WO2022122100A1 - A method for damping power oscillations in a power grid - Google Patents

A method for damping power oscillations in a power grid Download PDF

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Publication number
WO2022122100A1
WO2022122100A1 PCT/DK2021/050350 DK2021050350W WO2022122100A1 WO 2022122100 A1 WO2022122100 A1 WO 2022122100A1 DK 2021050350 W DK2021050350 W DK 2021050350W WO 2022122100 A1 WO2022122100 A1 WO 2022122100A1
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WO
WIPO (PCT)
Prior art keywords
power
chopper
grid
oscillation
modulation signal
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Application number
PCT/DK2021/050350
Other languages
French (fr)
Inventor
Hans Abildgaard
Original Assignee
Vestas Wind Systems A/S
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Application filed by Vestas Wind Systems A/S filed Critical Vestas Wind Systems A/S
Publication of WO2022122100A1 publication Critical patent/WO2022122100A1/en

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Classifications

    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/24—Arrangements for preventing or reducing oscillations of power in networks
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
    • H02J3/381—Dispersed generators
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J2300/00—Systems for supplying or distributing electric power characterised by decentralized, dispersed, or local generation
    • H02J2300/20—The dispersed energy generation being of renewable origin
    • H02J2300/28—The renewable source being wind energy

Definitions

  • the present invention relates to a method for controlling a wind turbine or a wind farm connected to a power grid in such a manner that power oscillations in the power grid are dampened without introducing serious mechanical oscillations in the wind turbine.
  • power oscillations may occur, e.g. in the form of low frequency inter-area oscillations. Such power oscillations may cause instability of the power grid, and it is therefore necessary to dampen or avoid power oscillations.
  • Normally power oscillations are handled by means of synchronous generators forming part of large and stable power producers, such as conventional power plants. In this case the power oscillations are dampened by appropriately injecting a signal in antiphase with a locally measured oscillation signal.
  • the low frequency inter-area power oscillations described above are dampened, since they are very likely to destabilize the power grid.
  • the frequency of these power oscillations is normally within the range 0.1-4.0 Hz.
  • Several mechanical eigenfrequencies of wind turbines, e.g. in drive train, tower, wind turbine blades, etc. are also within this frequency range. Therefore, controlling wind turbines in order to provide damping of these low frequency power oscillations may result in mechanical eigenfrequencies in the wind turbines being excited, thereby leading to excessive wear and loads on the wind turbines. This constitutes a serious limitation for wind turbines and therefore requires careful coordination between the mechanical design and power oscillation damping control of the wind turbines.
  • the invention provides a method for controlling a wind turbine connected to a power grid, the wind turbine comprising a generator and a converter coupled to the generator, the converter comprising a generator side inverter, a grid side inverter and a DC link interconnecting the generator side inverter and the grid side inverter, the wind turbine further comprising a DC chopper connected to the DC link, the method comprising the steps of:
  • the invention provides a method for controlling a wind turbine.
  • the term 'wind turbine' should be interpreted to mean an apparatus which converts energy of the wind into electrical energy.
  • Wind turbines normally comprise a tower with a nacelle mounted rotatably thereon via a yaw system.
  • a rotor comprising a hub and a number of wind turbine blades is mounted rotatably on the nacelle. Wind acting on the wind turbine blades thereby causes the rotor to rotate.
  • the mechanical energy represented by the rotations of the rotor is converted into electrical energy by a generator.
  • the wind turbine comprises a generator and a converter coupled to the generator.
  • the generator is, thus, connected to a power grid via the converter.
  • the converter comprises a generator side inverter, a grid side inverter and a DC link interconnecting the generator side inverter and the grid side inverter.
  • the wind turbine further comprises a DC chopper connected to the DC link.
  • DC choppers are normally used during grid contingencies, where the DC choppers may be activated in order to dump power in a resistor, and thereby reduce the active power supplied to the power grid. DC choppers may further be applied during emergency shutdown.
  • a power oscillation in the power grid is initially detected.
  • the power oscillation may be detected locally, at the position of the wind turbine, or it could be detected at some distance from the wind turbine, e.g. at a point of common coupling to the power grid.
  • the point of common coupling could be a point of common coupling between the wind farm and the power grid, but it is also within the scope of the present invention that the power oscillation is detected at a distance from the wind turbine when the wind turbine is a stand alone wind turbine, i.e. when the wind turbine does not form part of a wind farm.
  • a modulation signal for the DC chopper is generated.
  • the modulation signal is generated in accordance with the detected power oscillation, and thereby it takes properties of the detected power oscillation into account, e.g. in terms of amplitude, frequency, phase, etc., of the detected power oscillation.
  • the DC chopper is controlled according to the generated modulation signal for the DC chopper, i.e. the control of the DC chopper is modulated as prescribed by the modulation signal. Since the modulation signal was generated in accordance with the detected power oscillation, the DC chopper is also controlled in accordance with the detected power oscillation. More particularly, this is performed in such a manner that the power oscillation in the power grid is dampened. This could, e.g., be obtained by modulating the DC chopper in such a manner that the power supplied to the power grid via the grid side inverter counteracts the power oscillation, e.g. by supplying power which is in antiphase with the power oscillation in the power grid. Accordingly, the wind turbine contributes to damping of power oscillations in the power grid.
  • the generator side inverter is substantially decoupled from the power oscillation in the power grid, as well as from the variations in the power supplied by the wind turbine to the power grid, via the grid side inverter, in order to dampen the power oscillation in the power grid.
  • the control of the DC chopper results in modulation of the power provided by the grid side inverter to the power grid, without affecting the generator side inverter.
  • the power oscillation affects the generator, and thereby any mechanical systems connected to the generator, such as drive train, tower, wind turbine blades, etc.
  • mechanical eigenfrequencies in such mechanical systems are excited when the wind turbine provides damping of the power oscillation of the power grid, and excessive wear or loads on the wind turbine is therefore avoided.
  • the step of detecting a power oscillation in the power grid may comprise detecting a frequency of the power oscillation, and the modulation signal for the DC chopper may be generated in accordance with the detected frequency.
  • the generated modulation signal takes the frequency of the detected power oscillation into account.
  • the modulation signal may have substantially the same frequency as the detected power oscillation, and it may modulate the DC chopper in such a manner that the power provided by the grid side inverter is in antiphase with the power oscillation.
  • the step of controlling the DC chopper according to the generated modulation signal may comprise activating the DC chopper during positive half cycles of the detected power oscillation and deactivating the DC chopper during negative half cycles of the detected power oscillation.
  • the modulation signal may be a pulse width modulation (PWM) signal.
  • PWM pulse width modulation
  • a PWM signal is very suitable for providing a required modulation for damping power oscillations.
  • the step of detecting a power oscillation in the power grid may comprise monitoring a voltage signal of the power grid.
  • the power oscillation in the power grid is detected in the form of an oscillation in the voltage of the power grid.
  • the voltage may be monitored locally, i.e. at the position of the wind turbine.
  • the voltage may be monitored at a distance from the wind turbine, e.g. at or near a point of common coupling between a wind farm and the power grid, in the case that the wind turbine forms part of the wind farm.
  • local variations in voltage may occur in an internal power grid of a wind farm, and when this is the case a given wind turbine may detect a local power oscillation when there is no power oscillation in the power grid, or vice versa. Since the wind turbine should only provide power oscillation damping when there is actually a power oscillation in the power grid, the detected signal should preferably reflect the actual state of the power grid.
  • the power oscillation in the power grid may be detected in the form of an oscillation in current and/or active power.
  • the method may further comprise the steps of:
  • controlling the DC chopper in order to handle the fault ride through event before controlling the DC chopper according to the generated modulation signal.
  • the main purpose of DC choppers is to allow power to be absorbed during emergency shutdown and during grid contingencies, such as fault ride through events, the wind turbine thereby contributing to stabilizing the power grid during such events.
  • This is a service which the owner of the wind turbine may very well have committed to provide, and therefore it needs to be ensured that the DC chopper is in fact available for this purpose when a fault ride through event occurs. Therefore, according to this embodiment, when it turns out that a fault ride through event is occurring, the DC chopper is controlled in such a manner that handling of the fault ride through event is prioritized over handling of the power oscillation in the power grid. Thereby it is ensured that fault ride through events can always be handled, and power oscillations are only handled to the extent that the capacity of the DC chopper allows so when fault ride through events have been handled.
  • the method may further comprise the steps of:
  • the energy absorbed by the DC chopper is monitored in order to ensure that the DC chopper, in particular a DC chopper resistor, is not overloaded. Accordingly, if it is revealed that the energy absorbed by the DC chopper exceeds a predefined threshold value, an amplitude of the modulation signal for the DC chopper is limited. Thereby power dissipated in the DC chopper is reduced, and overloading of the DC chopper is avoided.
  • the energy absorbed by the DC chopper may, e.g., be monitored by monitoring a temperature of the DC chopper resistor.
  • the predefined threshold value may be a temperature threshold value, and the limitation of the amplitude of the modulation signal may be provided in order to avoid failure of the DC chopper resistor, due to high temperature.
  • the energy absorbed by the DC chopper may be monitored by monitoring power dissipated in the DC chopper resistor.
  • the predefined threshold value may also reflect a power dissipation level beyond which there is a risk of failure of the DC chopper resistor.
  • the energy absorbed by the DC chopper may be monitored by monitoring a current and a voltage at the DC chopper. Since the voltage at the DC link is already known, this will only require an additional measurement of the current.
  • the predefined threshold value may, e.g., be selected in such a manner that it is ensured that there is sufficient headroom in the capacity of the DC chopper to handle a specified number of fault ride through events.
  • the method may further comprise the step of adjusting a voltage setpoint of the wind turbine in the case that the step of controlling the DC chopper according to the generated modulation signal provides insufficient damping of the power oscillation in the power grid.
  • the wind turbine may form part of a wind farm, the wind farm comprising a plurality of wind turbines and a power plant controller being in communication connection with each of the wind turbines of the wind farm, and the step of generating a modulation signal for the DC chopper may be performed by the power plant controller, and the power plant controller may dispatch the generated modulation signal to the wind turbine.
  • the wind turbine forms part of a wind farm.
  • the term 'wind farm' should be interpreted to mean a plurality of wind turbines arranged within a specified geographical area, and which share some infrastructure, such as internal power grid, connection to an external power grid, substations, access roads, etc.
  • the wind farm further comprises a central power plant controller (PPC) being in communication connection with each of the wind turbines of the wind farm.
  • the power plant controller may, e.g., be responsible for the overall control of the wind farm, such as ensuring that obligations towards the power grid are fulfilled, dispatching operation setpoints to the wind turbines, etc.
  • the power plant controller may be in the form of a single unit, or it may be in the form of two or more units, and/or two or more separate control loops, each being responsible for the control of a subset of the wind turbines of the wind farm.
  • the step of generating a modulation signal for the DC chopper is performed by the power plant controller.
  • the power plant controller which determines how the DC chopper needs to be controlled, based on the detected oscillation in the power grid, in order to provide damping of the power oscillation.
  • the power plant controller dispatches the modulation signal to the wind turbine, in order to ensure that the DC chopper of the wind turbine is subsequently controlled in accordance with the modulation signal.
  • the power oscillation damping provided by the wind turbine is controlled by the power plant controller.
  • the power plant controller may ensure that the control of the wind turbine, in terms of providing power oscillation damping, is coordinated with other considerations at wind farm level, such as control of the other wind turbines, power oscillation damping provided by the entire wind farm, etc.
  • the step of detecting a power oscillation in the power grid may also be performed by the power plant controller.
  • the power plant controller may generate a modulation signal for the wind turbine, and possibly similar modulation signals for one or more of the other wind turbines of the wind farm, which ensures that the wind farm as a whole provides appropriate power oscillation damping for the power grid.
  • the generated modulation signal reflects the state of the power grid, rather than a condition occurring locally at the position of the wind turbine.
  • the modulation signal may be added to a power reference signal of the wind turbine. According to this embodiment, the power reference signal of the wind turbine is simply adjusted by the modulation signal, rather than providing the modulation signal as a separate signal.
  • the modulation signal and the power reference signal may be provided as a separate signals.
  • the invention provides a method for controlling a wind farm, the wind farm comprising a plurality of wind turbines connected to a power grid, and a power plant controller, the power plant controller being in communication connection with each of the wind turbines of the wind farm, each of the wind turbines comprising a generator and a converter coupled to the generator, the converter comprising a generator side inverter, a grid side inverter and a DC link interconnecting the generator side inverter and the grid side inverter, each wind turbine further comprising a DC chopper connected to the DC link, wherein the method comprises the steps of:
  • the power plant controller in response to the detected power oscillation, the power plant controller generating a modulation signal for the DC chopper of at least some of the wind turbines of the wind farm, the modulation signal being generated in accordance with the detected power oscillation,
  • the invention provides a method for controlling a wind farm comprising a plurality of wind turbines connected to a power grid.
  • the wind turbines of the wind farm may be of the kind described above with reference to the first aspect of the invention.
  • the wind farm further comprises a power plant controller being in communication connection with each of the wind turbines of the wind farm, as described above.
  • a power oscillation in the power grid is initially detected, e.g. as described above with reference to the first aspect of the invention.
  • the power plant controller In response to the detected power oscillation, the power plant controller generates a modulation signal for the DC chopper of at least some of the wind turbines of the wind farm.
  • the modulation signals are generated in accordance with the detected power oscillation, similarly to the modulation signal described above with reference to the first aspect of the invention.
  • the power plant controller then dispatches the generated modulation signals to the respective wind turbines. Accordingly, the power plant controller ensures that the wind farm as a whole provides the required power oscillation damping, and instructs the individual wind turbines to control their DC choppers in such a manner that this is obtained.
  • Fig. 1 is a diagrammatic view of a wind turbine being controlled in accordance with a method according to an embodiment of the invention
  • Fig. 2 illustrates steps performed in a method according to an embodiment of the invention
  • Fig. 3 illustrates a power oscillation signal and a generated modulation signal for a DC chopper for performing a method according to an embodiment of the invention
  • Fig. 4 illustrates control of a wind farm in accordance with a method according to an embodiment of the invention.
  • Fig. 1 is a diagrammatic view of a wind turbine 1 being controlled in accordance with a method according to an embodiment of the invention.
  • the wind turbine 1 comprises a rotor 2 with a number of wind turbine blades 3, two of which are shown, the rotor 2 being connected to a generator 4.
  • the wind turbine 1 further comprises a converter comprising a generator side inverter 5, a grid side inverter 6 and a DC link 7 interconnecting the generator side inverter 4 and the grid side inverter 6.
  • the converter is connected to the generator 4 via the generator side inverter 5 and to a power grid (not shown) via the grid side inverter 6 and a transformer 8.
  • a DC chopper 9 is connected to the DC link 7.
  • the wind turbine 1 of Fig. 1 may be operated in the following manner.
  • a power oscillation in the power grid is detected by means of a detection unit 10 and supplied to a control unit 11.
  • the control unit 11 In response to the detected power oscillation, the control unit 11 generates a modulation signal for the DC chopper 9.
  • the modulation signal is generated in accordance with the detected power oscillation, and thereby takes specific characteristics of the power oscillation into account, e.g. in terms of frequency, amplitude, phase, etc.
  • the modulation signal is supplied to the DC chopper 9, and the DC chopper 9 is subsequently controlled according to the modulation signal.
  • the modulation signal may cause the DC chopper 9 to be activated during positive half cycles of the power oscillation and to be deactivated during negative half cycles of the power oscillation.
  • the generator side inverter 5 is substantially decoupled from the power oscillation, as well as from the measures taken in order to dampen the power oscillation. Thereby the torque on the generator 4 is also essentially unaffected, and the risk of exciting mechanical eigenfrequencies of the wind turbine 1 is minimised.
  • Fig. 2 illustrates steps performed in a method according to an embodiment of the invention, in the form of control blocks.
  • the washout filter 14 derives a frequency spectrum from the detected power oscillation and supplies this to a phase compensation unit 15.
  • the phase compensation unit 15 corrects a phase shift introduced in the washout filter, and the result is supplied to a gain unit 16, where the signal is amplified.
  • the amplified signal is supplied to a limiter 17, which limits the maximum amplitude of the damping power.
  • the resulting modulation signal is supplied to a DC chopper controller in the form of an output 18.
  • Fig. 3 illustrates a power oscillation signal 19 and a generated modulation signal 20 for a DC chopper for performing a method according to an embodiment of the invention.
  • the DC chopper could, e.g., be the DC chopper 9 illustrated in Fig. 1.
  • the power oscillation signal 19 defines a frequency, an amplitude and a phase, and could, e.g., be a measured active power, voltage or current at a specified point in the power grid.
  • the modulation signal 20 varies with the same frequency as the power oscillation, and specifies that the DC chopper shall be activated during positive half cycles of the power oscillation 19, and that the DC chopper shall be deactivated during the negative half cycles of the power oscillation 19.
  • the required phase displacement between the DC chopper signal and the power grid signal will depend on the measured grid quantity.
  • Fig. 4 illustrates control of a wind farm 21 in accordance with a method according to an embodiment of the invention.
  • the wind farm 21 comprises a plurality of wind turbines 1, three of which are illustrated, connected to a power grid 22.
  • a power plant controller 23 is in communication connection with the wind turbines 1, and is adapted to provide control signals to the wind turbines 1, in order to ensure that the wind turbines 1 of the wind farm 21 in combination fulfil requirements of the power grid 22.
  • the power plant controller 23 In response to a detected power oscillation in the power grid 22, the power plant controller 23 generates modulation signals for DC choppers of at least some of the wind turbines 1 of the wind farm 21, and dispatches the generated modulation signals to the respective wind turbines 1. Subsequently the DC choppers of the wind turbines 1 are controlled in accordance with the modulation signals.
  • the power plant controller 23 may, e.g., dispatch the modulation signals to the wind turbines 1 in the form of an added signal to respective power reference signals which are also provided to the wind turbines 1 by the power plant controller 23.

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  • Engineering & Computer Science (AREA)
  • Power Engineering (AREA)
  • Control Of Eletrric Generators (AREA)

Abstract

A method for controlling a wind turbine (1) connected to a power grid (22) is disclosed. The wind turbine (1) comprises a generator (4) and a converter coupled to the generator (4), the converter comprising a generator side inverter (5), a grid side inverter (6) and a DC link (7) interconnecting the generator side inverter (5) and the grid side inverter (6). The wind turbine (1) further comprises a DC chopper (9) connected to the DC link (7). A power oscillation (19) in the power grid (22) is detected. In response to the detected power oscillation (19), a modulation signal (20) for the DC chopper (9) is generated, the modulation signal (20) being generated in accordance with the detected power oscillation (19). The DC chopper (9) is controlled according to the generated modulation signal (20), thereby providing damping of the power oscillation (19) in the power grid (22) while substantially decoupling the generator side inverter (5) from the power oscillation (19) in the power grid (22).

Description

A METHOD FOR. DAMPING POWER OSCILLATIONS IN A POWER GRID
FIELD OF THE INVENTION
The present invention relates to a method for controlling a wind turbine or a wind farm connected to a power grid in such a manner that power oscillations in the power grid are dampened without introducing serious mechanical oscillations in the wind turbine.
BACKGROUND OF THE INVENTION
In power grids power oscillations may occur, e.g. in the form of low frequency inter-area oscillations. Such power oscillations may cause instability of the power grid, and it is therefore necessary to dampen or avoid power oscillations.
Normally power oscillations are handled by means of synchronous generators forming part of large and stable power producers, such as conventional power plants. In this case the power oscillations are dampened by appropriately injecting a signal in antiphase with a locally measured oscillation signal.
However, as renewable power producers increasingly penetrate power grids, at the cost of conventional power plants, it is becoming more common that the grid operators require that inverter based renewable power producers are able to provide services which have previously been delivered by the conventional power plants, including contributing to damping of power oscillations in the power grid. Such requirements may, e.g., form part of the grid code for the power grid.
It is particularly relevant that the low frequency inter-area power oscillations described above are dampened, since they are very likely to destabilize the power grid. The frequency of these power oscillations is normally within the range 0.1-4.0 Hz. Several mechanical eigenfrequencies of wind turbines, e.g. in drive train, tower, wind turbine blades, etc., are also within this frequency range. Therefore, controlling wind turbines in order to provide damping of these low frequency power oscillations may result in mechanical eigenfrequencies in the wind turbines being excited, thereby leading to excessive wear and loads on the wind turbines. This constitutes a serious limitation for wind turbines and therefore requires careful coordination between the mechanical design and power oscillation damping control of the wind turbines.
DESCRIPTION OF THE INVENTION
It is an object of embodiments of the invention to provide a method for controlling a wind turbine connected to a power grid in such a manner that power oscillations in the power grid are dampened without subjecting the wind turbine to excessive wear.
It is a further object of embodiments of the invention to provide a method for controlling a wind farm connected to a power grid in such a manner that power oscillations in the power grid are dampened without subjecting the wind turbines of the wind farm to excessive wear.
According to a first aspect the invention provides a method for controlling a wind turbine connected to a power grid, the wind turbine comprising a generator and a converter coupled to the generator, the converter comprising a generator side inverter, a grid side inverter and a DC link interconnecting the generator side inverter and the grid side inverter, the wind turbine further comprising a DC chopper connected to the DC link, the method comprising the steps of:
- detecting a power oscillation in the power grid,
- in response to the detected power oscillation, generating a modulation signal for the DC chopper, the modulation signal being generated in accordance with the detected power oscillation, and
- controlling the DC chopper according to the generated modulation signal, thereby providing damping of the power oscillation in the power grid while substantially decoupling the generator side inverter from the power oscillation in the power grid.
Thus, according to the first aspect, the invention provides a method for controlling a wind turbine. In the present context the term 'wind turbine' should be interpreted to mean an apparatus which converts energy of the wind into electrical energy. Wind turbines normally comprise a tower with a nacelle mounted rotatably thereon via a yaw system. A rotor comprising a hub and a number of wind turbine blades is mounted rotatably on the nacelle. Wind acting on the wind turbine blades thereby causes the rotor to rotate. The mechanical energy represented by the rotations of the rotor is converted into electrical energy by a generator.
Thus, the wind turbine comprises a generator and a converter coupled to the generator. The generator is, thus, connected to a power grid via the converter. The converter comprises a generator side inverter, a grid side inverter and a DC link interconnecting the generator side inverter and the grid side inverter.
The wind turbine further comprises a DC chopper connected to the DC link. DC choppers are normally used during grid contingencies, where the DC choppers may be activated in order to dump power in a resistor, and thereby reduce the active power supplied to the power grid. DC choppers may further be applied during emergency shutdown.
In the method according to the first aspect of the invention a power oscillation in the power grid is initially detected. The power oscillation may be detected locally, at the position of the wind turbine, or it could be detected at some distance from the wind turbine, e.g. at a point of common coupling to the power grid. In the case that the wind turbine forms part of a wind farm, the point of common coupling could be a point of common coupling between the wind farm and the power grid, but it is also within the scope of the present invention that the power oscillation is detected at a distance from the wind turbine when the wind turbine is a stand alone wind turbine, i.e. when the wind turbine does not form part of a wind farm. Thus, at this stage it is established that power oscillation in the power grid is occurring, and that it is therefore necessary to take measures in order to dampen the power oscillation.
In response to the detected power oscillation, a modulation signal for the DC chopper is generated. The modulation signal is generated in accordance with the detected power oscillation, and thereby it takes properties of the detected power oscillation into account, e.g. in terms of amplitude, frequency, phase, etc., of the detected power oscillation.
Finally, the DC chopper is controlled according to the generated modulation signal for the DC chopper, i.e. the control of the DC chopper is modulated as prescribed by the modulation signal. Since the modulation signal was generated in accordance with the detected power oscillation, the DC chopper is also controlled in accordance with the detected power oscillation. More particularly, this is performed in such a manner that the power oscillation in the power grid is dampened. This could, e.g., be obtained by modulating the DC chopper in such a manner that the power supplied to the power grid via the grid side inverter counteracts the power oscillation, e.g. by supplying power which is in antiphase with the power oscillation in the power grid. Accordingly, the wind turbine contributes to damping of power oscillations in the power grid.
Furthermore, since the damping is obtained by appropriately controlling the DC chopper, the generator side inverter is substantially decoupled from the power oscillation in the power grid, as well as from the variations in the power supplied by the wind turbine to the power grid, via the grid side inverter, in order to dampen the power oscillation in the power grid. Instead, the control of the DC chopper results in modulation of the power provided by the grid side inverter to the power grid, without affecting the generator side inverter. Thereby it is also avoided that the power oscillation affects the generator, and thereby any mechanical systems connected to the generator, such as drive train, tower, wind turbine blades, etc. Thus, it is prevented that mechanical eigenfrequencies in such mechanical systems are excited when the wind turbine provides damping of the power oscillation of the power grid, and excessive wear or loads on the wind turbine is therefore avoided.
The step of detecting a power oscillation in the power grid may comprise detecting a frequency of the power oscillation, and the modulation signal for the DC chopper may be generated in accordance with the detected frequency.
According to this embodiment, the generated modulation signal takes the frequency of the detected power oscillation into account. For instance, the modulation signal may have substantially the same frequency as the detected power oscillation, and it may modulate the DC chopper in such a manner that the power provided by the grid side inverter is in antiphase with the power oscillation.
The step of controlling the DC chopper according to the generated modulation signal may comprise activating the DC chopper during positive half cycles of the detected power oscillation and deactivating the DC chopper during negative half cycles of the detected power oscillation.
When the DC chopper is activated, power is dumped in the DC chopper, and thereby the active power supplied to the power grid via the grid side inverter is correspondingly reduced. Therefore, by activating the DC chopper during positive half cycles of the detected power oscillation it is obtained that the active power supplied to the power grid is reduced during these positive half cycles. This contributes to damping the power oscillation. Furthermore, by deactivating the DC chopper during the negative half cycles, the normal active power supply is maintained during these negative half cycles.
The modulation signal may be a pulse width modulation (PWM) signal. A PWM signal is very suitable for providing a required modulation for damping power oscillations.
The step of detecting a power oscillation in the power grid may comprise monitoring a voltage signal of the power grid. According to this embodiment, the power oscillation in the power grid is detected in the form of an oscillation in the voltage of the power grid. The voltage may be monitored locally, i.e. at the position of the wind turbine. Alternatively or additionally, the voltage may be monitored at a distance from the wind turbine, e.g. at or near a point of common coupling between a wind farm and the power grid, in the case that the wind turbine forms part of the wind farm. In some cases it may be an advantage to monitor the voltage at a distance from the wind turbine because the local voltage is not necessarily representative for the voltage of the power grid. For instance, local variations in voltage may occur in an internal power grid of a wind farm, and when this is the case a given wind turbine may detect a local power oscillation when there is no power oscillation in the power grid, or vice versa. Since the wind turbine should only provide power oscillation damping when there is actually a power oscillation in the power grid, the detected signal should preferably reflect the actual state of the power grid.
Alternatively or additionally, the power oscillation in the power grid may be detected in the form of an oscillation in current and/or active power.
The method may further comprise the steps of:
- investigating whether a fault ride through event is occurring, and
- in the case that a fault ride through event is occurring, controlling the DC chopper in order to handle the fault ride through event before controlling the DC chopper according to the generated modulation signal.
As described above, the main purpose of DC choppers is to allow power to be absorbed during emergency shutdown and during grid contingencies, such as fault ride through events, the wind turbine thereby contributing to stabilizing the power grid during such events. This is a service which the owner of the wind turbine may very well have committed to provide, and therefore it needs to be ensured that the DC chopper is in fact available for this purpose when a fault ride through event occurs. Therefore, according to this embodiment, when it turns out that a fault ride through event is occurring, the DC chopper is controlled in such a manner that handling of the fault ride through event is prioritized over handling of the power oscillation in the power grid. Thereby it is ensured that fault ride through events can always be handled, and power oscillations are only handled to the extent that the capacity of the DC chopper allows so when fault ride through events have been handled.
The method may further comprise the steps of:
- monitoring energy absorbed by the DC chopper, and
- limiting an amplitude of the modulation signal for the DC chopper in the case that the absorbed energy exceeds a predefined threshold value.
According to this embodiment, the energy absorbed by the DC chopper is monitored in order to ensure that the DC chopper, in particular a DC chopper resistor, is not overloaded. Accordingly, if it is revealed that the energy absorbed by the DC chopper exceeds a predefined threshold value, an amplitude of the modulation signal for the DC chopper is limited. Thereby power dissipated in the DC chopper is reduced, and overloading of the DC chopper is avoided.
The energy absorbed by the DC chopper may, e.g., be monitored by monitoring a temperature of the DC chopper resistor. In this case the predefined threshold value may be a temperature threshold value, and the limitation of the amplitude of the modulation signal may be provided in order to avoid failure of the DC chopper resistor, due to high temperature.
Alternatively or additionally, the energy absorbed by the DC chopper may be monitored by monitoring power dissipated in the DC chopper resistor. In this case the predefined threshold value may also reflect a power dissipation level beyond which there is a risk of failure of the DC chopper resistor. Alternatively or additionally, the energy absorbed by the DC chopper may be monitored by monitoring a current and a voltage at the DC chopper. Since the voltage at the DC link is already known, this will only require an additional measurement of the current.
The predefined threshold value may, e.g., be selected in such a manner that it is ensured that there is sufficient headroom in the capacity of the DC chopper to handle a specified number of fault ride through events.
The method may further comprise the step of adjusting a voltage setpoint of the wind turbine in the case that the step of controlling the DC chopper according to the generated modulation signal provides insufficient damping of the power oscillation in the power grid.
According to this embodiment, if it turns out that providing power oscillation damping by modulating the DC chopper, and thereby modulating the active power provided by the wind turbine to the power grid, via the grid side inverter, is insufficient to dampen the power oscillation in the power grid, additional damping is provided by means of reactive power injection. This is obtained by adjusting the voltage setpoint of the wind turbine.
The wind turbine may form part of a wind farm, the wind farm comprising a plurality of wind turbines and a power plant controller being in communication connection with each of the wind turbines of the wind farm, and the step of generating a modulation signal for the DC chopper may be performed by the power plant controller, and the power plant controller may dispatch the generated modulation signal to the wind turbine.
According to this embodiment, the wind turbine forms part of a wind farm. In the present context the term 'wind farm' should be interpreted to mean a plurality of wind turbines arranged within a specified geographical area, and which share some infrastructure, such as internal power grid, connection to an external power grid, substations, access roads, etc. The wind farm further comprises a central power plant controller (PPC) being in communication connection with each of the wind turbines of the wind farm. The power plant controller may, e.g., be responsible for the overall control of the wind farm, such as ensuring that obligations towards the power grid are fulfilled, dispatching operation setpoints to the wind turbines, etc.
The power plant controller may be in the form of a single unit, or it may be in the form of two or more units, and/or two or more separate control loops, each being responsible for the control of a subset of the wind turbines of the wind farm.
According to this embodiment, the step of generating a modulation signal for the DC chopper is performed by the power plant controller. Thus, it is the power plant controller which determines how the DC chopper needs to be controlled, based on the detected oscillation in the power grid, in order to provide damping of the power oscillation. Once the power plant controller has generated the modulation signal, the power plant controller dispatches the modulation signal to the wind turbine, in order to ensure that the DC chopper of the wind turbine is subsequently controlled in accordance with the modulation signal.
According to this embodiment, the power oscillation damping provided by the wind turbine is controlled by the power plant controller. Thereby the power plant controller may ensure that the control of the wind turbine, in terms of providing power oscillation damping, is coordinated with other considerations at wind farm level, such as control of the other wind turbines, power oscillation damping provided by the entire wind farm, etc.
Furthermore, the step of detecting a power oscillation in the power grid may also be performed by the power plant controller. In this case the power plant controller may generate a modulation signal for the wind turbine, and possibly similar modulation signals for one or more of the other wind turbines of the wind farm, which ensures that the wind farm as a whole provides appropriate power oscillation damping for the power grid. Furthermore, it can be ensured that the generated modulation signal reflects the state of the power grid, rather than a condition occurring locally at the position of the wind turbine. The modulation signal may be added to a power reference signal of the wind turbine. According to this embodiment, the power reference signal of the wind turbine is simply adjusted by the modulation signal, rather than providing the modulation signal as a separate signal.
As an alternative, the modulation signal and the power reference signal may be provided as a separate signals.
According to a second aspect the invention provides a method for controlling a wind farm, the wind farm comprising a plurality of wind turbines connected to a power grid, and a power plant controller, the power plant controller being in communication connection with each of the wind turbines of the wind farm, each of the wind turbines comprising a generator and a converter coupled to the generator, the converter comprising a generator side inverter, a grid side inverter and a DC link interconnecting the generator side inverter and the grid side inverter, each wind turbine further comprising a DC chopper connected to the DC link, wherein the method comprises the steps of:
- detecting a power oscillation in the power grid,
- in response to the detected power oscillation, the power plant controller generating a modulation signal for the DC chopper of at least some of the wind turbines of the wind farm, the modulation signal being generated in accordance with the detected power oscillation,
- the power plant controller dispatching the generated modulation signals to the respective wind turbines, and
- the wind turbines controlling their respective DC choppers according to the generated modulation signals, thereby providing damping of the power oscillation in the power grid while substantially decoupling the generator side inverters from the power oscillation in the power grid. It should be noted that a person skilled in the art would readily recognise that any feature described in combination with the first aspect of the invention could also be combined with the second aspect of the invention, and vice versa. The remarks set forth above with reference to the first aspect of the invention are therefore equally applicable here.
According to the second aspect the invention provides a method for controlling a wind farm comprising a plurality of wind turbines connected to a power grid. The wind turbines of the wind farm may be of the kind described above with reference to the first aspect of the invention. The wind farm further comprises a power plant controller being in communication connection with each of the wind turbines of the wind farm, as described above.
In the method according to the second aspect of the invention, a power oscillation in the power grid is initially detected, e.g. as described above with reference to the first aspect of the invention.
In response to the detected power oscillation, the power plant controller generates a modulation signal for the DC chopper of at least some of the wind turbines of the wind farm. The modulation signals are generated in accordance with the detected power oscillation, similarly to the modulation signal described above with reference to the first aspect of the invention.
The power plant controller then dispatches the generated modulation signals to the respective wind turbines. Accordingly, the power plant controller ensures that the wind farm as a whole provides the required power oscillation damping, and instructs the individual wind turbines to control their DC choppers in such a manner that this is obtained.
Finally, the wind turbines control their respective DC choppers according to the generated modulation signals, in the manner described above with reference to the first aspect of the invention. BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described in further detail with reference to the accompanying drawings in which
Fig. 1 is a diagrammatic view of a wind turbine being controlled in accordance with a method according to an embodiment of the invention,
Fig. 2 illustrates steps performed in a method according to an embodiment of the invention,
Fig. 3 illustrates a power oscillation signal and a generated modulation signal for a DC chopper for performing a method according to an embodiment of the invention, and
Fig. 4 illustrates control of a wind farm in accordance with a method according to an embodiment of the invention.
DETAILED DESCRIPTION OF THE DRAWINGS
Fig. 1 is a diagrammatic view of a wind turbine 1 being controlled in accordance with a method according to an embodiment of the invention. The wind turbine 1 comprises a rotor 2 with a number of wind turbine blades 3, two of which are shown, the rotor 2 being connected to a generator 4. The wind turbine 1 further comprises a converter comprising a generator side inverter 5, a grid side inverter 6 and a DC link 7 interconnecting the generator side inverter 4 and the grid side inverter 6. The converter is connected to the generator 4 via the generator side inverter 5 and to a power grid (not shown) via the grid side inverter 6 and a transformer 8. A DC chopper 9 is connected to the DC link 7.
The wind turbine 1 of Fig. 1 may be operated in the following manner. A power oscillation in the power grid is detected by means of a detection unit 10 and supplied to a control unit 11. In response to the detected power oscillation, the control unit 11 generates a modulation signal for the DC chopper 9. The modulation signal is generated in accordance with the detected power oscillation, and thereby takes specific characteristics of the power oscillation into account, e.g. in terms of frequency, amplitude, phase, etc.
The modulation signal is supplied to the DC chopper 9, and the DC chopper 9 is subsequently controlled according to the modulation signal. For instance, the modulation signal may cause the DC chopper 9 to be activated during positive half cycles of the power oscillation and to be deactivated during negative half cycles of the power oscillation.
When the DC chopper 9 is activated, power from the DC link 7 is absorbed by the DC chopper 9, and thereby the power supplied to the power grid via the grid side inverter 6 is reduced. By activating the DC chopper 9 during positive half cycles of the power oscillation, the power oscillation is counteracted, and thereby dampened, during these positive half cycles.
However, since the damping of the power oscillation is provided by appropriately activating the DC chopper 9, the generator side inverter 5 is substantially decoupled from the power oscillation, as well as from the measures taken in order to dampen the power oscillation. Thereby the torque on the generator 4 is also essentially unaffected, and the risk of exciting mechanical eigenfrequencies of the wind turbine 1 is minimised.
Fig. 2 illustrates steps performed in a method according to an embodiment of the invention, in the form of control blocks.
A grid input 12, in the form of a detected power oscillation in a power grid, is supplied to a transducer 13, and further on to a washout filter 14. The washout filter 14 derives a frequency spectrum from the detected power oscillation and supplies this to a phase compensation unit 15. The phase compensation unit 15 corrects a phase shift introduced in the washout filter, and the result is supplied to a gain unit 16, where the signal is amplified. The amplified signal is supplied to a limiter 17, which limits the maximum amplitude of the damping power. Finally, the resulting modulation signal is supplied to a DC chopper controller in the form of an output 18.
Fig. 3 illustrates a power oscillation signal 19 and a generated modulation signal 20 for a DC chopper for performing a method according to an embodiment of the invention. The DC chopper could, e.g., be the DC chopper 9 illustrated in Fig. 1.
The power oscillation signal 19 defines a frequency, an amplitude and a phase, and could, e.g., be a measured active power, voltage or current at a specified point in the power grid.
The modulation signal 20 varies with the same frequency as the power oscillation, and specifies that the DC chopper shall be activated during positive half cycles of the power oscillation 19, and that the DC chopper shall be deactivated during the negative half cycles of the power oscillation 19. In order to provide damping of the active power oscillation, the required phase displacement between the DC chopper signal and the power grid signal will depend on the measured grid quantity.
Fig. 4 illustrates control of a wind farm 21 in accordance with a method according to an embodiment of the invention. The wind farm 21 comprises a plurality of wind turbines 1, three of which are illustrated, connected to a power grid 22. A power plant controller 23 is in communication connection with the wind turbines 1, and is adapted to provide control signals to the wind turbines 1, in order to ensure that the wind turbines 1 of the wind farm 21 in combination fulfil requirements of the power grid 22.
In response to a detected power oscillation in the power grid 22, the power plant controller 23 generates modulation signals for DC choppers of at least some of the wind turbines 1 of the wind farm 21, and dispatches the generated modulation signals to the respective wind turbines 1. Subsequently the DC choppers of the wind turbines 1 are controlled in accordance with the modulation signals. The power plant controller 23 may, e.g., dispatch the modulation signals to the wind turbines 1 in the form of an added signal to respective power reference signals which are also provided to the wind turbines 1 by the power plant controller 23.

Claims

1. A method for controlling a wind turbine (1) connected to a power grid (22), the wind turbine (1) comprising a generator (4) and a converter coupled to the generator (4), the converter comprising a generator side inverter (5), a grid side inverter (6) and a DC link (7) interconnecting the generator side inverter (5) and the grid side inverter (6), the wind turbine (1) further comprising a DC chopper (9) connected to the DC link (7), the method comprising the steps of:
- detecting a power oscillation (19) in the power grid (22),
- in response to the detected power oscillation (19), generating a modulation signal (20) for the DC chopper (9), the modulation signal (20) being generated in accordance with the detected power oscillation (19), and
- controlling the DC chopper (9) according to the generated modulation signal (20), thereby providing damping of the power oscillation (19) in the power grid (22) while substantially decoupling the generator side inverter (5) from the power oscillation (19) in the power grid (22).
2. A method according to claim 1, wherein the step of detecting a power oscillation (19) in the power grid (22) comprises detecting a frequency of the power oscillation (19), and wherein the modulation signal (20) for the DC chopper (9) is generated in accordance with the detected frequency.
3. A method according to claim 1 or 2, wherein the step of controlling the DC chopper (9) according to the generated modulation signal (20) comprises activating the DC chopper (9) during positive half cycles of the detected power oscillation (19) and deactivating the DC chopper (9) during negative half cycles of the detected power oscillation (19).
4. A method according to any of the preceding claims, wherein the modulation signal (20) is a pulse width modulation (PWM) signal.
5. A method according to any of the preceding claims, wherein the step of detecting a power oscillation (19) in the power grid (22) comprises monitoring a voltage signal of the power grid (22).
6. A method according to any of the preceding claims, further comprising the steps of:
- investigating whether a fault ride through event is occurring, and
- in the case that a fault ride through event is occurring, controlling the DC chopper (9) in order to handle the fault ride through event before controlling the DC chopper (9) according to the generated modulation signal (20).
7. A method according to any of the preceding claims, further comprising the steps of:
- monitoring energy absorbed by the DC chopper (9), and
- limiting an amplitude of the modulation signal (20) for the DC chopper (9) in the case that the absorbed energy exceeds a predefined threshold value.
8. A method according to any of the preceding claims, further comprising the step of adjusting a voltage setpoint of the wind turbine (1) in the case that the step of controlling the DC chopper (9) according to the generated modulation signal (20) provides insufficient damping of the power oscillation (19) in the power grid (22).
9. A method according to any of the preceding claims, wherein the wind turbine (1) forms part of a wind farm (21), the wind farm (21) comprising a plurality of wind turbines (1) and a power plant controller (23) being in communication connection with each of the wind turbines (1) of the wind farm (21), and wherein the step of generating a modulation signal (20) for the DC chopper (9) 18 is performed by the power plant controller (23), and wherein the power plant controller (23) dispatches the generated modulation signal (20) to the wind turbine (1).
10. A method according to any of the preceding claims, wherein the modulation signal (20) is added to a power reference signal of the wind turbine (1).
11. A method for controlling a wind farm (21), the wind farm (21) comprising a plurality of wind turbines (1) connected to a power grid (22), and a power plant controller (23), the power plant controller (23) being in communication connection with each of the wind turbines (1) of the wind farm (21), each of the wind turbines (1) comprising a generator (4) and a converter coupled to the generator (4), the converter comprising a generator side inverter (5), a grid side inverter (6) and a DC link (7) interconnecting the generator side inverter (5) and the grid side inverter (6), each wind turbine (1) further comprising a DC chopper (9) connected to the DC link (7), wherein the method comprises the steps of:
- detecting a power oscillation (19) in the power grid (22),
- in response to the detected power oscillation (19), the power plant controller (23) generating a modulation signal (20) for the DC chopper (9) of at least some of the wind turbines (1) of the wind farm (21), the modulation signal (20) being generated in accordance with the detected power oscillation (19),
- the power plant controller (23) dispatching the generated modulation signals (20) to the respective wind turbines (1), and
- the wind turbines (1) controlling their respective DC choppers (9) according to the generated modulation signals (20), thereby providing damping of the power oscillation (19) in the power grid (22) while substantially decoupling the generator side inverters (5) from the power oscillation (19) in the power grid (22).
PCT/DK2021/050350 2020-12-07 2021-12-01 A method for damping power oscillations in a power grid WO2022122100A1 (en)

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