GB2519673A

GB2519673A - Medium speed super-position Gearing

Info

Publication number: GB2519673A
Application number: GB1418898.1A
Authority: GB
Inventors: Adam Austin; James Day; Annabel Shahaj
Original assignee: Romax Technology Ltd
Current assignee: Romax Technology Ltd
Priority date: 2013-10-23
Filing date: 2014-10-23
Publication date: 2015-04-29
Also published as: GB201418898D0; GB201318721D0

Abstract

An electromechanical driveline, for electrical power generation from a power source such as a wind turbine, includes a motor 6, a generator 9, and a power splitting device 4 in the form of a planetary differential gear which is a second stage of a two stage gearbox. The differential 4 comprises a planet carrier connected via a first shaft 3 the power source, a sun connected via a second shaft 5 to the motor 6, and a ring gear connected via a third shaft 10 rotationally to the generator 9. The driveline also includes a controller 12 electrically connected to the motor 6) and a sensor 11connected to the controller 12 and configured to provide rotational speed information about the power source 1. In use, the controller 12 governs a supply of power to the motor 6 according to the rotational speed information and the motor 6 provides power to the power splitting device 4 via the second shaft 5 to maintain a rotational speed of the third shaft 10 within a selected controllable speed range.

Description

Medium speed super-position gearing The invention relates to the layout of an electromechanical driveline for power generation allowing decoupling of speed from the power source for speed control of the output shaft.

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 there are advantages available if the drivetrain can generate power at variable speeds.

A particular important aspect of the present invention is its application to drivelines for use in wind or water turbine installations. In early wind turbine designs involving constant speed squirrel cage induction generators, the requirement to maintain a constant generator speed was largely 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 electronic 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, and allow the wind turbine to be more efficient in terms of converting wind energy to rotational energy. Variable speed gearing as an alternative to power electronics on wind turbines is not a new idea, the 3MW [Si turbine built in Orkney in i982, made use of differential gearing.

The issues associated with needing to condition the variable power output from wind into a suitable form for grid connection, and enhancing operational envelopes via variable speed, would also apply for other renewable energy power sources such as wave, tidal and hydro, or 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.

The power split device as described above would be called a differential gear if it is done through conventional mechanical gearing. Another common term used in the wind energy arena for this type of gearing is superposition gearing, such as can be found in. U.S. Patent 81 54143 B2.

Prior art for power split devices involving differential gears in which an auxiliary machine is used to control the speed of the main generator such that it remains constant is well established.

In recent years a new class of wind turbine drivetrains has developed, that are perceived to bring cost of energy advantages through their increased reliability.

Conventional turbines that use gearboxes are typically now referred to as high speed turbines, and the overall gearbox ratio tends to be between 70 and 120. Having high gearbox ratios allows a smaller generator, and this can make the turbine cheaper in terms of capital expenditure, as the generator tends to be one of the most expensive parts of the wind turbine drivetrain. Unfortunately, reliability issues have been experienced by some operators with the high speed gearboxes. As such cost of energy produced by the turbine can be high. The target of future turbines is to reduce cost of energy. As such there has been a lot of interest in larger lower speed generators for wind turbines, some not using a gearbox at all, (this would be called a direct drive turbine), or alternatively using a lower ratio gearbox, that typically involves only two gear stages. This would be called a medium speed turbine. An example of this terminology can be found in US Patent 7935020 B2.

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 the layout of the drivetrain of a variable speed wind energy generator, that makes use of a variable gear ratio system. More specifically the invention relates to the layout and type of variable gearing in the gearbox. The invention is however not limited to the wind energy arena, it could be used for any generator drivetrain for generating alternating current electricity from a variable speed power supply source and supplying it to an electrical power grid; examples of variable speed powers sources are at the present time. wave, wind, hydro, and tidal. The invention relates specifically to the integration of the device within a medium speed gearbox. In the context of wind energy, a medium speed gearbox is a gearbox having two gear stages, rather than three stages.

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, an auxiliary electrical machine, and a planetary differential gear comprising: a sun shaft mechanically connected to the primary electrical machine; a ring gear mechanically connected to the auxiliary electrical machine; and a planet carrier mechanically connected to the power input; in which, the planetary differential gear forms a second stage of a two stage gearbox.

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; Figure 4 illustrates a schematic diagram of a wind turbine according to various embodiments of the invention; and Figure 5 illustrates a schematic of a power splitting device in the form of a planetary differential.

The drivetrain for a wind turbine with variable speed gearing would typically comprise; 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 auxiliary electrical machine (motor or motor-generator), 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. A schematic is shown in figure 3. In the present invention the power splitting device is assumed to be a planetary differential, as shown in figure 5.

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 of the type shown in Figure 5.

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 1 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 will 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 supply 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.

The present invention relates to the layout of the variable speed gearing, for a medium speed gearbox, (a medium speed gearbox is considered to have two stages and a ratio in the range 15 to 40). The variable speed gearing is achieved through the use of a planetary differential gear. The constant speed generator is attached to the sun shaft of the planetary differential, and an auxiliary motor-generator is used to drive the ring gear, such that a constant speed is maintained on the sun shaft, even though the input shaft from the power source (the planet carrier shaft) is moving at variable speed. The invention is the location of the planetary differential within the medium speed gearbox. Specifically the invention is that the planetary differential should form the second stage of a medium speed gearbox. This brings advantages, because if the differential gearing is located as the second stage of the medium speed gearbox, the torque required for the auxiliary motor generator, and hence the size of the motor-generator, can be smaller, reducing the cost. Conversely locating a planetary differential after a two stage medium speed gearbox is not advantageous.

The torque requirements for the auxiliary motor-generator, are reduced, but the overall solution, two fixed stages, and a third differential stage is essentially a complex 3 stage high speed gearbox, and as such is no longer a medium speed gearbox. This means that overall complexity is higher, and the higher speeds are problematic, there have been concerns that a lot of the problems seen on high speed gearboxes were speed related, this certainly could be true in the case of failures from bearing skidding on the 3 stage of a high speed gearbox. In short if there are more than two gear stages, and the speeds are higher it's no longer a medium speed gearbox.

The remaining option is to have the differential as the first stage of a two stage medium speed gearbox. This is likely to be better than a 3 stage gearbox, but is disadvantageous in that the torque requirements of the auxiliary motor-generator are higher.

Considering cost of energy, the optimal solution is having the differential stage as the second stage of the medium speed gearbox.

The advantage of the variable speed gearbox as opposed to conventional solutions that involve fixed ratio gearboxes and power electronics is related to the size of the turbine in terms of its rating in MW. The power electronics is a collection of switches. As turbine size increases so does the number of switches in the power electronics. The reliability of the power electronics derives from the reliability of each individual switch and the total number of switches. As such, at larger turbine sizes the power electronics become less reliable, because of the larger number of switches. Variable speed gearing removes the need for power electronics for the main generator. The auxiliary machine (typically a motor generator) still has power electronics but the required size is much smaller. It is even possible to not have any power electronics at all using an electro-hydraulic motor as the auxiliary machine.

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 environmental 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; an auxiliary electrical machine; a planetary differential gear comprising: a sun shaft mechanically connected to the primary electrical machine; a ring gear mechanically connected to the auxiliary electrical machine; and a planet carrier mechanically connected to the power input; in which, the planetary differential gear forms a second stage of a two stage gearbox.
2. An electromechanical driveline according to claim 1, in which the primary electrical machine is a generator.
3. An electromechanical driveline according to claim 1 or claim 2, in which the auxiliary electrical machine is a motor-generator.
4. An electrochemical driveline according to any preceding claim, in which the auxiliary machine is connected to power electronics which in turn are connected to the grid.
5. An electromechanical driveline according to any preceding claim, in which the differential gearing is located, as the first stage of a two stage medium speed gearbox.
6. An electromechanical driveline according to any preceding claim, in which the auxiliary machine is hydraulic rather than electrical
7. An electromechanical driveline according to any preceding claim in which the auxiliary machine is a combination of an electric motor and a hydraulic drive.
8. An electromechanical driveline according to any preceding claim in which the differential gearing has the main generator connected to the ring gear, and the auxiliary machine connected to the sun gear.
9. An electromechanical driveline according to any preceding claim in which the power input is connected to the sun gear, and the electrical machines are connected to the ring gear and planet carrier in either permutation.
10. An electromechanical driveline according to any preceding claim in which the power input is connected to the ring gear, and the electrical machines are connected to the sun gear and planet carrier in either permutation.
11. A medium speed turbine comprising an electromechanical driveline of any preceding claim.