Patent Publication Number: US-2023147903-A1

Title: Wind turbine and method of operating a wind turbine

Description:
The present disclosure relates generally to wind turbines and, more particularly, to method of operating a wind turbine and apparatus for operating a wind turbine. 
     BACKGROUND 
     Generally, a wind turbine includes a turbine that has a rotor that includes a rotatable hub assembly having multiple blades. The blades transform wind energy into a mechanical rotational torque that drives one or more generators via the rotor. The generators are sometimes, but not always, rotationally coupled to the rotor through a gearbox. The gearbox steps up the inherently low rotational speed of the rotor for the generator to efficiently convert the rotational mechanical energy to electrical energy, which is fed into a utility grid via at least one electrical connection. Gearless direct drive wind turbines also exist. The rotor, generator, gearbox and other components are typically mounted within a housing, or nacelle, that is positioned on top of a base that may be a truss or tubular tower. 
     Some wind turbine configurations include double-fed induction generators (DFIGs). Such configurations may also include power converters that are used to convert a frequency of generated electric power to a frequency substantially similar to a utility grid frequency. Moreover, such converters, in conjunction with the DFIG, also transmit electric power between the utility grid and the generator as well as transmit generator excitation power to a wound generator rotor from one of the connections to the electric utility grid connection. Alternatively, some wind turbine configurations include, but are not limited to, alternative types of induction generators, permanent magnet (PM) synchronous generators and electrically-excited synchronous generators and switched reluctance generators. These alternative configurations may also include power converters that are used to convert the frequencies as described above and transmit electrical power between the utility grid and the generator. 
     Known wind turbines have a plurality of mechanical and electrical components. Each electrical and/or mechanical component may have independent and/or different operating limitations, such as current, voltage, power, temperature and other limits, than other components. Moreover, known wind turbines typically are designed and/or assembled with predefined limits. To operate within such limits, the electrical and/or mechanical components may be operated based on certain decisions. Depending on the decisions, less or more desirable outcomes may result. The subject-matter described herein, is intended to address at least some of the challenges of working with operating limits for achieving desirable outcomes. 
     BRIEF DESCRIPTION 
     Aspects and advantages of the invention will be set forth in part in the following description, or may be obvious from the description, or may be learned through practice of the invention. 
     According to an aspect, there is provided a method of operating a wind turbine, the wind turbine including a double-fed induction generator having a generator stator and a generator rotor, a power conversion assembly including a rotor-side power converter and a line-side power converter, and a load, wherein the generator rotor is electrically coupled to the rotor-side power converter, wherein the line-side power converter and the load are electrically coupled to a grid via a low voltage winding of a transformer, and wherein a first sum of a rated current of the generator rotor and a rated current of the load is more than an upper current limit of the low voltage winding, the method including: when operating the generator at a sub-synchronous speed, providing a first current to the generator rotor, the first current being less than a normal generator rotor current, and providing a second current to the load, wherein a second sum of the first current and the second current is equal to or less than the upper current limit of the low voltage winding. 
     According to a further aspect, there is provided a method of operating a wind turbine, the wind turbine including a blade, a blade heating system, a double-fed induction generator having a generator stator and a generator rotor, a power conversion assembly including a rotor-side power converter and a line-side power converter, and a load, wherein the generator rotor is electrically coupled to the rotor-side power converter, wherein the line-side power converter and the load are electrically coupled to a grid via a low voltage winding of a transformer, and wherein a first sum of a rated current of the generator rotor and a rated current of the load is more than an upper current limit of the low voltage winding, the method including: determining ice is on the blade, determining a current wind speed, when the determined current wind speed is less than a cut-in wind speed, providing, to the blade heating system, a current up to a rated current of the blade heating system, and when the determined current wind speed is equal to or more than the cut-in wind speed and when the generator is at sub-synchronous speed, providing, to the blade heating system, a current up to a lower one of an unused current capacity of the low voltage winding and the rated current of the blade heating system. 
     According to another aspect, there is provided a wind turbine including: a blade, a double-fed induction generator having a generator stator and a generator rotor, a power conversion assembly including a rotor-side power converter and a line-side power converter, a load, and a controller, wherein the generator rotor is electrically coupled to the rotor-side power converter, wherein the line-side power converter and the load are electrically coupled to a grid via a low voltage winding of a transformer, and wherein a first sum of a rated current of the generator rotor and a rated current of the load is more than an upper current limit of the low voltage winding, wherein the controller is configured to perform the following: when the generator is at sub-synchronous speed, provide, by the rotor-side power converter, a first current to the generator rotor, the first current being less than a normal generator rotor current, and provide a second current to the load, wherein a second sum of the first current and the second current is equal to or less than the upper current limit of the low voltage winding. 
     According to yet further aspect, there is provided a computer-readable storage medium including instructions which, when executed by a computer, cause the computer to carry out the method according to an aspect described herein. 
     These and other aspects, embodiments, examples and advantages of the present invention will become better understood with reference to the following description and appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A full and enabling disclosure, including the best mode thereof, directed to one of ordinary skill in the art, is set forth in the specification, which makes reference to the appended figures, in which: 
         FIG.  1    is a perspective view of a wind turbine according to embodiments described herein; 
         FIG.  2    is a schematic view of an electrical and control system of a wind turbine according to embodiments described herein; 
         FIG.  3    is a block diagram of a method of operating a wind turbine according to an aspect described herein; 
         FIG.  4    is a block diagram of a method of operating a wind turbine according to embodiments described herein; 
         FIG.  5    is a block diagram of a method of operating a wind turbine according to embodiments described herein; 
         FIG.  6    is a block diagram of a method of operating a wind turbine according to embodiments described herein; 
         FIG.  7    is a block diagram of a method of operating a wind turbine according to embodiments described herein; 
         FIG.  8    is a block diagram of a method of operating a wind turbine according to embodiments described herein; 
         FIG.  9    is a block diagram of a method of operating a wind turbine according to embodiments described herein; 
         FIG.  10    is a block diagram of a method of operating a wind turbine according to embodiments described herein; 
         FIG.  11    is a block diagram of a method of operating a wind turbine according to embodiments described herein; and 
         FIG.  12    is a block diagram of a method of operating a wind turbine according to embodiments described herein. 
     
    
    
     DETAILED DESCRIPTION 
     Reference now will be made in detail to embodiments, one or more examples of which are illustrated in the drawings. Each example is provided for explaining the present disclosure. Features illustrated or described as part of one example or embodiment can be used with another example or embodiment. 
     As discussed above, the present disclosure relates to wind turbines and, more particularly, to method of operating a wind turbine and apparatus for operating a wind turbine. In general, any component in an electrical system of a wind turbine may be a bottleneck or limiting component. 
     For example, the slip ring of the generator may be a limiting component, for example, in terms of maximum current or power that can be drawn or provided by the electrical system. In another example, a transformer may be a limiting component, for example, a transformer between a distribution or collection grid and an individual wind turbine (or a group of wind turbines). For example, when current or power demands exceed a transformer rating, the demanded current or power cannot be drawn or provided. 
     Generally, more than one electrical system or load can be electrically coupled to the same winding of the transformer. For example, rotor windings of a double fed induction generator (DFIG) may be electrically coupled to the same low voltage winding of the transformer as an electrical load, such as a yaw system or an ice management system. When wind speeds are low, the rotor windings may consume power. When conditions are icy, the ice management system, such as a blade heating system may also consume power. 
     Accordingly, demanded current or power through the low voltage winding of the transformer may exceed a transformer rating, e.g. a maximum current rating of the low voltage winding. 
     Accordingly, a transformer may be a bottleneck or limiting component, for example, in terms of maximum current or power that can be drawn by or provided to rotor windings and blade heating system at the same time. 
     For addressing such a bottleneck, the transformer may be specified or upgraded appropriately. In case the transformer is specified or upgraded such as to simultaneously handle a maximum current or power demand of all power consumers, e.g. generator rotor and ice management system, such a transformer would generally be very large and expensive, that is generally space- and cost-ineffective. 
     In general, some components are more costly or more difficult to up-spec (up-rate) or upgrade than others. In the above examples, the transformer would generally be a more expensive component to upgrade than a slip ring, and/or more difficult to upgrade, for example, when the supporting frame has limited space or when available cooling is limited. 
     As an example, a transformer system typically involves a plurality of aspects such as grid side interface, impedance, arc flash, protection coordination, busbar, main fuse, and cooling, which contribute to the (cascading) cost and complexity of up-specifying or up-rating. As described herein, the transformer may be understood as the main transformer. 
     Accordingly, it is advantageous if the effectiveness of the system can be improved without involving an overly large and/or expensive transformer. In general, current or power is drawn by or provided to the rotor windings during sub-synchronous speed operation of the generator. Generally, a normal generator rotor current setpoint is set such as to maximize power production (for a current wind speed) within other considerations, such as grid side demands, mechanical loading limit(s), and/or (pre-determined) acoustic noise generation limit(s). 
     In general, a current or power demand of a load such a yaw system or an ice management system can be substantial. In particular, longer blades and/or harsher ambient conditions imposes increasing added load demands. Additionally, the current or power demands may be transitional and/or time-sensitive. 
     Accordingly, when a load and rotor windings are electrically coupled to the same low voltage winding of the transformer, and a total current demand of the rotor windings and the load exceeds a current rating of the low voltage winding, a current provided to the rotor windings may be reduced, in order to provide a demanded current to the load, such as to operate near or at an upper current limit of the low voltage winding. 
     In view of the above, it can be understood that generally there is a large number of operational conditions, where for each operational condition, there is a respective limiting condition, and for each limiting condition, a respective limiting component. Accordingly, the present disclosure provides beneficial insight, for improving the effectiveness of the system, to address the transformer as a limiting component. In particular, it is identified to address the low voltage winding of the transformer that is electrically coupled to rotor windings and a load. 
     The present disclosure provides further beneficial insight, for improving the effectiveness of the system, to control the current or power supplied to the rotor windings and the load, such as to operate within the upper current limit of the low voltage winding, thus improving an effectiveness of the system without involving an overly large and/or expensive transformer. The advantages are provided for both retrofit and new build applications. 
     In view of the above, the methods and apparatuses according to aspects, examples and embodiments described or illustrated herein enables the person of ordinary skill in the art to achieve at least some of the above described advantages. 
     Wind Turbine 
       FIG.  1    is a perspective view of a wind turbine  100  according to embodiments described herein. Wind turbine  100  includes a nacelle  102  housing a generator (not shown in  FIG.  1   ). Nacelle  102  is mounted on a tower  104  (a portion of tower  104  being shown in  FIG.  1   ). Tower  104  may have any suitable height that facilitates operation of wind turbine  100  as described herein. Wind turbine  100  also includes a rotor  106 . The rotor  106  may include three blades  108  attached to a rotating hub  110 . Alternatively, wind turbine  100  may include any number of blades  108  that facilitates operation of wind turbine  100  as described herein. The wind turbine  100  may include a gearbox (not shown in  FIG.  1   ) operatively coupled to rotor  106  and a generator (not shown in  FIG.  1   ). 
     According to an aspect, a wind turbine  100  includes a blade  108 , a double-fed induction generator  118  having a generator stator  120  and a generator rotor  122 , a power conversion assembly  210  including a rotor-side power converter  220  and a line-side power converter  222 , a load  340 , and a controller, wherein the generator rotor  122  is electrically coupled to the rotor-side power converter  220 , wherein the line-side power converter  222  and the load  340  are electrically coupled to a grid  390  via a low voltage winding  310  of a transformer  234 , and wherein a first sum of a rated current of the generator rotor  122  and a rated current of the load  340  is more than an upper current limit of the low voltage winding  310 . 
     According to the aspect, the controller is configured to perform the following: when the generator  118  is at sub-synchronous speed, provide, by the rotor-side power converter  220 , a first current to the generator rotor  122 , the first current being less than a normal generator rotor current, and provide a second current to the load  340 , wherein a second sum of the first current and the second current is equal to or less than the upper current limit of the low voltage winding  310 . 
     According to an aspect, a computer-readable storage medium comprising instructions which, when executed by a computer, cause the computer to carry out a method according to aspects, embodiments and examples described herein. 
     Electrical and Control System 
       FIG.  2    is a schematic view of an electrical and control system  200  that may be used with wind turbine  100  according to embodiments described herein. According to embodiments, the rotor  106  includes blades  108  coupled to hub  110 . According to embodiments, rotor  106  includes a low-speed shaft  112  rotatably coupled to hub  110 . According to embodiments, low-speed shaft  112  is coupled to a step-up gearbox  114  that is configured to step up the rotational speed of low-speed shaft  112  and transfer that speed to a high-speed shaft  116 . 
     In an example, gearbox  114  has a step-up ratio of approximately 70:1. In an example, low-speed shaft  112  rotating at approximately 20 revolutions per minute (rpm) coupled to gearbox  114  with an approximately 70:1 step-up ratio generates a speed for high-speed shaft  116  of approximately 1400 rpm. Alternatively, gearbox  114  may have any suitable step-up ratio that facilitates operation of wind turbine  100  as described herein. As a further alternative, wind turbine  100  may include a direct-drive generator that is rotatably coupled to rotor  106  without any intervening gearbox. 
     According to embodiments, high-speed shaft  116  is rotatably coupled to generator  118 . The generator  118  is double-fed induction generator (DFIG) includes a generator stator  120  and a generator rotor  122 . According to embodiments, generator  118  is a wound rotor, three-phase, asynchronous generator where the generator stator  120  is magnetically coupled to a generator rotor  122 . It can be understood that a generator rotor can generally have a plurality of permanent magnets in place of rotor windings. 
     According to embodiments, electrical and control system  200  includes a turbine controller  202 . Turbine controller  202  may include at least one processor and a memory, at least one processor input channel, at least one processor output channel, and/or at least one computer (none shown in  FIG.  2   ). As used herein, the term computer is not limited to integrated circuits referred to in the art as a computer, but broadly refers to a processor, a microcontroller, a microcomputer, a programmable logic controller (PLC), an application specific integrated circuit, and other programmable circuits (none shown in  FIG.  2   ), and these terms are used interchangeably herein. 
     According to embodiments, memory may include, but is not limited to, a computer-readable medium, such as a random access memory (RAM) (none shown in  FIG.  2   ). In an example, one or more storage devices, such as a floppy disk, a compact disc read only memory (CD-ROM), a magneto-optical disk (MOD), and/or a digital versatile disc (DVD) (none shown in  FIG.  2   ) may be used. In an example, additional input channels (not shown in  FIG.  2   ) may be, but are not limited to, computer peripherals associated with an operator interface such as a mouse and a keyboard (neither shown in  FIG.  2   ). In an example, additional output channels may include, but are not limited to, an operator interface monitor (not shown in  FIG.  2   ). 
     According to embodiments, processors for turbine controller  202  process information transmitted from a plurality of electrical and electronic devices that may include, but are not limited to, voltage and current transducers. In an example, RAM and/or storage devices store and transfer information and instructions to be executed by the processor. In an example, RAM and/or storage devices can be used to store and provide temporary variables, static (i.e., non-changing) information and instructions, or other intermediate information to the processors during execution of instructions by the processors. In an example, instructions that are executed include, but are not limited to, resident conversion and/or comparator algorithms. In an example, the execution of sequences of instructions is not limited to any specific combination of hardware circuitry and software instructions. 
     According to embodiments, generator stator  120  is electrically coupled to a stator synchronizing switch  206  via a stator bus  208 . In an example, to facilitate the DFIG configuration, generator rotor  122  is electrically coupled to a bi-directional power conversion assembly  210  via a rotor bus  212 . In an example, generator rotor  122  is electrically coupled to rotor bus  212  via any other device that facilitates operation of electrical and control system  200  as described herein. In an example, electrical and control system  200  is configured as a full power conversion system (not shown) that includes a full power conversion assembly (not shown in  FIG.  2   ) similar in design and operation to power conversion assembly  210  and electrically coupled to generator stator  120 . The full power conversion assembly may facilitate channeling electric power between generator stator  120  and an electric power transmission and distribution grid (not shown). 
     According to embodiments, stator bus  208  transmits three-phase power from generator stator  120  to stator synchronizing switch  206 . In an example, rotor bus  212  transmits three-phase power from generator rotor  122  to power conversion assembly  210 . In an example, stator synchronizing switch  206  is electrically coupled to a first main transformer circuit breaker  380  via a stator side system bus  384 . In an example, one or more fuses (not shown) are used instead of first main transformer circuit breaker  380 . In an example, neither fuses nor first main transformer circuit breaker  380  is used. 
     According to embodiments, power conversion assembly  210  includes a rotor filter  218  that is electrically coupled to generator rotor  122  via rotor bus  212 . In an example, rotor filter bus  219  electrically couples rotor filter  218  to a rotor-side power converter  220 . In an example, rotor-side power converter  220  is electrically coupled to line-side power converter  222 . In an example, rotor-side power converter  220  and line-side power converter  222  are power converter bridges including power semiconductors (not shown). In an example, rotor-side power converter  220  and line-side power converter  222  are configured in a three-phase, pulse width modulation (PWM) configuration including insulated gate bipolar transistor (IGBT) switching devices (not shown in  FIG.  2   ). In an example, rotor-side power converter  220  and line-side power converter  222  have any configuration using any switching devices that facilitate operation of electrical and control system  200  as described herein. In an example, power conversion assembly  210  is coupled in electronic data communication with turbine controller  202  to control the operation of rotor-side power converter  220  and line-side power converter  222 . 
     According to embodiments, a line-side power converter bus  223  electrically couples line-side power converter  222  to a line filter  224 . In an example, a line bus  225  electrically couples line filter  224  to a line contactor  226 . In an example, line contactor  226  is electrically coupled to a conversion circuit breaker  228  via a conversion circuit breaker bus  230 . In an example, conversion circuit breaker  228  is electrically coupled to a second main transformer circuit breaker  320  via a second system bus  324  and a connection bus  232 . In an example, line filter  224  is electrically coupled to rotor side system bus  324  directly via connection bus  232  and includes a protection scheme (not shown) configured to account for removal of line contactor  226  and conversion circuit breaker  228  from electrical and control system  200 . 
     A transformer  234  or main transformer  234  or electric power main transformer  234  is provided. In an example, the transformer  234  is configured to provide a step-change of voltage between voltage of the generator  118 , and voltage of the grid  390 . The term “grid  390 ” may be understood as a collection/distribution network/grid. 
     The transformer  234  includes a low voltage winding  310  or a first low voltage winding  310 . In an example, the transformer  234  includes a further low voltage winding  370  or a second low voltage winding  370 . In an example, the low voltage winding  310  is arranged. In an example, rotor side main transformer circuit breaker  320  is electrically coupled to the electric power main transformer  234  via a rotor side generator side bus  322 . In an example, stator side main transformer circuit breaker  380  is electrically coupled to the electric power main transformer  234  via a stator side generator side bus  382 . 
     According to embodiments, main transformer  234  is electrically coupled to a grid circuit breaker  238  via a breaker-side bus  240 . In an example, grid circuit breaker  238  is connected to the electric power transmission and distribution grid via a grid bus  242 . In an example, main transformer  234  is electrically coupled to one or more fuses (not shown), rather than to grid circuit breaker  238 , via breaker-side bus  240 . In an example, neither fuses nor grid circuit breaker  238  is used and main transformer  234  is coupled to the electric power transmission and distribution grid via breaker-side bus  240  and grid bus  242 . 
     According to embodiments, rotor-side power converter  220  is coupled in electrical communication with line-side power converter  222  via a single direct current (DC) link  244 . In an example, rotor-side power converter  220  and line-side power converter  222  are electrically coupled via individual and separate DC links (not shown in  FIG.  2   ). In an example, DC link  244  includes a positive rail  246 , a negative rail  248 , and at least one capacitor  250  coupled between positive rail  246  and negative rail  248 . In an example, capacitor  250  includes one or more capacitors configured in series and/or in parallel between positive rail  246  and negative rail  248 . 
     According to embodiments, turbine controller  202  is configured to receive a plurality of voltage and electric current measurement signals from a first set of voltage and electric current sensors  252 . In an example, turbine controller  202  is configured to monitor and control at least some of the operational variables associated with wind turbine  100 . In an example, each of three voltage and electric current sensors  252  are electrically coupled to each one of the three phases of grid bus  242 . In an example, voltage and electric current sensors are electrically coupled to stator side system bus  384  and/or rotor side system bus  324 . In an example, voltage and electric current sensors are electrically coupled to any portion of electrical and control system  200  that facilitates operation of electrical and control system  200  as described herein. In an example, turbine controller  202  is configured to receive any number of voltage and electric current measurement signals from any number of voltage and electric current sensors including, but not limited to, one voltage and electric current measurement signal from one transducer. 
     As shown in  FIG.  2   , electrical and control system  200  may include a converter controller  262  that is configured to receive a plurality of voltage and electric current measurement signals. According to embodiments, converter controller  262  receives voltage and electric current measurement signals from a second set of voltage and electric current sensors  254  coupled in electronic data communication with stator bus  208 . In an example, converter controller  262  receives a third set of voltage and electric current measurement signals from a third set of voltage and electric current sensors  256  coupled in electronic data communication with rotor bus  212 . In an example, converter controller  262  receives a fourth set of voltage and electric current measurement signals from a fourth set of voltage and electric current sensors  264  coupled in electronic data communication with conversion circuit breaker bus  230 . In an example, second set of voltage and electric current sensors  254  is substantially similar to first set of voltage and electric current sensors  252 . In an example, fourth set of voltage and electric current sensors  264  is substantially similar to third set of voltage and electric current sensors  256 . In an example, converter controller  262  is substantially similar to turbine controller  202  and is coupled in electronic data communication with turbine controller  202 . In an example, converter controller  262  is physically integrated within power conversion assembly  210 . In an example, converter controller  262  has a configuration that facilitates operation of electrical and control system  200  as described herein. 
     During operation, wind impacts blades  108  and blades  108  transform wind energy into a mechanical rotational torque. According to embodiments, mechanical rotational torque rotatably drives low-speed shaft  112  via hub  110 . In an example, low-speed shaft  112  drives gearbox  114  that subsequently steps up the low rotational speed of low-speed shaft  112  to drive high-speed shaft  116  at an increased rotational speed. In an example, high speed shaft  116  rotatably drives generator rotor  122 . In an example, a rotating magnetic field is induced by generator rotor  122  and a voltage is induced within generator stator  120  that is magnetically coupled to generator rotor  122 . 
     According to embodiments, generator  118  converts the rotational mechanical energy to a sinusoidal, three-phase alternating current (AC) electrical energy signal in generator stator  120 . In an example, electrical power is transmitted to main transformer  234  via stator bus  208 , stator synchronizing switch  206 , stator side system bus  384 , stator side main transformer circuit breaker  380  and stator side generator side bus  382 . In an example, main transformer  234  steps up the voltage amplitude of the electrical power and the transformed electrical power is transmitted to a grid via breaker-side bus  240 , grid circuit breaker  238  and grid bus  242 . 
     According to embodiments, a second electrical power transmission path is provided. In an example, electrical, three-phase, sinusoidal, AC power is generated within generator rotor  122  and is transmitted to power conversion assembly  210  via rotor bus  212 . In an example, within power conversion assembly  210 , the electrical power is transmitted to rotor filter  218  and the electrical power is modified for the rate of change of the PWM signals associated with rotor-side power converter  220 . In an example, rotor-side power converter  220  acts as a rectifier and rectifies the sinusoidal, three-phase AC power to DC power. In an example, DC power is transmitted into DC link  244 . In an example, capacitor  250  facilitates mitigating DC link  244  voltage amplitude variations by facilitating mitigation of a DC ripple associated with AC rectification. 
     According to embodiments, DC power is transmitted from DC link  244  to line-side power converter  222 . In an example, line-side power converter  222  acts as an inverter configured to convert the DC electrical power from DC link  244  to three-phase, sinusoidal AC electrical power with pre-determined voltages, currents, and frequencies. In an example, power conversion is monitored and controlled via converter controller  262 . In an example, converted AC power is transmitted from line-side power converter  222  to rotor side system bus  324  via line-side power converter bus  223  and line bus  225 , line contactor  226 , conversion circuit breaker bus  230 , conversion circuit breaker  228 , and connection bus  232 . In an example, line filter  224  compensates or adjusts for harmonic currents in the electric power transmitted from line-side power converter  222 . In an example, stator synchronizing switch  206  is configured to close to facilitate connecting the three-phase power from generator stator  120  with the three-phase power from power conversion assembly  210 . 
     According to embodiments, conversion circuit breaker  228 , first main transformer circuit breaker  380 , second main transformer circuit breaker  320 , and grid circuit breaker  238  are configured to disconnect corresponding buses, for example, when excessive current flow may damage the components of electrical and control system  200 . In an example, additional protection components are provided including line contactor  226 , which may be controlled to form a disconnect by opening a switch (not shown in  FIG.  2   ) corresponding to each line of line bus  225 . 
     According to embodiments, power conversion assembly  210  compensates or adjusts the frequency of the three-phase power from generator rotor  122  for changes, for example, in the wind speed at hub  110  and blades  108 . Accordingly, mechanical and electrical rotor frequencies can be decoupled from stator frequency. 
     According to embodiments, the bi-directional characteristics of power conversion assembly  210 , and specifically, the bi-directional characteristics of rotor-side power converter  220  and line-side power converter  222 , facilitate feeding back at least some of the generated electrical power into generator rotor  122 . In an example, electrical power is transmitted from rotor side system bus  324  to connection bus  232 . In an example, electrical power is transmitted through conversion circuit breaker  228  and conversion circuit breaker bus  230  into power conversion assembly  210 . 
     According to embodiments, electrical power is transmitted through line contactor  226 , line bus  225 , and line-side power converter bus  223  into line-side power converter  222 . In an example, line-side power converter  222  acts as a rectifier and rectifies the sinusoidal, three-phase AC power to DC power. In an example, DC power is transmitted into DC link  244 . In an example, capacitor  250  facilitates mitigating DC link  244  voltage amplitude variations by facilitating mitigation of a DC ripple sometimes associated with three-phase AC rectification. 
     According to embodiments, DC power is transmitted from DC link  244  to rotor-side power converter  220  and rotor-side power converter  220  acts as an inverter configured to convert the DC electrical power transmitted from DC link  244  to a three-phase, sinusoidal AC electrical power with pre-determined voltages, currents, and frequencies. In an example, power conversion is monitored and controlled via converter controller  262 . In an example, converted AC power is transmitted from rotor-side power converter  220  to rotor filter  218  via rotor filter bus  219 . In an example, converted AC power is transmitted to generator rotor  122  via rotor bus  212 , thereby facilitating sub-synchronous operation. 
     According to embodiments, power conversion assembly  210  is configured to receive control signals from turbine controller  202 . In an example, control signals are based on sensed conditions or operating characteristics of wind turbine  100  and electrical and control system  200 . In an example, control signals are received by turbine controller  202  and used to control operation of power conversion assembly  210 . Feedback from one or more sensors may be used by electrical and control system  200  to control power conversion assembly  210  via converter controller  262  including, for example, conversion circuit breaker bus  230 , stator bus and rotor bus voltages or current feedbacks via second set of voltage and electric current sensors  254 , third set of voltage and electric current sensors  256 , and fourth set of voltage and electric current sensors  264 . Using this feedback information, and for example, switching control signals, stator synchronizing switch control signals and system circuit breaker control (trip) signals may be generated in any known manner. 
     According to embodiments, for a grid voltage transient with predetermined characteristics, converter controller  262  will at least temporarily substantially suspend the IGBTs from conducting within line-side power converter  222 . Such suspension of operation of line-side power converter  222  can substantially mitigate electric power being channeled through power conversion assembly  210  to approximately zero. 
     Low Voltage Winding 
     According to an aspect, the line-side power converter  222  and the load  340  are electrically coupled to a grid  390  via a low voltage winding  310  of a transformer  234 . In an example, the low voltage winding  310  is a first low voltage winding  310 . In an example, the low voltage winding  310  of the transformer  234  is a first low voltage winding  310  of a plurality of low voltage windings of the transformer  234 . 
     According to embodiments, the generator stator  120  is electrically coupled to the grid  390  via a further low voltage winding  370  or a second low voltage winding  370  of the transformer  234 . In an example, the further low voltage winding  370  or second low voltage winding  370  is different from the low voltage winding  310  or first low voltage winding  310 . 
     According to embodiments, the transformer  234  includes the low voltage winding  310  (or first low voltage winding  310 ) and the further low voltage winding  370  (or second low voltage winding  370 ). In an example, the (main) transformer  234  includes the low voltage winding  310  (or first low voltage winding  310 ), and an auxiliary transformer (not shown) includes the further low voltage winding  370  (or second low voltage winding  370 ). In another example, the auxiliary transformer (not shown) includes the low voltage winding  310  and the main transformer  234  includes the further low voltage winding  370 . 
     Generator Rotor 
     According to an aspect, the double-fed induction generator  118  includes a generator stator and a generator rotor  122 . According to an aspect, generator rotor  122  is electrically coupled to rotor-side power converter  220 . According to an aspect, power conversion assembly  210  includes the rotor-side power converter  220  and line-side power converter  222 . According to an aspect, line-side power converter  222  and load  340  are electrically coupled to low voltage winding  310 . 
     According to embodiments, the generator rotor has a rated current. In an example, a controller, e.g. generator controller or turbine controller, is configured to control a current provided to the generator rotor, e.g. when the generator is at sub-synchronous speed. 
     Loads 
     According to embodiments, the load  340  includes a blade heating system  360 . In an example, the blade heating system  360  is part of an ice management system. In an example, the blade heating system  360  includes one or more heaters arranged to heat a respective blade  108  of the wind turbine  100 . In an example, the blade heating system  360  includes at least one heater arranged on each blade  108  of the wind turbine  100 . 
     According to embodiments, the blade heating system  360  is electrically coupled to the low voltage winding  310  via switchgear equipment  352  and/or slip ring. In an example, the switchgear equipment  352  for the load  340  includes at least one of fuse module, lock-out-tag-out box, protection switchgear, and current transformer. 
     According to embodiments, one or more further loads (not shown) is/are electrically coupled to the grid  390  via the low voltage winding  310 . One or more further loads can include for example, yaw system, gearbox system, pitch system, lubrication system, cooling system, electrical receptacles and lights, heaters and miscellaneous equipment. In an example, a further load, in particular, a yaw system, is electrically coupled to the grid  390  via the low voltage winding  310 . In an example, the wind turbine  100  includes one or more of a yaw system, gearbox system, pitch system, lubrication system, cooling system, electrical receptacles and lights, heaters and miscellaneous equipment. 
     According to embodiments, the load  340  is electrically coupled to the grid  390  via a first tap on the low voltage winding  310 . In an example, the one or more further loads (not shown) is/are electrically coupled to the grid  390  via a further tap on the low voltage winding  310 . In an example, the further tap is different from the first tap. In an example, the load  340  is supplied power via the low voltage winding  310  at a first voltage. In an example, the one or more further loads is/are supplied power via the low voltage winding  310  at a second voltage. In an example, the first voltage and the second voltage are different. For illustrative purpose, the first voltage can be 690 V and the second voltage can be 400 V. 
     Normal Generator Rotor Current 
     According to embodiments, the normal generator rotor current is a generator rotor current for operating at one of a plurality of upper limit portions, or at a lowest upper limit portion of the plurality of upper limit portions, wherein the plurality of upper limit portions comprises an upper limit portion of possible power production of the wind turbine  100  at the current wind speed, an upper limit portion of mechanical loading of the wind turbine  100  and an upper limit portion of acoustic noise generation for the wind turbine  100 . 
     According to embodiments, the normal generator rotor current is a generator rotor current for operating at an upper limit portion of possible power production of the wind turbine  100 . An upper limit portion of possible power production of the wind turbine  100  may be understood to be for a current wind speed. In an example, an upper limit of possible power production is a power production corresponding to a maximum power coefficient for a current wind speed. In an example, a current wind speed is a calculated wind speed or a measured wind speed, or a modelled wind speed, for a current time, or for most recent calculation or measurement of wind speed available. 
     According to embodiments, the normal generator rotor current is a generator rotor current for operating at an upper limit portion of mechanical loading of the wind turbine  100 . In an example, an upper limit of mechanical loading of the wind turbine  100  is based on a pre-determined and/or as-designed upper limit. 
     According to embodiments, the normal generator rotor current is a generator rotor current for operating at an upper limit portion of acoustic noise generation for the wind turbine  100 . In an example, an upper limit of acoustic noise generation of the wind turbine  100  is based on a pre-determined and/or as-designed upper limit (e.g. based on pre-determined regulation limits). 
     In an example, an upper limit portion is a portion including the upper limit. In an example, an upper limit portion is a portion from 90% of the upper limit and higher, from 95% of the upper limit and higher, or from 99% of the upper limit and higher. In an example, an upper limit portion is up to the upper limit. In an example, an upper limit portion is a portion from a first threshold and higher. In an example, the first threshold is a fixed percentage of the upper limit. In an example, the first threshold is based on a controller speed or a control margin. In an example, an upper limit portion is a portion within 10%, 5% or 1% of the upper limit. 
     In an example, the normal generator rotor current is a generator rotor current for operating at a setpoint, the setpoint being one of upper limit portion of possible power production of the wind turbine  100  at a current wind speed, mechanical loading of the wind turbine  100  and acoustic noise generation for the wind turbine  100 . 
     Upper Current Limit 
     According to embodiments, the low-voltage winding  310  includes an upper current limit. The upper current limit of the low voltage winding may be understood as a current capacity of the low voltage winding  310  or a maximum continuous current rating of the low voltage winding  310 . 
     In an example, a second sum of the first current (that is being provided to the generator rotor  122 ) and the second current (that is being provided to the load  340 ) is (selected such as to be) equal to or less than the upper current limit of the low voltage winding  310 . 
     According to embodiments, a difference between the second sum of the first current and the second current, and the upper current limit of the low voltage winding is less than 10% of the current capacity of the low voltage winding  310 , less than 5% of the current capacity of the low voltage winding, less than an operational buffer of current capacity. 
     According to embodiments, a third current through the low voltage winding  310 , the third current including the first current (that is being provided to the generator rotor  122 ) and the second current (that is being provided to the load  340 ) is (selected such as to be) substantially at the upper current limit of the low voltage winding  310 . The term ‘substantially’ may be understood as ‘within 10%’, ‘within 5%’, ‘within 2%’, ‘within a pre-determined margin’ or ‘within a fixed margin’. 
     Methods 
       FIG.  3    is a block diagram of a method of operating a wind turbine according to aspects described herein. According to the aspect seen in  FIG.  3   , the method of operating a wind turbine includes, when operating the generator  118  at a sub-synchronous speed, providing a first current  410  to the generator rotor  122 , the first current being less than a normal generator rotor current, and providing a second current  420  to the load  340 , wherein a second sum of the first current and the second current is equal to or less than the upper current limit of the low voltage winding  310 . 
     According to an aspect, wind turbine  100  includes a double-fed induction generator  118  having a generator stator  120  and a generator rotor  122 , a power conversion assembly  210  including a rotor-side power converter  220  and a line-side power converter  222 , and a load  340 . According to an aspect, the generator rotor  122  is electrically coupled to the rotor-side power converter  220 . According to an aspect, the line-side power converter  222  and the load  340  are electrically coupled to a grid  390  via a low voltage winding  310  of a transformer  234 . According to an aspect a first sum of a rated current of the generator rotor  122  and a rated current of the load  340  is more than an upper current limit of the low voltage winding  310 . 
     According to an aspect, a method of operating a wind turbine includes, when operating the generator  118  at a sub-synchronous speed, providing a first current  410  to the generator rotor  122 , the first current being less than a normal generator rotor current, and providing a second current  420  to the load  340 , wherein a second sum of the first current and the second current is equal to or less than the upper current limit of the low voltage winding  310 . 
     It may be understood that providing the first current  410  to the generator rotor  122  and providing the second current  420  to the load  340  may be performed at (substantially) the same time or concurrently. 
     It may be understood that a controller(s), such as (one or more of) a turbine controller, converter controller, power conversion assembly controller, generator controller, and/or wind farm controller may be configured to issue a setpoint to the power conversion assembly  210  (and/or to the rotor-side power converter  220 ) to provide a first current  410  to the generator rotor  122 . 
     It may be understood that a controller(s), such as (one or more of) a load controller, blade heating system controller, turbine controller, and/or wind farm controller may be configured (to control the load  340  or the blade heating system  360 ) to draw a second current  420 . 
       FIG.  4    is a block diagram of a method of operating a wind turbine according to embodiments described herein. According to the embodiment seen in  FIG.  4   , a method of operating a wind turbine includes drawing a third current  510  through the low-voltage winding  310 . 
     According to embodiments, a method of operating a wind turbine includes drawing a third current  510  through the low-voltage winding  310 , the third current including the first current and the second current, wherein the third current is substantially at the upper current limit of the low voltage winding  310 . 
     In an example, the second sum of the first current and the second current is at least 50%, at least 70%, or at least 90% of the third current. 
     It may be understood that drawing a third current  510  through the low-voltage winding  310  may be performed (or may be effected by a controller) concurrently with providing the first current  410  to the generator rotor  122  and providing the second current  420  to the load  340 . 
     It may be understood that a controller(s), such as (one or more of) a load controller, blade heating system controller, turbine controller, and/or wind farm controller may be configured (to control the load  340  or the blade heating system  360 ) to draw a third current  510  through the low-voltage winding  310 . 
       FIG.  5    is a block diagram of a method of operating a wind turbine according to embodiments described herein. According to the embodiment seen in  FIG.  5   , a method of operating a wind turbine includes determining  610  that setting a first generator rotor current setpoint to be the normal generator rotor current or setting a second load current setpoint to be the second current results in drawing a fourth current via the low voltage winding  310 , the fourth current being more than the upper current limit of the low voltage winding  310 . 
     According to the embodiment seen in  FIG.  5   , the method of operating a wind turbine includes determining  620  a first difference between the first generator rotor current setpoint and a preceding generator rotor current setpoint, or a second difference between the second load current setpoint and a preceding load current setpoint, is more than a third difference between the upper current limit of the low voltage winding  310  and a current already being drawn via the low voltage winding  310 . 
     According to embodiments, a method of operating a wind turbine includes, prior to providing the first current  410  to the generator rotor  122  and prior to providing the second current  420  to the load  340 , determining  610  that setting a first generator rotor current setpoint to be the normal generator rotor current or setting a second load current setpoint to be the second current results in drawing a fourth current via the low voltage winding  310 , the fourth current being more than the upper current limit of the low voltage winding  310 . 
     According to embodiments, a method of operating a wind turbine includes, determining  620  a first difference between the first generator rotor current setpoint and a preceding generator rotor current setpoint, or a second difference between the second load current setpoint and a preceding load current setpoint, is more than a third difference between the upper current limit of the low voltage winding  310  and a current already being drawn via the low voltage winding  310 . 
     It may be understood that a controller(s), such as (one or more of) a load controller, blade heating system controller, converter controller, power conversion assembly controller, generator controller, turbine controller, and/or wind farm controller may be configured to determine  610  that setting a first generator rotor current setpoint to be the normal generator rotor current or setting a second load current setpoint to be the second current results in drawing a fourth current via the low voltage winding  310 . 
     It may be understood that a controller(s), such as (one or more of) a load controller, blade heating system controller, converter controller, power conversion assembly controller, generator controller, turbine controller, and/or wind farm controller may be configured to determine  620  a first difference between the first generator rotor current setpoint and a preceding generator rotor current setpoint, or a second difference between the second load current setpoint and a preceding load current setpoint, is more than a third difference between the upper current limit of the low voltage winding  310  and a current already being drawn via the low voltage winding  310 . 
       FIG.  6    is a block diagram of a method of operating a wind turbine according to embodiments described herein. According to the embodiment seen in  FIG.  6   , a method of operating a wind turbine includes determining  710  that the normal generator rotor current is already being provided to the generator rotor  122  or that the second current is already being provided to the load  340 . 
     According to embodiments, a method of operating a wind turbine includes, prior to providing the first current  410  to the generator rotor  122  and prior to providing the second current  420  to the load  340 , determining  710  that the normal generator rotor current is already being provided to the generator rotor  122  or that the second current is already being provided to the load  340 . 
     It may be understood that a controller(s), such as (one or more of) a load controller, blade heating system controller, converter controller, power conversion assembly controller, generator controller, turbine controller, and/or wind farm controller may be configured to determine  710  that the normal generator rotor current is already being provided to the generator rotor  122  or that the second current is already being provided to the load  340 . 
       FIG.  7    is a block diagram of a method of operating a wind turbine according to embodiments described herein. According to the embodiment seen in  FIG.  7   , a method of operating a wind turbine includes issuing a torque setpoint  810  for operating the wind turbine  100  at a tip speed ratio that is lower than an optimum tip speed ratio. 
     According to embodiments, the optimum tip speed ratio is a tip speed ratio for operating at one of a plurality of upper limit portions, or at a lowest upper limit portion of the plurality of upper limit portions. According to embodiments, the plurality of upper limit portions includes an upper limit portion of possible power production of the wind turbine  100  at the current wind speed, an upper limit portion of mechanical loading of the wind turbine  100  and an upper limit portion of acoustic noise generation for the wind turbine. 
     It may be understood that a controller(s), such as (one or more of) a turbine controller, converter controller, power conversion assembly controller, generator controller, and/or wind farm controller may be configured to issue a torque setpoint  810  for operating the wind turbine  100  at a tip speed ratio that is lower than an optimum tip speed ratio. 
       FIG.  8    is a block diagram of a method of operating a wind turbine according to embodiments described herein. According to the embodiment seen in  FIG.  8   , a method of operating a wind turbine includes adjusting  910  the first current in inverse proportion to the second current. 
     It may be understood that a controller(s), such as (one or more of) a load controller, blade heating system controller, converter controller, power conversion assembly controller, generator controller, turbine controller, and/or wind farm controller may be configured to adjust  910  the first current in inverse proportion to the second current. 
     According to embodiments, a difference between the normal generator rotor current and the first current to the generator rotor  122  may be equal to or less than the rated current of the load  340 . Accordingly, maximum current drawable by the load is made available within an upper current limit of the low voltage winding and effectiveness of the system is thereby improved. 
     According to embodiments, the load  340  includes a blade heating system  360 . Accordingly, operation within an upper current limit of the low voltage winding is improved and effectiveness of the system is thereby improved. 
       FIG.  9    is a block diagram of a method of operating a wind turbine according to embodiments described herein. According to the embodiment seen in  FIG.  9   , a method of operating a wind turbine includes receiving a third request  1010  to increase a supply of current to the load  340 , and in response to receiving the third request, increasing an upper current limit  1020  of the low voltage winding  310 . 
     In an example, the third request  1010  to increase the supply of current to the load  340  is based on a decrease in ambient temperature. In an example, the third request  1010  to increase the supply of current to the load  340  includes a first amount of increase, the first amount of increase being in proportion to an amount of decrease in ambient temperature. 
     In an example, the increase in the upper current limit  1020  of the low voltage winding  310  is based on a decrease in ambient temperature. In an example, the increase in the upper current limit  1020  of the low voltage winding  310  includes a second amount of increase, the second amount of increase being in proportion to an amount of decrease in ambient temperature. 
     It may be understood that a controller(s), such as (one or more of) a load controller, blade heating system controller, converter controller, power conversion assembly controller, generator controller, turbine controller, and/or wind farm controller may be configured to receive a third request  1010  to increase a supply of current to the load  340 , and in response to receiving the third request, increasing an upper current limit  1020  of the low voltage winding  310 . 
     It may be understood that the upper current limit of the low voltage winding  310  may be a pre-determined value or (adjustable) constant. In an example, the upper current limit of the low voltage winding  310  is (a value) stored in or for a controller(s), such as (one or more of) a load controller, blade heating system controller, converter controller, power conversion assembly controller, generator controller, turbine controller, and/or wind farm controller. 
     Further Methods 
       FIG.  10    is a block diagram of a method of operating a wind turbine according to aspects described herein. According to the aspect seen in  FIG.  10   , the method of operating a wind turbine includes, determining ice is on the blade  1110 , determining a current wind speed  1120 , when the determined current wind speed is less than a cut-in wind speed, providing, to the blade heating system, a current up to a rated current of the blade heating system  1130 , and when the determined current wind speed is equal to or more than the cut-in wind speed and when the generator is at sub-synchronous speed, providing, to the blade heating system, a current up to a lower one of an unused current capacity of the low voltage winding and the rated current of the blade heating system  1140 . 
     According to the aspect, the wind turbine  100  includes a blade  108 , a blade heating system  360 , a double-fed induction generator  118  having a generator stator  120  and a generator rotor  122 , a power conversion assembly  210  including a rotor-side power converter  220  and a line-side power converter  222 , and a load  340 , wherein the generator rotor  122  is electrically coupled to the rotor-side power converter  220 , wherein the line-side power converter  222  and the load  340  are electrically coupled to a grid  390  via a low voltage winding  310  of a transformer  234 , and wherein a first sum of a rated current of the generator rotor  122  and a rated current of the load  340  is more than an upper current limit of the low voltage winding  310 . 
     In an example, determining ice is on the blade  1110  includes determining based on measurements from one or more of vibrating probe, weight sensor, optical sensor, and/or capacitive sensor. 
     In an example, a controller(s), such as (one or more of) a load controller, blade heating system controller, turbine controller, and/or wind farm controller may be configured to determine ice is on the blade  1110 . In an example, a controller(s), such as (one or more of) a turbine controller, generator controller, and/or wind farm controller may be configured to determine a current wind speed  1120 . 
     In an example, a controller(s), such as (one or more of) a load controller, blade heating system controller, turbine controller, and/or wind farm controller may be configured, when the determined current wind speed is less than a cut-in wind speed, (to control the load  340  or the blade heating system  360 ) to draw a current, for the blade heating system  360 , up to a rated current of the blade heating system  1130 . 
     In an example, a controller(s), such as (one or more of) a load controller, blade heating system controller, turbine controller, and/or wind farm controller may be configured, when the determined current wind speed is equal to or more than the cut-in wind speed and when the generator is at sub-synchronous speed, (to control the load  340  or the blade heating system  360 ) to draw a current, for the blade heating system  360 , up to a lower one of an unused current capacity of the low voltage winding and the rated current of the blade heating system  1140 . 
       FIG.  11    is a block diagram of a method of operating a wind turbine according to embodiments described herein. According to the embodiment seen in  FIG.  11   , the method of operating a wind turbine includes, when the determined current wind speed is more than the cut-in wind speed and when the generator is substantially at synchronous speed, providing, to the blade heating system, a current up to the rated current of the blade heating system  1150 . 
     In an example, a controller(s), such as (one or more of) a load controller, blade heating system controller, turbine controller, and/or wind farm controller may be configured, when the determined current wind speed is more than the cut-in wind speed and when the generator is substantially at synchronous speed, (to control the load  340  or the blade heating system  360 ) to draw a current, for the blade heating system  360 , up to the rated current of the blade heating system  1150 . 
       FIG.  12    is a block diagram of a method of operating a wind turbine according to embodiments described herein. According to the embodiment seen in  FIG.  12   , the method of operating a wind turbine includes, when the determined current wind speed is more than the cut-in wind speed and when the generator is at super-synchronous speed, providing, to the blade heating system, a current up to the rated current of the blade heating system  1160 . 
     In an example, a controller(s), such as (one or more of) a load controller, blade heating system controller, turbine controller, and/or wind farm controller may be configured, when the determined current wind speed is more than the cut-in wind speed and when the generator is at super-synchronous speed, (to control the load  340  or the blade heating system  360 ) to draw a current, for the blade heating system  360 , up to the rated current of the blade heating system  1160 . 
     In an example, the expression ‘substantially at synchronous speed’ may be a generator speed range that includes synchronous speed, for which a distinct or particular or synchronous speed (generator/turbine) control strategy/scheme is applicable or in effect. 
     In an example, it may be understood that there may be at least a first control algorithm/scheme for operating the generator ‘substantially at sub-synchronous speed’. In an example, it may be understood that there may be at least a second control algorithm/scheme for operating the generator ‘substantially at synchronous speed’. In an example, it may be understood that there may be at least a third control algorithm/scheme for operating the generator ‘substantially at super-synchronous speed’. 
     In an example, the expression ‘substantially at synchronous speed’ may be understood as within 5%, 2% or 1% of synchronous speed. In an example, the expression ‘substantially at synchronous speed’ may be understood as ‘at synchronous speed’. 
     Accordingly, when a rated current of the generator rotor and of the load or blade heating system, which are electrically coupled to the same low voltage winding, is more than an upper current limit of the low voltage winding, power production is optimum. Further, the maximum rated current of the load or blade heating system is made (substantially or fully) available during substantially synchronous and super-synchronous speeds when the generator rotor is drawing small or no current. 
     Accordingly, operation within an upper current limit of the low voltage winding is improved and effectiveness of the system is thereby improved. 
     The term “stator side main transformer circuit breaker  380 ” may be used interchangeably with “first main transformer circuit breaker  380 ”. The term “rotor side main transformer circuit breaker  320 ” may be used interchangeably with “second main transformer circuit breaker  320 ”. The term “stator side system bus  384 ” may be used interchangeably with “first system bus  384 ”. The term “rotor side system bus  324 ” may be used interchangeably with “second system bus  324 ”. The term “low voltage winding  310 ” may be used interchangeably with “first low voltage winding  310 ”. The term “second low voltage winding  370 ” may be used interchangeably with “further low voltage winding  370 ”. 
     The present written description uses embodiments and examples to provide enabling disclosure. The scope of the present invention is defined by the claims.