WO2010078903A2

WO2010078903A2 - A rotary mount for a turbine

Info

Publication number: WO2010078903A2
Application number: PCT/EP2009/008745
Authority: WO
Inventors: Teza Simanwe; Alan Stuart Kinson; Ellis Ful Hen Chong
Original assignee: Rolls-Royce Plc
Priority date: 2009-01-06
Filing date: 2009-12-08
Publication date: 2010-07-15
Also published as: KR101640807B1; GB2477710A; GB2477710B; KR20110102502A; CA2748945A1; GB0900073D0; WO2010078903A3; CA2748945C; GB201110456D0

Abstract

The invention relates to a rotary mount (8) for a turbine (1), such as a subsea tidal stream turbine. The mount comprises a fixed part (9), and a rotary part (10) configured for connection to said turbine (1), the rotary part (10) being mounted for rotation relative to the fixed part (9). The fixed part comprises a fixed conductor (52), and the rotary part comprises rotary conductor (47), the rotary conductor (47) being electrically connected to the turbine (1) and arranged for rotation relative to the fixed conductor (52). The arrangement is configured such that electrical energy generated by the turbine (1) is transferred via the rotary conductor (47) to the fixed conductor (52) either by direct conduction, or by induction, whilst permitting rotation therebetween. In one embodiment, the mount (8) takes the form of a rotary transformer comprising a plurality of rotary conductors (47), each comprising a primary coil having a plurality of turns wound on a rotary-core (44), and a plurality of fixed conductors, each comprising a secondary coil (52) having a plurality of turns wound on a fixed-core (48), the rotary-core (44) being mounted for rotation relative to the fixed-core (48).

Description

A ROTARY MOUNT FOR A TURBINE

The present invention relates to a rotary mount for a power generation turbine, and is particularly configured to accommodate movement of the turbine m yaw.

Tidal stream power is a significant renewable energy source, with an estimated production level of 17.5TWh/year in Britain. There is increasing interest in exploiting this energy source due to its dependable and predictable nature. However, the viability of offshore tidal stream power schemes is very much dependent on the successful transmission of the generated energy to shore.

In order to maximise energy capture, it has been proposed to design tidal stream generating units so that they can passively align themselves into the best stream, with complete 180° re-orientation occurring when the local tide floods or ebbs. This so-called passive yaw control is achieved by the manner in which the generating units are tethered to the seabed. In addition, it is preferable that the generating units be allowed to move in sympathy with all other normal sea movements, and this is again allowed by the tethering arrangement. These factors require the generating units to be light so as not to drag on the seabed, and also require the effective management of power transmission cables in order to prevent damage to the cables due to twisting and snagging as the generating units move .

It has also been proposed to provide tidal turbines, which instead of using passive yaw control, utilise active yaw control to align themselves towards the tidal stream when the tide changes direction. Active yaw control may be accomplished using a thruster or winch together with some form of counting mechanism, which inevitably introduces extra complication to the system and increases the initial installation costs and the ongoing maintenance costs.

Presently, there are no effective cable management systems suitable for use with active- or passive-yaw control tidal-stream turbine arrangements for fully submerged electrical cables, which allow 360° rotation of the turbines about an anchor point without electrical cables becoming tangled and possibly damaged. Current technologies for transferring power across subsea cable systems employ 'wet-mate' connectors, which are not designed for rotation. Furthermore the maximum voltage of

'wet-mate' connectors is generally restricted to 8kV.

Beyond this voltage level, the connectors would become prohibitively expensive and large in size, both of which restrict their use with tidal stream turbines. On top of that, the maximum voltage of 8kV limits the amount of power that can be exported through 'wet-mate' connectors. This is because as the power increases at a given voltage, the rating and hence the physical size of the subsea cable becomes excessive and highly impacts on its flexibility.

In addition, tidal stream generating units are of course fully submerged and thus subject to all movements of the sea. Conventional wet-mate connectors simply serve to connect cables exiting the nacelle of a tidal-stream turbine to the power transmission cables running from the generating site to the shore. These connectors are not specifically designed for cable management purposed in the sense of allowing movement of the tidal turbines. Cables served by these types of connectors will still twist and wrap round the tidal stream generating unit in sympathy with sea movement. Furthermore, wet mate connectors operate at low voltage, whereas higher voltages are required for economical power transmission. Some of the above described problems are shared with other types of turbine, for example, wind turbines, which are required to undergo yaw dependent on the prevailing wind direction. Active yaw systems are typically employed for this purpose. However such systems are typically limited to a fraction of a revolution of yaw in either direction m order to avoid excessive stresses in cables due to twisting.

It is an object of the present invention to provide an improved rotary mount for a turbine.

According to a first aspect of the present invention, there is provided a rotary mount for a turbine, the rotary mount comprising: a fixed part, and a rotary part mounted for rotation relative to the fixed part; the rotary part being configured for connection to said turbine; wherein the fixed part comprises a fixed conductor, and the rotary part comprises a rotary conductor, the rotary conductor being electrically connectable to the turbine and arranged for rotation relative to the fixed conductor. The arrangement of fixed and rotary parts is typically configured such that electrical energy generated by the turbine is transferred via the rotary conductor to the fixed conductor, whilst permitting rotation therebetween.

The fixed part may be configured to be secured relative to a rigid structure such as the ground, the seabed or a mount, base, tower or other anchoring structure .

According to a second aspect of the present invention, there is provided a subsea rotary mount for a tidal-stream turbine, the rotary mount comprising: a fixed part configured to be secured relative to the seabed, and a rotary part configured for connection to said turbine; the rotary part being mounted for rotation relative to the fixed part; wherein the fixed part comprises a fixed conductor, and the rotary part comprises rotary conductor, the rotary conductor being electrically connectable to the turbine and arranged for rotation relative to the fixed conductor, the arrangement being configured such that electrical energy generated by the turbine is transferred via the rotary conductor to the fixed conductor, whilst permitting rotation therebetween.

Preferably, the mount comprises a plurality of said fixed conductors and a plurality of said rotary conductors, wherein each rotary conductor is arranged for rotation relative to a respective fixed conductor.

As will be appreciated, it is common to transmit electrical power generated from turbines, such as tidal- stream turbines, as three-phase electric power. Accordingly, in a particularly preferred embodiment the mount comprises three said fixed conductors and three said rotary conductors arranged in pairs, each said pair of fixed and rotary conductors being configured to carry a respective phase of a three-phase supply of electrical energy generated by the turbine.

Advantageously, according to the second aspect, the or each rotary conductor may be inductively coupled to a respective said fixed conductor. Alternatively, however, the or each rotary conductor may be physically coupled to a respective said fixed conductor.

A particularly preferred configuration of rotary mount in accordance with the present invention takes the form of a transformer, wherein the or each rotary conductor comprises a primary coil having a plurality of turns wound on a rotary-core, and the or each fixed conductor comprises a secondary coil having a plurality of turns wound on a fixed-core, the rotary-core being mounted for rotation relative to the fixed-core. Preferably, the or each primary coil is arranged concentrically relative to the or each respective secondary coil .

In a favoured arrangement, said fixed-cores, and their associated secondary coils, are mounted in spaced apart relation such that the secondary coils are substantially coaxial .

In order to reduce flux coupling between the different phase transformers, said fixed cores are each mounted to a pillar of non-magnetic material. Further reductions in cross-coupling can also be achieved by inserting copper baffles between the individual phase transformers.

Preferably, said rotary-cores, and their associated primary coils, are mounted in spaced apart relation such that the primary coils are substantially coaxial.

It should be appreciated however, that the mount of the second aspect of the present invention does not need to be provided in the form of a rotary transformer, and so an alternative embodiment is configured such that the or each fixed conductor comprises a slip ring, and the or each rotary conductor comprises a brush biased into contact with a respective said slip ring.

In such an arrangement, the or each slip ring may be mounted on a pillar forming part of the fixed-part. It is preferred that the rotary part is mounted for substantially unrestricted 360° rotation relative to the fixed part .

It is envisaged that in some embodiments of the invention, a substantially hermetic seal will be provided between the rotary and fixed parts.

Preferably, the above-defined mount is at least partially filled with dielectric oil, and may be at least partially filled with pressurized Nitrogen gas. So that the invention may be more readily understood, and so that further features thereof may be appreciated, embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which:

Figure 1 is a side view showing a tethered array of tidal turbines, suitable for use with the present invention;

Figure 2 is an upstream view of the array of turbines illustrated in figure 1 ;

Figure 3 is a perspective view of a rotary mount in accordance with the present invention;

Figure 4 is a vertical cross-sectional view through the rotary mount of figure 3; and Figure 5 shows a rotary mount in accordance with another embodiment.

Embodiments of the present invention will now be described with reference to a turbine array. However it is to be noted that the turbine array may be substituted for a single turbine in accordance with the present invention. The one or more turbines are typically for electrical power generation and may be subsea turbines, such as tidal turbines, or else may comprise wind turbines.

Referring now in more detail to the drawings, figures 1 and 2 illustrate an array of tidal-stream turbines 1 provided under the surface 2 of the sea and tethered to the seabed 3. Each turbine 1 is mounted to a supporting frame

4, and the supporting frame 4 is connected via a tether arrangement 5 to an anchor unit 6 which is securely anchored to the seabed 3

As will be appreciated, the combination of the supporting frame 4 and the tidal turbines 1 is most preferably configured to be substantially neutrally buoyant in seawater, and the supporting frame 4 may incorporate one or more foils, or may be substantially foil-shaped, so as to permit natural orientation of the array of tidal turbines relative to the direction of tidal-stream flow, as indicated by arrows 7.

5 As will be explained in more detail below, the tether arrangement 5 is connected to the anchor unit 6 in a manner so as to allow pivotal movement between the tether arrangement 5 and the anchor unit 6 about a substantially horizontal axis, but also so as to allow rotational

10 movement relative to the anchor unit 6 about a substantially vertical axis, with a 360° range of movement. The array of tidal turbines 1 illustrated in figures 1 and 2 is thus tethered in a manner appropriate to allow passive yaw control of the turbines relative to the changing tidal-

15 stream flow 7. This means that the tidal turbines 1 are allowed to orientate themselves relative to the instant direction of tidal-stream flow 7.

Figure 3 illustrates a rotary mount 8 in accordance with the present invention which is specifically designed

20 to provide appropriate connection between the tether arrangement 5 and the anchor unit 6 in order to permit yaw control m the manner indicated above. The rotary mount 8 is substantially cylindrical in form and comprises two main parts, namely a lower fixed part 9 and an upper rotary part

25 10. As will be explained in more detail below, the rotary part 10 is mounted to the fixed part 9 for relative rotation about a substantially vertical axis of rotation 11.

The fixed part 9 of the rotary mount 8 is sized to be

30 received within an upwardly open cylindrical socket 12 formed in the uppermost surface of the anchor unit 6. The lowermost end of the fixed part 9 may thus be inserted into the socket 12 as indicated by the arrow in figure 3, and releasably secured to the anchor unit 6 by an appropriate releasable retaining arrangement (not illustrated) . As will be appreciated, when the fixed part 9 is secured within the socket 12 in this manner, the rotary part 10 will project upwardly so as to be substantially clear of the anchor unit 6 for substantially unrestricted rotation about the axis 11, as indicated by arrow 13.

As indicated above, the tether arrangement 5 must be mounted for pivotal movement relative to the anchor unit 6, and this is achieved by way of a pivotal connection between a tether arm 14 and the rotary part 10 of the mount 8, as illustrated in figure 3. The end of the tether arm 14 is provided with an outwardly directed tab 15 having an aperture 16 formed therethrough. A pair of spaced apart flanges 17 project outwardly from the rotary part 10 of the mount 8, the two flanges 17 each having a co-aligned aperture 18 formed therethrough. The tether arm 14 is thus pivotally connected to the mount 8 by inserting the tab 15 into the space formed between the two flanges 17, whereafter a locking pivot pin (not illustrated) is inserted through the aligned apertures 16, 18. The tether arm 14 is thus pivotally mounted to the rotary part 10 of the mount 8 for pivotal movement relative thereto about a pivot axis 19, as indicated schematically by arrow 20.

An electrical cable 21 runs through the centre of the tether arm 14, the cable 21 being electrically connected to the array of tidal turbines provided at the opposite end of the tether arrangement. At a position near the lowermost end of the tether arm 14, the cable 21 passes through the side wall of the tether arm 14, via an aperture and a grommet 22, from where it then passes to another aperture and associated grommet 23, formed in the uppermost end surface 24 of the rotary part 10 of the mount 8. The cable 21 thus extends into the mount 8 for electrical connection to the internal components of the mount 8 in a manner which will be described in more detail below. Between the two grommets 22, 23, there is provided a loop of cable 21 having sufficient length to allow appropriate pivotal movement of the tether arm 14 relative to the mount 8. Turning now to consider figure 4, a vertical cross section is illustrated through a rotary mount 8 in accordance with a preferred embodiment of the present invention. The mount 8 is illustrated having been inserted in the anchor socket 12, and it will be noted that a region of the fixed part 9 extends vertically upwardly, above the upper surface 25 of the anchor unit 6. As will be explained in more detail below, the particular mount 8 illustrated in figure 4 takes the form of a rotary three-phase step-up transformer comprising three discreet transformer stages A, B, C.

Located centrally within the fixed part 9, there is provided a vertically extending central post 26 which is arranged so as to lie substantially coaxially with the main axis of rotation 11. The central post 26 is supported by a number of support brackets 27 arranged in radial positions around the post and which bear against the side wall of the fixed part 9. The central post 26 extends beyond the uppermost extent of the fixed part 9 so as to project a short way into the rotary part 10. Arranged concentrically within the central post 26, there is provided an elongate central pillar 28 which is significantly longer than the central post 26 so as to extend over substantially the entire height of the rotary mount 8. The central pillar 28 is fixed relative to the post 26 and thus effectively forms part of the fixed part 9, although it extends substantially all the way through the rotary part 10. The uppermost end of the central pillar 28 is ]ournalled within a central bearing 29 provided at the upper end of the rotary part 10, and affixed to the under surface of the end plate 30. The bearing 29 thus serves to support the upper end of the rotary part 10 for rotation about the central pillar 28.

Depending downwardly from a peripheral edge of the end plate 30, there is provided a cylindrical side wall 31 forming the outer skin of the rotary part 10. At the lower end of the side wall 31, there is provided an annular inwardly directed flange 32 which defines a downwardly directed, substantially planar bearing surface 33. The bearing surface 33 is supported by an annular bearing 34 provided on an uppermost bearing surface 35 of the fixed part 9. The annular bearing 34, provided between the opposing bearing surfaces 33, 35, thus supports the lower region of the rotary part 10 for rotary movement relative to the fixed part 9, about the axis of rotation 11.

A short skirt 36 extends downwardly from the bearing flange 32, the outer surface of the skirt 36 being substantially contiguous with the outer surface of the cylindrical side wall 31. The skirt 36 thus extends around the uppermost end of the fixed part 9. An annular seal 37 is provided between the uppermost end of the fixed part 9 and the downwardly depending skirt 36, the seal 37 thus serving to substantially hermetically seal the internal chamber defined by the cylindrical side wall 31 of the rotary part 10.

As will be appreciated, it is conventional, for reasons of electrical efficiency, to generate electrical power from tidal-stream flows using three-phase turbine generators. As will therefore be seen, the cable 21 entering the rotary part 10 through the grommet 23 thus comprises three inner core cables 38, 39, 40, each of which carries electrical current generated by a respective phase of the generator. All three core cables 38, 39, 40, are supported by an inwardly directed cable support 41. Features of the three discreet transformer stages A, B, C, wxll now be described in more detail, with particular reference to the features of the uppermost transformer stage A. It should be appreciated, however, that the other transformer stages B, C have a substantially identical form to the uppermost transformer stage A.

An annular support bracket 42 is secured to the inner surface of the side wall 31 so as to extend radially inwardly therefrom. The annular support bracket 42 is provided with a central aperture 43 which is centred on the axis of rotation 11, and within which is secured a rotary- core 44. The rotary-core 44 may be made from any convenient material conventionally used for transformer cores such as, for example, iron, and may have a laminated structure in order ro reduce transformer losses arising from eddy- currents. The rotary-core 44 takes the form of a generally inverted cup-shape comprising an uppermost base 45 from which depends a peripheral side wall 46. The base 45 is provided with a central aperture in which a bearing is provided, the bearing being journalled to the central pillar 28, and thus permitting rotation of the rotary-core 44 relative to the fixed pillar 28.

The core cable 38 is electrically connected to a primary coil 47 which comprises a plurality of turns wound inside the rotary-core 44. The primary coil 47 may thus be considered to represent a rotary conductor and, by virtue of being wound on the rotary-core 44, the primary coil 47 is thus also arranged for rotation about the axis of rotation 11. The uppermost transformer stage A further comprises a fixed-core 48 which is fixedly mounted to the central pillar 28 and which sits within the primary coil 47 and its associated rotary-core 44. As will be appreciated, the fixed-core 48 will typically be made from the same core material as the rotary-core 44 and may again have a laminated construction in order to reduce transformer losses arising from eddy-currents.

The fixed-core 48 comprises a generally planar base part 49 from which extends an upwardly directed annular part 50. The annular part 50 sits within the side wall 46 of the rotary-core 44, and the base part 49 effectively closes the open cup-shaped structure of the rotary-core 44, with a peripheral region of the base part 49 lying in close proximity to the lowermost edge of the side wall 46. A small air gap 51 is thus formed between the rotary-core 44 and the fixed-core 48, and this air gap is preferably minimised in order to reduce transformer losses, but must be sufficient to ensure substantially free rotation between the rotary-core 44 and the fixed-core 48.

A secondary coil 52 comprising a plurality of terms is wound on the annular part 50 of the fixed-core 48, such that the secondary coil is spaced slightly inwardly of the primary coil 47. As illustrated in figure 4, the secondary coil 52 has a larger number of terms than the primary coil

47 which is necessary to ensure that the transformer is of a step-up configuration. As will be appreciated, the secondary coil 52 effectively represents a fixed conductor.

The secondary coil 52 is electrically connected to a respective core cable 53 of a power transmission cable 56 configured to transmit power generated by the tidal turbines 1 from the generation site, along the seabed to the shore. The core cable 53 extends downwardly within the central pillar 28, and exits the associated central post 26 via an aperture 57, together with core cable 54 which is electrically connected to the secondary coil of the second transformer stage B, and core cable 55 which is electrically connected to the secondary coil of the third transformer stage C. The power transmission cable 56 exits the fixed part 9 of the rotary mount 8 via an aperture and associated grommet 58 provided in the side wall of the fixed part 9.

As will therefore be appreciated, the arrangement described above and illustrated in figures 3 and 4 allows substantially unrestricted rotation between the rotary part 10 and the fixed part 9, over a range of movement of 360°, and is configured such that the rotary conductors represented by the primary coils 47 of each transformer stage are inductively coupled to the fixed conductors represented by the secondary coils 52 of each transformer stage. Thus, electrical energy generated by the turbine and conveyed by the core cables 38, 39, 40, is thus transferred via the rotary conductors to the fixed conductors whilst permitting rotation therebetween. Also, by virtue of the step-up configuration of the three transformer stages A, B, C, this arrangement allows simultaneous stepping up of the AC voltage, and hence corresponding stepping down of the current for reduced wire resistance power losses along the power transmission cable 56.

It is envisaged that a rotary mount in the form of a three-phase transformer of a type illustrated in figure 4 would typically be configured to perform all of the required voltage transformation between an optimum generation voltage (typically 690V) and an optimum distribution voltage (typically 33000V) . In such an arrangement, it will be appreciated that the Nacelle of each tidal turbine 1 will not need to house any transformer itself, and thus could be reduced significantly in size and weight, thereby improving its yaw control characteristics. Alternatively, however, it is envisaged that a rotary mount of a type illustrated in figure 4 may be used as part of a two-stage transformation process, for example stepping up from a voltage of approximately 6600V to 33000V, with a first transformation stage (stepping up from 690V to 6600V) being carried out by a more conventional transformer located within the turbine Nacelle.

In order to reduce flux coupling between the three discreet transformer stages A, B, C, it is proposed to make the fixed central pillar 28 from non-magnetic materials. It is also envisaged that further reductions in cross coupling could be achieved by inserting copper baffles between the discreet transformer stages. It is also envisaged that the interior volume of the rotary mount 8 will be at least part-filled with a dielectric oil, and most preferably by a biodegradable marine oil in order to provide cooling for the transformer. The rest of the unit space may be pressurised by a supply of regulated nitrogen or some other suitable inert gas in order to maintain optimum pressure within the transformer chamber and reduce differential pressure across the seal 37.

Whilst the present invention has been described above with specific reference to a rotary mount 8 m the form of a rotary transformer, it should be appreciated that the rotary mount does not necessarily have to take the form of a transformer at all. For example, figure 5 illustrates a variant forming an alternative embodiment of the present invention, which performs no voltage transformation function at all.

In relation to figure 5, the central post 26 extends further into the rotatable part 10, but does not enclose a central pillar 28 as in the case of the arrangement illustrated in figure 4. The central post 26 carries three discreet slip rings 59, each slip ring forming a respective electrical conductor mounted around the central post 26 and electrically insulated from the post The three slip rings 59 are spaced apart from one another in a similar manner to which the three discreet transformer stages A, B, C, of the arrangement of figure 4 as spaced apart from one another.

Each slip ring 59 is electrically connected to a respective core cable (not shown) forming part of the power transmission cable 56, the core cables running inside the hollow post 26.

Bearing against the outer surface of each slip ring 59 is a respective electrical brush 60, each brush 60 being electrically connected to a respective core cable 38, 39, 40, from the cable 21 and being supported by a respective support plate 61 extending radially inwardly from a side wall of the rotary part 10. As illustrated schematically at 62, each brush 60 is spring biased towards the respective slip ring 59. It will also be noted that each brush 60 has an arcuate contact face 63 configured to conform closely to the profile of the outer surface of the respective slip ring 59.

The arrangement of figure 5 thus still performs the function of permitting the transfer of electrical energy generated by the turbines 1, via the three brushes 60 and the three slip rings 59, whilst permitting rotation between the rotary part 10 of the mount and the fixed part 9. It will be appreciated that as the rotary part 10 rotates about the axis of rotation 11, relative to the fixed part 9, so the brushes 60 will rotate around the slip rings 59. However, by virtue of the biasing springs 62, the brushes 60 will always remain in physical electrical contact with the slip rings 59, thereby ensuring reliable transmission of electrical energy to the core cables of the power transmission cable 56.

In variants of the embodiment described above and illustrated m figure 5, it is envisaged that each of the three brushes 60 could be replaced with a plurality of smaller brushes arranged in slightly spaced apart relation to one another, and electrically connected to one another. This type of arrangement would be beneficial where brush- life is of particular concern, as it would increase the surface area of contact between the brushes of each electrical phase and their respective slip rings, thereby reducing the likelihood of localised hotspots arising as a result of friction between the slip rings and the brushes.

When used in this specification and claims, the terms "comprises" and "comprising" and variations thereof mean that the specified features, steps or integers are included. The terms are not to be interpreted to exclude the presence of other features, steps or components.

The features disclosed in the foregoing description, or in the following claims, or in the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for obtaining the disclosed results, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

While the invention has been described m conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention . The rotary mount of the present invention may be applied to a wind turbine, for which the rotary mount would typically be mounted on a conventional tower above the ground or other supporting structure. In place of a tether arrangement 5, a wind turbine arrangement would typically allow for mounting of a wind turbine housing or nacelle directly onto the rotary mount.

Claims

1. A rotary mount (8) for an electrical power turbine (1), the rotary mount (8) comprising: a fixed part (9), and a rotary part (10) mounted for rotation relative to the fixed part (9) ; the rotary part (10) being configured for connection to said turbine (1); wherein the fixed part (9) comprises a fixed conductor (52, 59), and the rotary part (10) comprises rotary conductor (47, 60), the rotary conductor (47, 60) being electrically connectable to the turbine (1) and arranged for rotation relative to the fixed conductor (52, 59), wherein the or each rotary conductor (47) is inductively coupled to a respective said fixed conductor (52) such that electrical energy generated by the turbine (1) is inductively transferred via the rotary conductor (47, 60) to the fixed conductor (52, 59), whilst permitting rotation therebetween.

2. A rotary mount according to claim 1 comprising a plurality of said fixed conductors (52, 59) and a plurality of said rotary conductors (47, 60) , wherein each rotary (47, 60) conductor is arranged for rotation relative to a respective fixed conductor (52, 59) .

3. A rotary mount according to claim 2, comprising three said fixed conductors (52, 59) and three said rotary conductors (47, 60) arranged in pairs, each said pair of fixed and rotary conductors being configured to carry a respective phase of a three-phase supply of electrical energy generated by the turbine (1) .

4. A rotary mount according to any preceding claim, wherein the or each rotary conductor (47) comprises a primary coil having a plurality of turns wound on a rotary- core (44), and the or each fixed conductor (52) comprises a secondary coil having a plurality of turns wound on a fixed-core (48) in the form of a transformer, the rotary- core being mounted for rotation relative to the fixed-core.

5 A rotary mount according to claim 5, wherein the or each primary coil (47) is arranged concentrically relative to the or each respective secondary coil (52) .

6. A rotary mount according to claim 4 or claim 5, wherein said fixed-cores (48), and their associated secondary coils (52), are mounted m spaced apart relation such that the secondary coils (52) are substantially coaxial .

7. A rotary mount according to claim 6, wherein said fixed cores (48) are each mounted to a pillar (28) of nonmagnetic material

8. A rotary mount according to any one of claims 4 to 7, wherein said rotary-cores (44), and their associated primary coils (47), are mounted in spaced apart relation such that the primary coils (47) are substantially coaxial.

9. A rotary mount according to any preceding claim, wherein the rotary part (10) is mounted for substantially unrestricted 360° rotation relative to the fixed part (9) .

10. A rotary mount according to any preceding claim, wherein a substantially hermetic seal (37) is provided between the rotary (10) and fixed parts (9) .

11. A rotary mount according to claim 10, at least partially filled with dielectric oil.

12. A rotary mount according to claim 10 or claim 11, at least partially filled with pressurized Nitrogen gas.

13. A subsea rotary mount (8) for a tidal-stream turbine (1), the rotary mount (8) comprising: a fixed part (9) configured to be secured relative to the seabed (3), and a rotary part (10) configured for connection to said turbine (1); the rotary part (10) being mounted for rotation relative to the fixed part (9) ; wherein the fixed part (9) comprises a fixed conductor (52, 59), and the rotary part (10) comprises rotary conductor (47, 60), the rotary conductor (47, 60) being electrically connectable to the turbine (1) and arranged for rotation relative to the fixed conductor (52, 59), the arrangement being configured such 5 that electrical energy generated by the turbine (1) is transferred via the rotary conductor (47, 60) to the fixed conductor (52, 59), whilst permitting rotation therebetween.

14. A subsea rotary mount according to claim 13, wherein 10 each rotary conductor (47) is inductively coupled to a respective fixed conductor (52) .

15. A subsea rotary mount according to claim 13, wherein the or each rotary conductor (60) is physically coupled to a respective said fixed conductor (59) .

15 16. A subsea rotary mount according to claim 15, wherein the or each fixed conductor comprises a slip ring (59) , and the or each rotary conductor comprises a brush (60) biased into contact with a respective said slip ring (59) .

17. A subsea rotary mount according to claim 16, wherein

20 the or each slip ring (59) is mounted on a pillar (26) forming part of the fixed-part (9) .