Abstract:
According to one aspect of the teachings herein, various feeder connection arrangements and architectures are disclosed, for collecting electricity from wind turbines in an offshore collection grid that operates at a fixed low frequency, e.g., at one third of the targeted utility grid frequency. Embodiments herein detail various feeder arrangements, such as the use of parallel feeder connections and cluster-based feeder arrangements where a centralized substation includes a common step-up transformer for outputting electricity at a stepped-up voltage, for low-frequency transmission to onshore equipment. Further aspects relate to advantageous generation arrangements, e.g., tower-based arrangements, for converting wind power into electrical power using, for example, medium-speed or high-speed gearboxes driving generators having a rated electrical frequency for full-power output in a range from about 50 Hz to about 150 Hz, with subsequent conversion to the fixed low frequency for off-shore collection.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims priority under 35 U.S.C. §119 from U.S. Provisional Patent Application No. 61/953,111 filed on 14 Mar. 2014, the content of said application incorporated herein by reference in its entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention generally relates to offshore wind turbines and particularly relates to obtaining electricity from offshore wind turbines. 
       BACKGROUND 
       [0003]    Typical large-scale offshore wind farm architectures include a plurality of wind turbines, along with generators and collection networks, for collecting the generated electricity and transmitting it to shore, e.g., via high-voltage DC, HVDC, or high-voltage AC, HVAC, transmission systems. The choice of HVAC or HVDC transmission depends mainly on the distance from the offshore wind farm to the onshore grid connection point. 
         [0004]    The use of low-frequency AC, LFAC, transmission at high voltages to the onshore grid connection point has also been considered. While LFAC transmission from the offshore wind farm requires additional frequency conversion equipment at the onshore grid connection point, its usage can extend the economic distance of HVAC connections between the offshore wind farm and the onshore grid connection point. 
         [0005]    In a known approach to low-frequency collection and transmission of electricity in offshore wind farms, low-speed generators produce AC outputs with a nominal frequency of 16.7 Hz or 20 Hz. The generated electricity is coupled into the LFAC transmission system using one or more boost transformers. However, it is recognized herein that this approach suffers from a number of disadvantages, including necessitating the use of undesirably large equipment. 
       SUMMARY 
       [0006]    According to one aspect of the teachings herein, various feeder connection arrangements and architectures are disclosed, for collecting electricity from wind turbines in an offshore collection grid that operates at a fixed low frequency, e.g., at one third of the targeted utility grid frequency. Embodiments herein detail various feeder arrangements, such as the use of parallel feeder connections and cluster-based feeder arrangements where a centralized substation includes a common step-up transformer for outputting electricity at a stepped-up voltage, for low-frequency transmission to onshore equipment. Further aspects relate to advantageous generation arrangements, e.g., tower-based arrangements, for converting wind power into electrical power using, for example, medium-speed or high-speed gearboxes driving generators having a rated electrical frequency for full-power output in a range from about 50 Hz to about 150 Hz, with subsequent conversion to the fixed low frequency. 
         [0007]    In an example embodiment, a system is configured for obtaining electricity in an offshore wind turbine farm. The system includes at least a first arrangement that comprises a gearbox, a generator, and an AC-to-AC converter. The gearbox is configured to mechanically convert a first variable rotational speed of a wind turbine into a corresponding higher second variable rotational speed. The generator has a rated electrical frequency for full-power output in a range from about 50 Hz to about 150 Hz, and is configured to be driven at the variable second rotational speed by an output of the gearbox. The generator thereby generates electricity at a correspondingly variable first frequency and the AC-to-AC converter is configured to convert the electricity from the generator into electricity output from the AC-to-AC converter at a fixed low frequency for off-shore collection at the fixed low frequency. The fixed low frequency is lower than the utility grid frequency, e.g., one-third of the frequency of the targeted onshore utility grid. 
         [0008]    In some embodiments, the first arrangement further comprises a step-up transformer connected between the generator and the AC-to-AC converter. The step-up transformer has a rated frequency corresponding to the rated electrical frequency of the generator and is configured to step up a voltage of the electricity output from the generator, and thereby output electricity at a stepped-up voltage. Correspondingly, the AC-to-AC converter is configured to convert the electricity output at the stepped-up voltage from step-up transformer. Thus, it will be understood that in some embodiments the AC-to-AC converter operates on the variable-frequency electricity as directly output from the generator, and in other embodiments it operates on the electricity output from a transformer that is connected between the AC-to-AC converter and the generator. 
         [0009]    In another embodiment, a method of obtaining electricity from an offshore wind turbine farm includes mechanically converting a variable first rotational speed of a wind turbine into a corresponding higher variable second rotational speed, and generating electricity at a variable first frequency using a generator having a rated electrical frequency for full-power output in a range from about 50 Hz to about 150 Hz. The generator is driven at the variable second rotational speed and the method further includes converting the variable-frequency electricity output from the generator into a fixed low frequency for offshore collection at the fixed low frequency. Conversion to the fixed low frequency may operate directly on the output from the generator, or may operate on the output of a step-up transformer that is driven by the output from the generator. The fixed low frequency is in a range from about 16 Hz to about 20 Hz, for example. 
         [0010]    In a further example embodiment, a system is configured for obtaining electricity in an offshore wind turbine farm that includes a plurality of wind turbines. The system includes an arrangement corresponding to each wind turbine. Each arrangement includes a gearbox, a generator, and an AC-to-AC converter. The gearbox is configured to mechanically convert a variable first low rotational speed of the corresponding wind turbine to a higher variable second rotational speed. The generator has a rated electrical frequency for full-power output in a range from about 50 Hz to about 150 Hz, and outputs electricity at a variable first frequency, based on being driven by the gearbox at the variable second rotational speed. Correspondingly, the AC-to-AC converter is configured to convert the electricity from the generator, either taken directly from the generator or through a step-up transformer, into output electricity at a fixed low frequency, which is lower than the grid frequency of the targeted onshore electrical grid. 
         [0011]    Further, the example system includes an offshore low-frequency collection grid that comprises one or more feeders. Each feeder is associated with one or more of the arrangements and is configured to collect the electricity output from the associated arrangements at the fixed low frequency. 
         [0012]    Of course, the present invention is not limited to the above features and advantages. Those of ordinary skill in the art will recognize additional features and advantages upon reading the following detailed description, and upon viewing the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is a block diagram of one embodiment of a system and arrangement for obtaining electricity from an offshore wind turbine farm. 
           [0014]      FIG. 2  is a logic flow diagram of one embodiment of a method of obtaining electricity from an offshore wind turbine. 
           [0015]      FIGS. 3A-3C  are block diagrams of alternate embodiments of feeder networks within an offshore low-frequency collection grid, for collecting electricity from a plurality of wind turbines. 
           [0016]      FIG. 4  is a block diagram of another embodiment of arrangements for obtaining electricity from respective offshore wind turbines and a corresponding embodiment for an offshore low-frequency collection grid. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]      FIG. 1  illustrates a plurality of like arrangements  10 - 1 ,  10 - 2 , . . . , and  10 -N, each of which is configured to obtain electricity in an offshore wind farm. More particularly, each arrangement is associated with a given wind turbine  8 , and includes a gearbox  12 , a generator  14 , an optional step-up transformer  16 , and an AC-to-AC converter  18 . Unless suffixes are needed for clarity, the reference numeral “ 10 ” will be used to refer to any given arrangement  10  in the singular sense, and to any given arrangements  10  in the plural sense. 
         [0018]    The plurality of arrangements  10  connect to a low-frequency offshore collection grid  20 , which includes one or more feeders  22 , shown here as feeders  22 - 1 ,  22 - 2 , . . . ,  22 -M. The value of M is an integer number generally less than the value of N—i.e., the number of arrangements  10 —inasmuch as each feeder  22  usually will be associated with more than one arrangement  10 . Broadly, however, each feeder  22  is coupled to one or more arrangements  10  among the plurality of arrangements  10  and collects the electricity from its associated arrangements  10  into the low-frequency offshore collection grid  20 . 
         [0019]    The diagram further depicts a number of protective devices  24  disposed at wind turbines  8  for coupling the corresponding arrangements  10  with the low-frequency offshore collection grid  20 . Further protective devices  24  are used within a central substation  30  that is included in the low-frequency offshore collection grid  20  for coupling feeders  22  and low-frequency collection transformer  28  with the bus  26 . In more detail, one sees that the output from the collection transformer  28 , also referred to as the “step-up transformer  28 ,” couples into a low-frequency high-voltage transmission system  32 , which includes one or more transmission lines  34  that carry the electricity output from the low-frequency offshore collection grid  20  to onshore equipment  36 . In turn, the onshore equipment  36  converts the electricity from the offshore wind into the correct frequency for the coupling into the onshore electric grid  38 , with or without further voltage adjustments. 
         [0020]    The onshore electric grid  38  comprises, for example, an onshore transmission system operating at 50 Hz or 60 Hz. In some embodiments, the low-frequency offshore collection grid  20  is configured to operate at one-third of the frequency of the onshore electric grid  38 , e.g., at about 16 Hz for a 50 Hz utility grid frequency and at about 20 Hz for a 60 Hz utility grid frequency. 
         [0021]    With these example details in mind, then, the diagram of  FIG. 1  can be understood as disclosing a system  40  that is configured for obtaining electricity in an offshore wind turbine farm. In a minimal configuration, the system  40  includes at least a first one of the previously described arrangements  10 . In some embodiments, that first arrangement  10  includes gearbox  12  that is configured to mechanically convert a variable first rotational speed of a wind turbine  8  into a higher variable second rotational speed. As a non-limiting example, the gearbox provides an input-to-output turns ratio of from about 10-to-1 to 100-to-1. 
         [0022]    The first arrangement  10  further includes a generator  14  having a rated electrical frequency for full-power output in a range from about 50 Hz to about 150 Hz. For example, the generator  14  has a rated electrical frequency of 75 Hz, for full-power output. It is desired herein to generate electricity at frequencies substantially higher than the rotational speed of the wind turbine  8 , and it will be appreciated that these higher frequencies can be obtained by mechanical gearing in the gearbox  12  and/or by configuring the number of electrical poles in the generator  14 . However, the actual frequency of the electricity output from the generator  14  at any given instant will be proportional to the rotational speed of the wind turbine  8  and will vary with the rotational speed of the wind turbine  8 . 
         [0023]    The electricity output from the generator  14  is referred to herein as having a variable first frequency, denoted as f 1  in the diagram. In a non-limiting example of actual operation, the first variable frequency may range from about 20 Hz to about 150 Hz, in dependence on actual wind speed. In more detail, the variable first frequency of the generated electricity may deviate or vary from the rated electrical frequency of the generator with variation of wind speed. For example, a generator  14  having a rated electrical frequency of 50 Hz for full-power output may generate electricity at a corresponding variable frequency in a range between about 20 Hz and about 50 Hz, according to changes in wind speed. At lower wind speeds, the generator may operate near 20 Hz, while at higher wind speeds, it may operate near 50 Hz. 
         [0024]    The example first arrangement  10  further includes an AC-to-AC converter  18  that is configured to convert the electricity from the generator  14  into electricity that is output from the AC-to-AC converter  18  at a fixed low frequency, denoted as f 2  in the diagram, for offshore collection at the fixed low frequency. The fixed low frequency is lower than the targeted utility grid frequency. In some situations it may be beneficial to choose this fixed low frequency to be a value of about one-third of the utility grid frequency, which is denoted as f 3  in the diagram. Note that the AC-to-AC converter  18  operates on the electricity output from the generator  14  directly in cases where the step-up transformer  16  is omitted, and indirectly in cases where the step-up transformer  16  is included. 
         [0025]    In that latter case, the first arrangement  10  further includes the step-up transformer  16  disposed or connected between the generator  14  and the AC-to-AC converter  18 . The step-up transformer  16  has a rated frequency that matches or corresponds to the rated electrical frequency of the generator  14  in the first arrangement. That is, the rated frequency of the transformer  16  complements the rated frequency of the generator  14  and the generally higher electrical frequencies obtained with the disclosed configuration of the arrangement  10  advantageously results in the step-up transformer  16  having a lighter and more compact build than would be practical if the transformer  16  were rated, for example, for operation at or below 20 Hz. 
         [0026]    The step-up transformer  16  is configured to step up a voltage of the electricity output from the generator  14 , and thereby output electricity at a stepped-up voltage. Correspondingly, the AC-to-AC converter is configured to convert the electricity output at the stepped-up voltage from the step-up transformer  16 . That is, the AC-to-AC converter  18  operates on the electricity at the stepped-up voltage. However, this electricity is still considered as being from the generator  14 , inasmuch as it is directly obtained by stepping up the output voltage of the generator  14 . 
         [0027]    In one example of such an embodiment, the generator  14  is configured to output electricity in a voltage range of about 690 V to about 13 KV and the step-up transformer  16  is configured to output electricity in a voltage range of about 13 KV to about 72 KV. In the same or other embodiments, the AC-to-AC converter  18  is configured to output electricity at a fixed low frequency in the range of about 16 Hz to about 20 Hz. See the circled number annotations in  FIG. 1  for reference. 
         [0028]    Referring to these circled annotation numbers as “Item” numbers, Item 1 denotes the variable first rotational speed of the wind turbine  8 . Item 2 denotes the higher variable second rotational speed of the gearbox output, as mechanically derived from the wind turbine input. Item 3 denotes the electricity output from the generator  14 , which has a first voltage and the variable first frequency. 
         [0029]    Continuing with the Item references, Item 4 denotes the electricity output from the step-up transformer  16 , having a stepped-up voltage relative to the generator voltage. This stepped-up voltage may be referred to as a second voltage level, which is higher than the first voltage level provided by the generator  14 . Because the step-up transformer  16  is included in some embodiments and not in others, the input to the AC-to-AC converter  18  is marked with Item 3 or Item 4, indicating that the AC-to-AC converter  18  may receive electricity at the first or second voltage level. In either case, the AC-to-AC converter  18  outputs electricity having a fixed low frequency, which is denoted as Item 5. It will be understood that the electricity at the output of the AC-to-AC converter  18  may be at the generator voltage, in embodiments that omit the step-up transformer  16 , or at the stepped-up voltage of the step-up transformer  16 , in embodiments that include the step-up transformer  16 . 
         [0030]    One further sees that the feeders  22  operate at whatever voltage is output from the AC-to-AC converters  18  that are coupled to each respective feeder  22 . Thus, the Item 5 designation is propagated into the low-frequency offshore collection grid  20  and is carried across the bus or buses  26  within the offshore collection grid  20 , for input to the substation step-up transformer  28 . Correspondingly, the step-up transformer  28  steps up the collection grid voltage to a higher voltage, which may be referred to as a third voltage level or a transmission voltage, denoted by Item 6. This latter designation indicates that the voltage output from the step-up transformer  28  is the voltage used for the low-frequency high-voltage transmission system  32 . 
         [0031]    While it is contemplated to have a system  40  that includes only a first arrangement  10  as set forth above, other embodiments of the system  40  include a plurality of like arrangements  10 , including the first arrangement  10 . Each arrangement  10  is associated with a corresponding one of the wind turbines  8  in an offshore wind farm and each includes a gearbox  12 , generator  14 , and AC-to-AC converter  18 . The “overall” system  40  in such embodiments further comprises one or more feeders  22  comprising an offshore low-frequency collection grid  20 . Each such feeder  22  is configured to collect the electricity output from the AC-to-AC converter  18  of each arrangement  10 . That is, each feeder  22  is associated with one or more of the arrangements  10  and is configured to “collect” the electricity output from the associated arrangements  10  at the fixed low frequency. 
         [0032]    The offshore low-frequency collection grid  20  includes a substation  30  having a common step-up transformer  28  that is configured to step up the electricity collected by one or more of the feeders  22 . Further, as previously noted, the offshore low-frequency collection grid is configured to output electricity at a stepped-up voltage for transmission to an onshore electric grid  38  via a low-frequency high-voltage transmission system  32 . In some embodiments, each feeder  22  is configured for parallel collection of the electricity output by those arrangements  10  among the plurality of arrangements  10  that are coupled to the feeder. 
         [0033]      FIG. 2  illustrates a related method  200  of obtaining electricity from an offshore wind turbine farm. The method  200  includes mechanically converting (Block  202 ) a variable first rotational speed of a wind turbine  8  into a corresponding higher variable second rotational speed, and generating (Block  204 ) electricity at a variable first frequency, based on driving a generator  14  at the variable second rotational speed. The generator  14  has a rated electrical frequency for full-power output in a range from about 50 Hz to about 150 Hz. Thus, while the nominal frequency of the electricity output from the generator  14  may be taken as its rated frequency, the actual electricity will have a variable first frequency that is a function of the wind speed. 
         [0034]    The method  200  thus includes converting (Block  208 ) electricity output from the generator  14  into electricity at a fixed low frequency for offshore collection at the fixed low frequency. The fixed low frequency is lower than the grid frequency of the onshore electric grid  38 . 
         [0035]    Some embodiments include the further step or operation of stepping up (Block  206 ) the voltage of the electricity output from the generator  14 , in advance of the conversion operation in Block  208 . For example, each arrangement  10  includes a step-up transformer  16  connected between the generator  14  and the AC-to-AC converter  18  in the same arrangement  10 . When included, the step-up transformer  16  has a rated electrical frequency that matches or otherwise corresponds to the rated electrical frequency of the generator  14 . 
         [0036]    The method  200  in some embodiments includes the further steps or operation of collecting (Block  210 ) the electricity output from the AC-to-AC converter used in Block  208  to obtain the electricity at the fixed low frequency, along with the electricity produced from any like converters  18  associated with other wind turbines  8  in the offshore wind farm, via a low-frequency offshore collection grid  20 , and stepping up (Block  212 ) the voltage of the electricity output from the low-frequency offshore collection grid  20 , for transmission to onshore equipment  36  via a low-frequency high-voltage transmission system  32 . The onshore equipment  36  provides whatever frequency and/or voltage adjustments are required with respect to the onshore electric grid  38 . 
         [0037]    Referring back to  FIG. 1  momentarily, the wind turbines  8  may be grouped and connected to different feeders  22  of the low-frequency offshore collection grid  20 . In embodiments where each arrangement  10  includes a step-up transformer  16  between the generator  14  and the AC-to-AC converter  18 , the output of the wind turbine  8  associated with each such arrangement  10  is made to “match” the desired voltage and frequency of the collection grid  20 . In other words, the variable frequency and variable voltage output of each generator  14 , which operates under varying wind speeds, is transformed to the rated frequency and rated voltage of the low-frequency offshore collection grid  20 —e.g., an rated frequency of 20 Hz and a rated voltage of 33 KV. Advantageously, then, such arrangements  10  allow multiple wind turbines  8  to be connected in parallel to a given feeder  22 . A feeder  22  operated at, say 33 KV, may transfer 30-50 MW of electric power economically. In a contemplated example, as many as ten wind turbines  8  are associated with a given feeder  22 , each having a rated capacity of 5 MW, with additional feeders  22  obtaining electricity from further pluralities of wind turbines  8 . The electricity is “collected” in parallel on each such feeder  22  and aggregated at the substation  30 . 
         [0038]    As non-limiting examples of other contemplated architectures,  FIGS. 3A-3C  illustrate various cluster-based collection architectures that are implemented by the low-frequency offshore collection grid  20  in various different embodiments. To appreciate these configurations, consider a system  40  in which generators  14  in the plurality of arrangements  10  are configured to output electricity in a voltage range of, say, 6.6 KV to 13.8 KV. Of course, higher output voltages may be configured, too. At such voltages, it is economical to couple the output of each generator  14  to the AC-to-AC converter  18  in the same arrangement  10 , without use of the intervening step-up transformer  16 . 
         [0039]    The cluster-based collection architectures of  FIGS. 3A-3C  are particularly interesting in such cases. For example,  FIG. 3A  illustrates an example cluster comprising eight arrangements  10 —each associated with a wind turbine  8 —that are connected to one cluster platform substation  30  directly. There may be multiple such clusters in the low-frequency offshore collection grid  20 . 
         [0040]      FIG. 3B  illustrates a similar cluster, but one in which nine arrangements  10  are included in the cluster, by virtue of including a wind turbine  8  and corresponding arrangement  10  directly on the same platform as used to support the substation  30 .  FIG. 3C  provides yet another variation in which fifteen wind turbines  8 —not explicitly shown—have their respective arrangements  10  connected to one cluster platform substation  30  directly, or via short feeders. 
         [0041]    Cluster collection of wind turbines may be more suitable for medium sized wind farms. The collected wind powers are aggregated at the cluster platform substation. Step-up transformers—e.g., a step-up transformer  28  acting as a common step-up transformer for the cluster—are used to boost the voltage of the low-frequency offshore collection grid  20  to a higher, transmission voltage, for transmission to onshore equipment  36 . 
         [0042]      FIG. 4  illustrates another variant of the cluster architecture, in which each arrangement  10  omits the AC-to-AC converter  18 , and AC-to-AC conversion to the fixed low-frequency is instead handled by one or more AC-to-AC converters  50  that are centrally located, preferably on the same platform used to support the substation  30 . Note that in the cluster-based architecture, the protective device  24  corresponding to a faulty wind turbine  8  or to a faulty arrangement  10  may be used to disconnect from the affected arrangement  10 . 
         [0043]    Notably, modifications and other embodiments of the disclosed invention(s) will come to mind to one skilled in the art having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the invention(s) is/are not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of this disclosure. Although specific terms may be employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.