Method for operating an electrolysis plant, and electrolysis plant

Abstract

The application relates to a method for operating an electrolysis plant (20) having an electrolyzer (14) and a power converter (10), the power converter (10) being set up to supply active DC power (P-DC) at a DC voltage (U-DC) from an AC grid (12) to the electrolyzer (14), the method comprising: • operating the power converter (10) in a first operating mode (BM1) in which the power converter (10) draws active AC power (P_AC) from the AC grid (12) and exchanges reactive AC power (Q_AC) for supporting the AC grid (12) with the AC grid (12), • monitoring the active AC power (P_AC) drawn from the AC grid (12) in the first operating mode (BM1) and the DC voltage (U_DC), and • changing the operating mode if the active AC power (P_AC) drawn from the AC grid (12) falls below a predefinable minimum power and the DC voltage (49) at the electrolyzer (49) falls below a predefinable minimum voltage (U_min). The application further relates to an electrolysis plant (20).

Description

[0001] METHOD FOR OPERATING AN ELECTROLYSIS PLANT AND ELECTROLYSIS PLANT

[0002] TECHNICAL AREA

[0003] The application concerns a procedure for operating an electrolysis plant and an electrolysis plant.

[0004] STATE OF THE ART

[0005] Hydrogen is often produced using an electrolyzer, which splits water into its elements, hydrogen and oxygen, through an electrolysis reaction. The electrolyzer can be supplied with DC power (direct current) via a power converter. The power converter can draw suitable electrical power from an AC network. When drawing such power from a public distribution network, certain regulatory requirements must be observed, particularly those that stipulate that an electrolysis plant, as a large consumer, contributes to stabilizing the distribution network. Furthermore, it may be desirable and / or required by regulations that large consumers remain connected to the network in the event of a fault and ride through it, a process known as fault ride-through (FRT).

[0006] From DE 10 2018 133 641 A1 a method for operating an electrolysis plant with a voltage-impeding power converter is described, with which an instantaneous reserve power can be made available to the AC network in particular.

[0007] In DE 10 2020 112 880 A1 a method for operating an electrolyzer in a normal operating mode and a standby operating mode is described, wherein the electrolyzer has capacitive properties in the standby operating mode.

[0008] CN 115377992 A describes a method for operating an electrolysis plant in which the AC mains voltage is monitored and analyzed. In normal operating mode, a DC-side bus voltage is stabilized using droop control to achieve stable electrolysis operation. The operating mode can be switched and, in the event of an AC mains fault, utilizes rapid optimization of reactive power output with simultaneous DC current limiting to ensure continued stable operation of the electrolysis plant.

[0009] TASK

[0010] The application is based on the task of providing a method for operating an electrolysis plant and an electrolysis plant with improved grid performance. SOLUTION

[0011] The problem is solved by a method with the features of independent claim 1 and an electrolysis plant with the features of independent claim 10. Embodiments of the application are specified in the dependent claims.

[0012] DESCRIPTION

[0013] An electrolysis plant comprises an electrolyzer and a power converter. The power converter is configured to supply the electrolyzer with DC power from an AC network at a DC voltage. One method for operating the electrolysis plant includes:

[0014] A) Operating the power converter in a first operating mode in which the power converter draws AC active power from the AC network and exchanges AC reactive power with the AC network to support the AC network,

[0015] B) Monitoring the AC active power and DC voltage drawn from the AC network in the first operating mode, and

[0016] C) Changing the operating mode if the AC active power drawn from the AC network falls below a predefined minimum power and the DC voltage falls below a predefined minimum voltage.

[0017] The power converter is designed to supply electrical power to the electrolyzer connected to its DC side. This power is drawn from the AC network connected to its AC side. The power converter is specifically designed as a rectifier, capable of converting AC active power from its AC side into DC active power and making it available on its DC side. The power conversion is achieved, for example, by means of a bridge circuit with controllable power switches. The power conversion can be controlled, for example, by the power converter's control unit via a control signal, particularly a clock signal, to the bridge circuit's power switches. By controlling the power switches, the AC active power drawn from the AC network, the DC active power supplied to the electrolyzer, and the AC reactive power exchanged with the AC network can be adjusted.The power converter can be configured to convert and transfer electrical power in any of the possible directions, and in particular can be operated as a rectifier and / or inverter.

[0018] The electrolyzer supplied by the power converter requires a base power supply for its regular operation. In addition, the power converter itself requires electrical power for its own operation and to cover power losses that occur during the transfer of active power or the exchange of reactive power with the AC grid.

[0019] The described method enables grid-supportive operation of one or more electrolysis plants on an AC grid. In particular, grid-connected electrolysis plants with at least one electrolyzer and at least one power converter, which can represent large loads in the range of several hundred kilowatts to several megawatts, can be operated in a grid-supportive manner as long as the minimum power and voltage are not undershot. The at least one electrolysis plant can be operated in a grid-supportive manner, especially in the event of a grid fault, for as long as possible within its operating limits. The grid fault can be detected and quantified by monitoring the DC voltage. The grid fault can be ridden through (FRT, Fault Ride Through), whereby the electrolysis plant remains connected to the AC grid and exchanges reactive power with the AC grid for grid support for as long as possible.This method makes it possible to go through the fault phase completely, so that, especially when the power returns, continued operation without large compensating currents is possible.

[0020] By monitoring the active power drawn from the AC grid relative to a minimum power level, the state of the electrolysis plant in its initial operating state can be monitored to determine whether it can be maintained or must be abandoned. In one embodiment of the method, the minimum power level essentially corresponds to the power loss of the power converter. This allows the initial operating mode to be maintained as long as it is possible to cover the power loss of the power converter, which occurs particularly during the operation of the bridge circuit and the exchange of reactive power, from the AC grid. Only if this is no longer possible is the operating mode changed. Alternatively, the minimum power level can comprise the power loss of the power converter and the base power of the electrolyzer, with the base power being, in particular, between 5% and 20% of the electrolyzer's rated power.This ensures that the first operating mode is maintained at least as long as regular operation of the electrolyzer can be covered by drawing AC power from the AC grid.

[0021] In one embodiment of the method, the process (C) switches from the first operating mode to a second operating mode. In the second operating mode, both the exchange of reactive power and the exchange of AC active power between the power converter and the AC grid are stopped. Grid support by the electrolysis plant is therefore discontinued in the second operating mode, but the power converter remains connected to the AC grid, thus avoiding the need to operate the grid disconnect switches between the power converter and the AC grid. No further power losses occur in the power converter due to power exchange with the AC grid, as no electrical power is being converted. In one embodiment of the method, the switching of the power converter is stopped in the second operating mode.This avoids switching losses and power requirements for clocking the power switches, especially for the driver circuits of semiconductor power switches in the bridge circuit of the power converter in the second operating mode.

[0022] The electrolyzer can, for example, be designed as a hydrogen electrolyzer, which splits water into its components, hydrogen and oxygen, using DC current in an electrolysis reaction. The electrolyzer typically exhibits a current-voltage characteristic, also known as the electrolysis characteristic. This characteristic can be divided into two regions. Below the open-circuit voltage, the electrolyzer exhibits predominantly capacitive behavior, and an electrolysis reaction does not yet occur, or at least not to a significant degree. At input voltages above the open-circuit voltage, the electrolyzer exhibits predominantly resistive behavior, which is caused by the electrolysis reaction taking place at these voltages.The speed of the electrolysis reaction, and thus the production rate of hydrogen, increases with increasing actual direct current and, according to the electrolysis characteristic curve, with increasing DC input voltage.

[0023] In one embodiment, the method has a third operating mode in which the power converter draws active power from the AC network and in which the electrolyzer is pre-charged to an open-circuit voltage, with the energy fed into the electrolyzer during the third operating mode being determined and stored. In this third operating mode, the electrolyzer is pre-charged by increasing the DC voltage of the power converter, which is applied to the electrolyzer as the DC input voltage, from a minimum voltage to the open-circuit voltage by supplying DC electrical power to the electrolyzer. The minimum voltage can be close to zero and, in particular, below the minimum voltage used in section C).

[0024] In one embodiment of the method, a characteristic value for grid support is provided, which can depend on the energy determined in the third operating mode and thus "stored" in the electrolyzer. The electrolyzer can make this energy available to the power converter in whole or in part as needed, for example, to at least temporarily compensate for the losses during the exchange of reactive power with the AC grid for grid support. The minimum voltage used in C) can, for example, be specified as the voltage down to which the electrolyzer is able to supply the power losses in the power converter without ultimately requiring the electrolysis system to be disconnected from the AC grid. The usable energy in the capacitive operation of the electrolyzer corresponds approximately to the active power supplied in the third operating mode to increase the DC voltage from the minimum voltage to the open-circuit voltage.When the power supply is restored, it may be necessary to recharge the electrolyzer to its open-circuit voltage, whereby the energy fed into the electrolyzer can be determined and stored.

[0025] In one embodiment of the method, the characteristic value for network support is transmitted to an AC network operator. During higher-level network planning, particularly the planning of the necessary equipment to ensure network stability, this characteristic value can then be incorporated by the AC network operator. The characteristic value can, in particular, include an FRT characteristic curve, which represents a possible duration of the first operating mode as a function of a reduced AC network voltage in the event of a fault.

[0026] In one embodiment of the method, the possible duration of the first operating mode is determined when the AC network voltage is reduced to a predefined value, depending on the energy determined in the third operating mode. This makes it possible to specify a duration for various network fault scenarios, indicating how long network support could be maintained at a predefined reduced network voltage in the event of a fault. The possible duration is maximized by the availability of energy from the electrolyzer in the first operating mode. This further simplifies network planning.

[0027] In one embodiment, the method comprises a normal operating mode in which the electrolyzer is supplied with DC active power from the AC grid, depending on a setpoint. The first operating mode is a grid support mode in which the reactive power supplied is adjusted depending on the grid voltage of the AC grid. Optionally, the second operating mode can be a standby mode in which the power converter remains connected to the AC grid when power conversion is stopped. Optionally, the third operating mode can be a pre-charge mode, which can be executed before starting or resuming the normal operating mode.

[0028] In normal operating mode, the power converter draws active AC current from the AC grid and converts it into DC power to supply the electrolyzer. During a grid fault, a voltage drop to a (significantly) reduced grid voltage causes the power converter to support the AC grid by injecting reactive AC power into it. Due to power limitations of the power converter, such as its maximum apparent power and / or a given and / or limited current of the active AC current within the power converter, the active power that the converter can draw from the AC grid decreases depending on the depth of the AC grid voltage drop. In other words, the deeper the grid voltage drop, the more apparent power is required for reactive power, and the less active AC power the power converter can convert into DC power in addition to the required reactive power.By monitoring the initial operating mode and switching between operating modes, it is possible to prevent the power converter from being unable to meet its own power requirements in the extreme case of a drop in the AC mains voltage to a low value, especially to a value close to zero. This can occur if the power converter can no longer draw any active power from the AC mains because its maximum apparent power would already be reached by the exchanged reactive power, and / or if the AC mains itself can no longer supply any AC active power.

[0029] An electrolysis plant comprises an electrolyzer and a power converter. The power converter is configured to supply the electrolyzer with DC active power from an AC grid at a DC voltage. The electrolysis plant has a first operating mode in which the power converter draws AC active power from the AC grid and exchanges AC reactive power with the AC grid to stabilize it. In this first operating mode, the AC active power drawn from the AC grid and the DC voltage at the electrolyzer are monitored. The electrolysis plant is configured to switch operating modes if, in the first operating mode, the drawn AC active power and the DC voltage fall below a predefined minimum level.

[0030] The described electrolysis plant, comprising at least one electrolyzer and at least one power converter, can be considered a large, grid-connected load with, for example, several hundred kilowatts to several megawatts, which can or must be operated in a grid-supportive manner within its operating limits in the event of a grid fault. In particular, the electrolysis plant can continue to exchange reactive power during grid faults with residual voltages of less than 20 percent, preferably less than 10 percent, and remains connected to the AC grid even at lower residual voltages. Optionally, the extraction of AC active power can continue upon grid restoration without significant equalizing currents.

[0031] BRIEF DESCRIPTION OF THE FIGURES

[0032] The registration process is further explained and described below using the examples shown in the figures.

[0033] Fig. 1 schematically shows an electrolysis plant.

[0034] Fig. 2 shows a first embodiment of the method.

[0035] Fig. 3 shows a second embodiment of the method. Fig. 4 shows exemplary current and voltage waveforms in the electrolysis plant.

[0036] Fig. 5 shows a section of the gradients from Figure 4.

[0037] The same reference symbols are used in the figures for identical or similar elements. The representations in the figures cannot be to scale.

[0038] FIGURE DESCRIPTION

[0039] Figure 1 shows an electrolysis system 20, which includes a power converter 10 and an electrolyzer 14. The power converter 10 has an AC side which is connected to an AC network 12.

[0040] The AC network 12 is configured as a three-phase AC voltage network, which provides its phase voltages 41 to the AC side of the power transformer 10. The corresponding phase currents 42 flow between the AC network 12 and the power transformer 10 on its AC side. The power transformer 10 thus exchanges AC active power P_AC and AC reactive power Q_AC with the AC network 12 on its AC side.

[0041] The DC voltage 49 is applied to the DC side of the power converter 10. The DC voltage 49 supplied by the power converter 10 is also present at the input of the electrolyzer 14. According to the current-voltage characteristic of the electrolyzer 14, the electrolyzer 14 is supplied with DC current 48 from the power converter 10, depending on the DC voltage 49. The power converter 10 thus supplies the electrolyzer 14 with DC active power P_DC on its DC side. This power can be set to a setpoint by means of suitable control of the switching of the power switches of the power converter 10, where the setpoint can include a DC current setpoint and / or a DC voltage setpoint.

[0042] Figure 2 shows a first embodiment of the method.

[0043] After the electrolysis plant 20 has started operation, a third operating mode, BM3, can optionally be run, which includes a pre-charging mode. In this pre-charging mode, the electrolyzer 14 can be pre-charged to its open-circuit voltage to start hydrogen production. Between a voltage of 0 V and the open-circuit voltage, the electrolyzer 14 exhibits essentially capacitive properties, with the capacitance ranging from a few hundred mF to a few F, depending on the type and design of the electrolyzer 14. During the pre-charging of the electrolyzer 14, this capacitance and the energy content stored in it during the pre-charging mode are determined. For this purpose, the current 48 and the voltage 49 can be measured over the pre-charging period. After start-up and the optional pre-charging, the electrolyzer 14 is supplied with DC active power P_DC from the AC network 12 in a normal operating mode NBM, depending on an active power setpoint.Optionally, in normal operating mode NBM, reactive power can also be exchanged between power converter 10 and AC network 12, for example depending on an externally specified static reactive power setpoint.

[0044] The first operating mode, BM1, is a grid support mode used in the event of a grid fault instead of the normal operating mode, NBM. In this mode, the reactive power exchanged with the AC grid 12 is adjusted depending on the phase voltages 41 of the AC grid 12. A grid fault relevant to this application may, in particular, involve a drop in the phase voltages 41 of the AC grid 12 to a value significantly below the nominal voltage of the AC grid 12. The depth of the drop in the phase voltages 41 can be specified by a residual voltage during the AC grid fault, which lies between 0% of the AC nominal voltage (short circuit) and 100% of the AC nominal voltage (normal operation).

[0045] In the first operating mode BM1, the electrolysis plant 20 thus acts to support the grid to the best of its ability through the (additional) exchange of reactive power and remains connected to the AC grid 12, particularly during the grid fault. During the passage through the grid fault (FRT mode), reactive power Q_AC is provided by the electrolysis plant 10 in the first operating mode BM1, depending on the depth of the phase voltage dip 41. For this purpose, the power transformer 10 generates reactive power Q_AC during the voltage dip, the magnitude of which can be predetermined by a Q(U) static function depending on the depth of the phase voltage dip 41. The provision of reactive power Q_AC for grid support by the power transformer 10 can take priority over the withdrawal of AC active power P_AC to supply the electrolyzer 14.Therefore, the DC active power P_DC must be reduced by the power converter 10 if the apparent power, as the sum of active power P_AC and reactive power Q_AC, exceeds the rated apparent power of the power converter 10. Additionally, the active power available from the AC network may already be reduced by the voltage dip, especially if the residual voltage of the AC network approaches 0 V.

[0046] A typical Q(U) static load is normalized to the nominal apparent power of the power converter 10, so that if the phase voltages 41 drop to 0 V, a reactive power Q_AC equal to the nominal apparent power may be required; in this case, the power converter 10 can no longer transfer any active power P_AC and must completely cease supplying the electrolyzer 14 with DC active power P_DC. Furthermore, at the latest when the phase voltages 41 drop to 0 V or close to 0 V, no active power P_AC is available from the AC network 12, so that the power converter 10's own power consumption and losses in generating the reactive power Q_AC in the power converter 10 can no longer be covered by the AC network 12. However, to continue cycling the power converter 10, active power at least equal to the power loss of the power converter 10 is necessary.The power loss of the power converter 10 is essentially a device characteristic that depends on internal efficiencies and lossy components of the power converter 10.

[0047] In the first operating mode BM1, the active power P_AC and the DC voltage 49 at the electrolyzer 14 are monitored. The first operating mode BM1 is used during the grid fault at least as long as the active power P_AC exceeds a minimum power and / or the DC voltage 49 exceeds a minimum voltage. As long as both the active power P_AC and the DC voltage 49 exceed their respective minimum values, the power loss of the power transformer 10 can be covered by the AC grid 12. Upon grid restoration in such a state, the normal operating mode NBM can be resumed immediately after the first operating mode BM1.

[0048] If the fault in the AC network 12 is severe, e.g., if the phase voltages 41 drop to <20% or even <10% of the nominal voltage, the AC active power P_AC still available from the AC network 12, taking into account the apparent power limit of the power transformer 10, is no longer sufficient to fully compensate for the losses in the power transformer 10. As a result, the DC voltage 49 at the electrolyzer 14 can fall below the open-circuit voltage of the electrolyzer 10 during the course of the network fault.

[0049] The electrolyzer 14 behaves like a capacitor when its DC voltage 49 is between zero and its open-circuit voltage. This property can be used in the event of a grid fault to continue operating the power converter 10. The energy stored in this capacitor can be used in the first operating mode BM1 to ride out the grid fault by drawing energy from it and using it to cover losses in the power converter 10 during reactive power exchange and for self-sufficiency.

[0050] If the available and / or transferable active power P_AC is insufficient to cover the power loss of the power converter 10 and the DC voltage 49 falls below the minimum voltage due to energy being drawn from the electrolyzer 14, the second operating mode BM2 can be activated according to the procedure. The second operating mode BM2 is a standby mode in which the power converter 10 remains connected to the AC network 12, but power exchange with the AC network 12 is interrupted, for example, by stopping the switching of the power converter 10's circuit breakers. Upon network restoration, the normal operating mode NBM can be resumed after the second operating mode BM2.

[0051] Figure 3 shows a second embodiment of the method. In this embodiment, after the electrolysis plant 20 has started operation, the third operating mode BM3 is used as the pre-charging mode. After pre-charging, the electrolyzer 10 is supplied with DC active power P_DC from the AC network 12 via the power converter 10 in normal operating mode NBM.

[0052] In the first operating mode BM1, the power converter 10 exchanges reactive power Q_AC with the AC network 12 during the grid fault, in accordance with the normative requirements. The resulting losses in the power converter 10 itself are primarily compensated by drawing AC active power P_AC from the AC network 12. In the first operating mode BM1, the DC voltage 49 at the electrolyzer 10 and the AC active power P_AC drawn by the power converter 10 from the AC network are monitored.

[0053] When the power supply returns in a situation where both the AC active power P_AC and the DC voltage 49 exceed their respective minimum values, it is possible to switch directly back to the normal operating mode NBM.

[0054] If the AC active power P_AC during the first operating mode BM1 is too low to cover the losses of the power converter 10 during the grid fault, power is drawn from the electrolyzer 14's capacitance, provided the DC voltage 49 is below the electrolyzer 14's open-circuit voltage and above the minimum voltage. Upon grid restoration in this situation, the system can optionally switch back to the third operating mode BM3 (not shown) to raise the DC voltage 49 to the open-circuit voltage and then return to the normal operating mode NBM.

[0055] If the DC voltage 49 at the electrolyzer 14 drops further during the first operating mode BM1 in the course of the grid fault and falls below the minimum voltage, the system switches to the second operating mode BM2. In this mode, the power transformer 10 remains connected to the AC grid 12, but the switching, and thus the exchange of active and reactive power with the AC grid 12 and electrolyzer 14, is stopped. When the grid is restored in this situation, the system switches back to the third operating mode BM3, and the DC voltage 49 is raised back to the open-circuit voltage to return to the normal operating mode NBM.

[0056] The third operating mode, BM3, is the pre-charging mode, which is executed before the normal operating mode, NBM, and in which the energy absorbed by the electrolyzer 10 is determined. Depending on the absorbed energy, a characteristic value can be determined and transmitted to the AC network operator, 12. This characteristic value can, in particular, include an FRT curve representing a possible duration of the first operating mode, BM1, as a function of the reduced AC network voltage 41 in the event of a fault.

[0057] The application enables the prediction of the minimum possible duration of full dynamic grid support (FRT Full) during the operation of a partially capacitive load, in particular an electrolyzer 14, via a power converter 10 on the AC grid 12. An advantage of the invention lies in the fact that the FRT capabilities of the electrolysis plant 20 are determined and communicated, in particular the minimum possible duration of the reactive power provision, if required by standards, depending on the residual AC voltage in the event of a fault. This information can be made available to a grid operator and adapted to any changing characteristics of the electrolysis plant, e.g., aging effects, through regular updates.This allows the behavior of a large consumer (here: an electrolysis plant for hydrogen production) to be simulated and evaluated during a network study, especially in the event of faults in the interconnected network, and the reaction of the large consumer to such faults, in particular the possible contribution of the large consumer to network support.

[0058] During a grid fault, the power converter 10, operating in its first mode BM1, exchanges reactive power with the AC grid 12 in accordance with the normative requirements. The resulting losses in the power converter 10 itself are primarily compensated from the AC grid 12. If the fault in the AC grid 12 is severe (e.g., AC voltage dropping to approximately U < 10% of the nominal voltage), the active power available from the AC grid 12 may no longer be sufficient to fully compensate for the losses in the power converter 10. The power converter 10 can still be operated, in particular by discharging the electrolyzer's capacity during the FRT (Fault Reduction Time) event and using it for the power converter 10's own power supply. Provided the losses of the power converter 10 at the relevant FRT operating points are known, a duration can be calculated and specified for which the electrolysis plant can provide complete dynamic grid support.If this duration is exceeded, the power converter 10 can be switched to a standby mode BM2, from which it can return directly or indirectly to the normal operating mode NBM upon grid restoration. In any case, a typical grid fault is fully experienced without the electrolysis plant 20 disconnecting from the AC grid.

[0059] Figure 4 shows exemplary current and voltage waveforms in the electrolysis plant 20. In the top two rows, the phase voltages 41 of the AC network 12 and the phase currents 42 on the AC side of the power transformer 10 are shown in the respective AC diagrams U_AC and l_AC. In the third row, the AC active current 44 drawn from the AC network 12 and the AC reactive current 45 exchanged with the AC network 12 are shown in an AC diagram l_AC. In the penultimate row, the DC current 48 at the electrolyzer 14 is shown in a DC diagram l_DC. In the last row, the DC voltage 49 at the electrolyzer 14 and the rectified average value 50 of the phase voltages 41 are shown in a DC diagram U_DC.

[0060] In Figure 4, the normal operating mode NBM is active from time zero to tO. The electrolyzer 14 is supplied with DC active power P_DC close to the rated power of the power converter 10. The DC voltage 49 and the DC current 48 at the electrolyzer 14 have values ​​correlated with each other via the electrolyzer characteristic curve, which, when multiplied together, yield the DC active power P_DC. (Almost) no reactive power Q_AC is exchanged with the AC network 12; the AC reactive current 45 is (almost) zero.

[0061] In the first operating mode BM1, from time tO to time t1, the phase voltages 41 in the AC network 12 are significantly reduced to approximately 5% of the nominal AC voltage, for example, due to a network fault. According to a potentially standardized Q(U) static behavior, after a short transient phase, AC reactive power Q_AC is exchanged with the AC network 12, and the AC reactive current 45 rises to a value close to the nominal power of the power converter 10. The DC current 48 at the electrolyzer initially drops to near zero and briefly reverses direction, so that power is briefly drawn from the electrolyzer 14. The DC voltage 49 initially drops, correlated with the DC current 48, to the open-circuit voltage of the electrolyzer 14 and then, due to the brief power draw from the electrolyzer 14's capacitance, further to the specified minimum voltage (see Figure 5).

[0062] If the minimum voltage is undershot at time t1 (see Figure 5), the system switches to the second operating mode BM2. In the second operating mode BM2, from time t1 to time t2, the power exchange between the power converter 10 and the AC network 12 is stopped. The AC active current 44, the AC reactive current 45, and the DC current 48 at the electrolyzer 10 are all zero, and the DC voltage 48 remains largely constant, slightly below the open-circuit voltage of the electrolyzer 14.

[0063] In the optional third operating mode BM3, starting at time t2, the time of grid restoration, the capacity of the electrolyzer 14 is recharged to the open-circuit voltage by means of the DC current 48 at the electrolyzer 14. In the third operating mode BM3, the power converter 10 is clocked for pre-charging the electrolyzer 14 until time t3 in such a way that excessively high DC currents 48 at the electrolyzer 14 are avoided.

[0064] Figure 5 shows, at the top, a section of the waveforms of the AC active current 44 and the AC reactive current 45 from Figure 4. Figure 5 below shows the corresponding section of the waveforms of the DC voltage 49 at the electrolyzer 14 and the rectification voltage 50. In normal operating mode NBM up to time t0, the electrolyzer 14 is supplied with DC active power P_DC from the AC network 12 via the power converter 10, and the DC voltage 49 and the DC current 48 have values ​​correlated with each other via the electrolyzer characteristic curve, which, when multiplied together, yield the DC active power P_DC. Only a very small reactive current 45 flows between the AC network 12 and the power converter 10, so that the exchanged reactive power Q_AC is very small and almost zero.

[0065] At time tO, the mains voltage of the AC network 12 collapses (see Figure 4). The AC reactive current 45 rises to a value close to the AC active current 44 in normal operating mode (NBM), while the AC active current 44 assumes values ​​of almost zero. The power converter 10 thus reaches its apparent power limit by supplying AC reactive power Q_AC and must therefore significantly reduce the AC active current 44, so that the AC active current falls below a minimum current l_min. At the same time, the DC voltage 49 at the electrolyzer 14 drops sharply, since the remaining AC active current 44, and thus the remaining AC active power P_AC, is used for the power converter 10's own supply, and therefore no more DC active power P_DC can be supplied to the electrolyzer 14.

[0066] Since the remaining AC active current 44 and the equivalent remaining AC active power P_AC fall below a minimum value, which can correspond in particular to the power loss of the power converter 10 during reactive power exchange, power is briefly drawn from the electrolyzer 14 for the self-supply of the power converter 10. The DC voltage 49 thus drops further and reaches the minimum voltage U_min at time t1, so that at time t2 the system switches to the second operating mode BM2. In the second operating mode BM2 from time t1, power exchange with the AC network 12 is discontinued and the AC reactive current 45 and the AC active current 44 assume values ​​of zero.

[0067] REFERENCE MARK LIST

[0068] 10 power converters

[0069] 12 AC network

[0070] 14 Electrolyzer

[0071] 20 Electrolysis plant

[0072] 41 phase voltages AC network

[0073] 42-phase AC-side power converter

[0074] 44 AC active current

[0075] 45 AC reactive power

[0076] 48 DC current at the electrolyzer

[0077] 49 DC voltage at the electrolyzer

[0078] 50 Equal reference value of the phase voltages

[0079] BM1 first operating mode

[0080] BM2 second operating mode

[0081] BM3 third operating mode

[0082] NBM normal operating mode P_AC AC active power

[0083] Q_AC AC reactive power P_DC DC active power U_AC AC voltage l_AC AC current

[0084] U_DC DC voltage l_DC DC current U_min minimum voltage I min minimum current

Claims

PATENT CLAIMS 1. Method for operating an electrolysis plant (20) comprising an electrolyzer (14) and a power converter (10), wherein the power converter (10) is configured to supply the electrolyzer (14) from an AC network (12) with DC active power (P_DC) at a DC voltage (49), wherein the method comprises: A) Operating the power converter (10) in a first operating mode (BM1) in which the power converter (10) draws AC active power (P_AC) from the AC network (12) and exchanges AC reactive power (Q_AC) with the AC network (12) to support the AC network (12), B) Monitoring the AC active power (P_AC) and the DC voltage (49) drawn from the AC network (12) in the first operating mode (BM1), and C) Changing the operating mode if the AC active power (P_AC) drawn from the AC network (12) falls below a predefined minimum power and the DC voltage (49) falls below a predefined minimum voltage (U_min).

2. Method according to claim 1, wherein the minimum power essentially corresponds to the power loss of the power converter (10) or comprises the power loss and a basic power of the electrolyzer (14).

3. Method according to claim 1 or 2, wherein in C) the first operating mode (BM1) is switched to a second operating mode (BM2), wherein in the second operating mode (BM2) both the exchange of reactive power (Q_AC) and the exchange of AC active power (P_AC) between the power converter (10) and the AC network (12) is stopped.

4. Method according to claim 3, wherein in the second operating mode (BM2) the clocking of the power converter (10) is stopped.

5. Method according to one of the preceding claims, wherein the method has a third operating mode (BM3) in which the power converter (10) extracts AC active power (P_AC) from the AC network (12) and in which the electrolyzer is precharged to an open-circuit voltage, wherein the energy fed into the electrolyzer (14) during the third operating mode (BM3) is determined and stored.

6. Method according to claim 5, wherein a characteristic value for network support is provided which depends on the energy determined in the third operating mode (BM3).

7. Method according to claim 6, wherein the characteristic value for network support is transmitted to an operator of the AC network (12).

8. Method according to one of claims 5 to 7, wherein, depending on the energy determined in the third operating mode (BM3), a possible duration of the first operating mode (BM1) is determined when a mains voltage of the AC network (12) is reduced to a predefinable value.

9. A method according to any of the preceding claims, wherein the method comprises a normal operating mode (NBM) in which the electrolyzer (14) is supplied with DC active power (P_DC) from the AC network (12) depending on an active power setpoint, wherein the first operating mode (BM1) is a grid support mode in which the injected reactive power (Q_AC) is set depending on the grid voltage of the AC network (12), wherein the second operating mode (BM2) is a standby mode in which the power converter (10) remains connected to the AC network (12), and / or wherein the third operating mode (BM3) is a pre-charge mode that can be executed before a start of the normal operating mode (NBM).

10. Electrolysis plant (20) comprising an electrolyzer (14) and a power converter (10), wherein the power converter (10) is configured to supply the electrolyzer (14) from an AC network (12) with DC active power (P_DC) at a DC voltage (49), wherein the electrolysis plant (20) has a first operating mode (BM1) in which the power converter (10) draws AC active power (P_AC) from the AC network (12) and feeds AC reactive power (Q_AC) into the AC network (12) to support the AC network (12), wherein in the first operating mode (BM1) the AC active power (P_AC) drawn from the AC network (12) and the DC voltage (49) at the electrolyzer (14) are monitored, wherein the electrolysis plant (20) is configured to change the operating mode if in the first Operating mode (BM1) the extracted AC active power (P_AC) falls below a predefined minimum power and the DC voltage (49) falls below a predefined minimum voltage (U_min).

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