Abstract:
The present invention relates to electricity generating equipment, particularly off-shore generators for generating electricity from tidal streams, marine currents or wave motion. In a generator ( 101 ) according to one embodiment, each three-phase set ( 113 ) of three coils ( 109 ) is connected to a separate rectifier ( 121 ), the output of which is subsequently inverted to produce a substantially constant-voltage, constant-frequency AC output, for onward transmission and/or direct connection with the outputs of other similar generators ( 101 ), or the grid.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to the conditioning of electrical power generated by electricity generating machines, for subsequent collection, connection and transmission, and relates particularly, but not exclusively to off-shore electricity generating apparatus for generating electricity from wind, tidal streams, constant marine currents or wave motion. 
       BACKGROUND OF THE INVENTION 
       [0002]    Various offshore power generation devices, such as offshore wind turbines, tidal turbines and wave energy converters, have been designed to generate electricity from the kinetic power of the wind, tides, waves and/or ocean currents. Such systems are often deployed as an array or ‘farm’ of multiple units arranged to share a common infrastructure. An important advantage of such arrays is that a single or dual export power cable can be used to connect the array of devices to shore, rather than using a separate power export cable for each device. As these cables are expensive, both to buy and install, minimising cable-laying requirements is a significant priority. 
         [0003]    However, a common problem associated with these systems is the electrical interconnection of devices in an array for onward transmission of the generated power. To combine the raw generated AC output of each device, these outputs must be in phase, and of the same frequency and voltage. Preferably, the outputs are also of the same frequency as the onshore grid power supply, to reduce or eliminate expensive additional onshore power processing. 
         [0004]    Induction generators (IG) are a type of generator capable of generating AC power. They can also be simple, robust and suited to a rotating central hub type geometry. However, IGs have several disadvantages for implementation in off-shore energy generation. Induction generators are unable to provide ‘Fault ride through’ for grid compliance and thus require expensive additional back up systems. They typically also dissipate 20% or more of captured power as heat, lowering efficiency and requiring effective heat dissipation. Induction generators can be expensive to manufacture, due in part to various geometrical constraints related to the close tolerances required both initially and latterly to be maintained in service. Induction generator devices are also not suited to implementation as linear generators or large diameter rim generator geometries due to the large mass of such devices, and the requirement of extremely high tolerances over large distances. 
         [0005]    An alternative is to use synchronous generators such as permanent magnet generators (PMG&#39;s), which are well suited to linear and rim generator geometries, in addition to hub generator geometries. However, synchronous generators generate variable frequency, variable voltage output power, the voltage and frequency being directly linked to the speed of the incident fluid flow. If adjacent devices are electrically connected they will try to drive each other synchronously, leading to a loss of power captured from the fluid medium due to driving adjacent generators and increased wear and tear in any generators driven as motors rather than generators due to this effect. Consequently, synchronous generators are not generally directly interconnected. 
         [0006]    WO2008006614A1 describes a permanent-magnet hydroelectric turbine, in which the output of each coil is rectified by means of a diode bridge or half bridge (i.e. a passive rectifier), and the rectifiers are connected in series and/or parallel to produce a varying DC generator output which can then be interconnected with the varying DC outputs of adjacent generators. A disadvantage of this technology is that it is limited to voltages within the operating range of commercially available bridge rectifiers, typically 1500-3000 V. Insulation requirements impose a further limitation on the maximum output voltage. At these low and medium voltages, power losses in the power export cable become important, effectively limiting the distance over which power can be transmitted (e.g. to shore and/or to other generators) to between a few hundred metres to a few kilometers depending on the voltage level used. A further disadvantage is that, in order to optimally interconnect the variable DC outputs of an array of synchronous generators, the generated power profiles of all the interconnected devices should be nearly identical, otherwise any generators generating less power may not add anything significant to total power output. The reduced power capture occurring when dissimilar DC output voltages are joined effectively limit the number of similar synchronous generators that can be interconnected. Yet another disadvantage is that a varying DC output requires relatively complex and expensive inversion equipment onshore to obtain grid compliant AC power. 
         [0007]    In the onshore wind turbine industry, asynchronous and synchronous generators are often used with two inverters, one for converting the AC output to fixed voltage DC, and the other for converting the fixed voltage DC to fixed voltage, fixed frequency AC. Typically the generated AC power is at a voltage below 1000V AC to reduce the electrical leakage potential and thus the insulation requirements. This ‘low voltage’ power can then be stepped up with a standard 3 phase transformer to higher voltage, 11 kV, 33 kV, 66 kV or other grid standard voltage as required for long distance power transmission. In such systems, multiple wind turbines can be interconnected for onward power transmission via a single or dual export power cable, and direct connection to the grid. 
         [0008]    However, the inversion technology developed for onshore wind turbines is highly complex, and includes many cut-out switches, safety circuits, and connection boards, and requires high levels of cooling. It is therefore not generally suited to the vibrating (or constantly moving), difficult to cool environment of offshore wind turbines, tidal turbines and wave energy converters, where maintenance is often impossible and always expensive. 
         [0009]    Consequently, adapting this technology for offshore wind, wave, tidal or similar devices would require raw AC power from device to be transmitted to a central power processing platform housing the inversion equipment. Such platforms would need to be constructed near the actual generating devices, to avoid transmission losses due to the relatively low generator output voltage, and to minimise cabling requirements. This would be very expensive, requiring the construction of either a physical platform or a sub-sea housing or pod rated to survive the highly energetic tidal, wave or similar regime and to isolate the inversion equipment from excess vibration and turbine related movement. Sub sea pods would have the added difficulties of being suitably pressure rated and leak proof at depth, and would require considerable heat dissipation capability (e.g a 1 MW generator with a typical 4% loss would need to dissipate 40 kW). Effective heat dissipation would require a large surface area, countering the requirement to keep the pod as small as possible for operational reasons and sealing reasons, or large external cooling fins, which would encourage marine growth, increase drag and make retrieval more difficult. Alternatively, active cooling with water and/or internal circulation air pumps would increase the complexity, cost and potential failure rate of the system. Retrieval of such a sub-sea pod for any maintenance, even for fuse changing, would be at significant effort and cost, and may be impossible in certain weather and/or tidal conditions. At present there are no commercially available inverters rated for large scale power conditioning under these conditions. 
         [0010]    Preferred embodiments of the present invention seek to overcome one or more of the above disadvantages of the prior art. 
       SUMMARY OF THE INVENTION 
       [0011]    According to an aspect of the present invention, there is provided an electrical generator, comprising: 
         [0012]    a first part having a plurality of magnetic field generating elements, said magnetic field generating elements being fixed in position relative to each other; and 
         [0013]    a second part, moveably mounted to said first part, and having a plurality of groups of windings, said windings being fixed in position relative to each other, each group comprising at least one winding, wherein relative movement between the windings and the magnetic field generating elements induces an electromotive force in each winding; 
         [0014]    power conditioning means connected to a plurality of said groups of windings for enabling the generator to output substantially fixed voltage, fixed frequency AC power, wherein the power conditioning means comprises: 
         [0000]    (a) a plurality of rectifiers mounted to a plurality of locations on said second part, for converting an a.c. electrical power output of at least one winding of a respective group of windings to d.c.; and
 
(b) at least one inverter for converting the output of at least one said rectifier to AC.
 
         [0015]    By providing a plurality of rectifiers mounted to a plurality of locations on said second part, rather than a single power conditioning unit, the power processed by each rectifier is reduced. Advantageously, this reduces the heat dissipation per rectifier. This also provides the advantage of enabling cooling of the rectifiers by providing the rectifiers at a plurality of locations on the second part, since the ratio of surface area to volume for the total rectifier means can be significantly increased. A further advantage is that, by dividing the power generated into manageable quantities, the power output can be processed using standard power electronics at low voltage. Advantageously, the fixed frequency, fixed voltage, AC output of the generator can be combined with the output of other generators and/or fed directly into the grid system without requiring further processing. 
         [0016]    At least one said rectifier may be a respective active rectifier for rectifying the electric power output of at least one winding of a respective group of windings to a substantially constant voltage DC output. 
         [0017]    Advantageously, this ensures optimal combination of the outputs from different windings, or from different generators. A further advantage is that the conversion of fixed DC to grid-compliant AC is less complex than the conversion of varying DC to grid-compliant AC. 
         [0018]    The power conditioning means may comprise a respective plurality of inverters connected in parallel. 
         [0019]    Providing a plurality of inverters in said power conditioning means improves the fault tolerance of the generator in the event of failure of a single inverter. 
         [0020]    At least one said rectifier may be located remote from at least one respective inverter to which said rectifier is connected. 
         [0021]    By providing at least one said inverter externally of the corresponding rectifier, the plurality of inverters may advantageously be housed in a common location, for example, to improve access for maintenance. A further advantage is that multiple rectifiers may connect to the same inverter or inverters. 
         [0022]    At least one said rectifier may be connected to a plurality of parallel-connected inverters. 
         [0023]    This improves the fault tolerance of the generator in the event of failure of a single inverter. 
         [0024]    Multiple rectifiers may be connected to at least one said inverter. 
         [0025]    Advantageously, the inverters need not be rated at the same capacity as the rectifiers, and can thus be selected according to availability. 
         [0026]    At least one said inverter may be adapted to output three-phase AC power. 
         [0027]    Advantageously, the output can be directly connected to the grid, and standard components for further processing of the output are readily available. 
         [0028]    At least one said inverter may be adapted to receive a reference signal for controlling the phase of the output of at least one respective inverter. 
         [0029]    Advantageously, the reference signal can be used to synchronise the outputs of each inverter, thereby enabling the outputs of the inverters to be combined. 
         [0030]    The reference signal may be used to synchronise the inverter output phases to the grid. 
         [0031]    The output of at least one said active rectifier may be less than 1000 V DC. 
         [0032]    Working at relatively low voltages provides the advantage that standard power electronics components can be used. 
         [0033]    The output of at least one said inverter may be less than 1000 V AC. 
         [0034]    Working at relatively low voltages provides the advantage that standard power electronics components can be used. 
         [0035]    The electrical generator may further comprise a transformer, for transforming the AC output of at least one said inverter to a higher AC voltage. 
         [0036]    Transforming the output to higher AC voltage at the generator itself reduces transmission losses when the power is transmitted to the grid. 
         [0037]    At least one said group of windings may comprise at least one respective set of n windings adapted to generate n different phases of electrical power. 
         [0038]    Advantageously, this smoothes the torque acting on the generator and the output of each rectifier and/or inverter. 
         [0039]    The windings of said set of n windings may be located adjacent to each other. 
         [0040]    At least one said group of windings may comprise respective multiple sets of n windings adapted to generate n different phases of electrical power, wherein the corresponding phases of each set are connected in series. 
         [0041]    Connecting more than one set of coils in series enables the voltage input to the rectifiers and/or the inverters to be selected to match the specifications of standard power electronics components. 
         [0042]    At least one said group of windings may comprise at least one respective set of three windings adapted to generate three-phase electrical power. 
         [0043]    Advantageously, this enables standard three-phase electronics components to be used. 
         [0044]    Said generator may be a linear generator. 
         [0045]    Advantageously, the windings of a linear generator are distributed along the length of the generator, enabling the rectifiers to be similarly distributed along the length of the generator for more efficient heat dissipation. 
         [0046]    Said generator may be a rim generator, comprising at least one rotor adapted to be rotatably mounted to a stator. 
         [0047]    Advantageously, the windings of a rim generator are distributed circumferentially around the generator, enabling the rectifiers and optionally inverters to be similarly distributed for more efficient heat dissipation. 
         [0048]    A plurality of said rectifiers may be distributed circumferentially around said stator. 
         [0049]    A plurality of said magnetic field generating elements may be permanent magnets. 
         [0050]    The generator may be adapted to be driven by a flow of water and/or air. 
         [0051]    The present invention provides particular advantages for use in offshore or marine applications, since it is costly to provide separate units for processing the power generated by one or more turbines, due to the requirements of heat dissipation, stability against currents, and providing a watertight seal. 
         [0052]    At least one said rectifier may be located adjacent a respective group of windings. 
         [0053]    Advantageously, distributing the rectifiers to be adjacent to respective groups of windings improves the dissipation of heat from said rectifiers. 
         [0054]    At least one said inverter may be located on said generator. 
         [0055]    Converting the output of each winding to AC at the generator, rather than transmitting DC to a distant site, reduces transmission losses. 
         [0056]    The electrical generator may further comprise a shield adapted to reduce penetration of the magnetic flux from the generator into the external environment, wherein a plurality of said rectifiers are located on the opposite side of said shield from said magnetic field generating elements. 
         [0057]    The output power of at least one said rectifier and/or inverter may be adjustable to compensate for failure of at least one further said rectifier and/or inverter. 
         [0058]    This provides the advantage of increasing the reliability of the apparatus. 
         [0059]    At least one said rectifier and/or inverter may be encapsulated in an electrically insulating material. 
         [0060]    This provides the advantage of enabling the rectifier and/or inverter to be passively cooled. 
         [0061]    At least one said rectifier may be adapted to passively lose substantially all internally generated waste heat. 
         [0062]    This provides the advantage of reducing the complexity and cost of the apparatus, and increasing its reliability. 
         [0063]    A plurality of said inverters may be mounted to a plurality of locations on said second part 
         [0064]    According to a further aspect of the present invention, there is provided an array of electrical generators, comprising a plurality of interconnected electrical generators as defined above, adapted to be connected to the public electricity grid. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0065]    A preferred embodiment of the present invention will now be described, by way of example only and not in any limitative sense, with reference to the accompanying drawings, in which: 
           [0066]      FIG. 1  illustrates a conventional permanent-magnet rim generator; 
           [0067]      FIG. 2  illustrates a generator according to a first embodiment of the invention; 
           [0068]      FIGS. 3A and 3B  each illustrate a portion of the generator shown in  FIG. 2 ; 
           [0069]      FIG. 4A  illustrates a connection schematic for the generator of  FIG. 2 ;  FIG. 4B  illustrates a connection schematic for an array of such generators; 
           [0070]      FIG. 5  illustrates a portion of a generator according to a second embodiment of the invention; 
           [0071]      FIGS. 6A and 6B  each illustrate a portion of a generator according to a third embodiment of the invention; 
           [0072]      FIG. 7  illustrates a connection schematic for an array of generator according to the third embodiment; 
           [0073]      FIG. 8  illustrates a portion of a generator according to a fourth embodiment of the invention; 
           [0074]      FIG. 9  illustrates a connection schematic for an array of generators according to a fifth embodiment of the invention; 
           [0075]      FIG. 10  illustrates a connection schematic for an array of generators according to a sixth embodiment of the invention; 
           [0076]      FIG. 11  illustrates a further connection schematic for an array of generators according to the invention. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0077]    The invention will be described with reference to a Rim Permanent Magnet Generator (RPMG). However, the invention is relevant to all wave, ocean current, tidal stream and other offshore electricity generating machinery, and can be applied to other types of generator, in particular linear generators, or generators having a large-diameter central hub. 
         [0078]    With reference to  FIG. 1 , a RPMG  1  generates electricity directly from the motion between an inner rim  3  and an outer rim  5 , one of which includes a number of permanent magnets  7  arranged with poles alternating north then south to provide a varying magnetic flux, and the other of which includes copper coils  9  within a stator section  5 . Relative rotation between the two rims  3 ,  5  induces an alternating Electro Motive Force (EMF) in the coils  9 . Movement between the rims  3 ,  5  is created by the action of water flowing over blades (not shown) attached to a surface of one of the rims  3 ,  5 . The coils and magnets  7  can be arranged in various configurations, in series and/or parallel, to produce an electrical output having the desired characteristics, e.g. 3-phase, 5-phase, 7-phase, N-phase, with varying degrees of redundancy, output voltage level, harmonic distortion characteristic, and other characteristics of the electrical output. It will be appreciated that in all such configurations the voltage and frequency of the output EMF will be directly proportional to the speed of relative rotation of the rims  3 ,  5 . As the rim speed cannot be perfectly controlled, the frequency and voltage of the raw power output will vary. 
         [0079]      FIG. 1  shows a conventional 3-phase generator  1 , configured with N coils  9  and 4N/3 magnets  7  arranged with poles alternating N-S-N-S . . . circumferentially around the inner rim  3 . The N coils  9  have a common neutral  11  and every third coil  9  is interconnected (that is, the output of the 1st, 4th, 7th, . . . , and (1+3N)th coils are interconnected, and likewise the outputs of the 2nd, 5th, 8th, . . . , and (2+3N)th coils and the 3rd, 6th, 9th, . . . , and (3+3N)th coils are respectively interconnected) in a combination of series and parallel arrangements to provide a 3-phase generator output. 
         [0080]      FIG. 2  shows a generator  101  according to a first embodiment of the present invention.  FIG. 3A  shows an enlarged section of the turbine  101  shown in  FIG. 2 . A series of coils  109  are arranged on a fixed stator, or outer rim  105 . A series of permanent magnets  107  are arranged on a steel flux ring  115  within the rim  103  of a rotor which spins under the influence of passing tide, current or wave motion. The poles of the magnets  107  alternate N-S-N-S . . . around the circumference of the inner rim  103 , to create a varying magnetic flux at each coil  109  when the rotor spins. In each coil  109 , the varying magnetic flux induces a varying EMF. 
         [0081]    An EMF shield  117  is located radially outward of the coils  19  to prevent leakage of the varying magnetic flux into the external environment. 
         [0082]    The ratio of magnets  109  to coils  107  is 4:3, such that each set  113  of three adjacent coils  109  generates three distinct electrical phases, separated by 120 degrees. Other ratios are also possible. In contrast to the conventional generator shown in  FIG. 1 , each set  113  of three coils  109  is wired as a separate three-phase machine, as shown in  FIG. 3B . In this embodiment the three coils  109  of each set  113  share a common neutral  111 . Output leads  119  from each group  113  of three coils  109  pass through or around the EMF shield  117  to an active rectifier  121  which converts the variable frequency, variable voltage AC into fixed voltage DC. An inverter  123  is directly connected to the rectifier  121  to convert the DC output of the rectifier into fixed-frequency, fixed-voltage AC. Each inverter  123  outputs to a common rail three-phase AC bus  124 . The rectifier  121  and the inverter  123  are preferably rated for operation within the standard ‘low’ voltage range of below 1000V. This allows standard power electronic components to be used. 
         [0083]    The rectifiers  121  and inverters  123  can be fabricated in solid state fashion and fully embedded within epoxy or similar matrix for installation around the circumference of the stator, adjacent the emf shield. The rectifiers  121  and inverters  123  could be integrated onto a chip, or could be provided by separate IGBTs. By positioning the rectifier  121  and inverter  123  radially outward of the EMF shield  117 , the power conditioning electronics are shielded from the varying magnetic flux. 
         [0084]    A control lead  125  feeds in a reference signal for phase control and other control signals for operation of the power conditioning electronics. For example, a control signal can be also be used to switch the circuits to ‘open circuit’ mode, in which no current is drawn from the coil, or to connect the coil outputs to a dump resistor  127  which will act to brake the rotor. The control cable can further be used to remotely control operation of the inverters  123  and to monitor power draw from that phase either separately from other phases or in synchronization with other phases around the turbine. 
         [0085]    In the present embodiment, each set  113  of three coils  109  are connected as a single 3-phase machine. This divides the total power generated by the generator  101  into smaller manageable amounts of power, each of which can be rectified and inverted using standard power electronics at low voltage. The generator  101  of the present embodiment includes forty-four sets of three coils  109  (i.e. N=44) and therefore includes forty-four separate power conditioning units  129 , each comprising a rectifier  121  and an inverter  123 . However, it will be appreciated that the person skilled in the art would select N according to the desired generator specifications. 
         [0086]    With each power conditioning unit  129  only handling a small amount of power (1/Nth of the total power), the heat dissipation per unit will be correspondingly lowered, (typically &lt;1 kW). The power conditioning units  129  are distributed around the outer rim  105 , close to the outer surface of the generator  101 , such that heat is efficiently dissipated over a large surface area into the ambient water stream for effective heat dissipation. 
         [0087]    Manufacturing each power conditioning unit  129  as an integrated solid state device, fully embedded and sealed within epoxy or a similar matrix, results in a system which is relatively robust against physical shocks or vibration, and less susceptible to leaks. 
         [0088]    Furthermore, by generating and processing power in N separate units, the generator  101  is highly fault tolerant. Should any individual 3-phase unit fail (due to failure within a single coil, rectifier or inverter), the faulty unit can be isolated from the remaining units and the turbine can continue to operate. The person skilled in the art will appreciate that the degree of fault tolerance can be engineered to meet various specifications (e.g. by increasing N). The capability to function at reduced capacity whilst carrying a fault significantly increases the mean time to maintenance, as compared with conventional generators. This is a significant advantage, as maintenance of off-shore machines is costly and can depend on the ambient conditions and availability of suitable vessels. 
         [0089]      FIG. 4A  shows a schematic of a generator  101  according the first embodiment described above, further comprising a transformer  131  for transforming the fixed frequency, fixed voltage 3-phase AC output of the inverters  123  to a higher voltage (typically 11 kV, 33 kV or 66 kV) suitable for long-distance power transmission, or for connection to the grid. Preferably, the 3-phase output voltage of the generator  101  is phase matched to a 3-phase reference signal from the grid. The high voltage power may be transmitted directly to shore  133  for connection to the grid. Alternatively, an array  135  or ‘farm’ of generators  101  may be connected together as shown in  FIG. 4B  for connection back to shore  133  and grid via a single export cable  137 . A dual export cable may be provided for redundancy. 
         [0090]      FIG. 5  shows a generator  201  according to a second embodiment of the invention. As in the first embodiment described above, permanent magnets  207  are arranged on a flux ring  215  on an inner rim  203 , and coils  209  are provided on an outer rim  205 . The generator  201  also includes a plurality of power conditioning units  229 , each comprising a combined back-to-back rectifier  221  and inverter  223 , located radially outward of an emf shield  217 . However, in the second embodiment, each power conditioning unit  229  conditions the power output from two adjacent 3-phase coil sets  213 . As in the first embodiment, the inverters  223  output to a common rail 3-phase AC bus  224 . The second embodiment could be modified such that each power conditioning unit  229  conditions the power output from a plurality of X&gt;2 3-phase sets  213  of coils  209 . 
         [0091]      FIG. 6  shows a generator  301  according to a third embodiment of the invention. As in the first embodiment described above, permanent magnets  307  are arranged on a flux ring  315  on an inner rim  303 , and coils  309  are provided on an outer rim  305 . Also, as in the first embodiment, individual active rectifiers  321  are connected to respective 3-phase sets  313  of coils  309 , and are located on the outer rim section  305  close to each 3-coil phase set  313 . However, the fixed DC output from each rectifier  321  is carried on a single bus  339  to a bank  341  of inverters  323 . The bank  329  of inverters  323  output to a common rail 3-phase AC bus  324 . Control signals may be input via a control cable  325  to the inverters for controlling the phase of the AC output. 
         [0092]    Since a one-to-one correspondence between the rectifiers  321  and inverters  323  is not necessary, the inverters  323  do not need to be rated at same capacity as the rectifiers  321 . This design flexibility enables the components to be specified according to those of widely available components. The bank  341  of inverters  323  may have a total capacity greater than the maximum rated output capacity of the rectifiers  321 , thereby allowing for failure of one or more of the inverters  323  to be tolerated. This further improves the fault tolerance of the machine  301 . 
         [0093]    The bank  341  of inverters  323  may be housed in a central, easy-to-access position, for example on the generator itself, or in a chamber above or near to the generator. The inverters  323  could be individually sealed, and cooled as required. Alternatively, the bank  341  of inverters  323  could be housed in a single pod, although this may then require additional cooling. 
         [0094]    In  FIG. 6 , two turbines  301  of a twin turbine configuration are connected to the same bank  341  of inverters  323 , which may be located in a central position between the twin turbines. However, each turbine  301  may be served by a separate bank  341  of inverters  323 . 
         [0095]      FIG. 7  shows a schematic of a generator  301  according the third embodiment described above, further comprising a transformer  331  for transforming the fixed frequency, fixed voltage 3-phase AC output of the inverters  323  to a higher voltage, and connected with an array  335  of similar generators  301  for connection back to shore  333  and grid via export cable  337 . 
         [0096]      FIG. 8  shows a generator  401  according to a fourth embodiment of the invention. As in the third embodiment described above, permanent magnets  407  are arranged on a flux ring  415  on an inner rim  403 , and coils  409  are provided on an outer rim  405 . The generator  401  also includes a plurality of active rectifiers  421 , located radially outward of an emf shield  417 , with the DC output of the each rectifier  421  connected via a common bus  439  to a centrally located bank  441  of inverters  423 , which output to a common rail 3-phase AC bus  424 . The fourth embodiment differs from the third embodiment in that each rectifier  421  rectifies the output from two adjacent 3-phase coil sets  413 . Alternatively, each rectifier  421  may rectify the output from a plurality of X&gt;2 sets  413  of coils  409 . 
         [0097]    In the above-described embodiments, three distinct electrical phases are generated, each at an angle of 120 degrees from the other. By generating three phases, or indeed any number of additional phases (polyphasic), a reasonably balanced output is obtained and standard, off-the-shelf power electronics and other components can be used. 
         [0098]    However, it will be appreciated that other configurations are possible, including grouping the coils in five-phase sets of coils as shown in  FIG. 9 .  FIG. 9  shows a schematic of a generator  501  according to a fifth embodiment, connected to an array  535  of other similar generators  501 . Here, each set  513  of coils  509  comprises five coils  509  generating five different phases of AC output. The generator could alternatively be configured with sets of coils comprising 3, 5, 7 . . . coils, for generating 3-, 5-, 7- . . . phase power. 
         [0099]    Other than the difference in the number of phases generated, the fifth embodiment is similar to the second and fourth embodiments, as individual rectifiers  521  rectify the output from two or more adjacent sets  513  of coils  509 , and the fixed DC output of the rectifiers is carried on a single bus  539  to a bank  541  of inverters  523 . Although multiple sets  513  of coils  509  are shown connected in parallel to each rectifier  521 , such sets  513  of coils  509  could alternatively be connected in series in order to increase the output voltage. 
         [0100]      FIG. 10  shows a schematic of a generator  601  according to a sixth embodiment, connected to an array  635  of other similar generators  601 . This embodiment is similar to the first embodiment, with the power generated by each set  613  of three coils  609  being processed by a power conditioning unit, located adjacent the respective coil set  613 . However, each power conditioning unit  613  includes a rectifier  621  connected to two inverters  623 . This embodiment thereby incorporates further redundancy or fault tolerance at the inversion stage as compared to the first embodiment described above. The fault tolerance could be increased further by increasing the number of inverters  623  connected to each rectifier  621 . As a further alternative, the number of rectifiers  621  could be increased, such that each coil set  613  fed a plurality of rectifiers  621 , the combined output of which fed one or more adjacent inverters  623 . 
         [0101]      FIG. 11  illustrates a further connection schematic for connecting the outputs of generators  701  according to any of the embodiments described above in an array  735 . The array  735  of generators  701  is divided in to sub-arrays  736 , the generators  701  of each sub-array  736  being connected together, and each sub-array  736  being connected to the next via a single or dual export cable  737 . 
         [0102]    Although the invention has been described with respect to a permanent-magnet rim generator, the invention is applicable to other types of generator. In particular, the skilled person will appreciate that the configurations of coils and power conditioning electronics, distributed circumferentially in the rim generator design described above, could instead be distributed linearly in a linear generator design. The present invention could also be applied to generators of the central hub type, having a large diameter hub, such that the configurations of coils and power conditioning electronics described above could be distributed around the hub. Similarly, the magnetic field may be generated by means other than permanent magnets. 
         [0103]    It will be appreciated by persons skilled in the art that the above embodiments have been described by way of example only, and not in any limitative sense, and that various alterations and modifications are possible without departure from the scope of the invention as defined by the appended claims.