Wind farm and method for operation of a wind farm

Abstract

The present invention relates to a method (300) for operating a wind turbine park (112) with at least two wind turbine plants (100) and a wind turbine park (112) thereof, where each of the wind turbine plants (100) includes an aerodynamic rotor (106) and each of the rotors (106) has an aerodynamic characteristic value.The method includes the following steps: taking a nominal power value of the wind turbine park (112), in particular a nominal value of the electrical power to be fed into the grid by the wind turbine park (112) (310); determining an actual power value of the wind turbine park (112) as the sum of the actual electrical power of the operated wind power plants (100) (320); determining the suitability of a power-reduced operating mode for each of the wind power plants (100) of the wind turbine park (112) based on the relevant aerodynamic characteristic value (340); operating each of the wind power plants (100) in a suitable operating mode and operating the wind turbine park (112) wind power plants (100) in such a way that the determined actual power value does not exceed the nominal power value taken (350).

Description

[0001] The present invention relates to a method for operating a wind farm with at least two wind turbines and an associated wind farm.

[0002] Wind turbines for generating electricity from wind are well-known. Typically, modern wind turbines contain an aerodynamic rotor with a horizontal axis, on which at least one rotor blade is arranged that can be rotated about its longitudinal axis.

[0003] Several wind turbines in a wind farm can be connected to a common grid connection point for feeding electrical energy into the electrical grid. In this case, a wind farm controller can be installed to control the operation of the individual wind turbines and ultimately coordinate the wind farm's feed-in to the electrical grid.

[0004] The operator of the electrical grid may impose requirements on the electrical power fed into the grid. A wind farm must comply with these grid-related requirements to meet applicable standards, for example. This includes, in particular, a requirement for a maximum electrical power output from the wind farm, which, for instance, must not exceed a threshold value provided by the grid, known as the power setpoint.

[0005] If, for example, the provided threshold is below the rated power of the wind farm and the prevailing wind conditions allow the wind turbines to operate above the provided threshold, the park controller must intervene to prevent the wind turbine from generating electrical power exceeding the provided threshold by operating in a power-optimized operating mode.

[0006] US 9,018,782 B2 discloses that wind turbines in a wind farm can be selectively overrated by measuring the difference between the rated and actual power plant outputs and deriving an overrating request signal based on this difference, which is then sent to each turbine. The same value can be sent to every turbine. Alternatively, each turbine can be assigned its own overrating amount based on turbine optimization. Overrating can also be used when external economic factors, such as energy costs, are sufficiently high to outweigh any potential adverse effects of overrating. The fatigue life of turbines and their critical components can also be considered when deciding whether and to what extent an overrating command should be implemented.

[0007] WO 2009 / 082204 A1 discloses a wind turbine park comprising a group of similar wind turbines, each turbine including a tower supporting a nacelle with a number of rotor blades. Each individual wind turbine is transformerless and includes a high-voltage asynchronous generator for AC power generation. The wind turbine park includes a central transformer for the group of similar transformerless wind turbines to transform the generated electricity to a higher voltage, a central control unit to optimize the power generation of the group of similar transformerless wind turbines, and an HVDC system to transmit the generated electricity in DC mode to a public power grid.

[0008] Against this background, it was an objective of the present invention to provide a method for operating a wind farm and a corresponding wind farm that enables standard-compliant operation as effectively as possible.

[0009] In one aspect, a method for operating a wind farm with at least two wind turbines is proposed, wherein the wind turbines each comprise an aerodynamic rotor, wherein the rotors each have an aerodynamic characteristic value, and wherein the method comprises the following steps: i) Obtaining a power setpoint of the wind farm, in particular a setpoint of the electrical power to be fed into the grid by the wind farm, ii) Determining an actual power value of the wind farm as the sum of the actual electrical powers of the operated wind turbines.The procedure further includes: iii) determining the permissibility of a power-reduced operating mode for each of the wind turbines in the wind farm based on the associated aerodynamic characteristic value, and iv) operating the wind turbines in the wind farm in such a way that each of the operated wind turbines is operated in a permissible operating mode and the determined actual power value does not exceed the obtained target power value.

[0010] The power setpoint can, for example, be provided by a grid operator and represent a maximum value for the electrical power to be fed into the electricity grid by the wind farm. It is known that a wind farm, in response to such a power setpoint, ensures that the fed-in electrical power does not exceed this setpoint.

[0011] The term "setpoint" here refers to a maximum value that can be achieved given sufficient wind. It is therefore a prerequisite that the wind conditions actually allow the setpoint to be reached. Wind turbines are typically operated in such a way that, taking various boundary conditions into account, the maximum amount of electrical energy is generated.

[0012] Even though it is therefore not a setpoint in the classical control engineering sense, the term has become established in the field of wind energy technology for the maximum value of the electrical power to be fed into the grid.

[0013] Ideally, the power setpoint can be set to any value between 0 kW (no power fed into the grid) and the rated power of the wind farm. This allows for stepless curtailment.

[0014] The actual power value that is determined is, in particular, an instantaneous value, preferably with high resolution, and most preferably describing a period of at most 1 second. In other words, the actual power value is not a power value averaged over a longer period, but can be described as the actual power value fed into the electrical supply network at the grid connection point at any given moment.

[0015] For this purpose, the actual power output can be measured at a central feed-in point of the wind farm, where the sum of the electrical outputs of the individual wind turbines converges. Alternatively or additionally, the individual wind turbines can transmit their respective actual power output values ​​via data communication to a central park computer, where the calculation of the actual power output is then performed by summing the individual transmitted values ​​from each wind turbine.

[0016] Both methods can be used to determine the actual performance value, which is particularly advantageous in order to detect any deviations between the two determined values ​​and thus identify malfunctions.

[0017] The aerodynamic characteristic value is a parameter that characterizes at least one aerodynamic property of the rotor blade. A particularly advantageous aerodynamic property is the occurrence of flow separation, especially flow separation on the pressure side in an outer region of the rotor blade, which is also known as negative stall. Other aerodynamic properties of the rotor blade can also be specified by the aerodynamic characteristic value.

[0018] The aerodynamic characteristic is not necessarily constant, but preferably depends on environmental and / or operating conditions. It is therefore particularly advantageous to provide the aerodynamic characteristic in a parameterized form. For example, the aerodynamic characteristic can depend on the wind conditions, in particular wind speed, turbulence intensity, wind shear, etc., and / or on operating conditions of the wind turbine, such as rotational speed, angle of attack, and / or the pitch angle of the rotor blades. Here, angle of attack preferably refers to an angle between the rotor axis and the wind direction.

[0019] Preferably, the aerodynamic characteristic allows for the assessment of the feasibility of a specific operating mode. Wind turbines are classically operated in a yield-maximizing mode, in which, for example, the rotor speed and / or the pitch angle of the rotor blades are regulated to maximize the generated electrical power. Other operating modes may have different boundary conditions, such as a maximum permissible noise level. Further operating modes can include power-reduced modes, in which the boundary condition for operation may be the maximum electrical power to be generated. This type of operation is primarily driven by the requirements of an electrical grid operator, although other factors may also necessitate this operating mode.The limitation of electrical power can preferably be stepless, ranging from no power generation to a rated or maximum power output of the wind turbine.

[0020] In some cases, a central park computer, which coordinates the operation of the wind farm, for example, can specify operating parameters for individual wind turbines, such as setting boundary conditions for the operating mode. These boundary conditions could include the aforementioned power limitations or other conditions.

[0021] A central park computer, which, for example, at least partially implements the method according to the disclosure, typically communicates operational requirements to the individual wind turbines. The park computer does not know immediately whether a wind turbine can meet these requirements or, for example, whether excessive throttling of one of the wind turbines might have undesirable consequences.

[0022] Using the aerodynamic characteristic value provided for each wind turbine, the feasibility of the intended operating mode can now be determined. This determination can be performed by the wind turbine itself, and the result transmitted, for example, to the central park computer. Alternatively or additionally, the park computer or another central control unit can perform the determination centrally for each individual wind turbine. For this purpose, the necessary aerodynamic characteristic values ​​can be provided to the central control unit or already pre-stored within it.

[0023] Preferably, the aerodynamic characteristic value is an unchanging value or an unchanging parameterized value that does not change or does not change significantly over the lifetime of the wind turbine.

[0024] Preferably, the method also includes determining, based on the obtained power setpoint, whether reduced-power operation of at least one of the wind turbines in the wind farm is necessary.

[0025] Reduced-power operation is only necessary if the wind turbines, even in optimized operation, generate a sum of actual electrical power outputs that exceeds the target power output. This is not the case, in particular, if the wind speed is too low, so that, for example, full-load operation of the respective wind turbines is unnecessary. Conversely, if the sum of the actual electrical power outputs exceeds the target power output, at least one wind turbine must be operated in a reduced-power mode to prevent the target power output from being exceeded.

[0026] It is known that a required power reduction is distributed evenly among the individual wind turbines of the wind farm. For example, the central park computer might request that all wind turbines be instructed to reduce their power output by a specific amount, e.g., an absolute amount such as 500 kW or a relative amount such as 10% of their rated power, or to a specific power level, e.g., an absolute amount such as 2 MW or a relative amount such as 50% of their rated power. The numerical values ​​given are, of course, only examples.

[0027] Alternatively, a non-uniform power limitation, i.e., a non-uniform throttling of the wind turbines, is also conceivable. The difficulty that is solved by the present disclosure is the prioritization and weighting of the power throttling of the individual wind turbines in the wind farm.

[0028] Preferably, at least one of the wind turbines in the wind farm is not operated so that at least one of the remaining wind turbines can be operated in a permissible operating mode.

[0029] It is a finding of the present disclosure that the heavily throttled operating modes for wind turbines can be particularly critical, especially in comparatively strong winds. Due to the typically high negative angles of attack in such operating modes, there is a risk of flow separation on the pressure side of the rotor blade, particularly in the outer areas, at high wind speeds. This phenomenon is also known as negative stall. Negative stall leads, for example, to high loads on the rotor blade.

[0030] By not operating at least one of the wind turbines, the curtailment of the remaining turbines can be less pronounced without exceeding the overall power target. This ensures that no wind turbine operates in an impermissible mode, such as a severely curtailed operating mode. In this context, "not operating" means that the wind turbine is in an operating mode in which no electrical power is generated. The turbine could, for example, be switched off or in idle mode, although other operating modes in which no electrical energy is generated are also conceivable.

[0031] Preferably, the aerodynamic characteristic value for flow separation, in particular on a pressure side of a rotor blade of the rotor, is indicative.

[0032] This allows flow separation to be avoided by providing the aerodynamic characteristic value, by marking operating modes that are critical for current environmental conditions as inadmissible and preferably not being used accordingly.

[0033] Preferably, the aerodynamic parameter includes a critical pitch angle, wherein flow separation occurs particularly when the critical pitch angle is exceeded.

[0034] The risk of flow separation increases the more the power output of a wind turbine has to be reduced. At constant power / rotation speed, this occurs when the blade turns out of the wind, which is referred to as increasing the pitch angle. Exceeding the critical pitch angle is therefore equivalent to the rotor blade being turned too far out of the wind, making flow separation possible.

[0035] Changing the pitch angle alters the angle of attack of the rotor blades to influence the rotor's aerodynamic efficiency. In particular, it is known to limit the rotor's aerodynamic efficiency in both full-load and reduced-power operation so that the desired electrical power at the generator output, for example, the generator's rated power or the limited set power, is not exceeded. If the rotor blade is set too aerodynamically unfavorably by increasing the pitch angle, flow separation on the pressure side is likely. Therefore, in this design, the aerodynamic parameter indicates whether the critical pitch angle is reached or not.

[0036] Preferably, the aerodynamic characteristic value is determined using a characteristic map.

[0037] A characteristic map is distinguished by its particularly simple implementation in information technology. Of course, other methods for determining the aerodynamic characteristic value based on parameters are also possible.

[0038] Preferably, the characteristic map determines the aerodynamic characteristic value as a function of a rotational speed of the wind turbine, a power level and optionally a degree of pollution of the rotor.

[0039] Preferably, each of the wind turbines is operated within the permissible range of the characteristic curve.

[0040] It is preferred that the entire characteristic map does not need to be continuously evaluated to obtain all permissible operating modes. For example, if a power increase or a power level is specified, the characteristic map for this power level can define the permissible operating modes. If no permissible operating mode is available for the requested power level, for example because the wind speed is too high or for other reasons, the wind turbine can, in one design, be operated at a different power level or shut down.

[0041] In this embodiment, the aerodynamic characteristic includes a critical pitch angle. It is particularly preferred that the pitch angle be compared with the critical pitch angle to determine the permissibility of the operating mode. The operating mode does not need to be set on the wind turbine; it is sufficient to use the aerodynamic characteristic to determine whether the resulting operation is likely to produce a pitch angle that lies outside the permissible range, especially above the critical pitch angle.

[0042] For higher shear values, the resulting pitch angle can be averaged over the rotor azimuth to obtain a mean pitch angle. This also reduces the impact of individual blade adjustments, where the pitch angle of individual rotor blades is partially or completely independently adjusted, and allows for a simple evaluation using the aerodynamic characteristic value.

[0043] Preferably, the operating mode is selected from the permissible operating modes of the individual systems based on at least one of the following criteria: a) remuneration; b) load; c) noise.

[0044] In this configuration, several operating modes from the available operating modes of the wind turbine are therefore permissible. The decision to be made is which of the permissible operating modes the wind turbine will select for operation. This selection can be made either by the wind turbine itself or by a central control unit, such as the park computer or a server.

[0045] Suitable criteria for selecting the operating mode from the permissible operating modes include compensation for the electrical power generated by the wind turbine, loads acting on the wind turbine, and / or noise emissions generated by the wind turbine. Accordingly, conditions are proposed from which, subject to the further constraint that the total power output of the wind turbines does not exceed the target power value and that the respective operating modes are permissible, the operating mode is ultimately selected.

[0046] It is possible that different wind turbines within a wind farm receive different monetary compensation payments for the energy they generate, known as feed-in tariffs. It is preferred that those wind turbines receiving a higher feed-in tariff are curtailed less frequently than those receiving a lower feed-in tariff.

[0047] Furthermore, one design may require that wind turbines located closer to residential areas be operated in a way that reduces noise emissions. While typically greater curtailment results in lower noise emissions, this can be reversed, especially with very strong curtailment. Therefore, the preferred approach is to optimize the operation of all wind turbines at relevant locations while maintaining the constraint of minimizing noise emissions.

[0048] Finally, it is preferable to consider residual loads or service lives when selecting the operating mode. Both options are conceivable, resulting either in an extension or a reduction of the remaining service life. It is particularly preferred that the remaining service lives of the wind turbines in the wind farm be aligned so that the renewal of the entire wind farm coincides as closely as possible with the end of service life of all wind turbines. Alternatively, the wind turbines can be renewed successively, in which case it is preferred that the wind turbines do not reach their respective end of service life simultaneously but rather at staggered intervals.

[0049] Preferably, the aerodynamic characteristic value is determined individually for each of the wind turbines.

[0050] This means that even in cases where several wind turbines of different types or from different manufacturers are operated in a shared wind farm, optimized operation of the wind farm and of each individual wind turbine is possible.

[0051] Preferably, determining the permissibility of a power-reduced operating mode for each of the wind turbines includes determining a target speed of the aerodynamic rotor, and the aerodynamic characteristic value is taken into account when determining the target speed of the aerodynamic rotor.

[0052] Preferably, the operation of the wind turbine in the power-reduced operating mode includes a reduction of the target speed in a first operating range and an increase of the target speed in a second operating range.

[0053] Preferably, the operation of the wind turbine in the power-reduced operating mode includes a change, in particular an increase, of the pitch angle of the rotor.

[0054] Preferably, the aerodynamic rotor is coupled with an electric generator, and the operation of the wind turbine in the power-reduced operating mode includes a change, in particular a reduction, of a generator torque of the electric generator.

[0055] Preferably, the power, in particular the electrical active power at the generator output, of the wind turbine in the power-reduced operating mode is reduced to a predetermined power value and / or by a predetermined power amount.

[0056] Preferably, at least one of the aerodynamic parameters is taken into account when determining at least one target rotational speed of one of the aerodynamic rotors.

[0057] Furthermore, a wind farm with at least two wind turbines and a wind farm control system is provided, wherein the wind farm control system is designed to carry out the method according to the invention.

[0058] The wind farm, according to this aspect, offers the same advantages as the previously described method and can also be advantageously combined with all preferred configurations.

[0059] Further advantages and special features are described below with reference to the attached figures. These show: Fig. 1 A schematic and exemplary representation of a wind energy plant; Fig. 2 A schematic and exemplary representation of a wind farm, and Fig. 3 A schematic and exemplary flowchart of a process.

[0060] Fig. 1 Figure 1 shows a schematic representation of a wind turbine. The wind turbine 100 has a tower 102 and a nacelle 104 on the tower 102. An aerodynamic rotor 106 with three rotor blades 108 and a spinner 110 is mounted on the nacelle 104. During operation of the wind turbine, the aerodynamic rotor 106 is set into rotation by the wind and thus also rotates an electrodynamic rotor or rotor of a generator 101, which is directly or indirectly coupled to the aerodynamic rotor 106. The electric generator is located in the nacelle 104 and generates electrical energy. The pitch angles of the rotor blades 108 can be changed by pitch motors at the rotor blade roots 109 of the respective rotor blades 108.

[0061] The wind turbine 100 has an electric generator 101, which is indicated in the nacelle 104. Electrical power can be generated by means of the generator 101. A feed-in unit 105 is provided for feeding electrical power into the grid; this unit can be specifically designed as an inverter. This allows a three-phase feed-in current and / or a three-phase feed-in voltage with amplitude, frequency, and phase to be generated for feeding into a grid connection point (PCC). This can be done directly or in conjunction with other wind turbines in a wind farm. A plant control unit 103 is provided for controlling the wind turbine 100 and the feed-in unit 105. The plant control unit 103 can also receive setpoint values ​​from external sources, in particular from a central park computer.

[0062] Figur 2 Figure 112 shows a wind farm with three exemplary wind turbines 100, which can be identical or different. The three wind turbines 100 thus represent, in principle, any number of wind turbines in a wind farm 112. The wind turbines 100 supply their power, namely the generated electricity, via an electrical park grid 114. The currents or power outputs of the individual wind turbines 100 are added together, and a transformer 116 is usually provided to step up the voltage in the park in order to feed it into the supply grid 120 at the feed-in point 118, which is also generally referred to as PCC. Fig. 2 This is only a simplified representation of a wind farm 112. For example, the park network 114 can be designed differently, for instance by including a transformer at the output of each wind turbine 100, to name just one other embodiment.

[0063] Wind farm 112 also has a central park computer 122, which can also be referred to as a central park control unit. This can be connected to the wind turbines 100 via data lines 124 or wirelessly, in order to exchange data with the wind turbines and, in particular, to receive measured values ​​from the wind turbines 100 and to transmit control values ​​to the wind turbines 100.

[0064] The central parking computer 122 is functionally understood as the component designed to execute the central parking control system. The central parking computer 122 and / or individual components thereof can also be located remotely, for example, on a server or in the cloud.

[0065] Fig. 3Figure 1 schematically and exemplarily shows a flowchart of a process 300 for operating a wind farm 112, as it is executed, for example, completely or partially on the central park computer 122 and / or the individual wind turbines 100 of the wind farm 112. The distribution of the execution of the process steps between the central park computer 122 and the individual wind turbines 100 can vary depending on the implementation.

[0066] An operator of a wind farm 112 guarantees preferably stepless power curtailment between 0-100% at the feed-in point 118 or grid connection point PCC. It is known to send the same active power control value from the park computer 122 to all wind turbines 100 of the wind farm 112. Method 300 enables individual turbine control with the operation of each wind turbine 100 with individual active power control values, whereby each wind turbine 100 can be subjected to different power curtailments. The central park computer 122 can either directly specify an active power control value to the wind turbines 100 or otherwise specify or communicate an operating mode to be controlled by the wind turbines 100.

[0067] Power reduction can be achieved classically by reducing the engine speed on the power-optimal operating curve, although other types of power reduction are also known.

[0068] Method 100 now makes it possible to consider the effects of power curtailment on rotor blade aerodynamics and thus, in particular at high wind speeds, the risk of flow separation on the pressure side of the profiles of the rotor blades 108 in the outer area of ​​the rotor blade 108. Such flow separations result, among other things, in loud wind noise, aeroelastic instabilities, and high loads. The sensitivity to this problem, which is particularly dependent on wind speed, varies across different types of wind turbines 100 and rotor blades 108.

[0069] Method 300 now enables individual turbine control at different power levels while maintaining the boundary condition of a separation-free airflow around the rotor blade. Additionally, this boundary condition allows for optimized operation based on feed-in tariffs, for example, when different wind turbines receive 100 different feed-in tariffs.

[0070] For this purpose, the procedure 300 initially comprises a step 310 of obtaining a power setpoint for the wind farm 112, in particular a setpoint for the electrical power to be fed into the grid by the wind farm 112. The power setpoint is provided, for example, by the grid operator of the electrical supply network and may require a power curtailment of the wind farm or at least some of the wind turbines 100 if sufficient wind power is available.

[0071] In step 320, an actual power output of the wind farm 112 is determined as the sum of the actual electrical outputs of the operated wind turbines 100. If the actual power output exceeds the target power output, the park computer 122 must intervene and curtail the power output to ensure that the target power output is maintained as the maximum value. If the actual power output is below the target power output, the park computer 122 can preferably ensure that each of the wind turbines 100 – unless excluded by other constraints such as noise generation – operates in a power-optimized operating mode. In this mode, the park computer 122 attempts to generate as much active electrical power as possible to reach the target power output, with the achievement of this target being dependent on the wind speed.

[0072] Step 330 of the procedure now involves verifying whether reduced-power operation of one of the wind turbines is fundamentally necessary. If not, curtailment of wind turbine 100 is not required, and each of the wind turbines 100 can be operated in optimized-power mode in step 335.

[0073] If a curtailment, i.e., reduced-power operation, of at least one of the wind turbines 100 is necessary, the permissibility of a reduced-power operating mode for each of the wind turbines 100 of the wind farm is determined in a further step 340 based on an associated aerodynamic characteristic value. The aerodynamic characteristic value is preferably designed as a characteristic curve and is a function of several input variables, for example, a rotational speed of the wind turbine 100, a power level, and / or a degree of soiling of the rotor. Preferably, an aerodynamic characteristic value can then be obtained as a function of the prevailing wind speed, which indicates whether or not flow separation occurs on the pressure side during operation in the operating mode to be checked under the current wind conditions.

[0074] An operating mode is generally permissible if the aerodynamic characteristic for the current wind conditions indicates that no flow separation occurs.

[0075] The aerodynamic characteristic value can be tabulated or available in the central park computer 122, or it can be available on the individual wind turbines 100, which then independently determine the permissibility of an operating mode.

[0076] In this step 340, the permissibility of only one specific, desired operating mode can be determined. For example, the central parking computer 122 can determine the permissibility of an operating mode with a specific power reduction. Alternatively, several different permissibilities of operating modes can also be determined in step 340. For example, the permissibility can be determined using the aerodynamic characteristic value for different levels of power reduction.

[0077] A particularly advantageous aspect is that it is not necessary to check the permissibility of all operating modes at all times; instead, the check is limited to potentially applicable operating modes. This allows the computing power requirements, for example those of the central parking computer 122, to be kept low, and the processing time until the result of step 340 is available to be reduced.

[0078] In a particularly preferred example, the aerodynamic characteristic value is a critical pitch angle that must not be exceeded under the given conditions for a specific operating mode in order for that operating mode to be deemed permissible.

[0079] Finally, in step 350, taking into account the permissibility determined in step 340, the wind turbines 100 are operated in such a way, i.e., the operating modes of the wind turbines 100 are set in such a way that each of the operated wind turbines 100 is operated in a permissible operating mode and the determined actual power value does not exceed the obtained target power value.

[0080] Depending on the circumstances, at least one of the wind turbines in the wind farm may need to be taken out of operation so that at least one of the remaining turbines can be operated in a permissible operating mode. This is particularly relevant for scenarios with heavily curtailed output and / or high wind speeds.

[0081] The selection of the respective operating mode from several permissible modes can be based in particular on a) remuneration, b) load and / or c) noise.

[0082] This allows for advantageous solutions to several operating scenarios.

[0083] At low wind speeds, wind turbines in the wind farm operate in reduced-power mode, for example, at a lowered rotational speed. As the wind speed increases, depending on the turbine type, wind turbines gradually enter a critical operating state. In this scenario, increasing the rotational speed once a critical wind speed is reached can shift the critical operating state to higher wind speeds. This is achieved by defining the permissible operating modes.

[0084] At high wind speeds, wind turbines, for example, operate at full load in a performance-optimized mode. If a power reduction is requested, wind turbines can enter a critical operating state, depending on the turbine type and the requested power level. In this scenario, the rated speed is preferably maintained during power reduction. A critical operating state is therefore shifted to lower power levels and / or higher wind speeds.

[0085] Determining the permissible operating modes allows operation at reduced speed up to a storm control mode, which is activated when a wind speed exceeds a certain threshold, as no negative stall occurs during operation. Preferably, compared to previously known control systems, the speed is increased from a predetermined wind speed up to the storm control mode. Alternatively, the speed is initially increased, followed, as the wind speed continues to rise, by a decrease to a value that is not critical for the occurrence of negative stall.

[0086] In summary, power reduction at high wind speeds in wind farms must take 112 aerodynamic boundary conditions into account. One proposed solution is to increase or maintain the rated rotational speed to shift critical operating points to higher wind speeds or lower power levels.

[0087] Alternatively, the described method can be combined with a controllable aerodynamic brake in the inner blade area and / or with energy storage solutions. The controllable aerodynamic brake could, for example, include flaps or similar movable components. Both additional concepts expand the application range of the wind farm 112 and the range of permissible operating modes of the individual wind turbines 100.

Claims

1. Method (300) for operating a wind farm (112) having at least two wind power installations (100), wherein the wind power installations (100) each comprise an aerodynamic rotor (106), wherein the rotors (106) each have an aerodynamic characteristic value, wherein the method comprises the following steps: - obtaining (310) a setpoint power value of the wind farm (112), in particular a setpoint value of the electrical power of the wind farm (112) that is to be fed into an electrical supply network, - ascertaining (320) an actual power value of the wind farm (112) as the sum of actual electrical powers of the operated wind power installations (100), characterized in that the method also comprises: - determining (340) a permissibility of a power-reduced operating mode of each of the wind power installations (100) of the wind farm (112) on the basis of the associated aerodynamic characteristic value, - operating (350) the wind power installations (100) of the wind farm (112) in such a way that each of the operated wind power installations (100) is operated in a permissible operating mode and the ascertained actual power value does not exceed the obtained setpoint power value.

2. Method (300) according to Claim 1, further comprising: - determining (330), on the basis of the obtained setpoint power value, whether a power-reduced operation of at least one of the wind power installations (100) of the wind farm (112) is required.

3. Method (300) according to Claim 1 or 2, wherein - at least one of the wind power installations (100) of the wind farm (112) is not operated in order for at least one of the remaining wind power installations (100) to be able to be operated in a permissible operating mode.

4. Method (300) according to one of the preceding claims, wherein the aerodynamic characteristic value is indicative of a flow separation, in particular on a pressure side of a rotor blade (108) of the rotor (106).

5. Method (300) according to Claim 4, wherein the aerodynamic characteristic value comprises a critical pitch angle, wherein the flow separation takes place in particular when the critical pitch angle is exceeded.

6. Method (300) according to one of the preceding claims, wherein the aerodynamic characteristic value is ascertained on the basis of a characteristic map, wherein the characteristic map determines the aerodynamic characteristic value in particular as a function of a rotational speed of the wind power installation (100), a power stage and optionally a degree of soiling of the rotor (106).

7. Method (300) according to Claim 6, wherein each of the wind power installations (100) is operated within the permissible area of the characteristic map.

8. Method (300) according to one of the preceding claims, wherein the operating mode is selected from the permissible operating modes of the individual installations on the basis of at least one of the following criteria: a) tariff b) load c) sound.

9. Method (300) according to one of the preceding claims, wherein the aerodynamic characteristic value is determined individually for each of the wind power installations.

10. Method (300) according to one of the preceding claims, wherein the determination of a permissibility of a power-reduced operating mode of each of the wind power installations (100) comprises a determination of a setpoint rotational speed of the aerodynamic rotor (106) and the aerodynamic characteristic value is taken into account in the determination of the setpoint rotational speed of the aerodynamic rotor, wherein the operation of the wind power installation (100) in the power-reduced operating mode comprises in particular a reduction of the setpoint rotational speed in a first operating range and an increase of the setpoint rotational speed in a second operating range.

11. Method (300) according to one of the preceding claims, wherein the operation of the wind power installation (100) in the power-reduced operating mode involves a change, in particular a reduction, in a pitch angle of the rotor (106).

12. Method (300) according to one of the preceding claims, wherein the aerodynamic rotor (106) is coupled to an electrical generator and the operation of the wind power installation (100) in the power-reduced operating mode involves a change, in particular a reduction, in a generator torque of the electrical generator.

13. Method (300) according to one of the preceding claims, wherein the power, in particular the active electrical power at the generator output, of the wind power installation (100) in the power-reduced operating mode is reduced to a predetermined power value and / or by a predetermined absolute power value.

14. Method (300) according to Claim 13, wherein at least one of the aerodynamic characteristic values is taken into account in the determination of at least one setpoint rotational speed of one of the aerodynamic rotors (106).

15. Wind farm (112) having at least two wind power installations (100) and a wind farm control system (122), wherein the wind farm control system (122) is designed to carry out the method (300) according to one of the preceding claims.