A method to limit current for grid forming wind turbine

Abstract

The invention relates to a method for controlling a wind power installation in order to limit current of the 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 a current to a grid, and a DC link electrically connected to an output of the machine side converter and an input of the line side converter, the line side converter having an output voltage and an output current, the method comprising: determining a grid voltage reference for controlling the line side converter; wherein the grid voltage reference comprises a summation of a first output voltage and a second output voltage; managing slow dynamics by means of a grid forming controller configured to control the first output voltage towards the grid voltage reference, and managing fast dynamics by means of a grid following controller configured to control the second output voltage towards the grid voltage reference; triggering a current limiter mode, when the output current reaches a threshold current value, operating the grid following controller, with a grid current controller (GCC) utilizing: a proportion (P) controller when the output current is below the threshold current value, and a proportional-integral (PI) controller when the output current reaches the threshold current value, maintaining the first output voltage of the grid forming controller to the output voltage level prior to triggering the current limiter, while operating in the current limiter mode, operating the line side converter according to the combination of the output voltage from the grid forming controller and the grid following controller.

Description

[0001] A METHOD TO LIMIT CURRENT FOR GRID FORMING WIND TURBINE

[0002] FIELD OF THE INVENTION

[0003] The invention relates to control of power generation in wind turbines, and particular to limit the current from the converter of the wind turbine.

[0004] BACKGROUND OF THE INVENTION

[0005] In order to allow a higher penetration of renewable energy sources, such as wind turbines, in the electrical grid it has been proposed requirements to equip the power converters of wind turbines with grid-forming properties similar to conventional synchronous generators. These requirements can be addressed, for example, by configuring the renewable power generating units as virtual synchronous machines, VSM.

[0006] When a wind turbine is configured to be operated as a VSM or according to other grid forming control schemes, grid disturbances, such as phase jumps, may lead to high peaks in the requested current from the inverter controller, and thereby a request of currents higher than the system being capable of delivering.

[0007] Accordingly, it is a problem that wind turbines configured operating according to a grid forming control scheme have a limitation in the over current capacity in response to grid disturbances.

[0008] SUMMARY OF THE INVENTION

[0009] It is an object of the invention to improve control of wind turbines comprising a power converter being configured to be controlled according to a grid forming control schemes in order to alleviate problems with overload currents.

[0010] According to a first aspect of the invention it is provided a method for controlling a wind power installation in order to limit current of the 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 a current to a grid, and a DC link electrically connected to an output of the machine side converter and an input of the line side converter, the line side converter having an output voltage and an output current, the method comprising: determining a grid voltage reference for controlling the line side converter; wherein the grid voltage reference comprises a summation of a first output voltage and a second output voltage; managing slow dynamics by means of a grid forming controller configured to control the first output voltage towards the grid voltage reference, and managing fast dynamics by means of a grid following controller configured to control the second output voltage towards the grid voltage reference; triggering a current limiter mode, when the output current reaches a threshold current value, operating the grid following controller, with a grid current controller (GCC) utilizing: a proportion (P) controller when the output current is below the threshold current value, and a proportional-integral (PI) controller when the output current reaches the threshold current value, maintaining the first output voltage of the grid forming controller to the output voltage level prior to triggering the current limiter, while operating in the current limiter mode, operating the line side converter according to the combination of the output voltage from the grid forming controller and the grid following controller.

[0011] According to embodiments of the invention, the method further comprising: gradually decreasing the output of the proportional-integral (PI) controller when the output current is below the threshold current value, until it reaches below a voltage threshold.

[0012] According to embodiments of the invention, the method further comprising: maintaining the current limiter mode until the voltage threshold has been reached. According to embodiments of the invention, the method further comprising: maintaining the current limiter mode for a predetermined period of time after the current level falls below the threshold current value.

[0013] According to embodiments of the invention, the method further comprising: entering a fault ride through (FRT) mode, and injecting a predetermined amount of reactive current into the grid in response to the fault ride through (FRT) mode.

[0014] According to embodiments of the invention, the method further comprising: utilizing a measure of the power supplied to the grid by the line side converter as power reference, processing the measure of the power supplied to the grid by the line side converter, and utilizing the processed measure of the power supplied to the grid by the line side converter as electrical machine power reference.

[0015] According to embodiments of the invention, the processing of the measure of the line side power comprises one or more from: subjecting the measure of the power supplied to the grid by the line side converter to a limitation of the rate of change when transient changes in the power supplied to the grid by the line side converter occur; low pass filtering the measure of the line side power and / or the result of the rate of change prior to determining the electrical machine power reference.

[0016] According to embodiments of the invention, the electrical machine power reference deviates from the measure of the line side power: compensating the difference in DC link voltage in relation to a DC link voltage reference caused by the power difference using a DC link voltage correction component.

[0017] According to embodiments of the invention, slow dynamics means a limited bandwidth with less than 5Hz.

[0018] According to embodiments of the invention, fast dynamics means frequencies above 5Hz. According to embodiments of the invention, the grid forming controller is configured to control the output voltage towards the grid voltage reference by determining a virtual synchronous machine angle.

[0019] According to a second aspect of the invention, a computer program comprising instructions which, when the program is executed by a computer, cause the computer to carry out the method according to embodiments of the invention.

[0020] According to a third aspect of the invention a wind power installation control system arranged to limit current of a 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 a current to a grid, and a DC link electrically connected to an output of the machine side converter and an input of the line side converter, the line side converter having an output voltage and an output current, the wind power installation control system being configured to perform the method according to to embodiments of the invention.

[0021] According to a forth aspect of the invention a wind power installation comprising a control system according to the third aspect of the invention.

[0022] BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Embodiments of the invention will be described, by way of example only, with reference to the drawings, in which:

[0024] Fig. 1 illustrates a wind turbine;

[0025] Fig. 2A illustrates an example of a power system of a wind turbine or a power generating unit; 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;

[0026] Fig. 3 illustrates an example of a combined grid forming and DC link control;

[0027] Fig. 4 illustrates an example of a combined grid forming and grid following;

[0028] Fig. 5 illustrates an example of a combined grid forming and grid following with a freeze function;

[0029] Fig. 6 illustrates an example of the control system;

[0030] Fig. 6a illustrates an zoom in of the current limitation function;

[0031] Fig. 7A illustrates a simulation of a FRT and current limiting event;

[0032] Fig. 7B illustrates a simulation of a FRT and current limiting event;

[0033] Fig. 8 illustrates a method of limiting changes of a generator power reference.

[0034] Fig. 9 illustrates a simulation of a FRT event and the DC link voltage.

[0035] DETAILED DESCRIPTION

[0036] The voltage source behavior of grid forming wind turbine makes their output currents highly dependent on external system conditions. In the event of large disturbances such as voltage drops or phase jumps at the point of connection, conventional synchronous generators can generally supply 5-7 p.u. overcurrent.

[0037] However, semiconductor-based converters used for wind turbine typically can only manage 1 .2-2 p.u. overcurrent, limiting their ability to maintain the voltage profile during normal operation. To effectively navigate and withstand these disturbances, implementing appropriate current-limiting control methods becomes essential for grid-forming wind turbines.

[0038] Fig. 1 shows a wind turbine 100 (WTG) comprising a tower 101 and a aerodynamic rotor 102 with at least one rotor blade 103, such as three blades. The aerodynamic 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 aerodynamic 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 generator having a rotor and a stator. 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 via the stator. The power converter comprises a generator 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.

[0039] Fig. 2A shows an example of a 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 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. The power system 200 further comprises a power converter 202. The power converter 202 comprises 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 island mode.

[0040] 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 210 and possibly via a wind turbine transformer 208 to the grid or power line 220. 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 210, forms a harmonic filter which converts the square wave voltage signals from the line side converter 204 to voltage sinusoidal signals.

[0041] The power line 220 may 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.

[0042] The power converter 202 may be full-scale converter configured according to different principles including forced-com mutated and line-commutated converters.

[0043] The power system 200 is only schematically illustrated and the system may be a three-phase system. However, principles of the described embodiments apply both to single and multi-phase systems.

[0044] The line side converter 204 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 line side converter 204 and for controlling the active power P and the reactive power Q generated by the line side converter 204. Fig. 2A shows that the grid voltage llgrid, here the voltage at the low voltage LV side of the transformer 208, can be measured. The grid voltage llgrid 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 Ugrid. 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.

[0045] 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.

[0046] References for the active and reactive power maey 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. The illustrated system may be utilized in grid forming control, e.g. based on a virtual synchronous machine angle 9VSM for active power control and where the voltage amplitude is provided through reactive power control. The voltage reference is the combination of the voltage amplitude and the voltage angle. 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 line side converter will immediately react to changes that occur on the grid and adjust the current injected into the grid, although within the limitations of the converter.

[0047] The present invention is to limit the current for grid forming turbine, wherein the combination of grid forming control (GFC) with grid following (GFL) operation is implemented (see Fig.3 and Fig. 4).

[0048] Fig. 4 shows the approach, where the GFC 340 and GFL 360 work in parallel, the outputs of the two controller are summed in the summation block 315, where the output voltage from both GFC and GFL is finally combined and sent to the line side converter (LSC) system 310. The LSC system 310 the output various signals, such as the VLalpha-beta used as input to the PWM modulator 265 for converter control. GFC 340 is responsible managing the steady state operation and is configured with a limited bandwidth less than 5Hz, while GFL 360 is specifically designed to handle fast dynamics required by DC voltage regulation and for current limitation.

[0049] Fig. 3 illustrates a general principle of having combined grid following and grid forming controllers. In the figure, the line side converter and the associated control of the line side converter is schematically illustrated by the box 310, also denoted “system”. This box also represents the measurements of the DC link voltage Ude, which, e.g., may be measured on the input side of the line side converter. The system box 310 may also represent the power converter, the generator, the grid, etc. The system box 310 is furthermore responsible for determining measures of the active power PL that goes into the grid as well as the reactive power QL being injected into the grid. These measures may, for example, be determined from the grid voltage Ugrid and the grid current Igrid, which, as may be measured according to the above or according to alternative voltages measurements as was also stated. The active and reactive currents may also be established from these measures.

[0050] 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. 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, 0VSM as is schematically indicated in fig.2B. In short, the synchronous machine angle acceleration (the double-time derivative of 0VSM) corresponds to the difference between a power reference Pref for a desired power output of the wind turbine and a grid power Pgrid that is actually supplied by the wind turbine to a power grid.

[0051] The synchronous machine angle 0VSM may be determined according to a grid forming converter scheme such as 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.

[0052] The synchronous machine angle 0VSM may be used to transform the signals from a rotating DQ frame into a non-rotating frame such as the a|3 or abc frame, or vice- versa. Based on the synchronous machine angle 0VSM and voltage magnitude reference Vqref, control signals for the desired active power and reactive power are determined.

[0053] The synchronous machine angle 0VSM may be defined in a rotating DQ frame defined by the angular position 0VSM and rotating with the frequency coVSM. 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.

[0054] 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.

[0055] 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. The voltage refence can also be a part of the grid forming.

[0056] The voltage magnitude reference Vqref may be defined in the DQ frame which rotates with the rotational speed coVSM of the virtual synchronous machine, which in a steady state condition may equal the fundamental frequency such as 50Hz 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. With regard to the DQ frame, it is to be noted that in the present description a generator notation of, e.g., Id, Iq, lid, llq, etc. is utilized, which differs from the general motor notation of, active and reactive currents and voltages.

[0057] As was mentioned, in a conventional synchronous machine the inherent inertia may be utilized for purposes of stabilizing the grid. 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.

[0058] Depending on the governing grid codes for a wind turbine power plant, the power converters of wind turbines may be required to be operated as virtual synchronous machines, at least for grid currents Igrid below a given overcurrent threshold. If the overcurrent threshold is high, grid disturbances like phase jumps 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. It is an objective of the present invention to provide a method which mitigates over currents in the converter system.

[0059] There are of course many conditions in the grid which may lead to situations where the current limiter is triggered. In principle the current outputted from the line side converter (LSC) is dictated by the voltage difference between the grid voltage and the LSC voltage, as the current flows in the grid choke of the converter, so if the grid voltage changes fast the converter controller can’t react fast enough. Such event may be phase jumps in the grid voltage, where the phase of the grid voltage at the terminals jumps, often because the impedance in the grid is changed as a result of branches in the grid has been disconnected or connected. Another source can be fault ride through (FRT) event where the voltage drops due to grid faults or where the grid voltage goes into a high voltage situation. FRT events, or at least for LVRT events, require the power source, either a wind turbine or a PV system, to deliver rated reactive current during the fault, this causes large reactive currents, whereas phase jumps often require steps in the active current as a result of the impedance between the converter and the grid being inductive.

[0060] Fig. 5 shows a more detailed embodiment of the invention, with similar blocks as in Fig. 4, with the GFC block 540 and the GFL block 560. The output Vgfc from the GFC block 540, is applied to a freeze block 545, before summing the outputs. The Freeze block maintain the Vgfc output level, once the current exceeds the threshold value, the current limiter is triggered into action. As the current limiter activates, the GFC output voltage is frozen, while the GFL output voltage is adjusted to modify the total output voltage to the modulator for limiting the current, the summed outputs are sent to the LSC system 510.

[0061] Once the current surpasses the threshold value, the current limiter is triggered into action. As the current limiter activates, the GFC output voltage is maintained by a freeze block 545, which is triggered by an ActivateFreeze input, the GFC output voltage is kept at its level due to the freeze function. While the GFL output voltage 561 is correspondingly adjusted and added to the GFC output voltage 541 , thereby reducing the overall output voltage to the modulator 265 and limiting the current.

[0062] Fig. 6 shows a full implementation in the system, where the GFL with the GCC block is shown in an enlarged version in Fig. 6A. The implementation of grid current controller (GCC) within the GFL 560 is in Fig 6A. There are two branches, one for the active current reference ( Iqref) 600, and one for the reactive power reference (Idref) 605. A current saturation algorithm block (CSA) 610 implements a D / Q current vector limitation, based on maximum allowed amplitude (with priority for reactive current supply). Each branch has a comparator 620 and 625, which generates the current errors. During normal operation, i.e. when not in current limiting mode, the GCC utilizes proportional (P) controllers 630 and 635. The selection elements 621 and 626, which can also been seen as the current limiters, they guides the errors signals depending on whether the current limit threshold has been exceeded or not. The active and reactive current references are selected to allow the flow of current resulting from the GFC control action.

[0063] During current limiting events, the GCC utilizes a proportional-integral (PI) controller 640 and 645 to empower the GCC control of the output current. The active and reactive current references are setup in a current reference saturation algorithm to restrict the total current to the maximum threshold value and to meet specific requirements.

[0064] When the current exceeds the specified threshold value, the integral gain from the PI controllers 640, 645 are enabled and the integrator output is initialized with a value configured to provide fast current limiting action.

[0065] When the current falls below the specified threshold, the integral gain from the PI controller is disabled and the integrator output is gradually reduced until the overall GFL voltage reaches below a prescribed voltage threshold, and the current limiter event ILim_Active is ended. The outputs from the controllers 630, 635, 640, 645 is again guided out of its respective controllers by selection blocks 631 , 636 depending on the current level. Following this, the GCC smoothly transitions from using a PI controller to use a P controller. This is not specifically reflected in Fig. 6, but the switch over in the selection blocks 631 , 636 ensure a smooth transition out of the current limiting mode.

[0066] The primary objective of system is to safeguard grid-forming wind turbines while preserving Grid Forming behavior unless when current limiting is required, thereby enhancing safety measures in operation. The present disclosure also ensures that the injection of reactive current meet the requirements in the event of large disturbances. The present disclosure also ensures that the injection of reactive current meet the requirements in the event of large disturbances. As already mentioned earlier, during FRT event there is a requirement from various grid operators, that the power producing system such as wind turbines has to deliver rated reactive current. Therefore, the input IdrefFRT 605 in Fig. 6, it is important that the system supports the grid by injection of reactive power, which serves the purpose of maintaining the voltage level, but is also feeding in current to ensure protection relays are tripped in order to clear fault up in the electrical power grid.

[0067] Without this current limiting method and system, the converter system would suffer from disconnection from the grid as the combined requirements of operating in a grid forming mode and injecting large amounts of reactive current, would push the system beyond its capacity.

[0068] Figure 7A and Figure 7B show two different simulations of events involving fault ride through (FRT) events, both showing the converter out voltage in alpha I beta system in the top graph, meaning the three voltage vectors have been projected into a two-vector system with 90 degrees displacement, known as a Clark transformation. The second graph shows the voltage output of the grid follow controller (560). The third graph shows the output current of the converter, with the current threshold value in the dotted line. The bottom graph shows the status of the control status flags FRT_Active and ILim_Active.

[0069] Fig. 7A shows an event where the grid voltage drops a time 0.9 T1a, as the grid voltage drops the FRT mode is activated (FRT_Active), this causes the current to be increased from 1 p.u. to exceed the threshold at around 1 .4 p.u. as the system is requested to provide reactive current. FRT mode could also involve injection of active current or a combination of both active and reactive current. The current threshold is exceeded, so the current limiter function is also activated (ILim_Active), thus, the GFL voltage output is increased as the integrator part of the PI controller increases its output, see 2ndgraph. The large transient in GFL voltage is due to DC voltage variation. Once DC voltage is stabilized, i.e. the voltage error is reduced, then the GFL voltage is minimized. The current level is only momentarily at the threshold level, and therefore is the current limiter function deactivated at time ~1 T2a, the GFL voltage is then starting to decrease again, the noise in the current signal causes the GFL voltage output to remain stable until the FRT event ends time 5.4 T3a, where the voltage returns to about 1 p.u. and thus the current also drops as the injection of reactive current ends. The drop in the current level gives some disturbance in the GFL voltage, here it is only the P controller that is active. The FRT mode is active until time 6 T4a, as it has some other functions in the system, which is not relevant for this invention.

[0070] Fig. 7B shows a different event also combined with a FRT event, at time 0.9 T1b the voltage drops and the current is increased do to reactive current injection this leads to the current level reaching the threshold and the current limiter mode is activated. In this event the current level remains at its threshold value until time 5.4 T2b. When looking at the GFL voltage one can see the voltage being increased because of the integrator until time 5.4 T2b, when the FRT event ends. Then the current falls below the specified threshold, the output voltage from GFL gradually decreases until it reaches below a voltage threshold and the current limiter mode ILim_Active is ended, this is in order to ensure transient free transfer in and out of the current limiter mode, with a smoothly transitions from its PI control mode to function as a P controller.

[0071] The primary objective of this approach is to safeguard grid-forming wind turbines by limiting the current to its designated reference, thereby enhancing safety measures in operation.

[0072] Both events in the Figs 7A and 7B, show that the current limiter function works and that the higher frequency content of the is handled by the GFL branch of the line side converter controller as explained earlier.

[0073] Fig. 8 shows a broader implementation of a converter system in a wind turbine operating in Grid Forming mode, in which the current limiter function can be applied to, the present invention is not limited to be implemented in the system of Fig. 8. During constant operating conditions, the power PL being injected into the grid by the line side converter will be essentially the same as the power PMSC being provided by the machine side converter. However, the power being injected into the grid PL may need to be compensated for, e.g., power being drawn by auxiliary devices of the wind power converter and / or losses and / or other powers. According to the present example, there is therefore a generator active power controller GAPC 320 that takes as input the power reference Pref_VMP, which represents the desired power output of the wind turbine, and the power PL being injected into the grid and outputs a machine side active power reference PMSC_ref. Hence the actual power to be produced by the electrical generator may be set to the desired power output compensated for losses etc. so that the actually injected power PL corresponds to the power reference Pref_VMP. The machine side active power reference PMSC_ref is hence used to control the electrical generator, using the generator power control GPC 330 and the machine side converter to obtain the desired power on the DC link.

[0074] References for the active Pref_VMP as well as for the reactive power QLref_VMP 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. The power reference may reflect the power that is extracted from the wind, and hence may change e.g. in accordance with what the wind turbine generator is currently producing. Thus, the power reference Pref_VMP may reflect e.g. power changes caused by changes in the wind. In this way a power balance on the drivetrain is also obtained. As an alternative to the turbine reference power being reference to GAPC either PL or Pref_VMP can be selected as refence in GAPC, and also a combination may be utilized as power reference.

[0075] As was stated, the line side converter controls the output voltage based on a voltage input, such as a voltage Va[3. This voltage input Va[3 consists, according to the present example, of two voltage components which will be explained in the following. The power PMSC provided to the DC link by the machine side converter is utilized by a grid forming control GFC 340 to determine an output voltage VGFC,a[3 to be output by the line side converter to obtain the desired power output, where, e.g. a grid forming control utilizing a virtual synchronous machine angle may control the active power being injected into the grid. It is to be noted that the output voltage component Va[3, GFC may be generated according to any suitable grid forming control scheme, and hence not limited to controlling the line side converter according to a virtual synchronous machine. For example, virtual oscillator grid forming, and / or moving average filtering grid forming may be utilized as alternatives to controlling the line side converter as a virtual synchronous machine. The grid forming control GFC 340 carries out the required calculations based on a power PLref which comprises the power being output by the machine side converter, and a further component PDCIref which is described below. In addition, a reactive power reference QLref is also used.

[0076] The control according to GFC 340 in fig. 3 is hence set out to control the line side converter according to a voltage reference. Use of this control alone, however, as is in general the case, exhibit drawbacks as explained above, since the electrical generator is no longer decoupled from the grid from a transient point of view in the same manner as when being controlled according to grid following control scheme because the rest of the system has to adapt to the control of the line side converter. The generator power control GPC will adapt to the power PL currently being input into the grid by the line side controller.

[0077] During normal operation there will be a balance between the power being output by the line side converter and the power being produced by the electrical machine. The power reference from the turbine Pref_VMP is respected since this reference provides information regarding the amount of power that can be injected into the grid according to the current power being extracted by the wind. However, if transients arise in the grid, the grid forming control GFC calls for the grid voltage to be maintained, and this will cause transients in the current that will be injected into the grid as a result of the maintaining of the voltage reference.

[0078] Thereby, there will also be a transient change in the power PL being injected into the grid by the line side converter, and as a consequence there will be a transient change in torque request from the electrical generator since a change in current output by the line side converter will be directly reflected by a change in the request for torque by the electrical generator. The DC link is in general very limited in terms of energy storage, and hence cannot account for sudden current changes. This means that power provided by the generator must immediately be delivered to the grid so that the DC link voltage can be kept at a desired level. The DC link voltage must be maintained within a lower and an upper limit in order to keep the converter operational, and this can hence only be ensured by keeping an energy balance between the electrical generator and the power injected into the grid. As a result transients will arise in the generator power / torque.

[0079] Fig.3 also illustrates a reactive power control GPRC, which is utilized for reactive power control. The reactive power control loop is required to ensure that reactive power is not unnecessarily produced. Still, both the active and the reactive power are required to form the overall output power, and it is therefore necessary to produce reactive power, e.g. in order to control the virtual electrical machine angle.

[0080] The reactive power control GPRC 370 takes as input a reactive power reference Qref_VMP, which represents the desired reactive power output of the wind turbine, which may be determined in a manner similar to the active power reference, and the reactive power QL being injected into the grid. The reactive power control GPRC 370 outputs a general reactive power reference QL_ref, which is provided to the grid forming control 340 and form part of the output voltage generated by the grid forming control. The reactive power control GPRC also outputs a reactive power reference Qref being input to the grid following control to be controlled in a manner similar to the active power grid following control, and which form part of the resulting output voltage component from the grid following control.

[0081] Fig. 8 illustrates a further embodiment according to embodiments of the invention. GAPC 420 and GPC 430 are similar to fig. 3, and therefore not discussed further. The embodiment of Fig. 8 can been seen as a system which allows several control features to be handled by the GFL branch 460. Where the current limiting function can be implemented as one of them.

[0082] With regard to the DC link control DCC 450 the determination 451 of the DC link voltage error signal lldcerr (squared) is illustrated, as well as the l-controller 452 for generation of the integral part PDCIref of the error signal, this may also be implemented without the integral part, i.e. a normal P-controller, and also the P- controller 453 for generation of the proportional part PDCPref of the error signal. These signals are used as described above with reference to fig. 3.

[0083] The proportional power component PDCPref being output by the DC link control DCC 450 is, as above, provided to a grid following control, schematically indicated by 460, where the power component PDCPref is first added together with the power PL being injected into the grid. The reason for this is that only the total currents Id and Iq can be measured, i.e. the combination of the outputs from the grid forming control and the grid following control. It is not possible to separately measure currents relating specifically to the grid forming control and the grid following control, respectively. The total power obtained from this addition, forming a grid following reference power Pgfl_ref, is then divided by the voltage II to form a grid following reference current lgfl_ref. The current Iq is subsequently subtracted from the grid following reference current lgfl_ref. This will have as result that the grid following GFL control component will follow the power reference Pgf l_ref which hence is representation of the power needed to correct the DC link voltage. The resulting current is subjected to a Pl-controller for generating a voltage component that then is utilized by a voltage generator 461 to generate the output voltage Va[3, GFL to be combined with the output Va[3, GFC from the grid forming control, and which compensates the current to be output by the line side converter so that the DC link voltage can be maintained. It is to be noted that this Pl-controller may alternatively be a P-controller.

[0084] The grid following control 460 also illustrates a similar generation of a reactive power voltage component which operates in the same manner and which also forms part of the grid following control output voltage being generated in the voltage generator 461 .

[0085] In addition to illustrating an example the grid following control more in detail, Fig. 8 also illustrates an example of a grid forming control algorithm 440 for determining a synchronous machine angle 9GFC of a virtual synchronous generator.

[0086] The synchronous machine angle 0GFC is determined based on a virtual synchronous machine control concept which aims at generating a power response which corresponds to the power response from a real synchronous generator, including the inertia of the synchronous generator.

[0087] A power error Perr is determined as a difference between PLref as defined above and the power PL being injected into the grid and a damping power PD determined according to the virtual synchronous model.

[0088] In response to a change in the grid power PL, e.g. due to an 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 0VSM to increase or decrease to reduce the power error Perr. Thus, in response to fluctuations in e.g. 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 PL is again following PLref.

[0089] The virtual synchronous model includes a closed loop where the virtual synchronous machine rotational speed from the grid forming control coGFC is determined based on a combination of a feedback of the damping power PD, and the power reference PLref for the desired active power output of the wind turbine, and the active grid power PL supplied by the wind turbine to the grid.

[0090] The inertial integration model 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.

[0091] 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 damping factor Dp dampens the performance of the control loop of the grid forming control.

[0092] The 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.

[0093] Fig. 8 further illustrates a decoupling virtual impedance 470, which may or may not be used, and is mainly used when there is a strong grid. In such situations a small variation in synchronous machine angle 0GFC may result in a high power difference. The virtual impedance 470 alters the voltage a little to provide a decoupling between the active and reactive power loop. This improves system stability. The figure also shows a current limiter 480 which may be used to ensure that currents do not exceed set limits by keeping the currents below such limits. During normal operation no such limitation is in general needed. The virtual impedance may be determined for one or more phases. By increasing the resistive and / or the reactive value of the virtual impedance, the output current drawn from the output of the line side inverter can be reduced. The virtual impedance may be used to reduce acceleration of the virtual synchronous generator during overcurrent situations and thereby the output current Igrid of the line side inverter. Fig. 8 further illustrates the reactive power control 490 of the grid forming control, which in a manner known per se provides the voltage amplitude, while the active power control provides the voltage angle. Hence the active power control and the reactive power control together form the voltage amplitude and angle that is output by the grid forming control. The reactive control is not discussed in detail, since the invention relates to active power control llref in the reactive power control could be a local setting, or be received from an external source.

[0094] Furthermore, as was indicated above, the invention allows for utilizing limits regarding, e.g., rate of change of the power / torque produced by the electrical generator, where this may be accounted for when determining the power reference being used by the machine side controller.

[0095] According to embodiments of the invention, the machine side power reference may instead be set following the applying of limits regarding, e.g., rate of change of the power / torque produced by the electrical generator, so that this may be accounted for when determining the power reference that form the basis for the machine side controller. Hence, according to embodiments of the invention, it may be further ensured that the electrical generator and drive train are not subjected to harmful transients, or at least that the transients are reduced through this control, but where still the grid following control according to the invention handles the differences in DC link voltage that this may give rise to.

[0096] Fig. 9 shows what happens to the DC link voltage during a the FRT event. The lowest graph shows the UDC voltage level and its reference voltage, where a drop in the line voltage, see first graph, at time 0.91 causes a disturbance in the UDC, and the current in the third graph triggers the current limitation at time 0.925. The second graph shows the activity in the GFL voltage.

[0097] Furthermore, with regard to the illustrated powers, i.e. , machine side power and line side power these may, as is known to a person skilled in the art, comprise power components that does not form part of the usable energy being injected into the grid. Such power components may, for example, comprise power losses, drivetrain damping power, power being consumed by auxiliary components etc. For example, the drivetrain damping power may be an AC power where ,e.g., a frequency in the order of 1 to 3 Hz may be utilized in an attempt to dampen low- frequency oscillations being inherent in the drivetrain when in use, where such oscillations may be a result, e.g., of drivetrain resonance frequencies. Power components of this kind may be comprised in the calculations when determining e.g. power reference levels in conventional grid forming control.

[0098] Part of the functionality of the generator control is to extract the desired average power and also dampen the drivetrain to account for the low-frequency oscillations resulting from resonance frequencies of the drivetrain.

[0099] A further advantage with the invention is that with regard to grid forming there may be a frequency interval in which no control is to be carried out and this also means that it may impose difficulties with regard to propagating e.g. the drive train damping power into the grid using the very slow control of up to e.g. a maximum 5 Hz that is still available, and the grid forming control which hence then it would also be used for the very slow variations of e.g. 1 to 2 Hz of the drive train damping power. This may therefore be difficult to fully account for in grid forming control algorithms.

Claims

CLAIMS1 . A method for controlling a wind power installation in order to limit current of the 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 a current to a grid, and a DC link electrically connected to an output of the machine side converter and an input of the line side converter, the line side converter having an output voltage and an output current, the method comprising: determining a grid voltage reference for controlling the line side converter; wherein the grid voltage reference comprises a summation of a first output voltage and a second output voltage; managing slow dynamics by means of a grid forming controller configured to control the first output voltage towards the grid voltage reference, and managing fast dynamics by means of a grid following controller configured to control the second output voltage towards the grid voltage reference; triggering a current limiter mode, when the output current reaches a threshold current value, operating the grid following controller, with a grid current controller (GCC) utilizing: a proportion (P) controller when the output current is below the threshold current value, and a proportional-integral (PI) controller when the output current reaches the threshold current value, maintaining the first output voltage of the grid forming controller to the output voltage level prior to triggering the current limiter, while operating in the current limiter mode, operating the line side converter according to the combination of the output voltage from the grid forming controller and the grid following controller.

2. Method according to claim 1 , further comprising: gradually decreasing the output of the proportional-integral (PI) controller when the output current is below the threshold current value, until it reaches below a voltage threshold.

3. Method according to claim 2, further comprising: maintaining the current limiter mode until the voltage threshold has been reached.

4. Method according to claim 1 , further comprising: maintaining the current limiter mode for a predetermined period of time after the current level falls below the threshold current value.

5. Method according to proceeding claims, further comprising: entering a fault ride through (FRT) mode, and injecting a predetermined amount of reactive current into the grid in response to the fault ride through (FRT) mode.

6. Method according to any one of the claims 1-5, the method further comprising: utilizing a measure of the power supplied to the grid by the line side converter as power reference, processing the measure of the power supplied to the grid by the line side converter, and utilizing the processed measure of the power supplied to the grid by the line side converter as electrical machine power reference.

7. Method according to claim 6, wherein the processing of the measure of the line side power comprises one or more from: subjecting the measure of the power supplied to the grid by the line side converter to a limitation of the rate of change when transient changes in the power supplied to the grid by the line side converter occur;low pass filtering the measure of the line side power and / or the result of the rate of change prior to determining the electrical machine power reference.

8. Method according to claim 6 or 7, wherein, when the electrical machine power reference deviates from the measure of the line side power: compensating the difference in DC link voltage in relation to a DC link voltage reference caused by the power difference using a DC link voltage correction component.

9. Method according to claim 1 , wherein slow dynamics means a limited bandwidth with less than 5Hz.

10. Method according to claim 1 , wherein fast dynamics means frequencies above 5Hz.11 . Method according to any one of the claims 1 -10, wherein: the grid forming controller is configured to control the output voltage towards the grid voltage reference by determining a virtual synchronous machine angle.

12. 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 11 .

13. 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 11 .

14. A wind power installation control system arranged to limit current of a 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 a current to a grid, and aDC link electrically connected to an output of the machine side converter and an input of the line side converter, the line side converter having an output voltage and an output current, the wind power installation control system being configured to perform the method according to any of the claims 1-11.

15. A wind power installation comprising a control system according to claim 14.

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