METHOD FOR OPERATING A WIND ENERGY PLANT

Description

[0001] The present invention relates to a method for operating a wind turbine connected to a grid connection point of an electrical supply network for generating and feeding electrical energy into an electrical supply network. The present invention further relates to a wind turbine connected to a grid connection point of an electrical supply network and to a wind farm connected to a grid connection point.

[0002] The process of feeding electrical energy into an electrical supply network, such as the European interconnected grid, which has a nominal network frequency of 50 Hz, is generally known. Electrical supply networks are generally considered to be in a stable state, meaning they are particularly stable with regard to their operating frequency. The network frequency typically fluctuates by only a few hundredths of the nominal network frequency.

[0003] Various faults in the electrical supply network, known as network faults, such as a short circuit, can cause the network to reach a critical state. The network then exhibits a disturbance. Disturbances can also arise from the disconnection of a large consumer or a large generating unit.

[0004] One consequence of such disturbances is, for example, the so-called overfrequency or critical overfrequency. In such a case, the power grid then exhibits a network frequency that is significantly higher than the nominal network frequency. For example, the network frequency might be 50.2 Hz, even though the nominal network frequency is only 50 Hz.

[0005] In such a case, almost all generating units typically reduce their output power depending on the grid frequency in order to contribute to balancing the power supply. This process is also known as overfrequency power reduction.

[0006] Overfrequency power reduction is fundamentally designed to stabilize the grid frequency during periods of high power surpluses. If this fails, the grid frequency continues to rise until it reaches a critical threshold, for example 51.5 Hz, at which point the generating units typically disconnect from the grid.

[0007] Wind turbines typically reduce their power output by adjusting the rotor blades, thereby decreasing the turbine's yield and consequently the electrical generator output. The wind turbine then feeds less electrical power into the grid after a time delay.

[0008] Once the grid frequency has normalized again, the electrical generator output is increased accordingly in order to feed more electrical power back into the supply grid with a time delay.

[0009] Due to the increasing penetration of the electrical supply network with renewable energy producers, such as wind turbines, which are largely coupled to the supply network electronically, for example with a power inverter, the future supply network structures or network topologies will continue to change.

[0010] In particular, the number of predominantly conventional power plants will decrease significantly, and with it the amount of inertial mass in the supply network, which has a damping effect, especially on frequency fluctuations, on the electrical supply network.

[0011] This can change the behavior of the grid and make stabilizing it increasingly difficult. Furthermore, it will necessitate that renewable energy producers contribute more to grid stabilization.

[0012] The following documents are considered to be of general state of the art: DE 10 2012 203 334 A1, DE 10 2013 206 119 A1 and DE 10 2014 104 287 A1.

[0013] WO 2010 / 108910 A2 describes a method for operating a wind energy plant by utilizing the inertia of a rotor-generator system.

[0014] DE 10 2008 034 531 A1 describes a method for controlling a converter of a wind turbine, wherein the converter is controlled in such a way that the reactive power to be supplied by the converter is controlled as a function of a change in an active power supplied.

[0015] EP 2 573 896 A1 Document D3 describes a method for controlling an offshore wind farm.

[0016] WO 03 / 030329 A1 describes a method for controlling an inverter of a wind energy plant depending on a phase angle.

[0017] CN 105 074 203 A describes a method for operating a wind energy plant connected to a grid connection point of an electrical supply network according to the preamble of claim 1.

[0018] The object of the present invention is therefore to address one of the aforementioned problems. In particular, a solution is to be proposed that makes it possible to address future control problems in the power grid using a wind turbine. At the very least, an alternative to what is currently known is to be proposed.

[0019] According to the invention, a method for controlling a wind energy plant connected to a grid connection point of an electrical supply network is proposed in accordance with claim 1.

[0020] In particular, a method for operating a wind turbine connected to a grid connection point of an electrical supply network is proposed, wherein the electrical supply network has a grid frequency and the wind turbine, which includes an electric generator with a generator rated power, is controllable depending on the grid frequency. This method includes the step of converting electrical power into thermal power, wherein the electrical power is drawn from the electrical supply network to support the grid frequency of the supply network when the grid frequency changes with a frequency gradient that exceeds a predetermined limit.

[0021] Preferably, the wind turbine uses an electric generator to produce electrical power for feeding into the electrical grid. This electrical power, or a portion thereof, is then fed into the electrical grid as feed-in power by the wind turbine, depending on the grid frequency.

[0022] If the mains frequency changes, the invention proposes to react to the change in the mains frequency in two stages, in particular by means of a first and a second support stage.

[0023] In the first support stage, the generator output is reduced depending on the grid frequency in order to correspondingly reduce the power fed into the grid. The power fed into the grid is thus reduced in the first support stage by reducing the generator output accordingly, for example, by adjusting the rotor blades of the wind turbine or by changing the generator's excitation current. This results in the generator producing, or being able to produce, less electrical power. Therefore, the power fed into the grid is reduced in the first support stage by adjusting the generator output.

[0024] In the second support stage, the electrical feed-in power is reduced so that it is less than the generator's electrical output. This can be achieved, for example, using a power chopper designed to convert electrical energy into thermal energy. The power chopper is specifically designed to convert large amounts of power, particularly the entire electrical feed-in power of the wind turbine. Preferably, the power chopper's output corresponds to the generator's power class.

[0025] A reduction in electrical feed-in power therefore refers in particular to a deliberate, technical reduction in power, which is clearly distinguishable from losses that usually occur.

[0026] Therefore, at least two support stages are provided for grid stabilization. Each support stage can contribute to stabilizing the electrical supply network in its own way. Specifically, it is proposed to reduce the electrical feed-in power in the first support stage by reducing the electrical generator output, and additionally or alternatively, to reduce it in the second support stage by using an energy sink, such as a power chopper. The power chopper is specifically designed to directly reduce the electrical feed-in power from the wind turbine.

[0027] The method thus enables a particularly rapid reduction in the electrical feed-in power of a wind turbine. Specifically, the proposed method makes it possible to completely reduce the electrical feed-in power within less than 0.4 seconds, so that the wind turbine, especially when needed, no longer feeds any power into the grid in the shortest possible time, preferably within one second.

[0028] Preferably, in the second support stage, the electrical feed-in power is reduced if the grid frequency changes with a frequency gradient that exceeds a predetermined limit gradient.

[0029] The second support stage is activated when the electrical grid exhibits a frequency gradient greater than a predetermined threshold. For example, if the frequency gradient of the electrical grid exceeds the predetermined threshold, the power chopper is activated to reduce the electrical feed-in power to the wind turbine.

[0030] A particular advantage of this approach is that it has significantly smaller time constants than adjusting the azimuth orientation of the wind turbine or the rotor blades. Therefore, in this second support stage, the power is reduced much more quickly than in the first. It has been recognized, however, that such a rapid reduction is not necessary in every situation. To quickly determine whether such a rapid reduction is necessary or desirable, it is proposed here to evaluate a change in grid frequency based on its frequency gradient. If its frequency gradient is below the threshold gradient, support in the first support stage, where the generator power is reduced, may be sufficient.

[0031] Preferably, the predetermined limit gradient is selected according to the grid connection point of the wind turbine; a predetermined limit gradient of at least 0.4 Hz per second, in particular of 0.5 Hz per second, has proven to be advantageous.

[0032] The grid frequency gradient itself can be determined by measuring the grid frequency over time. This can be done locally by the wind turbine or the wind farm containing the wind turbine, preferably by the wind farm control unit, or by the grid operator, who transmits the measured frequency gradient to the wind farm or the wind turbine. Alternatively, the grid operator can also transmit target values ​​for the wind farm or the wind turbine based on the measured frequency gradient.

[0033] Preferably, in the second support stage, the electrical feed-in power is reduced if the grid frequency is above a predetermined frequency value.

[0034] The second support stage is therefore preferably activated when the grid frequency is above a predetermined frequency value. This predetermined frequency value is higher than the nominal grid frequency; for example, the predetermined frequency value is 50.2 Hz for a nominal grid frequency of 50 Hz.

[0035] The second support stage then has a frequency deadband in which the second support stage is not initially triggered.

[0036] The reduction of the electrical feed-in power according to the second support stage can also or alternatively occur if the frequency gradient is above the limit gradient.

[0037] If both a limit gradient and an absolute frequency value are checked for the application of the second support stage, it may be provided that the second support stage is activated if only one of the two criteria is met, or if both criteria are met. An additional criterion, such as further limit values, may also be provided to consider both criteria together.

[0038] When combining both criteria, it can be stipulated that the system only checks outside the deadband frequency whether the grid frequency gradient exceeds the predetermined limit gradient. If both the grid frequency is above a predetermined frequency value and the grid frequency changes with a frequency gradient that exceeds a predetermined limit, the electrical feed-in power is reduced, and in particular, the second support stage is triggered or activated.

[0039] The predetermined frequency value is preferably in the range of 0.1% to 1% of the nominal network frequency, in particular in the range of 0.2% to 0.5%, and the preferred limit value is 0.4% of the nominal network frequency.

[0040] Preferably, in accordance with or in the second support stage, the electrical feed-in power is reduced if the electrical feed-in power is above a target power for a predetermined period and / or by at least a predetermined exceedance value, in particular above a target power that is smaller than the electrical generator power generated by the generator.

[0041] The second support stage is therefore preferentially activated, or the electrical feed-in power is reduced, when the electrical feed-in power exceeds a certain target power for a predetermined period. This is the case, for example, when the electrical feed-in power must be reduced at a gradient that the electric generator cannot or should not meet for technical reasons, e.g., because the nacelle is not turned out of the wind quickly enough, resulting in an excessively high yield from the wind turbine. The target power is then rapidly reduced according to the requirements, e.g., as a result of a control system or another specification, but the actual reduction, i.e., the actual power output, does not keep pace. The use of the second support stage is proposed for this situation.In such a case, the electrical feed-in power is reduced below the generator power produced at that moment, in particular by means of a power chopper.

[0042] A particular advantage here is that the wind turbine does not have to be subjected to unnecessarily high mechanical loads in order to achieve any target performance values; in particular, such a design enables a particularly gentle operation of the system, especially for the drive train of the wind turbine.

[0043] Preferably, the electrical feed-in power is reduced in accordance with or in the second support stage when the second support stage is requested, in particular by an operator of the electrical supply network or by a control center.

[0044] This allows, for example, the wind turbine to be operated by the grid operator as an energy sink or consumer, particularly of active power, even when it is not feeding electrical energy into the grid. Preferably, a control input for such an external request signal can be provided for this purpose.

[0045] This method thus enables a grid-supporting effect for wind turbines, even if the turbine's generator does not produce any electrical power to feed into the grid. This has a particularly beneficial effect on the electrical grid.

[0046] Preferably, the second support stage is only carried out after the first support stage has been completed.

[0047] The second support stage is therefore preferably activated only after the electrical generator output has been reduced depending on the grid frequency. Only if reducing the electrical generator output is no longer sufficient to provide a corresponding electrical feed-in power is the electrical feed-in power further reduced, for example by means of a power chopper.

[0048] Alternatively, the second support stage is implemented independently of the first support stage.

[0049] A particular advantage of this method is that it features two control loops which regulate the electrical feed-in power of a wind turbine based on two different parameters: the grid frequency deviation and the grid frequency gradient. This provides, or enables, both a fast and a slow control loop.

[0050] For example, if the grid frequency changes from 50 Hz with a frequency gradient greater than 0.5 Hz per second, the electrical feed-in power is preferably reduced by means of a power chopper. If, despite this measure, the grid frequency exceeds a predetermined target frequency, for example, 50.2 Hz, the electrical generator power is additionally reduced, for example, by adjusting the generator excitation. Reducing the electrical generator power, i.e., the first support stage, is thus carried out in addition to reducing the electrical feed-in power, i.e., the second support stage, particularly when necessary.

[0051] Preferably, the generation of electrical generator power for feeding into the electrical supply network is carried out with the electrical generator depending on the network frequency, in particular depending on a deviation of the network frequency from the nominal network frequency, wherein the electrical generator power is reduced if the network frequency is above a predetermined target frequency.

[0052] The generator is thus preferably controlled depending on the grid frequency. If the grid frequency exceeds a certain target frequency, for example 50 Hz or 50.2 Hz, the generator's electrical output is reduced so that the electrical feed-in power is also reduced. Conversely, if the grid frequency changes with a frequency gradient greater than, for example, 0.5 Hz per second, the feed-in power is preferably reduced independently of the generator's electrical output. Reducing the electrical feed-in power can be achieved by converting electrical generator output into thermal power using a power chopper. Particularly preferably, the reduction of the electrical feed-in power is accomplished by reducing the generator's electrical output.Preferably, the generator is controlled based on another parameter, in particular the deviation of the mains frequency from the nominal mains frequency; thus, the electric generator is not controlled, or not solely controlled, based on the frequency gradient. The frequency gradient can trigger frequency-dependent control.

[0053] Preferably, the reduction of the electrical feed-in power is carried out in such a way that the electrical feed-in power is equal to zero.

[0054] The wind turbine is thus designed to reduce its electrical feed-in power from rated power to zero within a very short time, for example, by means of power dissipation and / or power diversion. The wind turbine's power chopper has a corresponding performance class for this purpose, which differs significantly from conventional braking choppers or crowbars, particularly with regard to the maximum electrical power it can absorb, its operating time, and permissible power gradients.

[0055] Preferably, electrical power is drawn from the electrical supply network, particularly when the network frequency changes with a frequency gradient that exceeds a predetermined limit and / or the network frequency is above a predetermined target frequency.

[0056] By controlling the electrical generator output depending on a frequency deviation and the electrical feed-in output depending on a frequency gradient, it is made possible for the wind turbine to also absorb electrical power, in particular active power, from the electrical supply network in order to support the electrical supply network or to make a contribution to frequency support that is greater than the system or generator rated power or at least greater than the current generator output.

[0057] In a preferred embodiment, the wind turbine is configured to feed reactive power into the electrical grid and to draw active power from the electrical grid. For example, in a wind turbine with a full converter concept, the power chopper is arranged in the DC link of the full converter, and the inverter of the full converter is bidirectional in order to draw active power from the electrical grid and convert it into thermal power with the power chopper, while the wind turbine continues to feed reactive power into the electrical grid.

[0058] This enables at least two-quadrant operation, namely feeding in reactive power and extracting active power. Furthermore, the method can be applied to existing wind turbine models. Preferably, the extraction of electrical power, particularly active power, from the electrical grid is based on a frequency gradient and / or a frequency deviation.

[0059] It is preferably proposed that the predetermined limit of the frequency gradient be 0.5 Hz per second. Accordingly, the electrical feed-in power is only reduced when the grid frequency changes with a frequency gradient greater than 0.5 Hz per second. Below the preferred limit of 0.5 Hz per second, the feed-in power is thus adjusted exclusively via the electrical generator output. The electrical feed-in power is therefore essentially entirely determined by the electrical generator output below the preferred limit of 0.5 Hz per second. This means that the electrical power generated by the generator is fed into the electrical grid in its entirety, less any losses and the wind turbine's own power consumption.If the predetermined limit of 0.5 Hz per second is exceeded, the electrical feed-in power is further reduced, for example by converting electrical generator power into thermal power.

[0060] This method thus provides a way to reduce the input electrical power in a manner, and especially at a speed, that cannot usually be achieved with simple rotor blade adjustment. Furthermore, the method is particularly gentle on the wind turbine's generator, as its excitation for rapid power reduction does not need to be changed abruptly, but can be adjusted gradually with high time constants.

[0061] Depending on the network topology or as required, particularly by the network operator, the predetermined limit can also be between 0.5 Hz per second and 2 Hz per second, for example, 0.6 Hz per second or 1.2 Hz per second. This is particularly advantageous for low-power electrical supply networks, i.e., those that already exhibit a high frequency variance due to their topology, which is also permissible.

[0062] Preferably, the electrical feed-in power is reduced so that it equals the electrical generator power when the grid frequency changes with a frequency gradient that falls below the predetermined limit, and in particular when it falls below it again. According to this embodiment, the feed-in power is therefore only reduced below the generator power when the frequency gradient is high.

[0063] Therefore, if the electrical grid recovers, i.e., if the frequency gradient of the grid frequency normalizes again, thus decreasing and falling below the predetermined limit, the reduction of the feed-in power is preferentially discontinued. In other words, no further electrical generator power is converted into thermal power once the frequency gradient of the grid frequency falls below the predetermined limit.

[0064] According to a preferred embodiment, it was further recognized that a maximum limit of the predetermined gradient of 2 Hz per second is particularly advantageous, since the method can also be used as an alternative to a complete shutdown of the wind turbine.

[0065] The procedure can therefore also be used instead of shutting down the wind turbine, as would otherwise be initiated, for example, at 2 Hz per second.

[0066] Preferably, reducing the electrical input power includes consuming electrical power, in particular consuming at least part of the electrical generator power, which is done at least partially by means of a switching device for converting electrical power into thermal power.

[0067] Reducing the electrical feed-in power is achieved by consuming the generator's electrical output, or a portion thereof. This consumption is accomplished using a switching device that converts electrical power into thermal power, for example, via a high-power resistor circuit or a large chopper, particularly a power chopper. The wind turbine, the resistor circuit, or the chopper are designed to convert a sufficiently large amount of electrical power into thermal power in order to reduce the electrical feed-in power independently of the generator's electrical output.

[0068] Preferably, the switching device for converting electrical power into thermal power is configured to convert electrical power corresponding to the generator's rated power into thermal power for at least 3 seconds, and in particular for at least 5 seconds.

[0069] The switching device is therefore configured to convert the full generator output into thermal power for at least 3 seconds, reducing the electrical power input to zero. The wind turbine is thus configured to feed no electrical power into the electrical grid for at least 3 seconds, even though the generator is operating at its rated power and producing an electrical output equal to its rated power.

[0070] Particularly preferably, the switching device for converting electrical power into thermal power is configured to convert electrical power corresponding to twice the generator's rated power into thermal power for at least 3 seconds, and in particular for at least 5 seconds.

[0071] In a particularly preferred embodiment, the switching device is configured to absorb or convert into heat twice the generator's rated power for at least three seconds, preferably five seconds. The wind turbine is thus configured to feed no electrical power into the electrical grid for at least three seconds, even though the generator is operating at its rated power and producing an electrical output equal to the generator's rated power. Additionally, it can draw electrical power, particularly active power, from the electrical grid equal to the generator's rated power.

[0072] The switching device for converting electrical power into thermal power thus comprises at least one chopper or is designed as a chopper or resistor circuit and is preferably arranged in the DC link of an inverter of the wind turbine, in particular in the DC link of the full converter of the wind turbine.

[0073] The switching device is therefore of a corresponding size; in particular, it consists of a multitude of choppers arranged in parallel to each other in order to convert as much electrical power as possible into thermal power, especially heat, over a long period of time. Several or a multitude of choppers can also be referred to as a chopper or a chopper bank.

[0074] This method thus makes it possible to draw electrical power, particularly active power, from the electrical grid, even when the wind turbine itself is not feeding power into the grid. The wind turbine is then operated exclusively as a consumer, with this consumer function being enabled by a switching device described above or below. Furthermore, the wind turbine is designed to operate as an electrical consumer when needed, especially a large consumer with a rated output exceeding 1 MW.

[0075] According to the invention, a wind turbine comprising a generator with a generator rated power for generating electrical generator power is also proposed, wherein the wind turbine is prepared to be connected to a grid connection point of an electrical supply network in order to feed the electrical generator power or a part thereof into the electrical supply network as electrical feed-in power depending on a grid frequency of the electrical supply network, and wherein the wind turbine is configured to carry out a method described above or below.

[0076] The wind turbine is specifically designed so that, in a first support stage, the generator output is reduced depending on the grid frequency in order to correspondingly reduce the feed-in power, and in a second support stage, the feed-in power is reduced so that it is less than the generator output. For this purpose, the wind turbine preferably has a corresponding feed-in control unit that can both control the generator, or at least trigger generator control, and also control the second support stage. To control the second support stage, the feed-in control unit is specifically connected to a device for consuming electrical energy, in order to control this device so that it consumes energy to operate the second support stage.

[0077] Preferably, the wind turbine includes a switching device for converting electrical power into thermal power, wherein the switching device is configured to consume at least a portion of the electrical generator power in order to reduce the electrical feed-in power. In particular, the switching device is connected to the feed-in control unit so that the feed-in control unit can control the switching device to initiate the conversion of electrical power into thermal power.

[0078] Preferably, the switching device for converting electrical power into thermal power is configured to convert electrical power, which corresponds to a, preferably twice, generator rated power, into thermal power for at least 3 seconds, in particular for at least 5 seconds.

[0079] The switching device is particularly preferably designed as a chopper bank or power chopper bank and / or includes a rectifier.

[0080] The power chopper or power chopper bank can therefore be arranged both in the DC link and at the inverter output of the wind turbine to convert electrical generator power into thermal power in order to reduce the electrical feed-in power of the wind turbine. Preferably, the chopper bank has a diode rectifier at its input, which is configured to accept the electrical generator power at the output of an inverter of the wind turbine.

[0081] According to the invention, a wind farm is further proposed comprising at least one wind turbine as described above or below, wherein the wind farm has a wind farm control unit configured to send control signals to the wind turbines of the wind farm and to receive status signals provided by the wind turbines of the wind farm in order to determine a negative electrical wind farm power or energy.

[0082] The wind farm thus has a park control unit configured to determine the potential negative power or energy output of the individual wind turbines in the wind farm, provided by the wind turbines according to the invention, in particular by their switching devices, which are configured to execute a method described above or below. Negative power or energy is that by which the currently fed-in power or energy can be reduced. This allows, in particular, a reduction in the fed-in power to be planned or at least determined. Since considering the time over which the reduction can or should be carried out is also important, an energy-based approach is preferably proposed.

[0083] According to the invention, a method for controlling a wind farm described above or below is also proposed, comprising the steps of: querying status signals of the wind turbines, in particular the energy absorption readiness of the switching devices of the wind turbines, determining a negative electrical wind farm power or energy based on the queried status signals of the wind turbines and providing the determined negative electrical wind farm power or energy to a grid operator and / or a control center controlling the wind farm.

[0084] The measured negative power outputs or energies of the individual wind turbines, provided by the switching devices of the wind turbines, are aggregated to form a negative electrical wind farm power output or energy and provided, for example, to the grid operator as information. The grid operator can then, if necessary, access this provided negative wind farm power output or energy to support the electrical grid. The present invention will now be explained in more detail below by way of example embodiments with reference to the accompanying figures. Fig. 1 shows a schematic view of a preferred wind turbine. Fig. 2 shows a schematic diagram of an electrical train of a wind turbine for generating and feeding electrical energy into the grid according to one embodiment. Fig. 3 schematically shows a sequence of the inventive method for operating a wind turbine in a preferred embodiment. Fig. 4 schematically shows an overfrequency power input with a wind turbine according to one embodiment. Fig. 5 shows a schematic diagram of a wind farm for generating and feeding electrical energy into the grid according to one embodiment.

[0085] Fig. 1 shows a wind turbine 100 for generating and feeding electrical energy into an electrical supply network.

[0086] The wind turbine 100 comprises a tower 102 and a nacelle 104. An aerodynamic rotor 106 with three rotor blades 108 and a spinner 110 is arranged on the nacelle 104. During operation, the wind sets the rotor 106 into rotation, thereby driving an electric generator in the nacelle 104, the generator preferably being a 6-phase ring generator.

[0087] Fig. 2 simplified representation of an electrical strand of 200 in Fig. 1 shown wind turbine.

[0088] The electrical train 200 comprises a 6-phase ring generator 210 with a rated generator output for generating an electrical generator output P GEN, which is set in rotation by the wind via a mechanical drive train of the wind turbine to generate a 6-phase alternating current. The 6-phase alternating current is transferred from the electric generator 210 to the rectifier 220, which is connected to the 3-phase inverter 240 via a DC link 230. The 6-phase ring generator 210, which is designed as a synchronous generator, is controlled by an excitation 250 from the DC link 230, whereby the excitation can also be supplied from another source, in particular by a separate current controller.

[0089] The electrical string 200 thus features a full converter concept, in which the electrical feed-in power PEIN is fed into the grid 270 via the 3-phase inverter 240 and the wind turbine transformer 260. This grid 270 is typically a wind farm grid that feeds into an electrical supply network via a wind farm transformer.

[0090] To generate the three-phase current I₁, I₂, I₃ for each of the phases U, V, W, the inverter 240 is controlled using a tolerance band method. This control is performed by the controller 242, which uses a current sensor 244 to detect each of the three currents I₁, I₂, I₃ generated by the inverter 240. The controller 242 is thus configured to control each phase of the inverter 240 individually using the current sensor 244. For this purpose, a current setpoint ISOLL can be specified to the controller 242, depending on which the currents I₁, I₂, I₃ are adjusted. The current setpoint ISOLL is preferably calculated internally for each phase U, V, W and specified to the controller 242.

[0091] The electrical train 200 further comprises a switching device 280 for converting electrical power into thermal power, which is designed to convert electrical power equal to twice the generator's rated power into thermal power ΔP TH for at least 5 seconds.

[0092] The switching device 280 can be connected either (A) with a DC link 230 or (B) with the phases U, V, W between the inverter 240 and the wind turbine transformer 260 via a diode rectifier 282 in order to reduce the electrical feed-in power PIN so that the electrical feed-in power PIN is less than the electrical generator power PGEN when the grid frequency changes with a frequency gradient that exceeds a predetermined limit. The switching device 280 is thus designed to handle high power levels.

[0093] To reduce the electrical feed-in power P IN, the switching device 280 has a control input 284 to receive control signals S from the wind turbine control or the wind farm control, or to transmit or return further signals to the control systems.

[0094] For example, if the grid frequency changes with a frequency gradient exceeding a predetermined limit, the switching device 280 is activated to reduce the feed-in power PIN. Thus, the electrical generator power PGEN produced by the generator 210 is reduced by means of the switching device 280 so that the feed-in power PIN decreases. The switching device 280 is therefore a device for converting high electrical power into thermal power. Preferably, the switching device is configured as a chopper bank for converting large power and energy levels. The switching device 280 is also controlled depending on the grid frequency, in particular depending on a frequency gradient.

[0095] Fig. 3Figure 300 schematically shows a process of the inventive method for operating a wind turbine in a preferred embodiment. The method relates in particular to reducing the electrical feed-in power such that the electrical feed-in power is less than the electrical generator power when the grid frequency changes with a frequency gradient that exceeds a predetermined limit.

[0096] The generator of the wind turbine produces electrical power for feeding into the electrical grid, provided the grid is stable. This means, in particular, that the grid frequency fN essentially corresponds to the nominal grid frequency fNENN and that the frequency gradient of the grid frequency dfN / dt is smaller than the predetermined limit G. Block 310 indicates that the frequency gradient of the grid frequency dfN / dt is smaller than the predetermined limit G, and block 340 indicates that the grid frequency fN essentially corresponds to the nominal grid frequency fNENN.

[0097] If the frequency gradient of the mains frequency df N / dt is smaller than the predetermined limit value G, the switching device for converting electrical power into thermal power does not convert any thermal power ΔP TH. This is indicated by block 320.

[0098] The check to see if the frequency gradient of the mains frequency dfN / dt is smaller than the predetermined limit value G is performed dynamically, for example by recording the mains frequency fN and then averaging the dynamically recorded mains frequency fN over time t. The dynamic recording of the frequency gradient of the mains frequency dfN / dt is indicated by block 325.

[0099] If the frequency gradient of the mains frequency df N / dt is smaller than the predetermined limit value G, the switching device will continue to convert no thermal power P TH.

[0100] If the frequency gradient of the grid frequency dfN / dt exceeds the predetermined limit G, the switching device converts electrical power, in particular a portion of the electrical generator power PGEN, into thermal power PTH. This directly reduces the electrical feed-in power PIN. Block 330 indicates that the electrical feed-in power PIN is directly reduced by converting electrical power into thermal power ΔPTH.

[0101] If the frequency gradient of the mains frequency df N / dt is subsequently smaller than the predetermined limit value G, the switching device stops converting the electrical power.

[0102] If the frequency gradient of the mains frequency df N / dt continues to exceed the predetermined limit G, further electrical power is converted or the amount of converted electrical power is further increased.

[0103] To prevent overloading the switching device, its temperature is preferably monitored. This is indicated by block 335.

[0104] If the converted thermal energy ΔE TH exceeds a critical limit E KRIT, the electrical generator output P GEN is additionally reduced, or the electrical generator is shut down to such an extent that the wind turbine no longer feeds any electrical power into the grid. This is indicated by block 390.

[0105] The method according to the invention can therefore be carried out independently of the generator control, which regulates a generator depending on the mains frequency.

[0106] Preferably, the generator control is operated independently of the control of the switching device.

[0107] Provided that the grid frequency fN is substantially less than or equal to the nominal grid frequency fNENN, the generator preferably feeds the entire generated electrical generator power PGEN into the electrical supply network as electrical feed-in power PEIN. This is indicated by block 350.

[0108] The check to determine whether the grid frequency fN is less than or equal to the nominal grid frequency fNENN or less than or equal to a predetermined setpoint frequency fSOLL is indicated by block 355. For example, if the nominal grid frequency is 50 Hz and the setpoint frequency is 50.1 Hz, the generator's control system will then exhibit a kind of deadband. The grid frequency fN can also be detected by a wind farm control unit, which uses a comparison to determine whether the grid frequency fN exceeds the nominal grid frequency fNENN or the setpoint frequency fSOLL.

[0109] Provided that the grid frequency f N is essentially less than or equal to the nominal grid frequency f NENN or the target frequency f SOLL, the generator continues to feed the entire generated electrical generator power P GEN into the electrical supply network as electrical feed-in power P EIN.

[0110] If the grid frequency fN exceeds the nominal grid frequency fNENN or the target frequency fSOLL, the generator output PGEN is reduced. The generator then feeds a reduced output PGEN into the grid as feed-in power PEIN. Block 360 indicates that, depending on the grid frequency fN, the generator feeds a reduced output PEIN into the grid when the target frequency fSOLL is exceeded.

[0111] If the mains frequency f N now falls below or corresponds to the nominal mains frequency f NENN or the target frequency f SOLL, the electric generator maintains its state or is restarted.

[0112] If the grid frequency f N continues to exceed the nominal grid frequency f NENN or the target frequency f SOLL, the electrical generator output P GEN is reduced again or further.

[0113] If the grid frequency fN exceeds a critical frequency fKRIT, which is specified, for example, by the grid operator, the electrical generator output PGEN is significantly reduced, or the generator is brought into a state where it no longer produces any electrical power. Block 390 indicates that the electrical generator is shut down to such an extent that the wind turbine no longer feeds any electrical power into the grid.

[0114] Figure 4Figure 4 schematically shows an overfrequency power consumption of 400 with a wind turbine according to one embodiment. In particular, the mode of operation of the method according to the invention is shown by means of a fluctuation of the grid frequency f N.

[0115] In the upper diagram 410, the grid frequency fN is plotted against time t. The grid frequency is a few hundredths higher than the nominal grid frequency fNENN of 50 Hz, for example, 50.02 Hz, and fluctuates slightly. Thus, the electrical supply network behaves essentially frequency-stable up to time t1, meaning it exhibits no significant frequency deviation or steeper frequency gradient.

[0116] The central diagram 420 shows the electrical generator output P GEN over time. The generator is controlled according to the grid frequency f N and produces the slightly fluctuating generator output P GEN. Also shown in the central diagram 420 is the thermal power ΔP TH converted by the switching device. The switching device is controlled according to the frequency gradient df N / dt. Since the electrical supply network does not have a frequency gradient df N / dt that exceeds the predetermined limit G, no electrical power, in particular no electrical generator power, is converted into thermal power.

[0117] The electrical input power PIN is derived from the electrical generator power PGEN and the thermally converted power, and is essentially equal to the electrical generator power PGEN up to time t1. This is shown in diagram 430 below.

[0118] At time t1, the electrical supply network exhibits a disturbance in the form of a frequency gradient 412, which is greater than the predetermined limit value G. This can be detected, for example, by a measuring device.

[0119] The switching device, in particular the power chopper, is then activated. This is shown in the lower half-plane 422 of the middle diagram. The electric generator, which is controlled depending on a frequency deviation, remains unaffected by this, at least for the time being.

[0120] Switching on the switching device results in the conversion of both the total electrical generator output P GEN and a portion of the electrical power drawn from the grid to stabilize it into thermal power, specifically heat. This is illustrated in diagram 430. The switching device thus not only reduces the electrical feed-in power to zero by converting the generator output into thermal power, but also converts additional, excess power, particularly active power, from the grid into thermal power. At the connection point, the wind turbine therefore exhibits a negative power balance, specifically a negative active power balance.

[0121] As a result of this measure according to the invention, the grid frequency recovers slowly, so that the grid frequency approaches the nominal grid frequency with a frequency gradient 414, wherein the frequency gradient 414 is smaller than the predetermined limit value.

[0122] The switching device then slowly reduces its power consumption of the electrical generator power until the supply network is again in a frequency-stable state at time t2.

[0123] During the execution of the method according to the invention, the electrical generator power is preferably reduced depending on the grid frequency, particularly when a frequency deviation of more than 0.2 Hz from the nominal grid frequency lasts for more than 5 seconds. This is illustrated by way of example in the middle diagram 420 in section 424.

[0124] Figure 5 shows a wind farm 500 with three exemplary wind turbines 100 according to Figure 1The three wind turbines are thus representative of essentially any number of wind turbines in a wind farm 500. The wind turbines 100 provide their electrical feed-in power P INPUT via an electrical wind farm grid 570. These individual feed-in powers P INPUT are combined as wind farm power P PARK and fed into the supply grid 594 at a grid connection point PCC via the wind farm transformer 590, which steps up the voltage in the park.

[0125] Wind farm 500 is controlled by a wind farm control unit 542, also known as a Farm Control Unit (FCU). For this purpose, the wind farm control unit 542 uses measuring instruments 544 to detect the grid frequency, and in particular the frequency deviation and the frequency gradient. The wind farm control unit can also communicate with the individual wind turbines via the control lines 546. Specifically, status signals S from the wind turbines, such as the energy absorption readiness of their switching devices, can be queried via these lines. Based on these queried status signals S, the wind farm control unit 542 can calculate a negative electrical wind farm energy, i.e., the energy that the wind farm is ready to absorb. This calculated negative wind farm energy is then provided to a grid operator by means of a reduction signal R.The grid operator is thus informed at all times about how much electrical power, especially active power, the wind farm can absorb and can also request this power. The wind farm is therefore designed to function as a consumer of active power for at least 5 seconds, specifically with a negative power output corresponding to the wind farm's rated output.

Claims

1. A method for operating a wind power installation (100) which is connected to a network connection point of an electrical supply network, wherein the electrical supply network has a network nominal frequency (fNENN) and is operated at a network frequency (fN), and the wind power installation (100) which comprises an electrical generator (210) with a generator nominal power can be regulated on the basis of the network frequency (fN), characterized in that, that the method comprising the step of: - converting electrical power into thermal power (ΔPTH), wherein the electrical power is removed from the electrical supply network in order to support the network frequency of the supply network if the network frequency (fN) changes with a frequency gradient (dfN / dt), which exceeds a predetermined limit value.

2. A method for operating a wind power installation (100) which is connected to a network connection point of an electrical supply network as claimed in claim 1, comprising the steps of: - using the electrical generator (210) to produce an electrical generator power (PGEN) for feeding into the electrical supply network, - feeding the electrical generator power (PGEN) or a part of it into the electrical supply network as electrical feed-in power (PEIN) on the basis of the network frequency (fN), wherein - in a first supporting stage, the electrical generator power (PGEN) is reduced on the basis of the network frequency (fN) in order to accordingly reduce the electrical feed-in power (PEIN), and - in a second supporting stage, the electrical feed-in power (PEIN) is reduced such that the electrical feed-in power (PEIN) is less than the electrical generator power (PGEN).

3. The method as claimed in claim 1 or 2, characterized in that - according to the second supporting stage, the electrical feed-in power (PEIN) is reduced if the network frequency (fN) changes with a frequency gradient (dfN / dt) which exceeds a predetermined limiting gradient.

4. The method as claimed in one of the preceding claims, characterized in that - according to the second supporting stage, the electrical feed-in power (PEIN) is reduced if the network frequency (fN) is above a predetermined frequency value.

5. The method as claimed in one of the preceding claims, characterized in that - according to the second supporting stage, the electrical feed-in power (PEIN) is reduced if the electrical feed-in power (PEIN) is above a desired power for a predetermined period and / or at least by a predetermined exceedance value, in particular a desired power which is less than the electrical generator power (PGEN) produced by the generator.

6. The method as claimed in one of the preceding claims, characterized in that - according to the second supporting stage, the electrical feed-in power (PEIN) is reduced if the second supporting stage is requested, in particular by an operator of the electrical supply network or by a control room.

7. The method as claimed in one of the preceding claims, characterized in that - the second supporting stage is carried out only after the first supporting stage has been run through.

8. The method as claimed in one of claims 2 to 6, characterized in that - the second supporting stage can be carried out independently of the first supporting stage.

9. The method as claimed in one of the preceding claims, characterized in that - the electrical generator power (PGEN) for feeding into the electrical supply network is produced using the electrical generator (210) on the basis of the network frequency (fN), in particular on the basis of a deviation of the network frequency (fN) from the network nominal frequency (fNENN), wherein the electrical generator power is reduced if the network frequency (fN) is above a predetermined desired frequency.

10. The method as claimed in one of the preceding claims, characterized in that - the electrical feed-in power (PEIN) is reduced such that the electrical feed-in power is equal to zero.

11. The method as claimed in one of the preceding claims, also comprising the step of: - removing electrical power from the electrical supply network, in particular if the network frequency (fN) changes with a frequency gradient (dfN / dt) which exceeds a or the predetermined limiting gradient and / or the network frequency (fN) is above a or the predetermined frequency value.

12. The method as claimed in one of the preceding claims, characterized in that - a or the predetermined limiting gradient is: - 0.5 Hz per second or - between 0.5 Hz per second and 2 Hz per second or - 2 Hz per second.

13. The method as claimed in one of the preceding claims, characterized in that - the electrical feed-in power (PEIN) is reduced in the second supporting stage such that the electrical feed-in power (PEIN) is equal to the electrical generator power (PGEN) if the network frequency (fN) changes with a frequency gradient (dfN / dt) which undershoots the predetermined limiting gradient, in particular undershoots said gradient again.

14. The method as claimed in one of the preceding claims, characterized in that - the reduction of the electrical feed-in power (PEIN) in the second supporting stage comprises consuming electrical power, in particular electrical generator power (PGEN) and / or electrical power from the supply network, wherein the consumption is carried out at least partially, preferably completely, by means of a switching device (280) for converting electrical power into thermal power (ΔPTH).

15. The method as claimed in one of the preceding claims, characterized in that - a or the switching device for converting electrical power into thermal power is set up to convert electrical power corresponding to the generator nominal power into thermal power for at least three seconds, in particular at least five seconds.

16. The method as claimed in one of the preceding claims, characterized in that - a or the switching device (280) for converting electrical power into thermal power (ΔPTH) is set up to convert electrical power corresponding to twice the generator nominal power into thermal power (ΔPTH) for at least three seconds, in particular at least five seconds.

17. The method as claimed in one of claims 14 to 16, characterized in that - the switching device (280) comprises at least one chopper.

18. The method as claimed in one of claims 14 to 17, characterized in that - the switching device (280) is in the form of a chopper bank.

19. A wind power installation (100) comprising a generator with a generator nominal power for producing an electrical generator power (PGEN), wherein the wind power installation (100) is set up to be connected to a network connection point of an electrical supply network in order to feed the electrical generator power (PGEN) or a part of it into the electrical supply network as electrical feed-in power (PEIN) on the basis of a network frequency (fN) of the electrical supply network, wherein the wind power installation (100) is set up such that - in a first supporting stage, the electrical generator power (PGEN) is reduced on the basis of the network frequency (fN) in order to accordingly reduce the electrical feed-in power (PEIN), and - in a second supporting stage, the electrical feed-in power (PEIN) is reduced such that the electrical feed-in power (PEIN) is less than the electrical generator power (PGEN), the wind power installation (100) is set up to carry out a method as claimed in one of claims 1 to 18.

20. The wind power installation (100) as claimed in claim 19, characterized by - a switching device (280) for converting electrical power into thermal power (ΔPTH), wherein the switching device (280) is set up to consume at least part of the electrical generator power (PGEN) in order to reduce the electrical feed-in power (PEIN).

21. The wind power installation (100) as claimed in claim 20, characterized in that - the switching device (280) for converting electrical power into thermal power (ΔPTH) is set up to convert electrical power corresponding to a generator nominal power, preferably twice a generator nominal power, into thermal power (ΔPTH) for at least three seconds, in particular at least five seconds.

22. The wind power installation (100) as claimed in claim 20 or 21, characterized in that - the switching device (280) is in the form of a chopper bank and / or comprises a rectifier.

23. A wind farm (500) comprising at least one wind power installation (100) as claimed in one of claims 19 to 22, characterized by a wind farm control unit which is set up to transmit control signals (S) to the wind power installation (100) in the wind farm (500) and to receive status signals provided by the wind power installations (100) in the wind farm (500) in order to determine a negative electrical wind farm power or energy.