Grid forming control in a power system comprising a hydrogen electrolyzer stack

Abstract

According to the invention it is provided a method for controlling a grid connected power converter having a DC side with a DC link and an AC grid side, and being configured to control power supply to a hydrogen electrolyzer stack. The power supply to the hydrogen electrolyzer stack is controlled by controlling the DC link to thereby control hydrogen production. The method comprises: determining a grid voltage reference; providing a grid forming control for controlling at least the phase angle of the voltage of the power converter using a grid forming controller, operating according to a grid forming algorithm, the grid forming controller being configured to emulate inertia through control of the voltage of the power converter towards the grid voltage reference; the grid forming controller emulating inertia by charging and discharging an inherent capacitance of the electrolyzer stack; monitoring at least one operating parameter of the hydrogen electrolyzer stack; and limiting a change in charging level of the inherent capacitance based on the monitored operating parameter of the electrolyzer stack.

Description

[0001] GRID FORMING CONTROL IN A POWER SYSTEM COMPRISING A

[0002] HYDROGEN ELECTROLYZER STACK

[0003] FIELD OF THE INVENTION

[0004] The invention relates to control of grid connected power converters, and particular to grid connected power converters being utilized in grid forming control in a power system comprising a hydrogen electrolyzer stack.

[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 may be converted to a DC voltage to again be converted to an AC voltage that is adapted to the grid voltage.

[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 that provide grid stabilizing properties in a manner corresponding 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] Use of renewable energy may also be further facilitated using power-to-X conversion and reconversion, where, e.g., hydrogen electrolyzer stacks may be utilized to store energy for later use. Such hydrogen electrolyzer stack may be connected to a grid, such as a local grid, off-grid network, through a power converter that converts the grid voltage to a DC voltage powering the hydrogen electrolyzer stack, where the power consumption, and thereby load on the grid, is controlled by controlling the DC voltage. The hydrogen electrolyzer stack may be configured to generate hydrogen, where the hydrogen may be used in a fuel cell to generate electricity.

[0009] When a power converter connecting a renewable energy power source, such as 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 transient voltage and / or current peaks with potentially harmful transient mechanical loads of the renewable energy power source as result.

[0010] SUMMARY OF THE INVENTION

[0011] It is an object of the invention to provide a grid forming control for an electrical grid, in particular for an electrical grid where a power converter is utilized to connect a hydrogen electrolyzer stack to the grid, where the grid forming control may alleviate problems with transient mechanical loads of, e.g., renewable power sources to the grid.

[0012] According to a first aspect of the invention, it is provided a method for controlling a grid connected power converter having a DC side with a DC link and an AC grid side, and being configured to control power supply to a hydrogen electrolyzer stack. The power supply to the hydrogen electrolyzer stack is controlled by controlling the DC link to thereby control hydrogen production. The method comprises: determining a grid voltage reference; providing a grid forming control for controlling at least the phase angle of the output voltage of the power converter using a grid forming controller, operating according to a grid forming algorithm, the grid forming controller being configured to emulate inertia through control of the voltage of the power converter towards the grid voltage reference; the grid forming controller emulating inertia by charging and discharging an inherent capacitance of the electrolyzer stack; monitoring at least one operating parameter of the hydrogen electrolyzer stack; and limiting a change in charging level of the inherent capacitance based on the monitored operating parameter of the electrolyzer stack.

[0013] With regard to the power reference for the machine power control, the power being supplied to the grid may be feedforwarded for use as power reference, and / or a power reference may be received from, e.g., a power plant controller, PPC, or a grid operator, or be determined from active and reactive power references.

[0014] As was mentioned, requirements to equip power converters of renewable energy sources with grid-forming properties that exhibit similarities to the behavior of conventional synchronous generators may be desired or even required. This may be accomplished by operating the power converter according to a grid forming control scheme using a grid forming controller.

[0015] The power converter is then controlled to produce a voltage towards the grid 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 wind turbine and / or a wind farm, and where one or a plurality of power sources may be connected to the electrical grid for providing power. The grid voltage reference may comprise a phase angle, and may also comprise a voltage amplitude to be maintained by the power converter. The voltage reference may be determined according to any suitable criteria. For example, the renewable power source such as a wind turbine generator may be configured to 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 consumers. Grid forming hence allows a renewable energy power source such as a wind turbine generator to create a power grid by powering otherwise depowered power lines. The grid forming control thereby differs from the more conventional grid following control, where, instead, grid-following converters synchronize to the currently prevailing grid voltage amplitude and frequency, and adjusts the output voltage to track an external voltage reference, which is given by the currently prevailing voltage on the electrical grid. The voltage output by the line side converter during grid following is variable so that the current being injected into the grid is adapted in amplitude to correspond to the prevailing power that is delivered by the electrical generator. The drawback of such control is the lack of support for system stability.

[0016] Grid forming converters alleviate stability problems of this kind for grids comprising renewable energy sources, since the power converter, instead, may be configured to act as a grid stabilizer, where the output voltage is controlled towards a voltage reference and operates to maintain the voltage reference, irrespective of the actually prevailing grid voltage in order to support the maintaining of the intended voltage.

[0017] This 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 sources, such as wind turbine generator components.

[0018] According to the invention, a control method that provides grid forming control using a power converter is provided, but where the power converter is configured to control power supply to a hydrogen electrolyzer stack. This is performed in order to utilize inherent characteristics of the hydrogen electrolyzer stack to account for and alleviate the impact of DC link changes that may occur during the grid forming control. The power supply to the hydrogen electrolyzer stack, and thereby hydrogen production, is controlled by controlling the DC link, such as DC link voltage, using the power converter. A grid voltage reference is determined, e.g., according to the above, and a grid forming control for controlling at least the phase angle of the voltage, and according to embodiments also amplitude, of the power converter using a grid forming controller operating according to a grid forming algorithm is provided. The grid forming algorithm may be any suitable grid forming algorithm. The grid forming controller is configured to emulate inertia through control of the voltage of the power converter towards the grid voltage reference, where, according to the invention, the grid forming controller emulate inertia by charging and discharging an inherent capacitance of the electrolyzer stack.

[0019] According to the invention, the inherent capacitance of the electrolyzer stack is thereby utilized as an energy storage for storing and / or providing energy that the DC link needs to provide and / or absorb during grid forming control in order to allow the power converter to control the voltage towards the set grid voltage reference. At least one operating parameter of the hydrogen electrolyzer stack is monitored, and a change in charging level of the inherent capacitance is limited based on the monitored operating parameter of the electrolyzer stack so as to avoid subjecting the electrolyzer stack to harmful operating conditions.

[0020] Hence, according to the invention, a power converter that is used to control the powering of a hydrogen electrolyzer stack is used to provide grid forming control. This type of grid forming control may be used, e.g., in an environment where one or more renewable energy sources such as wind turbines are providing the power to the grid, where the renewable power source may be operating off-grid, i.e. , not connected to a national grid but being a local grid or be connected to a national grid.

[0021] In particular with regard to off-grid operation, the one or more, e.g., wind turbines will provide energy, although not at a constant rate but, in general, at a varying power rate due, inter alia, to fluctuations in the wind. This will provide a varying power supply, and thereby stability issues in case the consumers of the local grid do not momentarily adapt the power consumption in response to changes in the wind. According to the invention, at least part of the grid forming control is provided by a power converter using a hydrogen electrolyzer stack, which may partly or fully relieve the wind turbines from providing the grid forming control.

[0022] Furthermore, the grid forming control according to the invention is carried out in a manner taking at least one operation parameter of the hydrogen electrolyzer stack into account so as to ensure proper operation of the hydrogen electrolyzer stack during grid forming control. In this way it can be ensured that operating limits or other restrictions regarding the operation of the hydrogen electrolyzer stack are respected to avoid potentially harmful operating conditions of the electrolyzer stack.

[0023] According to embodiments of the invention, the grid forming control further comprises to control the DC link, such as DC link voltage level, to control the charging and discharging of the inherent capacitance of the electrolyzer stack. The power converter may hence be utilized to control the production of hydrogen of the hydrogen electrolyzer stack while also performing the grid forming control.

[0024] According to embodiments of the invention, the grid forming control further comprises to charge and discharge the inherent capacitance of the electrolyzer stack when controlling active power flow from and to the grid through the power converter, wherein charge stored by the inherent capacitance may be utilized to reinject power to the grid when controlling the voltage of the power converter towards the grid voltage reference.

[0025] The capacitance of the hydrogen electrolyzer stack is hence used as an energy storage being used by the grid forming control when controlling the voltage towards the grid voltage reference, where active power being absorbed during the grid forming control can be at least partly stored in the capacitance of the hydrogen electrolyzer stack, to thereby protect the general electrolyzer stack operation from harmful transients, and where this stored energy can also be utilised to inject power into the grid when required during the grid forming control. According to embodiments of the invention, a maximum and / or minimum allowable DC link voltage may be estimated based on the monitoring of the at least one operating parameter of the electrolyzer stack, and the grid forming controller may be controlled such that the maximum and / or minimum DC link voltage is maintained below the estimated maximum and / or minimum allowable DC link voltage.

[0026] The power consumption of the hydrogen electrolyzer stack is in general controlled by controlling the DC voltage powering the hydrogen electrolyzer stack, and grid forming control may be carried out such that the maximum and / or minimum DC link voltage is maintained below the estimated maximum and / or minimum allowable DC link voltage to ensure that the operating parameter of the electrolyzer stack is maintained within predetermined limits.

[0027] According to embodiments of the invention, it is further estimated a maximum ramp-up and / or ramp-down voltage, or power, when raising and / or lowering the DC link voltage from a first level to a second level when emulating inertia based on the monitoring of the at least one operating parameter of the electrolyzer stack. As stated, the power being consumed by the hydrogen electrolyzer stack is dependent on the DC voltage level powering it, and there may be restriction regarding changes in the voltage, and thereby power consumption changes, since this may cause undesired states in the electrolyzer stack. In particular the electrolyzer may need to be ramped relatively slowly to avoid undesired mixing of gases. For example, unfavorable operating conditions may result in hydrogen and oxygen mixing in the anode, which is highly undesirable. The raising and / or lowering of the DC link voltage may therefore be controlled such that the raising and / or lowering of the DC link voltage is maintained below the estimated maximum ramp-up and / or ramp-down voltage to ensure that the operation of the electrolyzer stack is maintained within predetermined limits. As an alternative, or addition, to a maximum ramp-up and / or ramp-down voltage, a maximum ramp-up and / or ramp-down power through the electrolyzer stack may be determined instead, where this can be controlled by suitably controlling the DC voltage. Furthermore, according to embodiments of the invention, the electrolyzer stack may form part of a plant of electrolyzer stacks, where each electrolyzer stack may be controlled by a power converter, and where the grid forming control may be carried out for the plant of electrolyzer stacks, where the inherent capacitance of one or more of the electrolyzer stacks may be utilized in the grid forming control. Different ramping levels may also be determined for different stacks.

[0028] According to embodiments of the invention, the grid forming control may further comprise to apply momentary DC link voltage changes exceeding the estimated maximum ramp-up and / or down voltages, wherein the momentary DC link grid voltage changes are controlled such that an average change in DC link voltage is maintained below the estimated maximum ramp-up and / or ramp-down voltage.

[0029] It is hence possible to exceed the maximum ramp-up or ramp-down voltage for short periods of time, but where the exceeding of the maximum limits is carried out for only very short periods of time, such that an average change in ramp-up or ramp-down voltage still is maintained below the estimated levels.

[0030] According to embodiments of the invention, the monitoring of the at least one operating parameter of the hydrogen electrolyzer stack comprises an estimation of the power flow through a membrane of the hydrogen electrolyzer stack, and the grid forming control further comprises to control the rate of change of the power flow through the hydrogen electrolyzer such that the rate of change of the power flow does not change more than a predetermined threshold.

[0031] Electrolyzer stacks may be sensitive to the rate of change of the power flowing through the stack, where the power is proportional to the DC voltage powering the stack. In case the power increases or decreases too quickly, undesired operating states may arise. According to embodiments of the invention, the grid forming control further comprises to control the DC link voltage based on the monitored operating parameter of the electrolyzer stack such that a reverse power flow through the electrolyzer stack is prevented.

[0032] For example, in case the DC link voltage becomes too low, the chemical reactions occurring in the electrolyzer stack may reverse, and hence start producing electricity and water from hydrogen and oxygen. This may be undesirable, and although the electrolyzer stack may be designed for operation in both ways this may not be the case, or desirable, and this may therefore be prevented by ensuring that the DC link voltage level is always maintained at a level where this will not occur.

[0033] On the other hand, the electrolyzer stack may instead be designed to operate also as a fuel cell, and hence produce electric power through a reversed flow through the electrolyzer stack. In such situations, operation of this kind may be utilized to produce power to assist in the grid forming control when needed. This may be the case, for example, when power is required to be injected into the grid during grid forming where the energy stored in the inherent capacitance is not sufficient, or where power otherwise provided to the grid is not sufficient in relation to the load currently provided by the consumers connected to the grid.

[0034] According to embodiments of the invention, therefore, the grid forming control may further comprise to control the voltage on the electrolyzer cell to become lower than a voltage at which the chemical reaction in electrolyzer cells is reversed, such that the cells in stack will start to act as fuel cell, thereby providing firm power back to the grid.

[0035] The monitoring of the at least one operating parameter of the hydrogen electrolyzer stack may also comprise, e.g., an estimation of gas quality in the stack, measurement of hydrogen in the oxygen stream, measurement of oxygen in the hydrogen stream, water temperature, lye temperature and any other process parameter in the electrolyzer system being of importance to the operation of the electrolyzer stack.

[0036] According to embodiments of the invention, the monitoring the at least one operating parameter of the electrolyzer stack utilizes a digital twin representation, i.e. a mathematical model, of the electrolyzer stack. The grid forming control influence on the operation of the hydrogen electrolyzer stack is then estimated utilizing the digital twin representation, and voltage and / or current limits used in the grid forming control is determined based on the monitoring of the electrolyzer stack utilizing the digital twin representation.

[0037] A model representation of the electrolyzer stack may hence be utilized to estimate the influence of different control strategies on the electrolyzer stack prior to actually carrying out the control actions, and estimate whether a control action will result in undesired operating states of the electrolyzer stack. The estimation may also be utilized to determine maximum / minimum limits according to the above by simulating various scenarios and determine allowable limits.

[0038] The monitoring of the at least one operating parameter of the electrolyzer stack may be carried out utilizing a membrane observer, where the membrane observer is configured to monitor at least one operating parameter regarding the operation of the electrolyzer stack by receiving signals regarding the status of the electrolyzer stack that allow determ ination / estimation of the various operating parameters exemplified above.

[0039] The membrane observer may be configured to provide the at least one operating parameter to a stack protector, where the stack protector then generates control limits to be used in the grid forming control, and provide limits, e.g., in regard of voltage / current changes in the control, based on the at least operating parameter.

[0040] According to embodiments of the invention, the power converter is connected to the grid through a transformer with a secondary side connected to the power converter and a primary side connected to the grid, where the transformer has a variable turns ratio to allow adjustment of the voltage ratio between a primary side voltage of the transformer and the secondary side voltage, the grid forming control further comprising: changing the turns ratio when the DC link voltage becomes below or a above a DC link voltage operating range of the hydrogen electrolyzer stack to return the DC link voltage to an operating range of the hydrogen electrolyzer stack.

[0041] It is hence possible to utilize a transformer in the grid forming control in situations where the transformer is configured with a variable turns ratio. This allows that the DC link voltage can be increased, if needed, by changing the turns ratio, and also be decreased, if needed, by suitable change of the turns ratio so that the DC voltage of the electrolyzer stack is kept within desired limits.

[0042] According to embodiments of the invention, the grid is an off-grid network, where the power in the off-grid network for powering the hydrogen electrolyzer is provided by at least one renewable energy source, such as at least one wind turbine. The method may then comprise to provide control signals for controlling the renewable energy source in dependence of the monitoring of the at least one operating parameter of the hydrogen electrolyzer stack.

[0043] A decrease, or increase, if possible, of the power provided by the renewable energy source may hence be requested by the control of the electrolyzer stack in dependence of the current operation of the stack, where, e.g., a decrease in power by the renewable energy source may be requested in case the electrolyzer stack is operating at or above its limits.

[0044] According to embodiments of the invention, the hydrogen electrolyzer stack is connected to the DC link through a DC / DC converter, where the DC / DC converter provides for use of a DC voltage in an operating range of the electrolyzer stack while also providing for a DC link voltage in an operating range of the power converter. Such a DC / DC converter may also be utilized to control the DC voltage over the electrolyzer stack to a voltage where the power through the stack is reversed (in case this is desired) to provide for a power transfer to the grid from the electrolyzer stack when this improves system stability.

[0045] Furthermore, wind turbines may need to mitigate loads or oscillations in drivetrain or tower. Drive train components in general exhibit fundamental frequency oscillations that may have a frequency being considerable lower than the grid frequency, such as, e.g., 1-10 Hz. The power output by the electrical generator can be compensated by a drive train damping power that dampens such drive train fundamental frequency oscillations, where the drive train damping power is compensated by supplying the drive train damping power to the grid.

[0046] This, in turn, can be compensated for by utilizing the electrolyzer stack to absorb these fluctuations through the grid forming control. In case more than one electrolyzer stack is present in the system drive train damping power may be distributed over more than one electrolyzer stack to increase the ability of the electrolyzer stacks to absorb power fluctuations. This kind of compensation may also be utilized for other powers occurring in the system and that it may be desirable to compensate for.

[0047] According to embodiments of the invention, the electrolyzer stack may form part of a hybrid power plant comprising, e.g., wind turbines and / or solar panels as power sources and additional electrolyzer stacks. The monitoring of the electrolyzer stack may include to monitor imminent weather changes, such as clouds and wind changes, that may affect power production, and control power consumption of the electrolyzer stacks, and, e.g., commence a turn-off of one or more electrolyzer stacks to prepare for an anticipated imminent drop in power production.

[0048] The invention also relates to a wind power installation control system, where the wind power installation comprises a wind turbine configured to supply power to a grid and a power converter controlled hydrogen electrolyzer. The wind power installation control system is configured to perform the method according to any of the above aspects. Furthermore, a wind power installation comprising a wind turbine control system of the discussed kind also form part of the invention.

[0049] BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Embodiments of the invention will be described, by way of example only, with reference to the drawings, in which:

[0051] Fig. 1 illustrates an example of grid forming control utilizing a hydrogen electrolyzer stack according to embodiments of the invention;

[0052] Fig. 2 illustrates control components arranged for controlling the generation of active power and reactive power during grid forming according to embodiments of the invention;

[0053] Fig. 3 illustrates an exemplary grid forming control algorithm utilizing a swing equation.

[0054] DETAILED DESCRIPTION

[0055] The invention will be exemplified in the following for an off-grid configuration comprising, e.g., a wind turbine for providing power to a local grid, and a hydrogen electrolyzer stack acting as load, but the invention is not limited to wind turbines as renewable power sources for providing power required to setup a grid. For example, solar power and / or one or more fuel cells may alternatively, or in addition, be used. A local grid may also comprise various additional energy sources and consumers. For example, an electrolyzer stack may also operate as a power source by operating as a fuel cell. The invention is also equally applicable for use in any grid, such as national / international transmission networks, comprising renewable energy power sources.

[0056] As was discussed above, power converters may be controlled according to different control strategies, where historically a grid following methodology has been utilized when injecting power of renewable energy sources. As was also mentioned, an advantage of the grid following control scheme is that since the power converter 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 the amount of power being injected into the grid still corresponds to the amount of power being provided to the DC link by the power converter of, e.g., a wind turbine.

[0057] However, as was also discussed, there may exist a requirement renewable power sources take part in the forming and supporting of the grid since such energy source in general lack large rotating masses, and assist in the maintaining of the stability of the grid when grid events affecting stability occur through a grid forming control. When a conventional synchronous machine is used to power a grid, the inherent inertia may be utilized for purposes of stabilizing the grid to avoid, e.g., frequency fluctuations. In case the energy source is operating in an island mode there may be no rotational mass at all. When a renewable energy source, such as a wind turbine, is to participate in grid stabilising measures, a modelled synchronous generator is utilised, where the increase or decrease of the kinetic energy of the modelled synchronous generator in response to a grid event causes changes in the control of the power converter.

[0058] When a wind turbine generator operates according to a grid forming control the power converter is operated to output a fixed voltage irrespective of any stability influencing negative events occurring on the grid. However, if there are transient changes in the grid during grid forming, there will be an immediate current response that follows automatically when continuing to control the output of the power converter towards the voltage reference. There will be a corresponding transient change in requirement for kinetic energy from the renewable energy source, and grid disturbances like phase jumps may transplant 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. For example, the aerodynamical rotor carrying the wings, the electrical generator as well as the gearbox interconnecting these components may be sensitive to torque spikes to high degree. It is therefore highly undesirable to have high torque spikes occurring in the drivetrain. This may provide a challenge when it comes to controlling a wind turbine generator according to a grid forming control scheme.

[0059] According to the invention, it is provided a method of mitigating problems of this kind, where grid forming control is instead, at least in part, provided by a power converter being utilized to control power supply to a hydrogen electrolyzer stack to reduce undesired stress on mechanical components of, e.g., wind turbine components.

[0060] Fig. 1 illustrates an exemplary embodiment of a solution according to the invention. Fig. 1 shows an example of a power system 100 of a hydrogen electrolyzer stack being utilized in grid forming of a grid 120, such as in a local grid I off-grid solution, where, e.g., a wind turbine or one or more other renewable energy power sources may be utilized to power the grid 120. The power system

[0061] 100 comprises a hydrogen electrolyzer stack 101 , which, inter alia, comprises an anode 102, a membrane 103 and a cathode 104. The hydrogen electrolyzer stack

[0062] 101 is powered by a DC voltage lldc, which is provided by a power converter 104.

[0063] The hydrogen electrolyzer stack splits water molecules into hydrogen H2 and oxygen O2 through the use of a DC current lei passing through the stack 101 , where the generated oxygen is ventilated away at the anode 102, and the generated hydrogen passes through the membrane 103 to be collected at the cathode 104 and stored, e.g., in tanks for subsequent use. The hydrogen electrolyzer stack 101 is hence a power-to-hydrogen gas storage device that can be utilised to convert power generated by, e.g., a fluctuating power source such as a wind turbine when power is available, to allow the energy stored in the generated hydrogen gas to be subsequently used as desired, e.g. by generating electric energy through a fuel cell. The hydrogen electrolyzer stack 101 consumes power in dependence of the DC voltage level, and hence the power consumption may be controlled through a control of the DC voltage level lldc powering the electrolyzer stack 101 . This hence provides a means for adapting power consumption to fluctuations in the power level produced, e.g., by a wind turbine so that the hydrogen production can be adapted to the currently available power. According to the invention, the hydrogen electrolyzer stack is further utilized to provide a grid forming control to assist in stabilising the grid, e.g., during transient grid events.

[0064] The power system 100 further comprises a line side DC / AC power converter 105 interconnecting a DC link 106 providing power to the electrolyzer stack 101 using the grid 120. The illustrated system further comprises a resistor, connected with a controllable switch 108. The resistor 107 and switch 108, form a power dissipation device, also known as chopper, for dissipating active power if the need for this arises which may be the case, e.g., to protect the electrolyzer stack 101 from excessive currents / voltages.

[0065] The DC link 106 may further comprise one or more DC link capacitors 109 which are charged by the DC output current from the power converter 105 generated from AC current on the AC side of the power converter 105.

[0066] The power converter 105 is connected to the grid or power line 120 through output inductors 110 and possibly a transformer 112. The transformer 112 has a secondary side towards the inductor arrangement and a primary side connected to the grid 120. In this example, although not illustrated specifically, the grid AC current is a 3-phase current, although the grid may also be a single-phase grid. Furthermore, harmonic filter capacitors 111 may be arranged between the conductors of the output, in order to form a harmonic filter together with inductors 111 , which converts any output square wave voltage signals from the power 104 to voltage sinusoidal signals. The grid 120 may, as discussed, be a local grid but may, in principle, be any type of grid and may be further form part of or be connected to a high voltage wide area transmission line network, e.g. via further transformers.

[0067] The power converter 105 may be full-scale converter configured according to different principles including forced-com mutated and line-commutated converters. The power system 120 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.

[0068] Fig. 1 further schematically illustrates that the grid voltage, Ugrid, according to the example illustrated as the voltage at the low voltage LV side of the transformer 112, can be measured. The grid voltage Ugrid can be utilized in the grid forming control using the power converter 105, and also for controlling the power converter 105 when powering the hydrogen electrolyzer stack 101 . In particular, the active power Pgrid may be determined 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 side of the transformer 112 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.

[0069] According to the invention grid forming control in the grid 120 is at least in part carried out by the power converter 105 that provide power to the hydrogen electrolyzer stack 101 . The grid forming control may be carried out utilizing any suitable grid forming control. The power converter 105 utilizes pulse width modulation (PWM) for converting the AC power into DC power. This is schematically illustrated in fig. 2, where a control system 250 is used for controlling the modulation of the power converter 105 and for controlling the active power P and the reactive power Q generated by the power converter 105. Fig. 2 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 of a power converter 204, such as power converter 105 of fig. 1 , according to a grid forming algorithm. 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.

[0070] References for the active and reactive power to be utilized in the grid forming 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.

[0071] The power converter 204 may be the power converter controlling the electrolyzer stack of fig. 1 , and may also be a power converter of the wind turbine of fig. 2, although this is not the object of the invention, and as stated, part of the object of the invention is to alleviate wind turbines from providing the grid forming control. The grid forming control may utilize grid forming algorithms of different kinds, where, for example, the active power, Pgrid, may be controlled using a virtual synchronous machine angle, denoted 9GFC as is schematically indicated in fig. 2.

[0072] 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. According to the invention, the inherent capacitance of the electrolyzer stack 101 is utilized instead.

[0073] The synchronous machine angle 9GFC 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.

[0074] 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.

[0075] 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.

[0076] 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 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.

[0077] The synchronous machine angle 9GFC 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 0GFC, 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.

[0078] Further to fig. 1 , the invention provides a grid forming control using the power converter 105 that control power supply to the hydrogen electrolyzer stack 101 , where the inherent characteristics of the hydrogen electrolyzer stack 101 is used to at least partially account for transient events, and alleviate DC link changes that may occur during the grid forming control. Since the power converter controlling the electrolyzer stack 101 is used for the grid forming functionality in the grid, this also means that a renewable power source providing the power to the grid need not necessarily provide such grid forming capabilities, and / or to a lesser extent.

[0079] The general power supply to the hydrogen electrolyzer stack, and thereby hydrogen production, is controlled by controlling the DC link voltage using the power converter 105. A grid voltage reference is determined, e.g., by receiving a reference provided, by an electrical grid operator, or be determined by the operator of the local grid. A grid forming control for controlling at least the phase angle of the output voltage of the power converter 105 is then carried out using, according to the present example, a grid forming algorithm in the form of a swing equation 130. An example of a swing example will be exemplified below with reference to fig. 3, but it is to be understood that the grid forming algorithm may be any suitable grid forming algorithm. During operation the grid forming control is configured to emulate inertia through control of the output voltage of the power converter 105 towards the grid voltage reference. It has been realized that an electrolyzer stack of the kind illustrated in fig. 1 has an inherent capacitance, schematically indicated by 140, which is comparatively large, in particular considerably larger than DC link capacitors such as capacitor 109, and thereby capable of storing a considerable amount of energy. According to the invention, the grid forming controller emulate inertia by charging and discharging the inherent capacitance of the electrolyzer stack. The inherent capacitance 140 of the electrolyzer stack 101 is hence utilized as an energy storage for absorbing and / or providing energy that is needed to maintain the DC link at a desired level during grid forming control in order to allow the power converter to control the output voltage towards the grid voltage reference.

[0080] Hence, in case the grid forming control results in a sudden DC link voltage reduction, e.g. caused by a grid event in the grid 120 that transplants to the DC link side of the power converter 105, charges stored in the inherent capacitance 140 may be utilized to stabilize the DC link 106 voltage and thereby also reduce any negative impact on the operation of the electrolyzer stack 101 caused by sudden changes in DC link voltage. Conversely, in case a grid event gives rise to a sudden surge in DC link voltage, this may at least partially be absorbed by the inherent capacitance 140, to reduce the impact on the DC link.

[0081] The operation of an electrolyzer stack is, however, sensitive to the DC link voltage that powers the stack, and therefore, according to the invention, at least one operating parameter of the hydrogen electrolyzer stack is monitored during the grid forming control, and a change in the charging level of the inherent capacitance is limited based on the monitored operating parameter of the electrolyzer stack. In this way the electrolyzer stack can be protected against too unfavorable operating conditions. The power converter 105 may consequently control the DC link voltage level to control the charging and discharging of the inherent capacitance of the electrolyzer stack 101. According to the illustrated example, a membrane observer 150 is utilized to monitor the current status of the electrolyzer stack 140, where the membrane observer 150 may be configured to monitor various parameters regarding the operation of the electrolyzer stack 101. For example, the membrane observer 150 may be configured to measure and / or estimate a differential pressure over the membrane 103 of the electrolyzer stack 101. The membrane observer 150 may also be configured to measure, e.g., one or more temperatures of the electrolyzer stack 101 . The monitoring of operating parameters of the hydrogen electrolyzer stack 101 may also comprise to estimate the power flow through the membrane 103 of the hydrogen electrolyzer stack 101 .

[0082] One or more of these electrolyzer stack operating parameters are then communicated to and utilized by a stack protector 151 that imposes limits on the grid forming control. That is, changes in DC link voltage that would result from an “unrestricted” control according to the swing equation 130 are subjected to a limitation in terms of the speed of the control so that the electrolyzer stack is not subjected to potentially harmful operating states. This applies similarly to temperatures, and rate of change of the power flow through the hydrogen electrolyzer such that the rate of change, or maximum level, of a temperature or the power flow does not change more than a predetermined threshold.

[0083] The invention hence provides a solution where a power converter for powering of a hydrogen electrolyzer stack is used to provide grid forming control, while simultaneously protecting the electrolyzer stack. This type of grid forming control may be used, e.g., in an environment where one or more renewable energy sources such as wind turbines are operating off-grid, i.e. , not connected to a national grid but a local grid. The one or more, e.g., wind turbines will provide energy, although not at a constant rate but, in general, at a varying power rate due, inter alia, to fluctuations in the wind. This will provide a varying power supply, and thereby stability issues in case the consumers of the local grid do not momentarily adapt the power consumption in response to changes in the wind. According to the invention, at least part of the grid forming control is provided by a power converter 105 using a hydrogen electrolyzer stack 101 , which may partly or fully relieve the wind turbines from providing grid forming control. Grid forming control using an electrolyzer stack may hence absorb transients using the inherent capacitance of the electrolyzer stack, and thereby relieve, e.g., a wind turbine from harmful transients. The electrolyzer stack may also be used to adapt the load in dependence of the currently available power, since the power consumed is dependent on the amplitude of the DC voltage that powers it.

[0084] The grid forming control controls the active power flow from and to the grid through the power converter where this can be at least partly accomplished through the use of charging and discharging of the inherent capacitance of the electrolyzer stack, to thereby reduce impact on the operation of the electrolyzer stack. Hence, charges stored by the inherent capacitance can be utilized to reinject power to the grid when controlling the output voltage of the power converter towards the grid voltage reference llref when such power flow is required to maintain system stability.

[0085] As was mentioned, the electrolyzer stack 101 consumes power in dependence of the prevailing voltage of the DC link, where a higher voltage in general results in higher power consumption, and hence higher production of hydrogen, and vice versa. However, there may be limits that the DC voltage feeding the electrolyzer stack should not exceed or go below, e.g., due to resulting chemical reactions in the electrolyzer stack. Such maximum and / or minimum allowable DC link voltages may be estimated by the membrane observer 150 based on the monitoring of the electrolyzer stack, and be communicated to the stack 151 which imposes limits on the grid forming control such that the limits are respected.

[0086] In addition to, or as an alternative to imposing maximum and minimum levels regarding the DC link voltage to be respected during the control, a maximum ramp-up and / or ramp-down voltage, i.e. speed of change of voltage level, when raising and / or lowering the DC link voltage may, instead or in addition, be estimated based on one or more parameters established by the stack observer 150. There may be restrictions regarding the speed at which the voltage changes, since this may give rise operating conditions in the electrolyzer stack that is to be avoided. For example, the limits may be set to avoid hydrogen and oxygen mixing with each other in the electrolyzer stack, since this may give rise to risk for explosion. Such situations may be avoided, by the stack protector 151 limiting the grid forming control so that the estimated maximum ramp-up and / or ramp-down voltages are respected. According to embodiments, the membrane observer may also be configured to shut down the operation of the electrolyzer stack to avoid certain operating states.

[0087] Although there may be limits regarding the maximum ramp-up and / or ramp-down voltages as explained above, momentary violations of may still be possible by, e.g. ramp-up being followed by an immediate ramp-down so that the average change in voltage still fulfills the set limits. According to embodiments of the invention the grid forming may therefore comprise to apply momentary DC link voltage changes exceeding the estimated maximum ramp-up and / or down voltages, but only in situations where the momentary DC link grid voltage change is controlled to be below permissible limits.

[0088] The DC link voltage may also be configured to be controlled such that the power flow through the electrolyzer stack 101 does not change, and start operating in the reverse direction. This would result in the electrolyzer stack starting to operate as a fuel cell and produce power to be supplied to the grid. This may be undesirable, and the electrolyzer stack 101 may also be of a kind that is not designed for such use.

[0089] However, according to embodiments of the invention, a reverse power flow through the electrolyzer stack, may be desirable, and hence operation of the electrolyzer stack as a fuel may be utilized in the grid forming control. As was mentioned above, the membrane observer may utilize various sensors to determine the current status of the electrolyzer stack, and providing signals to the stack protector in dependence of the sensor values. In addition to such use, the operating parameters of the electrolyzer stack may instead be monitored utilizing a digital twin representation of, i.e. , a model of, the electrolyzer stack. In this way, the membrane observer may estimate limit values using the current state of various parameters and the digital twin representation and estimate, e.g., maximum / minimum limits based on the current state of the electrolyzer stack.

[0090] Furthermore, fig. 1 illustrates the power converter being connected to the grid 120 through a transformer 112. Such a transformer may oftentimes have a fixed turns ratio. However, according to embodiments of the invention the transformer 112 may, instead, have a variable turns ratio N to allow for adjustment of the voltage ratio between the primary side voltage Upri of the transformer, i.e. grid voltage, and the secondary side voltage llsec, i.e., towards the power converter 105. This allows that in case the DC link voltage, due to the grid forming control, becomes, too low or high the turns ratio can be changed to raise or lower the DC link voltage level to again be within desired limits. When conditions stabilize the turns ratio may e.g. be reset to the previous setting.

[0091] It is also possible to use a DC / DC converter between the power converter 105 and the electrolyzer stack 101 , where this DC / DC converter can be utilized to maintain the voltage of the electrolyzer stack 101 within desired limits, while at the same time maintaining the operating voltage of the DC / AC converter at a desired level. In case the electrolyzer stack is configured for operation in both directions, this DC / DC converter may also be utilized to control the power flow of the electrolyzer stack to go in the opposite direction, and use this reversed power flow in the grid forming control to reinject power to the grid.

[0092] 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 0GFC of a virtual synchronous generator, i.e. for use according to fig. 1 .

[0093] 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. According to the grid forming control swing equation 300, a power error Perr is determined as a difference between a power reference Pref, which, e.g., may be the power setpoint to be consumed by the electrolyzer stack, and the actual power PLSC being consumed from the grid, and a damping power PD being determined according to the virtual synchronous model.

[0094] In response to a change in the consumed power, 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 0GFC 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, and thereby charge or discharge the inherent capacitance of the electrolyzer stack, to reach a new equilibrium condition. The new equilibrium is reached when PL is again following PLref.

[0095] 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 power converter to the grid. 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. 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. 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.

[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 controlling a grid connected power converter, the grid connected power converter having a DC side with a DC link and an AC grid side, and being configured to control power supply to a hydrogen electrolyzer stack, wherein the power supply to the hydrogen electrolyzer stack is controlled by controlling the DC link to thereby control hydrogen production, the method comprising: determining a grid voltage reference (Vref); providing a grid forming control for controlling at least the phase angle of the voltage of the power converter using a grid forming controller, operating according to a grid forming algorithm, the grid forming controller being configured to emulate inertia through control of the voltage of the power converter towards the grid voltage reference (Vref); the grid forming controller emulating inertia by charging and discharging an inherent capacitance of the electrolyzer stack; monitoring at least one operating parameter of the hydrogen electrolyzer stack; and limiting a change in charging level of the inherent capacitance based on the monitored operating parameter of the electrolyzer stack.

2. Method according to claim 1 , the grid forming control further comprising: controlling the DC link to control the charging and discharging of the inherent capacitance of the electrolyzer stack.

3. A method according to any one of the claims 1-2, the grid forming control further comprising: charging and discharging the inherent capacitance of the electrolyzer stack when controlling active power flow from and to the grid through the power converter, wherein charge stored by the inherent capacitance is utilized to reinject power to the grid when controlling the voltage of the power converter towards the grid voltage reference (Vref).

4. A method according to anyone of the claims 1 -3, further comprising: estimating a maximum and / or minimum allowable DC link voltage based on the monitoring of the at least one operating parameter of the electrolyzer stack, and controlling the grid forming controller such that the maximum and / or minimum DC link voltage is maintained below the estimated maximum and / or minimum allowable DC link voltage.

5. A method according to any one of the claims 1 -4, further comprising: estimating a maximum ramp-up and / or ramp-down voltage, and / or electrolyzer stack power, when raising and / or lowering the DC link voltage from a first level to a second level when emulating inertia based on the monitoring of the at least one operating parameter of the electrolyzer stack, and controlling the raising and / or lowering of the DC link voltage such that the raising and / or lowering of the DC link voltage, and / or electrolyzer stack power, is maintained below the estimated maximum ramp-up and / or ramp-down voltage.

6. A method according to claim 5, the grid forming further comprising: applying momentary DC link voltage changes exceeding the estimated maximum ramp-up and / or down voltages, wherein the momentary DC link grid voltage changes are controlled such that an average change in DC link voltage is maintained below the estimated maximum ramp-up and / or ramp-down voltage.

7. Method according to any one of the claims 1-6, wherein: the monitoring of the at least one operating parameter of the hydrogen electrolyzer stack comprises an estimation of the power flow through a membrane of the hydrogen electrolyzer stack, and the grid forming control comprising: controlling the rate of change of the power flow through the hydrogen electrolyzer such that the rate of change of the power flow does not change more than a predetermined threshold.

8. A method according to claim 7, the grid being an off-grid network, the power in the off-grid network for powering the hydrogen electrolyzer being provided by at least one renewable energy source, such as at least one wind turbine, the method further comprising: estimating imminent fluctuations in power produced by the renewable energy source, and controlling power production and power consumption based on the estimated fluctuations and the predetermined threshold for rate of change of the power flow through the electrolyzer stack.

9. A method according to any one of the claims 1-8, the grid forming control further comprising: controlling the DC link voltage based on the monitored operating parameter of the electrolyzer stack such that a reverse power flow through the electrolyzer stack is prevented.

10. Method according to any one of the claims 1-9, further comprising: monitoring the at least one operating parameter of the electrolyzer stack utilizing a digital twin representation of the electrolyzer stack, wherein the grid forming control influence on the operation of the hydrogen electrolyzer stack is estimated utilizing the digital twin representation, and wherein voltage and / or current limits used in the grid forming control is determined based on the monitoring of the electrolyzer stack utilizing the digital twin representation.11 . A method according to any one of the claims 1-10, the power converter being connected to the grid through a transformer with a secondary side connected to the power converter and a primary side connected to the grid, the transformer having a variable turns ratio (N) to allow adjustment of the voltage ratio between a primary side voltage (llpri) of the transformer and the secondary side voltage (llsec), the grid forming control further comprising: changing the turns ratio (N) when the DC link voltage becomes below or aabove a DC link voltage operating range of the hydrogen electrolyzer stack to return the DC link voltage to an operating range of the hydrogen electrolyzer stack.

12. A method according to any one of the claims 1 -11 , the grid being an off-grid network, the power in the off-grid network for powering the hydrogen electrolyzer being provided by at least one renewable energy source, such as at least one wind turbine, the method further comprising: providing control signals for controlling the renewable energy source in dependence of the monitoring of the at least one operating parameter of the hydrogen electrolyzer stack.

13. A method according to any one of the claims 1 -12, further comprising: connecting the hydrogen electrolyzer stack to the DC link through a DC / DC converter, the DC / DC converter providing for use of a DC voltage in an operating range of the electrolyzer stack while also providing for a DC link voltage in an operating range of the power converter.

14. A method according to any one of the claims 1 -13, wherein the power flow through the electrolyzer stack is reversible, the method further comprising, when an active power flow towards the grid is requested by the grid forming control: reversing the power flow through the electrolyzer stack to inject active power into the grid.

15. A method according to any one of the claims 1 -14, further comprising: monitoring the at least one operating parameter of the electrolyzer stack utilizing a membrane observer, the membrane observer being configured to monitor at least one operating parameter regarding the operation of the electrolyzer stack, and provide the at least one operating parameter to a stack protector, the stack protector generating, based on the at least operating parameter, control limits for the grid forming control.

16. 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 15.

17. 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 15.

18. A renewable energy power installation control system, the renewable power installation comprising a renewable energy power source, such as a wind turbine generator, configured to supply power to a grid and a power converter controlled hydrogen electrolyzer, the renewable power installation control system being configured to perform the method according to any of the claims 1-15.

19. A renewable energy power installation control system according to claim 18, further comprising a membrane observer for monitoring the at least one operating parameter of the electrolyzer stack, the membrane observer being configured to monitor parameters regarding the operation of the electrolyzer stack.

20. A renewable energy power installation control system according to claim 19, further comprising an electrolyzer stack protector, the stack protector being configured to determine limits for the grid forming control based on at least one electrolyzer stack operating parameter received from the membrane observer.21 . A renewable energy power installation comprising a renewable energy power installation control system according to any one of the claims 18-20.

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