Transformer coupled modular multilevel converter as rectifier for hydrogen electrolyser

Abstract

A system (100) and a method (1300) for controlling a system (100) are provided. The system comprises at least one transformer (110) connectable to an electrical grid (101) for galvanically isolating the system from the grid ground and for adapting an input voltage level associated with an alternating current received from the grid. The system further comprises a converter unit (115) connected to the at least one transformer and configured to convert the received alternating current to a direct current output between the anode and the cathode of the converter unit. The converter unit comprises at least one modular multilevel converter (120), the at least one modular multilevel converter comprising at least two converter branches (130). Each converter branch comprises at least one converter chamber (140) and at least one inductor (150). The at least two converter branches include one branch connected from the AC line of the at least one transformer to a positive pole, and another branch connected from the AC line of the at least one transformer to a negative pole. The system further includes an electrolyzer unit (160) disposed between the positive electrode and the negative electrode. The system further comprises a control unit (170) configured to control the direct current output from the converter unit to the electrolyser unit based on a reference value for driving the electrolyser unit.

Description

Technical Field

[0001] The present disclosure relates generally to the field of power conversion, and in particular to a method and system for a hydrogen electrolyzer rectifier in an electrolyzer station. More specifically, the present disclosure relates to a system including a converter arrangement for converting AC current to DC current for use with an electrolyzer unit and a method for controlling such a system. Background Art

[0002] In order to limit the impact and negative effects of climate change, it is necessary to reduce emissions of CO2 and other gases that contribute to global warming. One way to achieve carbon reduction (an area that is difficult to achieve using other methods) is through the use of hydrogen as an energy carrier. One way to produce hydrogen is through electrolysis, which uses electricity to separate water into oxygen and hydrogen. By using electricity from renewable energy sources, so-called "green hydrogen" can be produced.

[0003] Hydrogen can be produced to a large extent, for example, when there is an excess of renewable energy (such as wind or solar) and electricity prices are low. There are several electrolyser technologies that can be used to produce hydrogen. Regardless of the chemical reaction used in the process, the system typically includes at least one power electronic converter that transforms AC current from the grid into DC current.

[0004] One example used today for conversion is a 12-pulse thyristor rectifier. However, this solution is problematic and presents challenges, for example because the harmonics on the AC side will vary as the electrolyser load varies. Therefore, a harmonic filter needs to be used with the thyristor rectifier. An additional problem with using a thyristor rectifier is that the reactive power varies with the hydrogen produced, which needs to be addressed by using a STATCOM.

[0005] It would therefore be of interest to provide an improved method and system for converting AC current to DC current for use with an electrolyser cell. Summary of the invention

[0006] It is therefore an object of the present disclosure to provide a system and method that can provide controlled power conversion for use with an electrolyser unit.

[0007] To achieve this object, the present disclosure provides a system and a method for controlling the system as defined by the independent claims, the system comprising at least one transformer, a converter unit, an electrolyser unit, a control unit. Further embodiments are provided in the dependent claims.

[0008] According to a first aspect of the present disclosure, a system is provided. The system includes at least one transformer connectable to a power grid. The at least one transformer is configured to isolate the system from the power grid galvanically and to adapt an input voltage level associated with an alternating current received from the power grid. The system further includes a converter unit connected to the at least one transformer. The converter unit is configured to convert the received alternating current into a direct current output between a positive pole and a negative pole of the converter unit. The converter unit includes at least one modular multilevel converter. The at least one modular multilevel converter includes a converter branch. One converter branch is connected to the positive pole (of the converter unit) from the AC line of the at least one transformer, and another converter branch is connected to the negative pole (of the converter unit) from the AC line of the at least one transformer. Each converter branch includes at least one converter cell and at least one inductor. The system further includes an electrolyzer unit arranged between the positive pole and the negative pole of the converter unit. The system further includes a control unit configured to control the direct current output from the converter unit to the electrolyzer unit based on a reference value for driving the electrolyzer unit.

[0009] According to a second aspect of the present disclosure, a method for controlling a system is provided. The system includes at least one transformer connectable to a power grid, the at least one transformer being used to isolate the system from the power grid galvanically and to adapt the input voltage level of the alternating current received from the power grid. The system further includes a converter unit connected to the at least one transformer and configured to convert the received alternating current into a direct current output between the positive and negative poles of the converter unit. The converter unit includes at least one modular multilevel converter, the at least one modular multilevel converter including a converter branch. One converter branch is connected to the positive pole from the AC line of the at least one transformer, and another converter branch is connected to the negative pole from the AC line of the at least one transformer. Each converter branch includes at least one converter chamber and at least one inductor. The system further includes an electrolyzer unit arranged between the positive and negative poles.

[0010] The method comprises: receiving a reference value for driving an electrolyser unit and indicating a target value for a direct current output between a positive and a negative pole of a converter unit or for a voltage drop across a positive and a negative pole of the converter unit. The method further comprises: receiving information indicating an actual current value of the direct current output or an actual voltage value of the voltage drop. The actual current value may be determined based on a measured value of the direct current output or based on a measured value of the received alternating current followed by an estimate of the direct current output based on the measured received alternating current. The actual voltage value may be determined based on a voltage measurement representing the voltage across the positive and negative poles of the converter unit. The method further comprises: controlling a converter chamber of the at least one modular multilevel converter of the converter unit based on the received information and the received reference value.

[0011] By reference value is meant herein a control value to be used for operating the electrolyser unit. The reference value may for example be the volume or amount of hydrogen to be produced by the electrolyser unit or the rate at which the electrolyser unit is intended to produce hydrogen. Such a reference value may then indicate or at least represent a target value to be used for the operation of the converter unit in order to achieve the reference value. The target value derived from the reference value may for example be the level of direct current to be output between the positive and negative poles of the converter unit or the voltage level between the positive and negative poles of the converter unit. The target value derived from the reference value may also be both the level of direct current output between the poles and the voltage level between the poles.

[0012] It will be appreciated that a converter branch may be referred to as a converter arm, wherein the modular multilevel converter comprises an upper arm / branch and a lower arm / branch. Each phase typically comprises an upper arm and a lower arm, wherein the upper arm is connected to the positive pole of the converter unit and the lower arm is connected to the negative pole of the converter unit.

[0013] The positive and negative poles of the converter unit may also be referred to as the positive and negative terminals of the converter unit. Therefore, a system and method are provided, the first function of which is to convert the alternating current received from the grid into a direct current output to the electrolyzer unit using at least one modular multilevel converter. The amount of hydrogen produced via electrolysis is expected to increase in the next few years. Therefore, it is very advantageous to have a more reliable system and method for providing power to the electrolyzer. The present system provides advantages over other systems because the use of modular multilevel converters in the converter unit does not pollute the grid with harmonics or at least significantly reduces the occurrence of harmonics on the AC side. Further, compared with the prior art system based on the use of thyristor rectifiers, the need for harmonic filters on the AC side is reduced and may even be suppressed. Further, the system and method provide the possibility of controlling reactive power in the grid. The possibility of fast regulation of reactive power together with the system controlling the active power to the electrolyzer can be used to stabilize the grid with a high degree of renewable energy. Therefore, the need for adding passive or active reactive power compensation is reduced and may even be suppressed.

[0014] Further, since grid codes (ie, requirements for connected devices of the grid) can be met using the present system, the need for STATCOMs, which are typically connected to the common connection point between the system, renewable energy power plant and the grid, is reduced or even eliminated.

[0015] Further, the system provides a voltage and current with low harmonic content on the DC side, which is beneficial for the operation of the electrolyzer unit. The high flexibility of the system is further beneficial for handling and adapting the DC voltage or DC current of the electrolyzer unit over time, as the electrolyzer unit may degrade during its service life. Further, the present system and method may also be used to support an islanded grid. The electrolyzer unit may require a high rated current, and in the future when the electrolyzer unit is further developed, the rated current will likely increase even further. The modularity of the system provides a solution that can be used for future electrolyzer units that require a higher rated current than today's electrolyzers.

[0016] According to an embodiment, the converter unit includes a first number Z modular multilevel converters coupled in a parallel circuit. The first number Z is greater than 1. The advantage of this embodiment is that the flexibility of the system is improved. By coupling a plurality of modular multilevel converters in parallel, the rated current of the electrolyzer unit can be increased. This can be advantageous because different electrolyzer units may require different rated currents, thereby ensuring that the system can be used for any electrolyzer unit. Each of the modular multilevel converters can facilitate providing an equal portion of the current to the electrolyzer unit. For example, if Z is equal to 2, each modular multilevel converter can provide 50% of the required current to the electrolyzer unit. However, it should be understood that this is only an example, and the contribution of each of the modular multilevel converters may vary depending on the characteristics of the modular multilevel converter (such as the number and rated current of the converter chambers) and the method for controlling the system or even the condition of each modular multilevel converter. The first number Z can be any number greater than 1. However, as disclosed in the present disclosure, a system with only one modular multilevel converter seems reasonable.

[0017] According to an embodiment, the at least one transformer comprises a second number W of transformers. In other words, the system may include a number W of transformers. The second number W may be equal to the first number Z, so that each transformer is connected to a modular multilevel converter. The advantage of this embodiment is that each modular multilevel converter is connected to a transformer. The transformer is used both to adapt the incoming voltage to the modular multilevel converter and to isolate the modular multilevel converter galvanically. By providing a transformer for each modular multilevel converter, the flexibility of the system is increased, and the modular multilevel converters are individually isolated. Further, the system may be advantageous for handling very high alternating currents from the power grid.

[0018] According to an embodiment, the at least one transformer is a single transformer connected to a first number Z of modular multilevel converters. The advantage of this embodiment is that providing a transformer is a cheaper alternative. Depending on the number of modular multilevel converters, one transformer may be sufficient for the system. The transformer may be any type of transformer. The transformer may, for example, be a two-winding transformer. A two-winding transformer comprises one three-phase connection on the grid side and one three-phase connection on the converter side. The transformer may also be a three-winding transformer. A three-winding transformer comprises one three-phase connection on the grid side and two three-phase connections on the sides of the converters. A system using only one transformer may be advantageous in reducing the alternating current from the grid.

[0019] According to an embodiment, the at least one transformer comprises a second number W of transformers, and the second number W is less than the first number Z. At least one of the second number W of transformers can be connected to a plurality of modular multilevel converters. In this embodiment, one transformer can be coupled to a plurality of modular multilevel converters or to a single modular multilevel converter, thereby increasing the flexibility of the system. The transformer can be any kind of transformer. The transformer can be a two-winding transformer. The transformer can also be a two-winding transformer. The transformer can also include both a two-winding transformer and a three-winding transformer.

[0020] According to an embodiment, the at least one modular multilevel converter comprises six converter branches. The six converter branches comprise: three branches, each branch connected to the positive pole from the AC line of the at least one transformer; and three other branches, each branch connected to the negative pole of the converter unit from the corresponding AC line of the at least one transformer. In other words, the present embodiment provides a system for a three-phase AC side.

[0021] According to an embodiment, each converter branch comprises a plurality of serially connected converter chambers (or series connected converter chambers). It will be appreciated that if some of the serially connected converter chambers are not needed, or if one or more converter chambers cease to function for any reason, these chambers may be bypassed without disengaging or stopping the system.

[0022] According to an embodiment, the at least one inductor is arranged between the AC line and the plurality of serially connected converter chambers or between the serially connected converter chambers and the positive or negative pole.The inductor may also be referred to as a branch reactor and may be any type of branch reactor.

[0023] According to an embodiment, each converter branch comprises a plurality of inductors. At least one of the plurality of inductors is arranged between two converter chambers of the plurality of serially connected converter chambers. This embodiment is advantageous in the event of an internal ground fault. The inductor may be any type of branch reactor.

[0024] According to an embodiment, each converter chamber in the at least one converter chamber of each converter branch has a full-bridge topology. In other words, the at least one modular multilevel converter may be based on a full-bridge converter chamber. The full-bridge converter chamber may include any self-commutating semiconductor switches. These self-commutating semiconductor switches include at least insulated gate bipolar transistors (IGBTs), integrated gate-commutated thyristors (IGCTs), injection-enhanced gate transistors (IEGTs), gate turn-off thyristors (GTOs) and metal oxide semiconductor field effect transistors (MOSFETs).

[0025] According to an embodiment, each converter branch comprises at least one converter chamber having a full-bridge topology and at least one converter chamber having a half-bridge topology. Since the voltage applied to the electrolyzer cell (i.e., the voltage drop between the positive and negative poles of the converter cell) should be positive, the modular multilevel converter can use a mixture of full-bridge converter chambers and half-bridge converter chambers. This embodiment is advantageous in that the manufacturing cost of the half-bridge converter chamber can be lower than the manufacturing cost of the full-bridge converter chamber.

[0026] According to an embodiment, the control unit is configured to control the DC output to the electrolyzer unit by comparing the actual current value with the target current value or by comparing the actual voltage value with the target voltage value. The target current value and the target voltage value are derived from reference values. The actual current value is determined based on the measured value of the DC output, or based on the measured value of the received alternating current, followed by an estimated value of the DC output based on the measured received alternating current. The actual voltage value is determined based on the voltage measurement representing the voltage across the positive and negative poles of the converter unit. This embodiment is an example of how the control unit can control the system, and in particular the converter chamber of the at least one modular multilevel converter unit. Depending on the requirements of the electrolyzer unit, the DC output may change. Therefore, the control unit can control the modular multilevel converter so that the combined output current from the modular multilevel converter matches the current required for the operation of the electrolyzer unit. The control unit can also be configured to control the system based on the measured value representing the voltage between the positive and negative poles. Such a measured value may be, for example, the voltage between the poles, or it may correspond to a combined value of voltages measured across the electrolyser stack of the electrolyser unit or a combined value of voltages measured at each converter chamber. Further, the control unit may be configured to change from voltage control to current control as soon as a current starts to flow through the electrolyser unit, or at least as soon as a current above a certain threshold starts to flow through the electrolyser unit.

[0027] According to an embodiment, the system further includes at least one end inductor arranged between one of the converter branches and the positive or negative pole. In the present embodiment, at least one end inductor or branch reactor is added after the at least one modular multilevel converter. If the system has multiple modular multilevel converters, multiple end inductors may exist. These end inductors may, for example, be placed between the positive pole and the terminal connected to the positive pole of each modular multilevel converter. These end inductors may also be placed between the negative pole and the terminal connected to the negative pole of each modular multilevel converter. For each modular multilevel converter, the system may further include an end inductor coupled between the positive pole and the modular multilevel converter, and an end inductor coupled between the negative pole and the modular multilevel converter. The present embodiment provides current sharing, limits harmonics, and reduces the impact of faults.

[0028] According to an embodiment, the electrolyzer unit includes a plurality of electrolyzer stacks connected in series. The electrolyzer unit may include any type of electrolyzer stack. The electrolyzer stack may further include a plurality of electrolyzer chambers connected in series. The electrolyzer stack and chamber may be based on alkaline, proton exchange membrane (PEM) or solid oxide technology, for example. Compared with other types of electrolyzer units, the advantage of alkaline electrolyzer units is that they contain cheaper catalysts and have a higher lifespan. Compared with other electrolyzers, the advantage of PEM electrolyzer units is that they have higher current density, are more compact, have a smaller footprint, have a faster response, and allow more dynamic operation.

[0029] According to an embodiment, the system further comprises an electrolyzer protection unit. The electrolyzer protection unit is configured to limit the current through the electrolyzer unit based on information indicating the respective maximum voltages and maximum voltage limits of the electrolyzer stacks and / or is configured to cause bypassing of failed electrolyzer stacks. The electrolyzer stacks and possible multiple electrolyzer chambers in each stack may have different characteristics due to, for example, aging degradation, temperature differences and manufacturing processes. The electrolyzer protection unit may ensure that the voltage on a separate electrolyzer stack does not exceed the maximum value of the separate electrolyzer stack. The electrolyzer protection unit may further cause bypassing of failed electrolyzer stacks or electrolyzer chambers of these electrolyzer stacks.

[0030] According to an embodiment, the at least one converter chamber comprises a mechanical bypass switch configured to bypass the converter chamber.This embodiment is advantageous in case of a converter chamber failure.

[0031] According to an embodiment, the system may optionally include at least one of at least one filter and a surge suppressor arranged between the positive pole and the ground connection or between the negative pole and the ground connection. The at least one filter may include at least one resistor and at least one capacitor or at least one power electronic device configured to filter harmonics. The advantage of this embodiment is that the RC filter or the filter including the power electronic device can reduce and even remove any harmonics generated on the DC side. It is further envisioned that the system may include more than one filter. For example, there may be a filter coupled between the negative pole and the ground connection and a filter coupled between the positive pole and the ground connection. Further, there may be a filter coupled between each modular multilevel converter and the ground connection. A surge suppressor can be advantageous because it limits overvoltage.

[0032] According to an embodiment, the reference value indicates at least one of: an amount of hydrogen to be produced by the electrolyzer unit, a current to be conducted through the electrolyzer unit, a voltage to be applied to the electrolyzer unit, and a condition of the electrolyzer unit. The reference value may be any relevant information received from the electrolyzer unit indicating a voltage or current required to drive the electrolyzer unit.

[0033] According to an embodiment, controlling the converter chamber comprises switching from a voltage control mode to a current control mode when the DC power output has reached a predetermined threshold value.

[0034] Thus, when transitioning from a voltage control mode to a current control mode, the received information may change from indicating an actual voltage value to indicating an actual current value.

[0035] The voltage control mode means a control mode in which the control unit is configured to control the converter chamber so that a specific voltage is applied between the positive and negative poles of the converter unit. The current control mode means a control mode in which the control unit is configured to control the converter chamber so that a specific DC current is output between the positive and negative poles of the converter unit.

[0036] The control unit may be configured to switch between these two modes, for example when the DC power output reaches a predetermined threshold after system startup.

[0037] According to an embodiment, the converter unit comprises a number Y of operational modular multilevel converters. Controlling the converter unit may comprise increasing the current provided by the remaining operational modular multilevel converters if the number Y decreases. For example, the system may remain operational even if one or more modular multilevel converters experience a fault or malfunction. The modularity of the system allows the control unit to control the remaining operational modular multilevel converters so as to deliver the required DC output to the electrolyser unit (assuming that the remaining operational modular multilevel converters can handle the increased current).

[0038] According to another embodiment, still in the case of a converter unit comprising a number Y of operational modular multilevel converters, controlling the converter chamber comprises: adjusting the reference value if the number Y decreases. For example, in the case where the modular multilevel converters are already operating at maximum load, if one or more of the modular multilevel converters fail, the reference value may be adjusted. This may ensure that the system keeps delivering a DC output to the electrolyser unit.

[0039] Other objects, features and advantages of the disclosed embodiments will be apparent from the following detailed disclosure and from the accompanying drawings.

[0040] It should be noted that embodiments of the present disclosure relate to all possible combinations of features recited in the claims. Further, it will be appreciated that the various embodiments described for the system as defined according to the first aspect and the embodiments described for the method according to the second aspect are all combinable with each other. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] This and other aspects of the disclosure will now be described in more detail, with reference to the appended drawings showing embodiment(s) of the disclosure.

[0042] Figure 1a and Figure 1b A system according to an exemplary embodiment of the present disclosure is schematically shown.

[0043] Figure 2a and Figure 2b An exemplary converter chamber that may be used in a system according to an exemplary embodiment of the present disclosure is schematically shown.

[0044] Figures 3 to 11c A system according to an exemplary embodiment of the present disclosure is schematically shown.

[0045] Fig.12 A schematic diagram showing the voltage across an electrolyser cell as a function of the current at the electrolyser cell is shown.

[0046] Fig.13 A flow chart of a method according to an exemplary embodiment of the present disclosure is shown. DETAILED DESCRIPTION

[0047] Figure 1a A system 100 according to an exemplary embodiment of the present disclosure is schematically shown. The system 100 includes a transformer 110 connectable to a power grid 101 for isolating the system 100 from the power grid 101 galvanically and for adapting an input voltage level associated with an alternating current received from the power grid 101. The power grid 101 may be understood as a portion of a local, regional, national or international power grid to which the system 100 is connected. The power grid 101 may, for example, deliver excess electricity generated by renewable energy sources such as wind or solar energy in order to save the excess energy as hydrogen for later use. In the current embodiment, the power grid 101 has three connection lines for three phases. It is also conceivable to use the system 100 in other applications with fewer or more power grids 101.

[0048] The system 100 further includes a converter unit 115 connected to the transformer 110 and configured to convert the AC power received from the grid 101 into DC power, which is output between the positive and negative poles of the converter unit 115. Figure 1aIn the embodiment disclosed in , the converter unit 115 includes a modular multilevel converter 120. The modular multilevel converter 120 can be implemented as a double star AC / DC converter. The modular multilevel converter 120 includes six branches 130. Each converter branch includes four converter chambers 140 and an inductor 150. The inductor 150 can be any type of branch reactor. The six converter branches 130 include three converter branches 130 connected to the positive pole from the AC line of the transformer 110. The six converter branches 130 also include three converter branches 130 connected to the negative pole from the AC line of the transformer 110. Therefore, for each phase of the power grid 101, there are two converter branches 130. Further, the system 100 includes an electrolyzer unit 160 arranged between the positive and negative poles of the converter unit 115. The system 100 also includes a control unit 170, which is configured to control the converter chamber 140 of the converter unit 115 so that a certain DC power is output from the converter unit 115 to the electrolyzer unit 160.

[0049] In the present embodiment, the converter unit 115 comprises one modular multilevel converter 120. However, it is envisaged that the converter unit 115 may comprise more than one modular multilevel converter, such as, for example, a first number Z of modular multilevel converters coupled in a parallel circuit. The first number Z may then be greater than 1. Depending on the rated current required for operation of the electrolyser unit 160, a different number of modular multilevel converters 120 may be used. In other words, in general, the converter unit 115 may comprise a plurality of modular multilevel converters 120 electrically connected in parallel.

[0050] exist Figure 1a , the modular multilevel converter 120 comprises six converter branches (or arms) 130 for a three-phase system. However, depending on the intended application and the need for, for example, one, two or three phases, it will be appreciated that the modular multilevel converter 120 may comprise another number of converter branches 130. For example, if the grid 101 is to provide only one phase, the modular multilevel converter may comprise only four converter branches 130. In other words, in general, the modular multilevel converter 120 may comprise at least four converter branches 130.

[0051] Furthermore, for example, Figure 1a As illustrated in the system shown in , a modular multilevel converter 120 may include several phase legs, wherein one phase leg includes one converter branch 130 coupled from the AC line to the positive pole and one converter branch 130 coupled from the same AC line to the negative pole. Figure 1a The system 100 illustrated in comprises a modular multilevel converter 120 having three phase legs (one for each phase of the grid 101).

[0052] Figure 1a Each converter branch 130 in the embodiment includes four converter chambers 140 connected in series. However, this is for illustration purposes only, and the converter branch 130 may include a different number of converter chambers 140, ranging from 1 to any integer N. Utilizing multiple converter chambers 140, the modular multilevel converter 120 may deliver a higher direct current to the electrolyzer unit 160. Each converter chamber 140 may include a mechanical bypass switch configured to bypass the converter chamber 140. If one converter chamber 140 is damaged or fails, the converter chamber 140 may be bypassed, and the remaining converter chambers 140 may be configured to provide a higher voltage to compensate for the bypassed converter chamber 140.

[0053] The converter chamber 140 may have a full-bridge topology. The converter chamber 140 may also include a converter chamber 140 having a full-bridge topology and a converter chamber 140 having a half-bridge topology.

[0054] The electrolyzer unit 160 may include a plurality of electrolyzer stacks 165 connected in series. The electrolyzer stacks 165 may be connected in series between the positive and negative electrodes of the converter units. The electrolyzer stacks 165 may each include a plurality of electrolyzer chambers. The electrolyzer stacks and chambers 165 may be any hydrogen electrolyzer type. The electrolyzer stacks 165 may be based on alkaline, PEM or solid oxide technology, for example.

[0055] exist Figure 1a In the embodiment, the inductor 150 of each converter branch 130 is placed between the AC line and the converter chamber 140 of each converter branch 130. However, it seems reasonable to have other placements for the inductor 150, such as in combination with Figure 1b as described.

[0056] Figure 1a The embodiment in further includes two filters 180. The filter 180 may include at least one resistor and at least one capacitor, and the filter 180 may be any suitable RC filter. The filter 180 may also include at least one power electronic device configured to filter harmonics. The filter 180 may also be a combination of an RC filter and a power electronic device. In general, the filter 180 may be one of a passive filter and an active filter or a combination of a passive filter and an active filter. One filter 180 is arranged between the positive pole and the ground connection of the converter unit 115. Another filter 180 is arranged between the negative pole and the ground connection of the converter unit 115. The filter 180 can reduce the presence of harmonics in the system 100 and particularly on the DC side. Additionally, the system 100 may also include a surge suppressor arranged between one or both of the poles and the ground. The surge suppressor may be advantageous because it can limit overvoltage.

[0057] Figure 1b A system 100 according to an exemplary embodiment of the present disclosure is schematically shown. Figure 1b The system 190 is similar to Figure 1a , except that the inductor 150 is arranged between the converter chamber 140 and the positive and negative poles of the converter unit. Figure 1a and Figure 1b Alternative embodiments are illustrated in which the inductor may be connected at either the AC side of the branch or at the DC side of the branch. Figure 1b The system 190 in FIG. 1 may have more or less than four converter chambers 140 in each converter branch 130. It should be further understood that Figure 1b The system 190 in FIG. 1 may have more than one inductor 150 in each converter branch 130 .

[0058] Figure 2a and Figure 2b An exemplary converter chamber 140 that may be used in the system 100 according to an exemplary embodiment of the present disclosure is schematically illustrated. In certain embodiments of the present disclosure, each converter chamber 140 of the systems 100 and 190 has a full-bridge topology. Figure 2a A converter chamber 140 having a full-bridge topology is illustrated in FIG. Figure 2a The converter chamber 140 in the embodiment includes first to fourth semiconductor switches 145a, 145b, 145c, 145d connected in a full bridge configuration. The converter chamber 140 further includes an energy storage, which is generally implemented as a capacitor arrangement structure including at least one capacitor 142. The capacitor is configured to store electrical energy and thereby provide a voltage. The semiconductor switches 145a, 145b, 145c, 145d may include any self-commutated semiconductor switches. These self-commutated semiconductor switches include at least IGBT, IGCT, IEGT, GTO, MOSFET.

[0059] In certain embodiments of the present disclosure, each converter branch 130 may include at least one converter chamber 140 having a full-bridge topology and at least one converter chamber 140 having a half-bridge topology. Figure 2b 1 shows a converter chamber 140 with a half-bridge topology. The converter chamber 140 includes a first semiconductor switch 145a and a second semiconductor switch 145b connected in a half-bridge configuration. The converter chamber 140 further includes an energy storage, which is generally implemented as a capacitor arrangement structure including at least one capacitor 142. The capacitor is configured to store electrical energy and thereby provide a voltage. The semiconductor switches 145a, 145b may include any self-commutated semiconductor switches. These self-commutated semiconductor switches include at least IGBT, IGCT, IEGT, GTO, MOSFET.

[0060] Figure 3 A system 300 according to an exemplary embodiment of the present disclosure is schematically shown. Figure 3 The system 300 disclosed in is similar to previously disclosed systems and may include any of the features discussed with respect to the previous figures. Figure 3 In the system 300 shown in , each converter branch 130 includes a plurality of inductors 150, and at least one of the plurality of inductors 150 is arranged between two converter chambers 140 of the plurality of serially connected converter chambers 140. Figure 3 In the embodiment of the present invention, each of the inductors 150 is placed between two consecutively arranged converter chambers 140 in the converter branch 130. Such an arrangement of the inductors 150 provides improved handling of internal ground faults. It is also possible to envision a system having inductors 150 placed only between some of the converter chambers 140. It further seems reasonable to have inductors placed between the AC line and the converter chamber 140 or between the converter chamber and the positive or negative pole, such as Figure 1a and Figure 1b Combinations of these different placements of inductor 150 also appear to be plausible.

[0061] Figure 4 A system 400 according to an exemplary embodiment of the present disclosure is schematically illustrated. Figure 4 The converter unit 415 in the system 400 shown in the embodiment includes two modular multilevel converters 120 coupled in parallel circuits (i.e., in parallel on the DC side). Each modular multilevel converter 120 may include any one of the features disclosed above with respect to the previous figures. Coupling two modular multilevel converters 120 in parallel circuits allows a higher current to be provided to the electrolyzer unit 160. Each modular multilevel converter 120 may, for example, be configured to provide 50% of the current required by the electrolyzer unit 160. However, other configurations are possible. A modular multilevel converter 120 may, for example, be configured to provide a current greater than the current provided by another modular multilevel converter. The modular multilevel converters 120 may be similar in every respect, or may be different from each other. For example, one of the modular multilevel converters may include branches having more serially connected converter chambers than another modular multilevel converter.

[0062] Figure 4The modular multilevel converters 120 shown in FIG. 4 are coupled to the same two-winding transformer 110. However, it is possible that each of the modular multilevel converters 120 is connected to the grid 101 via its own two-winding transformer 110. Further, it is possible to have a three-winding transformer coupling the modular multilevel converter 120 to the grid 101. The converter unit 415 may also include more than two modular multilevel converters 120. In general, the converter unit 415 may include a first number Z of modular multilevel converters 120 coupled in a parallel circuit.

[0063] The system 400 further comprises a control unit 170 configured to control the DC power output from the converter unit 415. The control unit 170 may be configured to individually control each modular multilevel converter 120. Thus, if a fault is located in one of the modular multilevel converters 120, the control unit may cause the faulty modular multilevel converter to be decoupled while the load on the remaining modular multilevel converters 120 is increased.

[0064] Figure 5 A system 500 according to an exemplary embodiment of the present disclosure is schematically illustrated. Figure 5 The system 500 illustrated in FIG. 5 is similar to the system 500 described above with reference to, for example, Figure 4 However, Figure 5 The system 500 further includes two end inductors 285. Each end inductor 285 is placed between one of the modular multilevel converters 120 and the positive pole of the converter unit. The end inductors 285 can handle current sharing between the modular multilevel converters 120. The end inductors 285 can further limit harmonics and reduce the impact of possible faults.

[0065] Figure 6 A system 600 according to an exemplary embodiment of the present disclosure is schematically shown. Figure 6 The system 600 is similar to the reference Figure 5 The system 500 described herein, except Figure 6 The system 600 in FIG. 6 includes four end inductors 285. In the present system 600, two additional end inductors 285 are placed between the modular multilevel converter 120 and the negative pole of the converter cell.

[0066] Figure 7 A system 700 according to an exemplary embodiment of the present disclosure is schematically shown. Figure 7The converter unit 715 in the embodiment comprises three modular multilevel converters 120 coupled in a parallel circuit, i.e. the positive terminal of each of these modular multilevel converters is connected together to a common positive pole of the converter unit, and the negative terminal of each of these modular multilevel converters is connected together to a common negative pole of the converter unit. The modular multilevel converter 120 may have any of the features discussed above with respect to the embodiments illustrated in the previous figures.

[0067] Figure 8 A system 800 according to an exemplary embodiment of the present disclosure is schematically illustrated. Figure 8 The system 800 shown in FIG. Figure 4 The described system 400 is equivalent, except that the system 800 includes two transformers 110. Each transformer 110 is coupled to one modular multi-level converter 120. Such a system 800 may be beneficial for handling very high powers.

[0068] Fig. 9 A system 900 according to an exemplary embodiment of the present disclosure is schematically shown. Fig. 9 System 900 and Figure 8 , except that it includes four end inductors 285 arranged between the modular multilevel converter and each pole of the converter unit 115 (such as, for example, in Figure 6 In other words, the system 900 is connected to the AC side with Figure 8 The system 800 is arranged in a similar manner, but it is connected on the DC side with Figure 6 The system 600 is arranged in a similar manner.

[0069] Fig.10 A system 1000 according to an exemplary embodiment of the present disclosure is schematically shown. Fig.10 The system 1000 disclosed in Figure 4 The system 400 is described except that instead of the two-winding transformer 110, Fig.10 The system 1000 includes a three-winding transformer 1010. The three-winding transformer 1010 is configured to galvanically isolate the system 1000 and adapt an input voltage level associated with alternating current received from a grid.

[0070] Figures 11a to 11c A system 100 according to a further illustrative embodiment of the present disclosure is schematically shown. Figures 11a to 11c The systems 1100, 1110 and 1120 illustrated in FIG. 1 all comprise a converter unit including four modular multilevel converters 120. The difference between the systems 1100, 1110 and 1120 is the number of transformers 110. Fig.11a The system 1100 in FIG. 1 includes four transformers 110 , wherein each transformer 110 is coupled to a modular multi-level converter 120 . Fig.11b The system 1110 in EMBODIMENT 1 includes one single transformer 110 , wherein the transformer 110 is connected to four modular multilevel converters 120 . Fig.11c The system 1120 in comprises two transformers 110, wherein each transformer 110 is connected to two modular multilevel converters 120. Thus, these modular multilevel converters can be grouped such that they share a transformer on the AC side.

[0071] These examples illustrate that the converter unit may include a first number Z of modular multilevel converters 120 coupled in parallel circuits. The system may then further include a second number W of transformers 110 for galvanically isolating the system 100 and adapting an input voltage level associated with the alternating current received from the grid. The second number W may be the same as the first number Z (e.g., Fig.11a ), but it may also be lower than the first quantity Z (as shown in Fig.11b and Fig.11c ). The transformer 110 may be coupled to one or more modular multilevel converters 120. In other words, in this example, the system includes Z modular multilevel converters 120 and W transformers 110.

[0072] Fig.12 A schematic diagram illustrating the voltage across the electrolyser cell 160 as a function of the current at the electrolyser cell 160 is shown.

[0073] The electrolyzer unit 160 may include a plurality of electrolyzer stacks 165 connected in series. The electrolyzer stack 165 may include a plurality of electrolyzer chambers connected in series. The electrolyzer unit 160 is arranged between the positive and negative electrodes of the converter unit. The DC output from the converter unit to the electrolyzer unit 160 may be represented as I EL This current flows through the electrolyzer unit 160 and causes electrolysis to produce hydrogen. The voltage drop from the positive electrode to the negative electrode (ie, the voltage across the electrolyzer unit 160) can be expressed as U EL .like Fig.12 It can be seen that if the voltage U EL increases, the current I EL increase (and vice versa).

[0074] The control unit of the system may be configured to control the converter unit (and more specifically the switches of the converter chambers of the modular multilevel converter) based on a reference value at which the electrolyser unit is to be operated. The reference value may be, for example, the amount of hydrogen to be produced by the electrolyser unit (e.g., during a certain time period). Such a reference value may in turn indicate a voltage U 100 to be used for operation of the electrolyser unit 160. EL and / or current I EL The target values ​​of the voltage and current correspond to the voltage drop across the positive and negative poles of the converter unit and the current provided by the converter unit between the positive and negative poles.

[0075] Therefore, the control unit can be configured to control the system to reach the target value. This can be performed based on the received information indicating the voltage value or the current value. EL The electrolyser unit 160 will be run through, so the control unit may first control the system in voltage control mode until the absolute value of the current reaches a certain threshold value, and then change to current control mode.

[0076] The system may further include an electrolyzer protection unit. The electrolyzer protection unit may be configured to protect the electrolyzer unit. The electrolyzer unit may be configured to limit the current through the electrolyzer unit based on information indicating the respective maximum voltages of the electrolyzer stack 165 and the maximum voltage limit of the electrolyzer unit. The electrolyzer stack 165 required to drive the electrolyzer stack may vary depending on aging degradation, temperature differences, manufacturing processes, etc. This may also mean that the electrolyzer stack may each have a maximum voltage that it can handle without breaking down. Therefore, the electrolyzer protection unit may be configured to limit the current flowing through the electrolyzer unit based on these individual voltages. The electrolyzer protection unit may further be configured to cause a bypass switch to bypass a failed electrolyzer stack or bypass a failed electrolyzer chamber within the electrolyzer stack.

[0077] Fig.13 A flow chart of a method 1300 according to an exemplary embodiment of the present disclosure is shown. The method 1300 may be implemented in a control unit and configured to control a system according to any exemplary embodiment of the present disclosure, such as the systems shown in FIGS. 1 to 11 .

[0078] The method 1300 includes receiving 1310 a reference value for driving the electrolyzer unit. As mentioned above, the reference value may indicate a target value for a DC power output, or for a voltage drop across a positive and negative electrode of a converter unit, or for both a DC power output and a voltage drop. The reference value may be a voltage value or a current value. Alternatively, the reference value may be an amount of hydrogen to be produced by the electrolyzer unit. The reference value may be any reference value associated with the electrolyzer unit that may indicate a voltage or current for operating the electrolyzer unit.

[0079] The method 1300 further includes: receiving 1320 information indicating an actual current value of the DC output or an actual voltage value of the voltage drop. The actual current value may be determined based on a measured value of the DC output. The actual current value may also be determined based on a measured value of the received AC power, followed by an estimated value of the DC output based on the measured received AC power. The actual voltage value may be determined based on a voltage measurement representing the voltage across the positive and negative poles of the converter unit. The measurement representing the voltage across the positive and negative poles of the converter unit may be obtained by measuring the voltage across the positive and negative poles of the converter unit. The measurement representing the voltage across the positive and negative poles of the converter unit may also be a combined voltage (e.g., the sum of the individual voltage measurements) from each electrolyzer stack of the electrolyzer unit. The measurement representing the voltage across the positive and negative poles of the converter unit may also be obtained from the individual voltage measurements on each converter chamber in each converter branch of each modular multilevel converter or each converter unit.

[0080] The method 1300 further comprises the step of controlling 1330 a converter chamber of the at least one modular multilevel converter of the converter unit based on the received information and the received reference value. If the received reference value indicates a target value for the DC power output, controlling 1330 the chamber of the modular multilevel converter may control the DC power output from the modular multilevel converter. If the received reference value is a current value, controlling 1330 may be performed so that the system matches the DC power output to the current value.

[0081] In certain embodiments, method 1300 may include further steps. Controlling 1330 the converter chamber may include transitioning 1340 from a voltage control mode to a current control mode when the DC power output has reached a predetermined threshold. Thus, when transitioning 1340 from the voltage control mode to the current control mode, the received information may then change from indicating an actual voltage value to indicating an actual current value. Controlling the system in voltage control mode may be beneficial when starting the system, since in principle no current will flow through the electrolyser unit until the voltage drop across the poles of the converter unit reaches a certain value.

[0082] The converter units of the system may include a number Y of operational modular multilevel converters. The step of controlling 1330 the converter units may include increasing 1350 the current through the remaining operational modular multilevel converters if the number Y decreases. If one or more of the modular multilevel converters fail, the load on the remaining modular multilevel converters may increase. By allowing the load on the operational modular multilevel converters to increase, the direct output current to the electrolyzer unit may remain unchanged, thereby improving the stability of the system.

[0083] The step of controlling 1330 may further comprise: if the number Y decreases, changing 1360 the reference value. If the load on the modular multilevel converters is already at a maximum value and one or more of the modular multilevel converters fail, the load on the remaining modular multilevel converters cannot be increased. By changing the reference value instead, the control unit can still control the system. However, in such a case, the DC output delivered to the electrolyser unit may be reduced.

[0084] Although the invention has been illustrated in the drawings and the foregoing description, such illustrations are to be considered illustrative or exemplary rather than restrictive; the invention is not limited to the disclosed embodiments. Other variations of the disclosed embodiments may be understood and effected by a person skilled in the art in practicing the claimed invention from a study of the drawings, the disclosure and the appended claims. In the appended claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

Claims

1. A system (100), comprising: at least one transformer (110) connectable to a power grid (101), the at least one transformer for galvanically isolating the system from the power grid and for adapting an input voltage level associated with alternating current received from the power grid; a converter unit (115) connected to the at least one transformer and configured to convert the received alternating current into a direct current output between a positive pole and a negative pole of the converter unit, wherein the converter unit comprises at least one modular multilevel converter (120), the at least one modular multilevel converter comprising converter branches (130), wherein one converter branch is connected from an AC line of the at least one transformer to the positive pole and another converter branch is connected from the AC line of the at least one transformer to the negative pole, and wherein one converter branch comprises at least one converter chamber (140) and at least one inductor (150), an electrolyzer unit (160) arranged between the positive electrode and the negative electrode of the converter unit, and A control unit (170) is configured to control the DC power output from the converter unit to the electrolyzer unit based on a reference value for driving the electrolyzer unit.

2. The system according to claim 1, wherein: The converter unit comprises a first number Z of modular multi-level converters coupled in a parallel circuit, wherein the first number Z is greater than one.

3. The system according to claim 2, wherein: The at least one transformer comprises a second number W of transformers, wherein the second number W is equal to the first number Z, and wherein each transformer is connected to one modular multilevel converter.

4. The system according to claim 2, wherein: The at least one transformer is a single transformer connected to the first number Z of modular multilevel converters.

5. The system according to claim 2, wherein: The at least one transformer comprises a second number W of transformers, wherein the second number W is smaller than the first number Z, and wherein at least one of the second number W of transformers is connected to a plurality of modular multilevel converters.

6. A system according to any one of the preceding claims, wherein: The at least one modular multilevel converter comprises six converter branches, wherein the six converter branches comprise: three branches, each branch being connected from an AC line of the at least one transformer to the positive pole; and another three branches, each branch being connected from a corresponding AC line of the at least one transformer to the negative pole of the converter unit.

7. A system according to any one of the preceding claims, wherein: Each converter branch comprises a plurality of converter chambers connected in series.

8. The system according to claim 7, wherein: The at least one inductor is arranged between the AC line and the plurality of serially connected converter chambers or between the serially connected converter chambers and the positive electrode or the negative electrode.

9. The system according to claim 7, wherein: Each converter branch comprises a plurality of inductors, and wherein at least one of the plurality of inductors is arranged between two converter chambers of the plurality of serially connected converter chambers.

10. A system according to any one of the preceding claims, wherein: Each converter chamber of the at least one converter chamber of each converter branch has a full-bridge topology.

11. The system according to any one of claims 1 to 9, wherein: Each converter branch comprises at least one converter chamber having a full-bridge topology and at least one converter chamber having a half-bridge topology.

12. A system according to any one of the preceding claims, wherein: The control unit is configured to control the direct current output to the electrolyzer unit by comparing an actual current value with a target current value or by comparing an actual voltage value with a target voltage value, wherein the target current value and the target voltage value are derived from the reference value, wherein the actual current value is determined based on a measured value of the direct current output or based on a measured value of the received alternating current followed by an estimate of the direct current output based on the measured received alternating current, and wherein the actual voltage value is determined based on a voltage measurement representing the voltage across the positive and negative poles of the converter unit.

13. The system according to any of the preceding claims, further comprising at least one end inductor (285) arranged between one of the converter branches and the positive pole or the negative pole.

14. A system according to any one of the preceding claims, wherein: The electrolyser unit comprises a plurality of electrolyser stacks (165) connected in series.

15. The system of claim 14, further comprising an electrolyzer protection unit, wherein: The electrolyser protection unit is configured to limit current through the electrolyser unit based on information indicative of respective maximum voltages and maximum voltage limits of the electrolyser stack, and / or wherein the electrolyser protection unit is configured to cause a failed electrolyser stack to be bypassed.

16. A system according to any one of the preceding claims, wherein: The at least one converter chamber includes a mechanical bypass switch configured to bypass the converter chamber.

17. The system according to any of the preceding claims, further comprising at least one of a filter (180) and a surge suppressor arranged between the positive pole and the ground connection or between the negative pole and the ground connection, wherein: The at least one filter includes at least one resistor and at least one capacitor or at least one power electronic device configured to filter harmonics.

18. A system according to any one of the preceding claims, wherein: The reference value indicates at least one of: an amount of hydrogen to be produced by the electrolyzer unit, an electric current to be conducted through the electrolyzer unit, a voltage applied to the electrolyzer unit, and a condition of the electrolyzer unit.

19. A method (1300) for controlling a system (100), the system comprising: at least one transformer connectable to a power grid, the at least one transformer for galvanically isolating the system from the power grid and for adapting an input voltage level of an alternating current received from the power grid, a converter unit connected to the at least one transformer and configured to convert the received alternating current into a direct current output between a positive pole and a negative pole of the converter unit, wherein the converter unit comprises at least one modular multilevel converter, the at least one modular multilevel converter comprising converter branches, wherein one converter branch is connected from an AC line of the at least one transformer to the positive pole and another converter branch is connected from the AC line of the at least one transformer to the negative pole, and wherein each converter branch comprises at least one converter chamber and at least one inductor, and wherein the system further comprises an electrolyzer unit arranged between the positive pole and the negative pole of the converter unit, the method comprising: receiving (1310) a reference value for driving the electrolyzer unit and indicating a target value for the DC power output between the positive and negative electrodes of the converter unit or for a voltage drop across the positive and negative electrodes of the converter unit; receiving (1320) information indicative of an actual current value of the DC power output or an actual voltage value of the voltage drop, wherein the actual current value is determined based on a measured value of the DC power output or based on a measured value of the received AC power followed by an estimate of the DC power output based on the measured received AC power, and wherein the actual voltage value is determined based on a voltage measurement representing the voltage across the positive and negative poles of the converter unit; and The converter chamber of the at least one modular multilevel converter of the converter unit is controlled (1330) based on the received information and the received reference value.

20. The method according to claim 19, wherein: Controlling the converter chamber includes transitioning (1340) from a voltage control mode to a current control mode when the DC power output has reached a predetermined threshold.

21. The method according to claim 19 or 20, wherein: The converter unit comprises a number Y of operational modular multilevel converters, and wherein controlling the converter unit comprises increasing (1350) a current provided by the remaining operational modular multilevel converters if the number Y decreases.

22. The method according to any one of claims 19 to 21, wherein: The converter unit comprises a number Y of operational modular multi-level converters, and wherein controlling the converter chamber comprises adjusting (1360) the reference value if the number Y decreases.

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