POWER CONVERTER, ENERGY SUPPLY SYSTEM, METHOD FOR OPERATING A POWER CONVERTER AND METHOD FOR OPERATING A ENERGY SUPPLY SYSTEM

Abstract

The application relates to a power converter (10) for exchanging electrical power between a DC connection (DCA) and an AC connection (ACA), wherein the DC connection (DCA) is provided for connection to a DC unit (14), wherein the power converter (10) further comprises a DC intermediate circuit (16), a bridge circuit (20), an AC relay (18) and a control unit (26), wherein the DC intermediate circuit (16) is arranged between the DC connection (DCA) and the bridge circuit (20) and the AC relay (18) is arranged between the bridge circuit (20) and the AC connection (ACA), wherein the control unit (26) is designed to control power switches of the bridge circuit (20) such that the power converter (10) is operated with a voltage applied to the AC connection (ACA), and wherein the control unit (26) is further designed to, during the voltage applied operation (NB,NF) to detect a DC voltage (U_DC) at the DC intermediate circuit (16) and to disconnect the bridge circuit (20) from the AC connection (ACA) by opening the AC relay (18) when the DC voltage (U_DC) is below a limit voltage (U_GR). The application further relates to a method for operating a power converter, a power supply system and a method for operating a power supply system

Description

TECHNICAL FIELD

[0001] The application relates to a power converter for transmitting electrical power between a direct current (DC) side and an alternating current (AC) side. The application further relates to a power supply system comprising such a power converter and to methods for operating the power converter and the power supply system. STATE OF THE ART

[0002] Current-imprinting inverters, also called current-regulated inverters, generate an output alternating current from an input direct current, which can be single-phase or multi-phase. The output alternating voltage is determined by the load, which usually comprises an alternating voltage network. A current-regulated inverter appears like a current source at its output and is used in particular to feed electrical energy from a DC generator into an AC grid. A DC generator can in particular comprise a PV generator, a battery, or a fuel cell, which can generate a direct current in a suitable DC voltage range. Current-regulated inverters regulate the DC voltage applied to the DC generator at its DC connection to a setpoint by adjusting the AC current and therefore include detection and monitoring of the DC voltage within the AC control loop.

[0003] Voltage-injected inverters, also called voltage-regulated inverters, generate an AC output voltage from an input DC voltage, which can be single-phase or multi-phase. The shape and magnitude of the output currents are determined by the load, which can be arranged in an island grid, in particular. Within the scope of the voltage regulation of a known voltage-regulated inverter, the output AC voltage is fixed in terms of frequency and amplitude in the control loop. The input DC voltage is assumed to be largely constant and / or independent of the resulting AC current and, if necessary, tracked using a DC control loop. The AC control loop of a voltage-injected inverter can therefore be implemented without detecting and monitoring the DC voltage if the DC generator comprises a DC voltage source with a fixed DC voltage, in particular a battery or a fuel cell.

[0004] DE 10 2019 116 254 A1 discloses a method for operating a power generation system in which inverters of the power generation system can be operated either in a current-impressing or a voltage-impressing mode. Inverters connected to a PV generator as a direct current source are preferably operated in a current-impressing mode, and inverters connected to a storage device as a direct voltage source are preferably operated in a voltage-impressing mode.

[0005] From AT 512 611 A1, an inverter is known for feeding energy from photovoltaic modules into a public or private alternating voltage grid by converting the direct voltage generated by the photovoltaic modules into an alternating voltage, in which the feed-in is prevented or terminated if the direct voltage generated by the photovoltaic modules falls below a threshold value.

[0006] EP 4 184 738 A1 discloses an energy generation system in which fuel cells act as DC generators, feeding power either into an AC voltage grid via current-impressing inverters or into a microgrid via voltage-impressing inverters, the microgrid being supplied with electrical power either from the AC voltage grid or from the fuel cells. TASK

[0007] The application is based on the object of providing an improved power converter, an improved energy supply system and improved methods for their operation. SOLUTION

[0008] The object is achieved by a power converter having the features of claim 1, a method having the features of claim 12, a power supply system having the features of claim 13, and a method having the features of claim 17. Embodiments are specified in the dependent claims. DESCRIPTION

[0009] In a power converter for exchanging electrical power between a DC connection and an AC connection, the DC connection is provided for connection to a DC unit. The power converter further comprises a DC intermediate circuit, a bridge circuit, an AC relay, and a control unit, wherein the DC intermediate circuit is arranged between the DC connection and the bridge circuit, and the AC relay is arranged between the bridge circuit and the AC connection. The control unit is configured to control power switches of the bridge circuit such that the power converter is operated with a voltage applied to the AC connection. The control unit is further configured to detect a DC voltage at the DC intermediate circuit during voltage-applied operation and to disconnect the bridge circuit from the AC connection by opening the AC relay when the DC voltage is below a threshold voltage.

[0010] This allows the power converter to be protected from low DC voltages. Such low DC voltages can arise, for example, from a disconnection of the DC unit or a faulty DC unit. Such a potentially critical situation can be detected early by monitoring the DC voltage, and the AC relay can be opened to protect the bridge circuit if the DC voltage falls below the limit voltage.

[0011] The power converter can optionally further comprise a DC fuse or a DC relay, by means of which the bridge circuit can be connected to and disconnected from the DC unit. The power converter can thus be protected by the DC fuse or the DC relay. If the DC unit is disconnected from the power converter by the DC fuse or the DC relay, the DC voltage drops due to the continued voltage-impressing exchange of electrical power via the AC connection. The described protection can also be achieved in such a case by the described monitoring of the DC voltage and the described opening of the AC relay. The DC unit can optionally be provided with a DC bus, via which the DC unit can be connected to the power converter.

[0012] In one embodiment of the power converter, the AC terminal of the power converter is provided for connection to an AC load. The AC load can comprise an AC grid and / or AC generators. The AC grid can be a single-phase or multi-phase AC grid, which can be configured as a public or local AC grid. The AC generators can also be single-phase or multi-phase AC generators.

[0013] In the event of a drop in the DC voltage across the DC link, the power converter, particularly the power semiconductors of the bridge circuit, can be protected from excessive AC currents. In particular, protection can be achieved against uncontrolled reverse currents from an AC network into the DC link via the AC connection and freewheeling diodes of the bridge circuit's power switches. The threshold voltage can be selected to prevent the freewheeling diodes from becoming conductive, thus avoiding uncontrolled reverse currents, which, especially in a low-impedance DC link, can reach high amplitudes and damage the bridge circuit.

[0014] The voltage impressed on the AC side by the power converter can depend on the nominal voltage of connected AC generators and / or the voltage of the AC grid. The nominal voltage of the AC generators refers to the voltage during normal operation. The AC generators are designed to operate at the nominal voltage.

[0015] In one embodiment of the power converter, the limit voltage corresponds to a minimum voltage required for voltage-impressing operation. The minimum voltage depends on a peak value of the nominal voltage of the AC units and / or on a peak value of the voltage of the AC grid. The peak value of an AC voltage corresponds to the peak value of the AC voltage. In particular, the minimum voltage can correspond to the peak value, optionally increased by a margin value. This can ensure that the available DC voltage is large enough to generate the voltage to be impressed. If the available DC voltage is not large enough to generate the impressed voltage, the AC relay is opened and the power converter is disconnected from the AC grid.

[0016] In one embodiment of the power converter, the voltage-impressing mode can comprise a grid-forming mode and / or a grid-shaping mode. In grid-forming mode, the power converter is controlled so that it exhibits pure voltage source behavior at its AC connection. The output electrical power adapts to the load condition, allowing the power converter to, for example, create an island grid in grid-forming mode. Grid-forming mode can be used, for example, to supply AC generators.

[0017] In grid-forming operation, the power converter is controlled so that it can supply electrical power to an existing AC grid at its AC connection depending on a power setpoint from the DC unit. The power setpoint can be specified directly or indirectly by the DC unit or by a higher-level control system. In the event of grid faults, the power converter can adjust its power output so that, for example, an AC grid connected to the AC connection is supported by feeding in reactive power. Grid-forming operation can be intended, for example, for feeding into the AC grid and / or for parallel operation with the AC grid and connected AC units and, in particular, for the parallel supply of the AC units from the AC grid and the DC unit.

[0018] The power converter can therefore have grid-forming operation with an existing connection to the AC grid and grid-forming operation without a connection to the AC grid.

[0019] In embodiments, the DC unit connectable to the DC connection can comprise a DC generator, in particular a fuel cell or a battery. If the DC unit comprises one or more DC generators, the power converter acts as an inverter that can convert the electrical power generated by the DC generator(s) into AC power. This AC power can then be made available to the AC load via the AC connection of the power converter.

[0020] In further embodiments, the control unit of the power converter is configured to output a signal when the DC voltage is below a threshold voltage. The threshold voltage can, in particular, be above the limit voltage.

[0021] In embodiments, the power converter is configured to output the signal to a higher-level control unit and / or to a control unit of the DC generator, in particular to a control unit of the fuel cell. The higher-level control unit may also comprise the central control unit of a power supply system, wherein the power converter and the DC generator may be part of the power supply system.

[0022] The higher-level control unit and / or the control unit of the DC generator can react appropriately to the signal from the control unit of the power converter, e.g., by controlling or regulating the operation of other components of the power supply system.

[0023] The threshold voltage can depend on a nominal full-load voltage of the DC generator, in particular on a nominal full-load voltage of the fuel cell, whereby the threshold voltage can correspond in particular to the nominal full-load voltage. The nominal full-load voltage indicates the voltage applied to the DC generator during normal operation of the DC generator at its full power. The signal is therefore output when the actual voltage of the DC generator at the DC connection drops below the full-load voltage, which can indicate a fault condition in the fuel cell, particularly when a fuel cell is used as the DC generator. Outputting the signal then provides the opportunity to respond appropriately.

[0024] In one embodiment of the power converter, the control unit is configured to output the signal as long as the DC voltage is below the threshold voltage and above the limit voltage. Outputting the signal provides the ability to respond to the DC voltage that has dropped below the threshold voltage but is still above the limit voltage, in particular by the higher-level control unit influencing other parts of the power supply system, for example, auxiliary or secondary units of the fuel cell, with control interventions. If the limit voltage is reached or undershot, the power converter is disconnected from the AC grid, as described.

[0025] In one embodiment of the power converter, the control unit is configured to change the control of the semiconductor switches of the bridge circuit, in particular to interrupt it, if the DC voltage is below the threshold voltage and / or if the power converter receives a further signal from the higher-level control unit of the DC generator. In this embodiment, the DC voltage falling below the threshold voltage can be responded to by controlling the semiconductor switches differently or by interrupting their control entirely, thus leaving them permanently open. In the same way, the control of the semiconductor switches of the bridge circuit can be changed or interrupted in response to the further signal from the DC generator. The further signal can be generated by the higher-level control unit of the DC generator, for example, depending on the undervoltage signal and / or further information, e.g.on the status of the DC generator and its units.

[0026] In embodiments of the power converter, the control unit is configured to disconnect the bridge circuit from the AC terminal by opening the AC relay when the DC voltage remains below the threshold voltage for a predeterminable period of time. This allows an attempt to respond to the DC voltage falling below the threshold voltage in another way during the predeterminable period of time. If this attempt is unsuccessful, the power converter is disconnected from the AC terminal and any AC load connected to it by opening the AC relay.

[0027] The described power converter is configured to exchange electrical power between the DC connection and the AC connection, with the DC connection being provided for connection to the DC unit. The power converter comprises the DC intermediate circuit, the bridge circuit, and an AC relay, with the DC intermediate circuit being arranged between the DC connection and the bridge circuit. A method for operating the power converter comprises: • Controlling the power switches of the bridge circuit so that the power converter is operated with voltage applied to its AC connection. • Detecting a DC voltage at the DC link. • Disconnect the bridge circuit from the AC connection by opening the AC relay when the DC voltage is below a threshold voltage.

[0028] In one embodiment of the method, the AC connection is provided for connection to an AC load. The AC load comprises an AC grid and / or AC units, wherein the voltage impressed by the power converter at its AC connection depends on a nominal voltage of the AC units and / or on the voltage of the AC grid.

[0029] An energy supply system according to the invention comprises a DC generator and the described power converter. The power converter is configured, in particular, for supplying the AC units in parallel or independent of the grid. The AC units can be configured, in particular, as auxiliary or secondary units of the DC generator, i.e., the power converter can be configured, in particular, to supply electrical energy to AC units intended to maintain the operation of the DC generator.

[0030] In one embodiment of the energy supply system, the DC generator comprises a fuel cell and / or a battery. The AC units, in particular, comprise auxiliary units of the fuel cell and / or the battery, e.g., a redox flow battery. Such auxiliary units include, for example, a pump for the starting or end products of energy generation, a cooling system for the DC generator, or an on-board network for sensors and actuators for controlling and regulating the operation of the DC generator.

[0031] In one embodiment of the energy supply system, the DC generator and the power converter are connected to a DC bus via their DC connection, with another power converter being connected to the DC bus and configured to transmit electrical power from the DC bus to the AC grid. In the energy supply system, the DC unit can comprise the DC bus, via which the power converter is connected to the DC unit.

[0032] In the energy supply system, a connection between the DC bus and the AC grid can be established via both the power converter and the additional power converter. The rated power of the additional power converter can be at least five times higher than the rated power of the power converter. In this embodiment, the additional power converter can be configured to transfer large amounts of power between the DC unit and the AC grid, in particular power of the order of magnitude of the rated power of the DC unit. The power converter according to the invention can also be connected to the AC grid, for example, adjusting itself to its voltage at its AC connection, and, if necessary, supplying the AC units with an electrical operating power that is a fraction of the rated power of the DC unit.

[0033] In a method for operating the described power supply system, the power converter is operated in a grid-forming mode with a connection to the AC grid or in a grid-forming mode separated from the AC grid. The connection of the entire power supply system to the AC grid can be established and disconnected via an AC grid relay, and the connection of the power converter and the AC units to the AC grid can additionally be established and disconnected via an AC coupling relay.

[0034] In grid-forming mode, i.e., with the AC grid relay and AC coupling relay closed, the power converter feeds electrical power into the AC grid and the AC generators in parallel. Grid-forming mode can be used, in particular, in a start-up mode of the power supply system and / or deliberately activated to supply the AC generators in parallel with electrical power from the AC grid and, via the power converter, from the DC generator. The power supply system's internal power supply remains secure even in the event of an AC grid failure, as the power converter then switches to grid-forming mode.

[0035] In one embodiment of the method, the power converter in grid-forming mode supplies the AC units with electrical power, while the other power converter feeds electrical power from the DC generator into the AC grid via the closed AC grid relay, whereby the power converter and the AC units are separated from the AC grid by the AC coupling relay. In grid-forming mode, the power converter is therefore designed to establish an independent island grid to supply the AC units and to maintain it independently of the grid, particularly in the event of a grid failure. Grid-forming mode can therefore be used, for example, in a normal operating mode of the energy supply system and / or deliberately activated to decouple the AC units' internal power supply from the AC grid. BRIEF DESCRIPTION OF THE CHARACTERS

[0036] In the following, the application is further explained and described using exemplary embodiments shown in the figures. Fig. 1 shows a schematic diagram of a power converter. Fig. 2 shows an example of a DC voltage waveform. Fig. 3 shows another example of a DC voltage waveform. Fig. 4 schematically shows a method for operating the power converter. Fig. Figure 5 shows a schematic diagram of an energy supply system. Fig. 6 shows schematically a method for operating the power supply system.

[0037] The same reference numerals are used throughout the figures for identical or similar elements. The illustrations in the figures may not be to scale. FIGURE DESCRIPTION

[0038] Fig. 1 schematically shows a power converter 10 configured as a three-phase inverter. The power converter 10 has a DC terminal DCA and an AC terminal ACA. A three-phase AC load 12, which may include an AC grid 32 and / or AC units, is connected to the AC terminal ACA. A DC unit having a DC generator 14 is connected to the DC terminal DCA. In the example shown, the DC generator 14 is a fuel cell that generates electrical power by converting hydrogen (H2). The DC generator may alternatively or additionally comprise a battery and / or a DC generator.

[0039] The power converter 10 converts electrical DC power supplied to it via the DC connection DCA into electrical AC power, which it can output via its AC connection ACA. Optionally, the power converter 10 can also be configured for power transmission in the opposite direction, and the DC unit can include a DC load.

[0040] The power converter 10 has a bridge circuit 20 with clocked power switches. The bridge circuit 20 is designed such that the DC power can be converted into AC power via the clocking of the power switches. The bridge circuit 20 can be isolated from the DC generator 14 via a DC fuse 22. The bridge circuit 20 can be protected by the DC fuse 22, for example, from excessive currents flowing through the DC terminal DCA. The bridge circuit 20 can be isolated from and connected to the AC terminal ACA via an AC relay 18.

[0041] A DC intermediate circuit 16 is arranged between the DC connection DCA and the bridge circuit 20. The DC intermediate circuit 16 includes a capacitor. A DC voltage U_DC is applied to the DC intermediate circuit 16. If the DC generator 14 is connected to the DC connection DCA, as shown in Fig. 1, the DC voltage U_DC corresponds to the voltage at the DC generator 14.

[0042] The power converter 10 also has a control unit 26. The control unit 26 controls the bridge circuit 20. The control by the control unit 26 can, for example, clock the power switches of the bridge circuit 20 in order to convert the input-side DC voltage U_DC at the DC connection DCA or at the intermediate circuit 16 into an output-side AC voltage at the AC connection ACA that is suitable for delivering electrical AC power. The control unit 26 performs voltage-impressing control of the three-phase AC voltage U_AC, in which the AC voltage U_AC is set to a desired setpoint at a desired frequency. The AC current at the AC connection essentially results from the current parameters of the specific AC load 12. The control unit 26 monitors the DC voltage U_DC at the DC intermediate circuit 16 during voltage-impressing operation.

[0043] Depending on the detected DC voltage U_DC, the control unit 26 can interrupt the control of the bridge circuit 20, activate the AC relay 18, and / or output a signal SI. The signal SI is output to a control unit 15 of the DC generator. This allows the power converter 10 to be protected, in particular, from excessively low DC voltages U_DC at the DC link 16 and potentially resulting reverse currents.

[0044] A drop in the DC voltage U_DC can occur, for example, in the event of a fault. Such a fault could be the blowing of the DC fuse 22, which completely disconnects the DC link 16 from the DC generator 14 and causes the DC voltage U_DC to fall below a threshold voltage U_GR. A fault could also be a so-called partial short circuit in the DC generator 14, in which the DC voltage U_DC remains above the threshold voltage U_GR but can fall below a threshold voltage U_SW.

[0045] Fig. Figure 2 shows an example of a DC voltage curve U_DC at the intermediate circuit 16. Up to time t1, the DC voltage is above the threshold voltage U_SW, which is above the limit voltage U_GR. Up to time t1, the DC voltage is above the threshold voltage U_SW and also above the limit voltage U_GR.

[0046] At time t1, a fault occurs, e.g., due to the blowing of DC fuse 22. This fault causes the DC voltage U_DC at the intermediate circuit 16 to drop. At time t2, the DC voltage U_DC has fallen below the limit voltage U_GR. By detecting and monitoring the DC voltage U_DC, such a drop in the DC voltage U_DC below the limit voltage U_GR can be detected, and the AC relay 18 can be opened in a timely manner.

[0047] The limit voltage U_GR essentially corresponds to a voltage that is at least required as DC voltage U_DC to generate the impressed voltage U_AC. The limit voltage U_GR therefore advantageously depends on a peak voltage of the AC load 12. Below this limit voltage U_GR, freewheeling diodes, also called body diodes, of the power switch of the bridge circuit 20 can become conductive if the peak voltage of the AC load 12 is higher than the DC voltage below the limit voltage. In such a case, large currents could flow from the AC terminal ACA via the bridge circuit 20 to the DC terminal DCA, which would impair the service life of the bridge circuit 20 and could damage the power converter 10. This undesirable effect can be avoided by opening the AC relay 18 depending on the monitored DC voltage U_DC.

[0048] Fig. Figure 3 shows another example of a waveform of the DC voltage U_DC at the intermediate circuit 16. Up to time t1, the DC voltage U_DC is above the threshold voltage U_SW, which is above the limit voltage U_GR. Up to time t1, the DC voltage is above both the threshold voltage U_SW and the limit voltage U_GR.

[0049] At time t3, the DC voltage U_DC drops below the threshold voltage U_SW. The drop in the DC voltage U_DC below the threshold voltage U_SW can be caused, for example, by a fault in the DC generator 14. In particular, a partial short circuit in a fuel cell serving as the DC generator 14 can cause the DC voltage U_DC to drop. The threshold voltage U_SW corresponds, for example, to the nominal full-load voltage of the fuel cell, whereby a drop in the DC voltage U_DC below the nominal full-load voltage can indicate a fault in the DC generator 14.

[0050] At time t3, the control unit 26 detects that the DC voltage U_DC has fallen below the threshold voltage U_SW. The control unit 26 can then change the timing of the bridge circuit 20 to stabilize the DC voltage, e.g., by reducing the outflow of electrical energy from the DC intermediate circuit 16. The timing can also be completely stopped to interrupt the outflow of electrical energy from the DC intermediate circuit 16. The DC voltage U_DC can thereby stabilize below the threshold voltage U_SW but above the limit voltage U_GR. Alternatively, the timing of the bridge circuit 20 can continue unchanged to continue providing voltage-impressing AC power, in particular to continue to supply the AC load 12 in a stable manner, even though the DC voltage U_DC available for this purpose has dropped significantly compared to the normal state and is approaching the limit voltage U_GR.

[0051] If the DC voltage U_DC falls below the threshold voltage U_SW, a signal SI can be output alternatively or additionally. The signal SI is output, for example, to the control unit 15 of the DC generator 14. The control unit 15 can then respond to the signal SI and, by controlling the DC generator 14, attempt to locate and / or correct any fault in the DC generator 14 in order to stabilize the DC voltage U_DC. The DC voltage U_DC can stabilize below the threshold voltage U_SW but still remain above the limit voltage U_GR.

[0052] If the time t4 is reached and the DC voltage U_DC is still below the threshold voltage U_SW, a further signal can be sent and / or the AC relay can be opened by the control unit 26 and the bridge circuit 20 can thus be disconnected from the AC connection ACA.

[0053] A voltage drop of the DC voltage U_DC at the DC intermediate circuit 16 below the threshold voltage U_SW can be caused, for example, by an internal so-called partial short circuit, particularly in DC generators 14 that have fuel cells. Such an internal partial short circuit can be caused by a fault in the internal sensors of the fuel cell close to and in particular between the individual fuel cells of a fuel cell stack. Furthermore, a low resistance can occur within a fuel cell between the housing or casing of a stack of fuel cells due to inadequate insulation, e.g., when current-carrying lines, in particular connections between individual fuel cells of the stack and the common and possibly grounded casing, can come into conductive proximity to one another.

[0054] In the event of a partial short circuit in the fuel cell, the non-short-circuited cells continue to generate power, albeit at a reduced voltage, which may be below the threshold voltage U_SW. The threshold voltage U_SW may depend on or correspond to the nominal full-load voltage of all the cells in the stack. The full-load voltage is, in particular, the DC voltage present at the fuel cell at maximum possible generation power, i.e., the voltage at the base of a power-voltage characteristic curve of the fuel cell. In the event of a partial short circuit, the DC voltage is below the nominal full-load voltage of the fuel cell, particularly with a correspondingly reduced maximum generation power.In such a situation with a DC voltage below the threshold voltage U_SW but above the limit voltage U_GR, the fuel cell's auxiliary units 40 can and should continue to be supplied, allowing the fuel cell to continue operating, which is possible despite the fuel cell's reduced power in the event of a partial short circuit. Additionally, the SI signal can be used to initiate analysis and / or rectification of the potential fuel cell fault.

[0055] Fig. 4 schematically shows a method for operating the power converter 10, as it can be carried out, for example, in the control unit 26.

[0056] After starting the process 400, a check is made in 402 to determine whether the DC voltage U_DC is less than the threshold voltage U_GR. If this is the case (path "+"), the AC relay 18 is opened in 416, and the timing of the bridge circuit 20 is stopped.

[0057] If the check in 402 shows that the DC voltage U_DC is greater than or equal to the limit voltage U_GR (path "-"), a check is made in 404 to determine whether the DC voltage U_DC is less than the threshold voltage U_SW. If this is not the case (path "-"), monitoring continues with 402.

[0058] If the check in 404 shows that the DC voltage U_DC is less than the threshold voltage U_SW, path “+”, the signal SI is output in 406 and / or the clocking of the bridge circuit 20 is changed.

[0059] In 408, a counter is incremented to determine a preset time period. In 410, a check is made to determine whether the DC voltage U_DC is less than the threshold voltage U_SW. If this is not the case (path "-"), the DC voltage U_DC has risen above the threshold voltage U_SW again, and the counter is reset in 412. The process then continues with 404.

[0060] If it is determined in 410 that the DC voltage U_DC is still lower than the threshold voltage U_SW (path "+"), a check is performed in 414 to determine whether the DC voltage U_DC has since fallen below the threshold voltage U_GR or whether the counter has reached the specified time period. If this is not the case (path "-"), the process continues with 408, and the counter continues to count in 408.

[0061] If it is the case in 414, path “+”, that the DC voltage U_DC has fallen below the limit voltage U_GR or the counter has reached the preset time period, the AC relay 18 is opened in 416 and the clocking of the bridge circuit 20 is stopped.

[0062] To exit step 416, step 418 checks whether the DC voltage U_DC is greater than the limit voltage U_GR. If so, path "+" indicates that the process continues with step 402.

[0063] The method protects the power converter 10, and in particular its bridge circuit 20, from undesirable voltage conditions between the AC terminal ACA and the DC intermediate circuit 16, in particular from the freewheeling diodes of the bridge circuit 20 becoming conductive and the power flow through the power converter 10 becoming uncontrollable. This prevents a high reverse current through the freewheeling diodes of the bridge circuit, which would otherwise only be limited by the low impedance at the DC terminal DCA.

[0064] Fig. 5 shows a power supply system 50 comprising the power converter 10, the DC generator 14, a DC bus 38, and a further power converter 30. The power converter 10 and the further power converter 30 are connected to the DC generator 14 via the DC bus 38. The power converter 10 and the further power converter 30 can be connected on the AC side to the AC grid 32 via an AC grid relay 34 and can be disconnected from the AC grid 32 by opening the AC grid relay 34. A transformer (not shown) can be arranged between the further power converter 30 and the AC grid relay, in particular between the further power converter 30 and the connection to the AC terminal ACA of the power converter 10.

[0065] The DC generator 14 has AC units that include auxiliary units 40, which are connected to the AC terminal ACA of the power converter 10 via a connection 24. The AC units can be supplied with electrical energy from the power converter 10 via the connection 24.

[0066] The power supply system 50 further comprises an AC coupling relay 36, which is arranged between the AC terminal ACA of the power converter 10 and the AC grid relay 34. A transformer (not shown) can optionally be arranged between the AC terminal ACA of the power converter 10 and the AC coupling relay 36, in particular between the AC terminal ACA and the connection 24.

[0067] When the AC coupling relay 36 is closed, the power converter 10 is connected to the AC grid 32. At the same time, the auxiliary units 40 are connected to the AC grid 32 when the AC coupling relay 36 is closed. In this situation, the auxiliary units 40 are supplied with electrical power in parallel from the AC grid 32 and the power converter 10, as well as, if applicable, the additional power converter 30.

[0068] However, if the AC coupling relay 36 is open or opens during ongoing operation of the fuel cell, the auxiliary units 40 continue to be supplied by the power converter 10. The power converter 10 can therefore serve as an uninterruptible power supply (UPS) for the connection 24 and thus for the auxiliary units 40. In further embodiments, the energy generation system can comprise multiple power converters 10 that jointly supply the auxiliary units 40 with electrical power and have suitable voltage-injecting controls with automatic load sharing for this purpose.

[0069] Fig. 6 schematically shows a method for operating the energy supply system 50 in an exemplary embodiment according to Fig. 5 with the following procedural steps.

[0070] In step 600, a start of the energy supply system 50 is initiated, e.g., by the control unit 15 of the DC unit 14 or a higher-level control unit.

[0071] In step 602, the AC grid relay 34 is closed, connecting the power supply system to the AC grid 32. In step 604, the AC coupling relay 36 is closed, connecting the power converter 10 and the auxiliary units 40 of the DC generator 14 to the AC grid. Alternatively, steps 602 and 604 can be performed simultaneously or in reverse order.

[0072] In step 606, the power converter 10 outputs electrical power via its AC connection ACA, with the power converter 10 operating in parallel with the grid, applying voltage in a grid-forming mode NF. In step 608, the auxiliary units are supplied with electrical power taken from the AC grid and / or from the DC generator 14 via the power converter 10. During the grid-forming mode NF of the power converter 10 in steps 606 and 608, a power setpoint for the power converter 10 can be specified, e.g., by the control unit 15 of the electrolyzer. The power setpoint can, in particular, have a value of zero if the power converter 10 is merely to be in standby.Alternatively, or in preparation for the following steps, a power setpoint for the power converter 10 may be specified with a value that substantially corresponds to the power consumed by the auxiliary units 40 in order to minimize the electrical power drawn from the AC grid 32.

[0073] In step 610, the AC coupling relay 36 is opened, so that the power converter 10 and in particular the auxiliary units 40 are disconnected from the AC grid 32. In step 612, the power converter 10 outputs electrical power via its AC connection ACA, with the power converter 10 operating in a voltage-impressing manner in a grid-forming mode NB. In step 614, the auxiliary units 40 are (continued) supplied with electrical power, which is now drawn exclusively from the DC generator 14 via the power converter 10. Thus, the operation of the auxiliary units 40 is autonomous from step 610, i.e., from the opening of the AC coupling relay 36, i.e., independent of the AC grid 32.

[0074] For grid-forming operation NB to supply the auxiliary units 40, the power converter 10 operates with a voltage-impressing effect, so that the AC voltage at the AC terminal ACA of the power converter 10, and thus the AC voltage at the auxiliary units 40, lies within a permissible range. The control unit 26 of the power converter 10 can, in particular, comprise a droop control, in which an AC frequency at the AC terminal ACA is set depending on the power flowing via the AC terminal ACA. Such a droop control is particularly advantageous when the power converter 10 is implemented in duplicate and the electrical power required to supply the auxiliary units 40 is to be distributed equally between the two or more power converters 10.

[0075] In step 616, the energy supply system is in normal operation, which may in particular include a current-impressing feed-in of electrical power from the DC generator 14 via the additional power converter 30 into the AC grid 32. In normal operation, the additional power converter 30 can in particular feed a large portion of the electrical power generated by the DC generator 14 into the AC grid 32, while the power converter 10 uses only a fraction of the electrical power generated by the DC generator 14 to form the grid in order to supply the auxiliary units 40. The electrical power converted by the additional power converter 30 can be many times greater, at least by a factor of five, than the power converted by the power converter 10.

[0076] In step 618, during normal operation, a check is made to determine whether the DC voltage U_DC is within a normal range or whether it falls below the limit voltage U_GR and / or the threshold voltage U_SW. Step 618 can be carried out in particular according to Fig. 4 and include a repeated check of the DC voltage U_DC as well as a signaling of a fall below the threshold voltage U_SW, which, however, does not yet include leaving the normal operation of the energy supply system 50, compare steps 404 to 412 according to Fig. 4 and the associated description. As long as no undershoot of the limit voltage U_GR and no permanent undershoot of the threshold voltage U_SW is detected in step 618 (path "-"), the method branches back to step 616 and the power supply system remains in normal operation.

[0077] Normal operation in step 618 can be continued, at least with regard to the self-supply of the energy supply system 50, even if the additional power converter 30 should interrupt the feed into the AC grid 32, e.g., by disconnecting the additional power converter 30 from the AC grid 32, particularly in the event of a persistent fault in the AC grid 32. The supply of the auxiliary units is then still secured by the power converters 10, with the power converter 10 continuously operating in a voltage-impressing manner in a grid-forming mode NB during steps 612 to 618.

[0078] If it is determined in step 618 that the DC voltage U_DC continuously falls below the threshold voltage U_SW and the signaling of this undershoot to the control unit 15 has not resulted in an increase in the DC voltage, and / or that the DC voltage U_DC falls below the limit voltage U_GR once, path "+", the method branches to step 620. In step 620, the operation of the energy supply system 50 can be stopped entirely, in particular by stopping the timing and thus the feed-in of electrical AC power by the power converter 10 and the further power converter 30 and by opening the AC relay 18 of the power converter 10 and, if applicable, a corresponding AC relay in the further power converter 30.

[0079] Finally, in step 622, the power supply system 50 enters a standby state in which the power supply system 50 may be connected to the AC grid 32 by keeping the AC grid relay 34 (still) closed, but neither the additional power converter 30 feeds electrical power into the AC grid 32 nor the auxiliary units 15 are supplied by the power converter 10. Alternatively or additionally, the AC grid relay 34 may be opened in step 622.

[0080] As an alternative to the detection of an undervoltage in step 618, the standby state in step 622 can also be brought about due to an explicit stop command, wherein the stop command can be triggered in particular by the control unit 15 of the DC generator 14 or by a higher-level control unit.

[0081] From the standby state in step 622, the method can be restarted by means of a new start command in step 600. LIST OF REFERENCE SYMBOLS 10 power converters 12 AC load 14 DC generators 15 Control unit 16 DC link 18 AC relays 20 bridge circuit 22 DC fuse 24 Connection for AC units 26 Control unit 30 additional power converters 32 AC network 34 AC mains relays 36 AC coupling relays 38 DC bus 40 auxiliary units 50 Energy supply system 400-418 procedural steps 600-622 procedural steps DCA DC connector ACA AC connection NB network-forming operation NF network-forming operation U_DC DC voltage U_SW threshold voltage U_GR limit voltage U_AC AC Voltage SI Signal t1, t2, t3, t4 Times

Claims

[1] Power converter (10) for exchanging electrical power between a DC connection (DCA) and an AC connection (ACA), wherein the DC connection (DCA) is provided for connection to a DC unit (14) and the AC connection is provided for connection to an AC load (12), wherein the AC load (12) has an AC network (32) and / or AC units, wherein the power converter (10) further comprises a DC intermediate circuit (16), a bridge circuit (20), an AC relay (18) and a control unit (26), wherein the DC intermediate circuit (16) is arranged between the DC connection (DCA) and the bridge circuit (20) and the AC relay (18) is arranged between the bridge circuit (20) and the AC connection (ACA), wherein the control unit (26) is designed to control power switches of the bridge circuit (20) such that the power converter (10) at the AC connection (ACA) is operated with voltage impressing,wherein the voltage (U_AC) impressed by the power converter (10) depends on a nominal voltage of the AC units and / or on the voltage of the AC grid (32), and wherein the control unit (26) is further configured to detect a DC voltage (U_DC) at the DC intermediate circuit (16) during the voltage-impressing operation (NB, NF) and to disconnect the bridge circuit (20) from the AC connection (ACA) by opening the AC relay (18) when the DC voltage (U_DC) is below a limit voltage (U_GR), wherein the limit voltage (U_GR) corresponds to a minimum voltage required for the voltage-impressing operation (NB, NF), wherein the minimum voltage depends on a peak value of the nominal voltage of the AC units and / or on a peak value of the voltage of the AC grid (32). [2] Power converter according to claim 1, wherein the minimum voltage corresponds to the peak value, optionally increased by a distance value. [3] Power converter according to one of the preceding claims, wherein the voltage-impressing operation (NB, NF) comprises a grid-forming operation (NB) and / or a grid-shaping operation (NF), wherein the grid-forming operation (NB) is configured in particular for supplying the AC units and the grid-shaping operation (NF) is configured in particular for feeding into the AC network (32) and / or for parallel operation on the AC network (32) and the AC units. [4] Power converter according to claim 3, wherein the power converter (10) has the grid-forming operation (NF) with an existing connection to the AC grid (32) and the grid-forming operation (NB) without a connection to the AC grid (32). [5] Power converter according to one of the preceding claims, wherein the DC unit comprises a DC generator (14), in particular a fuel cell and / or a battery. [6] Power converter according to one of the preceding claims, wherein the control unit (26) is further configured to output a signal (SI) when the DC voltage (U_DC) is below a threshold voltage (U_SW). [7] Power converter according to claim 6, wherein the power converter (10) is configured to output the signal (SI) to a higher-level control unit and / or to a control unit (15) of the DC generator (14), in particular the fuel cell. [8] Power converter according to one of claims 6 or 7, wherein the threshold voltage (U_SW) depends on a nominal open-circuit voltage of the DC generator (14), in particular on a nominal open-circuit voltage of the fuel cell, wherein the threshold voltage (U_SW) can in particular correspond to the nominal open-circuit voltage. [9] Power converter according to one of claims 6 to 8, wherein the control unit (26) is arranged to output the signal (SI) as long as the DC voltage (U_DC) is above the limit voltage (U_GR). [10] Power converter according to one of claims 6 to 9, wherein the control unit (26) is further configured to change, in particular to interrupt, the control of the semiconductor switches of the bridge circuit (20) when the DC voltage (U_DC) is below the threshold voltage (U_SW) and / or when the power converter receives a further signal from the control unit (15) of the DC generator (14). [11] Power converter according to one of claims 6 to 10, wherein the control unit (26) is further configured to disconnect the bridge circuit (20) from the AC terminal (ACA) by opening the AC relay (18) when the DC voltage (U_DC) is below the threshold voltage (U_SW) for a predeterminable period of time. [12] A method for operating a power converter (10), wherein the power converter (10) is provided for exchanging electrical power between a DC connection (DCA) and an AC connection (ACA), wherein the DC connection (DCA) is provided for connecting a DC unit (14) and the AC connection is provided for connecting to an AC load (12), wherein the AC load (12) comprises an AC network (32) and / or AC units and the power converter (10) further comprises a DC intermediate circuit (16), a bridge circuit (20) and an AC relay (18), wherein the DC intermediate circuit (16) is arranged between the DC connection (DCA) and the bridge circuit (20), the method comprising: Controlling power switches of the bridge circuit (20) such that the power converter (10) is operated in a voltage-impressing manner at its AC connection (ACA), wherein the voltage (U_AC) impressed by the power converter (10) depends on a nominal voltage of the AC units and / or on the voltage of the AC network (32), Detecting a DC voltage (U_DC) at the DC intermediate circuit (18) and Disconnecting the bridge circuit (20) from the AC connection (ACA) by opening the AC relay (18) when the DC voltage (U_DC) is below a limit voltage (U_GR), wherein the limit voltage (U_GR) corresponds to a minimum voltage required for the voltage-impressing operation (NB, NF), wherein the minimum voltage depends on a peak value of the nominal voltage of the AC units and / or on a peak value of the voltage of the AC network (32). [13] Energy supply system comprising a DC generator (14) and a power converter (10) for exchanging electrical power between a DC connection (DCA) and an AC connection (ACA), wherein the DC connection (DCA) is connected to the DC generator (14) and the AC connection is connected to AC units and / or an AC network (32), wherein the power converter (10) is configured to supply the AC units and wherein the supply of the AC units serves for the self-supply of the DC generator (14), wherein the power converter (10) further comprises a DC intermediate circuit (16), a bridge circuit (20), an AC relay (18) and a control unit (26), wherein the DC intermediate circuit (16) is arranged between the DC connection (DCA) and the bridge circuit (20) and the AC relay (18) is arranged between the bridge circuit (20) and the AC connection (ACA) is arranged, wherein the control unit (26) is arranged to control power switches of the bridge circuit (20) in such a way thatthat the power converter (10) is operated in a voltage-impressing manner at the AC connection (ACA), wherein the voltage (U_AC) impressed by the power converter (10) depends on a nominal voltage of the AC units and / or on the voltage of the AC network (32), and wherein the control unit (26) is further configured to detect a DC voltage (U_DC) at the DC intermediate circuit (16) during the voltage-impressing operation (NB, NF) and to disconnect the bridge circuit (20) from the AC connection (ACA) by opening the AC relay (18) when the DC voltage (U_DC) is below a limit voltage (U_GR). [14] Power supply system according to claim 13, wherein the DC generator (14) comprises a fuel cell and wherein the AC units comprise auxiliary units (40) of the fuel cell. [15] Energy supply system according to claim 14, wherein the power converter (10) is connected to a DC bus (38) via its DC connection (DCA), wherein a further power converter (30) is connected to the DC bus (38), wherein the further power converter (30) is arranged to transmit electrical power from the DC bus (38) to the AC network (32). [16] Power supply system according to claim 15, wherein the rated power of the further power converter (30) is at least five times higher than the rated power of the power converter (10). [17] Method for operating an energy supply system according to one of claims 13 to 16, wherein the power converter (10) is operated in a network-forming operation (NF) with connection to the AC network (32) or in a network-forming operation (NB) separately from the AC network (32). [18] Method according to claim 17, wherein in the grid-forming operation (NB) the power converter (10) supplies the AC units with electrical power and in the grid-forming operation (NF) the power converter (10) feeds electrical power into the AC grid (32) and / or is operated in parallel on the AC grid (32) and on AC units. [19] Method according to claim 17 or 18, wherein the power converter (10) is operated in a network-forming manner separately from the AC network (32) when the further power converter (30) is separated from the AC network (32). [20] Method according to one of claims 17 to 19, wherein the grid-forming operation (NF) is used in a start-up mode and / or the grid-forming operation (NB) is used in a normal operating mode of the power generation plant.

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