CONTROL METHOD FOR STARTING UP A FUEL CELL SYSTEM

Description

[0001] The present invention relates to a control method for starting up a fuel cell system after a standstill, a corresponding control device and a computer program product.

[0002] In fuel cell systems for supplying a consumer with high-voltage power, such as in a stationary application or a powertrain with a large-scale electric drive for a commercial vehicle or ship, the output voltage from the fuel cell stacks is usually increased or boosted by a DC / DC converter to the voltage level of a high-voltage supply for the consumer.

[0003] Under full load or within a predetermined operating range, the voltage difference to be converted by the DC-DC converter is designed to be low for efficiency. To keep the voltage difference low, the output voltage of the fuel cell stack under load must be relatively high or close to the supply voltage of the consumer. In the case of high-voltage fuel cell systems, a maximum permissible operating voltage of the DC-DC converter on the low-voltage side represents a technical limitation when designing for the smallest possible and most efficient voltage difference to be converted under load. For semiconductor-based power electronics in voltage converters, the maximum permissible operating voltage is currently around 1200 V.

[0004] When the fuel cell stacks are disconnected from the electrical load, such as during fuel cell system startup, they generate an open-circuit voltage that can exceed the output voltage under load and, in particular, the maximum operating voltage of the DC-DC converter. If this condition occurs during the fuel cell system startup, the DC-DC converter would be damaged as soon as an electrical connection is established between the fuel cell stacks and the DC-DC converter. Therefore, a technique is needed to prevent overvoltage damage to a DC-DC converter during the startup of a high-voltage fuel cell system.

[0005] On the other hand, prior art fuel cell systems are known that connect an artificial load, or dummy load, to the fuel cell stacks when the open-circuit voltage becomes very high, in order to reduce it and prevent the cells' lifespan from being impaired by excessively high voltages. In this context, it can be problematic if, during the fuel cell system's startup, not all cells in the stack are yet uniformly or sufficiently supplied with fuel gas, especially if air is present in the system after a prolonged period of inactivity, i.e., if oxygen is present at both the cathode and the anode. If a load is applied in this state, in which a high open-circuit voltage can also be generated, polarity reversals or voltage potential inversions can occur at individual cells, leading to fuel depletion.Even if they occur only briefly, these are considerably more damaging to cell lifespan than temporary high open-circuit voltages. Therefore, there is a need for a technology that reliably prevents such potential reversals in cells.

[0006] DE102019119622 discloses a control method for starting up a fuel cell system with a DC voltage converter for increasing the voltage between an output voltage of the fuel cell unit and a supply voltage of a high-voltage supply section and a switching device for opening and closing an electrical connection between the load and the DC voltage converter.

[0007] One object of the invention is to provide a technique for starting up a fuel cell system that enables a higher output voltage of the fuel cell stacks under load, in particular a higher power output, in connection with which the open-circuit voltage of the fuel cell stacks can exceed a technically limited maximum operating voltage of the DC-DC converter. A further object of the invention is to provide a technique for starting up a fuel cell system that can both prevent overvoltage damage to a DC-DC converter and reliably prevent the occurrence of harmful potential reversals in the cells.

[0008] The aforementioned problem is solved by the control method with the features of claim 1. Further features and details of the invention will become apparent from the dependent claims, the description, and the drawings.

[0009] According to the invention, a control method for starting up a fuel cell system after a standstill is provided. The system typically comprises at least one fuel cell stack with a cathode supply section for supplying a cathode supply gas, a cathode discharge section for removing a cathode discharge gas, an anode supply section for supplying an anode supply gas, and an anode discharge section for removing an anode discharge gas; as well as a DC-DC converter for increasing the voltage between an output voltage of the fuel cell stack on a low-voltage side and a supply voltage of a high-voltage supply line on a high-voltage side; and a switch for opening and closing an electrical connection between the fuel cell stack and the DC-DC converter.

[0010] The control method according to the invention is characterized by the following steps: supplying the anode supply gas to the anode supply section; at least temporarily, blocking and / or throttling the cathode supply gas to the cathode supply section; monitoring an increasing open-circuit voltage of the fuel cell stack when the switch of the electrical connection is open.Provided that the open-circuit voltage exceeds a predetermined permissible threshold voltage with respect to the DC-DC converter, the control method according to the invention further comprises, in particular, the steps of carrying out and / or continuing the at least temporary blocking and / or throttling of the cathode supply gas until the open-circuit voltage has fallen back to or below the permissible threshold voltage; and of closing the switch of the electrical connection after the open-circuit voltage has fallen back to or below the permissible threshold voltage.

[0011] The invention thus provides for the first time a solution of providing for a reduction in oxygen during the start-up of the fuel cell system, and thereby causing a reduction in the open-circuit voltage before the fuel cell stacks are connected to the DC voltage converter.

[0012] The invention is further based on the understanding that during a standstill of the fuel cell system, air enters the system, or that air present in the cathode section also spreads or distributes itself towards the anode section. Therefore, when the fuel cell system restarts after a standstill, oxygen from the air remaining in the fuel cell stacks is always present, which can be used to start the electrochemical reaction until it is gradually consumed. If, due to a blockage of the supply of new oxygen or a significant reduction in the oxidation gas supply compared to the fuel gas supply, a shortage or consumption of oxygen occurs in the fuel cell stacks, the throughput of the electrochemical reaction in the cells decreases, and the open-circuit voltage is consequently lowered.

[0013] An advantage of the invention is that this measure allows the open-circuit voltage to be controlled after an increase during the start-up of the fuel cell system by limiting the oxygen and to be specifically reduced before connection with the DC voltage converter, thus preventing overvoltage damage to the latter.

[0014] A further advantage of the invention is that supersaturation of the fuel gas is achieved on the anode side, instead of the presence of oxygen on both the cathode and anode sides. This reliably prevents a potential reversal from occurring in a cell as soon as the fuel cell stacks are connected to a load under uneven fuel gas supply, as is the case in the prior art.

[0015] According to an advantageous aspect of the invention, the continuation of the at least temporary blocking and / or throttling of the cathode supply gas can include blocking and / or throttling that begins as soon as the anode supply gas is introduced. In this variant, the supply of the oxidation gas or air is not initiated until the fuel cell stacks are electrically connected. This allows the fuel cell system to be started up and put into operation as quickly as possible.

[0016] According to an alternative advantageous aspect of the invention, the implementation of at least temporary blocking and / or throttling of the cathode supply gas can include blocking and / or throttling that occurs as soon as the open-circuit voltage exceeds the permissible threshold voltage. This variant can be selected in a system design where the open-circuit voltage only occasionally exceeds critical values. This ensures that throttling of the gas supply only occurs when necessary for overvoltage protection.

[0017] According to a further advantageous aspect of the invention, the at least temporary blocking and / or throttling of the cathode supply gas can include a change in a throttling rate in response to monitoring the open-circuit voltage in relation to the permissible threshold voltage. This variant allows for more precise control of the oxygen deprivation at the cells, in order to ensure the electrochemical reaction from the outset and to better control it further as the open-circuit voltage develops.

[0018] According to a specific advantageous aspect of the invention, the at least temporary blocking and / or throttling of the cathode supply gas can include increasing the throttling rate in response to an exceedance of the open-circuit voltage relative to the permissible threshold voltage. In this variant, a lower throttling of the oxidation gas can initially be implemented, followed by a higher throttling in the case of critical voltage values, or a gradual change in the throttling rate depending on the rate of increase of the open-circuit voltage.

[0019] According to a further advantageous aspect of the invention, the DC-DC converter can include or be connected to a capacitor configured to be charged and discharged in a controllable manner; and the method can further include the following step: pre-charging the capacitor before closing the switch of the electrical connection. In this way, the capacitor provides an electrical capacitance that can be charged and discharged via the DC-DC converter. These charging and discharging processes are particularly controllable.

[0020] According to the preceding advantageous aspect of the invention, the control method with respect to the capacitor can comprise the following step: starting the pre-charging of the capacitor after the open-circuit voltage falls below the permissible threshold voltage. The capacitor can have an electrical resistance, which can also be referred to as a pre-charge resistor, so that high currents, especially current spikes, during charging and discharging of the capacitor can be avoided or at least smoothed out. As soon as the anode of the fuel cell unit is supplied, the voltage rises. The system then waits until the threshold value is undershot in order to perform the pre-charging of the capacitor. Following pre-charging, the switching device is closed. With this variant, the use of the capacitor is limited to cases in which the open-circuit voltage reaches critical levels.

[0021] According to the previous advantageous aspect of the invention, the capacitor can be connected to the high-voltage side of the DC-DC converter for pre-charging the capacitor on the high-voltage supply line.

[0022] Alternatively, according to the previous advantageous aspect of the invention, the capacitor can be connected to the low-voltage side of the DC-DC converter for precharging the capacitor on the fuel cell stack.

[0023] According to an advantageous aspect of the invention, the permissible threshold voltage can be predetermined with respect to a technically determined maximum operating voltage of the DC-DC converter on the low-voltage side.

[0024] The permissible threshold voltage can be predetermined below or equal to the maximum operating voltage of a silicon carbide (SiC) based semiconductor circuit of the DC-DC converter and / or preset within a range of 700 V to 900 V, preferably at or below approximately 800 V.

[0025] Alternatively, the upper threshold voltage can be predetermined below or equal to the maximum operating voltage of a gallium nitride (GaN) based semiconductor circuit of the DC / DC converter and / or preset within a range of 1100 V to 1300 V, preferably at or below about 1200 V.

[0026] The invention also includes a control device for a fuel cell system and a computer program product by which the steps of the control method according to the invention can be implemented when starting up the fuel cell system.

[0027] Thus, the control device and the computer program product offer the same advantages as have been explained in detail with reference to the control method according to the invention.

[0028] Further advantages, features, and details of the invention will become apparent from the following description, in which exemplary embodiments of the invention are described in detail with reference to the drawings. The drawings schematically show: Fig. 1 a schematic block diagram of a section of a fuel cell system in whose system context the control method according to the invention is implemented, Fig. 2 a comparison of diagrams of the voltage and current curves during the start-up of fuel cell stacks with and without air at the anode supply section and when applying the control method according to the invention, and Fig. 3 a diagram of the voltage and current curves as well as gas supply parameters over individual phases of the control method according to the invention.

[0029] Figure 1Figure 1 shows a section of a fuel cell system relevant to the control method, which is schematically represented as a block diagram. The fuel cell system comprises a fuel cell unit 10 with fuel cell stacks, each having an anode section (not shown) and a cathode section. The anode section of each fuel cell stack is connected to an anode supply section 12 for supplying anode supply gas, which supplies a fuel gas (Fx), in particular hydrogen, in a known manner. The cathode section of each fuel cell stack 10 is connected to a cathode supply section 14 for supplying cathode supply gas, which supplies an oxidation gas (Ox), in particular oxygen-containing air.Furthermore, each cathode section is connected to a cathode exhaust section for removing cathode gas, and each anode section is connected to a cathode exhaust section for removing cathode gas (not shown further). In the present embodiment, the fuel cell unit 10 is designed for large-scale power generation, such as in stationary, semi-stationary, or nautical applications, i.e., for self-sufficient building power supply or for propulsion of ships and other heavy-duty commercial vehicles, including on-board electrical systems and consumers. Under load, the fuel cell unit 10 can be designed to operate at a main operating point in a high-voltage range, for example, around approximately 750 V or approximately 1100 V.

[0030] The fuel cell unit 10 serves to generate electrical power for a high-voltage supply section 40 of the fuel cell system. The high-voltage supply section 40 supplies direct current power at a high supply voltage to loads connected to a drive train, an on-board electrical system, or a building's electrical network. In the illustrated embodiment, a first load 51, e.g., representing an electric drive, and a second load 52, e.g., representing an air conditioning system, are connected to the high-voltage supply section 40 to draw power generated by the fuel cell unit 10. The high-voltage supply section 40 is also connected to a battery storage system 44, which compensates for fluctuations in a dynamic load demand between generation and consumption and stabilizes the supply voltage level.

[0031] A DC-DC converter 30 is provided between the fuel cell unit 10 and the high-voltage supply section 40. The DC-DC converter 30 serves as a voltage booster, increasing the load-dependent output voltage of the fuel cell unit 10 to the supply voltage. For this purpose, the DC-DC converter 30 has a low-voltage side, to which the generated power is supplied to the fuel cell unit 10, and a high-voltage side, which passes the power on to the high-voltage supply section 40 at the increased voltage. Alternatively, a "buck / boost" configuration is also conceivable for the DC-DC converter 30, in which similar voltages are applied to both sides. Such a configuration is also encompassed by the present invention.Furthermore, the DC-DC converter 30 includes a capacitor 33, the capacitance of which helps to smooth out power-dependent fluctuations in the output voltage of the fuel cell unit 10 relative to the supply voltage of the high-voltage supply section 40, or to compensate for voltage and current spikes between the low-voltage side and the high-voltage side, which may occur, for example, during switching operations in electrical connections of the fuel cell system.

[0032] The DC-DC converter 30 also includes power electronics for variable voltage conversion, featuring transistors implemented on a silicon carbide (SiC) or gallium nitride (GaN) semiconductor. Due to the maximum reverse voltage across the band gaps of these semiconductor materials, there are limits to the maximum permissible operating voltage (Umax) that can be applied to the power electronics without damaging the circuitry. According to current technology, the maximum permissible voltages for silicon carbide (SiC) and gallium nitride (GaN) semiconductors are approximately 800 V and 1200 V, respectively.

[0033] An electrical connection between the fuel cell unit 10 and the DC-DC converter 30 can be disconnected by a switching device 20. The switching device 20 is designed with high-voltage-compatible switching technology, such as a circuit breaker. The switching device 20 is disconnected, among other times, during a standstill of the fuel cell system, when the fuel cell unit 10 is in a quiescent state without gas supply. Consequently, the switching device 20 must be closed again after restarting the fuel cell system following a standstill before the power generated by the fuel cell unit 10 can be supplied to the first load 51 and the second load 52.

[0034] Fig. 2Figure 1 shows a comparison of parameters resulting from the start-up of fuel cell stacks with (left) and without (right) air at the anode supply section and when applying the control method according to the invention.

[0035] Depending on the design of the piping, sealing, and valve technology in the fuel cell system, after a standstill of at least several hours, and especially after a standstill of approximately one day, it can be assumed that air has penetrated the entire fuel cell unit 10 or diffused through the membranes. In other words, this means that air is not only present in the cathode section of the fuel cell stack, but has also spread to the anode section.

[0036] When the fuel cell system is restarted, fuel gas is supplied via the anode supply section 12 and oxidation gas via the cathode supply section 14 to initiate the electrochemical reaction in the cells and ramp up power generation. Under normal operating conditions, this process continues, with a relatively high open-circuit voltage Uoc occurring relatively early, even in a supply state where air is still present on the cathode and anode sides of the cells, as shown by the upper voltage curve for the stack voltage Us and the lower set of voltage curves for the individual cell voltages Uc in the left diagram. Fig. 2 becomes apparent.

[0037] If the open-circuit voltage Uoc of the fuel cell unit 10 or a fuel cell stack exceeds a predetermined threshold voltage Uth of, for example, 400 V, voltage limiting (also known as "voltage clipping") is performed by diverting a set stack current Is across the fuel cell stack. This stack current Is, which is represented by a curve in the middle of the diagram, is diverted by an artificial load (also known as a "dummy load") or a resistor, which is electrically connected to the fuel cell stack for this purpose, or by connection to the DC-DC converter 30, which supplies power to the loads of the fuel cell system. This process is carefully controlled to prevent long-term damage to the cells caused by high open-circuit voltages Uoc.As a result of the controlled power dissipation via the stack current Is, the open-circuit voltage Uoc drops from about 600 V to the threshold voltage of 400 V or below in the interim, as shown.

[0038] Due to an incomplete supply or inhomogeneous distribution of the fuel gas to all cells of the fuel cell stack during startup, air may still be present on some cells, both on the cathode and anode sides. In this state, which can be considered a partial fuel shortage, there is a risk that, under the applied electrical load for voltage limiting, a potential reversal will occur in the affected cells, resulting in irreversible damage to the cell membrane. The wide dispersion of the lower set of voltage curves for the individual cell voltages Uc indicates an inhomogeneous fuel gas distribution in the fuel cell stack, with those voltage curves exhibiting a flatter profile or...The lowest voltage peak is specifically attributable to cells that are exposed to a lack of fuel and consequently to the risk of potential reversal when a load request is applied.

[0039] The diagram on the right shows the corresponding parameters of the stack voltage Us, individual cell voltages Uc, and the stack current Is plotted against the same time axis, which occur during the execution of the control method according to the invention. When the fuel cell system is restarted and a supply of fuel gas is initiated via the anode supply section 12, in the present embodiment, in both cases (left and right in the diagram) the current remains constant. Fig. 2 ) the supply of oxidation gas via the cathode supply section 14 is initially blocked. The right side of the Fig. 2This shows a situation in which there is no oxygen present on either the anode or cathode side of fuel cell unit 10. The voltage therefore only rises when the cathode side is supplied. In this case, voltage limiting occurs. This can be seen from the upper voltage curve for the stack voltage Us and the lower set of voltage curves for the individual cell voltages Uc in the right-hand diagram. Fig. 2As can be seen, due to the lack of oxygen at the anode, somewhat lower values ​​for the open-circuit voltage Uoc are assumed, with a significantly smaller scatter in the lower set of voltage curves. The latter is due to the fact that the fuel gas distribution in the fuel cell stacks is more homogeneous, since the time lag of an exclusive or at least predominant supply of fuel gas compared to the oxidation gas forces saturation with fuel gas on the anode side (in both in the Fig. 2 (as shown in the cases). As a result of the oxygen supply to the fuel cell stack on the right side of the Fig. 2 The tension is rising.

[0040] Fig. 3Figure 1 shows three phases of the control method according to the invention during the start-up of the fuel cell system. In the lower section, a series of cell voltages Uc of individual cells in a fuel cell stack and a stack current Is are plotted against the time axis, and in the upper section, the pressure of the supplied fuel gas Pf and the pressure of the supplied oxidation gas Po at the fuel cell stack are shown.

[0041] Phase 1 essentially corresponds to the control procedure previously described in connection with the Fig. 2 as described. However, due to a different dimensioning of the fuel cell system, the open-circuit voltage values ​​are higher than in Fig. 1 The open-circuit voltage Uoc in the system increases in conjunction with the pressure Pf of the opened fuel gas supply. Fig. 2to over 1000 V, wherein a maximum permissible operating voltage Umax of the associated DC voltage converter 30 on the low voltage side is about 1000 V, and a predetermined threshold voltage Uth is set at about 800 V in relation to this.

[0042] After a start-up time of approximately 10 seconds, the fuel gas supply is initiated, and Phase 1 begins. The open-circuit voltage (Uoc) then rises rapidly due to the increasing electrochemical reaction in the cells, which consumes the oxygen contained in the fuel cell stack. Without discharging any power, the open-circuit voltage (Uoc) remains at a high level between 800 V and 1000 V throughout Phase 1. As the oxygen is consumed, the electrochemical reaction in the cells slows down, and over a waiting period of up to 60 seconds, the open-circuit voltage (Uoc) gradually drops to approximately 800 V.Although this waiting period causes a certain delay in starting up the fuel cell system, and the cells remain at a high open-circuit voltage Uoc during this time, this circumstance, considering the risks, even in the long term, represents a lower potential for damage than a potential reversal that would be avoided in this way, which would be directly associated with irreversible damage to the cells.

[0043] Once the cell voltages UC drop from the open-circuit voltage Uoc to the threshold voltage Uth, the subsequent, optional phase 2 begins, during which the capacitor 33 of the DC-DC converter 30 is charged. By using the capacitor 33 together with a resistor during the charging and discharging process, the occurrence of a voltage spike on the low-voltage side of the DC-DC converter 30 during a switching operation for electrical connection can be reduced, thus protecting the capacitor 33.

[0044] At the end of phase 2, the oxidation gas supply is opened, as indicated by the vertically rising curve Po of the oxidation gas pressure, and immediately thereafter the switching device 20 is actuated to connect the fuel cell unit 10 to the DC-DC converter 30. This results in another increase in the open-circuit voltage Uoc of the fuel cell unit 10, which, however, is controllable by the previously achieved reduction to a certain voltage difference below the maximum permissible operating voltage Umax of the DC-DC converter 30 and a rapid, controlled sequence of the processes of starting the oxidation gas supply and closing the switching device 20.

[0045] Depending on the configuration and control of the charging circuit for capacitor 33, particularly when capacitor 33 is charged via a charging circuit to the low-current side of the DC-DC converter 30 from a stack current Is supplied by a fuel cell stack from the fuel cell unit 10, the open-circuit voltage Uoc can be reduced even further. Capacitor 33 is typically located on the low-voltage side. However, an arrangement on the high-voltage side is also conceivable, with pre-charging ensured by appropriate wiring. The resulting higher voltage differential then prevents the maximum permissible operating voltage Umax of the DC-DC converter 30 from being exceeded with even greater safety when the switching device 20 is closed.

[0046] Phase 3 essentially represents the beginning of the fully powered-up operating state of the fuel cell system, whereby, in the further course of the process, at least the pressure of the fuel gas supply Pf at the fuel cell unit 10 is variably adjusted depending on the total power demand of the first load 51 and the second load 52 from the high-voltage supply section 40. Thus, an operating point for the power to be generated at a uniform voltage level is possible by supporting the variable voltage conversion of the DC-DC converter 30 and the battery storage 40.

[0047] In one embodiment, the pre-charging step of the capacitor 33 of the DC-DC converter 30 in the control method involves connecting the capacitor 33 to the high-voltage side instead of the low-voltage side. By configuring a charging circuit accordingly, the capacitor 33 is charged from the high-voltage supply section 40 or from the battery storage 44, and not from the fuel cell unit 10, so that the charging process can take place independently of whether the switching device 20 is closed. Charging on the high-voltage side can be more controlled and / or faster than charging from the fuel cell unit 10 side.

[0048] In an alternative embodiment, the permissible threshold voltage Uth predetermined with respect to the DC voltage converter 30 is not designed to be below the technically determined maximum operating voltage Umax of a semiconductor circuit of the power electronics in the DC voltage converter 30, but the permissible threshold voltage Uth essentially corresponds to the maximum operating voltage Umax.

[0049] In one embodiment of the device, the control method is carried out by a dedicated control device (not shown), which is connected to the relevant system components of the fuel cell system, i.e., to the fuel cell unit 10 including the anode supply section 12 and the cathode supply section 14, the switching device 20, the DC voltage converter 30 including capacitor 33 and a charging circuit thereof, the high voltage supply section 40 including the battery storage 44 and the loads 51, 52, in particular to actuators, sensors and / or circuits thereof, at least in a signal connection, in order to instruct or also control the process steps.

[0050] In an alternative device-technical embodiment, the control method is carried out by an external control device in a system environment of the fuel cell system (not shown), which is connected indirectly or directly to the relevant system components at least via the aforementioned signal connection, wherein the aforementioned dedicated control device or the control method is implemented by means of a computer program on the hardware of the external control device.

[0051] The preceding explanations of the embodiments describe the present invention exclusively by way of examples. Reference symbol list

[0052] 10 Fuel cell unit 12 Anode feed section 14 Cathode feed section 20 Switching device 30 DC / DC converter 33 Capacitor 40 High-voltage supply section 44 Battery storage 51 First load 52 Second load Uoc Open-circuit voltage Uth Threshold voltage Umax Maximum permissible operating voltage Uc Cell voltage Us Stack voltage Is Stack current Pf Fuel gas supply pressure Po Oxidizing gas supply pressure

Claims

1. Control method for starting up a fuel cell system after a standstill, wherein the fuel cell system comprises: a fuel cell unit (10) with fuel cell stacks, each having a cathode supply section (14) for supplying a cathode supply gas, a cathode discharge section for removing a cathode discharge gas, an anode supply section (12) for supplying an anode supply gas, and an anode discharge section for removing an anode discharge gas; a DC-DC converter (30) for voltage increase between an output voltage of the fuel cell unit (10) on a low-voltage side and a supply voltage of a high-voltage supply section (40) on a high-voltage side; and a switching device (20) for opening and closing an electrical connection between the fuel cell unit (10) and the DC-DC converter (30), comprising the following steps: supplying the anode supply gas to the anode supply section (12); at least temporarily, blocking and / or throttling the cathode supply gas to the cathode supply section (14); monitoring an increasing open-circuit voltage (Uoc) of a fuel cell stack with the switching device (20) of the electrical connection open; and if the open-circuit voltage (Uoc) exceeds a predetermined permissible threshold voltage (Uth) with respect to the DC-DC converter (30): performing and / or continuing the at least temporary blocking and / or throttling of the cathode supply gas until the open-circuit voltage (Uoc) has dropped back to or below the permissible threshold voltage (Uth); and closing the switching device (20) of the electrical connection after the open-circuit voltage (Uoc) has dropped back to or below the permissible threshold voltage (Uth).

2. Control method for starting up a fuel cell system according to claim 1, wherein continuing the at least temporary blocking and / or throttling of the cathode supply gas comprises blocking and / or throttling that begins upon supply of the anode supply gas.

3. Control method for starting up a fuel cell system according to claim 1, wherein performing the at least temporary blocking and / or throttling of the cathode supply gas comprises blocking and / or throttling that begins as soon as the open-circuit voltage (Uoc) exceeds the permissible threshold voltage (Uth).

4. Control method for starting up a fuel cell system according to any of the preceding claims, wherein the at least temporary blocking and / or throttling of the cathode supply gas comprises increasing a throttling rate in response to an exceedance of the open-circuit voltage (Uoc) relative to the permissible threshold voltage (Uth).

5. Control method for starting up a fuel cell system according to any of the preceding claims, wherein the at least temporary blocking and / or throttling of the cathode supply gas comprises changing the throttling rate in response to monitoring the open-circuit voltage (Uoc) relative to the permissible threshold voltage (Uth).

6. Control method for starting up a fuel cell system according to any of the preceding claims, wherein the DC-DC converter (30) comprises or is connected to a capacitor (33) which is configured to be charged and discharged in a controllable manner. and the method further comprises the following step: pre-charging the capacitor (33) before closing the switching device (20) of the electrical connection.

7. Control method for starting up a fuel cell system according to claim 6, further comprising the following step: starting the pre-charging of the capacitor (33) after the open-circuit voltage (Uoc) falls below the permissible threshold voltage (Uth).

8. Control method for starting up a fuel cell system according to claim 6 or 7, further comprising the following step: connecting the capacitor (33) to the low-voltage side of the DC-DC converter (30) for pre-charging the capacitor (33) at the fuel cell unit (10).

9. Control method for starting up a fuel cell system according to claim 6 or 7, further comprising the following step: connecting the capacitor (33) to the high-voltage side of the DC-DC converter (30) for precharging the capacitor (33) at the high-voltage supply section (40).

10. Control method for starting up a fuel cell system according to any one of the preceding claims, wherein the permissible threshold voltage (Uth) is predetermined with respect to a technically determined maximum operating voltage (Umax) of the DC-DC converter (30) on the low-voltage side.

11. Control method for starting up a fuel cell system according to claim 10, wherein the permissible threshold voltage (Uth) is predetermined and / or preset within a range of 700 V to 900 V, preferably at or below approximately 800 V, and is less than or equal to the maximum operating voltage (Umax) of a silicon carbide (SiC)-based semiconductor circuit of the DC / DC converter (30).

12. Control method for starting up a fuel cell system according to claim 10, wherein the permissible threshold voltage (Uth) is predetermined and / or preset within a range of 1100 V to 1300 V, preferably at or below approximately 1200 V, and is less than or equal to the maximum operating voltage (Umax) of a gallium nitride (GaN)-based semiconductor circuit of the DC / DC converter.

13. Control device for controlling the fuel cell system and for carrying out the steps of the control method according to any one of claims 1 to 12.

14. Computer program product comprising instructions which, when the program is executed by a computer or a control device, cause it or the computer or control device to execute the control method according to any one of claims 1 to 12.