METHOD AND FEEDING DEVICE FOR FEEDING IN ELECTRICAL POWER

Description

[0001] The present invention relates to a method for feeding electrical power into an electrical supply network by means of an inverter-driven feed-in device, namely a wind turbine. The present invention also relates to a corresponding feed-in device, namely a wind turbine.

[0002] The process of feeding electrical power into the grid from a wind farm, or at least from a single wind turbine, is well-known. Modern wind turbines and wind farms use converter units to condition the electrical power generated from the wind for grid feed-in, in particular to adapt it to the specific feed-in situation with regard to frequency, phase, and amplitude.

[0003] It is also known to use wind turbines or wind farms to support the electrical supply network, which can also be referred to simply as grid support.

[0004] For such grid support, the power feed-in is specifically adapted to the concrete grid situation. This includes increasing or decreasing the fed-in power depending on the grid situation or grid demand. It is also possible to feed in reactive power depending on a grid situation or grid demand. A grid demand is, in particular, one that is directed by an operator of the electrical supply network, i.e., the grid operator, to the wind turbine or wind farm.

[0005] Often, fixed control rules are stored in the wind turbine or wind farm for such grid support. These control rules can include, for example, grid frequency-dependent power control, as described in US patent 6,891,281. Another example is voltage-dependent phase angle control, which specifies a relationship between the phase angle, which describes the phase difference between current and voltage, and an electrical voltage of the power grid or a voltage determined by the power grid. Such voltage-dependent phase angle control is described in US patent 6,965,174.It is also conceivable that, in the event of a rapid drop in frequency, a correspondingly short-term power increase could be provided as an instantaneous reserve, whereby this power increase could be derived from the inertia of the rotating rotor of the wind turbine. Such a variant can be found, for example, in US patent US 9,279,411.

[0006] Many of these support measures, especially the frequency-dependent power changes, assume a specific, currently common type of electrical grid. In particular, it is generally assumed that the electrical grid is fed by large power plants that use large synchronous generators directly coupled to the grid. For example, a frequency-dependent power change is based on this assumption, as it takes into account the behavior of such a directly coupled synchronous generator. In this scenario, such a synchronous generator accelerates when it cannot deliver the power with which it is mechanically driven, for example by a steam turbine, to the electrical grid.This increase in speed then leads, because it is electrically directly coupled to the electrical supply network, to an increase in the fed-in frequency, and this is essentially the reason why an increased frequency in the electrical supply network can indicate a power surplus in the electrical supply network.

[0007] Voltage effects in the electrical power grid are also frequently attributable to the behavior of such a directly coupled large synchronous generator. Firstly, these synchronous generators essentially maintain the voltage in the electrical power grid, with additional location-dependent variations primarily caused by transformers and transmission lines. Secondly, effects such as phase shifts can also be attributed to the behavior of synchronous generators.

[0008] If the structure of the electrical supply network changes, this may result in the planned network support measures no longer being suitable or no longer being as effective for network support.

[0009] The behavior of the electrical supply network is also influenced by how producers and consumers are distributed within it. For example, sections that are decentralized and connected to many wind turbines or wind farms behave differently than network sections with many consumers located near a large power plant.

[0010] The German Patent and Trademark Office has searched the following prior art in the priority application for the present application: EP 2 182 626 A1; DE 10 2014 214 151 A1; DE 10 2013 207 264 A1; US ​​6,891,281 B2; US 6,965,174 B2 and US 9,279,411 B2.

[0011] The European Patent Office has searched the following prior art for the present application: US 2013 / 215652 A1, US 2017 / 018932 A1, US 2001 / 048290 A1 and US 2017 / 018929 A1. Document US 2013 / 215652 A1 relates to a wind turbine that feeds electrical power into an electrical grid via a feed-in device. Document US 2017 / 018932 A1 relates to a system for controlling a DC / DC converter and an inverter for feeding electrical power from a photovoltaic system into an electrical grid. Document US 2001 / 048290 A1 relates to a control device for a turbine generator unit that feeds electrical power into an electrical grid and can switch between current-forming and voltage-forming operation. Document US 2017 / 018929 A1 concerns a procedure for controlling a micronetwork.

[0012] The present invention is therefore based on the objective of addressing at least one of the aforementioned problems. In particular, a solution is to be proposed to improve grid support through inverter-controlled feed-in devices, wind farms, and thus wind turbines, or, with regard to grid support or grid stabilization, to improve the feed-in by such inverter-controlled feed-in devices, i.e., to make the feed-in more grid-compatible. At the very least, an alternative solution to previously known solutions is to be proposed.

[0013] According to the invention, a method for feeding power into the grid according to claim 1 is proposed. Accordingly, electrical power is fed into an electrical supply network by means of a converter-controlled feed-in device. A wind turbine is proposed here as the converter-controlled feed-in device according to the invention. A converter-controlled feed-in device is one that generates the fed-in current or voltage by means of a converter or an inverter, namely with respect to frequency, phase, and amplitude. A harmonic component can also be adjusted, for example.

[0014] It is proposed that, for the injection of electrical power, at least one current-specifying operating mode and one voltage-specifying operating mode should be selectable. In the current-specifying operating mode, the current is controlled or regulated to a setpoint, while in the voltage-specifying operating mode, it is controlled or regulated to a setpoint. These two operating modes differ fundamentally in this respect. The current-specifying operating mode is implemented in such a way that a setpoint current to be injected is specified according to frequency, phase angle, and amplitude. This specification can be dynamic and change continuously during operation. In particular, the specified current setpoint can constantly adapt to the available power.Especially in the case of a wind turbine, this means that this target current can constantly adapt to the available wind power, i.e., to the power that the wind turbine can extract from the wind at any given moment.

[0015] According to the invention, such a converter-controlled feed-in device has several feed-in units, namely, in particular, several converters or inverters. According to the invention, several such units are present in a wind turbine. Often, one such feed-in unit can be located in a control cabinet.

[0016] The voltage-regulating or current-regulating operating mode can mean that each feed-in unit, i.e., each converter or inverter, or each control cabinet, is operated in the corresponding operating mode. This can therefore mean that each affected control cabinet feeds in voltage-regulating mode or each feeds in current-regulating mode.

[0017] According to the invention, a mixed operation is also proposed in which only some feed-in units have the predominant operating mode. Insofar as voltage-generating feed-in units dominate, one can also speak of a voltage-generating operation, or, if current-generating units dominate, one can speak of a current-generating operation.

[0018] If electrical power is fed into the system in current-determining operating mode in such a way that a feed-in current is specified for each instance, this can particularly mean that each feed-in unit specifies a corresponding feed-in current as a target current and then also generates such a specified feed-in current.

[0019] A central control unit can control the individual feed-in units and, for example, distribute available power, especially power available from wind, accordingly to the individual feed-in units, which then specify the target current to be fed in and thus set.

[0020] A wind farm can be operated in the same way, in which a central park control system is provided that can give corresponding setpoint values ​​to individual wind turbines, and in the wind turbines individual control devices can be provided that then control the individual feed-in units.

[0021] In current-regulating operating mode, a current is fed in according to a predefined setpoint. If the instantaneous value of the current changes, the corresponding feed-in unit reacts and attempts to counteract the change.

[0022] In voltage-regulating operation, a voltage is specified, and the relevant feed-in unit attempts to maintain its feed-in voltage at this specified value. The feed-in unit reacts accordingly to any voltage changes.

[0023] If the instantaneous voltage value at the output of the corresponding input unit changes, this can immediately trigger a response from the voltage-generating input unit. Such a voltage change can also be a frequency change or a phase shift in the voltage. The voltage-generating unit can react to this directly. This is what distinguishes it from a current-generating unit. The current-generating unit does not react to voltage changes in this respect.

[0024] Modern electrical power grids use voltage as their primary control parameter. This means that a grid voltage is specified, but not a grid current. The grid is designed to maintain a relatively constant voltage, while power input, output, and transmission are primarily determined by the current level. In other words, current is generally adjusted as needed and initially provides little information about the state of the electrical power grid. Of course, there are exceptions, such as a short circuit or an interruption that prevents the injection of electricity. Conversely, a voltage change typically indicates a change in a grid characteristic and can therefore provide a better and usually faster indication of a change in the electrical power grid.

[0025] In any case, changes in the electrical supply network are usually first noticeable as voltage changes; it is also possible that they manifest themselves only as voltage changes. Voltage-regulating units react to these changes instantly, regardless of any superimposed power control, while current-regulating units register such changes only later or possibly not at all. A current-regulating unit's reaction to a change in the electrical supply network is often only taken into account by changing its setpoint current, particularly if such a network change is measured and, due to a control regulation, leads to a modified current setpoint.

[0026] As a result, voltage-regulating feed-in units prove to be particularly effective as grid-regulating units, which, due to their operating principle, counteract changes in the electrical supply network especially quickly and thus particularly dynamically. Current-regulating feed-in units, on the other hand, are characterized by their tendency to react to events in the network with a delay before feeding current and thus power into the electrical supply network.

[0027] The proposed switching between current-regulating and voltage-regulating operation utilizes this special characteristic in the control system. This allows switching to a voltage-regulating operating mode when particularly dynamic support from the inverter-driven feed-in devices is required.

[0028] This also allows for a more responsive approach to changes in the electrical grid's topology. For example, if a nearby large power plant is shut down, whether temporarily or permanently, its regulatory effect on the grid is lost. In such a case, switching to a voltage-regulating operating mode could be proposed to compensate for some of the power plant's influence on the grid.

[0029] According to the invention, it is proposed that the current operating mode can be changed between the current-forming and the voltage-forming operating mode in several steps or in a smooth transition. This also makes it possible to select operating modes that are not exclusively voltage-forming or exclusively current-forming, but can be of varying proportions of either. This allows the proposed choice between current-forming and voltage-forming modes not only to be limited to extreme cases, but also to be dynamically adapted to corresponding network conditions.

[0030] The criteria described above or below for selecting between current-forming or voltage-forming operating modes can also be applied analogously to corresponding intermediate operating modes.

[0031] According to one embodiment, it is proposed that switching between the current-forming and the voltage-forming operating modes is performed depending on at least one of the following criteria, whereby it is also possible to select an operating mode between the current-forming and the voltage-forming modes depending on this criterion. The following criteria are proposed: a measure of voltage fluctuation in the electrical supply network, a current limiting measure which is a measure of how often voltage-generating units of the inverter-driven feed-in device have reached a current limit, a ratio of directly coupled synchronous generators feeding into the electrical supply network to inverter-driven feed-in devices feeding into the electrical supply network, wherein the currently fed-in power, in particular active power or a ratio of the currently fed-in power or active power, is used as a reference variable, a measure of frequency fluctuation in the electrical supply network, a measure of phase shifts in the network, a network property of the electrical supply network, in particular network sensitivity, an operating state of the inverter-driven feed-in device, in particular the currently fed-in power,in particular active power in relation to a rated power of the inverter-driven feed-in device, the proportion of voltage-generating feed-in devices feeding into the electrical supply network, the proportion of wind turbines feeding into the electrical supply network, the proportion of solar generators feeding into the electrical supply network, a short-circuit power at the grid connection point, a grid impedance at the grid connection point, a short-circuit current ratio at the grid connection point, the detection of sub-network formation, in particular islanding, and an external switching or selection signal, in particular a switching signal specified by an operator of the electrical supply network.

[0032] Therefore, one proposed criterion is to use a measure of voltage fluctuation in the electrical supply network. For example, an average fluctuation range can be used. Here, a voltage fluctuation can be recorded over a predetermined period, such as one minute, ten minutes, or one hour, by measuring the average of the maximum and minimum voltages, for example, relative to the RMS value of the network voltage. The difference between these averaged maximum and minimum voltage values ​​can be used as the fluctuation range. A threshold value can then be defined, and depending on whether the recorded fluctuation range exceeds this threshold, the voltage-regulating or current-regulating operating mode can be selected.Preferably, the voltage-regulating operating mode is selected when voltage fluctuations are high, i.e., when the fluctuation range is large and the limit value is exceeded. If the operating mode is changed between current-regulating and voltage-regulating modes in several steps, a further limit value or values ​​can be provided for corresponding intermediate operating modes.

[0033] One possible criterion proposed is a current limiting measure, which is a measure of how often voltage-regulating units of the inverter-driven power supply unit have reached a current limit. It has been observed that voltage-regulating units operate in such a way that a change in the instantaneous value of the voltage at the output of the corresponding power supply unit can immediately trigger a response from the voltage-regulating unit. The voltage-regulating unit attempts to maintain the voltage. This effect is desirable but can lead to a high current being injected by the voltage-regulating unit. If this current becomes so high that the unit reaches a current limit, this indicates that the unit can barely, or no longer, maintain the voltage.

[0034] Therefore, regularly reaching the current limit is taken as an indicator that more voltage-forming units are needed. Accordingly, it is proposed to switch to voltage-forming operation with more units, or at least to increase the proportion of voltage-forming units, depending on this.

[0035] Preferably, it is proposed to count how often the voltage-forming units of the feed-in device reach the current limit per unit in a given test period, to use this as the current limit frequency, and to switch to voltage-forming operation depending on this current limit frequency, or at least to increase the proportion of voltage-forming units.

[0036] Preferably, it is proposed that the current limiting measure, or the current limiting frequency, be taken into account as a function of at least one controller parameter of the voltage-generating unit. This can, in particular, be a controller gain or a controller time constant. Specifically, the at least one controller parameter can be taken into account via a weighting, whereby the weighting is chosen to be smaller the smaller the controller gain, namely the slope of a static function used for the voltage-generating unit. Such a static function, for an active-current-dependent frequency tracking, specifies a gain measure that indicates a linear relationship between the frequency to be set and the active current or power supplied.

[0037] For example, it is suggested that with a high controller gain, corresponding to a large controller gain or a small controller time constant, reaching the current limit is a strong indication that the proportion of voltage-forming units should be increased, compared to a weaker controller. A weaker controller is more likely to reach a limit than a stronger one. Therefore, reaching a limit is rather normal for a weaker controller and thus not very informative about the need for an additional voltage-forming unit. However, with a strong controller, even a small current limit value or low current limit frequency necessitates switching to voltage-forming operation, or at least increasing the proportion of voltage-forming units.

[0038] The ratio of directly coupled synchronous generators feeding into the electrical grid to inverter-driven feed-in devices provides information about the fundamental behavior and control characteristics of the electrical grid. In particular, directly coupled synchronous generators exhibit a certain, to put it simply, voltage-regulating behavior on the grid. The fewer such directly coupled synchronous generators influence the grid, the more voltage-regulating feed-in devices can be proposed to at least partially compensate for this behavior of the directly coupled synchronous generators.

[0039] The behavior is not determined by the number of generators, because directly coupled synchronous generators in large power plants regularly have a multiple of the rated power of a simple wind turbine; instead, a power-related reference value is used. Specifically, it is proposed to use the current active power outputs of the directly coupled synchronous generators on the one hand and the inverter-driven feed-in devices on the other. Alternatively, the rated power can also be used in each case.

[0040] Another criterion is the magnitude of frequency fluctuation. This frequency fluctuation measure can also be recorded similarly to the magnitude of voltage fluctuation, for example, by recording the frequency fluctuation range instead of the voltage fluctuation range. Here, too, it is preferably suggested that if the frequency fluctuation range is high, a voltage-forming operating mode should be selected, or at least the operating mode should be adjusted towards a voltage-forming operating mode.

[0041] According to another approach, a degree of phase shift in the grid is proposed as a criterion. Phase shifts occur during abrupt current changes, such as those that can happen during load shedding. These abrupt current changes then lead to phase shifts due to a corresponding voltage change across a grid impedance. It has been observed that phase shifts, in which the sinusoidal waveform of the grid voltage abruptly shifts, are prevented by synchronous generators directly coupled to the electrical supply network. However, such phase shifts occur particularly when there are few of these synchronous generators, or when they are located far away within the grid. The more synchronous generators operating in the electrical supply network, the more effectively they can prevent phase shifts. Directly coupled synchronous generators, in particular, can form a grid hotspot.The further a point in the electrical supply network is from the network's center of gravity, the greater the need for voltage-regulating units. It is therefore proposed that the more frequently and / or severely phase jumps occur, the sooner a point should be switched to voltage-regulating operation, or at least that the proportion of voltage-regulating units in the electrical supply network should be increased.

[0042] Similarly, a network property of the electrical supply network can serve as a criterion. In particular, network sensitivity is proposed. A network property is to be distinguished from an instantaneous state. An instantaneous state is, for example, the current frequency or the current voltage level. A network property, on the other hand, is a characteristic of the network, i.e., how the network reacts to a change. Therefore, the consideration of network sensitivity as a network property is particularly proposed. Network sensitivity is preferably defined here as the ratio of a voltage change in response to a change in injected active power, and this is referenced to a network connection point, namely the connection point where the inverter-driven feed-in device feeds in. Thus, a measure can be found of how the electrical supply network behaves in response to changes.Depending on the circumstances, a choice can be made between current-regulating and voltage-regulating operating modes, or a suitable intermediate stage can be selected. Preferably, a voltage-regulating operating mode is recommended in cases of high grid sensitivity, i.e., when the grid reacts strongly to changes in active power feed-in.

[0043] As a further criterion, the current operating state of the inverter-controlled feed-in device is proposed, in particular the currently fed-in power, especially the active power. This can be set in relation to a rated power of the inverter-controlled feed-in device. If the operating state is such that relatively little active power is fed in, the selection of the voltage-controlled operating mode can be proposed in order to improve the supporting characteristics of the inverter-controlled feed-in device.

[0044] The proportion of voltage-regulating feed-in devices feeding into the electrical grid can also be considered. The more voltage-regulating feed-in devices are already active in the electrical grid, the less a voltage-regulating operating mode needs to be selected. Here, too, the proportion of voltage-regulating feed-in devices should be viewed primarily in terms of their active power input.

[0045] As a further criterion, it is proposed to consider the proportion of wind turbines or wind farms feeding into the electrical grid. Here, too, the active power fed into the grid is proposed as a measure. By considering this active power, a distinction between the proportion of wind turbines and the proportion of wind farms is practically unnecessary. This power-related analysis of the proportion of wind turbines allows for consideration of how strongly wind fluctuations can affect the total power fed into the electrical grid, i.e., how large the proportion of active power fed into the grid is that depends on the wind. Here, too, a high proportion of wind turbines suggests a more voltage-regulating operation.

[0046] Similarly, the proportion of solar generators feeding into the electrical grid is proposed as a criterion. This is based on a similar consideration to that for the proportion of wind turbines, but here it relies on the consideration of active power fed into the grid as dependent on sunlight or light irradiance. The greater the proportion of solar-dependent active power, the more likely a voltage-regulating operating mode is proposed.

[0047] When considering the wind power share and / or the solar power share, the current feed-in situation can also be taken into account.

[0048] On the other hand, if there is a large proportion of wind turbines in the electrical supply network, it can also be suggested to choose a current-defining operating mode, because wind turbines as such already exhibit fast control characteristics when operating in the current-defining operating mode.

[0049] As a further criterion, the consideration of short-circuit power at the grid connection point is proposed. This refers to the short-circuit power that the electrical supply network could provide at the grid connection point in the event of a short circuit. In this respect, the short-circuit power at the grid connection point is also a network characteristic of the electrical supply network with respect to this connection point. The higher this short-circuit power, the stronger and more stable the electrical supply network is in this area. Accordingly, less regulation is required, and it is therefore proposed to switch to a voltage-regulating operating mode when the short-circuit power is low.

[0050] Network impedance at the grid connection point can also be used as a criterion, and the lower the network impedance there, the less need there is for a voltage-regulating operating mode. The voltage-regulating operating mode is therefore proposed when the network impedance is high, e.g., exceeding a predetermined limit.

[0051] A short-circuit current ratio at the grid connection point is another criterion used to select between a voltage-regulating or a current-regulating operating mode. The short-circuit current ratio represents the ratio of the short-circuit power available on the grid side at the grid connection point to the rated power of the feed-in device connected there. The lower the short-circuit current ratio, the more the grid connection point is utilized by the connected feed-in device. Accordingly, feed-in via such a feed-in device at the grid connection point is more stable the higher the short-circuit current ratio. For example, a short-circuit current ratio of 10, or even 6, typically represents a very stable state.From a short-circuit current ratio of 4 or less, it can be suggested to increase the proportion of voltage-forming feed-in units. Preferably, at a short-circuit current ratio of 2 or less, and particularly at a short-circuit current ratio of less than 1.5, it is suggested to select the voltage-forming operating mode.

[0052] Detecting subnetwork formation can also be a preferred criterion. Subnetwork formation refers to a situation in which parts of the electrical supply network have separated from one another. The remaining part, i.e., the remaining subnetwork where the grid connection point of the inverter-driven feed-in device is located, can be weakened by such subnetwork formation and prone to instability or at least oscillation. Accordingly, if subnetwork formation is detected, it is recommended to select the voltage-regulating operating mode.

[0053] A special case of subnetwork formation is islanding, where the subnetwork to which the feed-in device is connected via its grid connection point forms its own independent system. In this case, the generators connected to this island network must independently manage and control it. Such a situation can be better addressed, or at least more effectively addressed, by a voltage-regulating operating mode than by a primarily current-regulating operating mode.

[0054] Another criterion is that selection is made via an external switching signal. Such a switching signal can originate from an operator of the electrical supply network, i.e., the grid operator, thus enabling the grid operator to influence the control characteristics of this feed-in device. For example, the grid operator can use this signal to proactively switch the inverter-controlled feed-in device into a more regulating operating mode, namely the voltage-regulating operating mode.

[0055] Such a switching or selection signal can also be designed to set a mixed operating mode. A mixed operating mode is one that has or combines current-forming and voltage-forming characteristics. Such a combination, and thus such a mixed operating mode, can be achieved particularly by having some feed-in units operate as current-forming units and others as voltage-forming units.

[0056] The criteria used to switch between current-forming and voltage-forming operating modes, or to switch to a mixed operating mode, can also be combined. A combination of two or more characteristics can generally be implemented by normalizing each criterion to a uniform value. For example, each criterion could be assigned a value from 0 to 1, i.e., 0 to 100 percent. Such a value can then identify the respective criterion.

[0057] However, it is also possible to first convert the result of the criterion to a standardized value. This means that each criterion is initially considered and evaluated individually. The evaluation is then standardized. For example, the result of an evaluation might be that there is a weak need to switch to a voltage-limiting operating mode. This need could be expressed as, for example, 30 percent, or it might reveal a strong need for such a switch, which could be expressed as, for example, 70 percent. Similarly, for each criterion, such a need can be specified as a requirement value for switching or not switching, with a value from 0 to 1, i.e., 0 to 100 percent. If several criteria are then considered, their output requirement values—that is, the described values, which can range from 0 to 1 or 0 to 100 percent—can be averaged.

[0058] It is also conceivable that the criteria could be weighted, so that, for example, one criterion is weighted with a value of 5 and another with a value of 2. The combined consideration of the respective requirement values ​​of the criteria could then be carried out, for example, by multiplying each requirement value by the respective weighting of the underlying criterion and calculating an average for each criterion from the results obtained in this way. This average would then be calculated by adding up all the weighted requirement values ​​and dividing this sum by the sum of the weighting factors, to give just one example.

[0059] According to one embodiment, it is proposed that a switching control is provided for switching between the current-forming and the voltage-forming operating modes, which also includes selecting an intermediate position, and that this switching control has a switching criterion implemented. The switching criterion is thus checked in this switching control, and then a switch is performed if necessary.

[0060] Furthermore, or alternatively, the switching control switches depending on at least one of the criteria explained above. Thus, the criteria, or at least one of them, can be implemented by such a switching control.

[0061] Switching via the switching control can be achieved, for example, by the switching control sending a corresponding switching signal to the affected feed-in units, thereby switching them from a current-regulating operating mode to a voltage-regulating operating mode. Particularly for switching to a mixed operating mode, the switching control can be coupled to several feed-in units, setting some of these feed-in units to the current-regulating operating mode and others to the voltage-regulating operating mode. This allows the mixed operation to be controlled accordingly, depending on the number of feed-in units selected for the respective operating mode. A purely current-regulating or voltage-regulating operating mode is achieved when all of these voltage-regulating units are set to current-regulating or voltage-regulating operation, respectively, by the switching control.

[0062] Furthermore, or alternatively, it is proposed that an adaptation algorithm be implemented in the switching control. This algorithm can use a criterion and qualitatively and / or quantitatively evaluate changes after a switchover using a quality measure. Here, too, the consideration of a mixed operating mode is proposed, at least according to one embodiment.

[0063] It is now possible, for example, to evaluate whether a voltage fluctuation could be reduced after switching from a current-forming operating mode to a voltage-forming operating mode, and if so, how much it could be reduced. If, in this example, it is observed that the voltage fluctuation has decreased significantly, the switchover can generally be considered successful. If the effect was small, it might be worth considering switching to the voltage-forming operating mode earlier, i.e., when voltage fluctuations were lower.

[0064] Especially when considering mixed operation, to stick with the example of voltage fluctuation, the proportion of voltage-generating units, i.e., units that have switched to a voltage-generating operating mode, can be increased if it has turned out that the previously selected mixed mode, i.e., the previously selected mixed operating mode, was not yet sufficient.

[0065] The insights gained in this way, whether with or without mixed-mode operation, can be implemented and stored in such a way that the switching control is generally modified, or rather adapted. The changes to the switching algorithms identified here are then incorporated into future adjustments. In the simplest example, this could mean that, for instance, a voltage threshold for switching is changed in response to voltage fluctuations, and this modified threshold is used the next time. However, more complex relationships can also be adapted in this manner.

[0066] According to a further embodiment, it is proposed that a frequency-controlled operation can additionally be selected, in which the inverter-controlled feed-in device controls or regulates to a frequency setpoint, and in particular that it is possible to switch between frequency-controlled operation and a frequency-open operating mode. In the frequency-open operating mode, the inverter-controlled feed-in device regulates to a frequency detected in the electrical supply network. Thus, when the feed-in device is in frequency-open operating mode, a frequency detected in the electrical supply network is used as the basis, and the corresponding setpoint signal is specified accordingly. This means that in voltage-controlled operation, the voltage is specified with this frequency value, and in current-controlled operation, the current, i.e., the setpoint current, is specified with this frequency value.In frequency-controlled operation, a voltage signal with a predetermined frequency is generated, and this predetermined frequency can be adjusted to match the mains frequency. It is proposed that this frequency adjustment be performed with a large time constant.

[0067] If the feed-in device switches to frequency-controlling operation, which is particularly suitable for voltage-controlling operation, the corresponding setpoint—especially the voltage setpoint in voltage-controlling operation—is not adjusted to the frequency detected in the electrical supply network, or is adjusted only with a delay. Instead, a frequency is specified, and the voltage setpoint, i.e., the sequence of instantaneous values, is then set at this specified frequency. This allows the feed-in device to assume or support frequency control, or contribute to greater frequency stability. Such frequency-controlling operation is particularly advantageous in the case of detected subnetwork formation, especially in the case of detected islanding, and is therefore preferably recommended.

[0068] According to a further embodiment, it is proposed that the inverter-controlled power supply device outputs a voltage signal at a voltage output. This voltage output is connected to an inductor, so that, depending on the voltage signal, the voltage in the electrical supply network, and the inductor itself, an output current and an output voltage are generated at an output of the inductor. This results in an output voltage and an output current at the output of the inductor. These output voltage and current are used for further control steps.

[0069] Based on this, current control is used in the current-determining operating mode, and the described output current is fed back as the actual value. Accordingly, it is compared with a setpoint for the output current, in the sense of a control engineering setpoint-actual value comparison.

[0070] Alternatively, or in addition, a voltage regulator is used in voltage-simulating operating mode, and the described output voltage at the choke output is fed back as the actual value. This can then be compared with a target voltage value.

[0071] This feedback and the comparison of the output current or output voltage should be understood as a comparison of instantaneous values.

[0072] According to a further embodiment, it is proposed that switching or selection between the voltage-sensing operating mode and the current-sensing operating mode be implemented by a computer-implemented control system. This allows criteria, control rules, and resulting control commands to be implemented in the computer-implemented control system in a simple manner. Any changes can be achieved by reprogramming or installing an update. Furthermore, a more complex control system, including an adaptation algorithm, can also be implemented easily.

[0073] In particular, this implementation, using a computer-implemented control system, incorporates continuous feedback of the output current and voltage. Both values ​​are thus fed back, allowing the computer-implemented control system to access and select between the measured output current and output voltage as the actual value. This enables simple, short-term switching between operating modes.

[0074] It is particularly proposed that such a computer-implemented control system be provided at each feed-in unit. If necessary, switching requests can be sent from a higher-level central control unit to each of these computer-implemented control systems as a request signal.

[0075] According to a further embodiment, it is proposed that the inverter-controlled feed-in device has at least one inverter to generate the voltage signal as a pulsed voltage signal using a pulse pattern, wherein the pulse pattern is generated depending on the selected operating mode and the feedback actual value. Thus, if the current-forming operating mode is selected, the feedback actual value of the output current is used to set the instantaneous values ​​and compared with a corresponding target current, namely the sequence of instantaneous values. Similarly, if the voltage-forming operating mode is selected, the feedback actual voltage signal is used and compared with a voltage setpoint. This allows for simple switching between current-forming and voltage-forming operation.

[0076] According to a further embodiment, it is proposed that a current band or voltage band be specified as the setpoint. The output current or voltage to be generated then only needs to be realized to the extent that it lies within this current band or voltage band. This also prevents excessive control activity.

[0077] It can also be specifically stipulated that no regulation takes place as long as the actual current signal or voltage signal lies within the corresponding band, i.e., the current band or voltage band. This also refers to the sequence of instantaneous values.

[0078] According to the invention, a feed-in device according to claim 8 is also proposed, namely a wind turbine. This feed-in device is designed for feeding electrical power into an electrical supply network at a grid connection point. It has at least one converter or inverter for carrying out the feed-in, so that the feed-in device operates in converter-controlled mode. The converter or inverter ensures, in particular, that a feed-in signal with a corresponding voltage amplitude, frequency, and phase angle is generated for feeding power into the electrical supply network. Whether this is done by an inverter that has a DC voltage as its input supply source or by a converter that has an AC voltage as its input energy source is irrelevant, as long as the converter or inverter...The inverter outputs a corresponding signal with appropriate amplitude, frequency and phase.

[0079] It is now proposed that this feed-in device include at least one switching control for switching between a current-regulating and a voltage-regulating operating mode. Such a switching control can preferably be implemented by a computer-based control system.

[0080] The current-controlling operating mode is one in which the current is controlled or regulated to a setpoint, and the voltage-controlling operating mode is one in which the voltage is controlled or regulated to a setpoint.

[0081] According to the invention, at least one switching control also provides for switching or selecting a mixed operating mode.

[0082] According to the invention, the feed-in device operates as described in at least one embodiment of the described method for feeding electrical power into the electrical supply network.

[0083] This allows the possibilities and advantages described in connection with at least one embodiment of the feed-in method to be realized through this feed-in device.

[0084] The invention will now be explained in more detail below by way of example embodiments with reference to the accompanying figures. Figure 1 shows a wind turbine in a perspective view. Figure 2 shows a wind farm in a schematic representation. Figure 3 illustrates a geographical distribution of current-generating and voltage-generating feed-in units using the European interconnected grid as an example. Figure 4 schematically shows a section of an electrical supply network with several connected wind farms. Figure 5 schematically shows a representation with a current-generating feed-in unit and a voltage-generating feed-in unit. Figure 6 illustrates a section of an electrical supply network.

[0085] Figure 1 Figure 1 shows a wind turbine 100 with a tower 102 and a nacelle 104. A rotor 106 with three rotor blades 108 and a spinner 110 is mounted on the nacelle 104. During operation, the wind sets the rotor 106 into rotation, thereby driving a generator in the nacelle 104.

[0086] Figure 2Figure 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. 2This is only a simplified representation of a wind farm 112, which, for example, does not show a control system, although a control system is of course present. The park network 114 may also be designed differently, for example, by including a transformer at the output of each wind turbine 100, to name just one other example.

[0087] Figure 3 It shows a section of the outline of Europe, specifically a section where the European interconnected grid largely forms the electrical supply network. The following are illustrative: Figure 3 Wind farms and large power plants are shown distributed across the region. These are only examples of various other energy producers, and this representation is therefore not exhaustive for the European interconnected grid.

[0088] In any case, several large power plants (310) and various wind farms are shown. Specifically, wind farms that generate electricity (320) and wind farms that generate voltage (330) are depicted. For clarity, only a few reference numbers are shown, but the two different wind farms (320 and 330) can also be distinguished by their symbols. A simple wind turbine is used as the symbol for wind farms that generate electricity (320), and a simple wind turbine with a V at the base of the tower is used as the symbol for wind farms that generate voltage (330). The letter V at the base of the tower is intended to illustrate voltage generation.

[0089] The Figure 3This essentially illustrates that voltage-generating power sources are distributed across the geographical area shown, either as large power plants (310) or as voltage-generating wind farms (330). For example, although this does not reflect the current situation, no large power plants (310) but voltage-generating wind farms (330) are shown on the Iberian Peninsula, the Italian Peninsula, in Denmark, and in Brittany. However, not only voltage-generating wind farms but also electricity-generating wind farms (320) are shown there as well. Thus, a good distribution has been achieved.

[0090] The distribution shown is Figure 3However, it can be variable. For example, large power plants can be shut down. In Germany, in particular, several nuclear power plants will be shut down in the near future. Large power plants can also be temporarily switched on or off, especially gas-fired power plants or pumped-storage power plants that are spontaneously switched on or off to support the electrical grid, to name just a few examples. The amount of active power fed into the grid can also vary due to changes in wind conditions. For example, in the illustrative example of... Figure 3 to remain, to switch further wind farms 320 on the Iberian Peninsula, which are currently operating in a power-sharing mode, into a voltage-sharing or predominantly voltage-sharing operating mode if the voltage-sharing wind farms 330 shown, for example, feed in less active power due to weak winds and are therefore less able to regulate.

[0091] The implementation of such a comprehensive concept for an electrical supply network is in the Figure 4 illustrated. Basically, it shows Figure 4 Schematically, only an electrical supply network 440 is shown, of which essentially all the elements described below form a part. The electrical supply network 440 is nevertheless in the Figure 4 also represented by a common symbol.

[0092] Figure 4This section now shows three wind farms 450 as examples. The same reference symbol has been used for all three wind farms, although they may, of course, differ in their type or current operating mode. One of the wind farms 450, shown in the left half, is depicted in greater detail and shows three wind turbines 400 as examples. These three wind turbines, which can be representative of other wind turbines, may, for example, be identical in construction. For clarity, all three exemplary wind turbines are marked with the same reference symbol 400. Two of these wind turbines 400 are shown only as symbols, and one wind turbine 400 is shown as a dashed block to illustrate the structure being explained, which shows four feed-in units 460 as examples. Each of these feed-in units 460 can represent an inverter, or...for a control cabinet that contains such an inverter.

[0093] Each of these feed-in units 460 can be switched between current-forming operating mode I and voltage-forming operating mode V. This is symbolized in each feed-in unit 460 by a switch that can select between current-forming operating mode I and voltage-forming operating mode V. As an example, current-forming operating mode I is selected in one of the feed-in units 460, namely in the unit shown in the illustration. Figure 4 The feed-in unit shown below, whereas the other three exemplary feed-in units 460 symbolize by the switch position that they are operating in voltage-forming mode.

[0094] Such a selection can be made by the wind turbine controller 466. The wind turbine controller 466 is connected to each feed-in unit 460 via a wind turbine data bus 468. Corresponding control commands can be sent to the feed-in units 460 via this wind turbine data bus 468. Figure 4 This indicates that this data bus 468 sends a control command with the content I to the feed-in unit 460 shown at the very bottom, i.e., the control command to select the current-generating operating mode. Correspondingly, the other three exemplary feed-in units 460 receive the information V as a control command, i.e., the control command to select the voltage-generating operating mode V.

[0095] The wind turbine controller 466 can implement a higher-level control command from the wind farm control unit 470. The wind farm control unit 470 can then send corresponding control commands to the individual wind turbines 400 via a park data bus 472. In the Figure 4 In the illustrated example, the control command sent to the first wind turbine 400 could, for instance, be to operate at 75 percent voltage generation. This is illustrated as "V75" at the input of the wind turbine control unit 466 and can represent 75 percent voltage generation. The same signal, namely signal V75, is also sent to the other two wind turbines 400, but different signals can also be sent to the wind turbines 400.

[0096] The wind farm control unit 470, in turn, receives a selection signal transmitted by a grid operator 480. This is just one example; instead or additionally, the wind farm 450 or the wind farm control unit 470 can also switch or select a corresponding operating mode based on other criteria.

[0097] In the illustrative example of the Figure 4 However, it is planned that the network operator 480 transmits a corresponding selection signal to the wind farms 450 via an operator data connection 482. This operator data connection 482 does not require high transmission rates, because switching between current-regulating operating mode and voltage-regulating operating mode, or a mixed operating mode, is neither a constant occurrence nor particularly time-critical, and requires only a small amount of data for transmission.

[0098] Figure 5illustrates how a converter or inverter, i.e., a feed-in unit, can implement a current-forming operating mode or a voltage-forming operating mode.

[0099] Figure 5 Figure 800 illustrates a connection structure that can be connected to an electrical supply network 806 via a disconnect switch 802 and a connecting transformer 804. The connection structure 800 can, for example, be a connection between several current- or voltage-generating units that jointly provide electrical power via the connection structure. The connection structure can, for example, form or comprise a link between all power cabinets in a wind turbine.

[0100] For illustrative purposes, a voltage-saturating unit 808 and a current-saturating unit 810 are shown, which are themselves also part of the connection structure 800. Both units can, for example, be housed in a wind turbine. Furthermore, they are designed to be switchable, meaning they can operate either as a voltage-saturating unit or as a current-saturating unit. However, for the sake of simplicity, the corresponding sensors are not shown.

[0101] The voltage-forming unit 808 includes a voltage-forming inverter 812, which generates a voltage u(t) at its output and is measured specifically at the output of the indicated first output filter 814. The voltage u(t) is continuously measured and fed back to the first microcontroller 816. The first microcontroller 816 evaluates the instantaneous values ​​of this measured voltage u(t). These measured values, like the output voltage, are three-phase. To explain the Figure 6 However, it is not necessary to discuss this three-phase structure. This also applies to the stream-defining unit 810.

[0102] The first microcontroller 816 also receives a voltage setpoint u soll,w , which specifies the voltage u(t) to be set in terms of magnitude, frequency and phase.

[0103] This setpoint u soll,w is generated in the first inverter control unit 818. It depends on a setpoint voltage U soll,N and the measured values ​​U, I, f, φ, which are measured at the output of the first line choke 820.

[0104] The current-forming unit 810, which operates in a current-forming manner, includes a current-forming inverter 822, which operates similarly to the voltage-forming inverter 812, but regulates to an output current i(t). This output current i(t) is detected at the output of the indicated second output filter 824 and evaluated in the second microcontroller 826. The second microcontroller 826 thus receives a current setpoint itarget,w, which specifies the magnitude, frequency, and phase of the current i(t) to be generated. The second microcontroller 826 accordingly controls the switching operations in the current-forming inverter 822, as indicated by the reference symbol S. Incidentally, the first microcontroller 816 controls switching operations in the inverter 812.

[0105] The setpoint current itarget,w is determined in the second inverter control unit 828. It depends on the voltage U, the current I, the frequency f, and the phase angle φ, and these quantities are measured at the output of the second line choke 830. The second inverter control unit 828 also receives a setpoint voltage Utarget,N as an input. The first and second inverter control units 818 and 828 can also be combined into a single inverter control unit.

[0106] The voltage-generating unit 808 thus produces a first current I1, and the current-generating unit 810 produces a second current I2. These two currents I1 and I2 add up to the common current IG. This flows, for illustrative purposes, into the symbolic parking network 800. This is meant to be illustrative because the voltage-generating unit 808 and the current-generating unit 810 are also part of the parking network 800. Therefore, the common current IG flows into the remaining part of the parking network.

[0107] During operation, if, for example, a reactive power surge or a phase shift occurs in the park network 800, this is reflected in the total current IG. Because the output current I2 of the current-regulating unit 810 is controlled by this, a change in the common current IG initially only leads to a change in the first current I1 of the voltage-regulating unit 808.

[0108] The change in the common current IG initially led to a change in the first current I1, which was detected by the first inverter control unit 818. Based on this, the first inverter control unit 818 determines a new value for a voltage amplitude and / or a frequency, depending on a reactive power static or active power static. Accordingly, the voltage setpoint signal u sou,w is adjusted and transmitted to the first microcontroller 816. This then controls the voltage-regulating inverter 812 accordingly. This leads to a change in the voltage amplitude and / or the frequency of the voltage, which is measured by the current-regulating unit 810 at the output of the second line choke 830 and evaluated in the second inverter control unit 828.Depending on this, a new reactive power value and / or a new active power value is calculated, specifically based on the underlying reactive power or active power statics. Accordingly, a target current signal itarget,w is specified and transmitted to the second microcontroller 826. This then controls the current-generating inverter 822 accordingly. The result is that the second current I2 changes, which in turn changes the first current I1, leading to a further adjustment by the first inverter controller 818, again based on the respective statics, i.e., the reactive power statics and / or the active power statics.

[0109] As a result, the voltage-generating unit 808 and the current-generating unit 810 will adjust to each other in such a way that they feed in a corresponding active or reactive power according to the statics relevant to them at the same voltage deviation or the same frequency.

[0110] Figure 5 This describes the basic operation of a voltage-regulating unit 808 and a current-regulating unit, and these two can also be operated together and feed into the same grid. However, significantly more than two units can operate together. Each unit can operate independently as described above. It is now suggested that, depending on the situation, the focus should be placed on either voltage-regulating or current-regulating operation. In the simplified example of the Figure 5This would mean that both units are voltage-generating or current-generating. With many units, i.e., far more than two, it may be sufficient, depending on the situation, for most, or at least the majority, of units to be voltage-generating or current-generating.

[0111] Fig. 6 Figure 650 illustrates a network section 650 of an electrical supply network, comprising a large power plant 652, settlements 658, an industrial consumer 654, a city 656, and several wind farms WP1 to WP3. These elements each also represent other elements of their type and are, for illustrative purposes, connected to a common line via a transformer T. Each wind farm WP1 to WP3 forms a converter-controlled feed-in device.

[0112] The exemplary wind farms WP1 to WP3 can feed electrical power into the electrical supply network 650. These wind farms WP1 to WP3 generally operate in current-regulating mode, but can also include voltage-regulating feed-in units, and they can operate feed-in units in either current-regulating or voltage-regulating mode. They can also be controlled in a coordinated manner by the network control unit 651, particularly with regard to the control of setpoints. Each wind turbine controls and regulates instantaneous values ​​independently. The network control unit 651 can also control the switching of one or more wind farms WP1 to WP3 from current-regulating to voltage-regulating operation, or vice versa.It can also specify a proportion of current-forming or voltage-forming feed-in units and thereby change between a current-forming operating mode and a voltage-forming operating mode in several steps or in a smooth transition.

[0113] For the transmission of such specifications, including the transmission of setpoints, a transmission means is provided that transmits data via a data transmission channel 653 to the wind farms WP1 to WP3, where the data is entered via an interface. Each wind farm WP1 to WP3 can have a central park control unit to distribute the values ​​specified by the grid control unit 651 to the wind turbines of the wind farm. Each wind turbine has several feed-in units, and thus each wind turbine can also switch stepwise from a current-regulating operating mode to a voltage-regulating operating mode.

[0114] Communication between wind farms WP1 to WP3 and the grid control unit 651 can also be bidirectional, as illustrated by the respective representations of the data transmission channels with arrows pointing in both directions. Transmission can be wired, wireless, or a combination of both.

[0115] This allows the grid control unit 651 to also take data from the respective wind farm into account. For example, it can receive and evaluate information on current limits reached. The bidirectional information transmission also opens up the possibility for the wind farm to act as a measuring sensor, recording measured values ​​from the electrical supply network 650, in particular voltage and frequency, and transmitting this measurement data to the grid control unit for further use. This allows information, especially about grid conditions or grid characteristics, to be received and evaluated in order to then consider criteria for switching between the current-regulating and the voltage-regulating operating modes.

[0116] For example, a power plant, such as power plant 652, might be shut down. This can cause a phase shift and / or a frequency change. The grid control unit 651 can detect this, or it can receive this information from a grid operator. The grid control unit 651 can then signal wind farms WP1 to WP3, or at least one of them, to increase their voltage-regulating characteristics. The affected wind farms can then switch to a voltage-regulating operating mode, or at least increase the proportion of voltage-regulating units by switching.

[0117] The invention specifically recognizes that electrical supply networks can change. This can affect, for example, the European interconnected grid, which is mentioned here as an example and is referred to simply as the interconnected grid, also representing other interconnected grids. Through the substitution of conventional power plants with inverter-based, renewable energy plants, voltage-regulating generators can be lost to the interconnected system.

[0118] In undisturbed operation, the interconnected network can potentially be operated with very few, ideally evenly distributed, or even no voltage-generating units.

[0119] However, it was recognized that in disrupted operation, during network restoration or in the event of a black start, voltage-shaping behavior may be necessary, or at least helpful.

[0120] To achieve this, it is proposed that the installations, especially wind turbines or wind farms, then operate either as current-impacting or voltage-impacting systems depending on the characteristics of the grid and / or an operating point of the installations, and that these characteristics be switched dynamically.

[0121] An achievable advantage of this solution would be the avoidance of instabilities caused by a swarm of too many voltage-generating systems while simultaneously meeting the minimum demand for voltage-generating systems in the network.

Claims

1. A method for supplying electric power to an electric supply network (120) at a network connection point by means of a converter-controlled supply apparatus, namely a wind turbine, wherein - it is possible to choose at least between a current-impressing operating mode and a voltage-impressing operating mode for the purpose of supplying the electric power, and wherein - open-loop or closed-loop control to a desired current value is carried out in the current-impressing operating mode, and - open-loop or closed-loop control to a desired voltage value is carried out in the voltage-impressing operating mode, and wherein - an instantaneous operating mode can be changed between the current-impressing operating mode and the voltage-impressing operating mode in a plurality of steps or in a smooth transition, and in that - it is possible to set a mixed operating mode which has or combines a current-impressing property and a voltage-impressing property, characterized in that the converter-controlled supply apparatus comprises a plurality of supply units, and the mixed operating mode is set in that some of the supply units operate in a current-impressing manner and some of the supply units operate in a voltage-impressing manner.

2. The method as claimed in claim 1, characterized in that a changeover is made between the current-impressing operating mode and the voltage-impressing operating mode on the basis of at least one criterion from the list comprising - a measure of a voltage fluctuation in the electric supply network, - a current limitation measure which is a measure of how often voltage-impressing units of the converter-controlled supply apparatus have reached a current limitation, - a ratio of directly coupled synchronous generators supplying the electric supply network to converter-controlled supply apparatuses supplying the electric supply network, wherein the respectively currently supplied power, in particular the active power, or a ratio of the respectively supplied power and the active power, is used, in particular, as a reference variable, - a measure of a frequency fluctuation in the electric supply network, - a measure of phase jumps in the network, - a network property of the electric supply network, in particular a network sensitivity, - an operating state of the converter-controlled supply apparatus, in particular the currently supplied power, in particular the active power, with respect to a nominal power of the converter-controlled supply apparatus, - proportion of voltage-impressing supply apparatuses supplying the electric supply network, - proportion of wind turbines supplying the electric supply network, - proportion of solar generators supplying the electric supply network, - a short-circuit power at the network connection point, - a network impedance at the network connection point, - a short-circuit current ratio at the network connection point, - detection of the formation of a subnetwork, in particular the formation of an isolated network, and - an external changeover signal or selection signal, in particular a changeover signal predefined by an operator of the electric supply network.

3. The method as claimed in one of the preceding claims, characterized in that a changeover open-loop controller is provided for the purpose of changing over between the current-impressing operating mode and the voltage-impressing operating mode, wherein the changeover open-loop controller - has implemented a changeover criterion, - changes over on the basis of at least one criterion as claimed in claim 2, and / or - has implemented an adaptation algorithm which uses a criterion, in particular a criterion as claimed in claim 2, and qualitatively and / or quantitatively assesses changes after a changeover by means of a quality measure.

4. The method as claimed in one of the preceding claims, characterized in that it is additionally possible to choose a frequency-impressing mode in which the converter-controlled supply apparatus performs open-loop or closed-loop control to a desired frequency value, in particular in that it is possible to change over between the frequency-impressing mode and an open-frequency operating mode, wherein the converter-controlled supply apparatus performs closed-loop control to a frequency captured in the electric supply network in the open-frequency operating mode.

5. The method as claimed in one of the preceding claims, characterized in that the converter-controlled supply device outputs a voltage signal at a voltage output - the voltage output is connected to an inductor, with the result that an output current and an output voltage are produced at an output of the inductor on the basis of the voltage signal, a voltage in the electric supply network and the inductor, wherein - closed-loop current control is used in the current-impressing operating mode and the output current is fed back as an actual value, and / or - closed-loop voltage control is used in the voltage-impressing operating mode and the output voltage is fed back as an actual value.

6. The method as claimed in one of the preceding claims, characterized in that - a changeover or choice between the voltage-impressing operating mode and the current-impressing operating mode is performed by means of a computer-implemented open-loop controller, in particular in that - a capture operation for feeding back an output current or the output current and an output voltage or the output voltage is permanently carried out, and the computer-implemented open-loop controller can choose between the captured output current and the captured output voltage as the actual value which is fed back, and / or - the converter-controlled supply apparatus has at least one converter in order to generate the voltage signal as a pulsed voltage signal by means of a pulse pattern, and wherein the pulse pattern is generated on the basis of the selected operating mode and the actual value which has been fed back.

7. The method as claimed in one of the preceding claims, characterized in that - a current band or a voltage band is predefined as the desired current value or the desired voltage value.

8. A supply apparatus, namely a wind turbine (100), for supplying electric power to an electric supply network (120) at a network connection point (118), comprising - at least one converter or inverter (808, 810) for carrying out the supply operation, with the result that the supply apparatus operates in a converter-controlled manner, - at least one changeover open-loop controller (466) for changing over between a current-impressing operating mode and a voltage-impressing operating mode, wherein - open-loop or closed-loop control to a desired current value is carried out in the current-impressing operating mode, and - open-loop or closed-loop control to a desired voltage value is carried out in the voltage-impressing operating mode and - an instantaneous operating mode can be changed between the current-impressing operating mode and the voltage-impressing operating mode in a plurality of steps or in a smooth transition, and in that - it is possible to set a mixed operating mode which has or combines, a current-impressing property and a voltage-impressing property, characterized in that the converter-controlled supply apparatus comprises a plurality of supply units, and the mixed operating mode is set in that some of the supply units operate in a current-impressing manner and some of the supply units operate in a voltage-impressing manner, wherein the supply apparatus is configured to carry out a method as claimed in one of claims 1 to 7.