DE102022125719A1 - METHOD FOR OPERATING A POWER CONVERTER, CONTROL UNIT AND ELECTROLYSIS SYSTEM - Google Patents

Abstract

The application relates to a method for operating a power converter which is designed to transmit electrical power in current-setting operation between an AC voltage side and a DC voltage side, and which has a DC voltage intermediate circuit arranged on the DC voltage side, wherein the power converter sets an actual direct current and wherein in a first operating mode an intermediate circuit voltage is maintained between an upper threshold and a lower threshold, and wherein in a second operating mode at least one of the thresholds is changed such that the difference between the upper and lower threshold changes compared to the first operating mode.The application further relates to a control unit and an electrolysis system.

Description

TECHNICAL AREA

The invention relates to a method for operating a power converter, a control unit for operating the power converter and an electrolysis plant with power converter, control unit and electrolyzer.

STATE OF THE ART

In the DE102020112880 A method for operating an electrolyzer with two operating states is described. Normal operation with an input voltage at the electrolyzer greater than the open circuit voltage of the electrolyzer is provided. In normal operation, the electrolyzer is supplied with electrical power via a power converter from an AC network (AC: alternating current) and power is converted in the electrolyzer. A stand-by mode is also provided, in which an electrolyzer input voltage is kept below the open circuit voltage. The electrolyzer input voltage in stand-by operation is set via a clocked pre-charging circuit between the power converter and the electrolyzer.

A power converter in an electrolysis system can be operated in a current-controlled manner, i.e. the power converter adjusts the current that is taken from the AC network and fed to the electrolyzer based on a setpoint for the current or power of the electrolyzer, whereby the setpoint is specified by the electrolyzer or a higher-level control unit. The DC voltage (DC: direct current) on the electrolyzer is then adjusted based on the given current-voltage characteristic of the electrolyzer. This is referred to as current-controlled operation or current-setting operation. A control unit is higher-level, for example, if it controls or regulates more than one unit of the electrolysis system.

Power converters in electrolysis systems can have a voltage intermediate circuit on a DC side of the power converter. DC switches, in particular DC relays, can be provided between the power converter and the electrolyzer, which are prescribed in particular for normative reasons and can be designed to disconnect the rated current of the electrolyzer. In normal operation with closed DC switches, the intermediate circuit voltage then corresponds to the input voltage of the electrolyzer.

If the DC connection between the electrolyzer and the intermediate circuit is disconnected during operation, i.e. during the transfer of electrical power from the AC network via the power converter to the electrolyzer, in particular by opening the DC switches, it is possible to communicate this to the power converter in advance. This is possible, for example, if the DC disconnection is known in advance, i.e. is carried out according to plan, and/or is carried out manually with appropriate preparatory steps. After prior communication to the power converter, it can adapt its operating mode accordingly and/or in particular switch it off.

However, if the DC disconnection takes place without prior notification to the power converter, the power converter may be in normal operating mode at the time of disconnection, in which electrical power is transferred from its AC side to its DC side with the intermediate circuit. If the power converter remains in this operating mode after disconnection from the electrolyzer, the power converter continues to transfer current in current-setting mode according to the electrolyzer's current setpoint from the AC network to its DC side. Since the electrical power can no longer flow into the electrolyzer due to the DC-side disconnection, electrical charge is accumulated in the intermediate circuit of the power converter, which leads to an increase in the intermediate circuit voltage and can lead to the destruction of the power converter.

TASK

The application is based on the object of specifying a method and a control unit which improve the safe and reliable operation of a power converter.

SOLUTION

The object is achieved by a method having the features of claim 1 and a control unit having the features of claim 13. Embodiments are specified in the dependent patent claims, in particular claim 14 describes an embodiment in which the control unit is part of an electrolysis system.

DESCRIPTION

A power converter is designed to transfer electrical power in current-setting operation between an alternating voltage side, AC side, and a direct voltage side, DC side. The power converter sets an actual direct current. The power converter also has a direct voltage intermediate circuit on its direct voltage side.

In a method for operating the power converter, an intermediate circuit voltage is maintained between an upper threshold and a lower threshold in a first operating mode. In a second operating mode, at least one of the thresholds is changed such that the difference between the upper and lower thresholds changes compared to the first operating mode.

The intermediate circuit has a capacitance, for example a capacitor, to which the intermediate circuit voltage is applied. The power converter, which transfers electrical power between the alternating voltage side (AC side) and the direct voltage side (DC side), is operated in particular as a rectifier. On its alternating voltage side, for example, an AC network, such as an AC supply network, can be connected from which the electrical power can be drawn. In current-setting operation, the power converter sets the current on its DC side, i.e. it impresses it on its DC side and supplies connectable electrical power consumers on its DC side, e.g. DC devices, with electrical power at the set current strength. The intermediate circuit voltage corresponds to the DC output voltage of the power converter. The intermediate circuit voltage also corresponds to the input voltage that is applied to a DC device connected to the DC side of the power converter. The intermediate circuit voltage is kept in the range between the upper and lower threshold, in a so-called dead band. This dead band can be selected so that the intermediate circuit voltage, particularly in the first operating mode, is kept in a range in which the DC device can be operated reliably, for example corresponding to the input voltage range of a connectable electrolyzer.

For example, if an electrolyzer is connected to the DC side of the power converter, the lower threshold for the intermediate circuit voltage in the first operating mode could correspond to the open circuit voltage of the electrolyzer. The upper threshold in the first operating mode could, for example, be selected to correspond to the permissible maximum voltage of the electrolyzer, so that the rated power operating point of the electrolyzer lies in the dead band within the upper and lower thresholds of the intermediate circuit voltage.

In the second operating mode, one of the thresholds can be changed so that the difference between the upper and lower threshold is changed compared to the first operating mode. This can be used, for example, to operate a DC device connected to the DC side of the power converter in a different range of the DC voltage applied to its input. This can be desired for safety or maintenance reasons, for example. In the second operating mode, the DC output voltage of the power converter can therefore be in a different range compared to the first operating mode and can be adjusted, for example, to a maintenance state or a less dangerous state of the DC device.

In one embodiment of the method, in the second operating mode, at least one of the thresholds is changed such that the difference between the upper and lower thresholds is reduced compared to the first operating mode. In particular, the difference between the upper and lower thresholds can be reduced to a value of a few volts or to zero. This can be used, for example, to operate the DC device connected to the DC side of the power converter in a narrower range of the DC voltage applied to its input. This can be desirable, for example, for safety or maintenance reasons. In the second operating mode, the DC output voltage of the power converter can therefore be in a narrower range compared to the first operating mode and, in particular, can be limited to values in the lower range of the original dead band of the first operating mode. If the difference between the upper and lower thresholds is a few volts or zero, the intermediate circuit voltage and thus the DC output voltage of the power converter is kept close to or at a certain value.

In one embodiment of the method, the upper threshold of the intermediate circuit voltage is lowered in the second operating mode. In particular, the upper threshold is lowered to the lower threshold. This embodiment can offer advantages, for example, if safe operation of the power converter is to be ensured and a DC output voltage of the power converter is to be achieved that is at the lower threshold of the original dead band of the first operating mode and thus close to the open circuit voltage of a connected electrolyzer, so that the electrolyzer can continue to be operated immediately with an existing or recurring DC connection.

In one embodiment, a transition from the first to the second operating mode occurs when the intermediate circuit voltage exceeds an error threshold. The error threshold for the intermediate circuit voltage can, for example, be above the upper threshold. If the intermediate circuit voltage rises above this error threshold, the difference between the upper and lower thresholds is changed. In particular, the upper threshold can, for example, be lowered close to or to the lower threshold.

If a fault is detected in which the intermediate circuit voltage rises too far, namely beyond the fault threshold, the difference between the upper and lower thresholds can then be changed. This can be used, for example, to bring the system, in particular the intermediate circuit of the power converter, into a safe state. A safe state can, for example, consist of the intermediate circuit voltage being kept in a much narrower range than in the first operating state, close to the lower limit of the dead band of the first operating mode, by changing the upper and/or lower threshold. This can also be used, for example, to react to changes on the DC side of the power converter, for example a DC device previously connected in the first operating mode has been disconnected. If this is detected, for example by detecting an excessive increase in the intermediate circuit voltage, it can be reacted to by changing the upper and/or lower threshold. For example, the intermediate circuit may be protected by maintaining its voltage within a narrower range than in the first operating mode, with the narrower range preferably being close to the lower threshold of the original deadband of the first operating mode and optionally including this lower threshold.

In one embodiment, an electrolyzer is connected to the DC voltage side of the power converter. In the first operating mode, the electrolyzer is supplied with the actual direct current. The actual direct current is generated by the power converter from an AC actual current from the AC network. The intermediate circuit voltage depends on the actual direct current and an electrolysis characteristic curve of the electrolyzer when the electrolyzer is connected to the power converter and is supplied with the actual direct current.

The electrolyzer can, for example, be designed as a hydrogen electrolyzer that uses the actual direct current to convert water into its components hydrogen and oxygen in an electrolysis reaction. The electrolyzer typically has a current-voltage characteristic curve, which is also referred to as the electrolysis characteristic curve. The electrolysis characteristic curve can be divided into two areas. Below an open circuit voltage, the electrolyzer shows predominantly capacitive behavior, which is caused by the formation of double layers on the electrolyzer's electrodes. An electrolysis reaction does not take place at voltages below the open circuit voltage, or at least not to a significant extent. At input voltages above the open circuit voltage, the electrolyzer shows predominantly ohmic behavior, which is caused by the electrolysis reaction taking place at these voltages. The speed of the electrolysis reaction and thus the rate at which hydrogen is produced increases with increasing actual direct current and thus increasing input voltage. In current-setting operation of the power converter, the intermediate circuit voltage therefore depends on the actual direct current and the electrolysis characteristic of the electrolyzer. The actual direct current is set by the power converter, which is designed as an active rectifier.

It is advantageous if the lower threshold of the intermediate circuit voltage corresponds to the open circuit voltage of the electrolyzer or is above the open circuit voltage of the electrolyzer. This keeps the input voltage of the electrolyzer in a range in which electrolysis takes place through the power converter. The upper threshold of the intermediate circuit voltage can then, for example, in the first operating mode, be in a range that, according to the electrolysis characteristic, is above the rated power of the electrolyzer and at or below the permissible maximum voltage of the electrolyzer.

In one embodiment, in the first operating mode, the power converter sets the actual direct current according to a set target current, wherein the set target current is determined using a required target current. The required target current is specified, for example, by the electrolyzer or a higher-level control unit, or the electrolyzer or the higher-level control unit specifies a target power that can be converted into a required target current at a suitable point, taking the DC input voltage into account. The required target current can be specified, for example, by the electrolyzer based on its electrolysis characteristic curve and/or depending on its current and/or desired operating state. The set target current then depends on the required target current or the target power and is determined by the power converter or by a higher-level control that controls the power converter, so that the actual direct current is determined and set using the set target current or the target power.

In one embodiment, the set target current is determined from the required target current using a correction value. The set target current is obtained, for example, by subtracting the correction value from the required target current. The power converter can therefore be operated with a current control that has a required target current from the electrolyzer and a correction value. The correction value can be generated, for example, depending on the intermediate circuit voltage.

In one embodiment, the correction value is determined depending on the intermediate circuit voltage, the upper threshold and/or the lower threshold. The correction value is determined by a control system which aims to keep the intermediate circuit voltage in the first operating mode with an electrolyzer connected and in the second operating mode between the respectively valid upper and lower thresholds. This embodiment offers the advantage that the control system operates in the same current-controlled manner in the first operating mode with an electrolyzer connected and in the second operating mode and can therefore be used structurally unchanged. The control system aims to set the actual direct current in accordance with the set target current and at the same time to keep the intermediate circuit voltage between the upper and lower thresholds. By changing at least one of the thresholds in the second operating mode, changed boundary conditions can be taken into account. The control system itself does not have to be structurally adapted.

In one embodiment, the correction value in the second operating mode is temporarily greater than the required target current. Subtracting the correction value from the required target current can then result in a set target current with a sign opposite to the required target current. In such a situation, the power converter sets an actual direct current that transfers electrical power from the intermediate circuit to the AC network. This can counteract an excessively high intermediate circuit voltage and thus prevent the intermediate circuit from being overloaded by removing electrical charge from the intermediate circuit and keeping the intermediate circuit in a safe state.

If the power converter transfers electrical power from the DC link to the AC network, the power converter is operated as an active inverter. If the power converter transfers electrical power from the AC network to its DC side, the power converter is operated as an active rectifier. In both cases, this is current-controlled operation.

In one embodiment, the transition from the first to the second operating mode takes place alternatively or additionally depending on the correction value. This makes it possible to intercept another error case which manifests itself in conspicuous values for the correction value. In particular, the transition from the first to the second operating mode can take place when a limit value for the correction value is exceeded. In particular, this makes it possible to build in a safety mechanism which reacts to correction values which are too high and subsequently changes at least one of the thresholds for the intermediate circuit voltage in order to bring the system back to a safe state. The condition for the transition from the first to the second operating mode can in particular include both the error threshold for the intermediate circuit voltage and the limit value for the correction value, so that the transition to the second operating mode only takes place when both the intermediate circuit voltage and the correction value exceed their respective limit values.

In further embodiments, the transition from the second to the first operating mode takes place depending on the required target current. In this embodiment, the transition from the second to the first operating mode can be realized by reacting to a required target current that is, for example, in a safe or neutral range. In particular, with a required target current of zero, it is possible to transition from the second to the first operating mode. With a required target current of the electrolyzer of zero, it does not request any electrical power. Since no electrical power is now requested on the DC side of the power converter, there is no risk of charging the intermediate circuit too much, since there is no requirement to transfer power from the AC side to the DC side. It is therefore possible to return to the first operating mode.

The power converter can leave the second operating mode again and set the thresholds and thus the dead band to their original values when the electrolyzer has reacted to the lack of power supply and the electrolyzer's current setpoint has been set to zero. The electrolyzer can then be started up again by the power converter in the first operating mode, starting from idle.

In one embodiment, the upper threshold of the intermediate circuit voltage is determined depending on the intermediate circuit voltage, the correction value and/or the required target current. In this embodiment, it is possible to set the upper threshold dynamically depending on the current intermediate circuit voltage, the current correction value and/or the required target current. This enables the upper threshold to be determined taking into account at least these three values. Dangerous states of the overall system can thus be avoided.

The method can be used to define an error threshold for a DC voltage on the DC side of the power converter, in particular for the intermediate circuit voltage. If the DC voltage exceeds this error threshold, the power converter is put into a stand-by state in which no electrolysis takes place and the electrolyzer exhibits essentially capacitive behavior without significant power consumption. The switch to the stand-by state is carried out by changing the dead band of the DC voltage regulator, in particular by reducing it, in particular by reducing it to zero, so that the DC voltage is regulated by means of the correction value to a value at which the power converter remains in basically current-controlled operation and does not switch to passive rectifier operation.

The process can prevent the intermediate circuit from charging until it explodes, for example, or until other protective mechanisms kick in and possibly switch off the power converter, due to a continuous current supply in accordance with the required target current and a simultaneous lack of electrical power flowing into the electrolyzer. The process can detect such a case of impending overload at an early stage, with the power converter preventing the overload while remaining in current-controlled operation. In the second operating mode, the deadband of the DC voltage regulation is changed so that the power converter is de facto operated in standby mode in the second operating mode, with a DC voltage equal to or above the open circuit voltage of the (separated) electrolyzer.

The condition for changing the dead band can include a limit value for the correction value in addition to the error threshold for the DC voltage, so that the standby state is only entered when both the DC voltage and the correction value exceed the respective limit value. The correction value can exceed its limit value with a time delay after the DC voltage has exceeded the error threshold, whereby the time delay can depend in particular on the dynamics of the change in the DC voltage.

The power converter can exit the standby state and the deadband can be set to its original values once the electrolyzer has responded to the error and the electrolyzer's demand current has been set to zero. The electrolyzer can then be restarted from idle.

One of the methods described above can, for example, be carried out on a control unit for operating the power converter. The control unit can be arranged in the power converter and control the power switches of the power converter, via which the rectification and/or the inversion of the electrical current takes place and which implement the transfer of the electrical power between the AC side and the DC side. The control unit can also be designed as a higher-level control unit and perform control and/or regulation tasks for more than one device in the overall system.

An overall system with a control unit and power converter can be designed as an electrolysis system, for example. Such an electrolysis system has the previously described control unit, the electrolyzer and the power converter. The electrolyzer can be connected to the power converter via DC switches, in particular DC relays. The power converter is designed to transfer electrical power in current-setting operation between an AC voltage side and a DC voltage side. The power converter has the DC voltage intermediate circuit arranged on the DC voltage side. The power converter is designed and set up to set an actual direct current in current-setting operation and to supply the electrolyzer with electrical energy from the AC network when the DC switches are closed.

In one embodiment, the control unit is designed to maintain an intermediate circuit voltage between an upper threshold and a lower threshold. The lower threshold corresponds to the open circuit voltage of the electrolyzer or is a few volts above the open circuit voltage of the electrolyzer. As a result, the upper and lower thresholds of the intermediate circuit voltage are adapted to the electrolyzer of the electrolysis system and enable the electrolyzer to operate according to its electrolysis characteristic curve.

In the second operating mode, the DC switches in particular can be open. The second operating mode enables the control parameters of the power converter to be adjusted so that the intermediate circuit voltage is kept in a different range than in the first operating mode. This is also possible if the power converter is not informed in advance that the DC switches have been opened, i.e. it has no information to set the actual direct current it has set in advance to deviate from the required target current. The set target current and thus the actual direct current can then be adjusted, as described above, by adjusting the upper and/or lower threshold of the intermediate circuit voltage.

BRIEF DESCRIPTION OF THE CHARACTERS

• 1 zeigt schematisch ein Ausführungsbeispiel einer Elektrolyseanlage mit Leistungswandler und Steuereinheit,
• 2 zeigt schematisch ein Blockschaltbild eines Verfahrens zum Betrieb des Leistungswandlers und
• 3 zeigt schematisch beispielhafte Strom- und Spannungsverläufe in der Elektrolyseanlage.

In the following, the invention is further explained and described with reference to embodiments shown in the figures.

• 1 shows schematically an embodiment of an electrolysis plant with power converter and control unit,
• 2 shows a schematic block diagram of a method for operating the power converter and
• 3 shows schematic examples of current and voltage curves in the electrolysis plant.

In the figures, identical or similar elements are designated by the same reference numerals.

FIGURE DESCRIPTION

1 shows schematically an embodiment of an electrolysis system 10 with a power converter 16, which has an intermediate circuit ZWK. In the present example, the power converter 16 is designed as a rectifier, which can convert electrical power from alternating voltage to direct voltage. The power converter 16 is also able to act as an inverter and convert electrical power from direct voltage to alternating voltage. For electrical power conversion, the power converter 16 has in particular a bridge circuit with semiconductor switches. By suitable control of the semiconductor switches, a conversion of alternating voltage on an AC side of the power converter 16 into direct voltage on a DC side of the power converter 16 and vice versa can take place. The control of the semiconductor switches of the power converter 16 can take place in particular by means of clocked control signals, which are generated for example by a control unit of the power converter 16 (not shown). The parameters required to generate the control signals of the semiconductor switches of the power converter 16 are determined by a higher-level control of a control unit 20 and transmitted to the power converter 16. The control unit 20 can be designed as a processor or microcontroller with appropriate memory equipment and data input and output options.

The power converter 16 is connected on its AC side via an AC relay 14 to an AC network 12, for example an AC supply network, in particular a public AC voltage network. The power converter 16 can draw electrical power from the AC network 12 in the form of an actual AC current I_AC_ist. The power converter 16 can be operated in current-setting mode. In current-setting mode, the power converter 16 sets an actual direct current I_DC_ist on its DC side, which is set up to supply a DC device connected on the DC side with electrical power. In the example shown, the power converter 16 has an intermediate circuit ZWK on its DC side, which is designed, for example, as a capacitance in the form of a capacitor and can be arranged outside the power converter 16 as shown or is located inside a housing of the power converter 16 (not shown). An electrolyzer 22 is connected to the DC side of the power converter 16 via DC switch 18. By means of the actual direct current I_DC_act, the electrolyzer 22 can be supplied with electrical power from the AC network 12 via the power converter 16.

If the DC switches 18 are closed, the actual direct current I_DC_act provided by the power converter 16 can flow into the electrolyzer 22 and supply it with electrical power. This situation is in 1 represented by a dotted arrow I_DC_actual, which symbolizes a current flow from the power converter 16 via its intermediate circuit ZWK and the closed DC switches into the electrolyzer 22. The power converter 16 sets the actual direct current I_DC_actual according to a required target current I_DC_soll_1 from the electrolyzer 22. In normal operation, ie with closed DC switches 18 and an intermediate circuit voltage in the working range of the electrolyzer 22, the required target current I_DC_soll_1 largely corresponds to the set target current I_DC_soll_2. The set target current I_DC_soll_2 is supplied to the power converter 16 by the control unit 20. According to the required target current I_DC_soll, which the power converter 16 receives from the control unit 20, the actual direct current I_DC_ist is set on the DC side of the power converter 16.

An intermediate circuit voltage U_ZWK is dropped across the intermediate circuit ZWK of the power converter 16. This intermediate circuit voltage U_ZWK is measured and the measured value for the intermediate circuit voltage is transmitted from the power converter 16 to the control unit 20. If the DC switches 18 are opened, the actual direct current I_DC_ist provided by the power converter 16 cannot flow through the intermediate circuit ZWK into the electrolyzer 22. Instead, the current I_AC_ist set by the power converter 16 flows into the intermediate circuit ZWK and the intermediate circuit voltage U_ZWK increases as a result because the current cannot flow out of the intermediate circuit ZWK. This situation is in 1 represented by a dashed arrow I_DC_ist, the symbolizes a current flow from the power converter 16 into its intermediate circuit ZWK.

In 2 The control is shown using a block diagram as an example, as it can be carried out by the control unit 20. The control receives the required target current I_DC_soll_1 from the electrolyzer 22 as an input value. A correction value I_DC_k is added to the required target current I_DC_soll_1. The sum of the required target current I_DC_soll_1 and the correction value I_DC_k results in the set target current I_DC_soll_2, which is transmitted from the control in the control unit 20 to the power converter 16.

The correction value I_DC_k can be determined by the control in the control unit 20 by using the intermediate circuit voltage U_ZWK. A threshold generator 24 uses the intermediate circuit voltages to output the lower threshold U_ZWK_min and the upper threshold U_ZWK_max of the intermediate circuit voltage U_ZWK. The lower threshold U_ZWK_min can correspond, for example, to an open circuit voltage of the electrolyzer 22, below which no or no significant electrolysis takes place in the electrolyzer 22. The upper threshold U_ZWK_max can correspond, for example, to an intermediate circuit voltage which is in the range of a maximum permissible voltage of the electrolyzer 22, wherein the operating point at which the electrolyzer 22 operates in nominal operation with nominal power is below the upper threshold U_ZWK_max.

When the DC switches 18 are closed, the intermediate circuit voltage U_ZWK corresponds to the input voltage at the electrolyzer 22. A first operating mode of the power converter 16 corresponds to such an operation with the described thresholds U_ZWK_min, U_ZWK_max, closed DC switches 18 and connected electrolyzer 22.

In the upper path of 2 the difference between the upper threshold U_ZWK_max and the intermediate circuit voltage U_ZWK now forms an input signal of a first PI controller PI1. The first PI controller PI1 generates an intermediate correction value u by multiplying the difference between the upper threshold U_ZWK_max and the intermediate circuit voltage U_ZWK by a proportionality factor P and an integral factor I, which is fed to a first limiter B1. The first limiter B1 generates an output value y. Depending on the difference between the intermediate circuit voltage U_ZWK and the upper threshold U_ZWK_max, the output value y of the limiter is between 0 and 1.1 times the rated current of the electrolyzer I _n . The output value y corresponds to the intermediate correction value u if the intermediate correction value u is between 0 and 1.1 times the rated current of the electrolyzer I _n . However, if the intermediate correction value u is less than 0, which occurs in particular if the intermediate circuit voltage U_ZWK is less than the upper threshold U_ZWK_max, then the output value y is set to zero and also returned as a reset signal to the PI controller PI1 in order to reset the integral component of the PI controller PI1 (so-called anti-windup); in this case, the upper path of the control is in 2 practically inactive, since the upper threshold U_ZWK_max has not just been exceeded. However, if the intermediate correction value u exceeds 1.1 times the nominal current I _n , for example if the intermediate circuit voltage U_ZWK is significantly and continuously above the upper threshold U_ZWK_max, the output value y is limited to 1.1 times the nominal current. In this example, the correction value I_DC_k can therefore reach up to 1.1 times the nominal current I _n and thus in particular be greater than the required target current I_DC target_1. This can then result in a set target current I_DC_target_2 that has an opposite sign to the required target current I_DC_target_1. This can then in turn result in the direction of current flow in the power converter 16 being reversed compared to normal operation with the electrolyzer 22 connected and operating normally, so that electrical energy is taken from the DC side of the power converter 16, in particular from the intermediate circuit ZWK, and fed into the AC network 12. In such an operating mode, the power converter 16 then acts as an inverter and transfers energy from the intermediate circuit ZWK into the AC network 12, in particular when the intermediate circuit voltage U_ZWK is significantly and persistently above the upper threshold U_ZWK_max.

The lower path of regulation of 2 depends on the one hand on the lower threshold U_ZWK_min and on the other hand on the intermediate circuit voltage U_ZWK. A second PI controller PI2 generates a second intermediate correction value u by multiplying the difference between the lower threshold U_ZWK_min and the intermediate circuit voltage U_ZWK by a proportionality factor P and an integral factor I, which is fed to a second limiter B2. As an output value, the second limiter B2 generates a second output value y, which is added to the output value y of the first limiter B1. The second limiter also acts as a limiter in a similar way to the first limiter B1, but with regard to the lower threshold U_ZWK_min. The sum of the output values y of the upper and lower paths of the control results in the correction value I_DC_k. Depending on the value of the intermediate circuit voltage U_ZWK and the resulting Output value of the second PI controller PI2 decreases in the lower path of 2 the output value of the second limiter B2 has a value between 0 and negative 1.1 times the nominal current I _n of the electrolyzer 22. If, for example, the intermediate circuit voltage U_ZWK is significantly and persistently below the lower threshold U_ZWK_min, a current value can be generated as an output value that is a maximum of negative 1.1 times the nominal current of the electrolyzer 22. The value can then be added as a correction value I_DC_k to the required target current I_DC_soll_1 and results in the setting target current I_DC_soll_2 in a similar manner to that described with regard to the upper path. The correction value I_DC_k can also be so large here that electrical energy is taken from the intermediate circuit ZWK and fed into the AC network 12.

Will be in 1 the DC switches 18 are opened without the control of the power converter 16 having been adapted to this in advance, the power converter 16 initially continues to provide the actual direct current I_DC_ist on its DC side. However, since the DC switches 18 are then open, the electrical power cannot flow via the intermediate circuit ZWK into the electrolyzer 22, but remains in the intermediate circuit ZWK. This leads to more and more electrical charge being accumulated in the intermediate circuit ZWK and an excessively high intermediate circuit voltage U_ZWK can build up there. To prevent this, in response to the opening of the DC switches 18, in particular to a spontaneous opening without prior notice, which in particular causes a rapid increase in the intermediate circuit voltage, a switch can be made to a second operating mode in which the upper threshold U_ZWK_max and/or the lower threshold U_ZWK_min are adapted by the control of the control unit 20. To adjust the upper threshold U_ZWK_max, the intermediate circuit voltage U_ZWK and/or the correction value I_DC_k can be taken into account.

In relation to 3 An example of a possible adjustment of the upper threshold U_ZWK_max of the intermediate circuit voltage U_ZWK is explained using a time curve of the relevant voltages and currents. On a time axis t, 3 in the upper part of different voltages U and in the lower part of 3 Different currents I are plotted on a time axis t. In the upper part of 3 The lower threshold U_ZWK_min is shown as a solid line and the upper threshold U_ZWK_max is shown as a dashed line. The intermediate circuit voltage U_ZWK is also shown.

If the intermediate circuit voltage U_ZWK rises above an error threshold U_fehl, 3 shown in dashed lines in the upper part, in response to the increase in the intermediate circuit voltage U_ZWK above the error threshold U_fehl, the upper threshold U_ZWK_max is lowered to the lower threshold U_ZWK_min. Typical values for the error threshold U_fehl can be in the order of magnitude of 1000 volts, while the lower threshold U_ZWK_min and the upper threshold U_ZKW_min can be, for example, 400 and 900 volts respectively. The trigger for such an increase in the intermediate circuit voltage U_ZWK above the error threshold U_fehl can be, for example, the opening of the DC switch 18 at time t_0 without the control of the power converter 16 having been adjusted beforehand. As a result, the intermediate circuit voltage U_ZWK increases sharply from the error time t_0 and reaches the error threshold U_fehl. Then, at time t_1, the upper threshold U_ZWK_max is lowered so that the upper threshold U_ZWK_max is only a few volts above the lower threshold U_ZWK_min. The required target current I_DC_soll_1 remains at the previous value because the electrolyzer 22 continues to request the required target current I_DC_soll_1, which is required to maintain the electrolysis in the electrolyzer 22 at the desired output. However, the set target current I_DC_soll_2 from the control unit 20 to the power converter 16 drops because the upper threshold U_ZWK_max has been lowered and thus there is a large difference between the intermediate circuit voltage U_ZWK and the lowered upper threshold U_ZWK_max, which in turn results in a high intermediate correction value u via the PI controller PI1. The intermediate correction value u is limited by the limiter B1, in this example to a maximum of 1.1 times the nominal current of the power converter 16, and is added as correction value I_DC_k to the required target current I_DC_soll_1. By adding the correction value I_DC_k to the required target current I_DC_soll_1 in the order in relation to 2 described control, the set target current I_DC_soll_2 is reduced. As a result, the AC actual current I_AC_ist, which represents the power drawn from the AC network 12, also decreases. As previously described, the set target current I_DC_soll_2 and thus also the AC actual current I_AC_ist can become negative, so that electrical power is transferred from the DC side or from the intermediate circuit ZWK of the power converter 16 to the AC network 12. Due to this transfer of electrical power from the intermediate circuit ZWK to the AC network 12, the intermediate circuit voltage U_ZWK decreases again, as shown in the upper part of 3 shown.

If the required target current I_DC_soll_1 drops to 0, i.e. no more electrical power is required from the electrolyzer 22, then at time t_2 the second to the first Operating mode can be returned and the upper threshold U_ZWK_max can be raised back to its original value. This raising of the threshold is carried out, for example, by the threshold generator 24. If the electrolysis in the electrolyzer 22 is started again at a later point in time, the required target current IDC_soll_1 increases again and, with the DC switches 18 closed, electrical power from the AC network 12 can again be supplied to the electrolyzer 22 via the actual AC current I_AC_ist. The intermediate circuit voltage U_ZWK increases again accordingly and reaches a stable operating value at time t_3 with the required target current IDC_soll_1.

As previously described, a possible trigger for the transition from the first operating mode to the second operating mode is that the intermediate circuit voltage U_ZWK exceeds an error threshold U_fehl. It is also possible to trigger a transition from the first to the second operating mode if the correction value I_DC_k exceeds a certain threshold. This can also be an indication that the intermediate circuit voltage U_ZWK is leaving the thresholds assigned to it and is assuming values that are too high. In the second operating mode, this can then be counteracted and damage to the electrolysis system can be avoided.

LIST OF REFERENCE SYMBOLS

10

Electrolysis plant

12

AC network

14

AC Relay

16

Power converter

18

DC switch

20

Control unit

22

Electrolyzer

24

Threshold generator

B1, B2

Limiter

I_AC_ist

AC actual current

I_DC_ist

Actual direct current

I_DC_k

Correction value

I_DC_target_1

Demand target current from the electrolyzer

I_DC_target_2

Setting target current on power converter

In

Nominal current electrolyzer

PI1, PI2

PI controller

U_miss

Error threshold

U_ZWK_max

upper threshold

U_ZWK_min

lower threshold

U_ZWK

DC link voltage

ZWK

Intermediate circuit

QUOTES INCLUDED IN THE DESCRIPTION

This list of documents listed by the applicant was generated automatically and is included solely to provide the reader with better information. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions.

Cited patent literature

• EN 102020112880 [0002]

Claims (16)

Method for operating a power converter (16) which is designed to transmit electrical power in current-setting operation between an AC voltage side and a DC voltage side, and which has a DC voltage intermediate circuit (ZWK) arranged on the DC voltage side, wherein the power converter (16) sets an actual direct current (I_DC_ist) and wherein in a first operating mode an intermediate circuit voltage (U_ZWK) is maintained between an upper threshold (U_ZWK_max) and a lower threshold (U_ZWK_min), and wherein in a second operating mode at least one of the thresholds (U_ZWK_max, U_ZWK_min) is changed such that the difference between the upper and lower thresholds (U_ZWK_max, U_ZWK_min) changes compared to the first operating mode. Procedure according to Claim 1 , wherein in the second operating mode at least one of the thresholds (U_ZWK_max, U_ZWK_min) is changed such that the difference between the upper and the lower threshold (U_ZWK_max, U_ZWK_min) is reduced compared to the first operating mode, in particular reduced to a difference of zero. Procedure according to Claim 2 , whereby in the second operating mode the upper threshold (U_ZWK_max) is lowered, in particular to the lower threshold (U_ZWK_min). Method according to one of the preceding claims, wherein a transition from the first to the second operating mode occurs when the intermediate circuit voltage (U_ZWK) exceeds an error threshold (U_fehl). Method according to one of the preceding claims, wherein an electrolyzer (22) connected to the direct voltage side is supplied with the actual direct current (I_DC_ist) in the first operating mode, wherein the actual direct current (I_DC_ist) is generated by the power converter (16) from an AC actual current (I_AC_ist) from an AC network (12) connected to the alternating voltage side, and wherein the intermediate circuit voltage (U_ZWK) is regulated as a function of the actual direct current (I_DC_ist) and an electrolysis characteristic curve of the electrolyzer (22) when the electrolyzer (22) is supplied with the actual direct current (I_DC_ist) by the power converter (16). Procedure according to Claim 5 , where the lower threshold corresponds to or is above the open circuit voltage of the electrolyzer. Procedure according to Claim 5 or 6 , wherein the power converter (16) in the first operating mode sets the actual direct current (I_DC_actual) according to a setting target current (I_DC_setpoint_2), wherein the setting target current (I_DC_setpoint_2) is determined using a request target current (I_DC_setpoint_1), wherein the request target current (I_DC_setpoint_1) is specified by the electrolyzer (22). Procedure according to Claim 7 , whereby the setting target current (I_DC_soll_2) is determined from the request target current (I_DC_soll_1) using a correction value (I_DC_k). Procedure according to Claim 8 , wherein the correction value (I_DC_k) is determined as a function of the intermediate circuit voltage (U_ZWK), the upper threshold (U_ZWK_max) and/or the lower threshold (U_ZWK_min), wherein the correction value (I_DC_k) is determined by a control which aims to keep the intermediate circuit voltage (U_ZWK) in the first operating mode with the electrolyzer (22) connected and in the second operating mode between the respectively valid upper and lower thresholds (U_ZWK_max, U_ZWK_min). Procedure according to Claim 8 or 9 , wherein the correction value (I_DC_k) in the second operating mode is temporarily greater than the required target current (I_DC_soll_1) and a set target current (I_DC_soll_2) with a sign opposite to the required target current (I_DC_soll_1) results, so that the power converter (16) sets an actual direct current (I_DC_ist) which transfers electrical power from the intermediate circuit (ZWK) to the AC network (12). Method according to one of the Claims 8 until 10 , wherein the transition from the first to the second operating mode takes place as a function of the correction value (I_DC_k), wherein the transition from the first to the second operating mode requires in particular that a limit value for the correction value (I_DC_k) is exceeded. Method according to one of the Claims 7 until 11 , wherein the transition from the second to the first operating mode takes place as a function of the required target current (I_DC_soll_1), wherein the transition from the second to the first operating mode takes place in particular when the required target current (I_DC_soll_1) is equal to zero. Control unit (20) for operating a power converter (16) which is designed to transmit electrical power between an AC voltage side and a DC voltage side, and which has a DC voltage voltage side, wherein the power converter (16) sets an actual direct current (I_DC_ist) in current-setting operation, wherein the control unit (20) is designed and arranged to carry out a method according to one of the preceding claims. Electrolysis plant (10) comprising a control unit (20) according to Claim 13 , an electrolyzer (22) and a power converter (16), wherein the electrolyzer (22) can be connected to the power converter (16) via DC switches (18), wherein the power converter (16) is designed to transmit electrical power in current-setting operation between an AC voltage side and a DC voltage side, and wherein the power converter (16) has a DC voltage intermediate circuit (ZWK) arranged on the DC voltage side, wherein the power converter (16) is designed and set up to set an actual direct current (I_DC_actual) in current-setting operation and to supply the electrolyzer (22) with electrical energy from an AC network (12) when the DC switches (18) are closed. Electrolysis plant (10) according to Claim 14 , wherein the control unit (20) is configured to maintain an intermediate circuit voltage (U_ZWK) between an upper threshold (U_ZWK_max) and a lower threshold (U_ZWK_min), wherein the lower threshold (U_ZWK_min) corresponds to an open circuit voltage of the electrolyzer (22) or is above the open circuit voltage of the electrolyzer (22). Electrolysis plant (10) according to Claim 15 , wherein the DC switches (18) are open in the second operating mode.

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