wind turbines.

In the case of power generation applications such as wind turbine drivetrains, there are advantages available if the drivetrain can generate power at variable speeds. In early wind turbine designs involving constant speed squirrel cage induction generators, the requirement to maintain a constant generator speed was achieved aerodynamically via turbine pitch control. However pitch control is not sufficient for all wind speeds, and there are conditions where the turbine has to be disconnected from the grid.

Other wind turbines (which are commonly referred to as variable speed wind turbines) have a variable speed of the generator shaft in addition to aerodynamic pitch control. The output electrical power to the grid has to be of a suitable frequency and power to be compliant for grid connection. To achieve this, the variable speed generator shaft is accommodated in one of two ways: either via a variable speed generator such as a DFIG, or secondly using a synchronous generator with conditioning of the variable output power from that generator via a power electronics converter. The benefit of the variable speed machines is that they increase the operating envelope over which power can be generated by the wind turbine.

The issues associated with needing to condition the variable power output from wind into a suitable form for grid connection, and enhancing operational enveiopes via variable speed, would also appiy for other renewable energy power sources such as wave, tidal and hydro, (any application where the supplied power is variable).

An alternative approach is to use a variable speed drivetrain and a constant speed generator. U.S. Patent No. 7,936,078 B2 (Figure 2) discloses a variable ratio gearbox having first and second output shafts which rotate at speeds that are not in fixed proportion to the speed of rotation of said rotor shaft; a fixed speed large generator connected with the first output shaft; and a variable speed small generator connected with the second output shaft. The gearbox is for a wind turbine application, and utilises a power splitting device to split the power in such a way that one of the outputs is at a constant speed. This has the disadvantage of not having the flexibility to allow two outputs shafts able to rotate at varying speeds. It may in fact be preferable overall to tolerate a non-constant mildly varying output speed.

Present systems place all speed control on the generator side. The invention disclosed in U.S. Patent No. 7,936,078 B2 places all the burden of speed control on the transmission. Sharing the burden between both transmission and generator could potentially yield a cheaper more flexible overall system.

An additional disadvantage of the invention is that power is supplied to the grid via two routes, at low wind speed via a variable speed auxiliary generator and at high wind speed via both generators. It is better for power to be supplied primarily via the larger generator at all conditions with minimal power being supplied to the grid from the auxiliary generator. This can be achieved by taking power from the grid (using an electrical machine (motor-generator or motor) in place of the auxiliary generator, and running the electrical machine in motor mode) to allow generation via the larger generator at all operating conditions. As the power supply from the auxiliary electrical machine is reduced (or not present in the case that a just a motor is used), it reduces/removes the need for expensive power electronics associated with using variable speed auxiliary generator to supply energy to the grid.

In the foregoing, the term "electrical machine" is defined as either a generator a motor or a motor-generator, the term "generator" is defined as an electrical machine that generates electrical power from motion, the term "motor" is defined as an electrical machine that takes electrical power and provides motion, and the term "motor-generator" is defined as a machine that can run both as a motor and a generator.

The present invention relates to an improved generator drivetrain design for generating alternating current electricity from a variable speed power supply source and supplying it to an electrical power grid. Examples of such a variable speed power supply source include, wave, wind, hydro, and tidal. The primary object of the invention is to provide an output power shaft for connection to a generator that can be operated at selectable controllable speed ranges, including near constant speed ranges. The advantage of selectable speed ranges is that it allows the same drive train to be used with different generator types. An additional advantage of the device is that it provides capabilities to limit torque spikes that can arise from a fluctuating power supply.

According to a first aspect of the present invention, there is provided an electromechanical driveline for electrical power generation from a power source, the driveline comprising: a primary electrical machine which is a generator, an auxiliary electrical machine, and a power splitting device, in which the power splitting device comprises a first shaft rotationally connected to the power source, a second shaft rotationally connected to the motor, and a third shaft rotationally connected to the generator. The driveline also includes a controller electrically connected to the motor and a sensor connected to the controller and configured to provide rotational speed information about the power source. The auxiliary electrical machine is a motor, and in use, the controller governs a supply of power to the motor according to the rotational speed information and the motor provides power to the power splitting device via the second shaft to maintain a rotational speed of the third shaft within a selected controllable speed range.

Also disclosed is an electromechanical driveline for electrical power generation from a power source, the driveline comprising: a primary electrical machine which is a generator, an auxiliary electrical machine, and a power splitting device, in which the power splitting device comprises a first shaft rotationally connected to the power source, a second shaft rotationally connected to the motor, and a third shaft rotationally connected to the generator. The driveline also includes a controller electrically connected to the motor and a sensor connected to the controller and configured to provide rotational speed information about the power source. The auxiliary electrical machine is a motor-generator, and in use, the controller governs a supply of power to the motor-generator according to the rotational speed information and the motor-generator provides power to the power splitting device via the second shaft to maintain a rotational speed of the third shaft within a selected controllable speed range.

Preferably, the generator is connected to power electronics which in turn are connected to the grid.

Preferably, the power input shaft may be connected to the power split input shaft via a fixed ratio gearbox, or gear stage or equivalent torque and speed modifying device, but this need not always be inciuded.

Preferably, the power splitting device comprises a planetary gear set.

Preferably, the first shaft is connected to a planet carrier, the second shaft is connected to a ring gear and the third shaft is connected to a sun gear.

Preferably, the power input from the power source is connected to the sun shaft of a planetary differential, and the electrical machines are connected to the other two shafts, in either of the two permutations of that arrangement.

Preferably, the power input from the power source is connected to the ring gear shaft of a planetary differential, and the electrical machines are connected to the other two shafts, in either of the two permutations of that arrangement.

Preferably, the power input from the power source is connected to the sun gear shaft of a planetary differential, and the electrical machines are connected to the other two shafts, in either of the two permutations of that arrangement.

Preferably, the power from the input shaft can be transferred to an energy storage device (such as batteries, capacitors, fly wheels) such that power spikes arising from the power source can be managed to reduce the peak loads on the drive train.

Preferably, the power from the input shaft can be transferred to an energy dissipation device (such as resistors, brakes, etc) such that power spikes arising from the power source can be managed to reduce the peak loads on the drivetrain.

Preferably, there exists a brake and brake controller such that the shaft of the auxiliary electrical machine (motor or motor generator), can be locked when desired such that it does not rotate.

Preferably, there exists a second fixed speed gearbox between the power splitting device and the auxiliary electrical machine (motor or motor generator).

Preferably, the power source is a wind or water turbine.

Also included is a wind or water turbine comprising the electromechanical driveline disclosed above.

The invention will now be described solely by way of example and with reference to the accompanying drawings in which: Figure 1 is a schematic of how power is commonly split between two or more motor/generators; Figure 2 shows a power splitting wind driveline described in U.S. Patent No. No. 7,936,078 B2 in which one shaft always rotates at a constant speed; Figure 3 is a generic schematic of the present invention, comprising a fixed speed gearbox, a power splitting device, two motor/generators, a sensor and a controller allowing speed control of the motor/generators; and Figure 4 illustrates a schematic diagram of a wind turbine according to various embodiments of the invention.

In Figure 3, a shaft 1 is connected to a rotating power source that has a given speed and torque which can vary throughout an operating cycle. Power from the rotating source is transferred to a first shaft 3 of a power splitting device 4. Power splitting device 4 has a second shaft 4 rotationally connected to an auxiliary electrical machine 6, and a third shaft 10 rotationally connected to a primary electrical machine 9. Primary electrical machine 9 is connected to the grid, either directly or indirectly via a voltage transformer, and is used to supply power to the grid. Auxiliary electrical machine 6, which can be connected to power electronic converters, is used to supply power to, or take power from power splitting device 4 so that the operating speed of the primary electrical machine can be kept within a defined controllable speed range regardless of the input speed of shaft 1 defined by the power source.

The defined controllable speed range can be a range of speeds which varies by ±20% around a mean value. The range can be a range of speeds which varies by ±15% around a mean value. The defined controllable speed range can be a range of speeds which varies by ±10% around a mean value. The defined controllable speed range can be a range of speeds which varies by ±5% around a mean value. The defined controllable speed range can be a range of speeds which varies by ±1 % around a mean value. The defined controllable speed range can be a range of speeds which varies by ±0.1% around a mean value.

Power splitting device 4 can be a planetary differential.

A device 2 that allows modification of speed and torque at fixed ratio can be advantageously connected between the power source and power splitting device 4.

This can be a fixed ratio conventional gearbox, a magnetic gear set or other analogous device.

Additional components include: a shaft 3 connecting fixed ratio torque speed modifying device 2 and power splitting device 4; a shaft 5 connecting power splitting device 4 to auxiliary electrical machine 6; a shaft 10 connecting power splitting device 4 to primary electrical machine 9; an input speed measurement device 11; and a controller 12 used to control the auxiliary electrical machine such that the primary electrical machine (normally a generator) remains within a controllable speed range.

Primary electrical machine 9, which is typically a generator, can be selected depending on application. The rest of the system serves to condition the torque and speed that it receives. As such, a variety of different generator configurations can be utilised within the same drive train.

In a first embodiment of the invention, power input shaft I is connected via a fixed ratio gearbox 2 to the input of a planetary differential. The output from the sun gear of the differential shaft 10 is connected to a generator 9. The output from the ring gear shaft 5 is connected to a variable speed motor 6.

The variable speed motor 6 is used to supply power to the differential 4 such that the speed of the sun gear output shaft 10 can be kept within a defined controllable speed range. This shaft is connected to a generator 9 which in turn is connected to the grid. Controlling the speed range of the shaft such that it can be kept within defined limits reduces the requirements placed on the power electronics to condition power for supply to the grid.

Speed control of the primary generator shaft is achieved via speed sensor 11, and controller 12 that are used to control the speed/torque of the auxiliary motor 6 such that the generator shaft is kept within the desired speed range. The rotational speed of the power input shaft 1 is defined by the characteristics of the system and the available power from the power source (wave, wind, hydro, etc). At conditions where shaft power (and speed) is low, the auxiliary motor supplies power to the differential, such that the primary generator can continue supplying power of appropriate quality to the grid. At conditions of greater available power (and input-shaft speed), the auxiliary motor becomes almost redundant, and only a small amount of power is supplied to the planetary differential. If conditions are suitable the auxiliary motor can even be switched off.

If power available falls below a threshold level (cut in speed in the case of wind power), the primary generator wiil need to disconnect from the grid. If supply power is above a threshold level (cut out speed in the case of wind power), the primary generator will also need to disconnect from the grid.

Depending on the primary generator chosen, and grid requirements (applicable grid code) for the selected application, power electronics may or may not be required to condition the power for supply to the grid. The conditioning requirement is however greatly reduced compared with a device not including the speed control from the driveline.

In a second embodiment of the invention, the generic layout is again as shown in Figure 3. In this embodiment the auxiliary electrical machine is a motor-generator 6.

Speed control of the primary generator 9 is achieved through either supplying power to the differential (running in motor mode), or taking power from the differential, (running in generator mode).

This embodiment has the advantage that for a given application a smaller electrical machine can be used to control the speed, because it runs in both directions. This allows a smaller peak motor mode torque requirement.

As in the first embodiment input power/speed is defined primarily by the available power from the power source. At conditions of low input speed/power, the motor-generator runs in motor mode, and supplied power to the split. At conditions of higher available power, the motor-generator runs in generator mode taking power from the power split.

In this embodiment the power generated by the motor-generator would need to pass through power electronics to be conditioned such that it would be suitable for suppiy to the grid. However the ratios of the planetary differential are defined such that only a small amount power is generated at the higher running speeds via the motor-generator, thus reducing the requirements placed on (and expense of) the power electronics for this device.

All other aspects remain as described in the first embodiment.

In some cases, a second fixed ratio gearbox, or gear stage or equivalent torque and speed modifying device is connected between the power splitting device and the auxiliary electrical machine, on shaft 5 connecting power splitting device 4 to auxiliary electrical machine 6 (not shown in Figure 3).

In practice there are a variety of different ways that planetary power splitting device 4 can be connected in terms of which shaft is connected to the input, the generator and so on. These embodiments are shown in Table 1, and each has different speed torque characteristics, for a given ratio. -ii-

Table I -Power splitting device connections Embodiment Planet Carrier Ring Gear Sun Gear la Source Motor Generator I b Source Generator Motor I c Motor Generator Source id Generator Motor Source le Motor Source Generator if Generator Source Motor 2a Source Motor-Generator Generator 2b Source Generator Motor-Generator 2c Motor-Generator Generator Source 2d Generator Motor-Generator Source 2e Motor-Generator Source Generator 2f Generator Source Motor-Generator In a further embodiment, the auxiliary electrical machine is a hydraulic machine.

In all of the embodiments described energy storage devices (such as batteries, capacitors and fly wheels, hydraulic/pneumatic accumulators) or dissipation devices (such as brakes, resistors, etc), could be included for the purpose of load management. Via both approaches (storage/dissipation) power spikes arising from the power source can be managed to limit the peak loads experienced on the drivetrain.

The invention thus comprises: a shaft connected to a source of power (e.g. wave, wind hydro etc), with a given speed and torque for which both may vary throughout an operating cycle; a device that allows modification of speed and torque at fixed ratio (this could be a fixed ratio conventional gearbox, but could also be done with magnetic gear sets and other analogous devices); a power splitting device, (this could be but is not limited to a planetary differential); and two or more motor/generators that allow alternate power transfer paths between the power splitting device and the motor/generators.

Optionally, the invention can include a device to limit peak loads. This can be achieved by transferring some of supplied energy to an energy storage device (this could be electrical, mechanical or hydraulic or pneumatic), or by dissipating the energy and releasing it as heat.

The alternate power transfer paths provided by the plurality of motor/generators allow control of the output speed ranges experienced on the output shafts, allowing use of the device with a variety of generator types whilst cutting down on the associated requirements (for the various generator types) put on the power electronics to condition the electricity for supply to the grid.

Fig. 4 illustrates a schematic diagram of a wind turbine 40 according to various embodiments of the invention. The wind turbine 40 includes a nacelle 42 (which may also be referred to as a turbine housing), a support post 43, a rotor 44, a rotor shaft 46, a gear box 48 and a generator 50. The wind turbine 40 is arranged to convert wind energy to electrical energy. The wind turbine 40 may be installed off-shore or may be installed inland.

In the following, the terms downwind' and upwind' refer to an axial direction in relation rotor 44, which is upwind of electromechanical driveline 48; electromechanical driveline 48 is downwind of rotor 44.

The nacelle 42 houses the electromechanical driveline 48 and protects it from environmentai damage (e.g. caused by rain, snow etc). The support post 43 is connected to the nacelle 42 and to the earth (or to an anchored floating platform when located off-shore).

The rotor 44 is supported by the nacelle 42 and is arranged to rotate in response to the movement of air (wind) past the wind turbine 40. The electromechanical driveline 48 is connected to the rotor 44 via the rotor shaft 46 and is connected to the nacelle 42. The electromechanical driveline 48 is arranged to convert the relatively low angular frequency, high torque input from the rotor 44 into electrical energy 52.

Claims

Claims 1. An electromechanical driveline for electrical power generation from a power source, the driveline comprising: a primary electrical machine, in which the primary electrical machine is a generator; an auxiliary electrical machine; a power splitting device, in which the power splitting device comprises a first shaft rotationally connected to the power source, a second shaft rotationally connected to the auxiliary electrical machine, and a third shaft rotationally connected to the generator; a controller electrically connected to the auxiliary electrical machine; and a sensor connected to the controller and configured to provide rotational speed information about the power source; in which the auxiliary electrical machine is a motor, wherein, in use, the controller governs a supply of power to the motor according to the rotational speed information and the motor provides power to the power splitting device via the second shaft to maintain a rotational speed of the third shaft within a selected controllable speed range.
2. An electromechanical driveline for electrical power generation from a power source, the driveline comprising: a primary electrical machine, in which the primary electrical machine is a generator; an auxiliary electrical machine; a power splitting device, in which the power splitting device comprises a first shaft rotationally connected to the power source, a second shaft rotationally connected to the auxiliary electrical machine, and a third shaft rotationally connected to the generator; a controller electrically connected to the auxiliary electrical machine; and a sensor connected to the controller and configured to provide rotational speed information about the power source; in which the auxiliary electrical machine is a motor-generator, wherein, in use, the controller governs a supply of power to the motor-generator according to the rotational speed information and the motor-generator provides power to the power splitting device via the second shaft to maintain a rotational speed of the third shaft within a selected controllable speed range.
3. An electrochemical driveline according to claim 2, in which the motor-generator is connected to power electronics which in turn are connected to the grid.
4. An electromechanical driveline according to any preceding claim, in which the power input shaft is connected to the power split input shaft via a fixed ratio gearbox, or gear stage or equivalent torque and speed modifying device.
5. An electromechanical driveline according to any preceding claim, in which a second fixed ratio gearbox, or gear stage or equivalent torque and speed modifying device is connected between the power splitting device and the auxiliary electrical machine.
6. An electrochemical driveline according to any preceding claim, in which the generator is connected to power electronics which in turn are connected to the grid.
7. An electromechanical driveline according to any preceding claim, in which the power splitting device comprises a planetary gear set.
8. An electromechanical driveline according to claim 7, in which the first shaft is connected to a planet carrier, the second shaft is connected to a ring gear and the third shaft is connected to a sun gear.
9. An electromechanical driveline according to claim 7, in which the power input from the power source is connected to the sun shaft of a planetary differential, and the two electrical machines are connected to the other two shafts, in either of the two permutations of that arrangement.
10. An electromechanical driveline according to claim 7, in which the power input from the power source is connected to the ring gear shaft of a planetary differential, and the two electrical machines are connected to the other two shafts, in either of the two permutations of that arrangement.
11. An electromechanical driveline according to claim 7, in which the power input from the power source is connected to the sun gear shaft of a planetary differential, and the two electrical machines are connected to the other two shafts, in either of the two permutations of that arrangement.
12. An electromechanical driveline according to any of claims 1 to 11, in which the power from the input shaft can be transferred to an energy storage device including batteries, capacitors, or fly wheels such that power spikes arising from the power source can be managed to reduce the peak loads on the drivetrain.
13. An electromechanical driveline according to any of claims I to 11, in which the power from the input shaft can be transferred to an energy dissipation device including resistors, and brakes such that power spikes arising from the power source can be managed to reduce the peak loads on the drivetrain.
14. An electromechanical driveline according to any of the preceding claims, in which there exists a brake and brake controller such that the shaft of the auxiliary electrical machine, can be locked when desired such that it does not rotate.
15. An electromechanical driveline for electrical power generation from a power source, the driveline comprising: a primary electrical machine, in which the primary electrical machine is a generator; an auxiliary electrical machine; a power splitting device, in which the power splitting device comprises a first shaft rotationally connected to the power source, a second shaft rotationally connected to the auxiliary electrical machine, and a third shaft rotationally connected to the generator; a controller electrically connected to the auxiliary electrical machine; and a sensor connected to the controller and configured to provide rotational speed information about the power source; in which the auxiliary electrical machine is a hydraulic machine, wherein, in use, the controller governs a supply of power to the hydraulic machine according to the rotational speed information and the hydraulic machine provides power to the power splitting device via the second shaft to maintain a rotational speed of the third shaft within a selected controllable speed range.
16. An electromechanical driveline according to any preceding claim, in which the power source is a wind or water turbine.
17. An electromechanical driveline as described herein with reference to Figure 3.
18. A wind or water turbine as described herein with reference to Figure 4 and comprising the electromechanical driveline according to any preceding claim.