WIND TURBINE

Abstract

The invention provides a wind turbine, comprising: a turbine rotor comprising a set of turbine rotor blades and defining a rotor rotational axis, said turbine rotor mounted on a tower; an electrical generator for converting mechanical energy of said turbine rotor into electrical energy, comprising a generator rotor drivingly coupled to said turbine rotor and mounted on said tower; a transmission system coupling said turbine rotor to said generator rotor, and comprising: an upstream stepped planetary gearbox comprising a upstream ring gear drivingly coupled to said turbine rotor, upstream first planet gears drivingly coupled with said upstream ring gear, upstream second planet gears, each rotationally coupled with a first planet gear, and an upstream sun gear drivingly coupled to said upstream second and coupled to said generator rotor, wherein said upstream second planet gears are axially offset to one another.

Claims

1. A wind turbine, comprising: a turbine rotor comprising a set of turbine rotor blades and defining a rotor rotational axis, said turbine rotor mounted on a tower; an electrical generator for converting mechanical energy of said turbine rotor into electrical energy, comprising a generator rotor drivingly coupled to said turbine rotor and mounted on said tower; a transmission system coupling said turbine rotor to said generator rotor, and comprising an upstream stepped planetary gearbox comprising: a upstream ring gear drivingly coupled to said turbine rotor; upstream first planet gears drivingly coupled with said upstream ring gear; upstream second planet gears, each second planet gear rotationally coupled with a first planet gear; an upstream sun gear drivingly coupled to said upstream second planet gears and coupled to said generator rotor, and said upstream second planet gears are axially offset to one another.

2. The wind turbine of claim 1, wherein said upstream stepped planetary gearbox comprises at least four upstream second planetary gears, wherein said upstream second planetary gears of each set are functionally in an axial plane, and said axial planes are axially offset with respect to one another, allowing said second planetary gears to overlap.

3. The wind turbine of claim 1, comprising a common carrier rotationally carrying said upstream first and second planetary gears rotatable about their axes of rotation, rotationally carrying said upstream sun gear, and rotationally carrying said upstream ring gear.

4. The wind turbine of claim 1, wherein each said upstream first planetary gear is rotatably carried on a fixed pin and each said upstream second planetary gear is rotatably carried on a--said fixed pin, and said upstream first planetary gear and said upstream second planetary gear on said pin are rotationally coupled.

5. The wind turbine of claim 3, wherein said common carrier carries a ring pin rotatably carrying said upstream ring gear.

6. The wind turbine of claim 3, wherein said common carrier rotatably carries said upstream sun gear.

7. The wind turbine of claim 1, wherein said upstream sun gear is coupled to an output shaft via a flexible coupling.

8. The wind turbine of claim 1, wherein said transmission system further comprises a bevel gearbox coupled to said upstream sun gear.

9. The wind turbine of claim 1, wherein said upstream first planet gears are each rotatable about their planet rotational axes which have a fixed position with respect to said rotational axis.

10. The wind turbine of claim 2, wherein said upstream stepped planetary gearbox comprises at least two sets of at least 2 upstream second planetary gears.

11. The wind turbine of claim 2, wherein said upstream stepped planetary gearbox comprises at least two sets of at least 3 upstream second planetary gears.

12. The wind turbine of claim 3, wherein the wind turbine comprises planet pins fixed to said common carrier and each holding a said upstream first planet gear and a said upstream second planet gear, with said upstream first planet gear and said upstream second planet gear rotationally coupled.

13. The wind turbine of claim 4, wherein said upstream first planetary gear and said upstream second planetary gear on said pin are rotationally coupled via a flexible coupling.

14. The wind turbine of claim 4, wherein said upstream first planetary gear and said upstream second planetary gear on said pin are rotationally coupled via a shaft running through said fixed pin.

15. The wind turbine of claim 8, further comprising a downstream planetary gearbox coupling said bevel gearbox to said generator rotor.

16. The wind turbine of claim 8, wherein said bevel gearbox comprises a bevel gear coupled to said upstream sun gear, and two opposite bevel pinions coupled to said bevel gear, and one said bevel pinion coupled to a downstream ring gear and one said bevel pinion coupled to a downstream planet gear carrier, and a downstream sun gear coupled to said generator rotor.

17. The wind turbine of claim 9, wherein said planet rotational axes are functionally parallel to said rotational axis.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0112] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which:

[0113] FIG. 1 schematically depicts an embodiment of the wind turbine;

[0114] FIG. 2 a schematic cross sectional, schematic view of the interior of the gondola or nacelle part of FIG. 1;

[0115] FIG. 3 shows details of an embodiment of the transmission system and generator;

[0116] FIG. 4 shows an embodiment of a generator, in particular an outer-rotor permanent magnet generator, and

[0117] FIG. 5 an alternative transmission part.

[0118] The drawings are not necessarily on scale

DESCRIPTION OF PREFERRED EMBODIMENTS

[0119] FIG. 1 schematically shows an example of a two-bladed downwind wind turbine of the current invention. The wind turbine has a tower 18. Tower 18 carries a gondola or nacelle 30. Nacelle 30 rotatably holds a turbine rotor 1. In order to allow positioning the turbine rotor 1 in the wind, the nacelle 30 can be mounted rotatably on the tower 18, allowing setting a yaw angle of said nacelle 30.

[0120] The current design of the wind turbine was found to be in particular advantageous in a wind turbine having a capacity of 8-10 MW and even bigger, with matching rotor sizes for different IEC Wind Classes, because that represents the next major step in technology scaling. This is expected to at least partly require new innovative solutions for technologically enabling such scaling step and in parallel drive down lifetime based generating costs. In FIG. 1 a downwind wind turbine is illustrated, but the current design may also be applied to an upwind wind turbine.

[0121] The nacelle 30 comprises in this embodiment a helicopter deck 31 and has external cooling radiators 36.

[0122] In this embodiment, the nacelle 30 houses a drive train coupling the turbine rotor 1 via a transmission system 5 to a generator 6, see FIG. 2.

[0123] In FIGS. 2-4, the drive train with transmission system 5 and generator 6 will be explained in detail. The drawings are schematic, and not all elements and parts may be in their right size or mutual size. The drawing is a cross section along a plane defined by a rotor rotational axis R (striped line) and a tower longitudinal line L.

[0124] FIG. 2 shows an embodiment of the drive train with the turbine rotor 1 coupled via transmission system 5 to generator 6. FIG. 3 will explain an embodiment of the transmission system 5 and generator 6 in more detail, and FIG. 4 shows an embodiment of a generator 6 in more detail. The transmission system can be divided into parts 5A and 5B, that will be explained below. FIG. 5 shows an alternative embodiment of part 5A of the transmission system 5.

[0125] In the drawings, many instances of bearings will be indicated by the classical indication of rectangles with a cross inside. These do not always comprise a separate reference number, but is considered to be evident for a skilled person.

[0126] FIG. 2 shows that the wind turbine comprises a turbine rotor 1 which comprises a set of turbine blades and which defines the rotor rotational axis R (striped line). The turbine rotor 1 can have two or more blades. When having two blades, installation is simplified and use of the helicopter deck 31 for actual helicopter landing purposes is enabled, as can be seen in the drawing.

[0127] The turbine rotor 1 is here connected to a hollow turbine rotor shaft 2 which is here depicted of the type discussed above. The turbine rotor shaft 2 is provided with a mechanical lock 16 which allows locking the wind turbine in a locked position.

[0128] The nacelle or gondola 30 is rotatably mounted on tower 18 via a bearing 17 (schematic).

[0129] The turbine rotor shaft 2 is mounted into a housing 3 that is provided with front and end bearings and is tapered. This construction as such is known, described and patented by Eolotec, for instance. It comprises a front and rear, pre-loaded tapered roller bearing. In an alternative embodiment (not described), a functionally comparable solution could incorporate journal bearings. This housing 3 is attached to a frame attached to the nacelle. At the opposite end of the housing 3, the other parts of the drive trains extend.

[0130] Subsequently, the turbine rotor shaft 2 is coupled to a flexible coupling 4. Such a flexible or elastic coupling 4 can for instance be of the type already discussed. In an embodiment, a flexible coupling comprises two disks which are coupled using a flexible or elastic material.

[0131] The current embodiment of the wind turbine of FIG. 2 comprises a transmission system 5 coupled to the flexible coupling 4 which is subsequently coupled to a generator 6. The transmission system 5 in particular has two parts 5A and 5B that are mutually coupled and can be integrated into a common housing, in an embodiment. This will be explained in FIG. 3 and FIG. 5.

[0132] In the embodiment of FIG. 3, the transmission system 5 comprises subsequently an upstream planetary gearbox 7 drivingly coupled at one end with the turbine rotor 1, and at its other end with an incoming end of a bevel gearbox 8. In drawing 2, the upstream planetary gearbox in schematically indicated with 5A. The bevel gearbox 8 is at an outgoing end drivingly coupled to a downstream planetary gearbox 9. This is indicated 5B in drawing 2. The downstream planetary gearbox 9, in turn, is drivingly coupled to a generator rotor 10 of generator 6. In this respect, ‘upstream’ is towards the turbine rotor 1, and ‘downstream’ is towards the generator 6.

[0133] In FIG. 3, in this embodiment transmission system 5 has incoming upstream gearbox shaft 32 drivingly coupled to the flexible coupling 4 and an outgoing upstream gearbox shaft 33 drivingly coupled to the bevel gearbox 8. The transmission system 5 allows for the wind turbine to be designed for much larger power ratings.

[0134] First, we discuss the upstream planetary gearbox in the depicted embodiment of FIG. 3.

[0135] The upstream planetary gearbox 7 in this embodiment is of a modified planetary gear design, which is also referred to as a 1.5 stage gearbox. The upstream planetary gearbox 7 is also referred to as a multi-stage epicyclic gearing and enables a maximum step-up gear-ratio of at least i=1:10. Alternatively, the final set of gears together driving a central sun wheel at the output shaft could be skipped. Such an embodiment would result in a conventional 1-stage planetary gearbox with maximum step-up gear-ratio of around i=1:6.4, and as consequence increases input torque loads of the bevel gearbox 8. This in turn would lower generator revolutions for a given rotor speed and increase the dimensions and cost of the generator. The arrangement is likely less technically feasible for especially the overall drivetrain concept and in particular the upper ratings with associated higher rated torque levels.

[0136] The currently described embodiment of the upstream planetary gearbox 7 is as follows. The incoming upstream gearbox shaft 32 holds an upstream ring gear 19. Mounted in the housing are here two upstream planetary gear elements that each hold upstream first planetary gear 20, and upstream second planetary gear 21 representing main elements of the extra ‘0.5-stage’ of the gearbox. The upstream first planetary gear 20 and upstream second planetary gear 21 are rotationally fixedly mounted on a common shaft 35. The common shaft 35 is positionally fixed in a gearbox housing or alternatively inside a stationary planet carrier, keeping it positioned in the housing or in the stationary planet carrier. The common shafts 35 can rotate on their respective rotational axes. As the diameter of the upstream second planetary gear 21 is larger than the diameter of the upstream first planetary gear 20, an additional increase of rotational speed is provided. The upstream second planetary gears 21 engage and in operation drive an upstream sun gear 34. The upstream sun gear 34 is rotationally fixed on the outgoing gearbox upstream gearbox shaft 33. The drawing shows in the cross section two planetary gear sets. In practice, five or more planetary gear sets can be used, all engaging meshingly with the upstream sun gear 34, and positioned around the upstream sun gear 34.

[0137] In the current drive train, it is also possible to provide one or more additional gearboxes for further increasing the rotational speed of the generator 6. Using the current modified upstream planetary gearbox 7, the rotational speed is increased with a factor of about minimal 10. The first gear ration of the upstream ring gear 19 and the upstream first planetary gears 20 can be up to a maximum 1:6.4. The second gear ration of the upstream second planetary gears 21 and the upstream sun gear 34 can be up to 1:2 up to 1:4. Thus, the combined gear ratio can be up to 12 or more. This reduces load (torque) on the bevel gearbox 8.

[0138] In the current transmission system 5, upstream planetary gearbox 7 is drivingly coupled to bevel gearbox 8. We will now describe the currently depicted embodiment of the bevel gearbox 8 of FIG. 3.

[0139] The outgoing upstream gearbox shaft 33 is in a rotationally fixed manner and drivingly coupled to a gear wheel or bevel drive gear 11. In the current embodiment, bevel drive gear 11 has its toothed gearing here at an angle of between 60 and 70 degrees with respect to the rotor rotational axis R. Bevel drive gear 11 engages a first bevel pinion gear 12 and a second bevel pinion gear 13. Here, both the first bevel pinion gear 12 and second bevel pinion gear 13 have a conical shape. They taper towards the rotor rotational axis R. The teeth surface of the bevel drive gear 11 has a correspondingly conical shape. In alternative embodiments, the first and second bevel pinion gears 12, 13 can be inclined bevel gears, off-set gears, Zerol bevel gears, helical gears, spiral bevel gears, straight bevel gears or crown gears. The toothed part of the bevel drive gear 11 is adapted to the first and second bevel pinion gears 12, 13. The bevel gearbox 8 in fact defines a double, right angle transmission. Both bevel pinion gears 12, 13 in operation rotate in opposite directions.

[0140] In the current embodiment of FIG. 3, the transmission system 5 comprises a common housing 15. The second bevel pinion gear 13 comprises a second bevel pinion shaft 22 that is held in the common housing 15 using second pinion journal bearing 23.

[0141] First bevel pinion gear 12 in this embodiment comprises a hollow bevel pinion shaft 24 that is held in the common housing using a first pinion journal bearing 26.

[0142] In the embodiment depicted, the bevel pinion gears 12, 13 are inline on a common rotational axes R1. This rotational axis R1 here intersects the turbine rotor rotational axis R. There are known angled gearboxes that allow positioning such that the rotational axis of the pinion gears do not intersect the turbine rotor rotational axis R.

[0143] As mentioned earlier, the downstream planetary gearbox 9 can be drivingly coupled to the bevel gearbox 8 at various positions. In general, the generator 6 will be positioned near one of the bevel pinion gears 12, 13. This nearest bevel pinion gear will be indicated as the first bevel pinion gear 12. The generator 6 furthermore for easy construction will be positioned with its generator rotor rotational axis in line with the rotational axes R1 of both bevel pinion gears 12, 13. Usually, that line will cross the rotor shaft rotational axis R. Furthermore, in general the generator will be placed radially outside the first bevel pinion gear, or on the end of the first bevel gear that is remote from the rotor shaft rotational axis R.

[0144] The downstream planetary gearbox 9 can be drivingly coupled in general between the first and second bevel pinion gears 12, 13.

[0145] Alternatively, the downstream planetary gearbox 9 can be positioned radially outside the first bevel pinion 12 and between the first bevel pinion 12 and the generator 6. In other words, the downstream planetary gearbox 9 is positioned between the bevel pinions 12, 13 and the generator 6. It only requires a coupling shaft from the second bevel pinion 13 through the first bevel pinion and drivingly coupling to the downstream planetary gearbox 9. One end of the downstream planetary gearbox 9 is drivingly coupled to the first bevel pinion gear 12, and the other end of the downstream planetary gearbox 9 is drivingly coupled to the generator rotor 10. This places the generator 6 more remote from the rotor shaft rotational axis R.

[0146] Alternative, the downstream planetary gearbox 9 can be positioned radially outside the second bevel pinion gear 13. This position, most remote from the generator 6 and with both bevel pinion gears 12, 13 between the downstream planetary gearbox 9 and the generator 6, requires a coupling shaft from the first bevel pinion through the second bevel pinion and drivingly coupling to the downstream planetary gearbox 9, and a relatively long, concentric shaft coupling the downstream planetary gearbox 9 to the generator 6, this shaft further running through both bevel pinions. For easy and compact construction, it will/can be located between the rotational axis R and one of the bevel pinion gears 12, 13.

[0147] In FIG. 3, the first positioning of the downstream planetary gearbox 9 between the first and second bevel pinion gears 12, 13 is depicted. Here, the downstream planetary gearbox 9 is positioned close to the radial inner end of the first bevel pinion gear 12. One of the parts of the downstream planetary gearbox 9 may even be integrated with the first bevel pinion gear 12.

[0148] The downstream planetary gearbox 9 has the following general parts. A downstream ring gear 50, a downstream sun gear 51, and downstream planet gears 52 mounted on a downstream planet gear frame 53.

[0149] In the embodiment of FIG. 3, the parts of the downstream planetary gearbox 9 drivingly couples between the bevel gearbox 8 and the generator in the following way. The downstream planet gear frame or planet carrier 53 is drivingly coupled with the first bevel pinion gear 12. In particular, it can be attached to or integrated with the first bevel pinion gear 12. The downstream ring gear 50 is drivingly coupled to or with the second bevel pinion gear 13. This can for instance be done using a flexible shaft 55. In the current embodiment, the downstream ring gear 50 comprises a flexible coupling 54 coupled to the flexible shaft 55 that in turn is coupled to further flexible coupling 56 that is drivingly coupled to the second bevel pinion gear 13. Downstream sun gear 51 is drivingly coupled to/with a transmission system output shaft 27. In this embodiment, a flexible coupling or spline coupling 28 drivingly couples transmission system output shaft 27 to generator rotor shaft 29.

[0150] In order to protect the generator 6 especially from grid-induced events like sudden power outage, creating high instant drivetrain peak loads, in the current embodiment of FIG. 2 the inner drive shaft is coupled to generator 6 via an overload clutch or torque limiter. An overload clutch as such is known in the art.

[0151] Functionally, bevel gearbox 8 provides an additional gear ratio of between 1 and 10 to the transmission system 5 of the drive train. Furthermore, the bevel gearbox 8 provides an angled transmission (angular drive; bevel pinion; mitre gear; right angle bevel gearing; bell crank; angle drive) with two counter-rotating bevel pinion gears 12, 13. Thus, the current transmission system 5 first provides a gear ratio of up to a ‘gearbox engineering maximum’ in the range of 1:350-750. In particular, a practical step-up gear ratio range between 1:375-500 can be attained.

[0152] Thus, a rotational speed of the rotor of the generator of more than 3.000 rotations per minute may be possible. In an embodiment, up to 3.000-5.000 rotations per minute may be possible. This can result in a smaller generator diameter, for instance.

[0153] The rotor and the complete drive train are here mounted in the tower 18 perpendicular to a tower longitudinal axis L. In an alternative embodiment in particular in an upwind wind turbine, rotor and drive train can be mounted on the tower with the rotor rotational axis R at a tilt angle α away from a perpendicular (90°) coupling. The tilt angle α can be important in that allows an increase of the distance between the tower and the rotor (tip), thus minimizing chances of the blade tips hitting the tower and reducing the disturbing influence of the tower on the rotor. A tilt angle α is usually chosen between about 5 degrees and a maximum 10 degrees for not negatively impacting aerodynamic performance.

[0154] The housing of the transmission system 5 can be one single housing. Alternatively, the housing can be divided into two coupled housing parts, for instance having a split at the second planet gears 34. This may facilitate access and repair possibilities. The flexible coupling 4 in the current embodiment may be removed by de-boulting, for instance, and may be lifted out of the current drive train, thus providing space for subsequent removal of (part) of the housing. In an alternative embodiment, the upstream planetary gearbox 7 and the bevel gearbox 8 have each have a separate housing.

[0155] FIG. 3 also shows an embodiment of an electrical generator 6 in cross section. In FIG. 4, details of the generator 6 are shown in more detail. The electrical generator 6, shortly ‘generator’, has a housing 25. The housing 25 has fixing provisions for fixing the housing to the transmission system 5, here to the housing of the transmission system 5. Usually, the generator housing 25 will be housed inside the nacelle 30.

[0156] Generator 6 is of the outer rotor type and comprises an outer generator rotor 10 and an (inner) stator 38+57. The rotor 10 and stator 38+57 are concentric, and define an air gap 39 between them. The rotor 10 is coupled to the drive shaft 27 that results from the downstream planetary gearbox 9. In the current embodiment, the generator drive shaft 27 can coupled to the generator rotor 10 via a coupling 28, in an embodiment an overload clutch forming an integrated assembly with flexible coupling. In fact, here as an example of a possible coupling, a plate couples the transmission system output shaft 27 to the outer generator rotor 10. The flexible shaft or rigid shaft connecting the lower pinion 13 with the downstream ring gear may be coupled via a gear spline coupling as developed and patented by RENK AG of Germany or alternative design flexible coupling types. This to compensate for slight dynamic misalignment resulting from deflections and distortions, and to a lesser degree levy length changes. The gear spline or other design flexible couplings are attached to the downstream ring gear 50 respectively lower pinion 13 via the upper and lower adapter units 54 and 56.

[0157] The outer generator rotor 10 may comprise permanent magnets to provide alternation magnetic poles. The stator body 57 comprises coils 38 for inducing a voltage and a current. As the stator 11 in this embodiment is static with respect to the frame and nacelle, no power coupling, like wipers or sliding contacts or brushes, is needed. As discussed before, instead of the radial flux generator of FIG. 2, also an axial flux generator can be considered. In the current inventive concept, such a generator would also have a rotor and a stator that is static with respect to the nacelle.

[0158] In order to be able to resist or take up high torsion plus allowing some bending deflections or to reduce weight, the shafts 55 can be made from a fibre reinforced composite material. It provides a torque shaft. Suitable fibre reinforced composites comprise fibre material that is commercially sold under the names Dyneema, Aramid, and Kevlar. It was found, however, that in order to provide a high degree of rigidity and strength, carbon fibre reinforced composites are preferred.

[0159] In the current embodiment, the generator has one or two journal bearings, which are attached to the hollow generator pin 58, and hollow generator-rotor shaft 59 and forms the structural support of the generator-rotor 10.

[0160] In the current embodiment the generator 6 comprises a (light weight) generator housing 25. The generator 6 further comprises a cooling system. In the current embodiment, the cooling system comprises a combined gas and liquid cooling system.

[0161] The gas cooling system comprises a gas inlet 40 in the generator structural housing 14 and a gas outlet 41 in the generator housing 25. The inlet 40 and outlet 41 and air circulation pump and air-air or air-liquid heat exchanger are in the schematic drawing not drawn. The can also be as remote from one another as possible.

[0162] The gas cooling system, usually based upon air that circulates inside the generator 6, in an embodiment comprises air displacement means in the generator rotor. In the current embodiment, the generator rotor 10 is provided with vanes or fins and/or spokes and/or holes to set air inside the generator 6 in motion.

[0163] In an embodiment, the air displacement means on the rotor provide a pump function, displacing air from the gas inlet 40 to the gas outlet 41. The gas cooling system may comprise a pump device for circulating air through the generator housing 25. The gas cooling system in the current embodiment includes a heat exchanger gas-coupling the gas inlet 40 and the gas outlet 41. In the current embodiment, the heat exchanger is of the gas-liquid heat exchanger type. It allows the gas of the gas cooling system to exchange heat with liquid of the liquid cooling system which will be discussed further. In the discussed embodiment, in the gas cooling system, the inner generator rotor 11 is further provided with gas displacement means. Gas channels 28 are provided in the inner generator rotor 11 for further mixing or allowing mixing of gas inside the generator 38.

[0164] In an embodiment, the stator structural housing is hollow ring-shape body 57 in which cooling liquid circulates, and which is integral part of the generator temperature management system. The liquid inlet and liquid outlet 42, 43 are (not drawn) connected to a circulation pump and liquid-liquid or liquid-air heat exchanger.

[0165] In an embodiment, the stator housing incorporates at least one air inlet pipe or nozzle along the stator circumference along a fictive horizontal axis, and at least one gas channel or nozzle in the vertical plane. An air pump forces cooling air in between the stator coils and the air gap 39 where most of the generator heat is generated. If two or more gas channels are placed in the vertical plane, they can be positioned horizontal relative to the generator base or at various inclined positions to promote optimal cooling air mixing and heat dissipation performance.

[0166] In FIG. 5, an alternative embodiment of the upstream planetary gearbox 7 of the transmission system 5 is explained. In FIG. 3, the transmission system 5 is split in two parts, upstream transmission system part 5A and downstream transmission system part 5B. The downstream part 5B comprises the bevel gearbox 8 and downstream planetary gearbox 9. The upstream part 5A comprises the upstream planetary gearbox 7. The upstream planetary gearbox 7 in FIG. 3 comprises a 1.5 step planetary gearbox 7, also referred to as stepped planetary gearbox 7. For brevity, it will usually not be referred to as “upstream” as in the description of FIGS. 3 and 4. In FIG. 5, a redesign is made of that particular gearbox 7 to further optimise the current wind turbine design. It should be noted that the particular stepped planetary gearbox of FIG. 5 may also be applied in other wind turbine designs that do not comprise the bevel gearbox 8 discussed so far. It may for instance be used in a more traditional wind turbine design having a (one or more) further downstream planetary gearboxes and/or a ‘traditional spur gear coupling the stepped planetary gearbox of FIG. 5 to a generator 6. For brevity, in the description of FIG. 5, the addition “upstream” will not always be used. It is evident, however, that the stepped planetary gearbox 7 of FIG. 5 is close to and usually coupled directly to the turbine rotor shaft as first or upstream part of the transmission system 5.

[0167] The redesigned stepped planetary gearbox 7 (or 1.5-stage) design of FIG. 5 in an embodiment focuses at meeting the huge input torque demands linked to next-generation 12-16MW+ offshore wind turbines with matching 215-260 m rotor diameters. It incorporates journal bearings whenever possible and feasible, which is a key contributing factor to achieve a compact gearbox design with competitive torque density (Nm/kg).

[0168] Furthermore, the stepped planetary gearbox 7 of FIG. 5 and associated dimensioning enables matching specific power-rating combinations in dependence on IEC Class I-III, aimed at offering optimal LCOE performance for every specific wind climate. The gearbox input (rotor) side therefore comprises in an embodiment six planet gears instead of four in the original design for absorbing the corresponding 16-24 MNm+ range incoming torque levels. In particular, these are provided in two sets of planetary gear systems in two planet planes P1, P2 or respective first and second plane of upstream second planetary gear P1, P2. There may be more planes and sets, increasing complexity. The second planetary gears may also all be axially offset with respect to one another. This requires a complex alignment, but may allow even larger second planetary gears and increase transfer ratio. The six second planetary gears 21 may for instance be grouped into two sets of opposed second planetary gears 21 in two axially offset planes. This also allows larger diameters.

[0169] The layout of the stepped planetary gearbox 7 with (here) six planetary gears systems each comprising a first planetary gear 20 and a second planetary gear 21 thus comprises first planetary gears 20 rotating inside the ring gear 19 at the gearbox input side or upstream side which offers a first step-up ratio of in an embodiment 1:4.93. It also offers over one metre reduction in outer housing diameter, now at about 4100 mm. The second planetary gears 21, each in an embodiment featuring inclined or helical-shape teeth, downstream with respect to the first planetary gears 20, are individually attached to a shared drive shaft, sheared with a matching first planetary gear 20. All planetary gear systems are subdivided into two separate sets of three planetary gear systems each. These gear sets rotate in this embodiment in a separate plane (P1, P2) and together drive a ‘double’ or axially extended sun gear 34, and as an assembly represents the stepped planetary gearbox output stage. A second step-up ratio in the reference stepped gearbox 7 is in an embodiment about 1:3.73.

[0170] This offers a total step-up ratio of the specific first and second step-up ratio's is 1:18.33. This is a good first compromise between containing ring gear cost being a key gearbox cost driver and a parallel aim to maximise the step-up ratio. The latter focused at curbing size and cost of the bevel gear through a lower (remaining) step-up ratio required supplemented by a reduced torque to be transmitted.

[0171] The stepped planetary gearbox 7 of FIG. 5 comprises at least nine innovative features offering multiple benefits that will be explained below.

[0172] A first innovative feature is a compact central ‘tool’ carrier 62. The central carrier 62 comprises a structurally stiff element holding or carrying further main load-bearing elements. The central carrier 62 in the embodiment of FIG. 5 is part of the transmission housing. In the embodiment of FIG. 5, it couples an upstream housing part 65 and a right or downstream housing part 73. The elements attached to the central carrier 62 comprise (here) six planet gear pins (stationary shafts), each rotatably housing a planetary gears common shaft 35. Fixed to or near an upstream end the planetary gears common shaft 35 comprises a first planetary gear 20 rotatably fixedly attached to it. The planetary gears common shaft 35 thus rotates about planetary rotational axis Rp with its first planetary gear 20. The planetary gears common shaft 35 drives a second planetary gear 21. This second planetary gear 21 may be rotatably fixedly attached to the planetary gears common shaft 35. In the current embodiment of FIG. 5, a driving disk 70 is rotatably fixedly attached to the planetary gears common shaft 35. Via a flexible coupling 71, the driving disk 70 rotatingly drives the second planetary gear 21 (that planetary gear 21 is downstream from the first planetary gear 20). Furthermore, the central carrier 62 comprises a ring gear pin 63 fixedly attached or mounted to it. The ring gear pin 63 holds the ring gear 19, here attached via ring gear carrier disk 75 and a bearing 64.

[0173] Finally, the central carrier 62 holds a rear/upstream output shaft bearing 66 that bears the outgoing upstream gearbox shaft 33. As in FIG. 3, it can be attached to the bevel drive gear 11.

[0174] A second feature is the (upstream) ring gear carrier disk 75 bearing support at the spaciously designed static hollow shaft or ring gear pin 63, which is structurally stiff or provides structural rigidity. However, the ring gear carrier disk 75 provides an interface element that may introduce built-in design flexibility for promoting optimal load transfer between rotating upstream ring gear 19 and planet gears 20.

[0175] A third feature is a ‘tool carrier’ principle, comprising the already discussed central (tool) carrier 62 mentioned above, and further layout possibility, which enables a vertical gearbox split. The first split is between the outer left/upstream housing part 65 and the central carrier 62, and the second split between the right/downstream housing part 73 incorporating the bevel gearbox 8 and the secondary or downstream planetary gearbox 9, and in fact transmission system part 5B (FIG. 3). The central carrier 62 itself can also be individually removed and reassembled. The gearbox-splitting further enables ‘uncomplicated’ vertical gearbox assembly and up-tower repairs during the operational period without needing an expensive jack-up or other installation vessel.

[0176] The ‘tool carrier’ provided by the introduction of the central carrier 62 also offers a compact integrated gearbox system solution that minimizes negative gearbox and drivetrain interface impacts due to deflections and deformations. The latter are critical factors when designing large-scale mechanical drivetrains for turbines from about >10MW ratings. One crucial interface-related benefit of the new design is the near elimination of loads induced in the outer housing causes deflections and deformations being passed on to critical gearbox internals.

[0177] This is enabled by the fact that within the new design parameters the outer housing 65, 73 and central carrier 62 are linked only at an outer housing mounting ring. The main pin or ring gear pin 63 attached to the central carrier 62 directly supports the rotating ring gear 19, offering a structurally stiff and strong overall solution. The left housing part 65 of the gearbox housing remains directly linked to the MBU but via a shortened intermediate connection and now at a much larger radius for optimized load transfer.

[0178] A fourth innovative element is the (here six) stationary shafts or planet gear pin 61 supporting gearbox first planet gears 20 and second planet gears 21, coupled together via separate planetary gear torque/common shafts 35. This couples each matching first planetary gear 20 and second planetary gear 21 forming a planetary gear pair. The stationary shafts or planet gear pins 61 (three short, and three longer) are attached (here firmly pressed inside) to or mounted on the central carrier 62, creating a structurally strong and stiff interface connection.

[0179] Part of this solution is further that the planet gear support function and torque transfer function are split through applying a separate torque shaft 35 for each first planetary gear/second planetary gear set or pair. The planetary gear torque/common shafts/axles 35 in turn provide a mechanical linkage between the first planetary gears 20 and the second planetary gears 21. The first planetary gears 20 are rotationally fixedly coupled to their planetary gears common shaft/axle 35. This is for instance achieved via a spline connection, friction device or otherwise, and either rigid or with some built-in flexibility for optimizing the load distribution between ring gear and planetary gears.

[0180] Each of the first planetary gears 20 is rotationally coupled to one of the (“its”) second planetary gears 21. It is here proposed to use a flexible coupling. Furthermore, both the first planetary gear 20 and its coupled second planetary gear 21 are mounted via one or more bearings 72 on a fixed planetary gear shaft or planet gear pin 61. In this specific embodiment, a driving disk for the upstream second planetary gear 70 is rotationally fixed to the planetary gears common shaft 35. Via a flexible coupling 71, the driving disk 70 is drivingly, in particular rotationally fixedly, coupled to a second planetary gear 21. Thus, coupling of the first planetary gear 20 with its second planetary gear 21 is done via the chain formed by the planetary gears common shaft 35, the driving disk for upstream second planetary gear 70, and the flexible coupling 71. The mechanical linkage is thus via an intermediate driving element 70, supplemented by for instance a shrink fit and flexible element 71 in between driving element 70 and second planetary gear 21. A (stationary) planet gear pin 61 and torque shaft 35 combined solution further offers favourable materials fatigue performance compared to a single rotating shaft holding the first and second planetary gears 20, 21. This absorbs both bending moments and torque transfer loading.

[0181] Fifth innovative feature is that the stationary shafts or pins 61, 63 plus central carrier 62 solution eliminates negative impact of otherwise unavoidable (anti-clockwise with turbine rotor at left) shaft-gear moments of force. These moments of force result from the combinations of ring gear 19 with first planetary gears 20, and of second planetary gears 21 with sun gear 34. This is an inherent but perhaps not always recognised phenomenon for stepped planetary gearboxes like the one of FIG. 5. The issue itself is not easy to solve especially for alternative solutions with rotating shafts and two support bearings positioned in between the gears. If not addressed adequately, it could hamper gearbox integrity and lifetime.

[0182] Sixth innovative feature is the “axial displaced second planetary gears”. Here, second planetary gears 21 are in two planes P1, P2 at the stepped gearbox 7 output/downstream side. This feature allows substantially higher step-up ratios compared to an equivalent size stepped planetary gearbox but with a single row/plane of second/output planetary gears 21. A contributing reason is here that for a given ring gear pitch circle the maximum achievable step-up ratio goes down with increasing number of first planetary gears. Another contributing factor putting a limit to the maximum step-up ratio of ‘conventional’ stepped planetary gearboxes is that the second planetary gear circles could either touch or overlap each other, either one being functionally impossible. In the current example, there are six second planetary gears 21, arranged in two planes P1, P2 of each three second planetary gears 21. Other configurations, number of planes, number of second planetary gears etc. may be possible, for instance three planes of two secondary planetary gears each, but also two planes of four second planetary gears each, two planes of five second planetary gears each.

[0183] A seventh feature is a flexible linkage of each individual drive shaft 35 with a matching second planetary gear 21. This innovative arrangement allows slight movement of these gears out of their ‘natural’ plane of rotation.

[0184] The eighth feature involves a hollow sun-gear 34, fitting loosely over a tapering bevel gear shaft 33. This arrangement in a dual function serves both as support shaft for the bevel drive gear 11 and sun gear 34. Gearbox shaft 33 transfers sun gear 34 output torque to the bevel drive gear 11 input side. The sun gear 34 in this embodiment has a flexible mechanical linkage or coupling to the gearbox shaft 34. The overall arrangement eliminates the need for dedicated sun gear bearing(s) support. A further benefit is that it creates a ‘floating’ sun-gear through controlled flexibility of the linkage. This flexibility characteristic means here torsional stiff for optimal torque transfer, plus only minimal axial movement allowed, and finally some angular and parallel movements relative to the central rotational axis R allowed as essential. In this embodiment of FIG. 5, one end of gearbox shaft 33 has bearing 66 holding it in the central carrier 62, and the other end of gearbox shaft 33 has bearings 74 holding it here in the right or upstream structural bearing support 73. Here, the bevel drive gear is rotationally fixedly coupled to the gearbox shaft 33. A driving element, for instance a driving disk 67, is also rotationally fixedly coupled to the gearbox shaft 33. Sun gear 34 is freely arranged on the gearbox shaft 34. It comprises a flange 69 that is rotationally coupled to the driving disk 67 via a flexible sun gear coupling 68.

[0185] The ninth feature involves either opposed inclined teeth angles or an opposed helix shape when applying helically shaped teeth, for the two second planetary gear sets (in P1 and in P2) and the sun gear 34. In general, the sun gear 34 is axially extended. This measure enhances optimal interaction and load distribution within this complex dynamic sub-system, which involves the simultaneous movement of second planetary gears 21 in planes and with the matching axially extended sun gear 34. It is in parallel aimed at minimizing axial movements resulting from balancing axial loads at the axially extended sun gear 34.

[0186] A feature eleven involves the gearbox shaft 33 having an upstream end supported by an asymmetric spherical roller bearing 66, incorporated in the central carrier 62. This arrangement allows an uncomplicated solution for absorbing the substantially axial loading that originates from the bevel gearbox 8 (FIG. 3). It in addition offers essential extra systems flexibility in counteracting the possible negative impact of deflections and deformations in between the left and right parts of the gearbox as a whole.

[0187] Feature twelve relates to the following. The gearbox design of FIG. 5 can be shortened by about 800 mm, and the linkage between MBU and gearbox by another roughly 1000 mm. This as a key benefit allows the complete transmission system 5 to have its centre of mass closer to the tower centre or longitudinal axis. The mass of the transmission system 5 can be reduced by an estimated 80-125 tonnes.

[0188] The stepped planetary gearbox 7 of FIG. 3 has a large ring gear diameter and most likely ‘only’ four planets turning in a single plan. The individual second planetary gear outer circles of that design are already very close to each other. The maximum step-up ratio of the stepped planetary gearbox 7 of FIG. 3 is therefore in practical designs most likely limited to around 1:15. Furthermore gearbox mass and cost can both be considerable. With five or more second planetary gears 21 that may be required for higher, for instance 10MW and more, ratings with corresponding input torques, the maximum step-up ratio could even drop to between 1:10 and 1:12, unless ring gear diameter is again increased. This would finally result in a ‘dead-end’ strategy. The design of FIG. 5, or aspects of it, seeks to solve this. The features described above may be combined, like in FIG. 5. Also, separate elements of features may be used in the design of FIG. 3.

[0189] Design of FIGS. 3 and 5 Compared

TABLE-US-00002 FIG. 3 FIG. 5 Planets 4, indicative 12 MW 6, reference design 16 reference design MW/235 m Step-up ratio Maximum perhaps 1:15 At least 1:22 . . . 25 (no show stopper) Limiting Size and cost ring gear; Mainly size and cost ring gear factor(s) Deformations and deflections Critical Critical interfaces Not identified; tool carrier interfaces Multiple principle Modular Perhaps, with major Yes, up and eventually redesign downward Scalable Not easy, several ‘Easy’, at least up to 24- bottlenecks 26 MNm+ From 6 => 8 planets possible; But this would limit step-up ratio Limiting Number of planets; ring Mainly size and cost ring gear factor(s) gear, mass, Cost, deflections and deformations Housing split No Yes, left + right, and central Serviceability Below standard carrier State-of-the-art for offshore; up-tower Journal Partly, in right gearbox Most bearing positions bearings part Mass High Competitive Size Large Competitive Cost High Competitive

[0190] It will also be clear that the above description and drawings are included to illustrate some embodiments of the invention, and not to limit the scope of protection. Starting from this disclosure, many more embodiments will be evident to a skilled person. These embodiments are within the scope of protection and the essence of this invention and are obvious combinations of prior art techniques and the disclosure of this patent.

LIST OF REFERENCE NUMBERS

[0191] 1 Turbine rotor [0192] 2 turbine rotor shaft [0193] 3 housing of the turbine rotor shaft [0194] 4 flexible coupling [0195] 5 transmission system [0196] 5A upstream transmission part [0197] 5B downstream transmission part [0198] 6 generator [0199] 7 upstream planetary gearbox [0200] 8 bevel gearbox [0201] 9 downstream planetary gearbox [0202] 10 generator rotor [0203] 11 bevel drive gear [0204] 12 first bevel pinion gear [0205] 13 second bevel pinion gear [0206] 14 generator stator [0207] 15 transmission system common housing [0208] 16 holding brake+rotor lock [0209] 17 gondola jaw bearing [0210] 18 tower [0211] 19 upstream ring gear [0212] 20 upstream first planetary gear [0213] 21 upstream second planetary gear [0214] 22 second bevel pinion shaft [0215] 23 second pinion journal bearing [0216] 24 hollow bevel pinion shaft [0217] 25 Generator structural housing [0218] 26 first pinion journal bearing [0219] 27 transmission system output shaft [0220] 28 (spline) coupling [0221] 29 generator rotor shaft [0222] 30 gondola or nacelle [0223] 31 helicopter deck [0224] 32 incoming upstream gearbox shaft [0225] 33 outgoing upstream gearbox shaft [0226] 34 upstream sun gear [0227] 35 planetary gears common shaft [0228] 36 cooling radiator [0229] 37 Permanent magnets [0230] 38 stator coils [0231] 39 air gap [0232] 40 gas inlet [0233] 41 gas outlet [0234] 42 liquid inlet [0235] 43 liquid outlet [0236] 50 downstream ring gear [0237] 51 downstream sun gear [0238] 52 downstream planet gears [0239] 53 downstream planet gear frame [0240] 54 flexible coupling [0241] 55 flexible shaft [0242] 56 further flexible coupling [0243] 57 liquid cooling [0244] 58 generator pin (stationary shaft) for holding journal bearings [0245] XX generator journal bearings [0246] 59 hollow generator-rotor shaft [0247] 61 planet gear pin [0248] 62 central carrier [0249] 63 ring gear pin/main gearbox pin [0250] 64 ring gear bearing [0251] 65 upstream housing part [0252] 66 rear upstream gearbox shaft bearing [0253] 67 driving disk for upstream sun gear [0254] 68 flexible coupling for upstream sun gear [0255] 69 flange for upstream sun gear [0256] 70 driving disk for upstream second planetary gear [0257] 71 flexible coupling for driving disk for upstream second planetary gear [0258] 72 bearing for upstream planetary gears [0259] 73 right housing part [0260] 74 downstream bearing for upstream gearbox shaft [0261] 75 upstream ring gear carrier disk [0262] R rotor shaft rotational axis [0263] R1 first and second bevel pinion gear rotational axis [0264] L tower longitudinal axis [0265] P1 first plane of upstream second planetary gear [0266] P2 second plane of upstream second planetary gear [0267] Rp planetary gear rotational axis

[0268] The following clauses can be formulated to describe aspects of embodiments. Further, claims are defined at further pages. [0269] 1. A wind turbine, comprising: [0270] a turbine rotor comprising a set of turbine rotor blades and defining a rotor rotational axis, said turbine rotor mounted on a tower; [0271] an electrical generator for converting mechanical energy of said turbine rotor into electrical energy, comprising a generator rotor drivingly coupled to said turbine rotor and mounted on said tower; [0272] a transmission system coupling said turbine rotor to said generator rotor, and comprising: [0273] a bevel gearbox comprising a bevel drive gear coupled to the turbine rotor and a first bevel pinion gear and a second bevel pinion gear, with said first and second bevel pinion gears having a common rotational axis and in operation rotating counter directional; [0274] a downstream planetary gearbox comprising a downstream ring gear, downstream planet gears and a downstream sun gear, with said first bevel pinion gear drivingly coupled to one of said downstream ring gear and said downstream planet gears, said second bevel pinion gear drivingly coupled to another of said downstream planet gears and said downstream ring gear, and said generator rotor drivingly coupled to said downstream sun gear. [0275] 2. The wind turbine of clause 1, wherein said downstream planetary gearbox is provided between said first and second bevel pinion gear, in particular said downstream planetary gearbox is between said rotor rotational axis and one of said first and second bevel pinion gear. [0276] 3. The wind turbine of clause 1 or 2, wherein said first bevel pinion gear is connected to said downstream planet gears, said second bevel pinion gear is connected through a drive shaft to said downstream ring gear, and said downstream sun gear is connected via a transmission system output shaft to said generator rotor, wherein in particular said transmission system output shaft runs through said first bevel pinion gear. [0277] 4. The wind turbine of claims any one of the preceding clauses, wherein said downstream planetary gearbox comprises a downstream planet carrier for rotatably holding said downstream planet gears, wherein said one selected of said first bevel pinion gear, and second bevel pinion gear is coupled to said downstream planet carrier. [0278] 5. The wind turbine of any one of the preceding clauses, further comprising an upstream planetary gearbox, coupling said turbine rotor and said bevel gearbox, in particular said upstream planetary gearbox comprises a 1.5 stage planetary gearbox. [0279] 6. The wind turbine of preceding clause 5, wherein said upstream planetary gearbox comprises a planetary transmission, in particular comprising an upstream ring gear drivingly coupled to said turbine rotor, and an upstream sun gear drivingly coupled to said drive shaft gear. [0280] 7. The wind turbine of any one of the preceding clauses 5 and 6, wherein said upstream planetary gearbox comprises an upstream planet gear system having a first and second planetary gear on a common shaft, with first planetary gear drivingly coupled with said ring gear and said second planetary gear drivingly coupled with said sun gear. [0281] 8. The wind turbine of any one of the preceding clauses 5 or 6, wherein said upstream planetary gearbox provides a gear ratio of 10-15. [0282] 9. The wind turbine of any one of the preceding clauses, wherein said turbine rotor is mounted on said tower with its rotor rotational axis functionally perpendicular to a tower longitudinal axis. [0283] 10. The wind turbine of any one of the preceding clauses, wherein said turbine rotor is fixed to one end of a hollow turbine rotor shaft, said hollow turbine shaft extending through a housing with said housing fixed to a nacelle on said tower, and an opposite end of hollow turbine rotor shaft carrying said transmission system and said generator. [0284] 11. The wind turbine of any one of the preceding clauses, wherein said generator comprises a housing and a cooling system. [0285] 12. The wind turbine of the preceding clause 11, wherein said cooling system comprises a gas cooling system, said gas cooling system comprising a gas cooling inlet in said generator housing for entering a flow of cooling gas into said generator, and a gas cooling outlet for allowing gas to exit said generator housing. [0286] 13. The wind turbine of clause 12, wherein said generator rotor is provided with one or more fanes for setting said cooling gas inside said housing in motion, in particular designed for in operation inducing a flow of cooling gas from said cooling gas inlet to said cooling gas outlet. [0287] 14. The wind turbine of any one of clauses 11-13, wherein said stator is provided with one or more provisions, in particular passages, for setting said cooling gas inside said housing in motion, in particular designed for in operation inducing a flow of cooling gas from said cooling gas inlet to said cooling gas outlet. [0288] 15. The wind turbine of any one of clauses 13-14, wherein said gas cooling system comprises a heat exchanger for exchanging heat with a liquid flow.