EP3818609A1

EP3818609A1 - Method for controlling a wind farm

Info

Publication number: EP3818609A1
Application number: EP19737081.0A
Authority: EP
Inventors: Aramis SCHWANKA TREVISAN; Angelo MENDONCA; Jair CASSOLI
Original assignee: Wobben Properties GmbH
Current assignee: Wobben Properties GmbH
Priority date: 2018-07-06
Filing date: 2019-07-05
Publication date: 2021-05-12
Also published as: CN112368899A; WO2020008034A1; US11898539B2; CA3103352A1; US20210285420A1; DE102018116444A1

Abstract

The invention relates to a method for controlling a wind energy system (304), namely a wind turbine (100) or a wind farm comprising a plurality of wind turbines, for feeding electrical power from wind into an electrical supply grid (302), and for damping low-frequency oscillations, in particular subsynchronous resonances, in the electrical supply grid (302). The electrical supply grid (302) has a grid voltage having a nominal grid frequency. A damping controller having a closed control circuit (300) is used to damp the low-frequency oscillations. In order to damp the low-frequency oscillations, the damping controller controls the feeding of electrical power into the electrical supply grid (302) using a wind system controller (306). The damping controller is designed for a controlled system. The controlled system comprises the electrical supply grid (302) or a part thereof, the wind energy system (304) or a part thereof, and the wind system controller (306) or a part thereof. The damping controller is designed in such a way that the damping controller avoids weakly damped modes in the closed control circuit (300) and/or damps weakly damped modes of the controlled system in the closed control circuit (300).

Description

Method of controlling a wind farm

The present invention relates to a method for controlling a wind energy system, namely a wind energy installation or a wind farm comprising several wind energy installations, for feeding electrical power from wind into an electrical supply network, and for steaming low-frequency vibrations in the electrical supply network. The present invention also relates to a wind energy system, namely a wind energy installation or a wind farm comprising several wind energy installations for carrying out such damping.

An electrical supply network generally has a nominal network frequency of 50 Hz or 60 Hz. This nominal network frequency can also be referred to as the system frequency. The electrical supply network can also be referred to in a simplified and synonymous way as a supply network or network.

Low-frequency vibrations can occur in the supply network, which have frequencies below the nominal network frequency. These vibrations are often referred to as subsynchronous resonances (SSR) or subsynchronous frequencies. To this end, the Institute of Electrical and Electronics Engineers (IEEE) published a formal definition for subsynchronous resonance in 1990, namely:

"Subsynchronous resonance is an electrical system state in which an energy exchange takes place between an electrical network and a generator set at one or more natural frequencies of the combined system, which are below the synchronous frequency of the system", P.M. Anderson, B.L. Agrawal, J.E.

Van Ness: "Subsynchronous Resonance in Power Systems", IEEE Press 1990

One problem that can occur in a supply network is that the low-frequency vibrations can excite or amplify mechanical vibrations from synchronous generators directly coupled to the electrical supply network. This can damage the generators. If these are separated from the electrical supply network for protection, this can weaken the electrical supply network. Low-frequency oscillations in the area of a few hearts can also occur in the supply network between network sections, for example a low-frequency oscillation between a first network section in Germany and a second network section in France. Such oscillations can also lead to partial network shutdowns in the supply network. In the worst case, this can lead to a blackout.

Because the proportion of large power plants is declining in many countries, while decentralized, converter-based producers such as wind turbines or wind farms, for which the term wind energy system is used here as a generic term, or photovoltaic systems are being replaced, such decentralized converter-based producers are also gaining support for electrical power Supply network in importance.

The structure or structure of the electrical supply network can also change. The electrical supply network is also subject to constant changes due to connection and disconnection processes, maintenance work on supply lines or the influence of the weather. This also means that network capacities and network inductances or network impedances as a whole can change continuously. This means that even the low-frequency vibrations can change continuously, which makes their detection and damping difficult.

Since wind farms and wind energy systems, or wind energy systems, form part of the electrical supply network and play a key role in determining the network properties, they can thus help to stabilize the energy systems or the supply network and can be used as a means of damping undesired subsynchronous resonances. However, such wind farms must also be able to adapt to the changed network properties with regard to the low-frequency vibrations. The German Patent and Trademark Office researched the following prior art in the priority application for this application: DE 10 2015 219 407 A1, WO 201 1/033044 A2, CN 106300386 A.

The object of the present invention is therefore to address at least one of the problems mentioned above. In particular, a solution is to be proposed which at least enables a wind power plant or a wind farm to have a damping effect on low-frequency vibrations in the electrical supply network. At least an alternative solution to previously known solutions is to be proposed. According to the invention, a method according to claim 1 is proposed. It is used to control a wind energy system, that is to say a wind farm or a wind energy installation, for damping low-frequency vibrations, in particular subsynchronous resonances, in an electrical supply network into which this wind energy system feeds. The supply network has a network voltage with a nominal network frequency, and the low-frequency vibrations to be damped preferably have a lower frequency than half the nominal network frequency. In this respect, subsynchronous resonances also refer to vibrations that have a lower frequency than the system frequency, here the nominal network frequency. The low-frequency vibrations are therefore preferably less than 25 Hz or 30 Hz. Examples of characteristic frequency ranges for low-frequency vibrations, which are also known as “Power System Oscillations” (PSO), are approximately 0.2-3 Hz or 5-15 Hz, whereby the frequency ranges are not limited to this. These vibrations or oscillations with low frequency in a supply network can be differentiated into different types of vibrations or vibration categories, namely intraplant vibrations, control mode vibrations, interarea vibrations and local plant vibrations.

Intraplant vibrations refer to vibrations between several generation units connected to the electrical supply network in a supply network section. Control mode vibrations refer to vibrations caused by feedback control of generation units, consumer units or converter units connected to the electrical supply network. Interarea vibrations refer to vibrations between several supply network sections. Localplant vibrations refer to vibrations between a generation unit connected to the electrical supply network and the supply network.

In particular, the low-frequency vibrations can have values of 1 Hz and less. However, they can also reach up to five times the nominal network frequency. Low-frequency vibrations here are vibrations with a frequency of at most five times the nominal network frequency, preferably with a frequency that corresponds at most to the nominal network frequency. In particular, the low-frequency oscillation has no frequency that corresponds to a multiple of the nominal network frequency. It should be noted that the examination and consideration of low-frequency vibrations, especially the examination or ensuring system stability of the electrical supply network is used. This differs from an assessment of the network quality or signal quality of the voltage signal in the electrical supply network, in which harmonics are particularly important.

A damping control with a closed control loop is used to dampen the low-frequency vibrations. This damping control is prepared for damping the low-frequency vibrations and works in such a way that it controls the feeding of electrical power into the electrical supply network using a wind system controller.

The wind system control can be viewed as the control of the wind energy system and this includes the control of a feed unit, especially an inverter. The regulation or control carried out has the possibility of influencing the damping of any vibrations by feeding in electrical power. In addition, i.e. by changing the feed, the damping control can influence the electrical supply network and thus also achieve damping. It is now proposed that the damping control be designed for a special controlled system. The controlled system includes the electrical supply network, the wind energy system, and the wind system control. It is also possible that only a part of it is included, but in any case all three elements are considered. This is based in particular on the consideration that these three elements can each significantly influence or even trigger low-frequency vibrations in the electrical supply network.

Many known approaches aim to detect low-frequency vibrations and then - to put it graphically - to activate a counter signal, in any case to react to the detected vibration. The approach proposed here tries to avoid this and instead uses a regulation that is adapted to the system. This, ie the damping control, is designed so that it avoids weakly damped modes in the closed control loop and / or dampens weakly damped modes of the controlled system in the closed control loop. The concept thus assumes that the system has frequencies as a system property at which weakly damped vibrations can occur if a corresponding excitation occurs. If such an excitation occurs, there can then be correspondingly weakly damped modes, that is to say oscillations or modes of oscillation. At other frequencies, the system is not or much less likely to vibrate, even if there is an excitation. This system, which allows such vibrations in some frequency ranges, has been recognized here as the electrical supply network with the wind energy system and the wind system control. Therefore, these three elements are used as a controlled system. The wind system control can optionally be viewed as part of the wind energy system, but the wind system control and the wind energy system are expressly listed as elements of the controlled system. In addition to the electrical supply network or a part thereof, at least the wind system control and a further part of the wind energy system must be elements of the controlled system. The damping control is then adapted to this system and can thus avoid weakly damped modes in the closed control loop and / or dampen weakly damped modes of the controlled system in the closed control loop.

It is preferably proposed, namely according to the first variant, that no modes are examined in the uncontrolled case, i.e. in the open control loop, but the damping controller is designed directly in the closed control loop, namely in such a way that weakly damped modes are avoided in the closed control loop , This can e.g. mean that the damping control achieves a damping property in a frequency range in which no damping or only weak damping was previously available. Alternatively or additionally, namely according to the second variant, there are modes that would occur without this damping control, for example in the open control loop, which then no longer occur in the closed control loop or only with stronger damping.

An attenuation controller is therefore proposed which is suitable for the entire frequency range of the low-frequency vibrations, that is to say particularly for frequencies in the range below the nominal network frequency.

This is also based on the idea that it is not assumed that the network is rigid, but rather that it is assumed that changes are occurring in the network that the controller can take into account. The damping controller is therefore tailored to the entire controlled system and not to a specific frequency.

The proposed controller design can be a parameterization or also a specification or change of a control structure. For example. a state controller can be selected in a different order. Even the choice of a control type comes into consideration, such as a choice between a PID controller and a state controller.

Instead of an active or direct generation of damping signals by the wind energy plants or the wind farm, a suitable design of wind turbine and / or wind farm regulators for damping low-frequency vibrations is proposed by adapting the properties of the wind energy system. The wind energy system itself is taken into account as part of an oscillatory system.

According to one embodiment, it is proposed that a network vibration model be created for the design of the damping control and the network vibration model comprises a network model and a wind system model. The network model represents the electrical supply network or the part of it covered in the controlled system. This can take into account a structure of the electrical supply network and the consumers and generators connected to it. The structure particularly takes into account the line system, including the impedances of line sections and any transformers. As consumers, industrial plants and settlements or cities in particular can each be viewed as a simplified overall behavior, for example as impedance. Generators can be viewed in a simplified manner as a power source or, if necessary, more precisely, for example, as an electrical machine, especially for producers that are constructed as a conventional power plant. Information on this, that is to say information on creating such a network model, can be provided by a network operator who operates the electrical supply network. The network operator can thus provide such a model.

The wind system model depicts a behavior of the wind energy system. Technical details of the wind system are known and can be determined for the operator of the wind energy system, which also carries out the damping control here. The wind system model also includes the behavior of the wind system control. The behavior of the wind system control is also known or results from the design of the damping control and is then known. For this purpose, it is proposed to combine the two models of the supply network and of the wind farm or the wind energy installation in one overall model, namely the network vibration model. The network vibration model is therefore a model of the vibratory energy system, namely the entire supply network, or a part thereof, including the wind farm or wind energy plants. Accordingly, not only is the supply network modeled, but also the behavior of a wind farm with a park controller or a wind turbine with its turbine controller is taken into account. In addition to or instead of the wind system controller, a higher-level park controller that can control several wind farms is preferably proposed. In this case, the wind energy system also includes several wind farms. In this way, the influence of the wind turbine or the wind farm or even of the several wind farms on the supply network can be determined. In order to create the network vibration model, a calculation unit is provided, which can be arranged, for example, in a wind turbine. For this purpose, it is then proposed that at least one simplified, in particular linearized, work model is determined from the network vibration model, and the damping control is designed on the basis of the at least one simplified or linearized work model. At least one simplified or linearized working model is thus assumed. This is also based on the consideration that the network vibration model is non-linear. Basically, the network model, the wind system model and the wind system control can be non-linear. However, it is also possible to simplify the system, for example, by reducing the order of the network vibration model. Possibly. a linear system can result if the simplified working model no longer contains non-linearities. The existence of a non-linear system means that the system properties, particularly expected vibrations or modes, can depend not only on frequencies but also on amplitudes. An analysis or controller synthesis in the frequency domain, which is often desirable, is therefore at least difficult. It is therefore proposed to determine at least one working model linearized around a working point. The choice of the working point is important. For this purpose, for example, several working points can be selected, which will be described below. In particular, at least when the damping controller is used later, the respective operating point that was previously used for the design should be known. The system is in use The damping control in a similar operating point as in the design of the damping control used can be assumed to be a well-adjusted damping controller.

For example. The amount of electrical power feed-in can be an operating point, or at least partially define an operating point.

It is therefore assumed that the network model and the wind system model are known in principle or are at least available for creating the network vibration model. It can be in the form of a non-linear differential equation system, for example, in which the differential equations describe the network and the wind farm with the park control unit.

For example. the damping parameterization can be determined in such a way that first a system identification is carried out or at least a simplified system description is derived from the linearized or simplified working model. This can be done analytically, for example, by calculating or solving differential equations, or graphically using a pole-zero diagram or Bode diagram. Damping parameters or controller parameters that achieve a damping effect on the low-frequency vibrations are determined from this system identification or description of the simplified network model. For this purpose, for example, different controller parameters of the wind turbine are tested by simulation or by prototype test in order to recognize the change in the system behavior. In addition, different working points can be tested for the simplified working model. The controller is then designed in such a way that especially the undamped low-frequency vibrations, which result from the system identification from the simplified working model, are damped. According to one embodiment, control parameters, ie parameters of the damping control, such as time constants and gain parameters, are varied and results are compared and evaluated in a simulation.

According to one embodiment, it is proposed that, for the design of the damping controller, a plurality of linearized working models are determined by varying working points, and based on the several linearized working models of the varied working points, the damping control is designed such that it weakly damped modes for each of the several linearized working models closed loop avoids and / or weakly damped modes of the controlled system in the closed control loop.

Different working points are thus taken into account, preferably in such a way that the entire expected working area can be taken into account. The controller is designed so that damping control is created that achieves sufficiently damped behavior for all operating points and especially for the entire relevant frequency range. The controller design thus leads to a robust controller that does not require any adjustment for these operating points and the associated frequency range during operation. This robust controller design can be carried out analytically, for example, if a sufficient stability distance and / or a sufficiently damped eigenvalue selection is made for each linearized working model, e.g. by specifying an eigenvalue, i.e. the eigenvalues are selected, for example, with a sufficiently small imaginary component compared to their real component.

However, it is also possible to use simulations to simulate controllers for the various working models in order to test and, if necessary, adapt the robust controller until it achieves sufficient damping for all working models.

In particular, it can be done here that the linearized working models each form the controlled system. So the closed control loop with the linearized working models including the respective controller is checked, especially in a simulation.

In addition or as an alternative, the wind system control can be designed as a control which is robust with respect to variations in the controlled system, in particular with respect to variations in the varied operating points, namely by means of the simulation mentioned and / or the analytical design. Both can be combined if, for example, the controller is analytically designed as a robust controller, and the result is checked by simulations in a closed control loop and, if necessary, improved by adapting the damping controller.

At least one electrical voltage of the electrical supply network preferably forms an input variable and at least one electrical output current for feeding into the electrical supply network forms an output variable of the wind system controller. For this purpose, an electrical voltage is recorded in particular at the network connection point of the electrical supply network, or at a point at which a representative one Voltage or other variable can be measured, such as at an output terminal of an inverter of the wind energy system. The measurement is preferably carried out in three phases.

The output current that is fed into the electrical supply network by the wind energy system is provided as an output variable. The output current, which is in particular three-phase, can thus be regarded as the manipulated variable of the damping control. The damping control and thus the wind power system can influence the electrical supply network and dampen it if necessary. The output current is in particular initially provided as a setpoint for the damping control. It can then be output as the actual output current by means of the wind system controller, which can comprise one or more inverters, and fed into the electrical supply network.

According to one embodiment, it is proposed that

the damping control is designed as a multivariable control, with at least one electrical voltage of the electrical supply network forming an input variable and, in addition, at least one further variable forming an input variable, from the list comprising an electrical intermediate circuit voltage of a direct voltage intermediate circuit of an electrical inverter, a reactive power output by the inverter,

an electrical generator power of a generator of the wind energy installation or at least one of the plurality of wind energy installations, a rotational speed of the electrical generator and

at least one blade angle of adjustable rotor blades of the wind energy installation or at least one of the plurality of wind energy installations.

In addition or alternatively, it is proposed that

at least one electrical output current for feeding into the electrical supply network forms an output variable of the wind system control and also forms at least one further variable an output variable of the wind control, comprising from the list

the electrical intermediate circuit voltage of the direct voltage intermediate circuit,

the reactive power output by the inverter,

the electrical generator power, the speed of the electrical generator and at least one blade angle of the adjustable rotor blades.

In control engineering, a multivariable system is understood to mean a system that has several input variables and / or several output variables. A system with several state variables, but only one input variable and only one output variable is not a multi-variable system, but a higher-order single-variable system. Input variables are mostly measurement variables, or variables that have already been recorded elsewhere. The output variables of the control are often setpoints or manipulated variables and are then included in the system that is to be controlled. It is now proposed here that there is at least one further input variable and / or at least one further output variable in addition to the input variable of the electrical voltage and the output variable of the output current.

Via the intermediate circuit voltage as the input variable of the inverter, a reaction of the inverter to vibrations in the electrical supply network can in particular be recognized, or at least flow into the damping control.

The reactive power output can be a reaction of the regulation of the inverter to voltage fluctuations.

The generator power provides information about the state of the generator of the wind turbine and this can have an impact on network vibrations. In particular, a resonance of the generator to an oscillation can be read from this. The same applies to the speed of the generator.

A blade angle can provide information about the system status of the wind power installation, especially its generator. A change or activity of the blade angle or the blade angle adjustment can also be related to a vibration of the electrical supply network.

As output variables, especially manipulated variables, variables come into consideration which can counteract or prevent an oscillation. However, not only the sizes that are also useful as input variables and were mentioned above are particularly suitable for this. The DC link voltage of the DC link influences the feed in strength and quality. In particular, a high DC voltage enables a better and faster feed-in and thus also a change in the feed-in, but a high DC voltage in the DC link, however, can put a high load on the inverter and can therefore be avoided on the other hand. To do both, the DC voltage in the DC link varies.

A voltage in the electrical supply network can be influenced via the reactive power.

By controlling the generator power, the generator in particular can possibly be influenced in terms of a swinging characteristic and damped if the controller is designed accordingly. The same applies to the speed of the generator.

A mechanical vibration can be influenced in particular by means of the blade angle, in particular a targeted adjustment and control of the adjustment speed of the blade angle. It is preferably proposed that the design of the damping control is or comprises a damping parameterization and that the damping parameterization is determined from an eigenvalue analysis and / or modal analysis of the simplified working model.

In the eigenvalue analysis, the eigenvalues are considered, namely in the present case those of the closed control loop. An oscillation behavior, namely an oscillation frequency or natural frequency, and an attenuation and thus an attenuation behavior can also be derived from the position of the eigenvalues in the complex plane. Especially for the linearized and thus linear working model, the system property can be represented by eigenvalues. Variations in the damping parameterization, and thus variations in the controlled system as a whole, lead to changes in the eigenvalues and often an eigenvalue can also be assigned to a parameter or parameter set, so that desired eigenvalues can, as far as possible, be influenced by appropriate parameter selection or even selected ,

Modal analysis is based on a similar idea. In addition to the frequencies of system vibrations, i.e. vibrations that describe the system, the modal analysis also allows vibration modes to be taken into account. Here are under modes especially to understand their damping behavior in addition to the frequency of the respective vibration. The modal analysis thus records the frequencies along with damping properties.

In particular, the design is such that damping parameters are varied so that different working models with different eigenvalues result and that depending on the eigenvalues, a damping parameterization is selected and used for the damping control of the wind energy system. By varying the damping parameters, i.e. by varying the damping parameters, the eigenvalues also change and these can be represented, for example, in the complex s-plane and it can then be seen which parameter changes lead to which changes in the eigenvalues. Based on this, the parameterizations or the parameters can be selected.

It is preferably proposed that the network vibration model additionally comprises a generator model that simulates the behavior of synchronous generators directly coupled to the network in the electrical supply network.

With the additional generator model, the interactions of the synchronous generators directly coupled to the network are taken into account in the model formation. The modeling therefore not only includes a rigid network, but also takes into account interactions between synchronous generators. Such synchronous generators directly coupled to the electrical supply network are usually present in large power plants and it was recognized that vibrations between synchronous generators directly coupled to the electrical supply network can often be a cause of low-frequency vibrations. With the approach of including such synchronous generators in the network vibration model, the damping control can be adapted well to vibrations that could be caused by such synchronous generators.

It is preferably proposed that a plurality of damping parameter sets and / or several controller structures are selected and stored for the damping control, and that one or one of the several damping parameter sets or controller structures is selected for operation of the wind energy system and / or during operation of the wind energy system can be switched between them. This selection or change is preferably carried out as a function of a selection criterion. Such a selection criterion can be, for example, a signal from a network operator that informs the wind energy system of a structural change. It was recognized here that the properties of the electrical supply network in particular can change significantly. A robust controller can compensate for small changes without adjustment, or at least it is robust. In the case of major changes, this may no longer be sufficient. For example. can be a significant change that a large consumer, such as a steel foundry, is not working and is therefore actually not connected to the electrical supply network. The difference between whether this exemplary foundry is connected to the electrical grid or not can be a significant difference. Another significant difference can be whether an important transmission line of the electrical supply network is disconnected or not. For example. To give a clear example, a transmission line crossing the Ems is regularly cut when a large cruise ship is delivered from Papenburg via the Ems.

Both examples are usually known beforehand and can be taken into account in particular by means of a corresponding switchover signal when operating the wind energy system. The fact that they are known also means that the underlying descriptions of the electrical supply network are known, for example by means of differential equations which take these special features or differences into account. The damping control can therefore be designed specifically for these different circumstances. Accordingly, there are different damping regulations, at least different damping parameter sets, and depending on the change in the electrical supply network, different controller structures. In between, you can choose during operation of the wind energy system, preferably also during operation.

In addition, changes to the electrical supply network can also occur if the network is expanded or there are other changes in the network that lead to significantly changed network properties. Grds. but also result in changes in the wind energy system, which can be taken into account through different damping parameter sets or controller structures. Full load operation on the one hand and partial load operation on the other hand can be mentioned as an example. Not only is less power available through the wind system in part-load operation than in full-load operation, but wind-dependent output fluctuations can also occur in part-load operation.

It is preferably proposed that a switch is made between the different damping parameter sets depending on one of the following criteria, namely depending on an external signal from a network operator, depending on the time of day, depending on one Weekdays, depending on a calendar day, depending on a state of the electrical supply network, and / or depending on a state of the wind energy system.

The network operator in particular is aware of changes in its network, such as the announcement by a bulk consumer to disconnect from the network. Such or other events can be known in advance, including when they occur. Switching over to the time of day, the day of the week and / or a calendar day, i.e. a specific date, can be provided in such cases. An example is a bulk consumer, for example not working on weekends.

However, a change in the electrical supply network can also be identified, for example in terms of system technology, by evaluating, for example, reactions of the electrical supply network to changes in a feed size. For example, a change in the mains voltage due to a change in the active power and / or reactive power fed in can indicate properties and thus a state of the electrical supply network, e.g. how sensitive the electrical supply network is. A state of the wind energy system, particularly how much power can currently be fed in, can also be used as a criterion to switch between sets of damping parameters.

The different damping parameter sets can, for example, be stored on a storage unit. According to one embodiment, it is proposed that

the network vibration model

the network model has at least one first differential equation and

the wind system model has at least one second differential equation, wherein

the network vibration model is preferably described in a nonlinear state space representation

and or

the simplified working model is the linearized network vibration model for a selected working point, the simplified working model preferably being described in a linear representation of the state space. Non-linearities of both the electrical supply network and the wind energy system are thus taken into account in the network vibration model, if present. A nonlinear representation of the state space is generally chosen as the representation, as is represented in generalized form in the formula (1). The state variable legs x to x _n or their time derivatives x ₁ to x _{n are contained} in the state vector x or x. Input variables ui to u _m are combined in the input vector u. x = f (x, u) (1)

With this representation, the system under consideration, namely the electrical supply network or a relevant part thereof, together with the wind energy system or a relevant part thereof, and the damping controller can be described in an essentially complete system description, which also takes into account non-linearities.

The system described in this way can then be simplified by linearization in one operating point, and linearization methods such as the eigenvalue analysis can then be used by the linearization. Non-linear influences can be taken into account by different working points, that is, by performing the linearization for different working points.

The simplified working model, namely the network vibration model linearized for a selected working point, can be described in a linear representation of the state space and this is shown in general form by the formula (2). Here, too, the bold letters indicate vectors. Because of the linearization around a selected working point, the system of equations refers to the change in the working point, indicated by "A".

Ax = AAx + B u (2)

An operating point for the linearization can be defined, for example, by an active power fed in by the wind energy system and / or by a reactive power fed in by the wind energy system.

It is preferably proposed that the network model and additionally or alternatively the simplified network model be recorded or adapted continuously during operation in order to react to changes in the structure of the electrical supply network. In order to counter system changes in the supply network, which can arise, for example, from maintenance work on the network, from network expansion, from connecting and disconnecting generators and consumers in the network, it is proposed according to one embodiment to continuously determine the damping parameters and to adapt them during operation.

According to the invention, a wind energy system is also proposed, namely a wind energy installation or a wind farm which comprises one or more wind energy installations. Such a wind energy system is prepared for feeding electrical power from wind into an electrical supply network, and it is prepared for damping low-frequency vibrations, in particular subsynchronous resonances, in the electrical supply network. The starting point is a supply network that has a network voltage with a nominal network frequency and that the low-frequency vibrations to be damped have a lower frequency than half the nominal network frequency. The proposed wind energy system comprises a damping control for damping the low-frequency vibrations, a closed control loop in which the damping control is used, and a wind system control for feeding electrical power into the electrical supply network, the damping control for damping the low-frequency vibrations feeding electrical power into the electrical Controls supply network using wind system control.

The damping control is designed for a controlled system, and the controlled system comprises the electrical supply network or a part thereof, the wind energy system, or a part thereof, and the wind system controller, or a part thereof. The damping control is designed in such a way that it avoids weakly damped modes in the closed control loop and / or dampens weakly damped modes of the controlled system in the closed control loop.

In particular, the wind energy system is prepared to carry out a method for controlling a wind energy system and for damping low-frequency vibrations according to an embodiment described above. In particular, the wind energy system has a control device for this purpose, in particular a process computer, on which the damping control is implemented, in which the damping control is designed in whole or in part and / or in which damping parameter sets can be stored. A storage unit is preferably provided in order to store a plurality of damping parameter sets and / or a plurality of controller structures, which have been selected for selection, for the damping control and to keep them ready for operation, so that one or one of the several damping parameter sets or controller structures are selected for the operation of the wind energy system can and can be switched between them in the operation of the wind energy system.

In particular, a selection control is provided for this, in order to control a selection from the damping parameter sets or controller structures depending on a selection criterion. A change in the damping control can thus be implemented in a simple manner.

In summary, the present invention therefore aims to make the plant and wind farm controls more flexible when implementing network-critical projects, and in particular to take weakly damped network vibration modes into account when designing the controller. The controller parameters of the wind turbines or the wind farm are matched to the network by means of a network study or designed so that critical low-frequency network vibrations either do not occur or are strongly damped. Active detection of low-frequency vibrations is therefore not necessary. However, this does not exclude that the controller parameterization can also be adapted and designed for detected low-frequency vibrations in order to specifically dampen known low-frequency vibrations in the supply network.

The invention is explained below using exemplary embodiments with reference to the accompanying figures.

Figure 1 shows a wind turbine in a perspective view.

Figure 2 shows schematically a wind farm. FIG. 3 schematically shows a closed control loop which contains a network model and a wind system model, namely a park model.

FIG. 4 schematically shows a schematic sequence of the method according to the invention in one embodiment. FIG. 1 shows a wind energy installation 100 with a tower 102 and a nacelle 104. A rotor 106 with three rotor blades 108 and a spinner 110 is arranged on the nacelle 104. During operation, the rotor 106 is set into a rotary movement by the wind and thereby drives a generator in the nacelle 104. The wind energy installation 100 can be part of a wind farm or itself form a wind energy system.

FIG. 2 shows a wind farm 112 with three wind turbines 100 by way of example, which can be the same or different. The three wind energy plants 100 are therefore representative of basically any number of wind energy plants of a wind farm 112. The wind energy plants 100 provide their power, namely in particular the electricity generated, via an electrical parking network 114. The currents or powers of the individual wind turbines 100 generated in each case are added up and a transformer 116 is usually provided, which transforms up the voltage in the park in order to then feed into the supply network 120 at the feed-in point 118, which is also generally referred to as PCC. FIG. 2 is only a simplified illustration of a wind farm 1 12, which shows no control, for example, although of course there is a control. The parking network 114 can also be designed differently, for example, in that, for example, there is also a transformer at the output of each wind energy installation 100, to name just one other exemplary embodiment. Wind farm 112 is an example of a wind energy system. FIG. 3 schematically shows a closed control loop 300 according to one embodiment. The details of the wind energy system are in the foreground here. The control circuit 300 comprises an electrical supply network 302, the wind energy system 304 and the wind system control 306. In this embodiment, a wind energy installation forms the wind energy system 304. The measurement unit 308, also shown, can be assigned to the wind system control 306. The wind system controller 306 can be integrated in a wind turbine control or can coincide with it. In this respect, the wind system controller 306 can also be understood as part of the wind energy system. The wind turbine control takes into account all of the dynamics of the wind turbine that are controlled, and it will usually suffice to consider only a part of the wind turbine control for damping low-frequency vibrations, or only part of the wind turbine control when designing the damping control consider. As an illustrative explanation, which can also be considered as an embodiment, the inverter 310, which can also be referred to as a converter, forms the control-related actuator in the control circuit 300 and the electrical supply network 302 the system to be controlled. The inverter 310 outputs the three-phase feed current bc as a manipulated variable and feeds it into the electrical supply network 302, which thus forms the input variable for the electrical supply network. The electrical supply network then responds with a three-phase output voltage Uabc, which forms the actual variable for the system control, that is to say the damping regulator, and the dynamics of the measuring unit 308 are neglected in the wind system control by comparing three-phase output voltage Uabc with an ideal output voltage, that is to say with an ideal sinusoidal output voltage. However, other variants are also possible, in which the three-phase output voltage Uabc is, for example, only considered as an effective value and then, depending on its level, that is to say the level of the voltage, the control system operates, for example, outputs a reactive power setpoint to the inverter 310.

But even in this simplified view, in which the wind system controller 306 only controls the inverter, the dynamics of the wind energy system 304 flow in. First of all, the dynamics of the inverter 310 are already a dynamics of the wind energy system 304. In addition, however, other elements of the wind energy system 304 also have an effect on the dynamics of the inverter 310. For this purpose, generator 312 should be mentioned first. It supplies the inverter 310 with power and therefore the dynamics of the inverter 310 also depend on the dynamics of the generator 312. In addition, the inverter 310 controls the generator 312 at least partially. The inverter 310 and the generator 312 interact at least in such a way that the generator 312 supplies electrical power P _e to the inverter 310, whereas the inverter at least influences or even controls the mechanical power Pm of the generator 312.

However, it is also contemplated that the wind system controller 306 directly takes into account the dynamics of the generator 312, for example by controlling its excitation when the generator 312 is an externally excited synchronous generator. The dynamics of generator 312 can also be taken into account directly in that wind system controller 306 controls a stator current of generator 312 via converter 310. A drive train 314 shown can also influence the dynamics of the wind energy system and thus the damping control. It passes on the rotor speed of the aerodynamic rotor, which is illustrated here as aerodynamic block 316, as generator speed to the generator 312 and passes on the generator torque T _g to the rotor or the aerodynamic block 316 as drive torque Ta. Depending on the structure, vibrations can occur in the drive train 314, for example, which, however, are rather small in a gearless generator.

An effective wind speed Ve _ff , which is calculated from the actual wind speed V _w and a structural speed Vstmkt which is dependent on the structural dynamics of the rotor and which, depending on the sign, adds to the actual wind speed V _w acts on the rotor, that is to say the aerodynamic block 316 is subtracted from it. For this purpose, the structural dynamics block 318 is shown, which outputs such an equivalent structural speed Vstmkt from the structural dynamics, particularly of the rotor. For the consideration of the wind, the wind field block 320 is specified, which outputs the actual wind speed V _w .

The wind turbine control and thus the wind system control 306 can influence the aerodynamic block 316, via the pitch block 322, namely by adjusting the rotor blades, which is also referred to as pitching.

This means that there are many options for taking the dynamics of the wind energy system into account, both in the analysis and in the control. For the analytical consideration of the electrical supply network 302, a system description is taken into account, for example, as a system of differential equations. A system of differential equations can also be set up for the structure of the wind energy system 304, based on the structure shown in FIG. 3. Both differential equation systems, i.e. the electrical supply network, which can also be referred to as the network model, and the wind energy system, which can also be referred to as the wind system model, can together result in an entire network vibration model, which can then be used for controller design, as shown in the figure below 4 is described.

FIG. 4 schematically shows a method sequence 400 of the method according to the invention in accordance with an embodiment with several method steps. A network vibration model is created in a step S1. This includes the network model 404 and the wind system model 406. The network vibration model can be present, for example, in a state space representation, which has several differential equations as line entries having. This is represented by the formula (1) in FIG. 4, which has already been described above. In a step S2, a simplified working model is determined from the network vibration model that was determined in step S1. The simplified working model can be presented, for example, in a linear state space representation. This is represented by the formula (2) in FIG. 4, which has also already been described above.

In a step S3, a damping parameterization is determined from the simplified working model of step S2. The damping parameterization determined in this way is then implemented in the controller parameterization of an adjustable controller 415 (ACU) (“adjustable control unit”) in step S4. In step S5, the wind farm, which forms the wind energy system here, is then controlled with the damping parameterization implemented in order to reduce or avoid low-frequency vibrations in the supply network by changing the system behavior of the controlled wind farm. The control of the wind farm or the wind energy installation is carried out by the adjustable controller 415 (ACU). In addition, a generator model 414 is provided in FIG. 4, which in a further embodiment goes into the network model 404. This is shown by the dashed arrow connecting block 414 to block 404. Accordingly, in this embodiment, synchronous generators coupled directly to the network and their interactions in the dynamic network are taken into account. FIG. 4 also shows as a further embodiment that the damping parameterizations successively determined in step S3 are stored as damping parameter sets in a memory unit 416 (MEM). Several damping parameterizations are thus determined in step S3 and then stored or stored in the memory unit 416. A switch is made between these stored damping parameter sets depending on predefined criteria (CRIT) 418. If, for example, an external signal from a network operator has been received, according to which the electrical supply network has changed, it is possible to switch over to a damping parameter set prepared and stored for this purpose. For this purpose, a damping parameter set is selected accordingly and entered into the adjustable controller 415 (ACU) or parameterized with the new damping parameter set. This means that the controller parameterization can also be changed during operation, depending on specified criteria. In addition, two diagrams A and B are shown in FIG. 4 in step S3, which are intended to illustrate the method step of determining a damping parameterization from the simplified working model.

Diagram A shows an eigenvalue distribution in the complex s-plane, which has a real axis (Re) and an imaginary axis (Im). The complex s-plane in diagram A has a left and a right half-plane, the left half-plane describing the stable region in which the real part is less than zero. Conversely, the right, unstable half-plane is the area in which the real part is greater than zero. Several eigenvalues 420 and 422 are shown in the complex s-plane, the distribution of which was recorded for different controller parameterizations.

Jump responses for the different controller parameterizations are shown in diagram B. These can be stored as damping parameter sets. Diagrams A and B should also be considered together.

For this purpose, diagram responses for three different controller parameters in curves A1, B1, C1 are shown in diagram B, whereby these parameterizations are each assigned to the eigenvalues A1, A1 ^* , B1, B1 ^* or C1, C1 ^* from diagram A. Curve A1 shows a slower control behavior compared to curves B1 and C1. The three controller speeds of different speeds can also be read from some eigenvalues, which are shown in diagram A. It can be seen that a change in the controller parameterization (A1, B1, C1, diagram B) of the wind energy installation has a different influence on the eigenvalues 420, 422 in diagram A. For example, in contrast to the complex conjugate eigenvalue pair 422, the eigenvalues 420 change little or not at all due to the change in the controller parameterization. The eigenvalue pair 422, on the other hand, moves depending on the selected controller parameterization A1, B1, C1. This is indicated by the arrow in diagram A, which illustrates that the eigenvalue pair 422 moves to the right or left depending on the controller parameterization.

The eigenvalue pair 422 thus changes as a function of the controller parameterization of the wind energy installation or the wind farm and can be represented as a characteristic curve. The slow controller behavior, for example for controller parameterization A1 from diagram B, therefore leads to the eigenvalue pair 422 which can be influenced by the parameterization being shifted further to the left into the left half-plane of the complex plane. A faster control behavior, e.g. B. the controller parameterization C1 from diagram B, conversely leads to a shift of the eigenvalue pair 422 to the right in the direction of positive real values. The eigenvalues can be assigned to low-frequency vibrations or vibration modes. The closer an eigenvalue is to the right half-plane, the faster the system property that it describes. In this way, those eigenvalues can be selected that describe or come close to a desired behavior.

FIG. 4 essentially shows with the process sequence 400 a method with which a damping controller is designed in each case, which is therefore based on a network model. In step S3, parameters are varied in order to design a damping controller as a result. In order to design a further damping controller, in particular in order to obtain a further damping parameter set, the method has to be run through with a new network model, which is based on a changed network.

For example. the network operator can provide different network descriptions for different expected network situations. To this end, he can already provide a network model, or a network model in the desired form is created from another description of the network received from the network operator. For example. comes into consideration that the network operator provides system descriptions of the electrical supply network for different network situations, but the system descriptions first have to be adapted to the network connection point of the wind energy system under consideration. The process sequence 400 is then run through again with a new network model, and variations in the controller parameters can also be carried out again in step S3 in order to obtain the desired damping controller for this new network model. Figure 4 illustrates an important part of this process.

In the first step, the analytical models of the wind turbines or wind farms are combined with network equations.

In the second step, the weakly damped modes are examined. Depending on a particular requirement, the modes are placed by the suitable design of the wind turbine or wind farm parameters that can be influenced, that is to say they are specified or selected within the possible framework.

After a detailed investigation, which can be based on simulations or prototype tests, the resulting turbine code, ie a parameter set, is implemented in the wind turbine.

It was thus recognized that electrical energy systems, as electrical supply networks can also be called, are oscillatory systems which have natural modes below and above the system frequency, that is to say essentially below and above 50 Hz or 60 Hz. If these modes or vibrations are excited, such vibrations can impair the system stability if they are not sufficiently damped.

Wind turbines, also known as wind turbines, can help stabilize energy systems. It should also be noted that the lifespan of a wind turbine is many years, approx. 25 years, and that the energy system can change and develop significantly during this time.

If weakly damped network vibration modes are ideally identified before a wind farm connection, be it through direct information from the relevant network operator or based on simulation studies, these may still be taken into account when designing wind turbine and wind farm controllers.

This invention aims to make the plant and wind farm controls more flexible when implementing network-critical projects, so that weakly damped network vibration modes are still taken into account when designing the controller. Furthermore, these controller parameters of the system are coordinated or designed project-specifically by network studies, so that critical network vibrations either cannot occur or are strongly damped.

This invention was also based on the idea of not generating an active generation of damping signals by the turbines, but rather a suitable design of wind turbine and wind farm controllers for damping low-frequency vibrations, which are also referred to as power system oscillations, propose. The The proposal particularly aims at the fact that no immediate detection of oscillations is necessary.

One idea was to use combined equations for the system consisting of a wind farm and a grid. The "continuous" character of the solution is also important in this proposed solution. It does not require an online detection method to identify oscillations in power systems. As soon as necessary, and as far as possible, attenuation for a specific frequency, or a frequency range, is artificially generated by suitable design.

Since it is then a system characteristic, it always remains available.

Claims

Expectations

1. A method for controlling a wind energy system (304), namely a wind energy installation (100) or a wind farm comprising several wind energy installations, for feeding electrical power from wind into an electrical supply network (302), and for steaming low-frequency vibrations, in particular subsynchronous resonances, in the electrical supply network (302), the electrical supply network (302) having a mains voltage with a nominal network frequency, wherein

A damping control with a closed control circuit (300) is used to dampen the low-frequency vibrations, and the damping control to dampen the low-frequency vibrations controls the feeding of electrical power into the electrical supply network (302) using a wind system controller (306) a controlled system is designed and includes the controlled system

the electrical supply network (302) or a part thereof, the wind energy system (304), or a part thereof, and

the wind system controller (306), or a part thereof,

the damping control being designed in such a way that it avoids weakly damped modes in the closed control loop (300) and / or damps weakly damped modes of the controlled system in the closed control loop (300).

2. The method according to claim 1, characterized in that

to design the damping control

- a network vibration model is created and the network vibration model comprises:

a network model (404) which depicts the electrical supply network (302) or the part thereof included in the controlled system, taking into account the structure of the electrical supply network (302) and the consumers and generators connected to it, and - a

A wind system model (406) depicting a behavior of the wind energy system (304), the wind system model (406) also comprising the behavior of the wind system controller (306), and that at least one simplified working model, in particular linearized around an operating point, is determined from the network vibration model, and

the damping control is designed on the basis of the at least one simplified or linearized working model.

3. The method according to claim 1 or 2, characterized in that

several linearized working models are determined for the design of the damping controller by variation of working points, and

based on the multiple linearized working models of the varied operating points, the damping control is designed such that it is suitable for each of the multiple linearized working models,

weakly damped modes in the closed control loop (300) avoided and / or

weakly damped modes of the controlled system are damped in the closed control loop (300),

especially if the linearized working models each form the controlled system, and / or that

the wind system control is designed to be robust in relation to variations in the controlled system, in particular in relation to variations in the varied operating points.

4. The method according to any one of the preceding claims, characterized in that at least one electrical voltage of the electrical supply network (302) forms an input variable and

at least one electrical output current for feeding into the electrical supply network (302) is an output variable of the wind system controller

(306) forms.

5. The method according to any one of the preceding claims, characterized in that the damping control is designed as a multi-variable control, wherein at least one electrical voltage of the electrical supply network (302) forms an input variable and also forms at least one further variable an input variable, from the list an electrical intermediate circuit voltage of a direct voltage intermediate circuit of an electrical inverter, a reactive power output by the inverter (300), an electrical generator power of a generator (312) of the wind energy plant (100) or at least one of the several wind energy plants,

a rotational speed of the electrical generator (312) and - at least one blade angle of adjustable rotor blades of the wind energy installation (100) or at least one of the plurality of wind energy installations, and / or that

at least one electrical output current for feeding into the electrical supply network (302) forms an output variable of the wind system control (306) and also forms at least one further variable an output variable of the wind control, comprising from the list

the reactive power output by the inverter (300), - the electrical generator power,

the speed of the electrical generator (312) and at least one blade angle of the adjustable rotor blades.

6. The method according to any one of the preceding claims, characterized in that the design of the damping control is a damping parameterization or comprises and the damping parameterization is determined from an eigenvalue analysis and / or a modal analysis of the simplified working model, in particular that

When designing damping parameters, they are varied so that different working models with different eigenvalues result and that depending on the eigenvalues, a damping parameterization is selected and used for the damping control of the wind energy system (304).

7. The method according to any one of the preceding claims, characterized in that the network vibration model additionally comprises a generator model (414) which simulates the behavior of synchronous generators coupled directly to the network in the electrical supply network (302).

8. The method according to any one of the preceding claims, characterized in that for the damping control

several damping parameter sets and / or several controller structures can be determined and stored for selection, and that

- One or one is selected from the plurality of damping parameter sets or controller structures for the operation of the wind energy system (304) and it is possible to switch between them during operation of the wind energy system (304), in particular one selects or changes depending on a selection criterion. 9. The method according to any one of the preceding claims, characterized in that switching between different damping parameter sets, depending on at least one criterion from the list, comprising:

an external signal from a network operator,

a time of day

- a weekday,

a calendar day,

a state of the electrical supply network (302), and a state of the wind energy system (304).

10. The method according to any one of the preceding claims, characterized in that - the network vibration model

the network model (404) has at least one first differential equation and

the wind system model (406) has at least one second differential equation, wherein

- The network vibration model is preferably described in a non-linear state space representation

and or

the simplified working model is the linearized network vibration model for a selected working point, the simplified working model preferably being described in a linear representation of the state space.

1 1. The method according to any one of the preceding claims, characterized in that the network model (404) and / or the simplified network model (404) are continuously recorded or adjusted during operation to respond to changes in the structure of the electrical supply network (302) , 12. wind energy system (304), namely wind energy installation (100) or wind farm (1 12), which comprises one or more wind energy installations,

prepared for feeding electrical power from wind into an electrical supply network (302), and

prepared for steaming low-frequency vibrations, in particular subsynchronous resonances, in the electrical supply network (302), the supply network (302) having a mains voltage with a nominal network frequency, and the low-frequency vibrations to be vapor-coated having a frequency lower than half the nominal network frequency, and the wind energy system ( 304)

- a damping control for damping the low-frequency vibrations,

a closed control loop (300) in which the damping control is used,

a wind system controller (306) for feeding electrical power into the electrical supply network (302), the damping control for damping the low-frequency vibrations controlling feeding electrical power into the electrical supply network (302) using the wind system controller (306),

the damping control is designed for a controlled system and includes the controlled system

the wind system controller (306), or a part thereof,

13. Wind energy system (304) according to claim 12, characterized in that it is prepared to carry out a method for controlling a wind energy system (304) and for steaming low-frequency vibrations according to one of claims 1 to 1 1. 14. Wind energy system (304) according to claim 12 or 13, characterized in that

a storage unit (416) is provided to

for the damping control to store several damping parameter sets and / or several controller structures, which were determined for selection, and to keep them ready for operation, so that

for the operation of the wind energy system (304), one or one is selected from the plurality of damping parameter sets or controller structures and it is possible to switch between them during operation of the wind energy system (304), and that

- In particular, a selection control is provided in order to control a selection from the damping parameter sets or controller structures depending on a selection criterion.