Operating power converter

Abstract

A method of operating a power converter (104) is provided. The power converter may be electrically coupled to an electrical grid (108) and output converted electrical power to the electrical grid. The power converter may be operated in a first mode of operation, where the converter operates based on a first reference (epsilon1 *) generated by a virtual synchronous machine control scheme (201), and the power converter may also be operated in a second mode of operation, where the converter operates based on a second reference (i2 *) not generated by the virtual synchronous machine control scheme. The method (500) may include switching operation of the converter from the second operating mode to the first operating mode, where the switching operation may include obtaining (S10) one or more operating parameters while operating in the second operating mode, and estimating (S15) a parameterization of the virtual synchronous machine control scheme from the one or more operating parameters. The switching operation may further include parameterizing (S20) the virtual synchronous machine control scheme with the estimated parameterization, and switching (S25) to the first mode of operation by operating the virtual synchronous machine control scheme with the parameterization to generate a first reference ([epsilon] 1 *) and operating the converter based on the generated first reference.

Description

Technical Field

[0001] The present invention relates to a method of operating a power converter, in particular a wind turbine. It also relates to a computer program for performing such a method and a control system configured to perform such a method. It also relates to a power generation system comprising a power converter and such a control system. Background Art

[0002] The electrical power provided by a power generation system is typically fed into a power grid. To maintain the stability of the power grid, the generated electrical power must meet strict grid codes. Due to the increase in renewable energy sources, such as wind turbines or solar cells, converter-based power generation has become quite significant, and it is desirable to appropriately convert the generated electrical power and maintain stability. The converter is designed to routinely perform grid feeding / grid tracking operations or, more recently, grid forming operations. For grid feeding / grid tracking operations, the converter acts as a current source and synchronizes the electrical power output with the dynamics of the grid. For grid forming operations, the converter operates as a voltage source, applying voltage and frequency and participating in regulating the grid voltage and frequency.

[0003] Grid forming control schemes can implement a virtual synchronous machine. The virtual synchronous machine is used to simulate a synchronous generator in order to provide synthetic inertia for ancillary services and thus maintain grid stability. However, due to converter-based power generation, the virtual synchronous machine is considerably limited in its ability to output current. Compared with a real synchronous generator, the virtual synchronous generator cannot maintain its internal electromotive force by injecting a large amount of reactive current. Therefore, the virtual synchronous machine may not be able to track the provided reference electromotive force during a demand grid fault without exceeding the current limit.

[0004] For this reason, a current limit algorithm can be applied to the grid forming mode. However, during a grid fault, the current limit algorithm may cause misalignment between the reference grid voltage and the actual voltage. The misalignment may cause the d-axis and q-axis currents to no longer correspond to the active and / or reactive power, thus possibly resulting in uncontrolled active and reactive power.

[0005] To maintain controlled active and reactive power, during such a grid fault, a switch from the grid forming mode to the grid tracking mode is performed. In this way, the current limit operation is feasible without losing the alignment of the d-axis and q-axis.

[0006] However, when full power capability is restored, i.e., when the grid fault is cleared and the converter switches back to the grid-forming mode, known control schemes associated with the switching of the operating mode may create an oscillatory system response (i.e., a transient) of the converter. Similar to the resynchronization transient of a real synchronous generator after a grid fault, the transient may be caused by a mismatch between the actual operating point and the operating point required to provide the desired power flow after the switch.

[0007] In addition, an imbalance within the grid between the generation and demand of electrical power after a grid fault may inevitably introduce transients, which require the generators coupled to the grid to respond as soon as possible. However, known control schemes take a certain amount of time to resynchronize and are thus limited in terms of their response time and hence their contribution to grid stability.

[0008] Document EP3832829A1 relates to a grid-forming vector current control system configured to emulate a virtual synchronous machine. A virtual current source is connected in parallel to the virtual synchronous machine. SUMMARY OF THE INVENTION

[0009] Accordingly, there is a need to mitigate at least some of the above disadvantages and provide a solution that allows for more stable operation of the power converter, and in particular, a solution that allows for smoother and faster switching between the operating modes of such a power converter.

[0010] The features of the independent claims meet this need. The dependent claims describe embodiments of the invention.

[0011] According to one aspect of the invention, a method of operating a power converter is provided. The power converter is electrically coupled to a grid and outputs converted electrical power to the grid. The power converter is operable in a first operating mode, in which the converter operates based on a first reference generated by a virtual synchronous machine control scheme, and the power converter is also operable in a second operating mode, in which the converter operates based on a second reference that is not generated by the virtual synchronous machine control scheme. The method includes switching the operation of the converter from the second operating mode to the first operating mode. The switching operation includes: obtaining, while operating in the second operating mode, one or more operating parameters indicative of the current provided by the converter to the grid and / or the grid voltage; and estimating a parameterization of the virtual synchronous machine control scheme from the one or more operating parameters. The estimating includes calculating one or more values of the parameterization that will result in at least a portion of the one or more operating parameters obtained when the converter operates in the first operating mode based on the first reference generated by the virtual synchronous machine control scheme. The switching operation further includes parameterizing the virtual synchronous machine control scheme with the estimated parameterization and switching to the first operating mode by operating the virtual synchronous machine control scheme with the parameterization to generate the first reference and operating the converter based on the generated first reference.

[0012] The parameterization of the first operating mode of the power converter control can thus be determined based on one or more operating parameters that are obtained during operation in the second operating mode and set before switching to the first operating mode. In this way, a first reference is aligned with a second reference such that when the first operating mode takes over from the second operating mode, the operation in the first operating mode continues from the same operating point where the operation in the second operating mode stopped. Thus, the alignment can ensure that the actual operating point of the first operating mode of the converter corresponds to the operating point of the converter in the second operating mode at the time of switching. In other words, by taking over the corresponding operating point of the second operating mode, the power converter can smoothly transition to the first operating mode and then continue its operation from that operating point. Since the operating point is taken over and remains unchanged, control-related transients may not occur when it is switched. Therefore, the stability of the operation of the power converter is improved. In addition, since the switching is performed stably and since the first operating mode takes over control, the converter provides auxiliary services immediately after the switching. The transition between operating points is no longer critical for stability because the system is stably synchronized and continuously provides auxiliary services, thus contributing to the improvement of the power grid stability.

[0013] The converted electrical power can include active power and / or reactive power.

[0014] The power converter can include a converter for converting electrical energy. The power converter can convert alternating current (AC) to direct current (DC) and vice versa, or convert AC to AC. The power converter can change the voltage and / or frequency of the current. The power converter can include a DC link.

[0015] In one example, obtaining herein can include monitoring. For example, monitoring can be performed by at least one of a sensor, a sensing system, a model (especially a mathematical and / or physical model implemented in software), and a filter-based monitoring system (such as a state observer, a Kalman filter, etc.), but is not limited thereto. Monitoring can also include signal preprocessing and postprocessing, such as filtering. In addition, monitoring can utilize information received from hardware implemented for monitoring purposes (e.g., one or more sensors), and / or can utilize information received from hardware implemented for another purpose. Monitoring can also include estimation and / or prediction, preferably based on the received information.

[0016] During operation in the second operating mode, parameterization can be performed when the operating point can be monitored (e.g., when there are no excessive components above the Nyquist frequency caused by sudden voltage changes).

[0017] Any parameter or value herein (e.g., a parameter or value obtained, received, monitored, calculated, or estimated) may include a complex number. For example, such a parameter or value may include an amplitude and a phase angle / an independent variable associated with the amplitude. Additionally, the symbol j may represent the imaginary unit herein, and the symbol e may represent the Euler number herein. Thus, the complex parameter y may include an amplitude Y and a phase angle and may thus be represented as

[0018] In one example, the current provided by the converter to the power grid may include the current of the converted electrical power output to the power grid.

[0019] In one example, as an alternative or addition to one or more parameters indicating the current provided by the converter to the power grid when operating in the second operating mode, one or more operating parameters may include one or more parameters indicating a reference current according to which the converter provides current to the power grid when operating in the second operating mode.

[0020] In one example, at least part of the one or more operating parameters obtained may include the current provided by the converter to the power grid and / or the reference current when operating in the second operating mode, and the current of the converted electrical power is output to the power grid according to the reference current.

[0021] The first reference may include a reference voltage, in particular a reference voltage amplitude and a reference voltage phase angle. The reference voltage may be generated by a virtual synchronous machine control scheme. Preferably, the reference voltage may correspond to the electromotive force generated by the virtual synchronous machine control scheme.

[0022] The reference voltage may be a rotating reference voltage. The electromotive force may be a rotating electromotive force, the amplitude and phase angle of which are generated by the virtual synchronous machine control scheme.

[0023] The first operating mode may include a grid-forming mode. In the grid-forming mode, the power converter may operate as a voltage source. Preferably, in the first operating mode, the power converter outputs the converted electrical power according to a (externally received) reference power. The externally received reference power may be received from a transmission system operator.

[0024] The second operating mode may include a grid-following mode. In the grid-following mode, the power converter may operate as a current source. Preferably, operating in the second operating mode may include performing a fast fault current injection.

[0025] The power converter may operate in the first operating mode or the second operating mode. In other words, the power converter may operate only in one of the first operating mode and the second operating mode (simultaneously).

[0026] The power converter can operate based on a first reference or based on a second reference. In other words, the power converter can operate based on only one of the first reference and the second reference (simultaneously).

[0027] The power converter cannot operate in both the first operating mode and the second operating mode and / or based on the first reference and the second reference in parallel.

[0028] For example, in the case of a grid fault, the operation of the power converter can be switched from the first operating mode (grid-forming mode) to the second operating mode (grid-following mode), in which the converter does not need to provide a current exceeding the corresponding current limit. For example, during the operation in the second operating mode, a fast reactive current injection based on grid code requirements can be applied. When the grid has recovered from the grid fault and full power capability is restored, the converter can be switched from the second operating mode to the first operating mode. Due to parameterization, the first operating mode can take over the operating point from the second operating mode, and the transition is smooth and instantaneous. Therefore, the transition does not cause transients. Since the first operating mode starts from its estimated stable operating point, the stability of the operation of the power converter is improved.

[0029] The virtual synchronous machine control scheme can be parameterized such that, with the estimated parameterization, the virtual synchronous machine control scheme will generate a first reference that results in the obtained one or more operating parameters.

[0030] In this way, the obtained one or more operating parameters correspond to the one or more operating parameters previously obtained from the second operating mode.

[0031] According to one example, calculating one or more values of the parameterization can include a reverse calculation from the obtained one or more operating parameters to the parameterization.

[0032] According to one example, estimating the parameterization can include estimating a first reference based on one or more operating parameters. The estimation can include calculating one or more values of the first reference that will result in at least a portion of the one or more operating parameters obtained when the converter operates based on the estimated first reference.

[0033] According to one example, the second reference can be a reference current, and the obtained operating parameters can include the reference current or the obtained converter output current and grid voltage. Calculating one or more values of the first reference can include calculating a reference voltage amplitude and a reference phase angle based on the reference current or output current, grid voltage, and a virtual impedance associated with the virtual synchronous machine control scheme. Preferably, calculating one or more values of the first reference can include calculating a frequency indicating a change in the reference phase angle based on the grid voltage.

[0034] The virtual impedance can be predefined and preferably indicates the impedance of the virtual machine / virtual (synchronous) generator. Preferably, the impedance includes an inductive part.

[0035] According to one example, the calculation cycle can include generating values of a first and / or a second reference. Both parameterization and switching can be performed in one calculation cycle, or alternatively, parameterization can be performed in a first calculation cycle and switching can be performed in a second calculation cycle, and the first calculation cycle can be performed before the second calculation cycle.

[0036] According to one example, the method can include operating a power converter in a first operating mode, and operating in the first operating mode can include determining that a grid fault has occurred and, in response to that determination, switching from the first operating mode to a second operating mode. Additionally or alternatively, operating in the second operating mode can include obtaining an indication to switch to the first operating mode and, in response to the obtained indication, performing a switch from the second operating mode to the first operating mode. Preferably, the indication can be obtained when the grid recovers from the grid fault.

[0037] Note that after a grid event (i.e., a fault), the full power capability of the converter may no longer be available. As described above, in response to determining the grid event, it is switched to the second operating mode. Thus, when the converter recovers its full power capability, i.e., when the grid fault is cleared, the indication can be obtained.

[0038] In one example, the method can include operating a power converter in a first operating mode. Operating in the first operating mode can include determining that a first reference and / or a reference current determined based on the first reference exceeds a corresponding predefined threshold and, in response to the determination, switching from the first operating mode to a second operating mode.

[0039] According to one example, operating the power converter in the second operating mode can include stopping at least part of the calculation of the virtual synchronous machine control scheme. Preferably, the part includes one or more integrator units.

[0040] Since at least part of the calculation output of the virtual synchronous machine control scheme is no longer used, the calculation of the output can be stopped, and thus, the total number of calculation steps required in each calculation cycle can be reduced. Therefore, the calculation performance of the method can be improved.

[0041] According to one example, the virtual synchronous machine control scheme can include one or more integrator units, and each integrator unit in the one or more integrator units can output the sum of the integral of the signal input to the integrator unit and an initial value. Parameterization can include setting one or more initial values of the one or more integrator units based on an estimated parameterized value. Preferably, the integral of the signal is an integral with respect to time.

[0042] According to one example, one or more initial conditions may include one or more of an initial amplitude of a first reference, an initial phase angle of the first reference, and an initial frequency of the first reference.

[0043] Preferably, the initial amplitude and the initial phase angle may include an initial voltage amplitude and a voltage phase angle, respectively. More preferably, the initial frequency may include a derivative of the initial voltage phase angle.

[0044] It should be clear that the setting may be performed repeatedly. Thus, the setting may include a reset.

[0045] Since the output of the integrator unit may correspond to the (multidimensional) state of the state - space representation of the virtual - synchronous - machine control scheme, setting or resetting the initial conditions of the integrator unit corresponds to setting or resetting the state of the virtual - synchronous - machine control scheme. In other words, the virtual - synchronous - machine control scheme is (re)initialized. When setting or resetting the state associated with the first operating mode according to one or more operating parameters obtained while operating in the second operating mode, the actual states of the first operating mode and the second operating mode are aligned. Since the states (operating parameters) correspond to each other, there is no difference between the references generated when operating in the first operating mode (setting a second reference based on a first reference) or the second operating mode (independently setting the second reference) that correspond to the second reference. Thus, a smooth transition is possible.

[0046] In one example, the initial conditions may include at least one of an initial voltage phase angle, an initial voltage amplitude, and an initial speed.

[0047] According to one example, the current of the converted electric power output to the power grid may be controlled by a current control according to a controller reference current, and the second reference may be the reference current. The method may include, when the power controller operates in the second operating mode, inputting the second reference as the controller reference current to the current control, and the switching from the second operating mode to the first operating mode may include inputting a first reference current derived from the first reference as the controller reference current to the current control instead of the second reference current.

[0048] Switching the input to the current control to achieve the switching between the first and second operating modes is beneficial because reference currents for the current control are generated in both the first and second operating modes. Thus, when the reference current generated in the first operating mode and the reference current generated in the second operating mode are aligned with each other, i.e., when there is no difference between the two reference currents, switching the reference current fed to the current control results in a smooth transition between the operating modes.

[0049] In one example, the estimation parameterization includes estimating one or more reference powers of a virtual synchronous machine control scheme. Preferably, the estimation is based on a virtual impedance.

[0050] According to one example, in a first operating mode, the converted electrical power can be output to the power grid according to a reference power. The switching from a second operating mode to the first operating mode can include obtaining a power parameter indicative of the electrical power output to the power grid when operating in the second operating mode, and setting the reference power to the power parameter.

[0051] The reference power can include a reference active power and / or a reference reactive power. The converted electrical power output to the power grid can include an active power output according to the reference active power and / or a reactive power output according to the reference reactive power.

[0052] As a result, the control error between the electrical power output to the power grid and the reference power may be small or may not exist. Therefore, when the power converter switches from the second operating mode to the first operating mode, the power output to the power grid does not change suddenly. Thus, since the control error is forced to zero when the first operating mode takes over, the transition can be even smoother.

[0053] According to one example, the switching from the second operating mode to the first operating mode can further include transforming the reference power set to the power parameter into another reference power according to a predetermined transformation function. Preferably, the predetermined transformation function can depend on time.

[0054] The transformation function can be pre-determined based on the closed-loop dynamics of the virtual synchronous machine.

[0055] The another reference power can include the reference power according to which the power converter outputs the converted electrical power when the power converter operates in the first operating mode. The another reference power can include another reference active power and / or another reference reactive power. The another reference power can include an external reference power, which is preferably received from an external source, such as from the power grid or from an external controller, such as a power plant controller included in a wind farm. The external reference power can be received, for example, as a power scheduling command.

[0056] The transformation function can include a ramp function or a sigmoid function. The ramp function or the sigmoid function can define the transition between the reference power set to the power parameter and the another reference power, especially a time-related transition. For example, the ramp function or the sigmoid function can increase or decrease from the reference power set to the power parameter to the another reference power at an initial time within a predetermined time period.

[0057] It should be clear that the transformation function is not limited to the given exemplary implementation. The transformation function can include any function from zero to one. For example, the transformation function can include a step function.

[0058] The transformation can allow for a controlled transition from a reference power that does not succumb to control errors to another reference power. The dynamics of the transition between the reference powers can be predetermined by parameterizing the transformation function. Thus, the transition between the first and second operating modes can be even smoother. Since the power converter has been operating in the first operating mode during the transformation, ancillary services can be provided quickly, and thus the contribution to grid stability can be increased.

[0059] It should be clear that the switch to the first operating mode can be performed at any time, even without adjusting the reference power, and when an adjustment is applied, the transition to the first operating mode is optimized. However, when no adjustment is applied, a smooth transition between the operating modes is still performed. Not adjusting the reference power is equivalent to using a transformation function that includes a step function and is equivalent to starting operation in the first operating mode with a non-zero control error. This only results in a change in the output converted electrical power that does not follow the dynamics of the transformation function. However, the change is still limited / suppressed by the system behavior that defines the dynamics of the output converted electrical power in response to the input reference power.

[0060] According to another aspect of the present invention, a power converter is provided. The power converter is configured to be electrically coupled to a power grid and output converted electrical power to the power grid. The power converter can operate in a first operating mode, in which the converter operates based on a first reference generated by a virtual synchronous machine control scheme. The power converter can also operate in a second operating mode, in which the converter operates based on a second reference that is not generated by the virtual synchronous machine control scheme. The power converter is configured to perform any of the methods described herein.

[0061] According to another aspect of the present invention, a control system for controlling the operation of a power converter is provided. The power converter is configured to be electrically coupled to a power grid and output converted electrical power to the power grid. The power converter can operate in a first operating mode, in which the power converter operates based on a first reference generated by a virtual synchronous machine control scheme. The power converter can also operate in a second operating mode, in which the converter operates based on a second reference that is not generated by the virtual synchronous machine control scheme. The control system is configured to perform any of the methods described herein.

[0062] The control system can be configured to be coupled to the power converter. The control system can be an external control system of the power converter, i.e., not included in the power converter, or it can be an internal control system, i.e., included in the power converter.

[0063] The control system may include, for example, a processing unit and a memory unit. The memory unit stores control instructions which, when executed by the processing unit of the control system, cause the control system to perform any of the methods described herein. The processing unit may include, for example, a digital signal processor, an application specific integrated circuit, a field programmable gate array, a microprocessor, etc. The memory unit may include RAM, ROM, flash memory, a hard disk drive, etc.

[0064] According to another aspect of the present invention, a power generation system is provided. The power generation system includes any of the power converters described herein. The power converter is electrically coupled to the power grid and outputs the converted electrical power to the power grid. The power generation system further includes any of the control systems described herein and at least one power generation unit configured to supply electrical power to the power converter. The control system is coupled to the power converter to control the operation of the power converter.

[0065] The at least one power generation unit may include components capable of receiving and / or outputting electrical power, such as a generator (e.g., a generator of a wind turbine, in particular an asynchronous or synchronous generator), or a photovoltaic system (e.g., a photovoltaic module), or an energy storage system, etc., or a combination of these components. When the at least one power generation unit includes more than one power generation unit, the more than one power generation units may be coupled to each other by electrical coupling means electrically coupled to the power converter. For example, the more than one power generation units may supply electrical power to the power converter via a DC link.

[0066] According to another aspect of the present invention, a wind turbine is provided. The wind turbine is configured to supply the generated electrical power to any of the power converters described herein.

[0067] According to another aspect of the present invention, a computer program for controlling the operation of a power converter is provided. The computer program includes control instructions which, when executed by the processing unit of a control system that controls the operation of the power converter, cause the processing unit to perform any of the methods described herein. The computer program may be provided on a volatile or non-volatile storage medium or data carrier.

[0068] It should be understood that, without departing from the scope of the present invention, the above features and those to be explained below can be used not only in the respective combinations shown, but also in other combinations or alone. In particular, the features of different aspects and embodiments of the present invention can be combined with each other, unless stated to the contrary.

[0069] It should also be clear that the order of the method steps of the methods described herein is not limited to the order described. In addition, the methods are not limited to the number of steps described. The individual steps of the methods can be replaced, extended or not performed. BRIEF DESCRIPTION OF THE DRAWINGS

[0070] The foregoing and other features and advantages of the present invention will become more apparent from the following detailed description read in conjunction with the accompanying drawings. In the drawings, like reference numerals represent like elements.

[0071] Figure 1 is a schematic diagram showing a power generation system according to an example.

[0072] Figure 2 is a schematic diagram showing a control system and a signal flow diagram according to an example, and the signal flow diagram shows the operation of the control system.

[0073] Figure 3 is a schematic signal flow diagram according to an example, which shows the operation of a virtual synchronous machine implemented by a control system.

[0074] Figure 4 is a schematic signal flow diagram of a reference amplitude generation unit included in a virtual synchronous machine according to an example.

[0075] Figure 5 is a schematic flow diagram showing a method of operating a power converter according to an example. DETAILED DESCRIPTION

[0076] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. It should be understood that the following description of the embodiments is given only for the purpose of illustration and not in a limiting sense. It should be noted that the drawings are only regarded as schematic representations, and the elements in the drawings are not necessarily in proportion to each other. On the contrary, the representations of the various elements are chosen such that their functions and general purposes become apparent to those skilled in the art. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. Unless otherwise specified, the terms "comprising", "having", "including" and "containing" should be construed as open-ended terms (i.e., meaning "including, but not limited to").

[0077] Figure 1 is a schematic diagram showing a power generation system 100 according to an example.

[0078] The power generation system 100 may include a control system 101, a power converter 104, and a power generation unit 105. The power converter 104 may be configured to be electrically coupled to the power grid 108 and output converted electric power to the power grid 108. The converted electric power may be converted from electric power, and the electric power is provided to the power converter 104 from the power generation unit 105.

[0079] The conversion can be based on a control signal 109, which is generated by a control system 101 to control the operation of a power converter 104 and is provided to the power converter 104. For example, the control signal 109 can include a pulse width modulation signal to control switching devices included in the converter 104, such as semiconductor switches. The control system 101 can include a processing unit 102 and a memory unit 103. The memory unit 103 stores control instructions that, when executed by the processing unit 102 of the control system 101, cause the control system 101 to perform the operation of the power converter 104. To generate the control signal 109, the control system 103 can include an acquisition unit 106, which is configured to acquire one or more operating parameters indicating the actual grid voltage v and / or the actual current i injected by the converter 104 into the grid 108. The grid voltage v can be obtained as a complex number including an amplitude V and a phase angle The current i can be obtained as a complex number including an amplitude I and a phase angle . For example, the grid voltage v can be monitored, for example, by measuring at the point of common coupling of the grid 108 and the power converter 104 or the power generation system 100. For example, the current i injected by the converter 104 can be monitored, for example, by measuring at the output of the power converter 104 or the power generation system 100. The conversion can also be based on an external reference power obtained by the control system 101 from an external reference power supply unit 107. The external reference power can include an external reference active power P e * and an external reference reactive power Q e *.

[0080] Figure 2 FIG. is a schematic diagram showing a control system 101 and a signal flow diagram according to an example, and the signal flow diagram shows the operation of the control system 101.

[0081] The control system 101 can include a current control 208. The current control 208 can output a control signal 109 in response to the obtained complex controller reference current i* and the actual current i. Preferably, the output control signal 109 can further depend on the voltage v, for example, when a current feed term is implemented. Based on the switching state of the switch 207, the controller reference current i* is equal to the complex reference current i 1 * generated by a virtual synchronous machine control scheme (VSM control scheme) 201 or equal to the complex reference current i 2*, the switching of switch 207 between its two switch states can be controlled by a switching flag sf. For example, the switching flag can be represented as a one-bit signal, which can be implemented, for example, using a boolean data type, where each of the two states of this data type corresponds to one of the states of switch 207. For example, when the switching flag sf is activated, it can switch from the first operating mode to the second operating mode. And when the switching flag sf is deactivated, parameterization can be performed, and it can switch back to the first operating mode.

[0082] When the power converter 104 operates in a first operating mode, for example including a grid-forming mode, switch 207 is switched such that the reference current i* is equal to the reference current i 1 *, and when the power converter 104 operates in a second operating mode, for example including a grid-tracking mode, switch 207 is switched such that the reference current i* is equal to the reference current i 2 *.

[0083] In one example, in response to determining that a grid fault has occurred, it can switch from the first operating mode to the second operating mode. The grid fault can include, for example, voltage and / or frequency distortion. In another example, in response to determining that the grid has recovered from the grid fault or in response to a received switching request, it can switch from the second operating mode to the first operating mode. The switching flag sf can be generated accordingly.

[0084] The reference current i 2 * can be generated by the reference current generation unit 205 based on the voltage v and / or the current i. The current i 2 * can be generated by using, for example, a grid-tracking control scheme for performing a fast fault current injection operation. Thus, when the reference current i 2 * is fed into the current control 208, the electric power output to the grid 108 may no longer follow the obtained external reference power P e * and Q e *.

[0085] The reference current i 1 * can be generated by the VSM control scheme 201. The reference current i 1 * can be based on the external reference active power P e *, the external reference reactive power Q e *, the actual power P, the actual reactive power Q, and the grid voltage v.

[0086] The control system 101 may include a power determination unit 206 configured to determine the actual electric power output to the power grid 108 based on the obtained grid voltage v and the obtained current i. The determined actual electric power may include actual active power P and actual reactive power Q. The determination may be based on the following equations: and where V is the amplitude of the obtained grid voltage v and is the phase angle, where I is the amplitude of the obtained current i and is the phase angle, and where k is a constant gain.

[0087] The VSM control scheme 201 may include a virtual synchronous machine (VSM) 202. The VSM 202 generates a complex reference electromotive force ε e * based on an external reference active power P e *, an external reference reactive power Q 1 *, the actual power P, the actual reactive power Q, and the grid voltage v. The complex reference electromotive force ε 1 * may include a reference amplitude E* and a reference phase angle (as Figure 3 shown), similar to the electromotive force induced in the stator of a real synchronous generator. The reference electromotive force ε 1 * may be provided to a virtual-impedance-based reference current generation unit 203 to generate a reference current i 1 *. The virtual-impedance-based reference current generation unit 203 may implement the following equation: i 1 * = 1 / Z v (ε 1 * - v) eq.03, where Z v is the complex virtual impedance. The complex virtual impedance may be predefined and may include, for example, the impedance of the VSM 202. Preferably, the impedance may be inductive, i.e., a complex number including a positive imaginary part (similar to the stator leakage impedance of a real synchronous generator).

[0088] The VSM control scheme 201 may further obtain the complex reference electromotive force ε 2 *, which may be back-calculated by a virtual-impedance-based back-calculation unit 204. The reference electromotive force ε 2 * may be generated only when operating in a second operating mode, i.e., when i* is set to i 2 *. The reference electromotive force ε 2 * may be based on the reference current i 2 *, the grid voltage v, and the virtual impedance Zv Generation. Alternatively, the current i can be used instead of the reference current i 2 *. The virtual impedance-based inverse calculation unit 204 can inversely implement Equation eq.03 to generate the reference electromotive force ε by the following equation 2 *: ε 2 * = z v i 2 * + v eq.04.

[0089] However, it should be clear that the inverse calculation can be based on a model different from Equation eq.04, and Equation eq.04 is considered herein as an exemplary implementation of such a model.

[0090] Before the operation mode is switched from the second operation mode to the first operation mode, the reference electromotive force ε obtained by the inverse calculation 2 * can be used to parameterize the VSM control scheme 201, particularly the VSM 202. As a result of the parameterization, when subsequently switched from the second operation mode to the first operation mode, the reference electromotive force ε 1 * is initially set to the reference electromotive force ε 2 *, and correspondingly, the reference current i 1 * is initially set to the reference current i 2 *. In this way, the operation in the first operation mode continues from the same operating point where the operation in the second operation mode stops, and thus, the first operation mode smoothly takes over from the second operation mode.

[0091] Figure 3 is a schematic signal flow diagram according to an example, which shows the operation of the VSM 202 implemented by the control system.

[0092] Figure 3 The dashed arrows in indicate the optional parts of the signal flow diagram. Below, it is described without considering the optional parts of the flow diagram Figure 3 . Therefore, the optional inputs to the summing points 309, 310, 311, and 312 are considered to be set to zero.

[0093] The VSM 202 may include a reference phase angle generation unit 300 and a reference amplitude generation unit 350. The reference phase angle generation unit 300 can generate the reference phase angle of the reference electromotive force ε based on the external reference active power P e * and the active power P 1 * of the reference electromotive force The reference amplitude generation unit 350 can generate the reference amplitude E* of the reference electromotive force ε based on the external reference reactive power Q e *, the reactive power Q, and the grid voltage v 1 * of the reference electromotive force. The reference amplitude E* and the reference phase angle The complex reference electromotive force ε can be combined by the combining unit 318. 1 *.

[0094] Reference phase angle can be generated by the integrator unit 306. The integrator unit 306 can output the sum of the initial value of the reference phase angle and the integral of the virtual synchronous machine speed ω over time. The speed ω can be generated by the integrator unit 305. The integrator unit 305 can output the sum of the initial value of the speed ω and the integral of the virtual synchronous machine acceleration α over time. The acceleration α can be generated by the gain unit 302. The gain unit 302 can output the product of the gain and the active power control error output by the summing point 308. In one example, the gain of the gain unit 302 can be a constant and can be predetermined based on the fundamental frequency ω b of the VSM202 and a predetermined inertia J v . The control error output by the summing point 308 can be generated by subtracting the actual active power P from the reference active power P*. The reference active power P* can be output by the summing point 307 and includes the sum of the reference active power P t * and the P-f droop active power dP. The P-f droop active power dP can be generated by the P-f droop unit 303 based on the speed ω. The reference active power P t * can be generated by the transformation unit 301 based on the P-f droop active power dP, the active power P, and the external reference active power P e *.

[0095] When switching from the second operating mode to the first operating mode (which can be indicated by a switching flag sf, for example, by a rising or falling signal edge), the parameterization of the VSM202 is performed and thus the parameterization of the VSM control scheme 201 is performed. The parameterization can include setting or resetting the initial values of the integrator unit 305 and the integrator unit 306. Therefore, the initial value of the integrator unit 305 can be set to the initial speed ω 0 , and the initial value of the integrator unit 306 can be set to the initial phase angle Initial phase angle can be equal to the reference initial phase angle which is the phase angle of the complex reference electromotive force ε 2 *. The initial speed ω 0 can be equal to the reference initial speed ω E0 *, which can be generated by the differentiator unit 304 by differentiating the reference initial phase angle over time. The reference initial phase angle and the corresponding reference initial amplitude E 2 * of the reference electromotive force ε 0 * can be separately output by the separating unit 317.

[0096] In other words, when switching from the second operating mode to the first operating mode, the VSM control scheme 201 is set such that the calculation of the reference current i 1 * continues the operation starting from the last calculated reference current i 2 *. The operation in the first operating mode can be continued immediately based on the external reference power P e * and Q e *, or based on the reference power P Figure 4 * and Q t * generated by the transformation units 301 and 401 (as t shown), Figure 4 shown. In the latter case, the reference power P t * and Q t * can be transformed into or set to the external reference power P e * and Q e * according to a predetermined dynamics. The predetermined dynamics can be triggered by a switching flag sf and can be implemented by a predetermined transformation function.

[0097] For this purpose, the transformation unit 301 can implement a system of equations according to the following equations: R t *(t) = (P(t r ) - dP(t r ))(1 - N(t)) + P e *(t)N(t) eq.05, and where t is time, t r is the time point when switching to the first operating mode, t f is the time point when the transformation is completed, and f(t) is a predetermined transformation function.

[0098] The transformation function f(t) can include, for example, a ramp function or a sigmoid function, the output value of which increases from 0 to 1 with time, where it outputs 0 when t is equal to t r and outputs 1 when t is equal to t f . Different types of transformation functions can be selected based on the virtual synchronous machine closed-loop dynamics.

[0099] As can be seen from equations eq.05 and eq.06, for t ≤ t r , the control error output by the summing point 308 can be set to zero. Therefore, the acceleration α is also set to zero. Therefore, when switching to the first operating mode, the reference phase angle 1 of the reference electromotive force ε is equal to the reference initial phase angle 2 of the reference electromotive force ε During a subsequent calculation period, when it may have been operating in the first operating mode, the reference active power P t * can be smoothly transformed from P t * = P(t r ) - dP(t r ) to P t * = P e *(t), thereby reconnecting the VSM control scheme 201 to the external power reference P e *.

[0100] Consider the following Figure 3 alternative portion of the signal flow diagram shown. The alternative portion shows a modification of the control scheme of the reference phase angle generation unit 300 according to an exemplary implementation. The modification may include a feedforward control structure and a proportional signal path in parallel with the integrator units 305, 306. When starting the control scheme of the reference phase angle generation unit 300, the use of the alternative modification may require forcing / initial setting the speed ω to an estimated or predetermined value. According to this modification, the obtained feedforward speed ω FF and the first sum of the control error amplified by the gain unit 315 can be added to the output of the integrator unit 305 at the summing point 309. The gain unit 315 may include a (predetermined) constant such that the output of the gain unit 315 may include the product of the control error and the constant. In addition, the obtained feedforward phase angle and the second sum of the control error amplified by the gain unit 316 can be added to the output of the integrator unit 306 at the summing point 310. The gain unit 316 may include a (predetermined) constant such that the output of the gain unit 316 may include the product of the control error and the constant. Since in the example shown the summing points 309 and 310 are arranged downstream of the integrator units 305 and 306, the first sum and the second sum must be subtracted from the reference initial speed ω E0 * and the reference initial phase angle respectively. In this way, it is ensured that when switching from the second operating mode to the first operating mode, the speed ω is equal to the reference initial speed ω E0 * and the reference phase angle is equal to the reference initial phase angle

[0101] It should be clear from the above that the modification of the control scheme of the reference phase angle generation unit 300 may require modification of the calculation of the initial values of the integrator units 305, 306. In addition, the implementation of the transformation unit 301 may need to be modified.

[0102] Figure 4 is a schematic signal flow diagram of the reference amplitude generation unit 350 included in the virtual synchronous machine 202 according to one example.

[0103] WithFigure 3 Similarly, Figure 4 the dashed arrows in Figure 4 represent an optional part of the signal flow diagram. Below, the description is made without considering the optional part of the flow diagram Figure 4 . Thus, the optional inputs to summing points 409 and 411 are considered to be set to zero.

[0104] The reference amplitude generation unit 350 can generate a reference electromotive force ε e * based on an external reference reactive power Q 1 *, reactive power Q, and grid voltage v, and a reference amplitude E

[0105] The reference amplitude E* can be generated by an integrator unit 405. The integrator unit 405 can output the sum of the initial value of the reference amplitude E* and the integral over time of the output generated by a gain unit 402. The gain unit 402 can output the product of a gain and the reactive power control error output by a summing point 408. In one example, the gain of the gain unit 402 can be a constant and can be predetermined. The control error output by the summing point 408 can be generated by subtracting the actual reactive power Q from the reference reactive power Q*. The reference reactive power Q* can be output by the summing point 407 and includes the sum of the reference reactive power Q t * and the Q-V droop reactive power dQ. The Q-V droop reactive power dQ can be generated by a Q-V droop unit 403 based on the grid voltage v or based on the amplitude V of the grid voltage v. The reference reactive power Q t * can be generated by a transformation unit 401 based on the Q-V droop reactive power dQ, reactive power Q, and external reference reactive power Q e *.

[0106] As described above, when switching from the second operating mode to the first operating mode (which can be indicated by a switching flag sf), the parameterization of the VSM202 is performed and thus the parameterization of the VSM control scheme 201 is performed.

[0107] In addition to setting or resetting the initial values of the integrator units 305, 306, the parameterization can also include setting or resetting the initial value of the integrator unit 405. The initial value of the integrator unit 405 can be set to an initial amplitude E 0 . The initial amplitude E 0 can be equal to the reference initial amplitude E 0 *, which is the amplitude of the complex reference electromotive force ε 2 *.

[0108] As described above, after switching to the first operating mode, the operation in the first operating mode can continue immediately based on the external reference powers P e * and Q e *, or based on those generated by the transformation unit 301 (asFigure 3 shown) and the reference power P generated by the transformation unit 401 t * and Q t * continues.

[0109] For this purpose, the transformation unit 401 can implement a system of equations similar to the system of equations implemented by the transformation unit 301. The implementation of the transformation unit 401 can include a system of equations according to the following equations: Q e *(t) = (Q(t r ) - dQ(t r ))(1 - N(t)) + Q e *(t)N(t) eq.07, where N(t) is defined as shown above.

[0110] It can be seen from equation eq.07 that for t ≤ t r , the control error output by the summing point 408 can be set to zero. The input of the integrator unit 405 is therefore also set to zero. Therefore, when switching to the first operating mode, the reference amplitude E* of the reference electromotive force ε 1 * is equal to the reference initial amplitude E of the reference electromotive force ε 2 *. In subsequent calculation cycles, when it may have been operating in the first operating mode, the reference reactive power Q 0 * can be smoothly transformed from Q t * = Q(t t ) - dQ(t r ) to Q r * = Q t *(t), thereby reconnecting the VSM control scheme 201 to the external power reference Q e *. e * reconnects.

[0111] It should be clear that when performing the transformation of the reference power P t *, Q t * into the external reference power P e *, Q e *, the power generation system 100 may already be able to provide ancillary services. It should also be clear that, contrary to the setting or resetting of the initial values, the transformation can only be optionally performed. For example, when the initial values are set or reset but the transformation is not performed, the switch from the second operating mode to the first operating mode will still be smooth, but will be associated with a sudden change in the operating point in subsequent calculation cycles.

[0112] As can be seen from the above, when switching to the first operating mode, the reference phase angle 1 * of the reference electromotive force ε and the corresponding reference amplitude E* are equal to the reference electromotive force ε2 The reference initial phase angle of and the corresponding initial reference amplitude E 0 *. Since the reference electromotive force ε 1 * is equal to the reference electromotive force ε 2 *, when switching to the first operating mode, based on the reference electromotive force ε 1 the reference current i 1 * generated is equal to the reference current i 2 *. In this way, the operation can continue smoothly from the reference current i 1 * in the first operating mode, and this reference current i 1 * is generated based on the last generated reference current i 2 * and thus corresponds to the current i injected into the power grid based on the output of the current control 208 in response to the last generated reference current i 2 *.

[0113] Consider next Figure 4 an alternative part of the signal flow diagram shown. The alternative part shows a modification of the control scheme of the reference amplitude generation unit 350 according to an exemplary implementation. This modification may include a feedforward control structure and a proportional signal path in parallel with the integrator unit 405. According to this modification, the feedforward amplitude E FF of the obtained electromotive force and the third sum of the control error amplified by the gain unit 415 can be added to the output of the integrator unit 405 at the summing point 409. The gain unit 415 may include a (predetermined) constant such that the output of the gain unit 415 may include the product of the control error and the constant. Since in the example shown the summing point 409 is arranged downstream of the integrator unit 405, the third sum must be subtracted from the reference initial amplitude E 0 *. In this way, it is ensured that when switching from the second operating mode to the first operating mode, the reference amplitude E* is equal to the reference initial amplitude E 0 *.

[0114] It should be clear from the above that the modification of the control scheme of the reference amplitude generation unit 350 may require modification of the calculation of the initial value of the integrator unit 405. In addition, the implementation of the transformation unit 401 may need to be modified.

[0115] More generally, the parameterization of the control scheme of the first operating mode may include parameterizing all the states included in the control scheme that affect the reference current i 1 * such that when switching from the second operating mode to the first operating mode, the reference current i 1 * corresponds to the reference current i 2 *.

[0116] Note that, optionally, when operating in the second operating mode, the calculation of the outputs of at least the integrator units 305, 306, and / or 405 can be stopped or frozen at least to reduce the number of calculation steps required for the calculation cycle. It should be clear that when operating in the first operating mode, any remaining calculations unrelated to the second operating mode can be stopped or frozen, and vice versa.

[0117] Figure 5 FIG. 5 is a schematic flow chart showing a method 500 of operating a power converter 104 according to an example. The power converter can be electrically coupled to the power grid and can output converted electrical power to the power grid. The power converter can operate in a first operating mode, in which the converter operates based on a first reference generated by a virtual synchronous machine control, and the power converter can also operate in a second operating mode, in which the converter operates based on a second reference that is not generated by a virtual synchronous machine control scheme. In step S01, the method 500 can include operating the power converter in the second operating mode. In step S05, the method 500 can include obtaining a command / request to switch to the first operating mode. In response to the obtained command / request, the method 500 can include switching the operation of the converter from the second operating mode to the first operating mode, where the switching operation can include steps S10, S15, S20, and S25. In step S10, the method 500 can include obtaining one or more operating parameters when operating in the second operating mode, which indicate the current supplied by the converter to the power grid and / or the power grid voltage. In step S15, the method 500 can include estimating the parameterization for the virtual synchronous machine control scheme from the one or more operating parameters, where the estimation includes calculating one or more values of the parameterization, and the one or more values of the parameterization will result in at least a portion of the one or more operating parameters obtained when the converter operates in the first operating mode at the first reference generated by the virtual synchronous machine control scheme. Calculating the one or more values of the parameterization can include a backward calculation from the obtained one or more operating parameters to the parameterization. In step S20, the method 500 can include parameterizing the virtual synchronous machine control scheme with the estimated parameterization. In step S25, the method 500 can include switching to the first operating mode by operating the virtual synchronous machine control scheme with the parameterization to generate the first reference and operating the converter based on the generated first reference.

[0118] Although specific embodiments are disclosed herein, various changes and modifications can be made without departing from the scope of the present invention. This embodiment is considered illustrative in all respects and not restrictive, and all changes falling within the meaning and equivalent scope of the appended claims are intended to be included therein.

Claims

1. A method of operating a power converter (104), wherein the power converter is electrically coupled to an electrical grid (108) and outputs converted electrical power to the electrical grid, and wherein the power converter is operable in a first operating mode in which the converter operates based on a first reference (ε 1 *) generated by a virtual synchronous machine control scheme (201), and wherein the power converter is further operable in a second operating mode in which the converter operates based on a second reference (i 2 *) that is not generated by the virtual synchronous machine control scheme, wherein the method (500) includes switching the operation of the converter from the second operating mode to the first operating mode, wherein switching the operation Comprising: - Obtaining one or more operating parameters when operating in a second operating mode, which indicate the current (i, i 2 *) supplied by the converter to the electrical grid and / or the electrical grid voltage (v); - Estimate a parameterization for a virtual synchronous machine control scheme from one or more operating parameters, wherein the estimation includes calculating one or more values of the parameterization that will result in at least a portion of the one or more operating parameters obtained when the converter operates in this first reference (ε 1 *) generated by the virtual synchronous machine control scheme in this first operating mode; - Parameterizing the virtual synchronous machine control scheme with an estimated parameterization; and - By parameterizing the virtual synchronous machine control scheme to generate the first reference (ε 1 *) and starting to operate the converter based on the generated first reference, thereby switching to the first operating mode.

2. The method according to claim 1, wherein calculating one or more values of the parameterization comprises a reverse calculation from one or more obtained operating parameters to the parameterization.

3. The method according to claim 1 or 2, wherein estimating the parameterization comprises estimating the first reference value (ε 1 *) according to one or more operating parameters, wherein the estimating comprises calculating one or more values of the first reference (ε 1 *) that will result in at least a portion of one or more operating parameters obtained when the converter operates based on the estimated first reference value (ε 1 *).

4. The method according to claim 3, wherein the second reference (i 2 *) is a reference current, wherein the obtained operating parameters include the reference current and the grid voltage (v), and wherein calculating one or more values of the first reference (ε 1 *) includes calculating a reference voltage amplitude (E*) and a reference phase angle based on the reference current, the grid voltage (v), and a virtual impedance associated with the virtual synchronous machine control scheme (201) and / or wherein calculating one or more values of the first reference (ε 1 *) includes calculating a frequency indicative of a change in the reference phase angle.

5. The method according to any one of the preceding claims, wherein calculating a cycle comprises generating values of a first and / or second reference, and wherein both the parameterization and the switching are performed within one calculation cycle, or wherein the parameterization is performed in a first calculation cycle and the switching is performed in a second calculation cycle, and the first calculation cycle is performed before the second calculation cycle.

6. The method according to any one of the preceding claims, wherein the method comprises operating the power converter (104) in a first operating mode, and wherein operating in the first operating mode Comprises: Determining that a grid fault has occurred, and In response to said determination, switching from the first operating mode to a second operating mode, and / or wherein operating in the second operating mode comprises: Obtaining an indication to switch to the first operating mode, and In response to the obtained indication, Performing said switching from the second operating mode to the first operating mode, wherein preferably, when the grid recovers from a grid fault, the indication is obtained.

7. The method according to any one of the preceding claims, wherein, Operating the power converter (104) in the second operating mode comprises stopping at least part of the calculation of the virtual synchronous machine control scheme (201), wherein preferably, this part comprises one or more integrator units (305, 306, 405).

8. The method according to any one of the preceding claims, wherein the virtual synchronous machine control scheme (201) includes one or more integrator units (305, 306, 405), and each integrator unit in the one or more integrator units outputs an initial value (E 0 , ω 0 ) and the sum of the integrals of the signals input to the integrator unit, wherein the parameterization includes setting one or more initial values of one or more integrator units based on an estimated parameterized value (E 0 , ω 0 ).

9. The method according to claim 8, wherein, One or more initial conditions include the initial amplitude (E 1 *) of a first reference (ε 0 ), the initial phase angle of a first reference (ε 1 *), and / or the initial frequency (ω 0 ) of a first reference (ε 1 *).

10. The method according to any one of the preceding claims, wherein the current of the converted electric power output to the power grid is controlled by a current control (208) according to a controller reference current (i*), and wherein a second reference (i 2 *) is a reference current, wherein the method Comprises: When the power controller operates in the second operation mode, a second reference (i 2 *) is input as the controller reference current (i*) to the current control (208), and wherein the switching from the second operating mode to the first operating mode comprises: The first reference current (i 1 *) derived from the first reference (ε 1 *) instead of the second reference current (i 2 *) is input to the current control (208) as the controller reference current (i*).

11. The method according to any one of the preceding claims, wherein, In the first operating mode, the converted electrical power is output to the grid according to a reference power (P*, Q*), wherein the switching from the second operating mode to the first operating mode comprises: Obtaining power parameters (P, Q) when operating in the second operating mode, which indicate the electrical power output to the grid, and Setting the reference power (P*, Q*) to the power parameters (P, Q).

12. The method according to claim 11, wherein the switching from the second operating mode to the first operating mode further Comprises: The reference power (P*, Q*) set as a power parameter is transformed into another reference power (P e *, Q e *) according to a predetermined transformation function, where preferably the predetermined transformation function depends on time.

13. A control system for controlling the operation of a power converter, wherein the power converter (104) is configured to be electrically coupled to an electrical grid (108) and output converted electrical power to the electrical grid (108), and wherein the power converter can operate in a first operating mode, in which the power converter (104) operates based on a first reference (ε 1 *) generated by a virtual synchronous machine control scheme (201), and wherein the power converter can also operate in a second operating mode, in which the converter operates based on a second reference (i 2 *) that is not generated by the virtual synchronous machine control scheme, wherein the control system (101) is configured to perform the method according to any one of claims 1-12.

14. A power generation system, Comprising: A power converter (104) configured to be electrically coupled to a grid (108) and output converted electrical power to the grid (108); At least one power generation unit (105) configured to supply electrical power to the power converter (104); and The control system (101) according to claim 13, wherein the control system (101) is coupled to the power converter (104) to control the operation of the power converter.

15. A computer program for controlling the operation of a power converter, wherein the computer program comprises control instructions which, when executed by a processing unit (102) of a control system (101) for controlling the operation of the power converter, cause the processing unit (102) to perform the method according to any one of claims 1 - 12.

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