Method for operating a grid-forming electrical converter, grid-forming electrical converter and power supply network

Abstract

Method for operating a grid-forming electrical converter (1), comprising the steps of: - Determining (S1) an initial reference current value (Iref); - Determining (S2) a current scaling factor, wherein the current scaling factor is less than one if the determined reference current (Iref) is greater than a predetermined threshold for a maximum current, and the current scaling factor is otherwise set to one; - Adjusting (S3) the initial reference current value (Iref) using the current scaling factor to obtain an adjusted reference current value (Ireflimited); - Calculating (S4) an initial reference active power output using the adjusted reference current value;- Determining (S5) a power scaling factor, wherein the power scaling factor is not equal to one if the determined reference active power output is outside a predetermined power range for power exchange between the converter (1) and the power grid (4), and the power scaling factor is otherwise set to one; - Adjusting (S6) an active component of the adjusted reference current (Ireflimited) using the power scaling factor to obtain an adjusted reference active power output; and - Driving (S7) the grid-forming electrical converter (1), wherein the active power output and the reactive power output are set using the adjusted reference current value (Ireflimited).;

Description

Technical field The present invention relates to a method for operating a grid-forming power converter. The present invention further relates to a grid-forming power converter and a power supply network with such a grid-forming power converter. background As renewable energy generation becomes increasingly widespread, inverters play a crucial role. Inverters act as the link between a direct current (DC) grid with one or more power generation plants on one side and an alternating current (AC) grid with one or more consumers on the other. When feeding electrical energy into a power grid, a distinction is made between grid-following inverters and grid-forming inverters. Grid-forming and grid-following inverters differ fundamentally in their operation and application. Grid-forming inverters are capable of independently providing a stable grid voltage and frequency. They are particularly suitable for off-grid applications or microgrids, as they act as the primary voltage source and can thus maintain a grid structure. Grid-following inverters, on the other hand, require an existing grid voltage to which they adapt and synchronize. They are mainly used in systems where a stable grid voltage already exists, such as in traditional networks with centralized power sources.Due to their different operating principles, these inverters each fulfill specific requirements and thus complement each other in hybrid energy systems. Furthermore, the operating behavior of conventional inverters, especially grid-following inverters, differs from that previously observed in connection with large synchronous machines in power plants. While synchronous machines, due to their large mass and associated inertia, react instantly to influences such as load transients or short circuits in a grid-supportive manner, inverters powered by renewable energy generation systems typically exhibit significantly different operating and control behavior. In WO 2024 / 074 214 A1, a method for operating a grid-forming voltage converter is described – also in connection with renewable energy sources – whereby the operation of the voltage converter with currents exceeding a current threshold is to be prevented. For this purpose, an active power reference value is calculated, which is limited depending on at least the current threshold. An active power control unit is configured to adjust the active power output of the voltage converter, at least depending on the active power reference value. Together with the limitation of the voltage amplitude, this is intended to prevent exceeding the current threshold. Particularly in the context of inverters powered by renewable energy generation systems, it can be desirable for a grid-forming inverter to replicate the operating behavior of a conventional synchronous machine with its corresponding inertia, or at least closely approximate it. Grid-forming inverters capable of damping oscillations in the connected power grid are especially desirable. Furthermore, grid-forming inverters capable of maintaining grid-supporting behavior and feeding in sinusoidal currents during system-induced power limitation, while adhering to the inverter's power limits, are desirable. Finally, grid-forming inverters capable of safely and reliably returning to an unlimited operating state after power limitation are also desirable. This is achieved through the method and the electrical power converter as well as the power supply network of the independent patent claims. The present invention provides a method for operating a grid-forming electrical converter, a grid-forming electrical converter, and a power supply network with the features of the independent claims. Further advantageous embodiments are the subject of the dependent claims. According to a first aspect, a method for operating a grid-forming electrical converter is provided. The method includes a step for determining an initial reference current value. Furthermore, the method includes a step for determining a current scaling factor. The current scaling factor can be set to a value less than one if the determined reference current is greater than a predetermined threshold for a maximum current, for example, a maximum permissible current in the converter. Otherwise, the current scaling factor can be set to 1. The method also includes a step for adjusting the initial reference current value. In particular, the initial reference current value can be adjusted using the current scaling factor. This results in an adjusted reference current value. Finally, the method includes a step for calculating an initial reference active power output.This initial reference power output can be calculated, in particular, using the adjusted reference current value. Furthermore, the procedure includes a step for determining a power scaling factor. This power scaling factor can be set to a value other than one if the determined reference power output lies outside a predetermined power range for power exchange by the converter. Specifically, the power scaling factor can be set to a value less than one if the determined reference power output is greater than an upper limit for the power range. Otherwise, i.e., if the determined reference power output lies within the power range for power exchange, the power scaling factor can be set to 1. The procedure further includes a step for adjusting an active component of the adjusted reference current.The active power component of the adjusted reference current can be adjusted, in particular, using the power scaling factor. This allows for the creation of an adjusted reference active power output. Finally, the procedure includes a step for controlling the grid-forming electrical converter. Here, the active power output can be set using the adjusted reference active power output. Furthermore, the reactive power output can be set using the adjusted reference current value. For the procedure, it is irrelevant whether the current limiting is performed first, followed by the active power limiting, or in reverse order, with the active power limiting performed first and then the current limiting applied. According to a second aspect, a grid-forming electrical converter is provided. The converter comprises a DC input, a grid input, a converter circuit, and a control unit. The DC input is designed to be electrically coupled to a DC voltage source. The grid input is designed to be electrically coupled to a power supply network. The converter circuit is designed to convert the DC voltage provided at the DC input into an AC voltage. Furthermore, the converter circuit is designed to provide the AC voltage at the grid input. The control unit is designed to control the converter using a method according to the invention as described in the first aspect. According to a further aspect, a power supply network is provided. The power supply network comprises a grid-forming electrical power converter according to the invention, as described in the second aspect. The power supply network can, in particular, comprise a local island grid, a microgrid, or a sub-network. The present invention is based on the understanding that power converters, as commonly used for feeding electrical energy into an AC grid, exhibit operating characteristics that differ from those of a conventional synchronous machine. Grid-forming power converters are operated as voltage sources. The active and reactive power supplied by the power converter is determined during operation solely by adjusting the phase angle and voltage amplitude. A change in the external grid voltage thus leads to a change in power output by the power converter in order to support the grid. If a limit value of the power converter is reached during this control process, a limitation must be implemented within the power converter itself. Such limit values ​​include, for example, the maximum currents in the power converter and active power limits.The active power limits can vary depending on the current operating conditions, such as photovoltaic yield or the state of charge of any battery. During control, it must be ensured that any oscillations caused by the grid-forming converter can be dampened. Furthermore, it must be ensured that grid-supporting behavior is maintained even during converter limiting, with sinusoidal currents flowing. The converter's variable power limits, if any, must always be observed. Finally, it must be ensured that the grid-forming converter can reliably return to unrestricted operation after limiting. Based on this, it is therefore an objective of the present invention to provide a grid-forming power converter and a method for operating a grid-forming power converter that can meet the aforementioned requirements. In particular, the concept according to the invention enables a grid-forming power converter that can exhibit operating characteristics that closely approximate the operating characteristics of a synchronous machine, especially a synchronous machine for grid-forming energy feed-in into a power supply network. Several factors can be decisive in limiting the power output during the operation of a power converter. Firstly, the active power output can be limited by the active power currently supplied by a power generation device, such as a photovoltaic system. If the system includes a battery storage system or similar, its characteristics, such as its current state of charge and maximum power output or input, can also be considered for this limitation. Furthermore, the maximum permissible currents that the power converter can handle also represent a limitation that can affect not only the active power output or input but also the reactive power. The maximum permissible currents in a power converter can be limited, for example, by the dimensions of the components used, such as semiconductor switches. Thus, the maximum current in the power converter can result from the design, particularly from the implemented components. Specifications for the maximum permissible current in the power converter can be determined, for example, from the datasheets for the components used. If necessary, the maximum current can be further adjusted, particularly by reducing it, by adding a safety margin. This maximum current, which can be carried by the power converter, can limit both the active power and the reactive power output or input by the power converter. During operation, initial values ​​for active and reactive power at the converter can be influenced by parameters currently present in the connected AC grid, such as frequency and voltage amplitude. Furthermore, other specifications may exist that affect the limits of the active and reactive power that the converter can provide. For example, a maximum available active power may result from the current system characteristics of a power generation plant, such as a PV system, possibly in combination with a battery storage system. Additionally, specifications for the reactive power to be provided may exist, such as system-related specifications, user specifications, or similar. Furthermore, specifications for active or reactive power may also be received from a remote, possibly higher-level, control center.From these active and reactive power values, a corresponding initial reference current is derived, which will flow through the converter to meet these requirements. If this initial reference current lies within a permissible range for the converter, further control can be based on this reference current. Otherwise, the reference current can be scaled down using a current scaling factor to ensure compliance with the converter's current limits. Thus, in subsequent control operations, a current value reduced by the current scaling factor can be used instead of the initial current value, if necessary. Furthermore, the maximum available active power may be limited, for example, due to a limited power supply on the energy generation side – possibly also taking into account the state of charge of an electrical energy storage system on the generation side. Numerous factors can play a role here, such as the active power currently available from a photovoltaic system or the operating parameters of electrical energy storage systems, and these must be considered. In addition, negative power output, i.e., power input by the inverter, for example to an electrical energy storage system such as a battery, is also possible. Here, too, limiting factors, such as the maximum charging capacity of the energy storage system, can be taken into account. To take into account the limits for active power output – both positive and negative – a requirement derived from the current network parameters can, if necessary, be limited to the specified constraints. For this purpose, an initially determined reference active power output can be checked against the system-related limitations. If the initial reference active power exceeds the maximum or minimum (especially negative) possible active power output, a scaling factor, in particular a power scaling factor, can be determined to limit the control variable for active power output to the possible or permissible power range. Thus, adapted control variables for current and active power exchange are available for regulating the grid-forming converter, possibly using the described scaling factors. This ensures that the required limits are always reliably maintained during operation of the grid-forming converter. According to one embodiment, the predetermined power range for power exchange includes an upper threshold for maximum active power output. This upper threshold can be determined, for example, based on the currently available active power of a power generation device connected to the converter, such as a photovoltaic system, and / or operating parameters of an electrical energy storage device. Furthermore, the predetermined power range for power exchange can also include a lower threshold, which specifies, for example, a minimum power output or power input by the converter. Power input can be specified, for example, by a negative value for power output. According to one embodiment, determining the initial reference current value involves determining a voltage difference between a virtual setpoint voltage of the converter and an electrical voltage at the converter's grid connection. This virtual setpoint voltage can be an internal calculated value for the converter's control. In particular, this virtual setpoint voltage need not correspond to an actual electrical voltage present in the converter. Compared to a synchronous machine, whose behavior is to be at least partially replicated by the converter, the virtual setpoint voltage can, for example, correspond to the rotor voltage of such a synchronous machine. The virtual setpoint voltage can, for example, be determined based on power control.In other words, the virtual setpoint voltage of the converter is derived from the active and reactive power to be transmitted by the converter, i.e., the active and reactive power that the converter is to feed into the grid connection. In this case, the initial reference current value can be calculated using this determined voltage difference. In particular, a predetermined virtual impedance can also be taken into account for this calculation. For example, a predefined desired impedance, which the converter represents relative to the grid, can be chosen as the predetermined impedance. According to one embodiment, individual reference current values ​​are calculated for the positive-sequence, negative-sequence, and zero-sequence systems to determine the initial reference current value. The current scaling factor can then be calculated by summing these three currents for the positive-sequence, negative-sequence, and zero-sequence systems and subsequently determining the current scaling factor using a quotient of the predetermined threshold value for the maximum current and this sum. According to one embodiment, the predetermined threshold for the maximum power exchange of the converter can be adjusted using a value for an electrical voltage at the converter's DC link. The magnitude and, in particular, the rate of change of this electrical voltage at the DC link (DC link voltage) can be used, for example, to balance the power between the feeding energy generation system, a local energy storage device such as a battery or similar, and power output to or input from the connected power supply network. According to one embodiment, the upper threshold for power exchange by the converter can be reduced if the DC link voltage falls below a predetermined first threshold. This indicates a power deficit, so the power output limit is lowered in such a case. The upper threshold can also be increased if the DC link voltage exceeds a second threshold. The first and second thresholds can differ. According to one embodiment, a lower threshold for power exchange can be reduced if the voltage at the intermediate circuit falls below a third threshold. Alternatively, this lower threshold can be increased if a fourth threshold is exceeded. Here, too, the third and fourth thresholds can differ. The lower threshold for power exchange can also be negative, which corresponds to power transfer from the external power supply network to a local energy storage device. According to one embodiment, a virtual unlimited power output can be determined during a limitation of the initial reference current value or the initial reference active power output. In particular, the virtual setpoint voltage of the converter can be calculated using such a virtual unlimited power output. This enables a reliable return to the unlimited state. According to one embodiment, the virtual unlimited power output during limitation in the converter can be calculated using the value of the initial reference current. Furthermore, the grid voltage can also be taken into account. In other words, the virtual unlimited power is not reduced by a reduction in electrical current using the current scaling factor or the power scaling factor. According to one embodiment, the method further comprises the steps for determining a virtual frequency of the converter and for determining a grid frequency at the converter's grid connection. The converter's virtual frequency represents a computational parameter of the converter. This virtual frequency can, for example, correspond to the rotor frequency of a synchronous machine whose operating characteristics are to be at least partially replicated by the converter. Any suitable method can be used for this purpose. If the converter's virtual frequency and the grid frequency at the converter's grid connection differ, the converter's virtual frequency can be adjusted using this difference. During this adjustment, a predetermined damping factor, in particular a control with a predetermined damping term, can be provided.In particular, such a damping term can be used during power control to determine the virtual target voltage. According to one embodiment, the predetermined damping term can include a function that depends on the difference between the determined virtual frequency and the frequency at the grid connection of the power converter. The above embodiments and further developments can be combined with one another as appropriate. Further embodiments, further developments, and implementations of the invention also include combinations of features of the invention described previously or subsequently with regard to the exemplary embodiments, even if not explicitly mentioned. In particular, those skilled in the art will also add individual aspects as improvements or additions to the respective basic forms of the invention. Brief description of the drawings Further features and advantages of the invention are explained below with reference to the figures. Figure 1 shows a schematic representation of a power supply network with a power converter according to one embodiment; Figure 2 shows a schematic representation of a basic circuit diagram of an electrical power converter according to one embodiment; and Figure 3 shows a flowchart as it forms the basis of a process participant in a power converter according to one embodiment. Description of embodiments Fig. 1 shows a schematic block diagram of a power supply network 4 with an electrical converter 1 according to one embodiment. The power supply network 4 can be a multi-phase, in particular three-phase, AC network, such as a local network, a microgrid or a subnetwork of a larger power supply network. The power converter 1 can feed electrical energy from a power supply system 2 into the power supply network 4. For this purpose, the power converter 1 converts the electrical energy provided by the power supply system 2 as direct current into alternating current and makes this alternating current available at a grid connection for the power supply network 4. The power converter 1 is designed as a grid-forming converter. A grid-forming converter generates an alternating voltage and its own grid frequency, unlike a grid-commutated converter, which synchronizes with an existing power grid. As a "virtual machine," the grid-forming converter can regulate voltage, frequency, and phase angle, thus operating a stable power grid for consumers, either independently or in conjunction with other power converters. The power supply system 2 can include any components that generate electrical energy and provide it as direct current, e.g., a photovoltaic system 2a or an optional electrical energy storage device 2b, such as a battery. The energy storage device 2b can also provide electrical energy as needed, which is fed into the power supply network 4 by the power converter 1. Optionally, the energy storage device 2b can also be charged with electrical energy from the power supply network 4 via the power converter 1. This corresponds to a negative power output. Fig. 2 shows a schematic diagram of an electrical power converter 1 according to one embodiment. The power converter 1 comprises a DC link capacitor 13 which, as can be seen in Fig. 1, can be supplied by a DC voltage connection 11, which is connected, for example, to a power generation plant 2, and a grid connection 12 for coupling to a power supply network 4. The power converter 1 further comprises an inverter 10, which includes electrical switching elements such as MOSFETs or IGBTs. By appropriately controlling these switching elements, the inverter 10 generates a single-phase or multi-phase AC voltage from the DC voltage provided at the DC voltage terminal 11 and makes this available at the grid terminal 12. The DC voltage at the DC voltage terminal 11 can be buffered by an intermediate circuit capacitor 13. The voltage level of the AC voltage Vac at the mains connection 12 can be determined using a voltage sensor 111. The output current Iac provided by the power converter 1 can be measured with a suitable current sensor 112. Furthermore, filter components such as a filter choke, a filter capacitor, and / or a filter resistor can be provided at the mains connection 12. In this circuit diagram, Zgrid represents the (real) network impedance of the power supply network. The following describes possible control components of the grid-forming power converter 1. A measuring device 120 can calculate the active power output Pac and the reactive power output Qac based on the measured currents Iac and the voltages Vac at the grid connection 12 and determine the current grid frequency fac at the grid connection 12. The measured reactive power Qac is transmitted to the reactive power controller 132, which calculates the amplitude VampGF of the virtual target voltage from the measured actual reactive power value Qac and a reference value Qref, and forwards this to the calculation block 133. The reference value Qref for the reactive power can, for example, be a fixed, predefined value. Alternatively, it is also possible to specify this reference value Qref according to a user setting. In this way, for example, the converter 1 can provide the corresponding reactive power. It is also possible to adjust the reference value Qref using the measured parameters at the grid connection 12, for example, the measured voltage Vac, and depending on a nominal voltage for the power supply network 4. The determined actual value Pac of the active power and the grid frequency fac are passed to the active power controller 131. Taking into account a reference value Pref for the active power, the virtual frequency fGF is calculated. The integral of the virtual frequency fGF, limited to the periodicity of a circle between 0 and 360 degrees, yields the phase angle ThetaGF of the virtual setpoint voltage. This is calculated in calculation block 133. The integral for converting frequency to angle can also be calculated in block 131. The reference value Pref for the active power can, for example, come from a DC link controller. Alternatively, it is also possible to specify this reference value Pref according to a user setting. In this way, for example, the converter 1 can provide a corresponding active power, provided that the power supply system 2 can deliver the set active power.In principle, it is also possible to adjust the reference value Pref using the measured variables at the network connection 12 or the calculated variables from the calculation block 120, for example the determined frequency fac and depending on a nominal frequency for the energy supply network 4. The calculation block 133 determines the virtual target voltage VGF using the phase angle ThetaGF and the amplitude VampGF. This is passed, together with the mains voltage Vac, to a voltage regulator 134, which calculates the internal reference current Iref. This reference current Iref corresponds to the electric current resulting from the difference between the virtual target voltage VGF and the external applied voltage Vac across a freely selectable virtual impedance. The maximum electrical current in the inverter 10 of the power converter 1 can be limited by the current-carrying capacity of the components, especially the semiconductor switches, and by other factors such as thermal load. Furthermore, the power supplied by the energy generation plant 2, particularly in the case of renewable energy sources such as photovoltaics, represents an upper, i.e., maximum, limit to the power that can be fed into the grid 4. The intermediate circuit capacitor 13 can only compensate for short-term power fluctuations. To ensure compliance with the specified restrictions, a current and power monitoring system 140 is provided. It receives the initial reference current Iref, the mains voltage Vac, the maximum permissible current Imax, and limits for the maximum and minimum power output (Pmax and Pmin). At least some of these limits, such as the maximum permissible current Imax or, if applicable, the minimum power output (Pmin), can be set depending on the device. The current and power monitoring system 140 checks whether the power converter 1 can deliver the required current and power. If this is possible, the control unit 110 controls the inverter 10 according to the control parameters. If not, a limit can be imposed. First, it is checked whether the initial reference current Iref exceeds a predefined threshold for the maximum permissible current Imax. If so, a current scaling factor Scal_I is calculated such that the product of the initial reference current Iref and the current scaling factor Scal_I equals the maximum permissible current Imax. If the initial reference current does not exceed the threshold, the current scaling factor is set to 1. On the other hand, it is checked whether the active power to be delivered by the converter 1 to the power supply network 4 complies with the specified limits. To this end, it is checked whether the active power requirement Pac is within the limits that the power generation plant 2 can provide. If the active power to be delivered is outside the specified limits, a power scaling factor Scal_P is calculated to scale the active power requirement to a value that is within the limits. If the active power to be delivered is within the specified limits, Scal_P is set to 1. The active component of the reference current Iref is scaled using the scaling factor Scal_P, resulting in a scaled reference current Ireflimited. For example, if the active power output is above an upper limit for the active power range, a power scaling factor Scal_P, less than one, can be calculated. This limits the active power output to the upper limit of the active power range. If a positive lower limit exists for the active power range and the measured active power output is below this lower limit, a power scaling factor Scal_P, greater than one, can be calculated to raise the active power output to the lower limit. However, the lower limit for active power output in photovoltaic systems is usually zero or negative if a battery storage system or energy storage unit is present.In this case, the power scaling factor Scal_P is calculated to be less than one if the determined active power output, which is negative here and thus corresponds to active power input, is below the lower limit. However, for alternative energy generation plants, such as wind turbines, a positive lower limit, i.e., a limit greater than zero, may be specified. Subsequently, the control unit 110 regulates the control of the inverter 10 using the scaled reference current Ireflimited. The threshold values ​​for active power output can include fundamental limits such as a system-related maximum active power output PmaxDefault and a minimum active power output PminDefault. These values ​​can result, for example, from the design of the energy generation plant 2, such as the maximum output of a photovoltaic system or a lower limit below which the plant cannot be operated. A negative active power output, corresponding to an active power input, such as the maximum charging power of an energy storage system 2b, can also be specified as the minimum active power output. In addition to these system-related limits, further, usually stricter, limitations can be derived from the current operation of the energy generation plant 2. For example, the maximum output of a photovoltaic system may be below its maximum value under low solar irradiance. The maximum power output and / or input of an electrical energy storage device 2b may depend on parameters such as state of charge or operating temperature. A limit value unit 141 records the current system parameters of the energy generation plant 2 and determines the applicable limits for the maximum and minimum active power output Pmax and Pmin. For example, the current voltage Vlink at the intermediate circuit capacitor 13 can be recorded and evaluated. If the DC link voltage Vlink falls below a defined threshold, the maximum active power output Pmax can be reduced to account for the reduced energy generation. If the voltage at the DC link capacitor 13 rises again at a later time, the limitation of the maximum active power output Pmax is adjusted or removed. Similarly, the lower limit Pmin for active power output or, in the case of a negative value for Pmin, for active power input can be adjusted accordingly. If, as described above, power limitation is implemented in converter 1, it must subsequently be ensured that converter 1 can reliably release this limitation. Assume that converter 1 is operating at nominal power in power grid 4, and a fault condition occurs there requiring a power increase, the power limit of converter 1 would be reached immediately. In such a case, however, the internal control error would not increase further, since neither the previously set power value nor the actual power would change significantly. Therefore, there would be only a slight tendency to release the activated limitation. To counteract this effect, a virtual unlimited power can be used at the controller input during power limitation instead of the currently measured power. This allows the system to reliably return to the unlimited state as soon as the power requirements are met. Using unit 121, a virtual unlimited power can be calculated using the initial reference current Iref and the electrical voltage Vac at the network connection 12. In particular, a virtual unlimited active power Punlim and a virtual unlimited reactive power Qunlim can be calculated. In the event of a limitation, the current and power limiter 140 can output an "inLimit" signal. Depending on whether a limitation is currently active or not, either the actual active and reactive power Pac, Qac or the unlimited active and reactive power Punlim, Qunlim is passed on to the active power controller 131 or reactive power controller 132, respectively. This ensures that the power converter 1 can reliably exit the limited operating range after a limitation has occurred. To avoid discontinuities at the transition point between measured power in the unlimited case and virtual power in the limited case, the unlimited power in the unlimited case must always be compared with the measured power. The frequency in the power supply network 4 is typically subject to fluctuations. The electrical frequency in the power supply network 4 can vary depending on the imbalance between the feed-in and consumption of active power. For active power control 131, a discrepancy between the nominal frequency and the network frequency fac at the network connection 12, as well as a deviation between the virtual frequency fGF and the network frequency fac, can be significant. In contrast to classical grid-forming control systems, where the frequency difference between the virtual frequency fGF and the nominal frequency is preferentially used in the damping term, it proves advantageous to divide this control task. The deviation between the virtual frequency fGF and the grid frequency fac can only occur temporarily and is taken into account in a damping term in the active power controller 131. The deviation between the grid frequency fac and the nominal frequency can be considered in an optional frequency control in the reference value Pref for the active power. This approach has proven to be significantly more flexible compared to conventional methods, as frequency control and damping can be parameterized separately. Fig. 3 shows a flowchart for a method for operating a grid-forming electrical converter 1 according to one embodiment. During operation of the electrical converter 1, this method can be executed continuously and repeatedly, particularly periodically. The method typically ends when the converter 1 is switched off. The method can include any steps, such as those previously described in connection with the converter 1. Similarly, the converter 1 can also include any components or units suitable for implementing the method described below. Furthermore, it should be emphasized that the steps described below do not necessarily have to be executed in the order shown. Moreover, the individual steps can, where practical and technically feasible, be reversed in order or, if necessary, executed in parallel.The reference symbols of the individual steps S1 to S7 therefore do not represent a mandatory sequence to be executed. In step S1, an initial reference current value, Iref, is determined. Then, in step S2, a current scaling factor, Scal_I, is determined. This current scaling factor can be less than 1 if the determined reference current, Iref, is greater than a predetermined threshold for the maximum current, Imax. Otherwise, the current scaling factor, Scal_I, is set to 1. In step S3, the initial reference current value Iref is adjusted using the current scaling factor Scal_I, resulting in an adjusted reference current value. In step S4, an initial reference active power output is calculated. This reference active power output is calculated using the adjusted reference current value. In step S5, a power scaling factor, Scal_P, is calculated. This power scaling factor has a value less than one if the determined reference active power output lies outside a predetermined power range for power exchange between converter 1 and power grid 4. Otherwise, Scal_P is set to 1. In step S6, the active component of the reference current is adjusted using the power scaling factor Scal_P. Finally, in step S7, the grid-forming electrical converter 1 is controlled, whereby the active power output is set with the adjusted reference active power output and the reactive power output with the adjusted reference current value. The predetermined power range for the power exchange between converter 1 and power supply network 4 can include an upper threshold for the maximum active power output and / or a lower threshold for a minimum power output or, in the case of a negative threshold, for a maximum power input. Determining the initial reference current value can include a step to determine a voltage difference between the virtual setpoint voltage VGF of the converter 1 and the electrical voltage Vac at the grid connection 12 of the converter 1. The initial reference current value Iref can then be calculated using this voltage difference and a predetermined virtual impedance. This virtual impedance can, in particular, be a desired device impedance of the converter 1. The initial reference current value Iref can be calculated separately for the positive-sequence, negative-sequence, and zero-sequence systems. In this case, the current scaling factor is determined by the quotient of the predetermined threshold for the maximum current and the sum of the currents for the positive-sequence, negative-sequence, and zero-sequence systems. The predetermined threshold for the maximum power exchange between converter 1 and power supply network 4 can be adjusted, in particular, by the voltage across the DC link capacitor 13. The upper threshold for power exchange is reduced if the DC link voltage falls below a defined value. If the power converter 1 is in a power control limit state, an internal control variable, in particular the virtual setpoint voltage VGF, can be calculated during this limit state using a virtual unlimited power output. The unlimited power output during the limit state can be determined, in particular, from the initial reference current value Iref. If the virtual frequency of the converter deviates from the grid frequency at the converter's grid connection, this difference can be compensated for by adjusting a predetermined damping term. In particular, for power control, a damping term is used to determine the virtual setpoint voltage VGF, which takes into account the difference between the virtual frequency fGF of converter 1 and the grid frequency fac. In summary, the present invention relates to a grid-forming electrical power converter and a method for operating such a grid-forming electrical power converter. The control of the power converter is designed to enable stable operation and reliably implement the necessary limits on active and reactive power output.

Claims

Method for operating a grid-forming electrical power converter (1), comprising the steps of: - Determining (S1) an initial reference current value (Iref); - Determining (S2) a current scaling factor, wherein the current scaling factor is less than one if the determined reference current (Iref) is greater than a predetermined threshold for a maximum current, and the current scaling factor is otherwise set to one; - Adjusting (S3) the initial reference current value (Iref) using the current scaling factor to obtain an adjusted reference current value (Ireflimited); - Calculating (S4) an initial reference active power output using the adjusted reference current value;- Determining (S5) a power scaling factor, wherein the power scaling factor is not equal to one if the determined reference active power output is outside a predetermined power range for power exchange between the converter (1) and the power grid (4), and the power scaling factor is otherwise set to one; - Adjusting (S6) an active component of the adjusted reference current (Ireflimited) using the power scaling factor to obtain an adjusted reference active power output; and - Driving (S7) the grid-forming electrical converter (1), wherein the active power output and the reactive power output are set using the adjusted reference current value (Ireflimited).; Method according to claim 1, wherein the predetermined power range for power exchange comprises an upper threshold for maximum active power output and / or a lower threshold for minimum power output or maximum power input. Method according to claim 1 or 2, wherein determining (S1) the initial reference current value (Iref) comprises: - determining a voltage difference between a virtual setpoint voltage (VGF) of the converter (1) and an electrical voltage (Vac) at the grid connection (12) of the converter (1); and - calculating the initial reference current value (Iref) using the determined voltage difference and a predetermined virtual impedance. Method according to one of claims 1 to 3, wherein for determining (S1) the initial reference current value (Iref) for the positive system, the negative system and the zero system individual values ​​are determined and wherein the current scaling factor is determined using a quotient of a predetermined threshold value for the maximum current and the sum of the currents for positive system, negative system and zero system. Method according to one of claims 1 to 4, wherein the predetermined threshold for maximum power exchange is adjusted using a value for an electrical voltage (Vlink) at an intermediate circuit (13) of the power converter (1). Method according to claim 5, wherein the upper threshold for power exchange is reduced if the value for the electrical voltage (Vlink) at the intermediate circuit (13) falls below a predetermined first threshold, and / or wherein the upper threshold for power exchange is increased if the value for the electrical voltage (Vlink) at the intermediate circuit (13) rises above a predetermined second threshold. Method according to claim 5 or 6, wherein the lower threshold for power exchange is reduced if the value for the electrical voltage (Vlink) at the intermediate circuit (13) falls below a predetermined third threshold, and / or wherein the lower threshold for power exchange is increased if the value for the electrical voltage (Vlink) at the intermediate circuit (13) rises above a predetermined fourth threshold. Method according to any one of claims 1 to 7, wherein a virtual unlimited power output (Punlim, Qunlim) is determined during a limitation of the reference current value or the reference active power output. Method according to claim 8, wherein the virtual unlimited power output (Punlim, Qunlim) during a limitation of the active power output by the converter (1) is calculated using the initial reference current value (Iref). A method according to any one of claims 1 to 9, wherein the method further comprises: - determining a virtual frequency of the converter (1); - determining the grid frequency (fac) at the grid connection (12) of the converter (1); and - adjusting the virtual frequency of the converter (1) using a difference between the determined virtual frequency and the grid frequency (fac) at the grid connection (12) of the converter (1), wherein the adjustment is carried out using a predetermined damping term. Method according to claim 10, wherein the predetermined damping term comprises a function that depends on a difference between the determined virtual frequency and the grid frequency (fac) at the grid connection (12) of the converter (1). Network-forming electrical power converter (1), comprising: - a DC voltage connection (11) designed to be electrically coupled to a DC voltage source (2); - a grid connection (12) designed to be electrically coupled to a power supply network (4); - a power converter circuit (10) designed to convert the DC voltage provided at the DC voltage connection (11) into an AC voltage and to provide it at the grid connection (12); and - a control device designed to control the power converter circuit using a method according to any one of claims 1 to 9. Power converter (1) according to claim 12, wherein the DC voltage connection (11) is designed to be coupled with a photovoltaic system (2a) and / or an electrical energy storage device (2b). Power supply network (4) with a network-forming electrical power converter (1) according to claim 12 or 13 . Energy supply network (4) according to claim 14, wherein the energy supply network comprises a local island network, a microgrid or a subnetwork.

Images