Method and system for connecting renewable energy sources to power grid

Abstract

According to a first aspect of the present invention, there is provided a method of connecting a renewable energy source to an electrical grid by a power converter configured to supply electrical power from the renewable energy source to the electrical grid and further configured to form a converter as the electrical grid and / or to follow the converter, and further the renewable energy source is connected to the power grid through an inductor device having an inductance, the inductance of the inductor device being configured to be switchable between at least a first inductance and a second inductance higher than the first inductance. The method comprises: switching an inductance of the inductor device between a first inductance and a second inductance as a function of a change in at least one grid condition present in the grid, the at least one grid condition being determined as a measured or estimated value of the present grid strength, the method further includes switching the inductor device to a first inductance when the measurement of the current grid strength is below a threshold, and switching the inductor device to a second inductance when the measurement of the current grid strength is above the threshold.

Description

Technical Field

[0001] This invention relates to renewable energy, and more particularly to a method for connecting renewable energy to the power grid. Background Technology

[0002] Renewable energy sources, such as wind turbines, can be connected to the power grid using power converters that transform input voltage into output voltage according to existing grid requirements. In this way, for example, AC power generated by a wind turbine with a variable frequency can be converted to DC power, and then back to AC power at a frequency suitable for the grid. This has the advantage that the rotation of the wind turbine does not need to be synchronized with the grid's existing frequency.

[0003] Furthermore, to allow for higher penetration of renewable energy sources (such as wind turbines) into the grid, there may be a requirement to equip the power converters of such renewable energy sources with grid-forming characteristics similar to those of conventional synchronous generators. Unlike conventional grid following, where the power converter provides voltage based on the prevailing voltage of the grid, grid forming helps maintain grid stability. This can be addressed, for example, by configuring renewable energy generation units as virtual synchronous motors (VSMs).

[0004] Regarding grid following and grid formation, the operation of power converters may be affected by more or less critical operating conditions, which may depend on the current grid strength. Summary of the Invention

[0005] The purpose of this invention is to provide a method for connecting renewable energy to the power grid to mitigate problems that may arise when conditions change.

[0006] According to a first aspect of the invention, a method is provided for connecting renewable energy to a power grid via a power converter configured to supply electricity from the renewable energy to the power grid, and further configured to operate as a grid forming converter and / or a grid following converter, and further wherein the renewable energy is connected to the power grid via an inductor device having an inductance configured to switch between at least a first inductance and a second inductance, the second inductance being higher than the first inductance. The method includes: Based on a change in at least one current grid condition in the grid, the inductance of the inductor device is switched between a first inductor and a second inductor. The method further includes: wherein the at least one power grid condition is determined as a measured or estimated value of the current power grid strength; When the measured value of the current power grid strength is below a threshold, the inductor device is switched to the first inductor, and When the measured value of the current power grid strength is higher than the threshold, the inductor device is switched to the second inductor.

[0007] Renewable energy sources are typically connected to the grid using power converters that can operate according to grid follow control schemes and / or grid formation control schemes.

[0008] The power converter is then controlled to generate an output voltage based on a grid voltage reference, wherein the grid voltage reference may be provided, for example, by a grid operator or, for example, by the operator of a single wind turbine and / or wind farm, and wherein one or more power sources may be available to supply power to the grid.

[0009] A grid follower converter synchronizes with the current grid voltage amplitude and frequency, where the power converter adjusts its output voltage to track an external voltage reference given by the current grid voltage. In the event of a grid fault or unexpected event, the grid follower power converter will, in principle, continuously adjust its voltage to the current grid voltage, thus also applicable when the grid voltage deviates from the voltage the grid aims to maintain. Therefore, the grid follower converter does not support grid rigidity in this case, but simply follows the voltage changes that occur and continues to inject power generated by renewable energy sources (e.g., wind turbine generators) into the grid at the current operating voltage amplitude and angle, adjusting the injection current accordingly to keep the injected power substantially constant. This also means that during abnormal grid events during grid follower operation, the generator will be largely unaffected because the generated power will be continuously supplied to the DC link and continue to the grid regardless of the current grid voltage.

[0010] Alternatively or additionally, a DC-link chopper can be used to dissipate energy, at least within the capabilities of the DC chopper, while keeping the machine side unaffected. However, a significant drawback of grid-following control is the lack of support for system stability.

[0011] However, there may be a requirement to equip power converters connecting renewable energy to the grid with characteristics that exhibit behavior similar to that of conventional synchronous generators. In this way, stability issues arising in the grid can be mitigated by renewable energy. This can be achieved by using a grid forming controller that operates the power converter according to a grid forming control scheme, where the power converter is configured to act as a grid stabilizer.

[0012] The power converter is then configured to direct the output voltage toward the voltage reference control and maintain the voltage reference, unaffected by the actual prevailing grid voltage, in order to support the maintenance of the expected voltage.

[0013] In principle, grid formation control allows renewable energy sources, such as wind turbines, to be configured as, for example, a local grid, i.e., operating in an islanded, off-grid mode, where the voltage amplitude provided by grid formation can be adjusted according to the specific power demands of connected users. Therefore, in grid formation control, renewable energy sources such as wind turbines can create a grid by supplying power to previously de-energized power lines.

[0014] Therefore, grid formation improves grid stability, but as a result, grid events, especially transient changes, can impose harmful torque transients on the mechanical components of renewable energy sources, such as wind turbine generator components.

[0015] Furthermore, regardless of whether the power converter operates according to the grid formation or grid follow control scheme, the stability of the power converter operation depends on the strength of the grid.

[0016] When using grid-following converters (e.g., injecting electricity from renewable energy sources into the grid), a critical operating condition for the system is when the grid is weak. This is because, in such cases, even small variations in the actual power supplied by renewable energy can cause significant fluctuations in phase angle and voltage amplitude in the grid.

[0017] On the other hand, when a power converter operates as a grid-forming converter, critical operating conditions may occur when the grid is strong. During grid formation, the output voltage is fixed in a rotating coordinate system (e.g., the DQ coordinate system). However, this means that small changes in voltage amplitude or phase angle in the grid can lead to large changes in power. If the grid is very strong, oscillations may occur, and phase jumps may also occur in the grid. This will directly affect the grid-forming operation of the power converter and may lead to large surges in both active and reactive power.

[0018] In addition, there may be situations where the power grid is very strong at times during operation, but, for example, something may suddenly trip, causing the power grid to weaken.

[0019] According to the present invention, a solution is provided that mitigates problems arising from changes in grid strength, and the solution also adapts to the current grid strength. Renewable energy sources are connected to the grid via inductor devices with inductance. This is typically the case, but according to the present invention, the inductance of the inductor device is configured to switch between at least a first inductance and a second inductance higher than the first inductance. Therefore, the inductance can switch between two or more inductance values.

[0020] Furthermore, according to the invention, the inductance of the inductor device switches between a first inductor and a second inductor based on changes in at least one current grid condition in the grid. Therefore, the general concept is, for example, to connect renewable energy sources (e.g., using a power converter) to the grid in a manner that reduces the negative impacts of potential grid events. The reactance can vary according to the current state of the grid to reduce the negative impacts on renewable energy components and / or the power converter caused by changes in grid conditions (such as grid strength).

[0021] As mentioned above, changes in grid strength can have a significant impact on the operation of grid-connected renewable energy sources, and these impacts can be mitigated according to embodiments of the present invention.

[0022] According to an embodiment of the present invention, the method further includes determining a measure of grid strength by determining the short-circuit level at the connection point of the renewable energy source to the grid, and the method further includes: When the measured short-circuit level is below a threshold, the inductor device is switched to the first inductor, and When the measured short-circuit level is higher than the threshold, the inductor device is switched to the second inductor.

[0023] Therefore, when the short-circuit level is below the threshold, and the power grid is weak, the inductor device can switch to a lower inductance, while when the short-circuit level is above the threshold, and the power grid is strong, it switches to a higher inductance. Power grid strength can be measured or estimated based on any known principles, various methods of which are well described in the prior art.

[0024] According to an embodiment of the invention, the opposite control is performed, which may be based, for example, on whether the renewable energy source is connected to the grid via a power converter currently operating according to a grid formation or grid follow control scheme.

[0025] According to an embodiment of the present invention, the inductor device is configured to switch between at least three inductors, and the method further includes: Based on at least one current grid condition in the power grid, the inductance of the inductor device is switched between at least three different inductors.

[0026] Therefore, inductor devices can be further adapted to various existing grid conditions by allowing the use of multiple different inductors, thereby enabling the selection of inductor settings that provide optimal connectivity for renewable energy based on current conditions. For example, different thresholds can be used for different inductors.

[0027] According to an embodiment of the present invention, renewable energy is connected to the power grid via a power converter configured to supply electricity from the renewable energy to the power grid, and further configured to operate as a grid forming converter and / or a grid following converter.

[0028] For example, this provides a solution in which, when the grid-formed power converter operates in strong grid mode, there is a suitable inductance between the converter and the grid, while in weak grid mode, when the converter needs to operate as a voltage source, the inductor device can be partially or completely bypassed.

[0029] Therefore, the present invention allows the inductor device to adapt to the current operating mode and state of the power converter.

[0030] The power converter can be further configured to operate selectively as a grid forming converter and a grid following converter, and the method further includes: In addition to the existing grid conditions, the inductance of the inductor device is switched depending on whether the power converter is currently operating as a grid-forming converter or a grid-following converter.

[0031] Therefore, the inductor can adapt to the current operating state of the power converter and the current state of the power grid, thus using the most suitable settings for the current situation.

[0032] Therefore, the inductance of the inductor device can be adapted not only to the grid strength, but also to the current operating mode of the power converter that connects renewable energy to the grid.

[0033] Therefore, the inductor can be switched to an inductor suitable for the current grid conditions and the current operation of the power converter, and the inductor can be switched, for example, when the operation of the power converter is switched (if it is considered more advantageous for operation). Furthermore, according to embodiments of the invention, the inductor device can be configured with more than two inductors, and in this case, the inductors to be configured can be even more dependent on the grid conditions and the current operating mode of the power converter.

[0034] Depending on the operating environment, the switching of the inductance of the inductor device can also be controlled as follows: when the power converter operates as a grid follower converter, it switches to the first inductor, and when the power converter operates as a grid forming converter, it switches to the second inductor.

[0035] According to an embodiment of the present invention, the inductor device is configured to include one or more of the following: At least a first inductor portion and a second inductor portion, wherein when the inductance of the inductor device is switched to the first inductance level, at least one of the first and second inductor portions is configured to be bypassed; At least the first and second inductor portions are configured to be selectively connected in parallel or in series, wherein the inductance of the inductor device switches between the first inductor and the second inductor when switching between the parallel connection of the first and second inductor portions and the series connection of the first and second inductor portions.

[0036] Therefore, the present invention provides various ways to realize the switching of an inductor device between different inductors, wherein, as described above, more than two inductor sections can also be used to provide greater adaptability to the strength of the current power grid, and wherein these inductor sections can be bypassed or connected in parallel or series depending on the current conditions. Furthermore, for example, if bypassing, parallel connection, and series connection are available alternatives, two inductor sections can be used to provide more than two different inductors.

[0037] According to an embodiment of the invention, multiple renewable energy sources are configured to be connected to the power grid together via an inductor device. Therefore, the inductor device can be used, for example, to connect a wind farm comprising multiple wind turbines to the power grid, wherein the inductance of the connection of all power sources can be controlled together, for example, according to the grid strength.

[0038] According to an embodiment of the present invention, the inductor device is designed as an inductor having multiple connection points to allow adjustment of the inductance of the inductor device by bypassing and / or connecting the segments (parts) of the inductor in parallel. Switching between a first inductor, a second inductor, and possibly additional inductors in the inductor device can be performed by changing the connection points of the inductor device.

[0039] According to an embodiment of the invention, renewable energy can be connected to the power grid via an inductor device and a transformer, with the secondary side of the transformer facing the inductor device and the primary side connected to the power grid. This provides the use of a secondary voltage adapted to the operating voltage range of renewable energy, while allowing connection to a power grid with a specific grid voltage.

[0040] According to an embodiment of the invention, at least one grid condition is determined by the grid operator, who then communicates a request to switch between a first inductor and a second inductor in the inductor device based on the determined grid conditions. In this way, the grid operator can control the connection of various power sources to the grid and select appropriate inductors for the power sources based on the current state of the grid.

[0041] According to an embodiment of the invention, at least one power grid condition is based at least in part on whether the network is an off-grid network, wherein, for example, off-grid networks may generally be relatively weak, and this can be taken into account when selecting the inductor for connection.

[0042] Renewable energy sources can include at least one wind power generation device, comprising a rotor, a motor driven by the rotor, a power converter including a machine-side converter and a grid-side converter configured to supply power to the grid, and a DC link electrically connecting the output of the machine-side converter to the input of the grid-side converter, the power converter being connected to the grid via an inductor device. Therefore, the present invention can be used to mitigate the negative impacts of varying grid conditions, such as when connecting a wind turbine to the grid, and wherein the connection can be adjusted by adapting the inductance of the inductor device to the prevailing conditions and to the current operating state of the power converter.

[0043] The present invention also relates to a wind power generation device control system, wherein the wind power generation device includes a rotor, a generator driven by the rotor, and a power converter configured to supply power to the grid. The wind power generation device control system is configured to perform the method according to any of the foregoing aspects. Furthermore, wind turbine generators including wind turbine control systems of the type discussed also constitute a part of the present invention. Attached Figure Description

[0044] Embodiments of the invention will be described by way of example only with reference to the accompanying drawings, wherein: Figure 1 A wind turbine is shown; Figure 2A An example of a power system for a wind turbine or power generation unit is shown; Figure 2B The control components are shown, which are arranged to control the generation of active and reactive power supplied to the grid at the power output of the wind turbine or power generation unit; Figure 3 An exemplary power grid formation control algorithm utilizing the oscillation equation is shown; Figure 4 The general principle of an inductor device having multiple inductor sections according to the present invention is shown; Figure 5 An exemplary method according to an embodiment of the present invention is shown. Detailed Implementation

[0045] In the following text, renewable energy will be illustrated by way of wind turbines, but the invention is not limited to wind turbines as a power source connected to the grid using an inductor device according to the invention.

[0046] Figure 1A wind turbine 100 (WTG) is shown, comprising a tower 101 and a rotor 102 having at least one rotor blade 103 (e.g., three blades). The rotor is connected to a nacelle 104, which is mounted on top of the tower 101 and adapted to drive a generator within the nacelle via a drivetrain. The rotor 102 is rotatable by the action of wind. The wind-induced rotational energy of the rotor blades 103 is transmitted to the generator via a shaft (and typically, as in this example, via a gearbox). Thus, the wind turbine 100 is able to convert the kinetic energy of the wind into mechanical energy by means of the rotor blades, and subsequently into electrical energy by means of the generator. The generator is connected to a power converter, which may include a generator-side (or machine-side) converter and a grid-side converter. The machine-side converter converts the generator's alternating current (AC) power into direct current (DC) power, and the grid-side converter converts the DC power back into AC power for injection into the power grid.

[0047] Figure 2A More details are shown, such as those from Figure 1 An exemplary, non-limiting power system 200 for a wind turbine 100. The power system 200 includes a generator or power source 201 connected to the rotor 102 of the wind turbine 100 as described above, wherein the drivetrain typically includes a gearbox (not shown) connecting the rotor to the generator 201. The power system 200 also includes a power converter 202. According to this example, the power converter 202 includes a machine-side converter 203, a grid-side converter 204, and a DC link 205 therebetween, where a DC link voltage Udc is present in the DC link during use. The power converter 202 may also include a resistor 207 connected to a controllable switch 206. The resistor and switch form a power dissipation device, also known as a chopper 209, for dissipating active power when needed, for example, if the wind turbine is operating in off-grid, islanded mode.

[0048] DC link 205 includes one or more DC link capacitors that are charged by the DC output current from the machine-side converter 203 and supply DC power to the grid-side converter 204. The output AC current from the grid-side converter 204 can be supplied to the grid or power line 220 via the output inductor 206 and, possibly, as in this example, via the wind turbine transformer 208. The transformer 208 has a secondary side facing the inductor assembly and a primary side connected to the grid. In this example, the output AC current is a three-phase current output. Furthermore, a harmonic filter capacitor 216 can be arranged between the output conductors, forming a harmonic filter together with the inductor 206 that converts the square wave voltage signal from the grid-side converter 204 into a sinusoidal voltage signal. The invention relates to the inductor 206, which will be explained in detail below.

[0049] Further reference Figure 2APower line 220 may be, for example, a medium-voltage power bus that receives power from other wind turbines 100. Power line 220 may be connected to a high-voltage network, for example, via other transformers. Thus, power line 220 and one or more power systems 200 corresponding to the wind turbines constitute a wind farm or wind farm, which is arranged to supply power to a public power grid for power distribution. In this document, power line 220 and the high-voltage network are generally referred to as a power grid or power grid. According to embodiments of the invention, renewable energy sources are alternatively configured, for example, as a local power grid, i.e., operating in an islanded, off-grid mode, wherein the voltage amplitude can be adjusted according to the specific power demands of one or more connected consumers.

[0050] The power converter 202 can be a full-power converter configured according to different principles (including forced commutation and line commutation converters).

[0051] The power system 200 is shown schematically only, and the system can be a three-phase system as shown. However, the principles of the described implementation are applicable to single-phase and multi-phase systems.

[0052] The power converter 202 uses pulse width modulation (PWM) to convert direct current into alternating current. The control system 250 is used to control the modulation of the power converter 202 and to control the active power P and reactive power Q generated by the power converter 202.

[0053] Figure 2A The diagram shows the measurable grid voltage Ugrid, here the voltage on the low-voltage LV side of transformer 208. The grid voltage Ugrid can be used to control the converter's power output based on the active power Pgrid determined from the grid voltage Ugrid and grid current Igrid. The reactive power Qgrid can be similarly determined from Ugrid and Igrid. Alternatively, the grid voltage Ugrid can be measured on the high-voltage HV side of the transformer and corrected according to the transformer's turns ratio, or an internal voltage amplitude reference Vqref can be used instead of the measured voltage Ugrid. In an alternative, an internal voltage amplitude reference such as Vqref, Vdqref, or Vαβref can be used to determine Pgrid. The grid current Igrid supplied to the grid can also be measured.

[0054] Figure 2BAn example of control components 260 is shown, which are arranged to control the generation of active power Pgrid and reactive power Qgrid supplied to the grid at the power output 270 of the wind turbine 100. That is, control components 260 may be arranged to control the output active power Pgrid and the output voltage amplitude of the low-voltage side LV, or alternatively, to control the output active power Pgrid and the output reactive power Qgrid of the low-voltage side LV. Control components 260, such as coordinate transformation unit 266 and pulse width modulator 265, may form part of control system 250 or receive control signals from control system 250.

[0055] The active and reactive power references can be received from the power plant controller (PPC) or the grid operator, or determined based on active and reactive power references (e.g., from the grid operator).

[0056] As mentioned above, power converters can be controlled according to different control strategies, among which the grid-following method has historically been used. Also as previously stated, one advantage of the grid-following control scheme is that, because the grid-side controller controls the voltage based on the current (dominant) voltage on the grid, the power converter will immediately respond to changes occurring on the grid and, for example, adjust the current so that the amount of power injected into the grid still corresponds to the amount of power supplied by the wind turbine generator.

[0057] However, as mentioned earlier, there may be requests from, for example, grid operators, that wind turbines participate in grid formation and help maintain grid stability in the event of grid events that affect stability. When wind turbines operate according to a grid formation control scheme rather than a grid follow-up control scheme, the grid-side converter is operated to output a fixed voltage, regardless of any negative events occurring on the grid that affect stability.

[0058] This can be achieved by using a grid formation algorithm, which can be of various kinds. For example, active power Pgrid can be controlled using a virtual synchronous motor angle, denoted as θGFC, such as... Figure 2B As shown schematically in the diagram.

[0059] The synchronous motor angle θGFC can be determined based on a grid forming converter scheme, such as, but not limited to, a virtual synchronous motor control scheme. The grid forming converter scheme simulates the inherent rotating mass inertia of a conventional synchronous generator. By simulating inertia, the converter can provide improved grid stability by resisting changes in grid frequency through the grid forming converter model. That is, an increase in grid frequency leads to an increase in the kinetic energy and rotational frequency of the inertia, but the response time is determined by inertia. Conversely, a decrease in grid frequency leads to a decrease in the kinetic energy and frequency of the inertia, but the response time is determined by inertia. In a wind turbine, an increase or decrease in the simulated synchronous generator kinetic energy leads to an increase or decrease in the kinetic energy of rotor 102.

[0060] The synchronous motor angle θGFC can be defined in a rotating DQ coordinate system, defined by the angular position θGFC and rotating at a frequency ωGFC. Based on the synchronous motor angle θVSM, the angle of the control signal used for the pulse width modulator PWM 265, i.e., the modulated voltage signal, is determined and transformed to a non-rotating coordinate system, such as the αβ or abc coordinate system. The modulated voltage reference signal controls the active and reactive power Pgrid and Qgrid.

[0061] The coordinate transformation unit 266 transforms the control signal from the DQ coordinate system to the αβ or abc coordinate system and determines a sinusoidal voltage reference for the PWM 265. The coordinate transformation output signal from the coordinate transformation unit 266 is converted by the pulse width modulator PWM 265 into a modulation signal for the grid-side converter 204 to generate the desired active and reactive power and / or voltage amplitude.

[0062] The voltage amplitude reference Vqref is provided as a reference for the desired grid voltage or the desired reactive power Qgrid generated by converter 204. The voltage amplitude reference Vqref can be determined based on the difference between the reactive power reference Qref and the actual reactive power Qgrid delivered to the grid. Therefore, the reactive power Qgrid generated by grid-side converter 204 can be controlled based on the voltage amplitude reference Vqref.

[0063] The voltage amplitude reference Vqref can be defined in the DQ coordinate system, which rotates at the speed ωGFC of a virtual synchronous motor. Under steady-state conditions, this speed can be equal to the fundamental frequency of the AC grid voltage, such as 50Hz or 60Hz. The voltage amplitude reference Vqref, or its modified form as follows, can be transformed from the DQ coordinate system to the αβ or abc coordinate system and output from the coordinate transformation unit 266 as a control signal to the pulse width modulator PWM 265, thereby determining the modulation signal used for the grid-side converter 204.

[0064] Figure 3An exemplary grid formation control scheme 300 is shown, which utilizes an algorithm in the form of an oscillating equation to determine the voltage component used to determine the synchronous motor angle θGFC of the virtual synchronous generator, i.e., for... Figure 2B The situation is shown.

[0065] The synchronous motor angle θGFC is determined based on a virtual synchronous motor control concept, which aims to generate a power response corresponding to that of an actual synchronous generator (including the generator's inertia). According to the grid formation control swing equation 300, the power error Perr is determined as the difference between Pref (e.g., the power output from the machine-side converter 203 and generated by the wind turbine) and the power injected into the grid PLSC, as well as the damping power PD determined according to the virtual synchronous model.

[0066] In steady-state operation, the power supplied by the machine-side converter 203 corresponds to the power injected into the grid. However, in response to changes in the grid power PLSC, such as due to a decrease in grid voltage Ugrid and a corresponding increase in grid current Igrid, the power error Perr becomes non-zero. This causes the angle θGFC to increase or decrease to reduce the power error Perr. Therefore, in response to fluctuations in, for example, grid power Pgrid, the synthetic inertial response becomes non-zero, causing the virtual machine to accelerate or decelerate to reach a new equilibrium condition. A new equilibrium is reached when PLSC again follows Pref.

[0067] The virtual synchronization model includes a closed loop, in which the virtual synchronous motor speed ωGFC from the grid forming control swing equation 300 is determined based on a combination of the feedback of the damped power PD, the power reference Pref of the wind turbine's expected active power output, and the grid active power PLSC supplied to the grid by the wind turbine.

[0068] According to the example shown in the figure, the inertial integral model of the main power grid formation algorithm 300 is implemented as 1 / (2Hs), where H is the inertial time constant, 1 / s is the integral in the s-domain, and Perr is used as the input of the inertial integral model.

[0069] The damping power PD is determined as the difference between the grid speed ωg and the synchronous motor speed ωGFC multiplied by the damping factor Dp. The total synchronous motor angle θGFC is determined based on the integral of ω0 / s over the synchronous motor speed ωGFC, where ω0 is the rated synchronous generator speed.

[0070] Therefore, the power converter can simulate inertia. In a synchronous motor, inherent inertia is utilized by increasing or decreasing the motor speed. In a wind turbine, an increase or decrease in the kinetic energy of the simulated synchronous generator results in an increase or decrease in the kinetic energy of the rotor 102. If these changes are transient, the kinetic energy demand of the rotor 102, and therefore other mechanical components, will change accordingly. This can lead to peak power or peak torque on the generator side and in the drivetrain, and thus an undesirable increase in mechanical load.

[0071] For example, as mentioned earlier, the rotor carrying the blades, the generator, and the gearbox connecting these components can be highly sensitive to such operating torque peaks. Therefore, high torque peaks in the drivetrain are highly undesirable, as they can lead to excessive wear and reduce the expected lifespan of the components within it.

[0072] According to the present invention, a method for alleviating the above-mentioned problems is provided, which is applicable, for example, when a control scheme is formed using the power grid, but also applicable to other situations. This is achieved by controlling the inductance of an inductor device that connects renewable energy to the power grid.

[0073] Figure 4 The invention illustrates the general principles of a renewable energy source, in the form of a wind turbine generator 401, connected to the power grid 420 in a manner similar to that disclosed in Figure 2. Therefore, the generator voltage of the wind turbine undergoes AC / DC conversion (not shown), wherein energy from the DC link is injected into the grid via a grid-side converter 404, which forms part of the power converter. The grid-side converter 404 is controlled using a grid forming controller 405, for example, utilizing... Figure 3 The example of the oscillation equation.

[0074] As described above, the power converter can be connected to the power grid via an inductor device, wherein, with Figure 2A Unlike other inductors, the inductor device is configured such that its inductance can switch between at least a first inductance and a second inductance higher than the first inductance. This is in Figure 4 The inductor device 410, comprising inductor portions 411 and 412, is shown in the diagram. The inductors of the inductor device 410 are arranged to switch between a plurality of different inductors, wherein the switching of the inductors of the inductor device between different inductors can be performed according to a change in at least one current grid condition in the grid.

[0075] The grid conditions can be configured to be continuously determined, for example by a power converter controller or other computing device, so that the inductance can be changed once a change in grid conditions indicating the need for switching is detected. For example, the short-circuit level can be determined according to methods fully described in the prior art.

[0076] According to this example, inductor switching is performed using two circuit breakers 413 and 414, where circuit breaker 413 can be selectively set to a closed state to directly connect inductor section 411 to the low-voltage side of the transformer. Circuit breaker 413 can also be set to an open state, as shown by the dashed line. Furthermore, circuit breaker 414 can be set to a first state (shown by the dashed line) where the grid side of inductor section 411 is connected to the machine side of inductor section 412; a second state (the current state in the figure) where circuit breaker 414 directly connects inductor section 412 to the output of the grid-side converter 404; and a third state (indicated by the dotted line pointing to the upper right) where the circuit breaker is open. This allows for various inductor configurations, where inductor sections 411 and 412 can be selectively connected in series or in parallel, and either inductor section 411 or 412 can also be bypassed. Therefore, the illustrated embodiment provides various alternatives regarding the control of the inductance between the grid-side converter 404 and the transformer 408, allowing the inductance to adapt to current conditions on the power grid and also to the current operating state of the grid-side converter 404. Switching between different inductance levels can be performed using known combinations of series, parallel, or individual connections between multiple inductors. Although Figure 4 An implementation with a grid forming controller is shown, but the invention of switching inductor device inductance can also be used in an implementation with classic grid following control.

[0077] Figure 5 This illustrates how to adjust power grid conditions based on changes in grid conditions. Figure 4 An exemplary method for switching the inductance of an inductor device between different inductors is described. According to this example, the grid conditions used in the control are grid strength, and the current prevailing grid strength can be determined in any suitable manner. In step 501, it is determined whether the grid is considered weak or strong. In step 502, it is determined whether the power converter is currently operating according to a grid follow or grid formation control scheme. In step 503, given the determinations in steps 501 and 502, an appropriate inductance setting is selected for the inductor device 410, and in step 504, based on their previous settings, the inductor is switched by controlling one or both circuit breakers 413, 414. Figure 4 The inductor device 410 is configured as determined in step 503. According to... Figure 5 The method can be configured to execute continuously in order to respond quickly to changes in the power grid.

[0078] The inductor can also be selected, for example, based on whether the network is an off-grid network. As an alternative to determining grid conditions such as grid strength when selecting the inductor of the inductor device, according to an embodiment of the invention, at least one grid condition can be determined by the grid operator, and the grid operator can then request to switch the inductor device 410 from the current setting to another setting based on the grid conditions determined by the grid operator (which may also take into account the operation of various other renewable energy sources).

[0079] According to this example, the switching of the inductance of the inductor device between different inductors may therefore depend both on the grid conditions and on whether the power converter connecting the renewable energy to the grid operates as a grid-forming converter or a grid-following converter, wherein, for example, for a stronger grid, the inductance selected for a grid-following converter may be lower than that for a grid-forming converter, and vice versa.

[0080] According to embodiments of the present invention, determining the current operating mode of the power converter may not be part of the method, but the method can be performed for a specific operating mode of the power converter (grid following or grid formation). The power converter can also be designed to always operate according to a specific mode (e.g., grid formation). Therefore, step 502 is entirely optional.

[0081] As mentioned above, the stability of power converter operation depends on the current strength of the power grid. If the power converter (as in this example) operates as a grid-forming converter, even small changes in voltage amplitude or phase angle in the grid can lead to significant power variations, accompanied by oscillations and phase jumps. This directly affects the grid-forming converter and can potentially cause surges in both active and reactive power. According to this example, this situation is mitigated by adjusting the inductance of the inductor device 410 that connects the power converter to the grid.

[0082] For example, when the grid is relatively strong, the inductance can be increased to provide a "softer" connection to the grid. This is because when the inductance is high, for example, phase transitions are not transmitted as much to, for example, the generator of a wind turbine, but due to the inherent characteristics of higher inductance, the output of the power converter is smoothed.

[0083] Conversely, according to this example, when the power grid is determined to be relatively weak, the stability of the power grid can be increased by reducing the inductance of the inductor device.

[0084] The grid strength can be determined in any suitable manner, for example, by determining the short-circuit level at the connection point where the power converter is connected to the grid. The short-circuit level can be determined according to any suitable known method known to those skilled in the art. Thus, when the measured value of the short-circuit level is below a threshold, circuit breakers 413, 414 can be controlled to a predetermined setting to provide the desired inductance, and when the measured value of the short-circuit level is above the threshold, controlled to another predetermined setting.

[0085] Furthermore, despite Figure 4 An inductor device with two inductor sections is shown. It can provide two or more different inductor configurations as described above, but the inductor device can also include more than two inductor sections to provide even more possible and different inductances, for example, for different power grid conditions.

[0086] The inductor device can also be designed as an inductor with multiple connection points to provide various inductor sections and allow adjustment of the inductance of the inductor device. In this solution, for example, a suitable circuit breaker can be used to change the connection points of the inductor device to provide switching between inductors according to the invention.

[0087] Furthermore, although renewable energy is exemplified as being connected to the grid via a power converter, it is also considered that renewable energy can be directly connected to the grid, while still utilizing the principles of the present invention.

[0088] According to the example above, a single renewable energy source is connected to the grid. However, the invention is also applicable to systems in which multiple renewable energy sources are configured to be connected to the grid collectively via an inductor device. That is, the inductor device can be designed to collectively connect, for example, multiple wind turbines constituting a wind farm to the grid, where the power injected into the grid by the wind turbines can be individually controlled by power converters, but these power converters are collectively connected to the inductor device. In this case, the inductor connecting the group of wind turbines can be controlled according to the existing grid conditions.

[0089] This invention is not limited to the embodiments described above. Rather, this invention relates to and includes all different embodiments falling within the scope of the independent claims.

Claims

1. A method for connecting renewable energy to a power grid via a power converter, the power converter being configured to supply electricity from the renewable energy to the power grid and further configured to operate as a grid forming converter and / or a grid following converter, and further wherein the renewable energy is connected to the power grid via an inductor device having an inductance, the inductance of the inductor device being configured to switch between at least a first inductance and a second inductance, the second inductance being higher than the first inductance, the method comprising: Based on a change in at least one current grid condition in the grid, the inductance of the inductor device is switched between a first inductor and a second inductor. The method further includes: wherein the at least one power grid condition is determined as a measured or estimated value of the current power grid strength; When the measured value of the current power grid strength is below a threshold, the inductor device is switched to the first inductor, and When the measured value of the current power grid strength is higher than the threshold, the inductor device is switched to a second inductor.

2. The method according to claim 1, further comprising: The method further includes determining a measurement of grid strength by identifying the short-circuit level at the connection point between renewable energy sources and the grid. When the measured short-circuit level is below a threshold, the inductor device is switched to the first inductor, and When the measured value of the short circuit level is higher than the threshold, the inductor device is switched to a second inductor.

3. The method according to any one of claims 1-2, wherein the inductor device is configured to switch between at least three inductors, the method further comprising: The inductance of the inductor device is switched between the at least three different inductors according to the at least one current power grid condition in the power grid.

4. The method according to any one of claims 1-3, wherein the renewable energy source is connected to the grid via a power converter configured to supply electricity from the renewable energy source to the grid, and further configured to operate as a grid forming converter and / or a grid following converter.

5. The method of claim 4, wherein the power converter is configured to operate selectively as a grid forming converter and a grid following converter, the method further comprising: In addition to the existing grid conditions in the grid, the inductance of the inductor device is switched between a first inductor and a second inductor, depending on whether the power converter is currently operating as a grid-forming converter or a grid-following converter.

6. The method according to any one of claims 1-5, wherein the inductor device is configured to include one or more of the following: At least a first inductor portion and a second inductor portion, wherein when the inductance of the inductor device is switched to the first inductance level, at least one of the first and second inductor portions is configured to be bypassed; At least the first and second inductor portions are configured to be selectively connected in parallel or in series, wherein the inductance of the inductor device switches between the first inductor and the second inductor when switching between the parallel connection of the first and second inductor portions and the series connection of the first and second inductor portions.

7. The method according to any one of claims 1-6, wherein a plurality of renewable energy sources are configured to be connected to the power grid together via an inductor device.

8. The method according to any one of claims 1-7, wherein the inductor device is designed as an inductor having a plurality of connection points to allow adjustment of the inductance of the inductor device, the method further comprising: The connection point of the inductor device is changed to switch between a first inductor and a second inductor of the inductor device.

9. The method according to any one of claims 1-8, wherein the renewable energy source is connected to the power grid via an inductor device and a transformer, the secondary side of the transformer facing the inductor device, and the primary side of the transformer connected to the power grid.

10. The method according to any one of claims 1-9, wherein the at least one grid condition is determined by a grid operator, who communicates a request to switch between a first inductor and a second inductor of the inductor device based on the determined grid condition.

11. The method according to any one of claims 1-10, wherein the at least one power grid condition is based at least in part on whether the network is an off-grid network.

12. The method according to any one of claims 1-11, wherein the renewable energy source comprises at least one wind power generation device, the wind power generation device comprising: Rotor; motor driven by a rotor; A power converter that includes a generator-side converter and a grid-side converter, wherein the grid-side converter is configured to supply power to the grid; And a DC link connecting the output of the power converter on the generator side to the input of the grid-side converter, wherein the power converter is connected to the power grid via an inductor device.

13. A computer program comprising instructions that, when executed by a computer, cause the computer to perform the method according to any one of claims 1 to 12.

14. A computer-readable medium comprising instructions that, when executed by a computer, cause the computer to perform the method according to any one of claims 1 to 12.

15. A wind power generation device control system, the wind power generation device including a rotor, a generator driven by the rotor, and a power converter configured to supply power to a grid, the wind power generation device control system being configured to perform the method according to any one of claims 1-12.

16. A wind turbine generator comprising the wind turbine control system according to claim 15.

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