System and method for operating a wind farm during low wind speeds

Abstract

A method for operating a wind farm connected to a power grid that requires active power to vary with reactive power demand, the method comprising monitoring a wind speed at each of a plurality of wind turbines in the wind farm. When the wind speed is within a cut-in wind speed range, the method comprises determining a reactive power margin for the wind farm based on a reactive power demand at an active power output corresponding to the wind speed and a reactive power availability of each of the plurality of wind turbines at the wind speed. The method further comprises determining a possible minimum cut-in rotor speed for each of the plurality of wind turbines that satisfies the reactive power margin. In addition, the method comprises commanding each of the plurality of wind turbines to cut in and begin generating power at a possible minimum cut-in rotor speed that satisfies the reactive power margin.

Description

Technical Field

[0001] The present disclosure relates generally to wind turbines, and more particularly to a system and method for operating a wind farm having one or more wind turbines to increase efficiency during low wind speeds. Background Art

[0002] Generally, during operation of a wind turbine, wind strikes the rotor blades, and the blades convert wind energy into a mechanical torque that drives a low-speed shaft. The low-speed shaft drives a gearbox that then steps up the low rotational speed of the low-speed shaft to drive a high-speed shaft at an increased rotational speed, wherein the high-speed shaft rotatably drives the generator rotor. In many conventional wind turbine configurations, the generator is electrically coupled to a bidirectional power converter that includes a rotor side converter (RSC) connected to a line side converter (LSC) via a regulated DC link. The LSC converts the DC power on the DC link to an AC output power that is combined with power from the generator stator to provide a multi-phase power having a frequency that is substantially maintained at a grid bus frequency (e.g., 50 Hz or 60 Hz).

[0003] The above system is generally referred to as a doubly fed induction generator (DFIG) system, the operating principle of which includes that the rotor winding is connected to the grid via slip rings and the power converter controls the rotor current and voltage. The control of the rotor voltage and current enables the generator to remain synchronized with the grid frequency when the wind turbine speed changes (for example, the rotor frequency may be different from the grid frequency). Moreover, the main sources of reactive power from the DFIG system are from the RSC (generator stator side reactive power) and from the LSC (generator line side reactive power) via the generator. The use of a power converter (particularly the RSC) to control the rotor current / voltage makes it possible to adjust the reactive power (and active power) fed from the RSC to the grid independently of the rotational speed of the generator. In addition, the generator is able to import or export reactive power, which allows the system to support the grid during extreme voltage fluctuations on the grid.

[0004] Typically, the amount of reactive power supplied by a wind farm to the grid during steady-state and transient conditions is determined by regulatory requirements dictated by a grid operator, with a wind farm controller determining reactive power requirements imposed on each wind turbine within the wind farm. A local controller at each wind turbine receives and distributes reactive power requirements between generator sources (e.g., between generator-side reactive power and line-side reactive power).

[0005] Generally, the minimum speed (i.e., cut-in speed) of the DFIG is determined based on the voltage limit (also referred to herein as the rotor voltage) imposed by the RSC. Reducing or extending the minimum speed of the turbine increases the operating slip of the DFIG, which requires the RSC to operate at a higher voltage. Therefore, the upper limit for the rotor voltage usually determines the lowest possible rotor cut-in speed.

[0006] In addition to the minimum turbine speed, the reactive power demand from the DFIG also affects the voltage at the terminals of the RSC. In addition, as mentioned, most grid codes require rated reactive power support during the entire operation of the wind turbine (i.e., from cut-in speed to rated speed). Providing rated reactive power at minimum turbine speed further forces the RSC to operate at a higher voltage.

[0007] One way to reduce the rotor voltage is to reduce the amount of overexcited (or capacitive) reactive power and, if possible, increase the amount of underexcited (or inductive) reactive power produced by the generator. In addition, when wind speeds are close to the cut-in wind speed, most grid codes allow wind farms to produce less than full-load reactive power. For example, the Electric Reliability Council of Texas (ERCOT) generally allows zero reactive power capability when the farm operates below 10% of its rated power output. In contrast, for the rest of the United States, Federal Energy Regulatory Commission (FERC) requirements require reactive power capability to be proportional only to the active power produced by the wind farm.

[0008] Therefore, improved systems and methods for operating a wind turbine power system during low wind speeds to take advantage of the relaxation of reactive power requirements at low wind speeds would be welcome in the art. Accordingly, the present disclosure relates to systems and methods for operating a wind turbine power system during low wind speeds while also meeting reactive power requirements set by a grid code for that power output level. Summary of the invention

[0009] 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.

[0010] In one aspect, the present disclosure relates to a method for operating a wind farm connected to a power grid that requires reactive power demand to vary with active power. The wind farm has a plurality of wind turbines, each of which includes a generator connected to a power converter. The method includes monitoring the wind speed at each of the plurality of wind turbines in the wind farm. When the wind speed is within a cut-in wind speed range, the method includes determining the reactive power margin of the wind farm via a controller of the wind farm based on the reactive power demand under the active power output corresponding to the wind speed and the reactive power availability of each of the plurality of wind turbines at the wind speed. The method also includes determining via the controller the possible minimum cut-in rotor speed for each of the plurality of wind turbines that satisfies the reactive power margin. In addition, the method includes commanding each of the plurality of wind turbines to cut in and begin to generate power at the possible minimum cut-in rotor speed that satisfies the reactive power margin via the controller.

[0011] In an embodiment, the cut-in wind speed range may include wind speeds up to about 5 meters per second (m / s).

[0012] In one embodiment, determining a reactive power margin of a wind farm based on a reactive power demand at an active power output corresponding to a wind speed and a reactive power availability of each of a plurality of wind turbines at the wind speed may include determining a difference between the reactive power demand and a sum of the reactive power availability of each of the plurality of wind turbines, and determining a possible minimum cut-in rotor speed for each of the plurality of wind turbines based on the difference.

[0013] In another embodiment, the method may further include storing the reactive power availability of each of the plurality of wind turbines at different cut-in wind speeds in the controller. Additionally or alternatively, the method may include dynamically sending the reactive power availability of each of the plurality of wind turbines at different cut-in wind speeds to the controller via each of the plurality of wind turbines in real time.

[0014] In several embodiments, a method may include determining in real time a possible lowest cut-in rotor speed for each of a plurality of wind turbines that satisfies a reactive power margin.

[0015] In additional embodiments, when the active power output increases to exceed about 10% of the rated power, the method may include operating each of the plurality of wind turbines in the wind park according to its corresponding standard power-speed curve.

[0016] In yet another embodiment, during a training period for the wind farm, the method may include operating the wind farm at different reactive power set points and voltage set points via the controller. Furthermore, the method may include determining, via the controller, a change in voltage drop and reactive power between terminals of each of the plurality of wind turbines and a point of interconnection to the wind farm. Furthermore, the method may include converting, via the controller, a change in voltage drop and reactive power between terminals of each of the plurality of wind turbines and a point of interconnection to the wind farm into a quantity comparable to an equivalent circuit of a collector system of the wind farm. Furthermore, the method may include matching the quantity to the equivalent circuit of the collector system of the wind farm via the controller.

[0017] In an alternative embodiment, during a training period of the wind farm, the method may include calculating, via the controller, an amount of change in voltage drop and reactive power between terminals of each of the plurality of wind turbines and a point of interconnection of the wind farm. The method may also include matching the amount to an equivalent circuit of a collector system of the wind farm via the controller.

[0018] In yet another embodiment, during a training period of a wind farm, a method may include storing in a controller an amount of change in voltage drop and reactive power between terminals of each of a plurality of wind turbines and a point of interconnection of the wind farm in a lookup table, and matching the amount via the controller to an equivalent circuit of a collector system of the wind farm.

[0019] In certain embodiments, the generator may be a doubly-fed induction generator (DFIG).

[0020] In another aspect, the present disclosure relates to a wind farm connected to a power grid. The wind farm includes a plurality of wind turbines, each of which includes a generator coupled to a power converter and a turbine controller, and a controller communicatively coupled to the turbine controller. The controller is configured to perform a plurality of operations, including but not limited to: monitoring the active power output of the wind farm, and when the active power output is equal to or below a power threshold, calculating the reactive power demand of the wind farm based on the power output and an active power-reactive power (PQ) curve specified by the power grid, calculating the reactive power availability at the interconnection point of the wind farm, comparing the reactive power availability and the reactive power demand, and allowing one or more of the plurality of wind turbines of the wind farm to cut in and start generating power at the lowest possible cut-in rotor speed when the reactive power availability is greater than the reactive power demand.

[0021] In an embodiment, the plurality of operations may further include determining whether all wind turbines where the wind speed is above the cut-in wind speed are cut-in, and if so, determining whether the power coefficient of any of the plurality of wind turbines will improve with the step-down in rotor speed. In another embodiment, if the power coefficient of any of the plurality of wind turbines will improve with the step-down in rotor speed, the plurality of operations may further include commanding those wind turbines to reduce their speeds.

[0022] In one embodiment, the plurality of operations may include determining whether all wind turbines where the wind speed is above the cut-in wind speed are cut-in, and if not, allowing at least one additional wind turbine from the plurality of wind turbines to cut-in at the lowest possible cut-in rotor speed.

[0023] In further embodiments, the plurality of operations may include prohibiting the plurality of wind turbines from cutting in at the lowest possible cut in rotor speed when the power output is above the power threshold.

[0024] In yet another embodiment, the power threshold may be equal to or lower than 10% of the rated power.

[0025] In another embodiment, when the active power output increases above a power threshold, the plurality of operations may include operating the wind farm according to a standard power curve.

[0026] It should be understood that the method and wind farm may also include any combination of additional features and / or steps as described herein.

[0027] 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. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] A complete and enabling disclosure of the present invention, including the best mode thereof, to one of ordinary skill in the art is set forth in the specification with reference to the accompanying drawings, in which:

[0029] Figure 1 A perspective view showing an embodiment of a wind turbine according to the present disclosure;

[0030] Figure 2 Shown to be suitable for Figure 1 A schematic diagram of one embodiment of a wind turbine electrical power system for use with a wind turbine shown in;

[0031] Figure 3 A schematic diagram showing one embodiment of a power converter for a wind turbine according to the present disclosure;

[0032] Figure 4 shows a system configuration of an embodiment of a wind farm according to the present disclosure;

[0033] Figure 5 A flow chart showing one embodiment of a method according to the present disclosure for operating a wind farm connected to a power grid requiring reactive power demand to vary with active power output;

[0034] Figure 6A flow chart showing one embodiment of a method for matching reactive power demand at a point of interconnection of a wind farm with reactive power availability at turbine terminals according to the present disclosure;

[0035] Figure 7 a flow chart showing another embodiment of a method for matching reactive power demand at a point of interconnection of a wind farm with reactive power availability at turbine terminals according to the present disclosure; and

[0036] Figure 8 A flow chart showing another embodiment of a method according to the present disclosure for operating a wind park connected to a power grid requiring reactive power demand varying with active power. DETAILED DESCRIPTION

[0037] Reference will now be made in detail to embodiments of the present invention, one or more examples of which are shown in the drawings. Each example is provided as an explanation of the present invention, not a limitation of the present invention. In fact, it will be apparent to those skilled in the art that various modifications and variations may be made in the present invention without departing from the scope of the present invention. For example, the features shown or described as part of an embodiment may be used with another embodiment to produce yet other embodiments. Therefore, it is intended that the present invention covers such modifications and variations as fall within the scope of the appended claims and their equivalents.

[0038] The minimum cut-in rotor speed of a wind turbine is generally determined as the speed at which the wind turbine can produce real power while also being able to produce / absorb full reactive power within a specified terminal voltage range (such as a maximum voltage). The need to produce overexcited (or capacitive) full reactive power at the high end of the terminal voltage defines a maximum value for the rotor voltage and, therefore, a minimum value for the cut-in rotor speed. This results in very high RPMs for cut-in, which results in poor power coefficient (C) at low wind speeds. p ).

[0039] Most grid codes specify reactive power requirements at the point of interconnection (POI) of a wind farm having multiple wind turbines. Therefore, the present disclosure is directed to a system and method that can predict reactive power and voltage drop between turbine terminals and the POI in real time to allow wind turbines to cut in and stay at the lowest rotor speed (e.g., RPM) that allows sufficient margin to be met for reactive power requirements at the POI with respect to grid code requirements.

[0040] Accordingly, the present disclosure is directed to a system and method for operating a wind farm connected to a power grid that requires reactive power demand that varies with real power. More particularly, the system and method of the present disclosure includes determining a rotor speed (e.g., in RPM) at which an individual wind turbine will cut in and generate real power. The cut-in RPM will be the minimum RPM at which the wind turbine, along with all other wind turbines already online, will have sufficient reactive power margin to meet reactive power / voltage demand at the point of interconnection (POI) of the wind farm immediately after cut-in.

[0041] After cut-in, during low power output, the systems and methods of the present disclosure are also configured to ensure that the wind turbine is operating at an RPM sufficient to produce enough reactive power to meet the needs of the wind farm. The present disclosure achieves this goal by setting a lower RPM limit based on reactive power availability. Typically, when the power produced by the wind turbine is greater than about 10% of its power output, the rotational speed is naturally high enough to ensure that the rotor voltage is no longer a limiting factor in producing reactive power. Beyond this point, the present disclosure may abandon or disable this aspect of its control.

[0042] Now refer to the figure, Figure 1 A perspective view of one embodiment of a wind turbine 10 is shown. As shown, the wind turbine 10 generally includes a tower 12 extending from a support surface 14, a nacelle 16 mounted on the tower 12, and a rotor 18 coupled to the nacelle 16. The rotor 18 includes a rotatable hub 20 and at least one rotor blade 22 coupled to the hub 20 and extending outwardly from the hub 20. For example, in the illustrated embodiment, the rotor 18 includes three rotor blades 22. However, in alternative embodiments, the rotor 18 may include more or less than three rotor blades 22. Each rotor blade 22 may be spaced about the hub 20 to facilitate rotating the rotor 18 to enable kinetic energy to be converted from the wind into usable mechanical energy and subsequently into electrical energy. For example, as will be described below, the rotor 18 may be rotatably coupled to a generator 120 ( Figure 2 One or more wind conditions (such as wind speed and / or wind direction) may also be monitored via a wind sensor 24 (such as an anemometer) located on the nacelle 16 or any other suitable location near the wind turbine 10.

[0043] Wind power generation is typically performed by having a wind turbine generator 120 ( Figure 2 ) is provided in a wind farm of a large number (typically 100 or more) of wind turbines 10, wherein each individual wind turbine 10 typically experiences unique wind forces. Accordingly, the output power for each individual wind turbine generator 120 may vary from one wind turbine 10 to another wind turbine 10 within the wind farm.

[0044] As is generally understood, active power and reactive power are provided by each wind turbine generator 120. In some embodiments, the farm-level controller provides reactive power commands (Q) to the wind turbine generators 120 based on the needs of the transmission grid (which may be indicated by the grid operator or determined based on the grid voltage). cmd ). The reactive power requirement may be the same for each wind turbine generator. In an alternative control method, the reactive power command may be individually tailored to the wind turbine generators 120 in the wind farm based on the different power generation characteristics of the respective wind turbine generators 120. It should be appreciated that the present invention is not limited to the manner or method in which reactive power commands are generated for individual wind turbine generators 120.

[0045] Now refer to Figure 2 , a schematic diagram of one embodiment of a wind turbine DFIG power system 100 ("wind turbine system") according to aspects of the present disclosure is shown. Figure 2 While the present disclosure is generally described with reference to the system 100 shown in FIG. 1 , a person of ordinary skill in the art (using the disclosure provided herein) should understand that aspects of the present disclosure may also be applicable to other power generation systems, and as mentioned above, the present invention is not limited to wind turbine systems.

[0046] exist Figure 2 In the embodiment of the present invention, the wind turbine 10 ( Figure 1 ) can be optionally coupled to a gearbox 118, which in turn is coupled to a generator 120, which can be a doubly fed induction generator (DFIG). As shown, the DFIG 120 can be connected to a stator bus 154. In addition, as shown, a power converter 162 can be connected to the DFIG 120 via a rotor bus 156 and to the stator bus 154 via a line-side bus 188. Thus, the stator bus 154 can provide output multi-phase power (e.g., three-phase power) from the stator of the DFIG 120, and the rotor bus 156 can provide output multi-phase power (e.g., three-phase power) from the rotor of the DFIG 120. The power converter 162 can also include a rotor side converter (RSC) 166 and a line side converter (LSC) 168. The DFIG 120 is coupled to the rotor side converter 166 via the rotor bus 156. Additionally, the RSC 166 is coupled to the LSC 168 via the DC link 136, with a DC link capacitor 138 spanning the DC link 136. The LSC 168 is in turn coupled to a line side bus 188.

[0047] As about Figure 3 As will be discussed in greater detail, the RSC 166 and LSC 168 may be configured for a normal operating mode in a three-phase pulse width modulation (PWM) arrangement using insulated gate bipolar transistor (IGBT) switching elements.

[0048] In addition, the power converter 162 may be coupled to a controller 174 to control the operation of the rotor side converter 166 and the line side converter 168. It should be noted that the converter controller 174 may be configured as an interface between the power converter 162 and the local wind turbine control system 176 and may include any number of control devices. In one embodiment, the controller 174 may include a processing device (e.g., a microprocessor, a microcontroller, etc.) that executes computer-readable instructions stored in a computer-readable medium. The instructions, when executed by the processing device, may cause the processing device to perform operations, including providing control commands (e.g., switching frequency commands) to the switching elements of the power converter 162.

[0049] In a typical configuration, various line contactors and circuit breakers may also be included, including, for example, a grid circuit breaker 182, for isolating various components necessary for normal operation of the DFIG 120 during connection to or disconnection from a load, such as a grid 184. For example, a system circuit breaker 178 may couple the system bus 160 to a transformer 180, which may be coupled to a grid 184 via the grid circuit breaker 182. In alternative embodiments, fuses may replace some or all of the circuit breakers.

[0050] In operation, the AC power generated at the DFIG 120 by the rotating rotor 18 is provided to the grid 184 via a dual path, which is defined by the stator bus 154 and the rotor bus 156. On the rotor bus side 156, the sinusoidal multi-phase (e.g., three-phase) alternating current (AC) power is provided to the power converter 162. The rotor-side power converter 166 converts the AC power provided from the rotor bus 156 into direct current (DC) power, and provides the DC power to the DC link 136. As generally understood, the switching elements (i.e., IGBTs) used in the bridge circuit of the rotor-side power converter 166 can be modulated to convert the AC power provided from the rotor bus 156 into DC power suitable for the DC link 136.

[0051] In addition, the LSC 168 converts the DC power on the DC link 136 into an AC output power suitable for the grid 184. In particular, the switching elements (e.g., IGBTs) used in the bridge circuit of the LSC 168 may be modulated to convert the DC power on the DC link 136 into AC power on the line side bus 188. The AC power from the power converter 162 may be combined with the power from the stator of the DFIG 120 to provide a multi-phase power (e.g., three-phase power) having a frequency substantially maintained at the frequency of the grid 184 (e.g., 50 Hz or 60 Hz). In some cases, as mentioned, for a stand-alone DFIG wind turbine power system 100, reactive power may be primarily supplied by the RSC 166 via the generator 120 and the LSC 168.

[0052] In addition, various circuit breakers and switches (such as grid breaker 182, system breaker 178, stator synchronizing switch 158, converter breaker 186, and line contactor 172) may be included in wind turbine power system 100 to connect or disconnect corresponding buses, for example, when current is too large and may damage components of wind turbine power system 100 or for other operational considerations. Additional protection components may also be included in wind turbine power system 100.

[0053] Moreover, the power converter 162 may receive control signals from, for example, a local control system 176 (also referred to herein as a turbine controller) via a converter controller 174. The control signals may be based, among other things, on a sensed state or operating characteristic of the wind turbine power system 100. Typically, the control signals are provided for controlling the operation of the power converter 162. For example, feedback in the form of a sensed speed of the DFIG 120 may be used to control the conversion of the output power from the rotor bus 156 to maintain a suitable and balanced multi-phase (e.g., three-phase) power supply. Other feedback from other sensors may also be used by the controller 174 or the control system 176 to control the power converter 162, including, for example, stator and rotor bus voltage and current feedback. Using various forms of feedback information, switch control signals (e.g., gate timing commands for IGBTs), stator synchronization control signals, and circuit breaker signals may be generated.

[0054] Power converter 162 also compensates or adjusts the frequency of the three-phase power from the rotor for changes in wind speed, for example, at hub 20 and blades 22. Thus, the mechanical and electrical rotor frequencies are decoupled, and electrical stator and rotor frequency matching is facilitated, substantially independent of mechanical rotor speed.

[0055] Under some conditions, the bidirectional nature of power converter 162 (and in particular the bidirectional nature of LSC 168 and RSC 166) facilitates feeding back at least some of the generated electrical power into generator rotor 120. More particularly, electrical power may be transmitted from stator bus 154 to line side bus 188, and then through line contactor 172 and into power converter 162, in particular LSC 168, which acts as a rectifier and rectifies the sinusoidal three-phase AC power into DC power. The DC power is transmitted into DC link 136. Capacitor 138 facilitates mitigating DC link voltage amplitude variations by facilitating mitigation of DC ripple (sometimes associated with three-phase AC rectification).

[0056] The DC power is then transmitted to the RSC 166, which converts the DC electric power into three-phase sinusoidal AC electric power by adjusting the voltage, current and frequency. This conversion is monitored and controlled via the converter controller 174. The converted AC power is transmitted from the RSC 166 to the generator rotor via the rotor bus 156. In this way, the rotor current and voltage are controlled to facilitate generator reactive power control.

[0057] Now refer to Figure 3 , showing aspects according to the present disclosure Figure 2 Detailed schematic diagram of one embodiment of a power converter 162 is shown in . As shown, the RSC 166 includes a plurality of bridge circuits (e.g., H-bridge circuits), wherein each phase of the rotor bus 156 input to the rotor side converter 166 is coupled to a single bridge circuit. In addition, the LSC 168 may also include a plurality of bridge circuits. Similar to the rotor side converter 166, the line side converter 168 also includes a single bridge circuit for each output phase of the line side converter 168. In other embodiments, the line side converter 168, the rotor side converter 166, or both the line side converter 168 and the rotor side converter 166 may include parallel bridge circuits without departing from the scope of the present disclosure.

[0058] Each bridge circuit may generally include a plurality of switching elements (eg, IGBTs) connected in series with each other. Figure 3As shown in , each bridge circuit includes an upper IGBT (e.g., IGBT 212) and a lower IGBT (e.g., IGBT 214). In addition, a diode may be connected in parallel with each of the IGBTs. In an alternative embodiment, the parallel IGBTs and diodes may be used to increase the current rating of the converter. As generally understood, the line side converter 168 and the rotor side converter 166 may be controlled, for example, by providing control commands to the gates of the IGBTs using suitable driver circuits. For example, the converter controller 174 may provide suitable gate timing commands to the gates of the IGBTs of the bridge circuits. The control command may control the switching frequency of the IGBT to provide a desired output. It should be understood by those of ordinary skill in the art that, as an alternative to the IGBT, the power converter 162 may include any other suitable switching element.

[0059] Generally speaking Figure 4 , a block diagram of a wind farm 175 having a plurality of wind turbine systems 100 coupled to a transmission grid 184 (also referred to herein as a power grid) is shown. For example, as shown, each wind turbine system 100 may include a local turbine controller 176 responsive to the state of the controlled wind turbine generator. In one embodiment, the local controller 176 senses only the terminal voltage and current (via potential and current transformers), which are used by the local controller 176 to provide an appropriate response to cause the wind turbine generator 120 to provide a desired reactive power or power factor and voltage.

[0060] Each wind turbine system 100 may be coupled to a collector bus 183 via a generator connection transformer 180 to provide active power and reactive power (respectively labeled P and R) to the collector bus 183. wg and Q wg ). Generator connection transformers and collector buses are known in the art.

[0061] The wind farm 175 provides field-level active power and reactive power output (respectively marked as P and wf and Q wf The farm level controller 190 senses the wind farm output and the voltage at the common connection point 181 (also referred to herein as the interconnection point) with the grid 184 to provide a farm level reactive power command (farm level Q_Cmd).

[0062] A local reactive power command (operator Q_Cmd) is generated and transmitted to each individual wind turbine system 100 in the wind farm 175. In one embodiment, the total reactive power requirement (Q wf) may be divided substantially equally among the wind turbine systems 100 such that the local operator Q_Cmd command is the same for all wind turbine systems 100. In an alternative embodiment, the local operator Q_Cmd command may be based on the available reactive power value for that wind turbine generator 120. For example, an individual wind turbine generator 120 providing relatively more active power (Pwg) may receive a relatively smaller personalized operator Q_Cmd command and provide relatively less active power Pwg. wg The individual wind turbine generators 120 of the same type may receive relatively large personalized operator Q_Cmd commands. This reduces the curtailment of the individual wind turbine generators 120 providing relatively more active power, and thus increases the reactive power command (Q wf ) increases the active power (P) generated by the wind farm 175 wf ).

[0063] It should be appreciated that the converter controller 174, the local wind turbine controller 176, and the field level controller 190 may each correspond to any suitable computing device and / or any combination of computing devices. For example, the controller may include one or more processors and associated memory devices configured to perform a variety of computer-implemented functions. As used herein, the term "processor" refers not only to integrated circuits that are considered to be included in a computer in the art, but also to controllers, microcontrollers, microcomputers, programmable logic controllers (PLCs), application-specific integrated circuits, and other programmable circuits. In addition, the memory device may generally include memory elements, including but not limited to: computer-readable media (e.g., random access memory (RAM)), computer-readable non-volatile media (e.g., flash memory), floppy disks, compact disks-read only memories (CD-ROMs), magneto-optical disks (MODs), digital versatile disks (DVDs), and / or other suitable memory elements. Such memory devices may generally be configured to store suitable computer-readable instructions that, when implemented by a processor, configure the controller to perform various functions, such as the steps disclosed herein.

[0064] Now refer to Figure 5 , a flow chart illustrating an embodiment of a method 300 for operating a wind farm connected to a power grid that requires reactive power demand that varies with active power according to aspects of the present disclosure. In general, the method 300 is described herein as being implemented using, for example, the wind turbine power system 100 described above. However, it should be appreciated that the disclosed method 300 may be implemented using any other suitable wind power generation system configured to supply power (including reactive power) for application to a load (such as a power grid). Additionally, while Figure 5The steps performed in a particular order are depicted for the purpose of illustration and discussion, and the methods described herein are not limited to any particular order or arrangement. Those skilled in the art (using the disclosure provided herein) will appreciate that the various steps of the methods may be omitted, rearranged, combined and / or adjusted in various ways.

[0065] As shown at (302), the method 300 includes monitoring a wind speed at each of a plurality of wind turbines in the wind farm 175. As shown at (304), the method 300 includes determining whether the wind speed is within a cut-in wind speed range. More particularly, in an embodiment, the cut-in wind speed range may include wind speeds up to about 5 meters per second (m / s). However, in general, the cut-in wind speed or cut-in wind speed range generally refers to a wind speed that allows a wind turbine to begin generating active power.

[0066] If the wind speed is within the cut-in wind speed range, as shown at (306), method 300 includes determining, via, for example, a farm-level controller 190 of wind farm 175, a reactive power margin of wind farm 175 based on a reactive power demand at an active power output corresponding to the wind speed and a reactive power availability of each of a plurality of wind turbines at the wind speed.

[0067] Thus, as shown at (308), the method 300 includes determining, via the farm level controller 190, a possible minimum cut-in rotor speed for each of the plurality of wind turbines that satisfies the reactive power margin. In one embodiment, for example, the farm level controller 190 may determine the possible minimum cut-in rotor speed by determining a difference between a reactive power demand and a sum of reactive power availabilities for each of the plurality of wind turbines and determining the possible minimum cut-in rotor speed for each of the plurality of wind turbines based on the difference. More specifically, in one embodiment, the farm level controller 190 may be configured to monitor the difference between the reactive power demand and the reactive power availability in real time.

[0068] In another embodiment, the farm level controller 190 may store the reactive power availability of each of the plurality of wind turbines at different cut-in wind speeds in the farm level controller 190. In an alternative embodiment, the farm level controller 190 may dynamically receive the reactive power availability of each of the plurality of wind turbines in real time from each of the plurality of wind turbines.

[0069] Still refer to Figure 5 As shown at (310), the method 300 also includes commanding each of the plurality of wind turbines via the farm level controller to cut in and begin generating power at the lowest possible cut-in rotor speed that satisfies the reactive power margin. In one embodiment, the farm level controller 190 may also command the wind turbines in real time to change their rotor speed (RPM) as needed.

[0070] In general, if the wind speed continues to increase after the cut-in wind speed range (and thus the active power output for wind farm 175 increases to more than about 10% of rated power), the rotor speed of the wind turbines is expected to naturally increase to a point where the rotor voltage is no longer a limit. Therefore, beyond this point, farm level controller 190 can relinquish this aspect of control and allow the wind turbines of wind farm 175 to follow their standard power-speed curves.

[0071] Now refer to Figure 6 , a flow chart showing an embodiment of a method 400 for matching reactive power demand at the interconnection point 181 of a wind farm 175 with reactive power availability at the turbine terminals according to aspects of the present disclosure. A conventional approach for matching reactive power demand at the interconnection point with reactive power availability at the turbine terminals would be to mathematically convert reactive power availability from the turbine terminals to the interconnection point through an equivalent circuit of the collector system. However, in practice, it is difficult to obtain an equivalent circuit of the collector system. Therefore, Figure 6 Method 400 provides a "train and learn" approach.

[0072] In general, the method 400 is described herein as being implemented using, for example, the wind turbine power system 100 described above. However, it should be appreciated that the disclosed method 400 may be implemented using any other suitable power generation system configured to supply power (including reactive power) for application to a load (such as a power grid). Figure 6 The steps performed in a particular order are depicted for the purpose of illustration and discussion, and the methods described herein are not limited to any particular order or arrangement. Those skilled in the art (using the disclosure provided herein) will appreciate that the various steps of the methods may be omitted, rearranged, combined and / or adjusted in various ways.

[0073] As shown at (402), during a training or commissioning period of the wind farm 175, the method 400 may include operating the wind farm 175 at different reactive power and voltage set points within a low wind speed range via the farm-level controller 190. As shown at (404), during operation of the wind farm 175, the method 400 may include determining or recording, via the farm-level controller 190, a change in voltage drop and reactive power (e.g., loss or gain) between terminals of each of the plurality of wind turbines and a point of interconnection 181 of the wind farm 175.

[0074] Furthermore, as shown at (406), method 400 may include converting, via the farm level controller 190, the change in voltage drop and reactive power between the terminals of each of the plurality of wind turbines and the interconnection point 181 of the wind farm 175 into quantities comparable to an equivalent circuit (i.e., the same point in the system) of the collector system of the wind farm 175. In other words, as shown at (408), method 400 may include matching the quantities, via the farm level controller 190, to the equivalent circuit of the collector system of the wind farm 175.

[0075] In an alternative embodiment, farm level controller 190 may be programmed to calculate parameters of an equivalent circuit of a collector system of wind farm 175 with a relatively limited amount of training.

[0076] In an alternative embodiment, if Figure 7 , a flow chart illustrating another embodiment of a method 450 for matching reactive power demand at the interconnection point 181 of the wind farm 175 with reactive power availability at the turbine terminals according to aspects of the present disclosure. In general, the method 450 is described herein as being implemented using, for example, the wind turbine power system 100 described above. However, it should be appreciated that the disclosed method 450 may be implemented using any other suitable power generation system configured to supply power (including reactive power) for application to a load (such as a power grid). Additionally, while Figure 7 The steps performed in a particular order are depicted for the purpose of illustration and discussion, and the methods described herein are not limited to any particular order or arrangement. Those skilled in the art (using the disclosure provided herein) will appreciate that the various steps of the methods may be omitted, rearranged, combined and / or adjusted in various ways.

[0077] As shown at (452), during a training or commissioning period of the wind farm 175, the method 450 may include operating the wind farm 175 at different reactive power and voltage set points within a low wind speed range via the farm level controller 190. As shown at (454), during operation of the wind farm 175, the method 450 may include determining or recording, via the farm level controller 190, a change in voltage drop and reactive power (e.g., loss or gain) between terminals of each of the plurality of wind turbines and the interconnection point 181 of the wind farm 175.

[0078] Additionally, as shown at (456), the farm level controller 190 may generate ΔV (V poi -V term ) and ΔQ(Q poi -Q term). Moreover, as shown at (458), the field level controller 190 can then directly use the lookup table of ΔV and ΔQ values ​​to convert the POI quantity to the turbine quantity for the operating point (e.g., without having to fit it to the R, X, and B parameters of the collector system).

[0079] Now refer to Figure 8 , a flow chart showing another embodiment of a method 500 for operating a wind farm connected to a power grid that requires reactive power demand to vary with active power according to aspects of the present disclosure. In general, the method 500 is described herein as being implemented using, for example, the wind turbine power system 100 described above. However, it should be appreciated that the disclosed method 500 may be implemented using any other suitable wind power generation system configured to supply power (including reactive power) for application to a load (such as a power grid). Additionally, although Figure 8 The steps performed in a particular order are depicted for the purpose of illustration and discussion, and the methods described herein are not limited to any particular order or arrangement. Those skilled in the art (using the disclosure provided herein) will appreciate that the various steps of the methods may be omitted, rearranged, combined and / or adjusted in various ways.

[0080] As shown at (502), the method 500 begins. As shown at (504), the method 500 includes monitoring the active power output of the wind farm 175 and comparing the active power output to a power threshold (e.g., P lim For example, in one embodiment, the power threshold may be equal to or lower than 10% of the rated power, which is generally the threshold above which the rotor voltage is non-limiting.

[0081] As shown at (530) and (532), if the active power output of the wind farm 175 is above the power threshold, the farm level controller 190 may relinquish control of the speed management algorithm of the present disclosure and the algorithm ends. In other words, if the active power output of the wind farm 175 is above the power threshold, the farm level controller 190 may prohibit the wind turbines from cutting in at the lowest possible cut-in rotor speed. In such embodiments, when the active power output increases to exceed the power threshold, the farm level controller 190 may include operating the individual wind turbines of the wind farm 175 according to one or more standard power curves.

[0082] However, when the active power output is equal to or below the power threshold, as shown at (506), the method 500 includes calculating the reactive power demand Q of the wind farm 175 at the interconnection point 181. In addition, as shown at (508), the calculation at (506) can be based on the active power output and the active power-reactive power (PQ) curve specified by the power grid. As shown at (510), the method 500 can also include calculating the reactive power availability at the interconnection point 181 of the wind farm 175 based on the turbine availability and the BoP (Bondage Plant) loss. More specifically, as shown at (512), the calculation at (510) can be completed using a database of turbine Q availability for each speed (RPM) (e.g., the maximum over-excitation reactive power availability of each wind turbine in the speed range from the minimum cut-in speed (rpm) to the rpm at which the rotor voltage no longer limits the reactive power availability). Alternatively or additionally, as shown at (514), the calculation at (510) may be accomplished using a database of learned ΔQ and ΔV (i.e., gains / losses in reactive power and / or voltage drops). More specifically, in an embodiment, during commissioning of the wind farm 175, the farm-level controller 190 may operate at low power with grouped Q and V values ​​(which allows the farm-level controller 190 to learn how to account for BoP losses) and convert turbine terminal quantities to POI quantities.

[0083] Still refer to Figure 8 , as shown at (516), method 500 includes comparing reactive power availability and reactive power demand to determine whether the available reactive power is greater than the reactive power demand. If not, as shown at (518), the farm level controller 190 may direct all wind turbines to increase their speeds by a stepwise increase (e.g., an increase of approximately 15%), and then at (504), method 500 will restart. Alternatively, if the available reactive power is greater than the reactive power demand, as shown at (520), method 500 is configured to determine whether all wind turbines in wind farm 175 where the wind speed is above the cut-in wind speed threshold begin generating power. If not, as shown at (522), method 500 includes allowing the next wind turbine (or one or more wind turbines) to cut in at the lowest possible cut-in speed, and then restarting at (504).

[0084] Alternatively, if all wind turbines in wind farm 175 where the wind speed is above the cut-in wind speed threshold begin generating power, as shown at (524), method 500 is configured to determine the power coefficient (C P ) will increase with a gradual decrease in speed (e.g., about 1%). More specifically, as shown at (526), ​​the farm level controller 190 may determine the C of any of the wind turbines based on a lookup table relating turbine speeds to their respective power coefficients. PWill it increase with a gradual decrease in speed?

[0085] If, as shown at (528), the C of any of the wind turbines P will increase with a step-down in speed, then the method 500 is configured to direct or command those wind turbines to reduce their speed (RPM) by stepping down. The method 500 then restarts at (504).

[0086] Various aspects and embodiments of the present invention are defined by the following numbered clauses:

[0087] Clause 1. A method for operating a wind farm connected to a power grid requiring reactive power demand that varies with active power, the wind farm having a plurality of wind turbines each comprising a generator coupled to a power converter, the method comprising:

[0088] monitoring a wind speed at each of a plurality of wind turbines in a wind farm;

[0089] When the wind speed is within the cut-in wind speed range,

[0090] determining, via a controller of the wind farm, a reactive power margin of the wind farm based on a reactive power demand at an active power output corresponding to a wind speed and a reactive power availability of each of the plurality of wind turbines at the wind speed;

[0091] determining, via the controller, a possible lowest cut-in rotor speed for each of the plurality of wind turbines that satisfies the reactive power margin; and

[0092] Each of the plurality of wind turbines is commanded via a controller to cut in and begin producing power at a lowest possible cut in rotor speed that satisfies a reactive power margin.

[0093] Clause 2. The method of clause 1, wherein the cut-in wind speed range includes wind speeds up to about 5 meters per second (m / s).

[0094] Clause 3. The method of clause 1 or 2, wherein determining, via a controller of the wind farm, a reactive power margin of the wind farm based on a reactive power demand at an active power output corresponding to a wind speed and a reactive power availability of each of the plurality of wind turbines at the wind speed further comprises:

[0095] determining a difference between a reactive power demand and a sum of reactive power availabilities of each of the plurality of wind turbines; and

[0096] A possible lowest cut-in rotor speed for each of the plurality of wind turbines is determined based on the difference.

[0097] Clause 4. The method of clause 3, further comprising storing in the controller reactive power availability for each of the plurality of wind turbines at different cut-in wind speeds.

[0098] Clause 5. The method of clause 3, further comprising dynamically sending, via each of the plurality of wind turbines, reactive power availability of each of the plurality of wind turbines at different cut-in wind speeds to the controller in real time.

[0099] Clause 6. The method of any preceding clause, further comprising determining in real time a possible lowest cut-in rotor speed for each of the plurality of wind turbines that satisfies the reactive power margin.

[0100] Clause 7. The method of clause 1, further comprising operating each of the plurality of wind turbines in the wind farm according to its corresponding standard power-speed curve when the active power value increases to exceed about 10% of the rated power.

[0101] Clause 8. A method according to any of the preceding clauses, further comprising:

[0102] During the training period at the wind farm,

[0103] operating the wind farm at different reactive power set points and voltage set points via a controller;

[0104] determining, via a controller, a change in voltage drop and reactive power between terminals of each of the plurality of wind turbines and a point of interconnection of the wind farm;

[0105] converting, via a controller, a change in voltage drop and reactive power between terminals of each of the plurality of wind turbines and a point of interconnection of the wind farm into quantities comparable to an equivalent circuit of a collector system of the wind farm; and

[0106] This quantity is matched via a controller to the equivalent circuit of the collector system of the wind farm.

[0107] Clause 9. A method according to any of the preceding clauses, further comprising:

[0108] During the training period at the wind farm,

[0109] calculating, via the controller, an amount of change in voltage drop and reactive power between terminals of each of the plurality of wind turbines and a point of interconnection of the wind farm;

[0110] This quantity is matched via a controller to the equivalent circuit of the collector system of the wind farm.

[0111] Clause 10. A method according to any of the preceding clauses, further comprising:

[0112] During the training period at the wind farm,

[0113] In the controller, storing the amount of change in voltage drop and reactive power between terminals of each of the plurality of wind turbines and a point of interconnection of the wind farm in a lookup table; and

[0114] This quantity is matched via a controller to the equivalent circuit of the collector system of the wind farm.

[0115] Clause 11. The method of any preceding clause, wherein the generator comprises a doubly fed induction generator (DFIG).

[0116] Clause 12. A wind farm connected to a power grid, the wind farm comprising:

[0117] a plurality of wind turbines, each of the plurality of wind turbines comprising a generator coupled to a power converter and a turbine controller; and

[0118] A controller communicatively coupled to the turbine controller, the controller configured to perform a plurality of operations including:

[0119] Monitor the active power output of the wind farm;

[0120] When the active power output is equal to or lower than the power threshold,

[0121] Calculate the reactive power demand of the wind farm based on the power output and the active power-reactive power (PQ) curve specified by the power grid;

[0122] Calculate reactive power availability at the interconnection point of the wind farm;

[0123] Comparing reactive power availability and reactive power demand; and

[0124] When reactive power availability is greater than reactive power demand, one or more of the plurality of wind turbines of the wind farm are allowed to cut in and start producing power at the lowest possible cut in rotor speed.

[0125] Clause 13. The wind farm of clause 12, wherein the plurality of operations further comprises: determining whether all wind turbines of the plurality of wind turbines where the wind speed is above the cut-in wind speed are cut-in, and if so, determining whether the power coefficient of any one of the plurality of wind turbines will improve with the step-down in rotor speed.

[0126] Clause 14. The wind farm of clause 13, wherein if the power coefficient of any of the plurality of wind turbines would improve with a step reduction in rotor speed, the plurality of operations further comprises commanding those wind turbines to reduce their speed.

[0127] Clause 15. A wind farm as described in clause 12, 13 or 14, wherein the plurality of operations further comprises determining whether all wind turbines of the plurality of wind turbines where the wind speed is above the cut-in wind speed are cut-in, and if not, allowing at least one additional wind turbine from the plurality of wind turbines to cut-in at the lowest possible cut-in rotor speed.

[0128] Clause 16. A wind farm according to clause 12, 13, 14 or 15, wherein the power threshold is equal to or lower than 10% of the rated power.

[0129] Clause 17. The wind farm of clause 16, wherein when the active power output increases above the power threshold, the plurality of operations further comprises operating each of the plurality of wind turbines in the wind farm according to its respective standard power-speed curve.

[0130] Clause 18. A wind farm as described in clauses 12, 13, 14, 15, 16 or 17, wherein calculating the reactive power availability at the interconnection point of the wind farm further includes storing the reactive power availability of each of the plurality of wind turbines at different cut-in wind speeds in a controller, and calculating the reactive power availability at the interconnection point of the wind farm based on the stored reactive power availabilities of the plurality of wind turbines at different cut-in wind speeds.

[0131] Clause 19. A wind farm according to clauses 12, 13, 14, 15, 16, 17 or 18, wherein calculating the reactive power availability at the interconnection point of the wind farm further comprises dynamically sending the reactive power availability of each of the plurality of wind turbines to a controller in real time via each of the plurality of wind turbines, and calculating the reactive power availability at the interconnection point of the wind farm based on the calculated reactive power availabilities of the plurality of wind turbines at different cut-in wind speeds.

[0132] Clause 20. A wind farm according to clause 12, 13, 14, 15, 16, 17, 18 or 19, wherein the plurality of operations further comprises:

[0133] During the training period at the wind farm,

[0134] providing an amount of change in voltage drop and reactive power between terminals of each of the plurality of wind turbines and a point of interconnection to the wind farm;

[0135] This quantity is matched via a controller to the equivalent circuit of the collector system of the wind farm.

[0136] 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 device or system and performing any combined 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 with insubstantial differences from the literal language of the claims.

Claims

1. A method for operating a wind farm connected to a power grid, the power grid requiring reactive power demand varying with active power, the wind farm having a plurality of wind turbines, the plurality of wind turbines each comprising a generator coupled to a power converter, the method comprising: monitoring wind speed at each of the plurality of wind turbines in the wind farm; When the wind speed is within the cut-in wind speed range, determining, via a controller of the wind farm, a reactive power margin of the wind farm based on the reactive power demand at the active power output corresponding to the wind speed and a reactive power availability of each of the plurality of wind turbines at the wind speed; determining, via the controller, a possible lowest cut-in rotor speed for each of the plurality of wind turbines that satisfies the reactive power margin; as well as commanding, via the controller, each of the plurality of wind turbines to cut in and begin producing power at a lowest possible cut in rotor speed that satisfies the reactive power margin; The cut-in wind speed range refers to a wind speed that allows the wind turbine to start generating active power.

2. The method according to claim 1, characterized in that The cut-in wind speed range includes wind speeds up to 5 meters per second (m / s).

3. The method according to claim 1, characterized in that Determining, via a controller of the wind farm, a reactive power margin of the wind farm based on a reactive power demand at the active power output corresponding to the wind speed and a reactive power availability of each of the plurality of wind turbines at the wind speed further comprises: determining a difference between the reactive power demand and a sum of reactive power availabilities of each of the plurality of wind turbines; and A possible lowest cut-in rotor speed for each of the plurality of wind turbines is determined based on the difference.

4. The method according to claim 3, characterized in that The method also includes storing in the controller reactive power availability for each of the plurality of wind turbines at different cut-in wind speeds.

5. The method according to claim 3, characterized in that: The method also includes dynamically transmitting, via each of the plurality of wind turbines, reactive power availability of each of the plurality of wind turbines at different cut-in wind speeds to the controller in real time.

6. The method according to claim 1, characterized in that The method also includes determining in real time a possible lowest cut-in rotor speed for each of the plurality of wind turbines that satisfies the reactive power margin.

7. The method according to claim 1, characterized in that When the active power value increases to exceed 10% of the rated power, the method further includes operating each of the plurality of wind turbines in the wind park according to its corresponding standard power-speed curve.

8. The method according to claim 1, characterized in that The method further comprises: During a training period at the wind farm, operating the wind farm at different reactive power set points and voltage set points via the controller; determining, via the controller, a voltage drop and a change in reactive power between terminals of each of the plurality of wind turbines and a point of interconnection of the wind farm; converting, via the controller, a change in voltage drop and reactive power between terminals of each of the plurality of wind turbines and a point of interconnection of the wind farm into quantities comparable to an equivalent circuit of a collector system of the wind farm; and The quantity is matched via the controller to an equivalent electrical circuit of a collector system of the wind farm.

9. The method according to claim 1, characterized in that: The method further comprises: During a training period at the wind farm, calculating, via the controller, an amount of change in voltage drop and reactive power between terminals of each of the plurality of wind turbines and a point of interconnection of the wind farm; The quantity is matched via the controller to an equivalent electrical circuit of a collector system of the wind farm.

10. The method according to claim 1, characterized in that The method further comprises: During a training period at the wind farm, In the controller, storing the amount of change in voltage drop and reactive power between terminals of each of the plurality of wind turbines and a point of interconnection of the wind farm in a lookup table; and The quantity is matched via the controller to an equivalent electrical circuit of a collector system of the wind farm.

11. The method according to claim 1, characterized in that: The generator comprises a doubly-fed induction generator (DFIG).

12. A wind farm connected to a power grid, the wind farm comprising: a plurality of wind turbines, each of the plurality of wind turbines comprising a generator coupled to a power converter and a turbine controller; as well as A controller communicatively coupled to the turbine controller, the controller being configured for operating the wind farm according to the method of any one of claims 1 to 11.

13. The wind farm according to claim 12, characterized in that: The controller is also configured to determine whether all of the plurality of wind turbines where the wind speed is above the cut-in wind speed are cut-in, and if so, whether the power coefficient of any of the plurality of wind turbines will improve with the step-down in rotor speed.

14. The wind farm according to claim 13, characterized in that: The controller is further configured to command any of the plurality of wind turbines to reduce their speed if the power coefficient of those wind turbines would improve with a step reduction in rotor speed.

15. The wind farm according to claim 12, characterized in that: The controller is further configured to determine whether all wind turbines of the plurality of wind turbines where the wind speed is above the cut-in wind speed are cut-in, and if not, allow at least one additional wind turbine from the plurality of wind turbines to cut-in at the lowest possible cut-in rotor speed.

16. The wind farm according to claim 12, characterized in that: The power threshold is equal to or lower than 10% of the rated power.

17. The wind farm according to claim 16, characterized in that: When the active power output increases above the power threshold, the controller is further configured to operate each of the plurality of wind turbines in the wind park according to its corresponding standard power-speed curve.

18. The wind farm according to claim 14, characterized in that: The controller is also configured to perform a number of operations, including: Monitoring the active power output of the wind farm; When the active power output is equal to or lower than the power threshold, Calculating reactive power demand of the wind farm based on the active power output and an active power-reactive power (PQ) curve specified by the power grid; calculating reactive power availability at a point of interconnection of the wind farm; comparing the reactive power availability and the reactive power demand; and allowing one or more of the plurality of wind turbines of the wind farm to cut in and begin generating power at a possible lowest cut in rotor speed when the reactive power availability is greater than the reactive power demand; And wherein calculating the reactive power availability at the interconnection point of the wind farm includes storing the reactive power availability of each of the plurality of wind turbines at different cut-in wind speeds in the controller, and calculating the reactive power availability at the interconnection point of the wind farm based on the stored reactive power availability of the plurality of wind turbines at different cut-in wind speeds.

19. The wind farm according to claim 12, characterized in that: Calculating the reactive power availability at the interconnection point of the wind farm includes dynamically sending the reactive power availability of each of the plurality of wind turbines to the controller in real time via each of the plurality of wind turbines, and calculating the reactive power availability at the interconnection point of the wind farm based on the calculated reactive power availabilities of the plurality of wind turbines at different cut-in wind speeds.

20. The wind farm according to claim 12, characterized in that: The controller is also configured to: During a training period at the wind farm, providing an amount of change in voltage drop and reactive power between terminals of each of the plurality of wind turbines and a point of interconnection of the wind farm; The quantities are matched to the equivalent electrical circuit of the collector system of the wind farm.

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