System and method for estimating power grid strength

Abstract

A method for estimating the grid strength of a power grid connected to a renewable energy field having multiple renewable energy power systems includes measuring at least voltage, active power, and reactive power at a point of interconnection of the renewable energy field to the power grid. The method also includes determining the sensitivity of the voltage at the interconnection point to at least one of active power or reactive power. In addition, the method includes determining the grid strength of the power grid based on the sensitivity of the voltage at the interconnection point to at least one of active power or reactive power. In addition, the method includes dynamically determining at least one of an active power command or a reactive power command to the renewable energy field at the interconnection point based on the grid strength. In addition, the method includes distributing at least one of the active power command or the reactive power command to a single controller of the multiple renewable energy power systems and a field-level controller of the renewable energy field.

Description

Technical Field

[0001] The present disclosure relates generally to renewable energy farms, such as wind farms, and more particularly to systems and methods for estimating grid strength of a grid connected to the renewable energy farm. Background Art

[0002] Wind energy is considered to be one of the cleanest and most environmentally friendly energy sources currently available, and wind turbines are gaining more and more attention in this regard. Modern wind turbines typically include a tower, a generator, a gearbox, a nacelle, and one or more rotor blades. The rotor blades use the known airfoil principle to capture the kinetic energy of the wind. For example, the rotor blades typically have a cross-sectional profile of an airfoil so that during operation, the airflow over the blades creates a pressure difference between the sides. Therefore, a lift force directed from the pressure side to the suction side acts on the blades. The lift generates torque on the main rotor shaft, which is coupled to a generator for generating electricity.

[0003] During operation, the wind affects the rotor blades and the blades convert wind energy into a mechanical rotation torque, which rotatably drives the low-speed shaft. The low-speed shaft is configured to drive a gearbox, which subsequently increases the low rotation speed of the low-speed shaft, thereby driving the high-speed shaft with an increased rotation speed. The high-speed shaft is generally rotatably coupled to the generator so as to rotatably drive the generator rotor. In this way, a rotating magnetic field can be induced by the generator rotor, and a voltage can be induced in the generator stator that is magnetically coupled to the generator rotor. The associated electric power can be transmitted to the main transformer, which is usually connected to the power grid via a power grid breaker. Therefore, the main transformer increases the voltage amplitude of the electric power so that the converted electric power can be further transmitted to the power grid.

[0004] In many wind turbines, the generator may be electrically coupled to a bidirectional power converter including a rotor-side converter coupled to a line-side converter via a regulated DC link. Furthermore, a wind turbine power system may include a variety of generator types including, but not limited to, a doubly-fed induction generator (DFIG).

[0005] With the increasing penetration of renewable energy, transmission upgrades lag behind generation additions leading to a situation where grid capacity (strength) deteriorates as capacity additions increase. From an operational perspective, wind farms will be connected to a grid with reduced short circuit capability (weaker grid), leading to challenges such as voltage stability and power evacuation capabilities in unexpected situations. Short circuit ratio (SCR) or grid strength is usually assessed offline during the planning phase, while the true grid strength varies based on the grid operating status (loading level, compensation subsystem, outage, etc.).

[0006] In view of the foregoing, it would also be advantageous to provide improved systems and methods for estimating grid strength in real time. Summary of the invention

[0007] 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.

[0008] In one aspect, the present disclosure is directed to a method for estimating the grid strength of a power grid connected to a renewable energy field having a plurality of renewable energy power systems. The method includes measuring at least voltage, active power, and reactive power at a point of interconnection of the renewable energy field to the power grid. The method also includes determining the sensitivity of the voltage at the interconnection point to at least one of the active power or the reactive power. In addition, the method includes determining the grid strength of the power grid based on the sensitivity of the voltage at the interconnection point to at least one of the active power or the reactive power. In addition, the method includes dynamically determining at least one of an active power command or a reactive power command to the renewable energy field at the interconnection point based on the grid strength. In addition, the method includes distributing at least one of the active power command or the reactive power command to a single controller of the plurality of renewable energy power systems and a farm-level controller of the renewable energy field.

[0009] In one embodiment, the sensitivity of the voltage at the interconnection point to at least one of the active power or the reactive power increases as the grid weakens. Furthermore, the weakening of the grid corresponds to a decrease in the short circuit ratio of the generators of the renewable energy farm.

[0010] In another embodiment, the renewable energy field is tightly coupled to adjacent renewable energy fields. In such an embodiment, each of the adjacent renewable energy fields exhibits perturbations in power due to changes in wind conditions and / or grid conditions. As such, the method further includes modeling the grid as a linear time-invariant system.

[0011] In further embodiments, determining the grid strength of the grid based on the sensitivity of the voltage at the interconnection point to at least one of the active power or the reactive power may include: performing a frequency domain transform of each of the voltage, the active power, and the reactive power of the renewable energy field. For example, in some embodiments, the frequency domain transform may include a fast Fourier transform.

[0012] In additional embodiments, determining the grid strength of the power grid based on the sensitivity of the voltage at the interconnection point to at least one of the active power or the reactive power may include: decoupling the effect of the active power on the voltage from the effect of the reactive power on the voltage. In several embodiments, the method may further include calculating a derivative of the active power with respect to the voltage and a derivative of the reactive power with respect to the voltage.

[0013] In certain embodiments, dynamically determining at least one of the active power command or the reactive power command for the renewable energy field at the interconnection point based on the grid strength may include: dynamically determining the active power command and the reactive power command for the renewable energy field at the interconnection point based on the derivative of the active power with respect to the voltage and the derivative of the reactive power with respect to the voltage.

[0014] In yet another embodiment, the renewable energy field may include at least one of a wind field, a solar field, and an energy storage field, or a combination thereof.

[0015] In another aspect, the present disclosure is directed to a method for estimating the grid strength of a power grid connected to a renewable energy power system (such as a wind turbine). The method includes receiving at least a voltage from an interconnection point of the renewable energy power system to the power grid. The method also includes determining a voltage change in response to at least one of an active power injection or a reactive power injection at the interconnection point. In addition, the method includes determining the grid strength of the power grid according to the voltage change in response to at least one of an active power injection or a reactive power injection at the interconnection point. In addition, the method includes dynamically determining at least one of an active power command or a reactive power command to the renewable energy power system at the interconnection point based on the grid strength. In addition, the method includes causing at least one of the active power command or the reactive power command to be distributed to a controller of the renewable energy power system. It should also be understood that the method may also include any of the additional features and / or steps as described herein.

[0016] In yet another aspect, the present disclosure is directed to a system for estimating the grid strength of a grid connected to a wind farm having a plurality of wind turbines. The system includes: one or more sensors communicatively coupled to a point of interconnection of the wind farm for measuring at least voltage, active power, and reactive power. In addition, the system includes a field-level controller having at least one processor. The processor(s) are configured to perform a plurality of operations, including but not limited to: determining a sensitivity of the voltage at the point of interconnection to at least one of the active power or the reactive power; determining the grid strength of the grid based on the sensitivity of the voltage at the point of interconnection to at least one of the active power or the reactive power; and dynamically determining at least one of an active power command or a reactive power command for the wind farm at the point of interconnection based on the grid strength. In addition, the method includes causing at least one of the active power command or the reactive power command to be distributed to a single controller of the plurality of wind turbines. It should also be understood that the system may also include any of the additional features as described herein.

[0017] 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 present invention and together with the description serve to explain the principles of the present invention.

[0018] The present invention provides a set of technical solutions.

[0019] Technical Solution 1. A method for estimating the grid strength of a grid connected to a renewable energy field having a plurality of renewable energy power systems, the method comprising:

[0020] measuring at least voltage, active power and reactive power at a point of interconnection of the renewable energy field to the grid;

[0021] determining a sensitivity of the voltage at the interconnection point to at least one of the active power or the reactive power;

[0022] determining the grid strength of the grid based on the sensitivity of the voltage at the interconnection point to at least one of the active power or the reactive power;

[0023] dynamically determining at least one of an active power command or a reactive power command to the renewable energy field at the interconnection point based on the grid strength; and

[0024] At least one of the active power command or the reactive power command is distributed to a single controller of the plurality of renewable energy systems and a farm-level controller of the renewable energy farm.

[0025] Technical Solution 2. The method as described in Technical Solution 1, wherein the sensitivity of the voltage at the interconnection point to at least one of the active power or the reactive power increases as the power grid becomes weaker.

[0026] Technical Solution 3. The method as described in Technical Solution 2, wherein the weakening of the power grid corresponds to a reduction in the short-circuit ratio of the generator of the renewable energy field.

[0027] Technical Solution 4. A method as described in Technical Solution 1, wherein the renewable energy field is tightly coupled to adjacent renewable energy fields, each of the adjacent renewable energy fields exhibits power disturbances due to changes in wind conditions and / or grid conditions, and the method also includes modeling the grid as a linear non-time-varying system.

[0028] Technical Solution 5. The method of Technical Solution 1, wherein determining the grid strength of the grid according to the sensitivity of the voltage at the interconnection point to at least one of the active power or the reactive power further comprises:

[0029] A frequency domain transformation of each of the voltage, the active power and the reactive power of the renewable energy field is performed.

[0030] Technical Solution 6. A method as described in Technical Solution 5, wherein the frequency domain transform includes a fast Fourier transform.

[0031] Technical Solution 7. The method of Technical Solution 6, wherein determining the grid strength of the power grid according to the sensitivity of the voltage at the interconnection point to at least one of the active power or the reactive power further comprises:

[0032] The effect of the active power on the voltage is decoupled from the effect of the reactive power on the voltage.

[0033] Technical Solution 8. The method as described in Technical Solution 7 also includes calculating the derivative of the active power with respect to the voltage and the derivative of the reactive power with respect to the voltage.

[0034] Technical Solution 9. The method of Technical Solution 8, wherein dynamically determining at least one of the active power command or the reactive power command for the renewable energy field at the interconnection point based on the grid strength further comprises:

[0035] The active power command and the reactive power command for the renewable energy field at the interconnection point are dynamically determined based on the derivative of the active power with respect to the voltage and the derivative of the reactive power with respect to the voltage.

[0036] Technical Solution 10. The method as described in Technical Solution 1, wherein the renewable energy field includes at least one of a wind field, a solar field, and an energy storage field, or a combination thereof.

[0037] Technical Solution 11. A method for estimating the grid strength of a grid connected to a renewable energy power system, the method comprising:

[0038] receiving at least a voltage from a point of interconnection of said renewable energy power system to said grid;

[0039] determining a voltage change in response to at least one of active power injection or reactive power injection at the interconnection point;

[0040] determining the grid strength of the grid based on the voltage change in response to at least one of active power injection or reactive power injection at the interconnection point;

[0041] dynamically determining at least one of an active power command or a reactive power command to the renewable energy power system at the interconnection point based on the grid strength; and

[0042] At least one of the active power command or the reactive power command is caused to be distributed to a controller of the renewable energy power system.

[0043] Technical Solution 12. A system for estimating the grid strength of a grid connected to a wind farm having a plurality of renewable energy power systems, the system comprising:

[0044] one or more sensors communicatively coupled to a point of interconnection of the wind farm for measuring at least voltage, active power and reactive power;

[0045] A field level controller, the field level controller comprising at least one processor, the processor configured to perform a plurality of operations, the plurality of operations comprising:

[0046] determining a sensitivity of the voltage at the interconnection point to at least one of the active power or the reactive power;

[0047] determining the grid strength of the grid based on the sensitivity of the voltage at the interconnection point to at least one of the active power or the reactive power;

[0048] dynamically determining at least one of an active power command or a reactive power command to the wind farm at the interconnection point based on the grid strength; and

[0049] At least one of the active power command or the reactive power command is distributed to a single controller of a plurality of wind turbines.

[0050] Technical Solution 13. The system as described in Technical Solution 12, wherein the sensitivity of the voltage at the interconnection point to at least one of the active power or the reactive power increases as the power grid weakens.

[0051] Technical Solution 14. The system as described in Technical Solution 12, wherein the weakening of the power grid corresponds to a reduction in the short-circuit ratio of the generators of the wind farm.

[0052] Technical Solution 15. A system as described in Technical Solution 12, wherein the wind farm is tightly coupled to adjacent wind farms, each of the adjacent wind farms exhibits power disturbances due to changes in wind conditions and / or grid conditions, and the method also includes modeling the grid as a linear non-time-varying system.

[0053] Technical Solution 16. The system of Technical Solution 12, wherein determining the grid strength of the power grid according to the sensitivity of the voltage at the interconnection point to at least one of the active power or the reactive power further comprises:

[0054] A frequency domain transformation of each of the voltage, the active power, and the reactive power of the wind farm is performed.

[0055] Technical Solution 17. A system as described in Technical Solution 15, wherein the frequency domain transform includes a fast Fourier transform.

[0056] Technical Solution 18. The system of Technical Solution 17, wherein determining the grid strength of the power grid according to the sensitivity of the voltage at the interconnection point to at least one of the active power or the reactive power further comprises:

[0057] The effect of the active power on the voltage is decoupled from the effect of the reactive power on the voltage.

[0058] Technical Solution 19. The system as described in Technical Solution 18 also includes calculating the derivative of the active power with respect to the voltage and the derivative of the reactive power with respect to the voltage.

[0059] Technical Solution 20. The system according to Technical Solution 19, wherein dynamically determining at least one of the active power command or the reactive power command for the wind farm at the interconnection point based on the grid strength further comprises:

[0060] The active power command and the reactive power command for the wind farm at the interconnection point are dynamically determined according to the derivative of the active power with respect to the voltage and the derivative of the reactive power with respect to the voltage. BRIEF DESCRIPTION OF THE DRAWINGS

[0061] A full and enabling disclosure of the invention, including the best mode thereof, to one skilled in the art is set forth in this specification, which refers to the accompanying drawings, wherein:

[0062] Figure 1 illustrates a perspective view of a portion of one embodiment of a wind turbine according to the present disclosure;

[0063] Figure 2 The diagram shows the suitable Figure 1 A schematic diagram of one embodiment of an electrical and control system for use with a wind turbine is shown in;

[0064] Figure 3 illustrates a block diagram of one embodiment of suitable components that may be included in a controller according to the present disclosure;

[0065] Figure 4 A schematic diagram of an embodiment of a wind farm according to the present disclosure is illustrated;

[0066] Figure 5 A schematic diagram illustrating one embodiment of a plurality of wind farms connected to a power grid at an interconnection point according to the present disclosure is illustrated;

[0067] Figure 6 A flow chart illustrating one embodiment of a method for estimating the grid strength of a grid connected to a wind farm according to the present disclosure is illustrated;

[0068] Figure 7 A flow chart illustrating one embodiment of an algorithm 500 that may be implemented by a controller for determining a grid strength of a power grid according to the present disclosure; and

[0069] Figure 8 A graph illustrating one embodiment of the derivatives of voltage with respect to active power and reactive power versus line length (x-axis) for multiple wind farms according to the present disclosure. DETAILED DESCRIPTION

[0070] Now will be referred to in detail embodiments of the present invention, one or more examples in the described embodiments are illustrated in the drawings. Each example is provided in a manner to illustrate the present invention rather than to limit the present invention. In fact, various modifications and variations can be made in the present invention without departing from the scope or spirit of the present invention, which will be apparent to those skilled in the art. For example, the features of the parts illustrated or described as an embodiment can be used together with another embodiment, to produce another embodiment. Therefore, it is intended that the present invention encompasses such modifications and variations, because they fall within the scope of the appended claims and their equivalents.

[0071] In general, the present disclosure is directed to systems and methods for estimating voltage sensitivity to active or reactive power at an interconnection point, which is indicative of grid strength. Furthermore, the systems and methods of the present disclosure are able to reliably estimate voltage sensitivity even in situations where multiple farms are tightly coupled. Prior art methods only operate reliably in situations where farms are electrically far from one another, and therefore have limited application in preventing voltage control-based interactions between tightly coupled wind farms. Thus, the present disclosure utilizes a frequency domain approach to derive voltage sensitivity to active and reactive power injection through a wind farm.

[0072] Thus, the present disclosure has many advantages not presented in the prior art. For example, a wind farm connected to a weak grid (i.e., with a low short circuit ratio (SCR)) exhibits an inability to deliver the active and reactive power generated by its wind turbines. When delivering power into a weak grid, the POI and turbine terminal voltages may rise beyond the designed limits, and thus, active power, reactive power, or both are inevitably curtailed to bring the voltage back within limits. The curtailment of active power reduces the annual energy production (AEP) of the wind farm and thus results in a loss of customer revenue. The voltage control performance of a wind farm is also affected by the strength of the grid, especially in situations where the farms are closely clustered together. Stable operation may require sacrificial trips on wind turbines, which can be avoided by estimating voltage sensitivity and tuning the control based on real-time estimates, as described herein.

[0073] Now refer to the figure, Figure 1 A perspective view of a portion of an exemplary wind turbine 100 according to the present disclosure is illustrated, the exemplary wind turbine 100 being configured to implement methods and apparatus as described herein. Wind turbine 100 includes a nacelle 102, which typically houses a generator (not shown). Nacelle 102 is mounted on a tower 104 having any suitable height that facilitates operation of wind turbine 100, as described herein. Wind turbine 100 also includes a rotor 106 that includes three blades 108 attached to a rotating hub 110. Alternatively, wind turbine 100 may include any number of blades 108 that facilitate operation of wind turbine 100, as described herein.

[0074] refer to Figure 2 , illustrates a schematic diagram of an exemplary electrical and control system 200 that may be used with the wind turbine 100. During operation, wind impacts the blades 108 and the blades 108 transform wind energy into a mechanical rotational torque that rotatably drives a low-speed shaft 112 via a hub 110. The low-speed shaft 112 is configured to drive a gearbox 114, which then increases the low rotational speed of the low-speed shaft 112, thereby driving a high-speed shaft 116 at an increased rotational speed. The high-speed shaft 116 is generally rotatably coupled to a generator 118 so as to rotatably drive a generator rotor 122. In one embodiment, the generator 118 may be a wound rotor three-phase doubly fed induction (asynchronous) generator (DFIG) that includes a generator stator 120 magnetically coupled to a generator rotor 122. As such, a rotating magnetic field may be induced by the generator rotor 122, and a voltage may be induced within the generator stator 120 magnetically coupled to the generator rotor 122. In one embodiment, the generator 118 is configured to convert the rotational mechanical energy into a sinusoidal three-phase alternating current (AC) electrical energy signal in the generator stator 120. The associated power may be transmitted to the main transformer 234 via the stator bus 208, the stator synchronizing switch 206, the system bus 216, the main transformer breaker 214, and the generator side bus 236. The main transformer 234 increases the voltage amplitude of the power so that the transformed power may be further transmitted to the grid 266 via the breaker side bus 240, the grid breaker 238, and the grid bus 242.

[0075] Generator stator 120 may be electrically coupled to stator synchronizing switch 206 via stator bus 208. In one embodiment, to facilitate a DFIG configuration, generator rotor 122 is electrically coupled to bidirectional power conversion assembly 210 or power converter via rotor bus 212. Alternatively, generator rotor 122 may be electrically coupled to rotor bus 212 via any other device that facilitates operation of electrical and control system 200, as described herein. In further embodiments, stator synchronizing switch 206 may be electrically coupled to main transformer breaker 214 via system bus 216.

[0076] The power conversion assembly 210 may include a rotor filter 218 electrically coupled to the generator rotor 122 via the rotor bus 212. The rotor filter bus 219 electrically couples the rotor filter 218 to the rotor side power converter 220. In addition, the rotor side power converter 220 may be electrically coupled to the line side power converter 222 via a single direct current (DC) link 244. Alternatively, the rotor side power converter 220 and the line side power converter 222 may be electrically coupled via a single and independent DC link. In addition, as shown, the DC link 244 may include a positive rail 246, a negative rail 248, and at least one capacitor 250 coupled therebetween.

[0077] 24. In addition, the line side power converter bus 223 can electrically couple the line side power converter 222 to the line filter 224. In addition, the line bus 225 can electrically couple the line filter 224 to the line contactor 226. In addition, the line contactor 226 can be electrically coupled to the transfer breaker 228 via the transfer breaker bus 230. In addition, the transfer breaker 228 can be electrically coupled to the main transformer breaker 214 via the system bus 216 and the connection bus 232. The main transformer breaker 214 can be electrically coupled to the power main transformer 234 via the generator side bus 236. The main transformer 234 can be electrically coupled to the grid breaker 238 via the breaker side bus 240. The grid breaker 238 can be connected to the power transmission and distribution grid via the grid bus 242.

[0078] During operation, alternating current (AC) power generated by the rotation of the rotor 106 at the generator stator 120 is provided to the grid bus 242 via a dual path. The dual path is defined by the stator bus 208 and the rotor bus 212. On the rotor bus side 212, sinusoidal multi-phase (e.g., three-phase) AC power is provided to the power conversion assembly 210. The rotor-side power converter 220 converts the AC power provided from the rotor bus 212 into DC power and provides the DC power to the DC link 244. The switching elements (e.g., IGBTs) used in the bridge circuit of the rotor-side power converter 220 can be modulated to convert the AC power provided from the rotor bus 212 into DC power suitable for the DC link 244.

[0079] The line side converter 222 converts the DC power on the DC link 244 into an AC output power suitable for the grid bus 242. In particular, the switching elements (e.g., IGBTs) used in the bridge circuit of the line side power converter 222 can be modulated to convert the DC power on the DC link 244 into AC power on the line side bus 225. The AC power from the power conversion assembly 210 can be combined with the power from the stator 120 to provide multi-phase power (e.g., three-phase power) having a frequency that is substantially maintained at the frequency of the grid bus 242 (e.g., 50 Hz / 60 Hz). It should be understood that the rotor side power converter 220 and the line side power converter 222 can have any configuration using any switching device that facilitates the operation of the electrical and control system 200, as described herein.

[0080] In addition, power conversion assembly 210 can be coupled in electronic data communication with converter controller 262 and / or turbine controller 202, which are configured to control the operation of rotor-side power converter 220 and line-side power converter 222. For example, during operation, controller 202 can be configured to receive one or more voltage and / or current measurement signals from a first set of voltage and current sensors 252. Thus, controller 202 can be configured to monitor and control at least some of the operating variables associated with wind turbine 100 via sensors 252. In the illustrated embodiment, each of sensors 252 can be electrically coupled to each of the three phases of grid bus 242. Alternatively, sensors 252 can be electrically coupled to any portion of electrical and control system 200 that facilitates operation of electrical and control system 200, as described herein. In addition to the sensors described above, the sensors can also include a second set of voltage and current sensors 254, a third set of voltage and current sensors 256, a fourth set of voltage and current sensors 264 (all in Figure 2), and / or any other suitable sensor. In addition, voltage and current sensors 252, 254, 256, 264 may be configured to directly or indirectly measure the power output of wind turbine 100.

[0081] In addition, the converter controller 262 is configured to receive one or more voltage and current measurement signals. For example, as shown in the illustrated embodiment, the converter controller 262 receives voltage and current measurement signals from a second set of voltage and current sensors 254 coupled in electronic data communication with the stator bus 208. The converter controller 262 may also receive a third set and a fourth set of voltage and current measurement signals from a third set and a fourth set of voltage and current sensors 256, 264. In addition, the converter controller 262 may be configured with any of the features described herein with respect to the turbine controller 202. Furthermore, the converter controller 262 may be separate from or integral to the turbine controller 202.

[0082] Thus, wind turbine controller 202 and converter controller 262 are configured to control various components of wind turbine 100. Thus, as particularly Figure 3 As particularly shown in FIG. 1 , the controller(s) 202, 262 may include one or more processors 204 and associated memory devices 207 configured to perform various computer-implemented functions (e.g., execute methods, steps, calculations, and the like as disclosed herein and store related data). In addition, the controller 202 may also include a communication module 209 for facilitating communication between the controller 202 and various components of the wind turbine 100 (e.g., Figure 2 In addition, the communication module 209 may include a sensor interface 211 (e.g., one or more analog to digital converters) for allowing signals transmitted from one or more sensors to be converted into signals that can be understood and processed by the processor 204. It should be appreciated that the sensors (e.g., sensors 252, 254, 256, 264) can be communicatively coupled to the communication module 209 using any suitable components. For example, as in Figure 3 , the sensors 252, 254, 256, 264 may be coupled to the sensor interface 211 via a wired connection. However, in other embodiments, the sensors 252, 254, 256, 264 may be coupled to the sensor interface 211 via a wireless connection, such as by using any suitable wireless communication protocol known in the art. Thus, the processor 204 may be configured to receive one or more signals from the sensors.

[0083] As used herein, the term "processor" refers not only to integrated circuits included in computers in the art, but also to controllers, microcontrollers, microcomputers, programmable logic controllers (PLCs), application-specific integrated circuits, and other programmable circuits. The processor 204 is also configured to calculate advanced control algorithms and communicate with a variety of Ethernet or serial-based protocols (Modbus, OPC, CAN, etc.). In addition, (one or more) memory devices 207 generally may include (one or more) memory elements, which include but are not limited to computer-readable media (e.g., random access memory (RAM)), computer-readable non-volatile media (e.g., flash memory), floppy disks, compact disk read-only memory (CD-ROM), magneto-optical disks (MOD), digital versatile disks (DVD), and / or other suitable memory elements. Such (one or more) memory devices 207 generally can be configured to store suitable computer-readable instructions, which, when implemented by (one or more) processors 204, configure the controller 202 to perform various functions as described herein.

[0084] It should also be understood that any number or type of sensors may be employed within and at any location in wind turbine 100. For example, the sensors described herein may be temperature sensors, micro inertial measurement units (MIMUs), strain gauges, accelerometers, pressure sensors, humidity sensors, velocity sensors, strain gauges, accelerometers, airflow sensors, angle of attack sensors, vibration sensors, light detection and ranging (LIDAR) sensors, camera systems, fiber optic systems, anemometers, wind vanes, sonic detection and ranging (SODAR) sensors, infrared lasers, radiometers, pitot tubes, radiosonde wind gauges, other optical sensors, and / or any other suitable sensors.

[0085] Reference now Figure 4, it should also be understood that the wind turbine 100 described herein may be part of a wind farm 300 according to the present disclosure. As shown, the wind farm 300 may include a plurality of wind turbines 302, including the wind turbine 100 described above and a farm-level controller 304. For example, as shown in the illustrated embodiment, the wind farm 300 includes twelve wind turbines, including the wind turbine 100. However, in other embodiments, the wind farm 300 may include any other number of wind turbines, such as less than twelve wind turbines or greater than twelve wind turbines. In other embodiments, other energy generation sources, such as solar, chemical, geothermal, and / or thermal generation, may also be added to the wind farm 300. In one embodiment, the controller 202 of the wind turbine 100 may be communicatively coupled to the farm-level controller 304 via a wired connection, such as connecting the farm-level controller 304 via a suitable communication link 306 or a network (e.g., a suitable cable). Alternatively, controller 202 may be communicatively coupled to farm level controller 304 via a wireless connection, such as by using any suitable wireless communication protocol known in the art. Additionally, farm level controller 304 may be generally configured similarly to controller 202 of each of individual wind turbines 302 within wind farm 300.

[0086] In addition, as in Figure 5 As shown in , multiple wind farms 300 can be connected to the power grid 266 at a point of interconnection (POI) 308. In such an embodiment, as shown, the wind farm 300 can be closely coupled to other adjacent wind farms 300. In such an embodiment, each of the adjacent renewable energy fields 300 exhibits power disturbances due to changes in wind conditions and / or grid conditions. In this way, the frequency domain method can be used to decouple the power disturbance from the adjacent wind farm 300 to reduce the impact of the power disturbance from the adjacent wind farm 300 on the locally measured voltage. In addition, for short-term time periods (e.g., a few seconds), the power grid 266 can be reasonably assumed to be a linear non-time-varying system. Therefore, (one or more) controllers 202, 304 can be configured to model the power grid 266 as a linear non-time-varying system, which evaluates the response of linear and non-time-varying systems (e.g., active power or reactive power) to arbitrary input signals (e.g., voltage). In such an instance, power disturbances of certain frequencies will be converted into voltage changes of the same frequency. Methods for decoupling power disturbances from neighboring wind farms 300 and for estimating the grid strength of grid 266 using a frequency domain approach are described below with respect to Figure 6-8 And describe.

[0087] Reference now Figure 6 , a flow chart illustrating an embodiment of a method 400 for estimating the grid strength of a grid connected to a renewable energy farm. The method 400 may be used, for example, in the above reference Figure 1-6The discussed wind turbine 100 and / or wind farm 300 are implemented. Figure 7 The steps are depicted as being performed in a specific order for purposes of illustration and discussion. One skilled in the art, using the disclosure provided herein, will understand that the various steps of method 400 or any of the other methods disclosed herein may be simultaneously adapted, modified, rearranged, performed, or modified in various ways without departing from the scope of the present disclosure.

[0088] As shown at (402), method 400 includes measuring at least voltage, active power, and reactive power at a point of interconnection 308 of wind farm 300 (also referred to herein as a renewable energy farm) to grid 266. As shown at (404), method 400 includes determining a sensitivity of the voltage at the point of interconnection to at least one of active power or reactive power. In one embodiment, the sensitivity of the voltage at the point of interconnection to at least one of active power or reactive power increases as grid 266 weakens. In addition, a weakening of grid 266 corresponds to a decrease in the short circuit ratio of generators 120 of wind farm 300.

[0089] As shown at (406), method 400 includes determining the grid strength of grid 266 based on the sensitivity of the voltage at interconnection point 308 to active power and / or reactive power. More specifically, Figure 7 A flow chart illustrating one embodiment of an algorithm 500 that may be implemented by the controller(s) 202, 304 for determining the grid strength of the power grid 266. As shown at (502), the algorithm 500 begins. As shown at (504), the algorithm 500 receives measured active power, reactive power, and voltage from the POI 308. As shown at (506), the controller(s) 202, 304 are configured to determine the grid strength of the power grid 266 by performing a frequency domain transform of each of the voltage (V), active power (P), and reactive power (Q) of the wind farm 300. For example, in certain embodiments, the frequency domain transform may include a fast Fourier transform. As shown at (508), the algorithm 500 may include setting a magnitude (f) less than a threshold (μ) to zero (0) for each of the voltage (V), active power (P), and reactive power (Q) of the wind farm 300. As shown at (510), the algorithm 500 may include selecting a point where E{P(f)} / E{Q(f)} is greater than or equal to α and E{P(f)} / E{Q(f)} is less than or equal to β, which has the effect of decoupling the effects of active power and reactive power on voltage. As shown at (512), the algorithm 500 may include calculating various ratios of voltage relative to reactive power and active power (e.g., A(f)=E{V(f)} / E{Q(f)} and B(f)=E{V(f)} / E{P(f)}) at a frequency "f" that satisfies the α, β thresholds.

[0090] As shown at (514), the algorithm 500 may also include calculating the median / mean of the ratio from (512), thereby determining its derivative (e.g., dV / dQ=median(A(f)) or mean(A(f)) and dV / dP=median(B(f)) or mean(B(f))). The derivatives (e.g., dV / dQ and dV / dP) represent the sensitivity of the voltage at the interconnection point 308 to the active power and / or reactive power. For example, as shown at Figure 8 , a graph 600 illustrating one embodiment of a relationship of derivatives (e.g., dV / dQ and dV / dP) (y-axis) versus line length (x-axis) for multiple wind farms 300. As shown, an increase in the derivatives (e.g., dV / dQ and dV / dP) corresponds to a weakening of the grid / SCR. Therefore, by calculating the derivatives (e.g., dV / dQ and dV / dP), the (one or more) controllers 202, 304 can determine the strength of the grid. As shown at (516), the algorithm 500 is further configured to determine whether the derivatives (e.g., dV / dQ and dV / dP) are reliable. If so, the algorithm 500 moves to (518), as further described below. If not, as shown at (520), the algorithm 500 utilizes the last known sensitivity.

[0091] Therefore, returning to the reference Figure 6 , as shown at (408), the method 400 includes dynamically determining at least one of an active power command or a reactive power command for the wind farm 300 at the interconnection point based on the grid strength. Thus, as shown at (410), the method 400 also includes distributing the active power command and / or reactive power command to the single controller 202 of the plurality of wind turbines 302 and the farm-level controller 304 of the wind farm 300. For example, in some embodiments, dynamically determining at least one of an active power command or a reactive power command for the wind farm 300 at the interconnection point 308 based on the grid strength may include dynamically determining the active power command and the reactive power command for the wind farm 300 at the interconnection point 308 based on a derivative of the active power with respect to the voltage and a derivative of the reactive power with respect to the voltage.

[0092] More specifically, as in Figure 5 As shown at (518), the controller(s) 202, 304 may use sensitivities (e.g., derivatives) to determine the required delta Q (e.g., ΔQ) and delta P (e.g., ΔP) (if Q-capacity is exhausted) using, for example, equations (1) and (2) below:

[0093]

[0094] in is the voltage change at the interconnection point 308 , Vset is the voltage set point at the interconnection point 308 , and Vmeas is the measured voltage at the interconnection point 308 .

[0095] Still reference Figure 7 As shown at (522), the algorithm 500 then includes distributing the active P and reactive Q power commands to the individual wind turbine controllers 202 and the farm level controller 304. As shown at (524), the algorithm 500 ends.

[0096] Although the various methods and algorithms described herein are generally explained with respect to field-level control, it should also be understood that by utilizing turbine-level measurements, the same methods and algorithms can also be used to obtain turbine-level sensitivity, although the estimate may be relatively noisier. In such instances, further processing can be performed to reduce noise, including, for example, filtering, etc.

[0097] Exemplary embodiments of wind turbines, control systems for wind turbines, and methods of controlling wind turbines are described in detail above. The methods, wind turbines, and control systems are not limited to the specific embodiments described herein, but rather, components of the wind turbines and / or control systems and / or steps of the methods may be utilized independently and separately from other components and / or steps described herein. For example, the control systems and methods may also be used in conjunction with other wind turbine power systems and methods, and are not limited to practice with only the power systems as described herein. Rather, the exemplary embodiments may be implemented and used in conjunction with many other wind turbine or power system applications, such as solar power systems and energy storage power systems.

[0098] Although specific features of various embodiments of the present invention may be shown in the same figure rather than in other figures, this is only for convenience. According to the principles of the present invention, any feature of a figure may be referenced and / or claimed in combination with any feature in any other figure.

[0099] This written description uses examples to disclose the invention, including the best mode, and also to enable those skilled in the art to practice the invention, including making and using any device or system and performing any included method. 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 that are insubstantially different from the literal language of the claims.

Claims

1. A method for estimating the grid strength of a grid connected to a renewable energy field having a plurality of renewable energy power systems, the method comprising: measuring at least voltage, active power and reactive power at a point of interconnection of the renewable energy field to the grid; determining a sensitivity of the voltage at the interconnection point to at least one of the active power or the reactive power; determining a grid strength of the grid based on the sensitivity of the voltage at the interconnection point to at least one of the active power or the reactive power, comprising: performing a frequency domain transformation of each of the voltage, the active power, and the reactive power of the renewable energy field; dynamically determining at least one of an active power command or a reactive power command to the renewable energy field at the interconnection point based on the grid strength; and At least one of the active power command or the reactive power command is distributed to a single controller of the plurality of renewable energy systems and a farm-level controller of the renewable energy farm.

2. The method of claim 1, wherein the sensitivity of the voltage at the interconnection point to at least one of the active power or the reactive power increases as the grid weakens. 3 . The method as claimed in claim 2 , wherein a weakening of the grid corresponds to a reduction in the short-circuit ratio of the generators of the renewable energy farm.

4. The method of claim 1, wherein the renewable energy field is coupled to adjacent renewable energy fields, each of the adjacent renewable energy fields exhibiting power disturbances due to changes in wind conditions and / or grid conditions, the method further comprising modeling the grid as a linear, time-invariant system. The method of claim 1 , wherein the frequency domain transform comprises a Fast Fourier Transform.

6. The method of claim 5, wherein determining the grid strength of the grid based on the sensitivity of the voltage at the interconnection point to at least one of the active power or the reactive power further comprises: The effect of the active power on the voltage is decoupled from the effect of the reactive power on the voltage. 7 . The method of claim 6 , further comprising calculating a derivative of the active power with respect to the voltage and a derivative of the reactive power with respect to the voltage.

8. The method of claim 7, wherein dynamically determining at least one of the active power command or the reactive power command to the renewable energy field at the interconnection point based on the grid strength further comprises: The active power command and the reactive power command for the renewable energy field at the interconnection point are dynamically determined based on the derivative of the active power with respect to the voltage and the derivative of the reactive power with respect to the voltage.

9. The method of claim 1, wherein the renewable energy field comprises at least one of a wind field, a solar field, and an energy storage field, or a combination thereof.

10. A method for estimating the grid strength of a grid connected to a renewable energy power system, the method comprising: receiving at least a voltage from a point of interconnection of said renewable energy power system to said grid; determining a voltage change in response to at least one of active power injection or reactive power injection at the interconnection point; determining the grid strength of the grid based on the voltage change in response to at least one of active power injection or reactive power injection at the interconnection point, comprising: performing a frequency domain transform of each of the voltage, the active power, and the reactive power; dynamically determining at least one of an active power command or a reactive power command to the renewable energy power system at the point of interconnection based on the grid strength; and At least one of the active power command or the reactive power command is caused to be distributed to a controller of the renewable energy power system.

11. A system for estimating the grid strength of a grid connected to a wind farm having a plurality of renewable energy power systems, the system comprising: one or more sensors communicatively coupled to a point of interconnection of the wind farm for measuring at least voltage, active power and reactive power; A field level controller, the field level controller comprising at least one processor, the processor configured to perform a plurality of operations, the plurality of operations comprising: determining a sensitivity of the voltage at the interconnection point to at least one of the active power or the reactive power; Determining a grid strength of the grid based on the sensitivity of the voltage at the interconnection point to at least one of the active power or the reactive power comprises: performing a frequency domain transformation of each of the voltage, the active power, and the reactive power of the wind farm; dynamically determining at least one of an active power command or a reactive power command to the wind farm at the interconnection point based on the grid strength; and At least one of the active power command or the reactive power command is distributed to a single controller of a plurality of wind turbines.

12. The system of claim 11, wherein the sensitivity of the voltage at the interconnection point to at least one of the active power or the reactive power increases as the grid weakens.

13. The system of claim 12, wherein a weakening of the grid corresponds to a reduction in a short circuit ratio of a generator of the wind farm.

14. The system of claim 11, wherein the wind farm is coupled to adjacent wind farms, each of the adjacent wind farms exhibiting power disturbances due to changes in wind conditions and / or grid conditions, the plurality of operations further comprising modeling the grid as a linear, time-invariant system.

15. The system of claim 11, wherein the frequency domain transform comprises a Fast Fourier Transform.

16. The system of claim 15, wherein determining the grid strength of the grid based on the sensitivity of the voltage at the interconnection point to at least one of the active power or the reactive power further comprises: The effect of the active power on the voltage is decoupled from the effect of the reactive power on the voltage.

17. The system of claim 16, further comprising calculating a derivative of the active power with respect to the voltage and a derivative of the reactive power with respect to the voltage.

18. The system of claim 17, wherein dynamically determining at least one of the active power command or the reactive power command to the wind farm at the interconnection point based on the grid strength further comprises: The active power command and the reactive power command for the wind farm at the interconnection point are dynamically determined according to the derivative of the active power with respect to the voltage and the derivative of the reactive power with respect to the voltage.

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