Grid formation control

Abstract

A method of controlling a power generation system (10) is provided. The power generation system (10) may be electrically coupled to a power grid (200) and may include a power generation unit (20) that generates electrical power and a converter system (30) coupled to the power grid (200). The converter system (30) can convert at least the electrical power exchanged between the power generation unit (20) and the power grid (200). The method may include operating the converter system in a grid forming mode of operation in which the converter system (30) may control an exchange of electrical power with the power grid to support a grid voltage and / or a grid frequency. During perturbations in the power grid, the exchanged electrical power may include a stable component that may provide support. The method may further include controlling the power generation system to at least one of limit the stabilization component of the exchanged electrical power by controlling the converter system, force the stabilization component of the exchanged electrical power, and prevent the stabilization component of the exchanged electrical power.

Description

Technical Field

[0001] The present invention relates to a method of controlling a power generation system. It further relates to a computer program for executing such a method, and to a control system and a power generation system configured to execute such a method. Background Art

[0002] Today, the grid frequency is controlled by large power plants with synchronous generators, and the inertia of these machines helps to stabilize the power grid to a nominal grid frequency of 50 Hz or 60 Hz. The increasing use of renewable energy sources, such as wind turbines or photovoltaic solar systems, in or as power generation systems may provide less support to the power grid, since such renewable energy sources generate only little or no inertia. Furthermore, the power output of renewable energy sources varies depending on the prevailing environmental influences. However, with advanced converter control, the converter of the grid connection of the renewable energy source can emulate selected properties of a synchronous generator and operate in a grid forming mode in which the renewable energy source inherently exchanges power with the power grid, in particular in response to disturbances in the grid frequency or angle, to continuously support the grid frequency.

[0003] In response to a disturbance on the power grid, the amount of disturbance power exchanged with the power grid in the grid forming mode depends on the characteristics of the grid event and the grid forming control parameterization. Unlike synchronous machines, a power converter is a current limited device that is usually operated with only a small current margin. It is a power electronic device and, unlike a synchronous generator that has its inherent stored kinetic energy in its rotating mass, there is usually only very little energy stored in the device. The lack of an inherent ability to deliver power and energy during, for example, an inertial response poses a challenge to the grid forming control scheme. If a converter or generator system is pushed beyond its respective capabilities, it may be damaged or may disconnect during a disturbance.

[0004] In order to avoid such problems, the renewable energy sources of the grid forming operation can be designed so that the worst-case disturbance power can be delivered, for example by using appropriately rated hardware components that can provide both the power and energy requested by the power grid in the worst case. However, this leads to over-rating of system components on the hardware level and increased hardware effort, for example by including additional power sources as, for example, dedicated storage devices. In addition, because the amount of disturbance power depends on the grid forming control parameterization, the system should be parameterized so as to take into account both the worst-case operating point of the renewable energy sources and the worst-case grid events. However, such a rather conservative parameterization limits the stabilizing contribution of the renewable energy sources and may therefore lead to inefficient grid forming operations.

[0005] The document "Synchronous Power Controller with Flexible Droop Characteristics for Renewable Generation Systems", Weiyi Zhang et al., IEEE Transactions on Sustainable Energy, IEEE, USA, Vol. 7, No. 4, October 1, 2016, pp. 1572-1582, XP011623289, DOI: 10.1109 / TSTE.2016.2565059, relates to a synchronous power controller for a grid-connected converter of a renewable generation system with energy storage. Instead of replicating the swing equations of a synchronous machine, an alternative control structure is proposed by which the damping and the inherent droop slope can be independently configured to meet the requirements of both dynamics and frequency regulation.

[0006] Document EP 3734799 A1 relates to a method of controlling a wind turbine or a wind farm comprising at least one wind turbine connected to a utility grid for utility grid support. The method may comprise receiving a (utility grid support) control command from a utility grid manager and controlling the wind turbine according to a mode selected by the control command. The control command may instruct the wind turbine to switch from a current control mode to a grid forming operation mode.

[0007] Document WO 2020 / 254161 A1 relates to a method for controlling a wind turbine system connected to an electric power grid. The method comprises generating a wind turbine control signal based on a power control reference for controlling the power output of the wind turbine, monitoring the electric frequency of the electric power grid, and in response to detecting a change in the frequency in the electric power grid, activating a fast frequency support method, the fast frequency support method comprising adjusting the power control reference to cause an overproduction of power by the wind turbine, wherein the power control reference is determined by applying an adaptive gain function to a measure of the difference of the grid frequency from a nominal level. Summary of the invention

[0008] Therefore, there is a need to mitigate at least some of the above mentioned disadvantages and to improve the operation of a power generation system in a grid forming mode of operation.

[0009] This need is met by the features of the independent claim.The dependent claims describe embodiments and / or examples of the invention.

[0010] According to one aspect, a method for controlling a power generation system is provided. The power generation system may be electrically coupled to a power grid and may include a power generation unit generating electric power and a converter system coupled to the power grid. The converter system may at least convert the electric power exchanged between the power generation unit and the power grid. The method may include operating the converter system in a grid formation operation mode, in which the converter system may control the exchange of electric power with the power grid to support grid voltage and / or grid frequency. During a disturbance of the power grid, the exchanged electric power may include a stabilizing component that may provide the support (the stabilizing component may be particularly provided to resist or counteract the disturbance). The method may further include controlling the power generation system to perform at least one of the following: limiting the stabilizing component of the exchanged electric power, enforcing the stabilizing component of the exchanged electric power, and preventing the stabilizing component of the exchanged electric power by controlling the converter system.

[0011] In this way, during the grid formation operation, the electric power exchanged between the power generation system and the power grid can vary only within a predetermined range. Therefore, when a grid disturbance occurs, the system can be protected. It is possible to prevent the system from responding to an operation point that may be harmful to or overload the system components, may not be supported by the components at the hardware level, and / or may be inefficient. Therefore, the method can improve the efficiency of the power generation system. In addition, the burden on the system is reduced in a controlled manner, which can lead to higher robustness and life of the system. In addition, the parameterization of the grid formation control is no longer limited to the worst case, and therefore, is no longer limited to conservative and quite inefficient parameterization. Alternatively, the grid formation control can be parameterized so as to respond more forcefully to grid disturbances that occur regularly, thereby producing a more robust power grid. In addition, the power generation system at the hardware level may not be required to be overrated. In short, the operation of the power generation system in the grid formation operation mode can be improved.

[0012] Alternatively, the forcing may include temporarily inhibiting a response to a deviating operating point of the power generation system.

[0013] Limiting the stable component may in particular occur by applying one or more corresponding limits to the stable component, in particular by keeping the stable component within one or more predefined, in particular static or dynamic limits. Alternatively, one or more limits may be applied to a power reference, a parameter indicative of a power reference or the electrical power exchanged with the power grid in order to limit the stable component.

[0014] The limiting, forcing and / or blocking can be performed by a control system that controls the power generation system, and the control system can be configured to implement one, two or all three of these control functions. For example, it can be configured to be able to perform at least the limiting function, or at least the limiting function and the blocking function, or can be configured to be able to perform the limiting function, the blocking function and the forcing function, or any other combination. Which function is performed can depend, for example, on one or a combination of the properties of the disturbance, the operating parameters of the power generation system and the settings of the control system, as explained in more detail below. For example, the stabilizing component may be limited only; may be limited first and then blocked; may be forced only; may be forced first, then optionally limited and then blocked; or may be directly blocked. Other combinations are possible.

[0015] The limiting, enforcing and / or blocking may be performed in response to obtaining (eg, generating or receiving) a limiting signal, an enforcing signal and / or a blocking signal, or in response to obtaining a parameter indicative of at least one of limiting, enforcing and blocking.

[0016] A power generation unit may be, but is not limited to, a component capable of receiving and / or outputting electrical power, such as a generator (e.g. a generator of a wind turbine, in particular an asynchronous or synchronous generator), or a photovoltaic system (e.g. a photovoltaic module), or an energy storage system.

[0017] A power generation system may include one or more power generation units.

[0018] The converter system may be a power converter for converting electrical energy. A power converter may convert alternating current (AC) to direct current (DC), and vice versa, or convert AC to AC. The converter may change the frequency of the voltage or current, or provide a combination of these. The converter system may include a DC link.

[0019] The grid voltage may be a grid voltage amplitude and / or the grid frequency may be at least one of a grid current or a frequency of the grid voltage.

[0020] Electrical power may include active power and / or reactive power.

[0021] The stabilizing component may be at least a part of the exchanged electric power.It may also be possible that the exchanged electric power comprises only or consists of the stabilizing component.

[0022] A disturbance of the power grid may be, for example, a deviation of the grid voltage and / or grid frequency from the grid voltage and / or grid frequency before the event, wherein before the event (i.e., before the disturbance occurs), the power system may be operated at a rated voltage and / or rated frequency. A disturbance may be a grid voltage and / or grid frequency that is higher or lower than the corresponding rated values, for example, higher or lower than a nominal grid frequency of, for example, 50 Hz or 60 Hz. For example, a disturbance may be a frequency fluctuation or a voltage sag.

[0023] The power generation system may be controlled to exchange electric power with the power grid according to a reference power, and the method may further include generating or receiving a parameter indicating at least one of limiting, forcing and blocking, and the method may further include limiting, forcing and blocking at least one of the power reference according to the parameter.

[0024] According to one example, limiting the stabilization component may include limiting the amplitude of the electric power exchanged with the power grid, and / or limiting the power ratio (e.g., the rate of change of the electric power) of the electric power exchanged with the power grid. This may occur by applying a corresponding amplitude limit and / or a corresponding ratio limit.

[0025] Limitation of the amplitude or power ratio can reduce the load on the power generation system.For example, the power characteristics can be limited so that the burden on the power generation system or some of its components is tolerable for the system.

[0026] The exchanged electrical power may include a stabilizing component and an additional component, and during a disturbance, the exchange of the additional component may be at least one of maintained, limited, blocked and forced. In one example, the exchange of the additional component may be at least one of maintained, limited, blocked and forced instead of or in addition to the stabilizing component. It should be clear that the purpose of the present invention is to limit the burden on the power generation system when a disturbance occurs. Therefore, any component of the (total) exchanged power may be limited or blocked to achieve a reduction while maintaining or forcing certain components, for example due to grid requirements, such as, for example, the required inertial response of the power generation system.

[0027] According to an example, the method may comprise operating the power generation system such that the power generation system performs maximum power point tracking, in particular during a grid forming mode. Renewable energy sources may conventionally be operated in such a manner. When a wind turbine is operated with maximum power point tracking, it may not be possible to extract any additional power from the wind. Furthermore, any reduction in rotor speed may result in a less optimal operating point (less power generation) if, for example, additional power is extracted.

[0028] Such an approach may allow the power generation system to generate a maximized power output when operating in a grid forming mode. Because the converter system may be controlled to limit or prevent the stabilizing power that may otherwise be required in such a mode, the system may be operated while avoiding pushing the power generation system into undesirable operating points. For a wind turbine, undesirable operating points may include a cut-out if the rotor speed is reduced to the point where the wind turbine stops, or if the resulting power reduction during the recovery period is too severe compared to the positive contribution of the stabilizing power.

[0029] According to one example, the method may include monitoring a grid voltage and / or a grid frequency to generate a monitored grid voltage and / or a monitored grid frequency, determining an operating mode based on the monitored grid voltage and / or the monitored grid frequency, and modifying at least one of limiting, forcing and preventing a stabilizing component based on the operating mode.

[0030] Modifications based on the operating mode can allow the action of limiting, forcing or blocking to be instantly adapted to the currently monitored grid characteristics. For example, the modification may include a modification of how and when the action is applied and / or enabling / disabling one of the actions. As a result, the power generation system operates more efficiently.

[0031] In this document, monitoring may be performed, for example, by at least one of a sensor, a sensing system, a model implemented in software, and a filter-based monitoring system, but is not limited thereto, and the filter-based monitoring system may be, for example, a state observer, a Kalman filter, etc. Monitoring may further include signal pre-processing and post-processing, such as filtering. In addition, monitoring may utilize information obtained from hardware (e.g., one or more sensors) implemented for monitoring purposes, and / or may utilize information obtained from hardware implemented for another purpose. Monitoring may further include estimation and / or prediction, preferably based on the information obtained.

[0032] According to one example, the method may include decoupling the power generation system from the power grid when the stabilizing component is blocked, which may occur, for example, if the power generation system fails to limit the stabilizing component in amplitude or speed. The blocking may in particular include tripping the power generation system, in particular to prevent additional stabilizing components from being drawn.

[0033] The decoupling or disconnection may be hardware based, for example by means of circuit breakers.Decoupling or disconnection from the grid may be advantageous when a stabilizing component is blocked, as it prevents the generation system from exchanging electrical power to such an extent that it may be harmful to the system.

[0034] According to an example, the method may comprise determining a disturbance power and / or a disturbance energy that may need to be exchanged with the power grid in response to the occurrence of the disturbance. At least one of the limitation, forcing and blocking of the stabilizing component may be based at least on the disturbance power and / or the disturbance energy. Preferably, at least one of the limitation, forcing and blocking of the stabilizing component may further be based on one or more operating parameters of the power generation system, and in particular on one or more operating parameters of the power generation unit and / or on one or more operating parameters of the converter system.

[0035] When the power generation system is connected to the power grid, the disturbance power may be the power difference between the (actual) electric power exchanged between the power generation system and the power grid and the reference electric power to be exchanged. The disturbance energy may be the integral of the disturbance power over time. In particular, the disturbance power may correspond to or may be a stable component. The reference electric power may correspond to a power set point provided by a system controller (e.g. a wind turbine controller or a wind farm controller).

[0036] The one or more operating parameters of the system / component may be, for example, one or more parameters of the system / component when the system / component is operated at a certain operating point. For a wind turbine, these may include wind speed, yaw angle of the nacelle, pitch angle of the rotor blades, rotational speed of the rotor of the wind turbine or generator, current or voltage, etc.

[0037] Whether and / or how the stabilizing component is limited, forced or prevented may depend on the severity of the disturbance and in particular on the amount of power and energy required for stabilization in response to such a disturbance. It may additionally or alternatively depend on the current operating point of the power generation system. For example, when the power required for stabilization is higher than the maximum power that the system can provide (e.g., in total or based on the current operating point), the stabilizing component may be limited or prevented. It is also possible that in order to respond to the disturbance, the system will need to operate in a state of power overproduction that is harmful to the system for a long enough time, for example, due to the continued generation of electrical overcurrents or because the mechanical torque increases beyond the design limit. In this case, the stabilizing component may also be limited, or prevented as a last resort. It is also possible that in order to respond to the disturbance, the operating point of the power generation system needs to be offset from the efficient operating point of the power generation system to a less efficient operating point. Returning from a less efficient operating point may require a large effort, such as a large amount of electrical power. For example, when it is assumed that the power generation unit is a wind turbine generator, such an offset of the operating point may include a significant deceleration of the wind turbine. In order to limit the extent to which the system offsets to an inefficient operating point, the stabilizing component may be limited.

[0038] Thus, such an approach may be advantageous because the power generation system may only respond to such an extent that the response does not involve a risk of efficiency deficits or system failure.

[0039] According to one example, the power generation system may be controlled so as to exchange power with the power grid according to a reference power. The method may include monitoring the power exchanged with the power grid to generate the monitored power. The disturbance power may be determined based on the difference between the monitored power and the reference power. Preferably, the method may further include filtering the difference before determining the disturbance power. The reference power and the monitored power may be, but are not limited to, a reference active power and a monitored active power, respectively. The filter may be, for example, a low-pass filter, in particular a mean filter or a moving average filter.

[0040] Such an approach may be beneficial because the disturbance power is calculated based on intermediate results available in the control system of the power generation system. Therefore, the computational performance may be increased. Optionally, the filtering may filter high frequency related signal parts of the disturbance power so that the system is prevented from responding to signal parts, such as those related to noise. Therefore, the system may operate more efficiently.

[0041] According to one example, a power controller may control a power output of a power generation unit, and the method may include providing a reference power by the power controller. The power controller may be, for example, a wind turbine controller that controls power output (e.g., directly or indirectly by providing a set point for a converter system), or a solar unit or solar power plant controller, etc.

[0042] The converter system may be controlled by a converter system controller and the reference power may be provided to the converter system controller. The method may include generating or receiving a parameter indicating at least one of limiting, forcing and blocking. The method may include limiting, forcing and blocking at least one of the power reference in accordance with the parameter.

[0043] In one example, the reference power that is limited, enforced, and / or blocked may include a magnitude and / or rate of change that is lower than the magnitude and / or rate of change of the reference power before limiting, enforcing, and / or blocking.

[0044] In one example, the monitored power may include monitoring a grid voltage and a grid current to generate a monitored grid voltage and a monitored grid current, respectively. The monitored power is generated by deriving the monitored power from the monitored grid voltage and the monitored grid current.

[0045] According to an example, the method may further include integrating the parameter indicating the disturbance power. The disturbance energy may be determined based on the integrated parameter. For example, the parameter may be the difference between the reference power and the monitored power, or the disturbance power itself, rather than the parameter indicating the disturbance power. Preferably, the method may further include filtering the integrated parameter before determining the disturbance energy. The filter may be, for example, a low-pass filter, particularly a mean filter or a moving average filter.

[0046] According to one example, the method may include determining that the disturbance power and / or the disturbance energy exceeds a corresponding threshold value, and / or determining that when exchanging the stabilizing component with the power grid, at least one of the operating parameters of the power generation system (particularly the power generation unit and / or the converter system) exceeds a corresponding threshold value. The stabilizing component may be limited in response to the threshold being exceeded. If the disturbance power and / or the disturbance energy cannot be kept within the corresponding threshold value, and / or at least one operating parameter of the power generation system cannot be kept within the corresponding threshold value by limiting the stabilizing component, the stabilizing component may be blocked, for example, by tripping the power generation unit.

[0047] The threshold value may, for example, be predetermined or predefined. The threshold value may include a limit on an (internal and / or external) operating parameter of a power generation system or a component of a power generation system. In particular, the threshold value for the internal operating parameter may be based on the hardware capabilities and / or hardware limitations of the system. For example, the threshold value may include a power limit and / or a stored energy limit. The power / energy limit may indicate the maximum power / energy that the power generation system or a component of the power generation system can provide. Alternatively or additionally, the threshold value may include a current limit and / or a voltage limit. The threshold value may further include a state of charge limit of a DC link, for example, of a converter. In the case where the power generation unit includes a generator, the threshold value may alternatively or additionally include a speed limit, a change in speed limit and / or a torque limit. In the case where the power generation unit includes an energy storage device, the threshold value may, for example, include a state of charge limit. The threshold value may depend on the operating point of the power generation system, so that the allowable amplitude and / or rate of change of the stable component may depend on one or more operating conditions of the power generation system. The thresholds may additionally or alternatively include limits associated with one or more internal or external operating parameters, such as, when the power generation system includes a wind turbine, a wind speed limit, a rotor or generator speed limit, an active power output limit, an available power measurement or estimate, or a wind intensity limit.

[0048] Thus, the power generation system can be prevented from responding to such a disturbance, a response that would be harmful to the system (for example, when the response would overload the system) can be prevented, and / or a response that would cause the current operating point to shift towards inefficient operation (for example, when the response would significantly slow the rotor speed from its pre-event operating point) can be prevented.

[0049] According to another aspect, a control system for controlling a power generation system is provided. The power generation system may be configured to be electrically coupled to a power grid and may include a power generation unit configured to generate electric power and a converter system configured to be coupled to the power grid. The converter system may be configured to at least convert the electric power exchanged between the power generation unit and the power grid. The control system may be configured to perform any of the methods described herein.

[0050] The control system may, for example, include a processing unit and a memory, the memory storing control instructions which, when executed by the processing unit of the control system, cause the control system to perform any of the methods described herein. The processing unit may, for example, include a digital signal processor, an application specific integrated circuit, a field programmable gate array, a microprocessor, etc. The memory may include RAM, ROM, flash memory, a hard disk drive, etc.

[0051] The control system may include multiple control systems. Multiple control systems may be communicatively coupled to each other, and / or the control system may be composed of one or more components of the power generation system. Each of the multiple control systems may include a processing unit and a memory unit. The control system may be configured to perform any method described herein, wherein the steps of the method may be distributed on multiple control systems for execution.

[0052] For example, the control system may include: a power controller configured to control the operation of the power generation unit and / or the entire power generation system; and a converter system controller (e.g., a converter control unit, CCU) configured to control the operation of the converter system. In particular, in the case of a wind turbine, the control system may include: a power controller in the form of a wind turbine controller configured to control the operation of the wind turbine (which may include the provision of a power reference); and a converter system controller configured to control the operation of the converter system.

[0053] The control system may be included in the power generation system. Alternatively, the control system may be a separate unit.

[0054] According to another aspect, a power generation system is provided. The power generation system may be configured to be electrically coupled to a power grid. The power generation system may include a power generation unit configured to generate electric power and a converter system configured to be coupled to the power grid. The converter system may be configured to convert at least a portion of the electric power exchanged between the power generation unit and the power grid. The power generation system may include any one of the control systems described herein.

[0055] The power generation system may be controlled by a first controller (e.g., a power controller) and the converter system may be controlled by a second controller (e.g., a converter system controller). The first controller may be configured to provide control commands to the second controller, the control commands controlling the exchange of electric power between the power generation system and the power grid. The first controller may further be configured to provide limits on the control commands to the second controller. The power generation system may be configured to apply limits via a control system so as to limit the exchange of electric power. The limits may be performed by the second controller, in particular via converter control, so that the disturbance power is within the received limits.

[0056] In one example, the power generation unit may be an electrical generator of a wind turbine. In another example, the power generation unit may be a photovoltaic solar system, which may be controlled by the first controller.

[0057] According to another aspect, a computer program for controlling a power generation system is provided. The computer program may include control instructions which, when executed by one or more processing units, cause the one or more processing units to perform any one of the methods described herein. The computer program may be provided on a volatile or non-volatile storage medium or data carrier.

[0058] It is to be understood that the features mentioned above and those to be explained below may be used not only in the indicated respective combinations, but also in other combinations or alone, without departing from the scope of the present invention. In particular, the features of different aspects, embodiments and / or examples of the present invention may be combined with each other, unless indicated otherwise. BRIEF DESCRIPTION OF THE DRAWINGS

[0059] The foregoing and other features and advantages of the present invention will become further apparent from the following detailed description read in conjunction with the accompanying drawings, in which like reference numerals refer to like elements.

[0060] Figure 1 is a schematic diagram showing a power generation system and a control system according to one example, and illustrating the signal flow that controls the operation of the power generation system.

[0061] Figure 2 is a schematic diagram showing a control system according to one example and illustrating the signal flow to control the operation of the power generation system.

[0062] Figure 3 A first graph showing a change in frequency over time, and a second graph and a third graph showing a migration of the injected power over time are exemplarily illustrated.

[0063] Figure 4is a schematic flow chart illustrating a method 400 of controlling a power generation system according to one example. DETAILED DESCRIPTION

[0064] Hereinafter, embodiments and / or examples of the present invention will be described in detail with reference to the accompanying drawings. It is to be understood that the following description of embodiments and / or examples is given only for the purpose of illustration and is not adopted in a limiting sense. It should be noted that the accompanying drawings are only regarded as schematic representations, and the elements in the accompanying drawings are not necessarily proportional to each other. More specifically, the representations of various elements are selected so that their functions and general purposes become apparent to those skilled in the art. As used herein, the singular forms "one", "an" and "the" are intended to include plural forms as well, unless the context clearly indicates otherwise. The terms "include", "have", "include" and "include" are to be interpreted as opening terms (i.e., meaning "including, but not limited to"), unless otherwise specified.

[0065] In the following, the description and explanations herein relate to a power generation system comprising a power generation unit, which is an electrical generator of a wind turbine. However, it should be clear that the power generation unit may be a different type of power source, such as a photovoltaic system, and may be coupled to an energy storage system (such as a battery system or a thermal energy storage device), etc. The power generation unit may also include a combination of multiple power sources.

[0066] It should be clear that descriptions and explanations herein which are restricted to one or more specific wind turbines may apply to other or all wind turbines of a wind park comprising the specific wind turbine(s).

[0067] Figure 1 is a schematic diagram showing a power generation system 10 and a control system 50 of a wind turbine 100. The figure further shows a signal flow diagram illustrating the operation of controlling the power generation system 10 according to one example. The power generation system 10 may include the control system 50. The control system 50 may control the operation of the power generation system 10. The power generation system 10 may be coupled to a power grid 200 and may exchange electrical power P with the power grid 200.

[0068] The electric power P may include active and / or reactive power. The power P may include at least a stable component that is exchanged with the power grid when a grid disturbance occurs. In particular, the power P may consist of a stable component or include only a stable component. The power generation system 10 may include a power generation unit 20 and a converter system 30. The power generation unit 20 may generate / export and / or store / import electric power. In one example, the power generation unit 20 may be an electrical generator of a wind turbine 100, which is driven by the rotor of the wind turbine using wind power. The converter system 30 may be coupled to the power grid 200 and the power generation unit 20, and may convert at least a portion, preferably all, of the electric power P imported or exported from the power grid to the power grid. The converter system 30 and / or the power generation unit 20 may be controlled by a control system 50. The control system 50 may include a wind turbine controller 60 and a converter control 70. Each of the control systems 60, 70 may include a processing unit 61, 71 and a memory unit 62, 72. Wind turbine controller 60 may control operation of power generation system 10 and may at least indirectly (e.g., via units 70, 30) control power generation by generator 20. Converter control 70 may control operation of converter system 30. Control systems 60, 70 may be communicatively coupled to one another.

[0069] The converter system 30 can convert the electric power bidirectionally exchanged between the power generation unit 20 and the power grid 200. The converter system 30 can be operated in a grid formation mode, in which the converter system is controlled so as to behave as an (ideal) voltage source, in particular, as a Thevenin source and impedance. The voltage source can be controlled in frequency and in amplitude, and the impedance, the real impedance or a combination of the two can be simulated. Therefore, the power generation system 10 can be operated so as to maintain the voltage amplitude, the voltage frequency and / or the voltage phase angle to control the electric power (active power and reactive power) exchanged with the power grid 200. In the grid formation mode, the power generation system 10 can inherently provide instantaneous power to the load that changes suddenly in the power system, and thus react to external disturbances on the power grid. Such disturbances can, for example, include deviations of the actual grid voltage or the actual grid frequency from the grid voltage or the grid frequency before the event, respectively. The voltage and frequency before the event can be close to the rated voltage and rated frequency of the power system. The severity of the disturbance can classify the disturbance as a normal operating disturbance, a severe disturbance or an extreme disturbance. Depending on the severity of the disturbance, the initial response of the grid formation to the disturbance may shift the power generation system to an operating point that is potentially harmful to the power generation system or at least inefficient or accompanied by a subsequent loss of power production.

[0070] When the converter system 30 is operated in the grid forming mode, the uncontrolled inherent response of the power generation system 10 to the grid disturbance in order to support the power grid (e.g., without evaluating the system feasibility and / or without restrictions) may therefore be disadvantageous because the response to the disturbance may require an amount of electrical power that exceeds the given system capacity and / or may shift the power generation system 10 into an inefficient operating point. In order to avoid the power generation system 10 being shifted into such an unfavorable operating point, the control system 50 can be configured to consider potential operating points in which the power generation system 10 can operate when the system responds to the disturbance that occurs. When it is determined that responding to the disturbance is disadvantageous, the control system 50 can control the power generation system 10 so that these disadvantages are avoided, such as being prevented or rejected, or at least reduced or limited. The amount of electrical power and energy exchanged between the power grid 200 and the power generation system 10 in response to the disturbance to support the power grid is referred to herein as a stabilizing component.

[0071] When a disturbance occurs, the converter control 70 may determine a disturbance power Pd and a disturbance energy Ed. The disturbance power Pd and the disturbance energy Ed may indicate the electrical power and energy that need to be exchanged with the power grid 200 in response to the disturbance. The determined disturbance power Pd and the disturbance energy Ed may be provided to the wind turbine controller 60. The wind turbine controller 60 may determine one or more limits li based at least on the operating point of the wind turbine in terms of at least rotor speed or active power output (but potentially also including pitch angle, available wind estimation or measurement). If the impact of the disturbance on the wind turbine operating point makes further limits necessary, the wind turbine controller 60 may select to update the limits li based on the received disturbance power Pd and the disturbance energy Ed, and may return the limits li and one or more reference signals ref to the converter control 70. The limits may be predetermined or preprogrammed, and are preferably stored in a lookup table. For example, given a wind turbine rotor speed and / or active power output, and preferably given one or more additional operating parameters of the power generation system 10, the lookup table may store and return one or more limits. The determined disturbance power and / or disturbance energy may be used in comparison with the expected response of the wind turbine mechanical system to allow additional corrective action and limits in the event that the actual response deviates too far from the expected response. The limit li may indicate a power limit. For example, the limit li may include a limit on the magnitude / amplitude of the exchanged power and / or a limit on the power ratio of the exchanged power (i.e., the derivative of the exchanged power over time or the change of the exchanged power over time). Alternatively, the limit li may be represented by a torque, and more specifically, by a limit on the magnitude / amplitude of the torque and / or the derivative of the torque over time or the change of the torque over time. One or more reference signals ref may, for example, include a power reference according to which the power generation system 10 operates during normal operation.

[0072] The converter control 70 may control the converter system 30 so that during the grid forming operation, the converter system 30 operates according to the received reference signal ref and within the limits li. For example, the converter system 30 may be controlled so as to limit the stabilizing component and, therefore, so as to limit the exchanged power P. Additionally or alternatively, the converter control 70 may receive a blocking / rejection signal, in response to which the converter system may block / reject the provision of the stabilizing component. For example, the stabilizing component may be blocked when it is determined that performing a disturbance response may be harmful, particularly destructive, to the power generation system 10, or in the event that the converter fails to limit the amount and / or rate of change of the disturbance power.

[0073] Additionally or alternatively, the control system may receive a forcing signal in response to which the control system may force the control system of the power generator to provide a stabilizing component. For a wind turbine, the forcing step may include temporarily preventing the normal speed / power control from responding to the decelerated rotor speed. Forcing the response may be subject to the same or similar limitations on the response as when the stabilizing component is not forced. This may mean that if the wind turbine moves into an operating point that will cause an undesirable post-event response, the forcing is lifted. For example, the stabilizing component may be forced when stabilizing power is required by a grid code or market commitment, such as an inertial power response committed by a contract.

[0074] Figure 2 is a schematic diagram showing a control system 50 according to one example and a signal flow diagram illustrating the operation of controlling the power generation system 10. The operations included in the left dashed box may be included in and performed by the wind turbine controller 60. The operations included in the right dashed box may be included in and performed by the converter control 70. However, it should be clear that the configuration shown for performing these operations is not limiting and the operations may also be distributed in a different manner among the depicted controls and / or even distributed among the controls. Figure 2 Other controls not depicted but included in or accessible by the power generation system 10 are included in the power generation system 10. Figure 2 As can be seen in Figure 1 The reference signal ref shown in may comprise a reference power Pr (particularly a wind turbine generator reference power), according to which the power generation system 10 exchanges power P with the power grid 200 .

[0075] The reference power Pr can be generated by a speed / power control 66 included in the wind turbine controller 60. The generation can be based on actual operating parameters. The speed / power control 66 can obtain the actual operating parameters of the wind turbine. The parameters can include an actual rotation speed rsp (which indicates the rotation of the wind turbine rotor or the rotation of the generator rotor), an actual wind speed wsp, and an actual pitch angle pa. The actual operating parameters can be obtained by monitoring, in particular by measuring with a sensor system or by estimating. For example, the wind speed wsp can be estimated or received from an external source. The speed / power control 66 can perform maximum power point tracking. The speed / power control can further obtain one or more limits li2, which can be provided by a higher level controller (e.g., a wind farm controller) and can, for example, indicate a power reduction. In addition, the speed / power control 66 can output a logic signal lo indicating the operating mode in which the speed / power controller 66 is currently operating. The power limit evaluation unit 65 may determine one or more limits li based on the monitored actual operating parameters, i.e. wind speed wsp, rotation speed rsp and pitch angle pa, and based on the determined disturbance power Pd, the determined disturbance energy Ed and the determined reference power Pr. Thus, the output limit li may be adapted to the current operating parameters and / or the severity of the disturbance.

[0076] In one example, the power limit evaluation unit 65 may compare one or more predetermined thresholds to the inputs of the power limit evaluation unit 65 and / or parameters derived from the inputs. For example, the thresholds may include power limits and / or energy limits that indicate the maximum power / energy that the power generation system 10 or a component of the power generation system 10 can provide in the current operating point of the system or its components. When one or more of the thresholds are exceeded, the power output of the system may be limited or blocked. The limits li may then be obtained, for example, by a lookup table. In an alternative example, at least one of the inputs of the power limit evaluation unit 65 may be fed directly into the lookup table and the result may be output as the limits li. In another example, the limits li may be calculated / derived from at least one of the inputs of the power limit evaluation unit 65, for example based on a mathematical model / equation or based on a simulation model. The power limit evaluation unit 65 may further take into account hardware-based limits, such as a maximum hardware-limited power output or a maximum hardware-limited energy storage capacity, both of which may depend on the wind turbine operating point.

[0077] The logic signal lo and the mode signal m can be used to modify the operation of the power limit evaluation unit 65. Such modifications can include how and when to perform the limitation of the stable component. The modification can additionally include information about whether the speed / power control should change mode to force the stable component. The mode signal m can be generated by the state machine 67, and the generation can be based on the actual grid frequency f (e.g., grid voltage frequency) and the actual grid voltage v. The actual grid frequency f and the actual grid voltage v can be monitored.

[0078] One or more limits li may be provided to the core control 75 and combined with the reference power Pr. In one example, during a power system frequency event, the actual active power output of the power generation system may deviate from the provided reference ref, such as the reference power Pr. As long as the total active power output is within the limits li provided to the converter system, no additional action is required by the converter system. However, if the actual active power flow exceeds one or more of the received limits, the converter system engages additional control features to reject the disturbance power so that the active power flow is within the received limits li.

[0079] The core control 75 may be included in the converter control 70 that implements the grid formation control. Therefore, the core control 75 may include direct voltage control or cascade voltage control, which cascade voltage control includes internal current control. The reference voltage vr that can be output by the core control 75 can be converted into a corresponding switching command sc by means of a pulse width modulation modulator unit 77. The switching command sc can be provided to the power electronic components of the converter system 30, such as semiconductor switches (such as IGBTs), so as to switch the power electronic components so that the actual power P exchanged with the power grid 200 in normal operation corresponds to the reference power Pr. The core control 75 can operate using the actual grid voltage and the actual grid current. The actual grid voltage and the actual grid current can be monitored. Preferably, the actual grid voltage can be the actual grid voltage vdq in the dq frame, and the actual grid current can be the actual grid current idq in the dq frame. The voltage vdq and the current idq can be generated by using d / q transformation and transforming the actual grid voltage vabc in the abc frame and the actual grid current iabc in the abc frame into the dq frame based on the determined phase angle θth. The actual voltage vabc and the actual current iabc may be monitored, for example measured, by means of the feedback measurement unit 76. The feedback measurement unit 76 may further output an actual power P derived from the voltage vabc and the current iabc, which is the power actually exchanged (imported or exported) with the power grid 200.

[0080] The actual power P output by the converter system 50 may follow the reference power Pr received from the speed power controller 66 during steady-state operation. When a grid disturbance (e.g., frequency fluctuation) occurs, the actual power P may deviate from the reference power Pr, and thus a control error may occur. In response to the control error, the control system 50 may control the power generation system 10 so that the control error is eliminated. Therefore, additional power (i.e., a stable component) for responding to such a grid disturbance may be exchanged with the grid, i.e., exported from or imported into the grid. The additionally imported or exported power may cause a (potentially harmful) burden on the power generation system 10, or may shift the operating point of the system 10 to an inefficient operating point. Therefore, depending on the severity of the disturbance, it may be beneficial to limit or prevent the additionally imported or exported power. Therefore, the power limit evaluation unit 65 may take into account the disturbance power Pd and the disturbance energy Ed. The disturbance power Pd may be generated by subtracting the actual power P from the reference power Pr at the coupling point 79, and preferably, the disturbance power Pd is generated by filtering the difference with the filter 74. The filter 74 may, for example, implement low-pass filtering, for example, for noise reduction. The disturbance energy Ed may be generated by subtracting the actual power P from the reference power Pr and integrating the difference over time using an integrator 78. Preferably, the integrated difference may be filtered with a filter 73. The filter 73 may, for example, implement low-pass filtering, for example, for noise reduction. Alternatively, the disturbance energy Ed may be obtained by integrating the power disturbance Pd over time.

[0081] It should be clear that the control scheme described above and in particular the limit signal li can be used to implement each of the limitations considered, such as amplitude limitations on active power and / or power rate of change. The following is an example of a possible implementation: for limitations, the limit signal li may include a maximum value and / or a minimum value indicating a range of allowed stable power and / or maximum rate of change of power, including zero. If the converter system fails to enforce the received limits, the control system may ultimately decide to cease operation to protect the hardware, for example by tripping the power generation system (e.g. opening one or more open switches). Thus, preventing stable power may include limiting the stable power to zero and / or tripping the power generation system. If the control system is configured to enforce a stabilizing component, the control system will ensure that normal control actions such as speed / power controls do not attempt to override the stabilizing component when entering a frequency event unless the power limiting block has found that the response is harmful to the hardware or post-event operation. Therefore, if the control system is configured to enforce a stabilizing component, the control system will ensure that normal control actions such as speed / power controls do not attempt to override the stabilizing component when entering a frequency event, ... cancel the stabilizing component when entering a frequency event. Figure 1 and 2 The control scheme shown in can be used to implement all modes (alone or in any combination): Restriction Mode, Blocking Mode and Enforcement Mode.

[0082] Figure 3The first (upper) graph and the second (middle) and third (lower) graphs are exemplarily illustrated, the first (upper) graph showing the change of frequency over time, and each of the second and third graphs showing the disturbance power (stable component) Pdu, Pdl injected over time. In response to the grid disturbance, the disturbance power Pdu, Pdl is exchanged with the power grid 200, which is Figure 3 In the diagram, the frequency variation / frequency fluctuation (curve f) is exemplarily represented by the frequency variation / frequency fluctuation in the first diagram. The unrestricted disturbance power Pdu can represent the stable component actually exchanged when the exchanged power P (stable component) is not restricted (i.e., unrestricted). The restricted disturbance power Pdl can represent the stable component actually exchanged when the exchanged power P (stable component) is restricted. As mentioned, the disturbance power Pd can be estimated as / derived from the deviation of the actual power P from the reference power Pr, which may occur when the power generation system 10 experiences a sudden change in grid characteristics (e.g., grid frequency or grid voltage). In such a case, the grid dynamics may cause additional power import or export to the converter. The import or export is time-limited because the converter control 70 continues to control the power generation system 10 to the received power reference Pr. Figure 3 That process is exemplarily depicted in .

[0083] Figure 3 The first graph of depicts frequency fluctuations, for example, a decrease in frequency from 50 Hz to 47 Hz in response to a sudden increase in grid load. The power generation system 10 can react to such frequency fluctuations by additionally injecting disturbance power Pdu into the grid 200. As outlined above, it can be determined / assessed that the magnitude of the required additional power Pdu may exceed a predetermined threshold, or that injecting the additional power Pdu may shift the system 10 into an inefficient operating point. In response to such a determination / assessment (which may be performed before power generation responds to a disturbance having a stabilizing component to counteract the disturbance), the required unrestricted power Pdu may be limited by a limit li so that the power generation system 10 can only exchange the limited disturbance power Pdl with the grid. Figure 3 The second and third graphs of show that the application of the limit li reduces the power actually exchanged with the power grid during restricted operation to approximately 50% of the power exchanged during unrestricted operation. Figure 3The third graph of deviates from the second graph in that a damping parameter of the power generation system 10 that affects the response of the power generation system 10 to a grid disturbance is set to a first value for the second graph and to a second value that is lower than the first value for the third graph. Due to the setting of the damping parameter, the second graph depicts a non-oscillatory system response and the third graph depicts a damped oscillatory system response. Both the second and third graphs include a step function (from 0 to about 0.04 MW), which is exemplarily represented as a corresponding reference signal, such as a reference power signal Pr. The reference signal can be modified (e.g., limited) by applying a limit li to generate a modified (limited) step function (from 0 to about 0.02 MW). Therefore, the output power of the power generation system can be modified accordingly.

[0084] Figure 4 is a schematic flow chart illustrating a method 400 according to one example, which may be performed by the control system 50 . Figure 4 The order of the method steps in the method is not limited to the order shown. The method is further not limited to the number of steps shown. Some steps of the method may not be implemented, may be replaced or expanded.

[0085] Figure 4 An exemplary method of controlling a power generation system is shown. Figure 1 As shown in , a power generation system may be electrically coupled to a power grid and may include a power generation unit generating electric power and a converter system coupled to the power grid. The converter system may at least convert the electric power exchanged between the power generation unit and the power grid. In step S401, the converter system operates in a grid formation operation mode, in which the converter system controls the exchange of electric power with the power grid to support the grid voltage and / or grid frequency. The exchanged electric power may include a stabilizing component that provides this support during a grid disturbance. Based on the operating point and / or operating conditions of the power generation system, one or more limits on the disturbance power are sent to the converter system. In step S402, a disturbance occurs on the power grid, such as a drop in grid frequency, a voltage sag, etc.; this may be detected. In step S403, the disturbance power and / or disturbance energy exchanged with the power grid in response to the disturbance is determined. As mentioned above, this may occur by monitoring the actual power supplied to the power grid and determining its difference from a power reference that may be provided by a controller (such as a wind turbine controller).

[0086] In step S404, it is determined whether to limit, force or prevent a stable component of the exchanged electric power that would be required to respond to the disturbance based on the disturbance power and / or the disturbance energy. Based on this determination, the converter system can be controlled to limit the stable component of the exchanged electric power, and the control system of the power generation system can force the stable component of the exchanged electric power or prevent the stable component of the exchanged electric power, respectively (step S405). The power limit evaluation unit 65 can compare the expected and actual response of the power generation system with the monitored disturbance power during the event, and can update the limit signal accordingly. For example, a corresponding limit command can be provided from the power limit evaluation unit 65 to the core control 75, such as Figure 2 The stable component of the electric power exchanged with the power grid can be limited accordingly, as Figure 3 , as shown in the second and third graphs of . In particular, the power generation system 10 can continue to operate at its undisturbed power output / exchange, and the additional stabilizing component of the exchanged electric power caused by the disturbance can be limited as described herein, for example to avoid damage to system components, avoid excessive deceleration of the wind turbine rotor, etc. Excessive deceleration can also, for example, trigger a condition for suspending the forcing of the stabilizing component if, for example, it is determined that continued forcing will result in a post-event response that will not demonstrate a contribution to the stabilizing component. If the converter system fails to limit the stabilizing component to the received limit value, the power limit evaluation unit 65 can command the power generation system to disconnect from the power system. Therefore, significant benefits can be achieved using this exemplary method.

[0087] As mentioned above, the method may be performed by a power generation system of a wind turbine, but may also be performed correspondingly in another power generation system, for example in a solar power plant.

[0088] Although specific embodiments and / or examples are disclosed herein, various changes and modifications may be made without departing from the scope of the present invention. The present embodiments and / or examples are considered to be illustrative and non-restrictive in all aspects, and all changes falling within the meaning and equivalent scope of the appended claims are intended to be included therein.

Claims

1. A method of controlling a power generation system (10), wherein: The power generation system is electrically coupled to a power grid (200) and comprises a power generation unit (20) generating electric power and a converter system (30) coupled to the power grid (200), wherein the converter system at least converts the electric power exchanged between the power generation unit (20) and the power grid (200), and wherein the method comprises: The method further comprises: operating the converter system (30) in a grid forming operation mode in which the converter system controls the exchange of electric power with the power grid to support a grid voltage and / or a grid frequency, wherein during disturbances of the power grid the exchanged electric power comprises a stabilizing component providing the support, wherein the method further comprises: Controlling the power generation system to perform at least one of the following: limiting the stable component of the exchanged electric power by controlling the converter system (30); Enforcing a steady component of the electrical power exchanged; and prevents the stable component of the electric power exchanged, wherein the forcing comprises temporarily preventing a control response to a deviating operating point of the power generation system, and Wherein, the method comprises: determining a disturbance power (Pd) and / or a disturbance energy (Ed) that needs to be exchanged with the power grid (200) in response to the occurrence of a disturbance, and The limiting, forcing and preventing of the stabilizing component are respectively based on at least the disturbance power and / or the disturbance energy, and further based on one or more operating parameters of the power generation system (10).

2. The method according to claim 1, wherein: The stabilizing component of limiting the exchanged electric power comprises: limiting the magnitude of electric power exchanged with the power grid (200); and / or The rate of change of electrical power exchanged with the power grid is limited.

3. The method according to claim 1 or 2, wherein: The method comprises: determining one or more limits (li) based at least on the disturbance power (Pd) and / or the disturbance energy (Ed) and / or the one or more operating parameters of the power generation system (10); and The one or more limits are applied to limit the electrical power exchanged with the power grid.

4. A method according to any one of the preceding claims, wherein: The method comprises: monitoring the grid voltage and / or the grid frequency to generate a monitored grid voltage and / or a monitored grid frequency; determining an operating mode based on said monitored grid voltage and / or said monitored grid frequency; and At least one of limiting, forcing and preventing the stabilizing component is modified based on the operating mode (m).

5. A method according to any one of the preceding claims, wherein: The method comprises: When the stabilizing component is blocked, the power generation system (10) is decoupled from the power grid (200).

6. A method according to any one of the preceding claims, wherein: The power generation system (10) is controlled to exchange power with the power grid (200) according to a reference power (Pr), and wherein the method comprises: monitoring power exchanged with the power grid to generate monitored power, wherein a disturbance power (Pd) is determined based on a difference between the monitored power (P) and the reference power, and wherein, preferably, the method further comprises: The difference is filtered before determining the disturbance power.

7. The method according to claim 6, wherein: A power controller (66) controls the power output of the power generation unit (20), and wherein the method comprises: The reference power (Pr) is provided by the power controller.

8. The method according to claim 6 or 7, wherein: Monitoring the power comprises: A grid voltage and a grid current are monitored to generate a monitored grid voltage (vabc) and a monitored grid current (iabc), respectively, wherein the monitored power (P) is generated by deriving a monitored power from the monitored grid voltage and the monitored grid current.

9. A method according to any one of the preceding claims, wherein: The method further comprises: Integrating a parameter indicative of the disturbance power (Pd), wherein the disturbance energy (Ed) is determined based on the integrated parameter, Preferably, the method further comprises: The integrated parameter is filtered before determining the disturbance energy.

10. The method according to any one of the preceding claims, wherein: The method comprises: determining that the disturbance power (Pd) and / or the disturbance energy (Ed) exceeds corresponding thresholds, Therein, in response to the threshold being exceeded, the stabilizing component is limited or prevented.

11. A method according to any one of the preceding claims, wherein: The method comprises: determining that when exchanging the stabilizing component with the power grid (200), at least one of the operating parameters of the power generation system (10), in particular at least one of the operating parameters of the power generation unit (20) and / or the converter system (30), exceeds a corresponding threshold value, Therein, the stabilizing component is limited or prevented in response to the threshold being exceeded.

12. A control system (50) for controlling a power generation system (10), wherein: The power generation system is configured to be electrically coupled to an electric power grid (200), and comprises a power generation unit (20) configured to generate electric power and a converter system (30) configured to be coupled to the power grid, wherein the converter system is configured to at least convert the electric power exchanged between the power generation unit and the power grid, and wherein the control system is configured to perform the method according to any one of claims 1-11.

13. A power generation system (10), wherein: The power generation system is configured to be electrically coupled to an electric power grid (200), and wherein the power generation system comprises: a power generation unit (20) configured to generate electrical power; a converter system (30) configured to be coupled to the power grid, wherein the converter system is configured to at least convert electric power exchanged between the power generation unit and the power grid; and The control system (50) of claim 12, configured to control the power generation system.

14. The power generation system according to claim 13, wherein: The power generation unit (20) is an electrical generator of a wind turbine.

15. A computer program for controlling a power generation system (10), wherein: The computer program comprises control instructions which, when executed by one or more processing units (61, 71), cause the one or more processing units to perform a method according to any one of claims 1-11.

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