Abstract:
A system and method for use in controlling a wind turbine&#39;s power output by a wind turbine controller. The method includes determining a predicted wind speed for the wind turbine, determining a current wind turbine power output, and determining a predicted wind turbine power output utilizing the predicted wind speed. The method also includes comparing the current wind turbine power output to the predicted wind turbine power output and adjusting the wind turbine power output based on the comparison of the current wind turbine power output and the predicted wind turbine power output.

Description:
BACKGROUND OF THE INVENTION 
     The subject matter described herein relates generally to operating wind turbines and, more particularly, to adjusting the power output of one or more wind turbines to provide a relatively more uniform output thereby improving the system frequency and other system control objectives such as scheduled power interchange. 
     Electricity generated from wind power can be highly variable due to the variations in wind speed and direction. This variation may cause rapid increases or drops in energy output delivered by wind turbines and wind plants to the power grid, which in turn, may have an adverse effect on the power grid. Because of the adverse effects on the power grid, various adverse cost impacts may occur, including that a wind farm operator may be required to pay a monetary penalty for providing more or less power than is typically produced, that the grid operator may need to run more expensive reserve generation or may incur fines for violating scheduled power interchange with neighboring systems. There is a need for wind power generation system that maintains a relatively more steady power output when connected to the power grid. 
     BRIEF DESCRIPTION OF THE INVENTION 
     In one aspect, a system for use in controlling a wind turbine&#39;s power output by a wind turbine controller is provided. The method includes determining a predicted wind speed for the wind turbine, determining a current wind turbine power output, and determining a predicted wind turbine power output utilizing the predicted wind speed. The method also includes comparing the current wind turbine power output to the predicted wind turbine power output and adjusting the wind turbine power output based on the comparison of the current wind turbine power output and the predicted wind turbine power output. 
     In another aspect, a system for use in operating a plurality of wind turbines controlling a wind turbine&#39;s power output is provided. The system includes a plurality of wind turbine controllers, each wind turbine controller of the plurality of wind turbine controllers operatively coupled to a wind turbine of a plurality of wind turbines, and a site controller coupled in communication with the plurality of wind turbine controllers and configured to determine a predicted wind speed for the wind turbine, determine a current wind turbine power output, and determine a predicted wind turbine power output utilizing the predicted wind speed. The site controller is also configured to compare the current wind turbine power output to the predicted wind turbine power output, and adjust the wind turbine power output based on the comparison of the current wind turbine power output and the predicted wind turbine power output. 
     In another aspect, a device for use in controlling a wind turbine&#39;s power output is provided. The device includes a memory device configured to store a target power output range, a processor coupled to the memory device and programmed to: determine a predicted wind speed for the wind turbine, determine a current wind turbine power output, determine a predicted wind turbine power output utilizing the predicted wind speed, and compare the current wind turbine power output to the predicted wind turbine power output, and a communication interface coupled to the processor and configured to adjust the wind turbine power output based on the comparison of the current wind turbine power output and the predicted wind turbine power output of at least a first wind turbine controller of the plurality of wind turbine controllers. 
     In yet another aspect, one or more computer readable storage media having computer-executable instructions embodied thereon are provided. The one or more computer readable storage media include computer-executable instructions embodied thereon, wherein when executed by at least one processor, the computer-executable instructions cause the processor to determine a predicted wind speed for a wind turbine, determine a current wind turbine power output, and determine a predicted wind turbine power output utilizing the predicted wind speed. The one or more computer readable storage media also include computer-executable instructions that cause the processor to compare the current wind turbine power output to the predicted wind turbine power output, and adjust the wind turbine power output based on the comparison of the current wind turbine power output and the predicted wind turbine power output. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of an exemplary wind turbine. 
         FIG. 2  is a block diagram illustrating an exemplary wind turbine controller for use with the wind turbine shown in  FIG. 1 . 
         FIG. 3  is a block diagram illustrating an exemplary computing device. 
         FIG. 4  is a block diagram illustrating an exemplary system for use in operating one or more wind turbines, such as the wind turbine shown in  FIG. 1 . 
         FIG. 5  is a flowchart of an exemplary method for use in operating one or more wind turbines using the system shown in  FIG. 4 . 
         FIG. 6  is an exemplary graph showing the relationship between power output of wind turbine  100  versus time using the method shown in  FIG. 5 . 
         FIG. 7  is an exemplary graph showing the relationship between power output of wind turbine  100  versus time using the method shown in  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The embodiments described herein facilitate operating one or more wind turbines in a site by maintaining a relatively steady power output in response to meteorological conditions. Power output may be determined by direct measurement and/or by calculating wind speeds based on wind turbine characteristics (e.g., wind turbine dimensions, blade geometry, and/or blade surface roughness) and/or operating conditions (e.g., wind speed and/or wind direction). When a power output level deviates from a target power output level, an operational adjustment may be transmitted to one or more wind turbine controllers that are coupled to the wind turbines. 
     A target power output level may include, without limitation, a power output level defined by a regulation (e.g., enacted by a municipality or other government body), by a command issued by the power system operator, by a contractual or property-based obligation, or by a preference of an operator of a wind turbine site. 
     In some embodiments, in response to a predicted wind speed that would correspond to a predicted power output level below the target power output level, an operational adjustment is calculated to gradually decrease the power output to avoid severe power output changes. In other embodiments, in response to a predicted wind speed that would correspond to a predicted power output level at or above the target power output level, an operational adjustment is calculated to permit a gradual increase of the power output to avoid an unacceptably rapid increase in power output. In other embodiments, in response to a predicted wind speed that would correspond to a predicted power output level that would correspond to a rate-of-change at or above the target power output threshold, an operational adjustment is calculated to permit a gradual change of the power output to avoid an unacceptably rapid change in the rate-of-change of the power output. 
     Embodiments are described herein with reference to geographic positions. As used herein the term “geographic position” refers to a point in a two-dimensional or three-dimensional space. For example, a geographic position may be expressed in two dimensions as a latitude and a longitude, or in three dimensions as a latitude, a longitude, and an elevation. 
       FIG. 1  is a perspective view of an exemplary wind turbine  100 . Wind turbine  100  includes a nacelle  102  that houses a generator (not shown in  FIG. 1 ). Nacelle  102  is mounted on a tower  104  (only a portion of tower  104  is shown in  FIG. 1 ). Tower  104  may have any suitable height that facilitates operation of wind turbine  100  as described herein. In an exemplary embodiment, wind turbine  100  also includes a rotor  106  that includes three rotor blades  108  coupled to a rotating hub  110 . Alternatively, wind turbine  100  may include any number of rotor blades  108  that enable operation of wind turbine  100  as described herein. In an exemplary embodiment, wind turbine  100  includes a gearbox (not shown) that is rotatably coupled to rotor  106  and to the generator, and an energy storage device (not shown) including, but not limited to, a capacitor, a battery, a flywheel, and rotor  106 . In one embodiment, the energy storage device is configured to allow access to an inherent inertia of the wind turbine  100  using a device such as, but not limited to, a flywheel, and rotor  106 . 
     In some embodiments, wind turbine  100  includes one or more sensors  120  and/or control devices  135  (shown in  FIG. 2 ). Sensors  120  sense or detect wind turbine operating conditions. For example, sensor(s)  120  may include a wind speed and/or a direction sensor (e.g., an anemometer), an ambient air temperature sensor, an air density sensor, an atmospheric pressure sensor, a humidity sensor, a power output sensor, a blade pitch sensor, a turbine speed sensor, a gear ratio sensor, and/or any sensor suitable for use with wind turbine  100 . Each sensor  120  is located according to its function. For example, an anemometer may be positioned on an outside surface of nacelle  102 , such that the anemometer is exposed to air surrounding wind turbine  100 . Each sensor  120  generates and transmits one or more signals corresponding to a detected operating condition. For example, an anemometer transmits a signal indicating a wind speed and/or a wind direction. In the exemplary embodiment, sensor  120  is a light detection and ranging (LIDAR) system sensor and is configured to predict wind speeds. Moreover, each sensor  120  may transmit a signal continuously, periodically, or only once, for example, though other signal timings are also contemplated. Furthermore, each sensor  120  may transmit a signal either in an analog form or in a digital form. In one embodiment, a forecast for meteorological data is utilized in place of sensor  120 . 
     Control devices  135  are configured to control an operation of wind turbine  100  and may include, without limitation, a brake, a relay, a motor, a solenoid, and/or a servomechanism. A control device  135  may adjust a physical configuration of wind turbine  100 , such as an angle or pitch of rotor blades  108  and/or an orientation of nacelle  102  or rotor  106  with respect to tower  104 . 
       FIG. 2  is a block diagram illustrating an exemplary wind turbine controller  200  for use with wind turbine  100 . Wind turbine controller  200  includes a processor  205  for executing instructions and a memory device  210  configured to store data, such as computer-executable instructions and operating parameters. Wind turbine controller  200  also includes a communication interface  215 . Communication interface  215  is configured to be coupled in signal communication with one or more remote devices, such as another wind turbine controller  200  and/or a computing device (shown in  FIG. 3 ). 
     In some embodiments, wind turbine controller  200  includes one or more sensor interfaces  220 . Sensor interface  220  is configured to be communicatively coupled to one or more sensors  120 , such as a first sensor  125  and a second sensor  130 , and may be configured to receive one or more signals from each sensor  120 . Sensor interface  220  facilitates monitoring and/or operating wind turbine  100 . For example, wind turbine controller  200  may monitor operating conditions (e.g., wind speed, wind direction, rotor speed, and/or power output) of wind turbine  100  based on signals provided by sensors  120 . In one embodiment, the wind turbine controller  200  is configured to calculate a power output produced by the corresponding wind turbine  100  based on one or more wind turbine characteristics (e.g., wind turbine dimensions and/or a rotor blade geometry), one or more operating parameters (e.g., a wind speed, a wind direction, a rotor blade tip speed, or a rotor blade pitch angle), and/or an operational state (e.g., disabled, curtailed, or normal) of a wind turbine  100 . 
     In an exemplary embodiment, processor  205  executes one or more monitoring software applications and/or control software applications. A software application may produce one or more operating parameters that indicate an operating condition, and memory device  210  may be configured to store the operating parameters. For example, a history of operating parameters may be stored in memory device  210 . 
     In some embodiments, wind turbine controller  200  also includes a control interface  225 , which is configured to be communicatively coupled to one or more control devices  135 , such as a first control device  140  and a second control device  145 . In one embodiment, wind turbine control interface  225  is configured to operate control device  135  including a brake to prevent rotor  106  (shown in  FIG. 1 ) from rotating. In addition, or in the alternative, wind turbine control interface  225  may operate a control device  135  including a blade pitch servomechanism to adjust one or more rotor blades  108  (shown in  FIG. 1 ) to a desired and/or predetermined pitch. In an alternative embodiment, electrical power and torque are operated by control device  135 . The brake, the blade pitch servomechanism, and the electrical power may be operated by the same control device  135  or a first control device  135  and a second control device  135 . In the exemplary embodiment, wind turbine controller  200  is configured to operate control devices  135  to achieve a desired power output. 
       FIG. 3  is a block diagram illustrating an exemplary computing device  300 . Computing device  300  includes a processor  305  for executing instructions. In some embodiments, executable instructions are stored in a memory device  310 . Memory device  310  is any device allowing information, such as executable instructions and/or other data, to be stored and retrieved. 
     In some embodiments, computing device  300  includes at least one presentation device  315  for presenting information to user  320 . Presentation device  315  is any component capable of conveying information to user  320 . Presentation device  315  may include, without limitation, a display device (e.g., a liquid crystal display (LCD), organic light emitting diode (OLED) display, or “electronic ink” display) and/or an audio output device (e.g., a speaker or headphones). In some embodiments, presentation device  315  includes an output adapter, such as a video adapter and/or an audio adapter. An output adapter is operatively coupled to processor  305  and configured to be operatively coupled to an output device, such as a display device or an audio output device. 
     In some embodiments, computing device  300  includes an input device  325  for receiving input from user  320 . Input device  325  may include, for example, a keyboard, a pointing device, a mouse, a stylus, a touch sensitive panel (e.g., a touch pad or a touch screen), a gyroscope, an accelerometer, a position detector, and/or an audio input device. A single component, such as a touch screen, may function as both an output device of presentation device  315  and input device  325 . Computing device  300  also includes a communication interface  330 , which is configured to be communicatively coupled to one or more wind turbine controllers  200  and/or one or more other computing devices  300 . 
     Stored in memory device  310  are, for example, computer readable instructions for determining and responding to power output levels, providing a user interface to user  320  via presentation device  315 , and/or receiving and processing input (e.g., target power output levels) from input device  325 . In addition, or alternatively, memory device  310  may be configured to store target power output levels, measured power output levels, calculated power output levels, and/or any other data suitable for use with the methods described herein. 
       FIG. 4  is a block diagram illustrating an exemplary system  400  for use in operating one or more wind turbines  100 . System  400  includes a network  405 . For example, network  405  may include, without limitation, the Internet, a local area network (LAN), a wide area network (WAN), a wireless LAN (WLAN), a mesh network, and/or a virtual private network (VPN). 
     In an exemplary embodiment, a wind turbine site  410  includes a plurality of wind turbines  100 , each of which includes a wind turbine controller  200 . One or more computing devices  300  (shown in  FIG. 3 ), such as a site controller  415 , are configured to be coupled in signal communication with wind turbine controllers  200  via network  405 . 
     In an exemplary embodiment, site controller  415  is positioned at wind turbine site  410 . Alternatively, site controller  415  may be positioned outside wind turbine site  410 . For example, site controller  415  may be communicatively coupled to wind turbine controllers  200  at a plurality of wind turbine sites  410 . 
     Each of site controller  415  and wind turbine controller  200  includes a processor, (shown in  FIGS. 2 and 3 ). A processor may include a processing unit, such as, without limitation, an integrated circuit (IC), an application specific integrated circuit (ASIC), a microcomputer, a programmable logic controller (PLC), and/or any other programmable circuit. A processor may include multiple processing units (e.g., in a multi-core configuration). Each of site controller  415  and wind turbine controller  200  is configurable to perform the operations described herein by programming the corresponding processor. For example, a processor may be programmed by encoding an operation as one or more executable instructions and providing the executable instructions to the processor in a memory device (also shown in  FIGS. 2 and 3 ) that is coupled to the processor. A memory device may include, without limitation, one or more random access memory (RAM) devices, one or more storage devices, and/or one or more computer readable media. 
     In some embodiments, one or more wind speed sensors  420  are coupled in communication with site controller  415 . Wind speed sensors  420  are configured to provide a predicted wind speed for indicating a wind speed corresponding to a geographic position. In one embodiment, sensors  120  of one or more wind turbines  100  include a wind speed sensor  420 . Wind speed sensors  420  may be further configured to provide a direction associated with a predicted wind speed measurement. For example, a wind speed sensor  420  may provide wind level measurements associated with a plurality of directions at a single geographic position. 
     In an exemplary embodiment, system  400  enables operation of wind turbines  100  such that a relatively balanced system power output is maintained. System  400  may further enable operation of wind turbines  100  such that power output of site  410  is optimized within the bounds of the target power output levels. 
       FIG. 5  is a flowchart of an exemplary method  500  for use in operating one or more wind turbines  100  (shown in  FIG. 1 ) using system  400  (shown in  FIG. 4 ). All or a portion of method  500  may be performed by one or more computing devices  300  (shown in  FIG. 3 ), such as, without limitation, wind turbine controller  200 , and/or site controller  415  (shown in  FIGS. 2 and 4 ). In the exemplary embodiment, site controller  415  determines  505  predicted wind speeds associated with wind turbines  100  using sensors  120  and/or wind speed sensors  420 . The predicted wind speeds may be determined  505  in a time frame ranging from 1 second to 10 minutes in advance. In the exemplary embodiment, the predicted wind speeds are determined  505  at least one second in advance. 
     In the exemplary embodiment, the current power output of the wind turbine  100  is determined  510  by at least one of the wind turbine controller  200  and the site controller  415 . In one embodiment, wind turbine controller  200  calculates a power output level for wind turbine  100  and transmits the calculated power output level to site controller  415 . In an alternative embodiment, wind turbine controller  200  transmits the wind turbine characteristics, operating parameters, and/or operational state to site controller  415  and site controller  415  calculates the power output level produced by wind turbine  100 . 
     Whether performed by wind turbine controller  200  or site controller  415 , determining  510  a current power output level produced by a wind turbine  100  provides a current power output level associated with a corresponding geographic position at certain operating parameters. 
     In addition, or alternatively, wind speed sensors  420  may provide wind speed level measurements indicating a wind speed level associated with a geographic position and, optionally, with a plurality of directions. In some embodiments, wind speed sensor  420  is included as a sensor  120  of one or more wind turbines  100 . Site controller  415  determines  510  a current power output at one or more geographic positions based on the calculated and/or measured wind speed. When measured wind speed is used, the current power output may also be determined  510 . 
     In the exemplary embodiment, site controller  415  determines  515  if the predicted wind speed allows wind turbine  100  to produce power within a target power output range, using the determined  505  predicted wind speeds. In one embodiment, the target power output range is expressed as a specific power level in the range of zero to 100% of wind turbine rating, with deadband requirement which specifies the required accuracy in the range from zero to 100 percent of turbine rating. Alternatively, the target power output range can be any range that enables the disclosure to function as described herein. In an alternative embodiment, site controller  415  determines  515  if the predicted wind speed allow wind turbine  100  to produce a power output within a threshold value. The threshold value may be defined in absolute (e.g., 2 kW, 3 kW, or 5 kW) or relative (e.g., 3%, 5%, or 10%) terms. 
     In an alternative embodiment, site controller  415  determines  515  if the predicted wind speed will cause wind turbine  100  to produce a rate-of-change of power output outside of a threshold value. In one embodiment, the threshold value is may be defined in absolute (e.g., 2 MW/min, −2 MW/Min, −5 MW/min) or relative (e.g., +3%/min, −4%/min, −5%/10 minutes). 
     If the predicted wind speed will not allow wind turbine  100  to produce power within the target power output range or threshold value, site controller  415  calculates  520  a desired power output level for one or more wind turbines  100 . The calculated  520  desired output level is a power output level that can be supported by the determined  505  predicted wind speeds. 
     Based on the calculated  520  desired power output level associated with wind turbine  100 , site controller  415  determines  525  an operational adjustment. In one embodiment, determining  525  an operational adjustment includes transmitting the desired maximum power output level associated with a wind turbine  100  to a corresponding wind turbine controller  200 . In such an embodiment, wind turbine controller  200  is configured to adjust operating parameters of wind turbine  100  to achieve the desired power output. In an alternate embodiment, site controller  415  is configured to determine one or more operating parameters (e.g., a rotor blade pitch angle, a maximum rotor blade speed, and/or a maximum torque) for wind turbine  100  and transmit to wind turbine controller  200  an operational adjustment that includes the determined operating parameters. 
     Operational adjustments may be determined  525  such that a difference between a predicted power output, corresponding to a predicted wind speed, and the target power output level decreases when the operational adjustment is applied. In one embodiment, when the predicted power output corresponding to a predicted wind speed will be below the target power output range or threshold value, an operational adjustment is determined  525  to reduce the power output to prevent a severe loss of power. In an alternative embodiment, when the predicted wind speed will cause wind turbine  100  to produce a predicted power output above the target power output range or threshold value, an operational adjustment may be determined  525  to reduce the power output to maintain the power output within the target power output range or threshold value. In yet another alternative embodiment, if the current power output is below the target power output range or threshold value and the predicted power output will produce a predicted power output within the target power output range or threshold value, an operational adjustment may be determined  525  to increase the power output. 
     In the exemplary embodiment, when a decision to decrease the power output of wind turbine  100  is determined  525 , site controller  415  gradually reduces  530  the power output to the calculated  520  desired output level. In an alternative embodiment, if a decision to increase the power output is determined  525 , site controller  415  gradually increases  530  the power output to the calculated  520  desired output level. In the exemplary embodiment, to prevent the power output levels from varying dramatically, if at any time during the gradual increase/decrease of power output the current wind cannot sustain the gradual increase/decrease of power output, site controller  415  utilizes  535  energy stored within the energy storage device (not shown). When the gradual adjustment  530  has finished, wind turbine  100  is maintained  540  in the current operating conditions. 
       FIGS. 6 and 7  are exemplary graphs showing the relationship between power output of wind turbine  100  versus time using the method shown in  FIG. 5 . A solid line  602  represents the power output resulting from a change in wind speed. A dashed line  604  represents the power output gradually adjusting  530  as a result of implementing the determined  525  operational adjustments. Portion  606  of solid line  602  represents the determined  510  current power output and portion  608  of solid line  602  represents the calculated  520  desired output level of wind turbine  100 . 
     In one embodiment, as shown in  FIG. 6 , if site controller  415  makes a determination  525  to reduce the power output in response to a low predicted power output, an operational adjustment to gradually decrease  530  the power output to the calculated  520  desired output level is initialized at point  620 . The power output is gradually decreased  530  until the power output is below the calculated  520  desired output level, such as point  608 . From point  622  to point  624 , power output is above what can be produced from the current wind alone. When the power output arrives at the point  622 , at which point the power output cannot be sustained by the current wind, site controller  415  utilizes energy stored in the energy storage device until the current wind can sustain the power output, such as point  624 . 
     At point  624 , the power output falls below the calculated  520  desired output level until point  626 . From point  624  until point  626 , wind turbine  100  returns an equal amount of energy used between points  622  to  624  to the energy storage device. At point  626 , the power output is kept in line with the calculated  520  desired output level and wind turbine  100  is maintained  540  at the current operating conditions. 
     In an alternative embodiment, as shown in  FIG. 7 , if site controller  415  makes a determination  525  to increase the power output in response to a high predicted power output, an operational adjustment to gradually increase  530  the power output to the calculated  520  desired power output level is initialized at point  630 . The power output is gradually increased  530  to the calculated  520  desired output level. From point  630  to point  632 , power output is above what can be produced from the current wind alone. When the gradual increase  530  of power output is initialized at point  630 , at which point the power output cannot be sustained by the current wind, site controller  415  utilizes energy stored in the energy storage device until the current wind can sustain the power output, such as point  632 . 
     From point  632 , the power output continues to gradually increase towards the calculated  520  desired output level. In one embodiment, when the power output is capable of maintaining the calculated  520  desired output level, at point  634 , site controller  415  maintains the power output below the calculated  520  desired output level and returns power to the energy storage device until an equal amount of energy used between points  630  to  632  is returned to the energy storage device. The power output is then maintained  540  at the current operating conditions at the calculated  520  desired output level. 
     Method  500  may be performed repeatedly (e.g., continuously, periodically, or upon request), enabling continual adjustment to operation of wind turbines  100  in site  410 . For example, as the wind direction changes, the power output level at second geographic position  430  may increase, while the power output level at first geographic position  425  decreases. Accordingly, operational adjustments may be determined  525  and transmitted to ensure the target power output range or threshold value at second geographic position  430  is not exceeded. 
     Similarly, if a wind turbine, such as first wind turbine  435 , is disabled for maintenance or repair, or is operated at a reduced level of operation for any reason, the power output level decreases, and site controller  415  may automatically increase a desired maximum power output level associated with a second wind turbine  440  and a third wind turbine  445 , such that the power output of second wind turbine  440  and third wind turbine  445  is increased. When first wind turbine  435  is activated again, power output produced by first wind turbine  435  is reflected in the aggregate power output level, and site controller  415  may adjust the desired maximum power output level of second wind turbine  440  and third wind turbine  445  downward to ensure compliance with the target power output ranges or threshold levels. 
     Embodiments provided herein facilitate automatically and continually adjusting the operation of wind turbines based on a power output level at one or more geographic positions that are associated with a target power output range or threshold value. Adjusting wind turbine operation as described herein enables an operator of a wind turbine site to provide a relatively constant power output, and/or a relatively lower rate-of-change of power output when faced with variable meteorological conditions. 
     The methods described herein may be encoded as executable instructions embodied in a computer readable storage medium including, without limitation, a memory device of a computing device. Such instructions, when executed by a processor, cause the processor to perform at least a portion of the methods described herein. 
     Exemplary embodiments of a wind turbine control system are described above in detail. The system, devices, wind turbine, and included assemblies are not limited to the specific embodiments described herein, but rather each component may be utilized independently and separately from other components described herein. 
     This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.