Method and system for connecting a renewable energy source to a grid

Abstract

According to a first aspect of the invention, it is provided a method for connecting a renewable energy source to a grid, through a power converter, the power converter being configured to supply power from the renewable power source to the grid, and further being configured to operate as a grid forming converter and / or a grid following converter, and further the renewable energy source being connected to the grid through an inductor arrangement having an inductance, the inductance of the inductor arrangement being configured to be switchable between at least a first inductance and a second inductance, the second inductance being higher than the first inductance, the method comprising: switching the inductance of the inductor arrangement between the first inductance and the second inductance in dependence of a change of at least one grid condition prevailing in the grid wherein the at least one grid condition is determined as a measurement or estimation of a prevailing grid strength, the method further comprising: switching the inductor arrangement to the first inductance when the measurement of a prevailing grid strength is below a threshold, and switching the inductor arrangement to the second inductance when the measurement of a prevailing grid strength is above the threshold.

Description

[0001] METHOD AND SYSTEM FOR CONNECTING A RENEWABLE ENERGY

[0002] SOURCE TO A GRID

[0003] FIELD OF THE INVENTION

[0004] The invention relates to renewable energy sources, and in particular to a method for connecting a renewable energy source to a grid.

[0005] BACKGROUND OF THE INVENTION

[0006] Renewable energy sources, such as, e.g., wind turbines may be connected to an electrical grid through the use of a power converter, where the power converter converts an input voltage to an output voltage according to prevailing grid requirements. In this way, e.g., AC power generated by a wind turbine with variable frequency may be converted to a DC power to again be converted to an AC power with a frequency that is suitable to the grid. This has the advantage that the wind turbine need not rotate in synchronism with the prevailing frequency of the grid.

[0007] Furthermore, in order to allow a higher penetration of renewable energy sources, such as wind turbines, in an electrical grid there may exist requirements to equip power converters of such renewable energy sources with grid forming properties similar to conventional synchronous generators. In difference to conventional grid following, where the power converter provides a voltage in accordance with the prevailing voltage of the grid, the grid forming may assist in maintaining stability of a grid. This can be addressed, for example, by configuring the renewable power generating units as virtual synchronous machines, VSM.

[0008] With regard to both grid following and grid forming, the operation of the power converter may be subjected to more or less critical operating conditions, where such operating conditions may depend on the currently prevailing grid strength. SUMMARY OF THE INVENTION

[0009] It is an object of the invention to provide a method for connecting a renewable energy source to a grid in order to alleviate problems that may arise when conditions change.

[0010] According to a first aspect of the invention, it is provided a method for connecting a renewable energy source to a grid, through a power converter, the power converter being configured to supply power from the renewable power source to the grid, and further being configured to operate as a grid forming converter and / or a grid following converter, and further the renewable energy source being connected to the grid through an inductor arrangement having an inductance, the inductance of the inductor arrangement being configured to be switchable between at least a first inductance and a second inductance, the second inductance being higher than the first inductance, the method comprising: switching the inductance of the inductor arrangement between the first inductance and the second inductance in dependence of a change of at least one grid condition prevailing in the grid wherein the at least one grid condition is determined as a measurement or estimation of a prevailing grid strength, the method further comprising: switching the inductor arrangement to the first inductance when the measurement of a prevailing grid strength is below a threshold, and switching the inductor arrangement to the second inductance when the measurement of a prevailing grid strength is above the threshold.

[0011] Renewable energy sources are oftentimes connected to a grid through the use of a power converter, where such power converters may operate according to a grid following control scheme and / or a grid forming control scheme.

[0012] The power converter is then controlled to produce an output voltage according to a grid voltage reference, where the grid voltage reference may be a reference provided, e.g., by an electrical grid operator, or an operator of, e.g., a single wind turbine and / or a wind farm, and where one or a plurality of power sources may be used to power a grid.

[0013] Grid following converters synchronize to the currently prevailing grid voltage amplitude and frequency, where the power converter adjusts the output voltage to track an external voltage reference, which is given by the currently prevailing voltage on the electrical grid. In case a fault or an unexpected event occurs in the electrical grid, a grid following power converter will, in principle, continuously adjust the voltage to the currently prevailing voltage on the grid, hence also in situations when the grid voltage deviates from the voltage the grid is intended to maintain. The grid following converter hence does not support the rigidity of the grid in situations of this kind, but simply follows voltage changes as they occur, and continue to provide power produced by the renewable energy power source, e.g., an electrical generator of a wind turbine, to the grid at the voltage amplitude and angle that the grid currently is operating at by adjusting the injected current in accordance therewith so that the injected power remains essentially the same. This also means that the electrical generator will be essentially unaffected when abnormal grid events occur during grid following operation, since the produced power is continuously delivered to the DC link and onwards towards the grid irrespective of the prevailing grid voltage.

[0014] Alternatively, or in addition, a DC link chopper may be used to dissipate the energy, still keeping the machine side unaffected, at least within the capability of the DC chopper. However, again, the apparent drawback of grid following control is the lack of support for system stability.

[0015] However, there may exist requirements to equip power converters that connect renewable energy sources to a grid with properties that exhibit similarities to the behavior of conventional synchronous generators. In this way, stability problems occurring in the grid may be alleviated by the renewable power source. This may be accomplished by operating the power converter according to a grid forming control scheme using a grid forming controller where the power converter instead is configured to act as a grid stabilizer.

[0016] The power converter is then configured to control the output voltage towards a voltage reference and maintain the voltage reference, irrespective of the actually prevailing grid voltage in order to support the maintaining of the intended voltage.

[0017] In principle, grid forming control allows that, for example, a renewable power source such as a wind turbine generator may be set up, e.g., as a local electrical grid, i.e. , operate in an island, off-grid, mode, where the voltage amplitude provided by the grid forming may be adapted according to the particular need for power of the connected consumers. Hence, in grid forming control, a renewable energy power source such as a wind turbine generator may create a power grid by powering otherwise depowered power lines.

[0018] Grid forming hence improves the stability of the grid, but as a result grid events, in particular transient changes, may impose harmful torque transients, e.g., on mechanical components of the renewable energy source, such as wind turbine generator components.

[0019] Furthermore, irrespective of whether the power converter is operating according to a grid forming or a grid following control scheme, the stability of the operation of the power converter may also depend on the strength / weakness of the grid.

[0020] When a grid following converter is utilised, e.g., to inject power of a renewable energy source into a grid a critical operating condition in the system is when the grid is weak. This is because in such situations a small change in actual power provided by the renewable energy source may cause the phase angle and voltage amplitude in the grid to fluctuate to a large extent.

[0021] On the other hand, a critical operating condition may occur for a power converter is operating as a grid forming converter when the grid is strong. The output voltage is fixed in a rotating frame (such as, e.g., a DQ frame) during grid forming. This, however, means that small changes in the voltage amplitude or phase angle in the grid may result in very large variations in power. If the grid is very strong oscillations may occur, and there may also be phase jumps occurring in the grid. This will directly affect the grid forming operation of the power converter, and possibly cause a large surge in active power and reactive power.

[0022] In addition, there may exist situations where the grid sometimes is very strong during operation, but where, e.g., something may suddenly trip and cause the grid to become weak instead.

[0023] According to the invention, it is provided a solution that mitigates problems arising when grid strength varies, and which also adapts to the prevailing grid strength. The renewable energy source is connected to the grid through an inductor arrangement having an inductance. This may oftentimes be case, but according to the invention the inductance of the inductor arrangement is configured to be switchable between at least a first inductance and a second, higher than the first, inductance. Hence the inductance may be switchable between two or more inductance values.

[0024] Furthermore, according to the invention, the inductance of the inductor arrangement is switched between the first inductance and the second inductance in dependence of a change of at least one grid condition prevailing in the grid. A general idea is hence to connect the renewable power source, e.g., using a power converter, to a grid in a manner that may reduce negative impact of grid events that may occur. The reactance can be varied in dependence of the prevailing status of the grid so as to reduce negative impact, e.g., on renewable energy source components and / or a power converter caused by changes in grid conditions, such as grid strength.

[0025] As was discussed above, changes in grid strength may have a large impact on the operation of grid connected renewable power sources, and such impacts may be alleviated according to embodiments of the invention. According to embodiments of the invention, the method further comprises to determine the measure of grid strength through a determination of a short circuit level of the connection point of the renewable energy source to the grid, the method further comprising: switching the inductor arrangement to the first inductance when the measurement of the short circuit level is below a threshold, and switching the inductor arrangement to the second inductance when the measurement of the short circuit level is above the threshold.

[0026] Hence, the inductor arrangement may be switched to a lower inductance when the short circuit level is lower than a threshold, and the grid hence weaker, than when the short circuit level is above the threshold, and the grid hence is stronger. The grid strength may be measured or estimated according to any known principle, where various methods are well described in the prior art.

[0027] According to embodiments of the invention the opposite control is instead carried out, where this may depend, e.g., on whether the renewable energy source is connected to the grid through a power converter currently operating according to a grid forming or grid following control scheme.

[0028] According to embodiments of the invention, the inductor arrangement is configured to be switchable between at least three inductances, and the method further comprises: switching the inductance of the inductor arrangement between the at least three different inductances in dependence of the at least one grid condition prevailing in the grid.

[0029] Consequently, the inductor arrangement may be further adapted to various different prevailing grid conditions by allowing use of a plurality of different inductances, so that an inductance setting can be selected that provides for the most optimal connection of the renewable energy source in dependence on prevailing conditions. For example, different thresholds may be utilized for different inductances.

[0030] According to embodiments of the invention, the renewable energy source is connected to the grid through a power converter, the power converter being configured to supply power from the renewable power source to the grid, and further being configured to operate as a grid forming converter and / or a grid following converter.

[0031] For example, this provides for a solution where, when a grid forming controlled power converter is operating in a strong grid mode there is a suitable inductance between the converter and the grid, whereas in a weak grid mode where the converter needs to act as a voltage source, the inductor arrangement can be bypassed, e.g. in part, or fully.

[0032] The invention hence allows that the inductance arrangement is adapted to the manner in which, and also the state that the power converter is currently operating in.

[0033] The power converter may further be configured to be selectively operated as a grid forming converter and a grid following converter, the method further comprising: switching the inductance of the inductor arrangement in dependence of, in addition to the grid condition prevailing in the grid, whether the power converter is currently operating as a grid forming converter or a grid following converter.

[0034] The inductance may hence be adapted to the current operating state of the power converter as well as the current status of the grid, so that the most suitable setting is used for the prevailing situation.

[0035] It is hence possible to adapt the inductance of the inductor arrangement in dependence not only of grid strength, but also in dependence on the manner in which a power converter connecting the renewable energy source to the grid currently is operating.

[0036] The inductance may hence be switched to an inductance that is suitable for the current grid conditions and the current operation of the power converter, and be switched, e.g., when the operation of the power converter is switched in case this is considered to be better for the operation. Furthermore, according to embodiments of the invention, the inductor arrangement can be set to more than two inductances, and in such cases the inductance to be set can depend even more on both grid condition and current operating mode of the power converter.

[0037] Depending on the operating circumstances, the switching of the inductance of the inductor arrangement may also be controlled such that it is switched to the first inductance when the power converter is operating as a grid following converter, and to the second inductance when the power converter is operating as a grid forming converter.

[0038] According to embodiments of the invention the inductor arrangement is configured to comprise one or more from: at least a first inductor portion and a second inductor portion, wherein at least one of the first and second inductor portion is configured to be bypassed when switching the inductance of the inductor arrangement to the first inductance level; at least a first and a second inductor portion being configured to be selectively connected in parallel or in series, wherein the inductance of the inductor arrangement is switched between the first inductance and the second inductance when switching between the parallel connection of the first and a second inductor portion and the series connection of the first and a second inductor portion.

[0039] The invention hence provides for various different manners of effectuating the switching between the different inductances of the inductor arrangement, where, as stated above, more than two inductor portions may also be utilized to provide for larger possibilities in adaption to the prevailing grid strength, and where such inductor portions may be bypassed or connected in parallel or series according to the prevailing conditions. Also, in case, e.g., bypass, parallel connection and serial connection are available alternatives, the two inductor portions may be utilized to provide for more than two different inductances.

[0040] According to embodiments of the invention, a plurality of renewable power sources are configured to be collectively connected to the grid through the inductor arrangement. The inductor arrangement may hence be utilized to connect, e.g., a wind power plant comprising a plurality of wind turbines to a grid, where the inductance of the connection of all power sources may be controlled collectively, e.g., in dependence of the grid strength.

[0041] According to embodiments of the invention, the inductor arrangement is designed as an inductor having a plurality of connection points to allow adjustment of the inductance of the inductor arrangement by bypassing, and or connecting in parallel, segments of the inductor. The switching between the first inductance and the second inductance, and possible additional inductances, of the inductor arrangement may be carried out by changing the connection point of the inductor arrangement.

[0042] According to embodiments of the invention, the renewable power source may be connected to the grid through the inductor arrangement and a transformer with a secondary side towards the inductor arrangement and a primary side connected to the grid. This provides for use of a secondary voltage being adapted to the operating voltage range of the renewable power source, while allowing connection to a grid having a particular grid voltage.

[0043] According to embodiments of the invention, the at least one grid condition is determined by a grid operator, the grid operator communicating requests for switching between the first and the second inductance of the inductor arrangement in dependence of the determined grid condition. In this way a grid operator may control the connection of various power sources to a grid, and select suitable inductances for the power sources in dependence of the prevailing status of the grid.

[0044] According to embodiments of the invention the at least one grid condition is at least partly based on whether or not the network is an off-grid network, where, e.g., an off-grid network may oftentimes be comparatively weaker, where this hence can be taken into account when selecting inductance of the connection.

[0045] The renewable energy source may comprise at least one wind power installation, the wind power installation comprising a rotor, an electrical machine driven by the rotor, a power converter comprising a machine side converter and a line side converter configured to supply power to a grid, and a DC link electrically connecting the output of the machine side converter to an input of the line side converter, the power converter being connected to the grid through the inductor arrangement. The invention may hence be utilized to reduce, e.g. negative impacts of changing grid conditions when connecting a wind turbine to a grid, and where the connection may be adapted to the prevailing conditions by adapting the inductance of the inductor arrangement to the prevailing situation, and to the current operating state of the power converter.

[0046] The invention also relates to a wind power installation control system, where the wind power installation comprises a rotor, an electrical generator driven by the rotor, and a power converter configured to supply power to a grid. The wind power installation control system is configured to perform the method according to any of the above aspects. Furthermore, a wind turbine generator comprising a wind turbine control system of the discussed kind also form part of the invention.

[0047] BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Embodiments of the invention will be described, by way of example only, with reference to the drawings, in which: Fig. 1 illustrates a wind turbine;

[0049] Fig. 2A illustrates an example of a power system of a wind turbine or a power generating unit;

[0050] Fig. 2B illustrates control components arranged for controlling the generation of active power and reactive power supplied to the grid at the power output of the wind turbine or power generating unit;

[0051] Fig. 3 illustrates an exemplary grid forming control algorithm utilizing a swing equation;

[0052] Fig. 4 illustrates a general principle according to the invention utilizing an inductor arrangement having a plurality of inductor portions;

[0053] Fig. 5 illustrates an exemplary method according to embodiments of the invention.

[0054] DETAILED DESCRIPTION

[0055] The invention will be exemplified for a renewable energy source in the form of a wind turbine in the following, but the invention is not limited to wind turbines as power sources to be connected to a grid utilizing an inductor arrangement according to the invention.

[0056] Fig. 1 shows a wind turbine 100 (WTG) comprising a tower 101 and a rotor 102 with at least one rotor blade 103, such as three blades. The rotor is connected to a nacelle 104 which is mounted on top of the tower 101 and being adapted to drive an electrical generator situated inside the nacelle via a drive train. The rotor 102 is rotatable by action of the wind. The wind induced rotational energy of the rotor blades 103 is transferred via a shaft, and oftentimes, as in the present case, a gearbox, to an electrical generator. The wind turbine 100 is hence capable of converting kinetic energy of the wind into mechanical energy by means of the rotor blades and, subsequently, into electric power by means of the electrical generator. The electrical generator is connected to a power converter which may comprise a generator, or machine, side converter and a line side converter. The machine side converter converts the generator AC power into DC power and the line side converter converts the DC power into an AC power for injection into the grid.

[0057] Fig. 2A shows an exemplary, non-limiting power system 200 of a wind turbine such as the wind turbine 100 of fig. 1 more in detail. The power system 200 comprises an electrical generator, or electric power source, 201 , which, according to the above, is connected to the rotor 102 of the wind turbine 100, where oftentimes the drive train comprises a gearbox (not shown) connecting the rotor to the electrical generator 201 . The power system 200 further comprises a power converter 202. The power converter 202 comprises, according to the present example, a machine side converter 203, a line side converter 204 and a DC link 205 therebetween, where, in use, a DC link voltage lldc is present. The power converter 202 may further comprise a resistor 207 connected with a controllable switch 206. The resistor and switch form a power dissipation device, also known as a chopper 209, for dissipating active power if the need for this arises which may be the case, e.g., if the wind turbine operates in an off-grid, island, mode.

[0058] The DC link 205 comprises one or more DC link capacitors which are charged by the DC output current from the machine side converter 203, and which supplies DC power to the line side converter 204. The output AC current from the line side converter 204 may be supplied via output inductors 206 and possibly, as in the present example, via a wind turbine transformer 208 to the grid or power line 220. The transformer 208 has a secondary side towards the inductor arrangement and a primary side connected to the grid. In this example, the output AC current is a 3- phase current output. Furthermore, harmonic filter capacitors 216 may be arranged between the conductors of the output, which together with the inductors 206, forms a harmonic filter which converts the square wave voltage signals from the line side converter 204 to voltage sinusoidal signals. The present invention relates to the inductors 206, and will be explained in detail further below. With further reference to fig. 2A, the power line 220 may, e.g., be a medium voltage power bus which receives power from other wind turbines 100. The power line 220 may be connected to a high voltage network, e.g. via further transformers. Thus, the power line 220 and one or more power systems 200 of corresponding wind turbines constitutes a wind power plant or park arranged to supply power to a utility grid for distribution of electrical power. The power line 220 and the high voltage network is commonly referred to as a power grid, or grid, herein.

[0059] According to embodiments of the invention, the renewable power source is instead set up, e.g., as a local electrical grid, i.e. , operate in an island, off-grid, mode, where the voltage amplitude may be adapted according to the particular need for power of the connected consumer(s).

[0060] The power converter 202 may be a full-scale converter configured according to different principles including forced-com mutated and line-commutated converters.

[0061] The power system 200 is only schematically illustrated, and the system may as illustrated be a three-phase system. However, principles of the described embodiments apply both to single and multi-phase systems.

[0062] The power converter 202 utilizes pulse width modulation (PWM) for converting the DC power into AC power. The control system 250 is used for controlling the modulation of the power converter 202 and for controlling the active power P and the reactive power Q generated by the power converter 202.

[0063] Fig. 2A shows that the grid voltage Ugrid, here the voltage at the low voltage LV side of the transformer 208, can be measured. The grid voltage Ugrid can be used for controlling the power output of the converter, based on determining the active power Pgrid from grid voltage Ugrid and grid current Igrid. The reactive power Qgrid may similarly be determined from Ugrid and Igrid. Alternatively, the grid voltage Ugrid may be measured on the high voltage HV side of the transformer and corrected based on the turns ratio of the transformer, or the internal voltage magnitude reference Vqref is used instead of the measured voltage llgrid . In an alternative, an internal voltage magnitude reference such as Vqref, Vdqref or Va[3ref may be used for determining Pgrid. The grid current Igrid supplied to the grid can also be measured.

[0064] Fig. 2B shows an example of control components 260 arranged for controlling 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, the control components 260 may be arranged for controlling the output active power Pgrid and the output voltage magnitude at the low voltage side LV, alternatively for controlling the output active power Pgrid and the output reactive power Qgrid at the low voltage side LV. The control components 260 such as the frame conversion unit 266 and the pulse width modulator 265 may form part of the control system 250 or receive control signals from the control system 250.

[0065] References for the active and reactive power may be received from a power plant controller, PPC, or a grid operator, or be determined from active and reactive power references, e.g. from the grid operator.

[0066] As was discussed above, the power converter may be controlled according to different control strategies, where historically a grid following methodology has been utilized. As was also mentioned, an advantage of the grid following control scheme is that since the line side controller controls the voltage in accordance with the prevailing voltage on the grid, the power converter will immediately react to changes that occur on the grid and adjust, e.g., the current so that still the amount of power being injected into the grid corresponds to the amount of power being provided by the generator of the wind turbine.

[0067] However, as was also discussed, there may exist a requirement from, e.g., a grid operator that the wind turbine generator take part in the forming of the grid and assists in the maintaining of the stability of the grid when stability affecting grid events occur. When the wind turbine generator operates according to a grid forming control scheme instead of a grid following control scheme, the line side converter is operated to output a fixed voltage irrespective of any stability influencing negative events occurring on the grid.

[0068] This may be accomplished through the use of a grid forming algorithm, where such grid forming algorithms may be of different kinds. For example, the active power, Pgrid, may be controlled using a virtual synchronous machine angle, denoted 0GFC as is schematically indicated in fig.2B

[0069] The synchronous machine angle 0GFC may be determined according to a grid forming converter scheme such as, but not limited to, a virtual synchronous machine control scheme. A grid forming converter scheme models the inherent rotating mass inertia of conventional synchronous generators. By modelling the inertia, the converter may provide improved grid stability by the grid forming converter model opposing changes in grid frequency. That is, an increase in the grid frequency causes an increase of the kinetic energy and rotation frequency of the inertia but with a response time determined by the inertia. Oppositely, a decrease in the grid frequency causes a decrease of the kinetic energy and frequency of the inertia, but with a response time determined by the inertia. In a wind turbine, the increase or decrease of the kinetic energy of the modelled synchronous generator causes an increase or decrease of the kinetic energy of the rotor 102.

[0070] The synchronous machine angle 0GFC may be defined in a rotating DQ frame defined by the angular position 0GFC and rotating with the frequency coGFC. Based on the synchronous machine angle 0VSM, control signals, i.e. the angle of the modulation voltage signals for the pulse-width-modulator PWM, 265 are determined and transformed into a non-rotating frame such as the a|3 or abc frame. The modulation voltage reference signal controls the active and reactive power Pgrid and Qgrid. The frame conversion unit 266 transforms the control signal from the DQ frame into the a|3 or abc frame and determines the sinusoidal voltage references for the PWM 265. The frame converted output signals from the frame conversion unit 266 are converted by the pulse-width-modulator PWM, 265 into a modulation signal for the grid side converter 204 in order to generate the desired active power and reactive power and / or voltage magnitude.

[0071] The voltage magnitude reference Vqref is provided as a reference for a desired grid voltage or a desired reactive power Qgrid to be generated by the converter 204. The voltage magnitude reference Vqref may be determined based on a difference between a reactive power reference Qref and an actual reactive power Qgrid delivered to the grid. Thus, the reactive power Qgrid to be generated by the line side converter 204 can be controlled based on a voltage magnitude reference Vqref.

[0072] The voltage magnitude reference Vqref may be defined in the DQ frame which rotates with the rotational speed coGFC of the virtual synchronous machine, which in a steady state condition may equal the fundamental frequency such as 50Hz or 60 Hz of the AC grid voltage. The voltage magnitude reference Vqref, or a modification thereof as described in the following, may be converted from the DQ frame to the a|3 or abc frame and outputted from the frame conversion unit 266 as a control signal to the pulse-width-modulator PWM, 265 which determines the modulation signal for the grid side converter 204.

[0073] Fig. 3 illustrates an exemplary grid forming control scheme 300 utilizing an algorithm in the form of a swing equation for determining a voltage component that is used in the determining of a synchronous machine angle 9GFC of a virtual synchronous generator, i.e. for use according to fig. 2B.

[0074] The synchronous machine angle 0GFC is determined based on a virtual synchronous machine control concept which aims at generating a power response that corresponds to the power response from a real synchronous generator, including the inertia of the synchronous generator. According to the grid forming control swing equation 300, a power error Perr is determined as a difference between Pref, which, e.g., may be the power being output by the machine side converter 203 and hence produced by the wind turbine, and the power PLSC being injected into the grid and a damping power PD being determined according to the virtual synchronous model.

[0075] In steady state operation the power provided by the machine side converter 203 will correspond to the power being injected into the grid. However, in response to a change in the grid power PLSC, e.g., due to a decrease in the grid voltage Ugrid and a corresponding increase in the grid current Igrid, the power error Perr becomes non-zero, which causes the angle 9GFC to increase or decrease to reduce the power error Perr. Thus, in response to fluctuations, e.g., in the grid power Pgrid, the synthetic inertial response value becomes non-zero, which causes the virtual machine to either accelerate or decelerate to reach a new equilibrium condition. The new equilibrium is reached when PLSC is again following Pref.

[0076] The virtual synchronous model includes a closed loop where the virtual synchronous machine rotational speed coGFC from the grid forming control swing equation 300 is determined based on a combination of a feedback of the damping power PD, and the power reference Pref for the desired active power output of the wind turbine, and the active grid power PLSC supplied by the wind turbine to the grid.

[0077] The inertial integration model of the main grid forming algorithm 300 is according to the illustrated example implemented as 1 / (2Hs) where H is the inertia time constant and 1 / s is the integration in s-domain where Perr is used as input for the inertial integration model.

[0078] The damping power PD is determined as the difference between the rotational speed of the grid cog and the synchronous machine rotational speed coGFC multiplied with the damping factor Dp. The overall synchronous machine angle 0GFC is determined based on an integration of the synchronous machine rotational speed coGFC according to coO / s, where coO is the rated synchronous generator speed.

[0079] The power converter may hence simulate inertia. In a synchronous machine the inherent inertia is utilized by increasing or decreasing the machine speed. In a wind turbine, the increase or decrease of the kinetic energy of the modelled synchronous generator causes an increase or decrease of the kinetic energy of the rotor 102. If these changes are transient, there will be a corresponding transient change in requirement for kinetic energy of the rotor 102, and thereby also other mechanical components. This may lead to a high power peak or torque peak in the generator side and drive train and consequently cause an undesired increase in the mechanical load.

[0080] For example, as was stated, the rotor carrying the wings, the electrical generator as well as the gearbox interconnecting these components may be sensitive to torque spikes to high degree in such operation. It is therefore highly undesirable to have high torque spikes occurring in the drivetrain since this may provide excessive wear and reduce lifetime expectancy of the components therein.

[0081] According to the invention, it is provided a method of mitigating problems, e.g., when a grid forming control scheme is utilized, but also for other situations, where this is carried out by controlling the inductance of the inductor arrangement connecting the renewable energy source to the grid.

[0082] Fig. 4 illustrates a general principle according to the invention where a renewable power source in the form of a wind turbine generator 401 is connected to a grid 420 in a manner similar to what is disclosed in fig. 2. Hence the generator voltage of the wind turbine is subjected to an AC / DC conversion (not shown), where energy from the DC link is injected into the grid through a line side converter 404 forming part of the power converter. The line side converter 404 is controlled using a grid forming controller 405, e.g. utilizing a swing equation as exemplified in fig. 3 above.

[0083] As was mentioned above, the power converter may be connected to the grid through an inductor arrangement, where, in difference to fig. 2A, the inductor arrangement is configured such that the inductance of the inductor arrangement can switched between at least a first inductance and a second, higher than the first, inductance. This is illustrated in fig. 4 by the inductor arrangement 410 comprising inductor portions 411 , 412. The inductance of the inductor arrangement 410 is arranged to be switchable between a plurality of different inductances, where switching of the inductance of the inductor arrangement between inductances can be carried out in dependence of a change of at least one grid condition prevailing in the grid.

[0084] The grid condition may be configured to be continuously determined, e.g., by the power converter controller or other computing means, so that a change in inductance can be effected as soon as the change in grid condition that indicates a request for switching is detected. For example, a short circuit level may be determined according to methods well described in the prior art.

[0085] According to the present example, the switching of the inductance is carried out using two circuit breakers 413, 414, where the circuit breaker 413 can be selectively set to a closed state connecting the inductor portion 411 directly to the transformer low voltage side. The circuit breaker 413 can also be set to an open state, indicated by dashed line. Furthermore, the circuit breaker 414 can be set to a first state (illustrated by dashed line) where the line side of the inductor portion 411 is connected to the machine side of the inductor portion 412, a second state (the present state in the figure) where the circuit breaker 414 connects inductor portion 412 directly to the output of the line side converter 404, and a third state (indicated by dotted line towards upper right) where the circuit breaker is open. This allows for various inductance settings, where the inductor portions 411 , 412 may selectively be connected in series or in parallel, and either of the inductor portions 411 , 412 may also be bypassed. The illustrated embodiment hence provides for various alternatives in regard of controlling the inductance between the line side converter 404 and the transformer 408, so that the inductance can be adapted to prevailing conditions on the grid, and also in dependence of the current operating state of the line side converter 404. The switching between different inductance levels can be performed by known combinations of serial, parallel or single connections among a number of inductors. Although Fig. 4 shows an embodiment with a grid forming controller, the invention of switching of the inductance of the inductor arrangement, can also be used in embodiments with a classic grid following control.

[0086] Fig. 5 illustrates an exemplary method for switching the inductance of the inductor arrangement in fig. 4 between different inductances in dependence of a change of a grid condition. According to the example, the grid condition being utilized in the control is grid strength, and the currently prevailing grid strength can be determined in any suitable manner, and 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 following or grid forming control scheme. In step 503 an appropriate inductance setting of the inductor arrangement 410 is selected in view of the determinations in step 501 and 502, and in step 504 the inductor arrangement 410 in fig. 4 is set to the setting determined in step 503 through control of one or both circuit breakers 413, 414 in dependence of their previous setting. The method according to fig. 5 may be arranged to be carried out continuously so as to quickly react to changes in the grid.

[0087] The inductance may also be selected, e.g., in dependence of whether or not the network is an off-grid network. As an alternative to determining the grid condition, such as grid strength, when selecting inductance of the inductor arrangement, the at least one grid condition may, according to embodiments of the invention, be determined by a grid operator instead, and where the grid operator may then request a switching from a current setting of the inductor arrangement 410 to another in dependence of a grid condition determined by the grid operator, where this determination also may take operation of various other renewable energy sources into account.

[0088] According to the present example, the switching of the inductance of the inductor arrangement between inductances may hence depend both on grid conditions, and also whether the power converter connecting the renewable power source to the grid is operating as a grid forming or a grid following converter, where, e.g. for stronger grids, the selected inductance may be lower for a grid following converter than a grid forming converter, and vice versa.

[0089] According to embodiments of the invention, the determination of the mode in which the power converter is currently operating may not form part of the method, but the method may be carried out for the particular mode (grid following or grid forming) in which the power converter is currently operating. The power converter may also be designed to always operate according to a particular mode, such as grid forming. Step 502 is hence entirely optional.

[0090] As was mentioned above, the stability of the operation of the power converter depends on the current strength of the grid, and in case the power converter, as in the present example, is operating as a grid forming converter, even small changes in voltage amplitude or phase angle in the grid may result in large variations in power, with oscillations and phase jumps which will directly affect the grid forming converter and possibly cause a surge in active power and reactive power. This is mitigated according to the present example by adapting the inductance of the inductor arrangement 410 connecting the power converter to the grid.

[0091] For example, when the grid is comparatively strong the inductance can be increased to provide a “softer” connection to the grid. This is because when the inductance is higher, e.g., a phase jump will not transplant as much towards, e.g., the generator of a wind turbine, but the higher inductance will smoothen the output of the power converter due to the inherent properties of the higher inductance. Conversely, according to the present example, when it is determined that the grid is comparatively weaker, the grid stability may be increased by, instead, reducing the inductance of the inductor arrangement.

[0092] The strength of the grid may be determined in any suitable manner, and, e.g., through a determination of a short circuit level of the connection point of the power converter to the grid. The short circuit level may be determined according to any suitable known method as is known to the person skilled in the art. Hence the circuit breakers 413, 414 may be controlled to a predetermined setting when the measurement of the short circuit level is below a threshold, to thereby provide a desired inductance, and to another predetermined setting when the measurement of the short circuit level is above the threshold.

[0093] Furthermore, although fig. 4 illustrates an inductor arrangement having two inductor portions, which in itself may provide for two or even more different inductance settings as explained above, the inductor arrangement may also comprise more than two inductor portions to provide for even further possible and different inductances, e.g., to be used for different grid conditions.

[0094] The inductor arrangement may also be designed as an inductor having a plurality of connection points to provide for the various inductor portions, and allow adjustment of the inductance of the inductor arrangement. In such solutions the connection point of the inductor arrangement can be changed, e.g., using suitable circuit breakers, to provide for the switching between inductances according to the invention.

[0095] Furthermore, although the renewable energy source has been exemplified as being connected to a grid through a power converter, it is also contemplated that the renewable energy source may be connected directly to the grid, still utilizing the principles of the invention. According to the above example, a single renewable energy source is connected to a grid. The invention is, however, applicable also in a system where a plurality of renewable power sources are configured to be collectively connected to the grid through the inductor arrangement. That is, the inductor arrangement may designed to be used to collectively connect, e.g., a plurality of wind turbines forming a wind farm, to a grid, where the power injected into the grid by wind turbines may be individually controlled by power converters, but where the power converters in turn are collectively connected to an inductor arrangement. In this case the inductance connecting the group of wind turbines may be controlled in dependence of prevailing grid conditions.

[0096] The present invention is not limited to the above-described embodiments. Instead, the present invention relates to, and encompasses all different embodiments being included within the scope of the independent claims.

Claims

CLAIMS1 . A method for connecting a renewable energy source to a grid, through a power converter, the power converter being configured to supply power from the renewable power source to the grid, and further being configured to operate as a grid forming converter and / or a grid following converter, and further the renewable energy source being connected to the grid through an inductor arrangement having an inductance, the inductance of the inductor arrangement being configured to be switchable between at least a first inductance and a second inductance, the second inductance being higher than the first inductance, the method comprising: switching the inductance of the inductor arrangement between the first inductance and the second inductance in dependence of a change of at least one grid condition prevailing in the grid wherein the at least one grid condition is determined as a measurement or estimation of a prevailing grid strength, the method further comprising: switching the inductor arrangement to the first inductance when the measurement of a prevailing grid strength is below a threshold, and switching the inductor arrangement to the second inductance when the measurement of a prevailing grid strength is above the threshold.

2. A method according to claim 1 , further comprising: determining the measure of grid strength through a determination of a short circuit level of the connection point of the renewable energy source to the grid, the method further comprising: switching the inductor arrangement to the first inductance when the measurement of the short circuit level is below a threshold, and switching the inductor arrangement to the second inductance when the measurement of the short circuit level is above the threshold.

3. A method according to any one of the claims 1-2, wherein the inductor arrangement is configured to be switchable between at least three inductances,the method further comprising: switching the inductance of the inductor arrangement between the at least three different inductances in dependence of the at least one grid condition prevailing in the grid.

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

5. A method according to claim 4, the power converter being configured to be selectively operated as a grid forming converter and a grid following converter, the method further comprising: switching the inductance of the inductor arrangement between the first inductance and the second inductance in dependence of, in addition to the grid condition prevailing in the grid, whether the power converter is currently operating as a grid forming converter or a grid following converter.

6. A method according to any one of the claims 1-5, where in the inductor arrangement is configured to comprise one or more from: at least a first inductor portion and a second inductor portion, wherein at least one of the first and second inductor portion is configured to be bypassed when switching the inductance of the inductor arrangement to the first inductance level; at least a first and a second inductor portion being configured to be selectively connected in parallel or in series, wherein the inductance of the inductor arrangement is switched between the first inductance and the second inductance when switching between the parallel connection of the first and a second inductor portion and the series connection of the first and a second inductor portion.

7. A method according to any one of the claims 1-6, wherein a plurality of renewable power sources are configured to be collectively connected to the grid through the inductor arrangement.

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

9. A method according to any one of the claims 1-8, the renewable power source being connected to the grid through the inductor arrangement and a transformer with a secondary side towards the inductor arrangement and a primary side connected to the grid.

10. A method according to any one of the claims 1-9, wherein the at least one grid condition is determined by a grid operator, the grid operator communicating requests for switching between the first and the second inductance of the inductor arrangement in dependence of the determined grid condition.11 . A method according to any one of the claims 1 -10, wherein the at least one grid condition is at least partly based on whether or not the network is an off-grid network.

12. A method according to any one of the claims 1-11 , wherein the renewable energy source comprises at least one wind power installation, the wind power installation comprising a rotor, an electrical machine driven by the rotor, a power converter comprising a machine side converter and a line side converter configured to supply power to a grid, and a DC link electrically connecting theoutput of the machine side converter to an input of the line side converter, the power converter being connected to the grid through the inductor arrangement.

13. A computer program comprising instructions which, when the program is executed by a computer, cause the computer to carry out the method according to any one of the claims 1 to 12.

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

15. A wind power installation control system, the wind power installation comprising a rotor, an electrical generator driven by the rotor, and a power converter configured to supply power to a grid, the wind power installation control system being configured to perform the method according to any of the claims 1-12.

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

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