Patent Publication Number: US-2022228558-A1

Title: Systems and methods for operating a power generating asset

Description:
FIELD 
     The present disclosure relates in general to power generating assets, and more particularly to systems and methods for determining a state-of-health rating for an energy storage device of a power generating asset. 
     BACKGROUND 
     Power generating assets may take a variety of forms, including but not limited to assets which rely on renewable and/or nonrenewable sources of energy. Such power generating assets may generally be considered one of the cleanest, most environmentally friendly energy sources presently available. For example, wind turbines have gained increased attention in this regard. A modern wind turbine typically includes a tower, a generator, a gearbox, a nacelle, and one or more rotor blades. The nacelle includes a rotor assembly coupled to the gearbox and to the generator. The rotor assembly and the gearbox are mounted on a bedplate support frame located within the nacelle. The rotor blades capture kinetic energy of wind using known airfoil principles. The rotor blades transmit the kinetic energy in the form of rotational energy so as to turn a shaft coupling the rotor blades to a gearbox, or if a gearbox is not used, directly to the generator. The generator then converts the mechanical energy to electrical energy and the electrical energy may be transmitted to a converter and/or a transformer housed within the tower and subsequently deployed to a utility grid. Modern wind power generation systems typically take the form of a wind farm having multiple wind turbine generators that are operable to supply power to a transmission system providing power to a power grid. 
     Certain power generating assets may also include at least one energy storage device, such as a battery or capacitor, which may serve as a backup power supply to a component of the power generating asset. During normal operation, components of the wind turbine, such as pitch drive motors, may be driven by power supplied by the power grid. However, in some instances, such as during a transient grid event, these components may be driven by the energy storage devices. Thus, the utilization of an energy storage device may ensure that the power generating asset may be controlled even in the absence of grid power. For example, the utilization of an energy storage device coupled to the pitch drive system of the wind turbine may permit the pitching of the rotor blades toward feather in the event grid power is lost. This, in turn, may prevent an overspeed event from negatively impacting the wind turbine. 
     Accordingly, it may be desirable to ensure that the energy storage is capable of operating when needed. As such, it may be desirable to determine a state of health of the energy storage device on a regular basis. 
     Thus, the art is continuously seeking new and improved systems and methods that address the aforementioned issues. As such, the present disclosure is directed to systems and methods for determining a state-of-health rating for an energy storage device of a power generating asset so as to ensure the energy storage device remains operable. 
     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. 
     In one aspect, the present disclosure is directed to a method for operating a power generating asset. The power generating asset may have an energy storage device operably coupled to a component of the power generating asset. The method may include initiating with a controller, which may be a controller of the power generating asset, a state-change event for an energy storage device. The state-change event may define a plurality of sampling intervals. The method may also include determining, via the controller, a change in a first and a second electrical condition of the energy storage device at each of the sampling intervals of the state-change event. Additionally, the method may include determining, via the controller, an actual equivalent series resistance (ESR) function for the energy storage device based on the change in the first and second electrical conditions at each of the sampling intervals of the state-change event. Further, the method may include determining, via the controller, a state-of-health rating for the energy storage device based on the actual ESR function of the energy storage device. The method may also include implementing a control action based on the state-of-health rating. 
     In an embodiment, determining the state-of-health rating for the energy storage device may include modeling, via the controller, a nominal ESR function for the energy storage device at an ambient temperature. The nominal ESR function may correspond to a maximal state-of-health rating at the ambient temperature. The method may include modeling, via the controller, at least one potential ESR function for the energy storage device at the ambient temperature. The potential ESR function(s) may indicate an increased ESR value relative to a nominal ESR value as a function of the second electrical condition. The potential ESR function(s) may correspond to a reduced state-of-health rating at the ambient temperature. Additionally, the method may include consolidating, via the controller, the nominal ESR function and the potential ESR function(s) into a look-up table, a graphical representation, and/or an algorithm so as to establish a correlation between the actual ESR function and the state-of-health rating for the energy storage device at the ambient temperature as a function of the second electrical condition of the energy storage device. 
     In an additional embodiment, the method may include defining, via the controller, a plurality of temperature intervals across a nominal operating range of temperatures for the energy storage device. A temperature interval of the plurality of temperature intervals may correspond to the ambient temperature. The method may also include determining, via the controller, the nominal ESR function and the potential ESR function(s) at each temperature interval of the plurality of temperature intervals. Additionally, the method may include establishing a correlation between the actual ESR function and the state-of-health rating for the energy storage device at each temperature interval as a function of the second electrical condition of the energy storage device. 
     In a further embodiment, determining the state-of-health rating for the energy storage device may also include determining the state-of-health rating corresponding to the determined actual ESR function at the ambient temperature of the energy storage device based on the correlation between the actual ESR function and the state-of-health rating. 
     In yet a further embodiment, the first electrical condition may be a voltage. Determining a change in the first electrical condition of the energy storage device may also include determining a difference between an open-circuit voltage of the energy storage device and an instantaneous voltage of the energy storage device at each of the sampling intervals of the state-change event. 
     In an embodiment, implementing the control action may include detecting an approach of the state-of-health rating to a state-of-health threshold and generating an alert to facilitate scheduling of a maintenance event. 
     In an additional embodiment, the state-change event may include a scheduled test event and/or a manipulation of the energy storage device during operation of the power generating asset. 
     In a further embodiment, the scheduled test event may be accomplished in conjunction with at least one additional component test of the power generating asset and results in an updating of the state-of-health rating. 
     In yet a further embodiment, the state-change event may include a discharging event of the energy storage device or a charging event of the energy storage device. 
     In an embodiment, determining the actual ESR function may include determining the actual ESR function as a function of the second electrical condition and independent of a first electrical condition discharge profile and a second electrical condition discharge profile. 
     In an additional embodiment, the method may include receiving, via the controller, a cycle count and/or a time count elapsed from an installation date. The method may also include determining, via the controller, a correlation between the state-of-health rating and the received cycle count and/or time count. The correlation may be indicative of a rate of degradation of the energy storage device per cycle count and/or, time count. Additionally, the method may include determining a state-of-health threshold for the energy storage device. Based on the correlation of the state-of-health threshold, the method may also include determining, via the controller, a number of cycles and/or time until the state-of-health threshold is reached. 
     In another aspect, the present disclosure is directed to a method for operating an energy storage device. The method may include initiating, with the controller, a discharge event for the energy storage device. The discharge event may define a plurality of sampling intervals. The method may also include determining, via the controller, a change in a voltage and a current of the energy storage device at each of the plurality of sampling intervals of the discharge event. Additionally, the method may include determining, via the controller, an actual equivalent series resistance (ESR) function for the energy storage device based on the change in the voltage and the current at each of the plurality of sampling intervals of the discharge event. Further, the method may include determining, via the controller, a state-of-health rating for the energy storage device based on the actual ESR function of the energy storage device. Additionally, the method may include implementing a control action based on the state-of-health rating. 
     In another aspect, the present disclosure is directed to a system for operating a power generating asset. The system may include an energy storage device operably coupled to a component of the power generating asset. The system may also include a controller communicatively coupled to the energy storage device. The controller may include at least one processor configured to perform a plurality of operations. The plurality of operations may include any of the operations and/or features described herein. 
     These and other features, aspects and advantages of the present invention will become better understood with reference to the following description and appended claims. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A full and enabling disclosure of the present invention, 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  illustrates a perspective view of one embodiment of a power generating asset configured as a wind turbine according to the present disclosure; 
         FIG. 2  illustrates a perspective, internal view of one embodiment of a nacelle of the wind turbine according to the present disclosure; 
         FIG. 3  illustrates a schematic diagram of one embodiment of an energy storage device operably coupled to a component of the power generating asset according to the present disclosure; 
         FIG. 4  illustrates a block diagram of one embodiment of a controller for use with the power generating asset according to the present disclosure; 
         FIG. 5  illustrates a flow diagram of one embodiment of a control logic of a system for operating a power generating asset according to the present disclosure; 
         FIG. 6  illustrates a graphical representation of a relationship between changes in a first and a second electrical condition and an equivalent series resistance for an energy storage device of the power generating asset according to the present disclosure; and 
         FIG. 7  illustrates a graphical representation of nominal, actual, and potential equivalent series resistance functions relative to a second electrical condition according to the present disclosure. 
     
    
    
     Repeat use of reference characters in the present specification and drawings is intended to represent the same or analogous features or elements of the present invention. 
     DETAILED DESCRIPTION 
     Reference now will be made in detail to embodiments of the invention, one or more examples of which are illustrated in the drawings. Each example is provided by way of explanation of the invention, not limitation of the invention. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope or spirit of the invention. For instance, features illustrated or described as part of one embodiment can be used with another embodiment to yield a still further embodiment. Thus, it is intended that the present invention covers such modifications and variations as come within the scope of the appended claims and their equivalents. 
     As used herein, the terms “first”, “second”, and “third” may be used interchangeably to distinguish one component from another and are not intended to signify location or importance of the individual components. 
     The terms “coupled,” “fixed,” “attached to,” and the like refer to both direct coupling, fixing, or attaching, as well as indirect coupling, fixing, or attaching through one or more intermediate components or features, unless otherwise specified herein. 
     Approximating language, as used herein throughout the specification and claims, is applied to modify any quantitative representation that could permissibly vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term or terms, such as “about”, “approximately”, and “substantially”, are not to be limited to the precise value specified. In at least some instances, the approximating language may correspond to the precision of an instrument for measuring the value, or the precision of the methods or machines for constructing or manufacturing the components and/or systems. For example, the approximating language may refer to being within a 10 percent margin. 
     Here and throughout the specification and claims, range limitations are combined and interchanged, such ranges are identified and include all the sub-ranges contained therein unless context or language indicates otherwise. For example, all ranges disclosed herein are inclusive of the endpoints, and the endpoints are independently combinable with each other. 
     Generally, the present disclosure is directed to systems and methods for operating a power generating asset. The power generating asset, such as a wind turbine, may have an energy storage device operably coupled to at least one component. The energy storage device may, for example, be a battery, a capacitor, or other suitable energy storage device which permits the continued, or emergency, operation of the component should grid power be disrupted. By way of illustration, when configured as a wind turbine, the pitch system of the wind turbine may be equipped with an energy storage device in order to ensure that the pitch system remains operable in the event of a power failure. This, in turn, may permit the pitching of the rotor blades to feather in order to slow the wind turbine as necessary even if the power from the power grid is interrupted. It should be appreciated that the ability to control a component of the power generating asset regardless of whether grid power is being received may facilitate the transition of the power generating asset to a safe operating mode and, thereby serve to prevent/mitigate potential damage to the power generating asset. Therefore, it may be desirable to determine a state of health of the energy storage device in order to ensure the energy storage device may be fully operational when called upon. 
     In order to determine the state of health of the energy storage device, the present disclosure requires the initiation of a state-change event, such as a charge or a discharge event, for the energy storage device. During the state-change event, the change in a first electrical condition and a second electrical condition of the energy storage device, such as a change in voltage and/or current, may be determined. From the change in conditions, an actual equivalent series resistance (ESR) function for the energy storage device may be determined. The ESR may represent the internal resistance of the energy storage device as seen during a charging or discharging event. The magnitude of ESR may vary depending on the particular characteristics (e.g. materials used, construction quality, degree of degradation, cell chemistry, ambient temperature) of the energy storage device. Accordingly, the ESR may be compared to a nominal ESR and/or projected ESR. This comparison may, for example, include plotting the actual ESR function and the nominal ESR function relative to the second electrical condition (e.g. current). A comparison of the ESR values at a specified value of the second electrical condition may indicate a decline in the state of health of the power generating asset. 
     For example, at a specific current level, the nominal ESR function may indicate a first ESR value, while at the same current level, the actual ESR function may indicate a second ESR value. A second ESR value which is greater than the first ESR value may be indicative of a decline in the state of health of the energy storage device. In other words, for a given current and temperature, the ESR of the energy storage device may increase as the remaining lifespan of the energy storage device decreases. It should be appreciated that detecting this degradation in the energy storage device may facilitate the generation of an alarm and/or a maintenance schedule. 
     Referring now to the drawings,  FIG. 1  illustrates a perspective view of one embodiment of a power generating asset  100  according to the present disclosure. As shown, the power generating asset  100  may be configured as a wind turbine  114 . In an additional embodiment, the power generating asset  100  may, for example, be configured as a solar power generating asset, a hydroelectric plant, a fossil fuel generator, and/or a hybrid power generating asset. As shown, the wind turbine  114  may generally include a tower  102  extending from a support surface  104 , a nacelle  106 , mounted on the tower  102 , and a rotor  108  coupled to the nacelle  106 . The rotor  108  may include a rotatable hub  110  and at least one rotor blade  112  coupled to, and extending outwardly from, the hub  110 . For example, in the illustrated embodiment, the rotor  108  includes three rotor blades  112 . However, in an additional embodiment, the rotor  108  may include more or less than three rotor blades  112 . Each rotor blade  112  may be spaced about the hub  110  to facilitate rotating the rotor  108  to enable kinetic energy to be transferred from the wind into usable mechanical energy, and subsequently, electrical energy. For instance, the hub  110  may be rotatably coupled to an electric generator  118  ( FIG. 2 ) positioned within the nacelle  106  to permit electrical energy to be produced. 
     The power generating asset  100  may also include a controller  200 . When configured as a wind turbine  114 , the controller  200  may be configured as a turbine controller centralized within the nacelle  106 . However, in other embodiments, the controller  200  may be located within any other component of the wind turbine  114  or at a location outside the wind turbine  114 . Further, the controller  200  may be communicatively coupled to any number of the components of the power generating asset  100  in order to control the components. As such, the controller  200  may include a computer or other suitable processing unit. Thus, in several embodiments, the controller  200  may include suitable computer-readable instructions that, when implemented, configure the controller  200  to perform various different functions, such as receiving, transmitting and/or executing wind turbine control signals. 
     Referring now to  FIG. 2 , a simplified, internal view of one embodiment of the nacelle  106  of the wind turbine  114  shown in  FIG. 1  is illustrated. As shown, the generator  118  may be coupled to the rotor  108  for producing electrical power from the rotational energy generated by the rotor  108 . For example, as shown in the illustrated embodiment, the rotor  108  may include a rotor shaft  122  coupled to the hub  110  for rotation therewith. The rotor shaft  122  may be rotatably supported by a main bearing  144 . The rotor shaft  122  may, in turn, be rotatably coupled to a high-speed shaft  124  of the generator  118  through a gearbox  126  connected to a bedplate support frame  136 . As is generally understood, the rotor shaft  122  may provide a low-speed, high-torque input to the gearbox  126  in response to rotation of the rotor blades  112  and the hub  110 . The gearbox  126  may then be configured to convert the low-speed, high-torque input to a high-speed, low-torque output to drive the high-speed shaft  124  and, thus, the generator  118 . 
     Each rotor blade  112  may also include a pitch control mechanism  120  configured to rotate each rotor blade  112  about its pitch axis  116 . Each pitch control mechanism  120  may include a pitch drive motor  128 , a pitch drive gearbox  130 , and a pitch drive pinion  132 . In such embodiments, the pitch drive motor  128  may be coupled to the pitch drive gearbox  130  so that the pitch drive motor  128  imparts mechanical force to the pitch drive gearbox  130 . Similarly, the pitch drive gearbox  130  may be coupled to the pitch drive pinion  132  for rotation therewith. The pitch drive pinion  132  may, in turn, be in rotational engagement with a pitch bearing  134  coupled between the hub  110  and a corresponding rotor blade  112  such that rotation of the pitch drive pinion  132  causes rotation of the pitch bearing  134 . Thus, in such embodiments, rotation of the pitch drive motor  128  drives the pitch drive gearbox  130  and the pitch drive pinion  132 , thereby rotating the pitch bearing  134  and the rotor blade(s)  112  about the pitch axis  116 . 
     It should be appreciated that pitching the rotor blade(s)  112  about the pitch axis  116  may alter an angle of attack between the rotor blade(s)  112  and an apparent wind. Accordingly, the rotor blade(s)  112  may pitch to feather when the rotor blade(s)  112  rotates about the pitch axis  116  towards alignment with the apparent wind and to power when the rotor blade(s) rotates towards an orientation generally perpendicular to the apparent wind. It should be further appreciated that pitching to feather generally depowers the rotor blade(s)  112  as a result of a reduction in the resultant lift. 
     Similarly, the wind turbine  114  may include one or more yaw drive mechanisms  138  communicatively coupled to the controller  200 , with each yaw drive mechanism(s)  138  being configured to change the angle of the nacelle  106  relative to the wind (e.g., by engaging a yaw bearing  140  of the wind turbine  114 ). It should be appreciated that the controller  200  may direct the yawing of the nacelle  106  and/or the pitching of the rotor blades  112  so as to aerodynamically orient the wind turbine  114  relative to a wind acting on the wind turbine  114 , thereby facilitating power production. 
     In an embodiment, the power generating asset  100  may also include an environmental sensor  156  configured for gathering data indicative of one or more environmental conditions. The environmental sensor  156  may be operably coupled to the controller  200 . Thus, in an embodiment, the environmental sensor(s)  156  may, for example, be a wind vane, an anemometer, a lidar sensor, thermometer, barometer, or any other suitable sensor. The data gathered by the environmental sensor(s)  156  may include measures of wind speed, wind direction, wind shear, wind gust, wind veer, atmospheric pressure, and/or ambient temperature. In at least one embodiment, the environmental sensor(s)  156  may be mounted to the power generating asset  100  (e.g., to the nacelle  106  at a location downwind of the rotor  108 ). For example, the environmental sensor(s)  156  may, in alternative embodiments, be coupled to, or integrated with, the rotor  108  and/or positioned within the nacelle  106 . It should be appreciated that the environmental sensor(s)  156  may include a network of sensors and may be positioned away from the power generating asset  100 . 
     It should also be appreciated that, as used herein, the term “monitor” and variations thereof indicates that the various sensors of the power generating asset  100  may be configured to provide a direct measurement of the parameters being monitored or an indirect measurement of such parameters. Thus, the sensors described herein may, for example, be used to generate signals relating to the parameter being monitored, which can then be utilized by the controller  200  to determine a condition or response of the power generating asset  100  and/or a component thereof. 
     Referring now to  FIG. 3 , a schematic diagram of one embodiment of an energy storage device  150  operably coupled to a component  142  of the power generating asset  100  is illustrated. In an embodiment such as depicted in  FIG. 3 , the component  142  may be configured as the pitch control mechanism  120  of the wind turbine  114 . In an embodiment, the component  142  may be communicatively coupled to the power grid  146 . Additionally, the component  142  may be communicatively coupled to at least one energy storage device  150 . In various embodiments, the energy storage device(s)  150  may be a single battery, capacitor, and/or other suitable energy storage device or pluralities thereof. 
     In an embodiment, at least one condition sensor  158  may be communicatively coupled to the energy storage device(s)  150 . The condition sensor(s)  158  may be configured to monitor at least a first electrical condition and a second electrical condition  302 ,  304  ( FIG. 5 ) of the energy storage device(s)  150 . For example, in monitoring the electrical conditions  302 ,  304 , the condition sensor(s)  158  may monitor an open-circuit voltage, an instantaneous voltage, and/or a current of the energy storage device(s)  150 . Accordingly, the condition sensor(s)  158  may, in an embodiment, be an ammeter, a voltmeter, an ohmmeter, and/or any other suitable sensor for monitoring the electrical conditions  302 ,  304  of the energy storage device(s)  150 . 
     During normal operation of the power generating asset  100 , the component  142  may be driven by the power grid  146 . However, in some instances, such as during an adverse grid event or grid loss, the component  142  may be powered by the energy storage device(s)  150 . Therefore, in an embodiment, the energy storage device(s)  150  may be configured as an uninterrupted power supply. Accordingly, the energy storage device(s)  150  may, in an embodiment, be utilized in a top-of-charge application. For example, when configured as pitch control mechanism  120 , the pitch drive motor  128  may utilize power from the energy storage device(s)  150  in order to pitch the rotor blade(s)  112  toward feather in response to an adverse grid event. In an additional example, the component  142  may be the controller  200  and the energy storage device(s)  150  may provide an uninterrupted power source to the controller  200  in response to a loss of grid power. In yet a further example, the energy storage device(s)  150  may be operably coupled to the generator  118  and configured to receive at least a portion of the power output of the generator  118  and, in certain instances, deliver the received portion of the power to the power grid  146 . It should be appreciated that if control of the component  142  relies on the energy storage device(s)  150  (i.e., due to a loss of grid power), it may be desirable to ensure that the energy storage device(s)  150  are capable of operating when called upon. Thus, the controller  200  may, in an embodiment, be configured to determine the state of health of the energy storage device(s)  150  on a regular basis. 
     Referring now to  FIGS. 4-7 , multiple embodiments of a system  300  for operating the power generating asset  100 , e.g. the wind turbine  114 , according to the present disclosure are presented. As shown particularly in  FIG. 4 , a schematic diagram of one embodiment of suitable components that may be included within the system  300  is illustrated. For example, as shown, the system  300  may include the controller  200  communicatively coupled to the environmental sensor(s)  156  and/or the condition sensor(s)  158 . Further, as shown, the controller  200  includes one or more processor(s)  206  and associated memory device(s)  208  configured to perform a variety of computer-implemented functions (e.g., performing the methods, steps, calculations and the like and storing relevant data as disclosed herein). Additionally, the controller  200 , may also include a communications module  210  to facilitate communications between the controller  200 , and the various components of the power generating asset  100 . Further, the communications module  210  may include a sensor interface  212  (e.g., one or more analog-to-digital converters) to permit signals transmitted from the sensor(s)  156 ,  158  to be converted into signals that can be understood and processed by the processors  206 . It should be appreciated that the sensor(s)  156 ,  158  may be communicatively coupled to the communications module  210  using any suitable means. For example, the sensor(s)  156 ,  158  may be coupled to the sensor interface  212  via a wired connection. However, in other embodiments, the sensor(s)  156 ,  158  may be coupled to the sensor interface  212  via a wireless connection, such as by using any suitable wireless communications protocol known in the art. Additionally, the communications module  210  may also be operably coupled to an operating state control module  214  configured to implement a control action based on a determination of the state of health of the energy storage device(s)  150 . 
     As used herein, the term “processor” refers not only to integrated circuits referred to in the art as being included in a computer, but also refers to a controller, a microcontroller, a microcomputer, a programmable logic controller (PLC), an application specific integrated circuit, and other programmable circuits. Additionally, the memory device(s)  208  may generally comprise memory element(s) including, but not limited to, computer readable medium (e.g., random access memory (RAM)), computer readable non-volatile medium (e.g., a flash memory), a floppy disk, a compact disc-read only memory (CD-ROM), a magneto-optical disk (MOD), a digital versatile disc (DVD) and/or other suitable memory elements. Such memory device(s)  208  may generally be configured to store suitable computer-readable instructions that, when implemented by the processor(s)  206 , configure the controller  200  to perform various functions including, but not limited to, operating a power generating asset  100  as described herein, as well as various other suitable computer-implemented functions. 
     Referring particularly to  FIG. 5 , the controller  200  of the system  300  may be configured to initiate a state-change event  306  for the energy storage device(s)  150 . The state-change event  306  may define a plurality of sampling intervals. In an embodiment, the plurality of sampling intervals may be time intervals, the sum of which may correspond to a duration of the state-change event  306 . As depicted at  308 , the controller  200  may, in an embodiment, determine a change in the first and second electrical conditions  302 ,  304  at each of the plurality of sampling intervals of the state-change event  306 . From the change in the first and second electrical conditions  302 ,  304 , the controller  200  may determine an actual equivalent series resistance (ESR) value (ESR t ) at each of the plurality of sampling intervals of the state-change event  306 . The controller  200  may then utilize the ESR value (ESR t ) to determine an actual ESR function  310  for the energy storage device(s)  150 . In an embodiment, the controller  200  may determine a state-of-health rating  312  for the energy storage device(s)  150  based on the actual ESR function  310  of the energy storage device(s)  150 . Based on the state-of-health rating  312 , in an embodiment, a control action  314  may be implemented. 
     In an embodiment, the state-change event  306  may be a discharge event. In an additional embodiment, the state-change event  306  may be a charging event. Accordingly, the state-change event  306  may, for example, be associated with a scheduled test event of the energy storage device(s)  150  and/or the component  142 . Additionally, the state-change event  306  may, in an embodiment, be associated with a manipulation (e.g. usage) of the energy storage device(s) during an operation of the power generating asset  100 . In other words, in an embodiment, the scheduled test event may be accomplished in conjunction with at least one additional component test of the power generating asset  100  and may result in an updating of the state-of-health rating  312 . 
     In order to determine the state-of-health rating  312  for the energy storage device(s)  150 , the controller  200  may, as depicted at  308 , determine the change in the first and second electrical conditions  302 ,  304  which may occur during the state-change event  306 . In an embodiment, the change may be detected, received, and/or computed at each of the sampling intervals defined by the state-change event  306 . Accordingly, the change in the first and second electrical conditions  302 ,  304  may reflect a difference between an initial state of the first and second electrical conditions  302 ,  304  and a state of the electrical conditions at each of the sampling intervals. 
     As depicted in  FIG. 6 , in an embodiment, the first electrical condition  302  may, for example, be a voltage (V). In such an embodiment, determining a change in the first electrical condition  302  of the energy storage device(s)  150  may include determining a difference between an open-circuit voltage (V OC ) of the energy storage device(s)  150  and an instantaneous voltage (V t ) of the energy storage device(s)  150  at each of the plurality of sampling intervals of the state-change event  306 . The open-circuit voltage (V OC ) may represent a no-load voltage, or a rest potential, of the energy storage device(s)  150 . The open circuit voltage (V OC ) may depend on a state of charge of the energy storage device(s)  150 . In an embodiment wherein the energy storage device(s)  150  is configured as a backup energy source, such as an uninterrupted power supply, the open-circuit voltage (V OC ) may be considered to be a top-of-charge voltage. As such, the open-circuit voltage (V OC ) may be a known/nominal value. In an additional embodiment, the open-circuit voltage (V OC ) may be measured prior to the initiation of the state-change event  306 . 
     In an additional embodiment, the second electrical condition  304  may, for example, be a current (I). Accordingly, determining a change in the second electrical condition  304  of the energy storage device(s)  150  may include determining a difference between the initial current (I i ) of the energy storage device(s)  150  immediately preceding the state-change event  306  and the instantaneous current (I t ) of the energy storage device(s)  150  at each of the plurality of sampling intervals of the state-change event  306 . In an embodiment wherein the energy storage device(s)  150  is neither charging nor discharging prior to the initiation of the state-change event  306 , the initial current (I i ) of the energy storage device(s)  150  preceding the state-change event  306  may be zero. As such, the change in the current (I) may equal the instantaneous current (I t ) measured by the condition sensor(s)  158  at each of the plurality of sampling intervals during the state-change event  306 . In an additional embodiment, wherein the energy storage device(s)  150  is in a charging or discharging state immediately prior to the state-change event  306 , the initial current (I i ) may be measured by the condition sensor(s)  158  at the instant of initiation of the state-change event  306 . 
     It should be appreciated that, in an embodiment, it may be desirable to pre-filter the first and second electrical conditions  302 ,  304  in order to ensure the fidelity of the resultant state-of-health rating  312  for the energy storage device(s)  150 . For example, in an embodiment, the controller  200  may be configured to monitor the current (I) prior to the initiation of the state-change event  306 . The current (I) may, in an embodiment, indicate a recharging operation for the energy storage device(s)  150 . As such, a current (I) above a specified threshold may indicate a degree of charging that may yield uncertainty regarding the open-circuit voltage (V OC ). In such an embodiment, the controller  200  may be configured to refrain from determining the state-of-health rating  312  for the state-change event  306  because uncertainty regarding the open-circuit voltage (V OC ) exists. Similarly, in an alternative embodiment, a current (I) below the specified threshold may indicate that the recharging operation is complete and that the open-circuit voltage (V OC ) may be considered to be a top-of-charge voltage. 
     As particularly depicted in  FIGS. 5 and 6 , in an embodiment, the controller  200  may utilize the changes in the first and second electrical conditions  302 ,  304  to determine the corresponding ESR value (ESR t ) at each of the plurality of sampling intervals of the state-change event  306 . The controller  200  may then utilize the ESR values (ESR t ) to determine an actual ESR function  310 . Accordingly, such an embodiment may normalize the actual ESR function  310  so that the actual ESR function  310  is invariant to a discharge history and discharge profiles, thereby establishing a tractable metric. For example, in an embodiment, the actual ESR function  310  may be a function of the second electrical condition  304  (e.g., current). Further, in an embodiment, the actual ESR function  310  may be independent of a first electrical condition  302  discharge profile and a second electrical condition  304  discharge profile. 
     In an embodiment wherein the first electrical condition  302  is the voltage (V) and the second electrical condition  304  is the current (I), the ESR of the energy storage device(s)  150  may be a function of the instantaneous current (I t ). As such, the ESR values (ESR t ) may be determined using the following equation: 
       ESR t   =V   OC   −V   t   /I   i   −I   t   (Equation 1)
 
     In an embodiment, a degradation of the energy storage device(s)  150  may be manifest in an increasing ESR for a given second electrical condition (e.g. current). As such, the controller  200  may determine a state-of-health rating  312  based on the actual ESR function  310 . In order to facilitate the determination of the state-of-health rating  312 , in an embodiment, the controller  200  may model a nominal ESR function  316  at an ambient temperature  318 . The nominal ESR function  316  may represent the degree of internal resistance for a non-degraded energy storage device(s)  150 . Therefore, the nominal ESR function  316  may correlate to a state-of-health rating  312  of a maximal value (e.g., 1, 100%, etc.) which may decrease with the degradation of the energy storage device(s)  150 . It should, however, be appreciated that in an additional embodiment, the state-of-health rating  312  may be indicative of a degree of degradation and may therefore be a minimal value (e.g., 0, 0%, etc.) which may increase in proportion to an increasing degradation of the energy storage device(s)  150 . It should be appreciated that when the actual ESR function  310  corresponds to the nominal ESR function  316 , the state-of-health rating  312  may be considered to be a non-degraded state-of-health rating. 
     In addition to modeling the nominal ESR function  316 , the controller  200  may, in an embodiment, also model at least one potential ESR function  320  at the ambient temperature  318 . The potential ESR function(s)  320  may, as depicted in  FIG. 7 , indicate an increased ESR value  322  relative to a nominal ESR value  324  as a function of the second electrical condition  304 . For example, the potential ESR function(s)  320  may indicate that for a given current  326 , the energy storage device(s)  150  may present a greater degree of resistance (e.g., points  322 ) than at the nominal ESR value  324 . This increase in the ESR value  322  may be indicative of a degradation of the energy storage device(s)  150 . Therefore, the potential ESR function(s)  320  may correspond to a degraded/reduced state-of-health rating  330  at the same ambient temperature  318 . In an embodiment wherein the non-degraded state-of-health rating  328  represents a maximal value, the degraded state-of-health rating  330  may represent a percentage of the maximum value (e.g., 0.9, 0.8, 0.7, etc.). Similarly, in an embodiment wherein the non-degraded state-of-health rating  328  represents a minimal value, the degraded state-of-health rating  330  may represent a degree of degradation (e.g., 0.1, 0.2, 0.3, etc.). 
     As depicted at  332  of  FIG. 5 , in an embodiment, the controller  200  may consolidate the nominal ESR function  316  and the potential ESR function(s)  320  into a look-up table, a graphical representation, and/or an algorithm. This consolidation may establish a correlation  334  between the actual ESR function  310  and the state-of-health rating  312  for the energy storage device(s)  150  at the ambient temperature  318  as a function of the second electrical condition  304  of the energy storage device(s)  150 . Accordingly, determining the state-of-health rating  312  for the energy storage device(s)  150  may include the utilization of the look-up table, graphic orientation, and/or algorithm to determine the state-of-health rating  312  corresponding to the actual ESR function  310 , which may be based off of the changes in the electrical conditions during the state-change event  306 . 
     It should be appreciated that the level of internal resistance of the energy storage device(s)  150  may, for example, vary with the ambient temperature  318  regardless of any degradation of the energy storage device(s)  150 . As such, in an embodiment, the controller  200  may define a plurality of temperature intervals  336  across a nominal operating range of temperatures for the energy storage device(s)  150 . In an embodiment, at least one of the plurality of temperature intervals  336  may correspond to or overlap the ambient temperature  318  at the initiation of the state-change event  306 . The plurality of temperature intervals  336  may, in an embodiment, correspond to a plurality of less than or equal to ten-degree temperature intervals  336 . 
     Insofar as the nominal ESR function  316  and potential ESR function(s)  320  may vary at different ambient temperatures at which the energy storage device(s)  150  may operate, in an embodiment, the controller  200  may determine the nominal ESR function  316  and the potential ESR function(s)  320  at each of the plurality of temperature intervals  336 . In an embodiment, the nominal ESR functions  316  and the potential ESR function(s)s  320  at each of the plurality of temperature intervals  336  may be assembled into a multi-dimensional, look-up table, a multi-dimensional graphical representation, and/or an algorithm. Such an assemblage may facilitate the establishment of a correlation between the actual ESR function  310  and the state-of-health rating  312  as a function of the second electrical condition  304  of the energy storage device(s)  150  at each temperature interval of the plurality of temperature intervals  336 . In other words, given an ambient temperature  318  within the nominal operating range of temperatures for the energy storage device(s)  150  and an actual ESR function  310  based on the determined electrical condition change for the state-change event  306 , the correlations (such as those assembled into the multi-dimensional, look-up table, a multi-dimensional graphical representation, and/or an algorithm) may be utilized to determine the state-of-health rating  312  for the energy storage device(s)  150 . 
     Additionally, in an embodiment, the controller  200  may correlate the state-of-health rating  312  determined for the ambient temperature to a state-of-health rating  312  at a second temperature via a correlation function. For example, in an embodiment wherein the second temperature is lower than the ambient temperature, the state-of-health rating  312  at the second temperature may indicate a decreased capability of the energy storage device(s)  150  than is indicated for the energy storage device(s)  150  operating at the ambient temperature. It should be appreciated that the relationship between the state of health rating and the ambient/forecasted temperatures may be employed to predict the ability of the energy storage device(s)  150  to provide the necessary power when required during a forecasted period (e.g. winter or summer). 
     In an embodiment, the system  300  may implement the control action  314  based on the state-of-health rating  312 . For example, in an embodiment, the control action  314  may include generating an alert  338 . The generation of the alert  338  may facilitate the scheduling of a maintenance event. Accordingly, the alert  338  may include an auditory signal, a visual signal, a notification, a system input, and/or any other system which may identify the state-of-health rating  312  to an operator. It should be appreciated that the control action  314 , as described herein, may further include any suitable command or constraint by the controller  200 . For example, in an embodiment, the control action  314  may include temporarily de-rating the power generating asset  100 . Additionally, in an embodiment, the control action  314  may include limiting an operation of at least one component of the power generating asset. For example, the control action  314  may limit a pitching of a rotor blade  112  and/or a yawing of the nacelle  106  of the wind turbine  114  when the state-of-health rating  312  indicates that the energy storage device(s)  150  may not have sufficient health to respond in the expected manner when called upon. 
     As depicted in  FIG. 5 , in implementing the control action  314 , the controller  200  may, in an embodiment, receive a state-of-health threshold  340 . The state-of-health threshold  340  may be indicative of a degree of degradation of the component  142  at which a control action may be desirable. In such an embodiment, the controller  200  may, at  342 , detect an approach of the state-of-health rating  312  to the state-of-health threshold  340 . In response to detecting the approach of the state-of-health rating  312  to the state-of-health threshold  340 , the controller  200  may implement the control action  314  by generating the alert  338  to facilitate scheduling of the maintenance event. 
     Referring still to  FIG. 5 , in an embodiment, the controller  200  of the system  300  may receive a cycle count  344  and/or a time count  346  elapsed from an installation date of the energy storage device(s)  150 . The controller  200  may then determine a correlation  348  between the state-of-health rating  312  and the cycle count  344  and/or time count  346 . The correlation  348  may be indicative of a rate of degradation of the energy storage device(s)  150  per the cycle count  344  and/or the time count  346 . Based on the correlation  348  and the state-of-health threshold  340 , the controller  200  may, as depicted at  350 , in an embodiment, determine a number of cycles and/or time remaining until the state-of-health threshold  340  is reached. In other words, in an embodiment, the controller  200  may determine a rate of degradation of the energy storage device(s)  150  in terms of the cycle/time count  344 ,  346  and utilize the rate of degradation to project a remaining service life until maintenance of the energy storage device(s)  150  is required. 
     Furthermore, the skilled artisan will recognize the interchangeability of various features from different embodiments. Similarly, the various method steps and features described, as well as other known equivalents for each such methods and feature, can be mixed and matched by one of ordinary skill in this art to construct additional systems and techniques in accordance with principles of this disclosure. Of course, it is to be understood that not necessarily all such objects or advantages described above may be achieved in accordance with any particular embodiment. Thus, for example, those skilled in the art will recognize that the systems and techniques described herein may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other objects or advantages as may be taught or suggested 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 include 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 languages of the claims. 
     Further aspects of the invention are provided by the subject matter of the following clauses: 
     Clause 1. A method for operating a power generating asset, the power generating asset having an energy storage device operably coupled to a component of the power generating asset, the method comprising: initiating, with a controller, a state-change event for an energy storage device, the state-change event defining a plurality of sampling intervals; determining, via the controller, a change in a first electrical condition and a second electrical condition of the energy storage device at each of the plurality of sampling intervals of the state-change event; determining, via the controller, an actual equivalent series resistance (ESR) function for the energy storage device based on the change in the first and second electrical conditions at each of the plurality of sampling intervals of the state-change event; determining, via the controller, a state-of-health rating for the energy storage device based on the actual ESR function of the energy storage device; and implementing a control action based on the state-of-health rating. 
     Clause 2. The method of clause 1, wherein determining the state-of-health rating for the energy storage device further comprises: modeling, via the controller, a nominal ESR function for the energy storage device at an ambient temperature, wherein the nominal ESR function corresponds to a maximal state-of-health rating at the ambient temperature; modeling, via the controller, at least one potential ESR function for the energy storage device at the ambient temperature, the at least one potential ESR function indicating an increased ESR value relative to a nominal ESR value as a function of the second electrical condition, the at least one potential ESR function corresponding to a reduced state-of-health rating at the ambient temperature; and consolidating, via the controller, the nominal ESR function and the at least one potential ESR function into at least one of a look-up table, a graphical representation, and an algorithm so as to establish a correlation between the actual ESR function and the state-of-health rating for the energy storage device at the ambient temperature as a function of the second electrical condition of the energy storage device. 
     Clause 3. The method of any preceding clause, further comprising: defining, via the controller, a plurality of temperature intervals across a nominal operating range of temperatures for the energy storage device, wherein a temperature interval of the plurality of temperature intervals corresponds to the ambient temperature; determining, via the controller, the nominal ESR function and the at least one potential ESR function at each temperature interval of the plurality of temperature intervals; and establishing a correlation between the actual ESR function and the state-of-health rating for the energy storage device at each temperature interval as a function of the second electrical condition of the energy storage device. 
     Clause 4. The method of any preceding clause, wherein determining the state-of-health rating for the energy storage device further comprises determining the state-of-health rating corresponding to the determined actual ESR function at the ambient temperature of the energy storage device based on the correlation between the actual ESR function and the state-of-health rating. 
     Clause 5. The method of any preceding clause, wherein the first electrical condition comprises a voltage, and wherein determining a change in the first electrical condition of the energy storage device further comprises determining a difference between an open-circuit voltage of the energy storage device and an instantaneous voltage of the energy storage device at each of the plurality of sampling intervals of the state-change event. 
     Clause 6. The method of any preceding clause, wherein implementing the control action further comprises: detecting an approach of the state-of-health rating to a state-of-health threshold; and generating an alert to facilitate scheduling of a maintenance event. 
     Clause 7. The method of any preceding clause, wherein the state-change event comprises at least one of a scheduled test event and a manipulation of the energy storage device during an operation of the power generating asset. 
     Clause 8. The method of any preceding clause, wherein the scheduled test event is accomplished in conjunction with at least one additional component test of the power generating asset and results in an updating of the state-of-health rating. 
     Clause 9. The method of any preceding clause, wherein the state-change event comprises a discharging event of the energy storage device or a charging event of the energy storage device. 
     Clause 10. The method of any preceding clause, wherein determining the actual ESR function further comprises determining the actual ESR function as a function of the second electrical condition and independent of a first electrical condition discharge profile and a second electrical condition discharge profile. 
     Clause 11. The method of any preceding clause, further comprising: receiving, via the controller, at least one of a cycle count and a time count elapsed from an installation date; determining, via the controller, a correlation between the state-of-health rating and the received at least one of the cycle count and the time count, wherein the correlation is indicative of a rate of degradation of the energy to storage device per at least one of the cycle count and the time count; determining a state-of-health threshold for the energy storage device; and based on the correlation and the state-of-health threshold, determining, via the controller, at least one of a number of cycles and time until the state-of-health threshold is reached. 
     Clause 12. A method for operating an energy storage device, the method comprising: initiating, with a controller, a discharge event for the energy storage device, the discharge event defining a plurality of sampling intervals; determining, via the controller, a change in a voltage and a current of the energy storage device at each of the plurality of sampling intervals of the discharge event; determining, via the controller, an actual equivalent series resistance (ESR) function for the energy storage device based on the change in the voltage and the current at each of the plurality of sampling intervals of the discharge event; determining, via the controller, a state-of-health rating for the energy storage device based on the actual ESR function of the energy storage device; and implementing a control action based on the state-of-health rating. 
     Clause 13. The method of any preceding clause, wherein determining state-of-health rating for the energy storage device further comprises: defining, via the controller, a plurality of temperature intervals across a nominal operating range of temperatures for the energy storage device, wherein a temperature interval of the plurality of temperature intervals corresponds to the ambient temperature; modeling, via the controller, a nominal ESR function for the energy storage device at the ambient temperature, wherein the nominal ESR function corresponds to a maximal state-of-health rating at the ambient temperature; modeling, via the controller, at least one potential ESR function for the energy storage device at the ambient temperature, the at least one potential ESR function indicating an increased ESR value relative to a nominal ESR value as a function of current, the at least one potential ESR function corresponding to a reduced state-of-health rating at the ambient temperature; determining, via the controller, the nominal ESR function and the at least one potential ESR function at each temperature interval of the plurality of temperature intervals; establishing a correlation between the nominal ESR function and the at least one potential ESR function at each temperature interval as a function of the current of the energy storage device; and determining the state-of-health rating corresponding to the determined actual ESR function at the ambient temperature of the energy storage device based on the correlation between the actual ESR function and the state-of-health rating. 
     Clause 14. The method of any preceding clause, wherein determining the actual ESR function further comprises determining the actual ESR function as a function of current and independent of a voltage discharge profile and a current discharge profile. 
     Clause 15. The method of any preceding clause, wherein determining a change in the voltage of the energy storage device further comprises determining a difference between an open-circuit voltage of the energy storage device and an instantaneous voltage of the energy storage device at each of the plurality of sampling intervals of the state-change event. 
     Clause 16. The method of any preceding clause, wherein implementing the control action further comprises: detecting an approach of the state-of-health rating to a state-of-health threshold; and generating an alert to facilitate scheduling of a maintenance event. 
     Clause 17. A system for operating a power generating asset, the system comprising: an energy storage device operably coupled to a component of the power generating asset; and a controller communicatively coupled to the energy storage device, the controller comprising at least one processor configured to perform a plurality of operations, the plurality of operations comprising: initiating a state-change event for an energy storage device, the state-change event defining a plurality of sampling intervals, determining a change in a first and a second electrical condition of the energy storage device at each of the plurality of sampling intervals of the state-change event, determining an actual equivalent series resistance (ESR) function for the energy storage device based on the change in first and the second electrical conditions at each of the plurality of sampling intervals of the state-change event, determining a state-of-health rating for the energy storage device based on the actual ESR function of the energy storage device, and implementing a control action based on the state-of-health rating. 
     Clause 18. The system of any preceding clause, wherein determining the state-of-health rating for the energy storage device further comprises: modeling a nominal ESR function for the energy storage device at an ambient temperature, wherein the nominal ESR function corresponds to a maximal state-of-health rating at the ambient temperature; modeling at least one potential ESR function for the energy storage device at the ambient temperature, the at least one potential ESR function indicating an increased ESR value relative to a nominal ESR value as a function of second electrical condition, the at least one potential ESR function corresponding to a reduced state-of-health rating at the ambient temperature; and consolidating the nominal ESR function and the at least one potential ESR function into at least one of a look-up table, a graphical representation, and an algorithm so as to establish a correlation between the actual ESR function and state-of-health rating for the energy storage device at the ambient temperature as a function of the second electrical condition of the energy storage device. 
     Clause 19. The system of any preceding clause, further comprising: defining a plurality of temperature intervals across a nominal operating range of temperatures for the energy storage device, wherein a temperature interval of the plurality of temperature intervals corresponds to the ambient temperature; determining the nominal ESR function and the at least one potential ESR function at each temperature interval of the plurality of temperature intervals; and establishing a correlation between the actual ESR function and state-of-health rating for the energy storage device at each temperature interval as a function of the second electrical condition of the energy storage device. 
     Clause 20. The system of any preceding clause, wherein determining the state-of-health rating for the energy storage device further comprises determining the state-of-health rating corresponding to the determined actual ESR function at the ambient temperature of the energy storage device based on the correlation between the actual ESR function and the state-of-health rating.