Field of the invention

[0001] The present invention relates to an electrolysis power converter system. The present invention further relates to a method for controlling an AC-to-DC conversion arrangement to provide power to an electrolytic cell arrangement.

Background of the invention

[0002] Greenhouse-gas emissions and climate change have a worldwide impact on the environment. To mitigate climate change, huge efforts are currently made to drive a renewable energy transition.

[0003] One cornerstone of this transition is the production of non-fossil fuels for use in transportation. Particularly within the transport industry, battery -based solutions are not viable, since battery technologies do not provide a sufficient energy density in the near future. Furthermore, typical batteries require rare materials, which are currently in high demand.

[0004] A promising approach for replacing fossil fuels is provided by the power-to- x concept. Within power-to-x, surplus electric power is converted into a form which can be used later, for example converted into a non-fossil fuel.

[0005] One major technological player for facilitating power-to-x is electrolysis technologies, in which electrical energy can be used to decompose water into oxygen and hydrogen gas. The hydrogen can be used directly as an energy source, or further steps may convert the hydrogen into another suitable gas, such as methane.

[0006] Although the principles of hydrogen production via electrolysis are relatively simple, it is challenging to effectively implement large scale hydrogen production in a feasible manner. Namely, the power-to-x concept is primarily meaningful whenever surplus electrical energy is available, preferably from renewable energy sources. Spending fossil fuels to produce non-fossil fuels somewhat deflates the power-to-x concept, at least in the renewable energy transition context. [0007] However, production of renewable electrical energy, for instance via wind power or solar power, suffers from being unpredictable as it relies largely on circumstances which cannot be controlled, such as wind and sun. And unfortunately, variances in available electrical energy is an inferior starting point for operating an electrolysis facility. Particularly, large-scale electrolysis production cannot be halted and commenced at will in an effective manner. For example, since the electrolysis process is temperature dependent, electrolysis facilities are preferably operated at a specific target temperature, but by halting production, the actual temperature will depart from the target temperature, leading to inefficient resumption of production.

[0008] Furthermore, power conversion from AC power, for example from a domestic power grid, to DC power for electrolysis can have certain restrictions imposed by the technology used for conversion, which further complicates efficient and flexible operation of electrolysis facilities.

Summary of the invention

[0009] The inventors have identified the above-mentioned problems and challenges related to electrolysis power converter systems, and subsequently made the below- described invention which may improve such systems.

[0010] An aspect of the invention relates to an electrolysis power converter system comprising: an electrolytic cell arrangement configured to use electrical energy to decompose water into hydrogen and oxygen, wherein said electrolytic cell arrangement is associated with an electrolysis voltage which represents a lower threshold above which substantial decomposition of water occurs in electrolytic cells of said electrolytic cell arrangement; an AC-to-DC conversion arrangement coupled to an AC electrical power grid and to said electrolytic cell arrangement, wherein said AC-to-DC conversion arrangement is configured to convert AC power from said AC electrical power grid into DC power to said electrolytic cell arrangement via an AC-to-DC switch converter which converts an AC voltage of said AC power to establish said DC power; and a converter controller arrangement configured to control said AC-to-DC conversion arrangement according to at least two converter control modes and configured to switch between said at least two converter control modes, wherein said at least two converter control modes comprise a standby mode and a production mode, wherein said standby mode comprises controlling said AC-to-DC conversion arrangement to convert said AC power such that said DC voltage of said DC power is smaller than said electrolysis voltage, wherein said production mode comprises controlling said AC-to-DC conversion arrangement to convert said AC power such that said DC voltage is larger than said electrolysis voltage.

[0011] By having an electrolysis power conversion system with an AC-to-DC conversion arrangement which can be controlled according to a standby mode and a production mode, the flexibility of everyday operation is improved, which is advantageous.

[0012] In particular, having an available standby mode, in which AC power is still converted, but in which the DC power is smaller than the electrolysis voltage, the AC- to-DC conversion arrangement can rapidly be switched into and away from the production mode. In contrast, if the AC-to-DC conversion arrangement had to be turned entirely off in between periods of production, switching in and out of production mode would potentially be more complicated, time-consuming, and powerconsuming.

[0013] Electrolytic cells of electrolytic cell arrangements in electrolysis facilities are typically operated at temperatures significantly above ambient temperatures. In fact, production processes are often designed for such high temperatures. As production is paused or halted, temperatures will typically decrease gradually, which provides a sub- optimal starting point when resuming production. By having a standby mode in which AC power is converted to a DC voltage smaller than the electrolysis voltage, production may be paused or halted, while potentially maintaining a small current through the electrolytic cells, effectively reduce the heat loss which would otherwise occur, which is advantageous for effectively resuming production once production mode is initiated again. Note particularly that, even though the actual current through electrolytic cells is relatively small below the electrolysis voltage, most of the energy of the current may be dissipated as heat in the electrolytic cells. In contrast, while in production mode, a significant amount of energy is also spent to decompose water.

[0014] The prospects of the invention may in particular be evaluated in light of the ongoing renewable energy transition, in which fossil fuels are to be gradually replaced by renewable energy. Here electrolysis power converter systems may play a vital role, since they may permit production of non-fossil fuels based on surplus energy of the power grid. However, energy production may generally become more unpredictable and unreliable, due to reliance on factors such as wind and sun. Consequently, actual implementation of electrolysis power converter systems which operate in a feasible manner, based on renewable power sources, is challenging. Namely, such systems are primarily meaningful if they can operate in periods with surplus electrical energy from renewable sources. The invention may advantageously permit such implementation, in which primarily renewable energy is used, and in which substantial amounts of nonfossil fuels are nevertheless produced. Particularly, with implementation of the invention, fragmented distributions of production periods may be more feasible, which is advantageous.

[0015] In addition, the flexibility offered by the invention may improve the load on power grids, which is advantageous. In particular, the flexibility may permit current to be drawn in periods with surplus power, while current demand may be reduced in periods in which power is sparse, which is advantageous.

[0016] Moreover, the flexibility offered by the invention may advantageously improve localized systems in which renewable power sources (such as wind turbines) are directly connected to an electrolysis power converter system (for example, without connection to a utility power grid), since such systems are particularly vulnerable to variations in power production.

[0017] Furthermore, the standby mode may be used as a step for commencing production in said electrolysis power converter system, which is advantageous. In particular, suddenly powering an electrolysis power converter system may damage components and/or result in electrolytic cells being operated under sub-optimal conditions. By using the standby mode as an intermediate step while powering on an electrolysis power converter system, such problems may be avoided, which is advantageous.

[0018] An electrolysis power converter system may be understood as facility which may draw electrical AC power from, e.g., a domestic power grid, to produce hydrogen through electrolysis in an electrolytic cell arrangement. Such hydrogen production typically requires DC power, and hence, electrolysis power converter systems according to the invention further comprises an AC-to-DC conversion arrangement which is configured to convert AC power from an AC electrical power grid into DC power for the electrolysis.

[0019] An electrolytic cell may be understood as an electrochemical cell that uses electrical energy to drive a non-spontaneous redox reduction to decompose water into hydrogen and oxygen. An electrolytic cell typically has three component parts: an electrolyte and two electrodes. The electrolyte is typically a solution of water in which ions are dissolved. Furthermore, an electrolytic cell may comprise a diaphragm separating the electrodes. An example of electrolysis is alkaline water electrolysis where electrodes operate in a liquid alkaline electrolyte solution.

[0020] An electrolytic cell arrangement may be understood as an arrangement of a plurality of electrolytic cells which are collectively configured to receive DC power (for example from an AC-to-DC conversion arrangement) such that a desired electrochemical reaction can be carried out in the individual electrolytic cells of the electrolytic cell arrangement. In an electrolytic cell arrangement, the electrolytic cells are typically grouped, such that the electrolytic cells of a given group are serially connected to, e.g., an AC-to-DC conversion arrangement. A group of electrolytic cells may be arranged in one or more electrolysis stacks. Several groups of electrolytic cells may then be parallelly connected to, e.g., that AC-to-DC conversion arrangement. In addition, an electrolytic cell arrangement may comprise additional well-known components, such as piping conducting gasses and/or liquids for carrying out the electrochemical process, separator tanks, gas storages, liquid storages, pumps, etc.

[0021] The electrolytic cell arrangement is associated with an electrolysis voltage which represents a lower threshold above which substantial decomposition of water occurs in electrolytic cells. In other words, when a DC voltage of the DC power provided by an AC-to-DC conversion arrangement lies above the electrolysis voltage, substantial decomposition of water occurs. And when that DC voltage lies below the electrolysis voltage, substantial decomposition of water does not occur. Given a single electrolytic cell at approximately 20 degrees Celsius, an example of a minimum voltage required to perform electrolysis is 1.23 V. However, note that this value is temperature dependent. This voltage, for a single electrolytic cell, may also be referred to as the reversible voltage. When several electrolytic cells are serially connected, the total DC voltage required may increase linearly with the number of cells serially connected. For example, if a single electrolytic cell requires a minimum voltage of 1.20 V, then 100 serially connected electrolytic cells collectively require a minimum voltage of 120 V. Such a voltage determined collectively by electrolytic cells of the electrolytic cell arrangement is referred to as the electrolysis voltage. It may also be referred to as a collective electrolysis voltage (of the electrolytic cell arrangement). Note that the electrolysis voltage may be temperature dependent.

[0022] Even through an electrolytic cell may be associated with a minimum voltage for electrolysis, for example 1.23 V, note that small amounts of current may be conducted through the cell and/or miniscule amounts of decomposition may occur below this voltage. Such a minimum voltage is thus not necessarily a hard boundary for current to run. However, above thus voltage, the current versus voltage may typically increase linearly versus voltage, at least approximately, as a result of the electrochemical reaction. And the slope/derivative of the current versus voltage is larger above this threshold than below this threshold.

[0023] An AC-to-DC switch converter may be understood as a switch-based AC-to- DC converter. As an example, an AC-to-DC switch converter may have converter legs, wherein each of the converter legs are connected to a separate phase of an AC electrical power grid and connected between a positive DC connection and a negative DC connection to the electrolytic cell arrangement. Further, each of the converter legs comprises two switching arrangements for coupling a given phase of the AC electrical power grid to any of the positive DC connection and negative DC connection to enable conversion of AC power to DC power by controlling the switching arrangements of the separate legs. Each of the switching arrangement typically comprises passive rectification unit, such as a diode, and an active switch unit. The active switch unit is typically a high-current transistor switch. Examples of active switch units are insulated-gate bipolar transistors (IGBTs), power MOSFETS, and gate controlled thyristors.

[0024] An AC-to-DC conversion arrangement may be understood as the power apparatus system configured to convert the AC power provided by the AC electrical power grid into DC power directly usable by the electrolytic cell arrangement. In some embodiments, the AC-to-DC conversion arrangement may just consist of an AC-to- DC switch converter. But in other embodiments, the AC-to-DC conversion arrangement comprises the AC-to-DC conversion arrangement as well as additional components, for example, in some embodiments of the invention, the AC-to-DC conversion arrangement comprises a DC-to-DC converter connected between the AC- to-DC switch converter and the electrolytic cell arrangement.

[0025] AC electrical power grid may be domestic utility power grid, or may be a more local power grid, such as a dedicated AC electrical power grid (separate from a domestic power grid) to which a wind farm outputs power.

[0026] A converter controller arrangement may be understood as a system, an apparatus, one or more processors, specially adapted circuitry, or any combination thereof, configured to control operation of at least the AC-to-DC conversion arrangement, and thereby, at least partially control operation of the electrolysis power converter system. A converter controller arrangement may also be in communication with the electrolytic cell arrangement, for example to monitor and/or regulate temperature of electrolytic cells. The converter controller arrangement is at least configured to control conversion of AC power into DC power, for example by controlling operation of switch arrangements of the AC-to-DC switch converter.

[0027] In embodiments of the invention, the converter controller arrangement is configured to control the AC-to-DC conversion arrangement according to at least two converter control modes and is configured to switch between these at least two converter control modes. A converter control mode may be understood as a particular scheme upon which AC power is converted into DC power, which thereby influences operation of the electrolysis power converter system and, potentially, hydrogen production and power consumption thereof. In typical embodiments of the invention, the different converter control modes relate particularly to the DC voltage of the DC power outputted by the AC-to-DC conversion arrangement. The DC voltage relatively to the electrolysis voltage determines whether substantial decomposition of water occurs in the electrolytic cell arrangement. Further, the degree to which the DC voltage surpasses the electrolysis voltage determines the rate of decomposition, and thereby the rate of hydrogen production. The established DC voltage may for example be controlled by controlling switching arrangements of an AC-to-DC switch converter, or by controlling other parts of the AC-to-DC conversion arrangement, such as a DC- to-DC converter.

[0028] A production mode may be understood as a mode which the AC-to-DC conversion arrangement is controlled according to whenever production of hydrogen is required. Thus, the DC voltage is larger than the electrolysis voltage of the electrolytic cell arrangement. The production mode may optionally comprise regulation of the DC voltage to control the current consumption and production rate to an appropriate or desired level. [0029] A standby mode may be understood as a mode upon which the AC-to-DC conversion arrangement may be controlled whenever a period of temporary termination of production is required. Accordingly, the DC voltage in this mode is smaller than the electrolysis voltage. Nevertheless, note that the AC-to-DC conversion arrangement is still controlled to actually convert some AC power into DC power. Thus, this mode is distinct from simply powering off the electrolysis power converter system or powering off the AC-to-DC conversion arrangement. The generated DC voltage may for example be at least half of the electrolysis voltage. This may ensure the AC-to-DC conversion arrangement can rapidly be switched into the production mode when required to and may ensure that the temperature of electrolytic cells decrease less during the termination of production.

[0030] Note that embodiments of the invention are not limited to two converter control modes. In some embodiments of the invention, the at least two converter control modes are at least three converter control modes. In some embodiments of the invention, the at least two converter control modes are at least four converter control modes. In some embodiments of the invention, the at least two converter control modes are at least five converter control modes.

[0031] In embodiments of the invention, said AC-to-DC switch converter is an AC- to-DC switch boost converter.

[0032] It should be noted, that the AC-to-DC switch converter preferably is implemented as a three-phased active rectifier as illustrated on fig 2.

[0033] In embodiments of the invention, said standby mode is an unregulated standby mode in which said DC voltage is unregulated.

[0034] In embodiments of the invention, said AC-to-DC switch converter converts said AC power such that said DC voltage at least corresponds to a peak value of said AC voltage whenever said AC-to-DC conversion arrangement is controlled according to said standby mode. [0035] It should be noted, that the lowest controllable DC voltage corresponds to the peak value of the AC voltage. Control of a higher DC voltage require active control of the switches.

[0036] In embodiments of the invention, said production mode is a regulated production mode in which said DC voltage and / or current is regulated. More specific, the DC voltage and current can e.g. be regulated to achieve either constant current, constant voltage or constant power output to the electrolyser.

[0037] Having the standby mode being an unregulated standby mode may in particular be the case in embodiments in which the AC-to-DC switch converter is an AC-to-DC switch boost converter. Whenever an AC-to-DC switch boost converter is operated, it minimally generates a DC voltage corresponding approximately to the peak value of the AC voltage. Operation of an AC-to-DC switch boost converter at a DC voltage corresponding approximately to the peak value of the AC voltage corresponds to the DC voltage being unregulated. The DC voltage may then be regulated to be increased above the peak value of the AC voltage and above the electrolysis voltage, such that production can be performed in, e.g., the production mode. More specific, during the standby mode, it is preferable to control the DC voltage, however in the production mode, it is preferable to control the DC current instead, because the hydrogen production is proportional to the current density in the electrolysis cells, whereas the DC voltage will gradually drop during stable hydrogen production due to temperature increase in the electrolyser. Therefore, in the production mode, it is mainly the DC current that is controlled.

[0038] Since an AC-to-DC switch boost converter minimally outputs a DC voltage corresponding approximately to the peak value of the AC voltage, having an AC-to- DC switch boost converter may impose certain restrictions to embodiments of the invention, for example since arbitrarily small values of DC voltage may not be available (except in embodiments having additional apparatus, such as a DC-to-DC converter for reducing the DC voltage between the AC-to-DC switch converter and the electrolysis cell arrangement). Accordingly, in embodiments of the invention the properties of the electrolytic cell arrangement and the properties AC-to-DC conversion arrangement are matched such that the minimum voltage produced by the AC-to-DC switch converter lies below the electrolysis voltage. Thereby, simplified switching between the standby mode and the production mode may be enabled, which is advantageous. Further, facilitation of the at least two modes may be implemented using a minimalistic AC-to-DC conversion arrangement, which is advantageous. Switching between the modes may simply corresponds to switching between whether the DC voltage is regulated or not.

[0039] In addition, having DC voltage corresponding to a peak value of said AC voltage may be considered as a relatively high voltage in the context of electrolysis. Having a large voltage permits a reduction of currents, which in turn enables cheaper components and less conductive material, such as copper, to be used, which is advantageous.

[0040] The production mode being a regulated production mode may be understood as the DC voltage being continuously or regularly adjusted to modify or optimize the actual production of the electrolysis facility.

[0041] Thus, at least in some embodiments, the standby mode and the production mode are not only different in relation to whether the DC voltage provided in these modes is smaller or larger than the electrolysis voltage, but also different as to the whether the DC voltage is regulated within these modes.

[0042] Having the production mode being regulated may ensure optimal production of the electrolytic cell arrangement which is advantageous. In particular, having a regulated production mode is advantageous in combination with an unregulated standby mode, which ensures that control capacity is primarily spent during production, and not during standby.

[0043] However, note that in some other embodiments, the standby mode may optionally be regulated, for example regulated to control a temperature of the electrolysis arrangement. [0044] In embodiments of the invention, said AC-to-DC conversion arrangement comprises a DC-to-DC converter connected between said AC-to-DC switch converter and said electrolytic cell arrangement.

[0045] Including a DC-to-DC converter into the AC-to-DC conversion arrangement may permit a greater liberty to adjust and regulate the DC voltage provided by the AC- to-DC conversion arrangement, which is advantageous. The DC-to-DC converter may for example be a DC-to-DC switch converter. The DC-to-DC converter may for example be controller by the converter controller arrangement. Further, the converter controller arrangement may be configured to switch between the standby mode and the production mode by controlling a conversion ratio of the DC-to-DC converter. Here, the conversion ratio is the ratio between the outputted voltage and the inputted voltage of DC-o-DC converter. In some embodiments, this conversion ratio is controlled to switch between the standby mode and the production mode.

[0046] In embodiments of the invention, said DC voltage is from 10 percent to 99 percent of said electrolysis voltage when said AC-to-DC conversion arrangement is controlled according to said standby mode, for example from 20 percent to 98 percent, for example from 30 percent to 97 percent, for example from 40 percent to 96 percent, such as from 50 percent to 95 percent.

[0047] In embodiments of the invention, said electrolysis voltage is at least 500 V, for example at least 600 V, for example at least 700 V, such as at least 1100 V.

[0048] In addition, the electrolysis voltage may be at most 2000 V, for example at most 1500 V, for example at most 1200 V, such as at most 1000 V.

[0049] In embodiments of the invention, said DC voltage is at least 800 V when said AC-to-DC conversion arrangement is controlled according to said production mode, for example at least 900 V, such as at least 1000 V.

[0050] In addition, the DC voltage may be at most 3000 V when said AC-to-DC conversion arrangement is controlled according to said production mode, for example at most 2500 V, for example at most 2000 V, such as at most 1500 V. [0051] Generally, the above exemplified voltages may be considered as relatively high voltages within the field of electrolysis. Operating at such voltages may not have been considered previously within the field of electrolysis, since hydrogen production at very large scales was simply not relevant. A straightforward approach to increase hydrogen production, from a starting point of a smaller electrolysis facility, is to multiply the number of electrolysis facilities. In contrast, the invention proposes to operate at high voltages, which was previously not viable. As a consequence of operating at high voltages, a reduction of currents may be permitted, which in turn enables cheaper components and less conductive material, such as copper, to be used, which is advantageous.

[0052] In embodiments of the invention, said converter controller arrangement is configured to switch between said at least two converter control modes based on an external control signal.

[0053] An external control signal may, for example, be established via input from a human operator. Or, for example, established by an automated centralized control system which manages renewable energy.

[0054] In embodiments of the invention, said converter controller arrangement is configured to switch between said at least two converter control modes based on monitoring said AC power of said AC electrical power grid.

[0055] Such monitoring may for example be performed by the converter controller arrangement, by a separate monitoring device, by a human operator, or by an automated centralized control system.

[0056] By monitoring the AC power, the switching between the at least two converter control modes may be optimized, which is advantageous. Particularly, the standby mode may be utilized to ensure that production can be paused and unpaused efficiently based on variations of the available AC power. [0057] In embodiments of the invention, said converter controller arrangement is configured to switch between said at least two converter control modes based on power production of an associated renewable energy production facility.

[0058] For example, an associated renewable energy production facility (such as a wind farm or a solar power facility) may be directly connected to the electrolysis power converter system without connection to a domestic power grid. In such systems, the available power is particularly susceptible to variations. Accordingly, having a standby mode which can be utilized based on such local power production, being prone to variations, is advantageous.

[0059] In embodiments of the invention, said converter controller arrangement is configured to automatically switch from said standby mode to said production mode based on presence of excess power in said AC electrical power grid.

[0060] In embodiments of the inventio, said standby mode comprises an active standby mode and a passive standby mode.

[0061] The active standby mode is advantageous in that it has the effect, that energy produced by a fuel cell supplied with hydrogen produced by the electrolytic cell arrangement, e.g. via a hydrogen storage, can be controlled so as to perform auxiliary services to the utility grid (or other loads) connected to the power converter system. This control is possible by controlling the active switching units of the semiconductor switches. During the passive standby mode, the active switch units of the semiconductors switches are opened and thus only the passive rectification units may conduct a minor current i.e. no current is conducted to the electrolytic cell and the electrolytic process is terminated.

[0062] In embodiments of the invention, said converter controller arrangement is configured to automatically switch from said production mode to said standby mode based on absence of excess power in said AC electrical power grid or based on an alarm signal. [0063] This is advantageous in that if something is wrong in process equipment such as the conversion equipment, electrolytic cell arrangement, hydrogen storage and compressing parts, etc. the production of hydrogen can be terminated to avoid potential hazardous situations.

[0064] In embodiments of the invention, said converter controller arrangement is configured to automatically switch from said standby mode to said production mode based on presence of excess renewable power in said AC electrical power grid.

[0065] Renewable power may be generated by wind turbines, photovoltaic systems, wave energy systems, thermic systems, etc.

[0066] In embodiments of the invention, said converter controller arrangement is configured to automatically switch from said production mode to said standby mode based on absence of excess renewable power in said AC electrical power grid.

[0067] Automatic switching from production mode to standby mode and vice versa may be based on monitoring the AC power of the AC electrical power grid.

[0068] Whether excess power qualifies as being renewable power may for example be determined by monitoring phase, voltage, frequency, or current on the AC electrical power grid. Any of these parameters may for example be compared to one or more associated thresholds, and upon exceeding such a threshold, a switch of modes is performed.

[0069] In embodiments of the invention, said converter controller arrangement is configured to switch between said at least two converter control modes based on available gas storage.

[0070] An electrolyser facility may typically be associated with a gas storage, in which hydrogen, oxygen, or another gas produced by the electrolysis facility can be stored. If there is little capacity in the gas storage to receive any more gas, i.e. there is little to none available gas storage, switching to the standby mode may be advantageous, since energy may be better used elsewhere on, e.g., the AC electrical power grid. Hence, using the available gas storage as basis for switching between the standby mode and the production mode, may ensure that production is efficiently paused and unpaused depending on the actual capacity for receiving such gases by a storage, which is advantageous.

[0071] In embodiments of the invention, said converter controller arrangement is configured to switch between said at least two converter control modes based on gas demand.

[0072] The demand for gas, such as hydrogen, oxygen, or another gas produced by the electrolysis facility, may vary over time depending on the needs of the industries consuming these gases, such as the transport sector. Hence, using such a gas demand as basis for switching between the standby mode and the production mode, may ensure that production is efficiently paused and unpaused depending on the needs of the relevant industries, which is advantageous.

[0073] In embodiments of the invention, said at least two converter control modes comprise a termination mode comprising controlling said AC-to-DC conversion arrangement to terminate conversion of said AC power into said DC power.

[0074] A termination mode may be considered as turning off the AC-to-DC conversion arrangement. In particular, the termination mode is distinct from the standby mode, in which some conversion of power still occurs.

[0075] In embodiments of the invention, said at least two converter control modes comprises a temporary pre-charge mode, wherein said converter controller arrangement is configured to control said AC-to-DC conversion arrangement according to said temporary pre-charge mode between controlling said AC-to-DC conversion arrangement according to said termination mode and said standby mode.

[0076] In embodiments of the invention, said temporary pre-charge mode comprises said DC voltage being lower than when controlling said AC-to-DC conversion arrangement according to said standby mode. [0077] In embodiments of the invention, said temporary pre-charge mode comprises gradually increasing said DC power prior to controlling said AC-to-DC conversion arrangement according to said standby mode.

[0078] By having a temporary pre-charge mode, a gradual/stepwise current or voltage build-up may be ensured to reduce risk of damaging parts of the electrolysis power converter system, such as the electrolytic cell arrangement or the AC-to-DC conversion arrangement, which is advantageous. An example of gradually increasing the DC power is gradually increasing the DC voltage.

[0079] In embodiments of the invention, said at least two converter control modes comprises a heat-up mode, wherein said converter controller arrangement increases the DC voltage so as to increase power input and thereby electrically heating the electrolytic cell.

[0080] The heat-up mode is advantageous in that the temperature of the electrolytic cell stack increases faster. Since hydrogen production is better when the electrolytic cell stack is warm compared to when it is cold, this leads to bringing the hydrogen production faster to higher / maximum production. More specific, the heat-up mode may be implemented by temporary, during start-up of the hydrogen production process, overload to conversion arrangement to generate heat warming up the electrolytic cell stacks.

[0081] In embodiments of the invention, said electrolysis power converter system comprises pre-charge circuit system arranged between said AC-to-DC switch converter and said AC electrical power grid, wherein said pre-charge circuit system is configured to implement said temporary pre-charge mode.

[0082] To implement the temporary pre-charge mode, a pre-charge circuit system may be integrated into the electrolysis power converter system. In practice, to implement the pre-charge mode, a pre-charge circuit system capable of gradually increasing the DC power may be used. [0083] A pre-charge circuit system may be considered as part of the AC-to-DC conversion arrangement, as it affects the provided DC voltage, given a particular AC voltage.

[0084] The converter controller arrangement may be configured to control the precharge circuit, such that the temporary pre-charge mode can be controlled via the converter controller arrangement.

[0085] The pre-charge circuit system may be considered as part of the AC-to-DC conversion arrangement. In other words, in some embodiments, said AC-to-DC conversion arrangement comprises said pre-charge circuit system.

[0086] In embodiments of the invention, a first pre-charge circuit path of said two pre-charge circuit system comprises a dampening element and a decoupling switch, wherein said first pre-charge circuit path is configured to provide AC power to said AC-to-DC switch converter during said temporary pre-charge mode.

[0087] In embodiments of the invention, said temporary pre-charge mode terminates when the DC voltage in said AC-to-DC conversion arrangement reaches a predefined pre-charge threshold or when a predetermined period of time has passed.

[0088] The pre-charge threshold may be between 50% and 90% of the final (unregulated) C voltage

[0089] The predetermined period of time may be between 1 and 10 seconds.

[0090] In embodiments of the invention, a second pre-charge circuit path of said two pre-charge circuit system comprises a circuit breaker configured to couple said AC electrical power grid to said AC-to-DC switch converter during said standby mode and during said production mode.

[0091] In embodiments of the invention, said first pre-charge circuit path and said second pre-charge circuit path parallelly couple said AC-to-DC switch converter to said AC electrical power grid. [0092] One or more switches and a dampening element may be a simple and efficient way of facilitating the temporary pre-charge mode, which is advantageous.

[0093] In particular, such a system enables a gradually increased said DC power or DC voltage despite potential limitations of the AC-to-DC switch converter, which may, at least in some embodiments, have a minimum DC voltage set by the AC voltage of the AC electrical power grid, which could otherwise prohibit such a gradual increase.

[0094] The invention relates to an electrolysis power converter system specified in any of the paragraphs 10-93 implementing a method specified in any of paragraphs 95-112.

[0095] An aspect of the invention relates to a method for controlling an AC-to-DC conversion arrangement to provide DC power to an electrolytic cell arrangement, wherein said electrolytic cell arrangement is configured to use electrical energy to decompose water into hydrogen and oxygen and is associated with an electrolysis voltage which represents a lower threshold above which substantial decomposition of water occurs in electrolytic cells of said electrolytic cell arrangement, wherein said method comprising the steps of: converting AC power from an AC electrical power grid into DC power to said electrolytic cell arrangement via an AC-to-DC switch converter of said AC-to- DC conversion arrangement, wherein said AC-to-DC switch converter converts an AC voltage of said AC power to establish said DC power; controlling said AC-to-DC conversion arrangement according to a standby mode in which said AC power is converted such that a DC voltage of said DC power is smaller than said electrolysis voltage; and controlling said AC-to-DC conversion arrangement according to a production mode in which said AC power is converted such that said DC voltage is larger than said electrolysis voltage. [0096] Generally, methods according to embodiments of the invention may have any of the same or similar advantages as electrolysis power converter systems according to embodiments of the invention.

[0097] The steps of controlling the AC-to-DC conversion arrangement according to the standby mode and controlling the AC-to-DC conversion arrangement according to the production mode may be performed by a converter controller arrangement configured to control the AC-to-DC conversion arrangement according to these modes, optionally referred to as converter control modes. In other words, in some embodiments, the converter controller arrangement is configured to control the AC- to-DC conversion arrangement to at least two converter controller modes and, optionally, configured to switch between the at least two converter control modes, wherein the at least two converter control modes comprise the standby mode and the production mode.

[0098] In embodiments of the invention, said method comprises a step of controlling said AC-to-DC conversion arrangement according to a temporary pre-charge mode in which said DC voltage is lower than during said step of controlling said AC-to-DC conversion arrangement according to said standby mode.

[0099] In embodiments of the invention, said method comprises a step of controlling said AC-to-DC conversion arrangement according to a temporary pre-charge mode in which said DC power is gradually increased.

[0100] In more specific examples, the method comprises a step of controlling said AC-to-DC conversion arrangement according to a temporary pre-charge mode in which said DC voltage is gradually increased, for example while said DC voltage is lower than the electrolysis voltage, for example until performing the step of controlling the AC-to-DC conversion arrangement according to said standby mode.

[0101] In embodiments of the invention, said method for controlling said AC-to-DC conversion arrangement to provide said DC power to said electrolytic cell arrangement is a method for controlling said AC-to-DC conversion arrangement to provide said DC power to said electrolytic cell arrangement to commence operation of said electrolytic cell arrangement, wherein said method comprises a sequential execution of: said step of controlling said AC-to-DC conversion arrangement according to said temporary pre-charge mode; then said step of controlling said AC-to-DC conversion arrangement according to said standby mode; and then said step of controlling said AC-to-DC conversion arrangement according to said production mode.

[0102] A sequential execution may be understood as performing one method step at a time in a sequence. In embodiments of the invention, firstly, the step of controlling the AC-to-DC conversion arrangement according to the temporary pre-charge mode is performed. Then, secondly, the step of controlling the AC-to-DC conversion arrangement according to the standby mode is performed. And then, thirdly, the step of controlling the AC-to-DC conversion arrangement according to the production mode is performed.

[0103] This sequential execution of steps may ensure that operation of the electrolytic cell arrangement is smoothly, efficiently, and safely commenced, which is advantageous.

[0104] In embodiments of the invention, said AC-to-DC conversion arrangement is controlled according to said standby mode in at least 5 seconds, for example in at least 30 seconds, such as at least 300 seconds.

[0105] By controlling the AC-to-DC conversion arrangement according to the standby mode in minimum durations as exemplified above, the advantages offered by systems and methods according to the invention, such as minimizing the temperature reduction, may have a particularly significant impact, which is advantageous.

[0106] In embodiments of the invention, said standby mode is an unregulated standby mode in which said DC voltage is unregulated. [0107] In embodiments of the invention, said production mode is a regulated production mode in which said DC voltage is regulated.

[0108] In embodiments of the invention, said production mode is a regulated production mode in which said DC voltage is regulated based on an amount of excess power in said AC electrical power grid.

[0109] In embodiments of the invention, said production mode is a regulated production mode in which said DC voltage is regulated based on an amount of excess renewable power in said AC electrical power grid.

[0110] In embodiments of the invention, said production mode is a regulated production mode in which said DC voltage is regulated based on available gas storage.

[0111] In embodiments of the invention, said production mode is a regulated production mode in which said DC voltage is regulated based on available gas demand.

[0112] Regulating the DC voltage based on excess (renewable) power in said AC electrical power grid, available gas storage, available gas demand, or any combination thereof may ensure that production in the regulated production mode is optimized with respect to these parameters. In particular, regulation based on excess renewable power in said AC electrical power grid is advantageous, since this may ensures that an electrolysis facility may be operated according to the core principles of power-to-x, in which primarily renewable electrical energy should be converted into a form which can be used later, which is advantageous.

[0113] The invention relates to a method specified in any of paragraphs 94-112 implemented in an electrolysis power converter system specified in any of the paragraphs 10-93. The drawings

[0114] Various embodiments of the invention will in the following be described with reference to the drawings where fig. 1 illustrates an electrolysis power converter system according to an embodiment of the invention, fig. 2 illustrates an example of an AC-to-DC switch converter, fig. 3 illustrates method steps according to an embodiment of the invention, fig. 4 illustrates DC voltages in the standby mode and in the production mode according to an embodiment of the invention, fig. 5 illustrates an electrolysis power converter system comprising a transformer and a DC-to-DC converter according to an embodiment of the invention, fig. 6 illustrates an electrolysis power converter system comprising a pre-charge circuit system according to an embodiment of the invention, fig. 7 illustrates DC voltages to commence operation of an electrolytic cell arrangement according to an embodiment of the invention, fig. 8 illustrates a qualitative example of current as a function of voltage in relation to electrolysis of water, fig. 9 illustrates a qualitative example of voltage as a function of current density with a stacked plot of various contribution to that voltage in relation to electrolysis of water, and fig. 10 illustrates a qualitative example of temperature-dependency of voltages in relation to electrolysis of water. Detailed description

[0115] Fig. 1 illustrates an electrolysis power converter system 1 according to an embodiment of the invention. The electrolysis power converter system 1 comprises an electrolytic cell arrangement 2, am AC-to-DC conversion arrangement 4, and a converter controller arrangement 7.

[0116] The electrolytic cell arrangement 2 comprises a plurality of electrolytic cells 3. Some of the electrolytic cells 3 are connected in series, and several series of electrolytic cells 3 are connected in parallel (as seen from the AC-to-DC conversion arrangement). Collectively, the electrolytic cell arrangement 2 is associated with an electrolysis voltage, which represents a lower threshold above which substantial decomposition of water occurs in the electrolytic cells 3 of the electrolytic cell arrangement 2. The electrolysis voltage is typically determined by the minimum voltage required across an individual electrolytic cell 3 to perform decomposition within that cell 3 in combination with the number of electrolytic cells 3 which are connected in series. In the exemplary illustration of fig. 1, electrolytic cells 3 are connected in series in two groups, which in turn are connected in parallel to the AC- to-DC conversion arrangement.

[0117] The embodiment further comprises an AC-to-DC conversion arrangement 4 which is coupled to an AC electrical power grid 5 and the electrolysis cell arrangement 2. The AC-to-DC conversion arrangement 4 is configured to convert AC power from AC electrical power grid 5 into DC power to the electrolytic cell arrangement 2. In this embodiment, this conversion is performed using an AC-to-DC switch converter 6 which converts an AC voltage of the AC power to establish the DC power, and namely a DC voltage of this DC power. The AC-to-DC switch converter 6 used in this particular embodiment is illustrated in further detail in figure 2.

[0118] Even though referred to as a power grid 5, the supply / load connected to the conversion arrangement 4 could also be a generator of a wind turbine or any other type of renewable energy generator. The energy production from a wind turbine, may be partly or fully be utilized as supply for the electrolytic cell arrangement via the conversion arrangement 4. The wind turbine may be one wind turbine of a wind farm or it may be a wind turbine operated in island mode.

[0119] The AC-to-DC conversion arrangement 4 and particularly the AC-to-DC switch converter 6 is controlled by the converter controller arrangement 7. This converter controller arrangement 7 is in particular configured to control the DC voltage which the AC-to-DC conversion arrangement outputs to the electrolytic cell arrangement 2 via their mutual connection. This control is implemented via two distinct converter control modes upon which the converter controller arrangement 7 controls the AC-to-DC conversion arrangement 4. That is, either the converter controller arrangement 7 controls the AC-to-DC conversion arrangement 4 according to one of these two converter control modes or the converter controller arrangement 7 controls the AC-to-DC conversion arrangement 4 according to the other of these two converter control modes. The converter controller arrangement 7 is further configured to switch between these two control modes, and thereby switch the AC-to-DC conversion arrangement 4 from being controlled according to one of the modes into being controlled according to one other of the converter control modes.

[0120] The converter control modes comprises a standby mode, and a production mode. In the standby mode, the AC-to-DC conversion arrangement 4 is controlled to convert the AC power such that the DC voltage is smaller than the electrolysis voltage. In contrast, in the production mode, the AC-to-DC conversion arrangement 4 is controlled to convert the AC power such that the DC voltage is larger than the electrolysis voltage. Accordingly, in the production mode, water is decomposed into hydrogen and oxygen in electrolysis cells 3 of the electrolytic cell arrangement 2. And in the standby mode, water is not substantially decomposed into hydrogen and oxygen. However, AC power may still converted into DC voltage, and resultingly, the temperature reduction in the electrolytic cells may be reduced, in comparison with a complete termination of conversion of AC power.

[0121] It should be noted, that the standby mode may include both a passive standby mode and an active standby mode. During the passive standby mode, the switching unit 11 of the switching arrangements 10 (the semiconductor switches) are opened and thus, only a minor current is conducted by the passive rectification unit 12 thereof. During the active standby mode, the controllability of the semiconductor switches 10 may be utilized to control electricity generated by a fuel cell provided with hydrogen from a storage or from the hydrogen electrolytic cell arrangement 2. In this way, the conversion arrangement 4 may perform auxiliary services to / for the connected utility grid, generator or the like. Such auxiliary services may include voltage control, reactive power supply or other grid support, etc. to a utility grid, magnetizing a connected generator, charging of connected energy storage, supplying power to auxiliary loads, etc. depending on what is connected to the conversion arrangement 4.

[0122] As a result of controlling the AC-to-DC conversion arrangement according to these modes, it is thus possible to efficiently resume production in the electrolysis power converter system 1 after a temporary pause of production facilitated by the standby mode. The AC power can rapidly be converted to a DC voltage which lies above the electrolysis voltage instead of below the DC voltage, and the temperature reduction, as a consequence of not being in the production mode, is minimized.

[0123] In this embodiment, the converter controller arrangement 7 is configured to switch between the two converter control modes based on input from a human operator. In practice, such an input may for example be facilitated by a physical switch toggled by the human operator, of via a computer-implemented command supplied via a programming interface operatable by the human operator.

[0124] Fig. 2 illustrates an example of an AC-to-DC switch converter 6. The illustrated switch converter 6 is used in the electrolysis power converter system 1 illustrated in fig. 1. As illustrated, the switch converter 6 comprises a DC link capacitor and a filter inductor on each of the AC phase input 9a-9c.

[0125] The switch converter 6 comprises three converter legs 25a-c, each comprising two switching arrangements 10, and each of the converter legs 25a-c connected to two DC connections 8a, 8b and an individual AC phase input 9a, 9b, 9c. The switch converter 6 thus has three AC phase inputs 9a, 9b, 9c, each corresponding to a phase received by an AC electrical power grid. The two DC connections 8a, 8b correspond to a positive DC connection 8a and a negative DC connection 8b, through which DC power, hereunder a DC voltage, can be provided to, e.g., an electrolytic cell arrangement 2. Each of these AC phase inputs 9a, 9b, 9c are then connected to the positive DC connection 8a through a separate switching arrangement 10, and to the negative DC connection 8b through another separate switching arrangement 10. Each of the switching arrangement comprises an active switch unit 11 and a passive rectification unit (of opposite directionality) 12.

[0126] Each of the separate inverter legs 25a, 25b, 25c are configured to separately and selectively couple the separate AC phase inputs 9a, 9b, 9c to either the positive DC connection or the negative DC connection to establish a DC voltage accordingly. This coupling is performed by providing control signals (also referred to as gate signals) to the active switch units 11 of the different switching arrangements. In general, positive voltages of the AC phase inputs 9a, 9b, 9c are coupled to the positive DC connection 8a, and negative voltages of the AC phase inputs 9a, 9b, 9c are coupled to the negative DC connection 8b. The duration and/or the magnitude of the control/gate signals may be regulated to control the established DC power and/or DC voltage. In practice, a converter controller arrangement provides the individual control signals to the active switch units 10. Via these control signals, the converter control arrangement is then capable of, for example, controlling the AC-to-DC conversion arrangement (via controlling the AC-to-DC switch converter 6) according to a standby mode or according to a production mode. In particular, control signals are still provided to the active switch units 11 while control according to the standby mode is performed, but in such a manner that the established DC voltage is below the electrolysis voltage.

[0127] In the exemplary AC-to-DC switch converter of fig. 2, the active switch units are IGBTs, but in other embodiments, other types of active switch units may be used, such as thyristors. Note also that, in other embodiments, switching arrangements may not necessarily have a passive rectification unit.

[0128] Fig. 3 illustrates method steps S1-S3 according to an embodiment of the invention. The embodiment relates to a method for controlling an AC-to-DC conversion arrangement to provide DC power to an electrolytic cell arrangement. The electrolytic cell arrangement is configured to use electrical energy to decompose water into hydrogen and oxygen, and is associated with an electrolysis voltage which represents a lower threshold above which substantial decomposition of water occurs in electrolytic cells of said electrolytic cell arrangement.

[0129] The method comprises a step SI of converting AC power from an AC electrical power grid into DC power to the electrolytic cell arrangement via an AC-to- DC switch converter of the AC-to-DC conversion arrangement. The AC-to-DC switch converter converts an AC voltage of the AC power to establish the DC power. Accordingly, a DC voltage of that DC power is also typically established.

[0130] The method further comprises a step S2 of controlling the AC-to-DC conversion arrangement according to a standby mode in which the AC power is converted such that the DC voltage of the DC power is smaller than the electrolysis voltage. Nevertheless, note that the AC-to-DC conversion arrangement does perform conversion of AC power into DC power while being controlled according to the standby mode. Accordingly, a current is typically conducted through the electrolytic cell arrangement while the AC-to-DC conversion arrangement is being controlled according to the standby mode.

[0131] The method further comprises a step S3 of controlling the AC-to-DC conversion arrangement according to a production mode, in which the AC power is converted such that the DC voltage is larger than the electrolysis voltage. Accordingly, substantial decomposition of water occurs in electrolytic cells of the electrolytic cell arrangement while the AC-to-DC conversion arrangement is being controlling according to the production mode.

[0132] Note that embodiments of the invention are not limited to a particular sequence in which method steps are to be performed, although the AC-to-DC conversion arrangement is typically only controlled according to one of the converter control modes (standby mode and production mode) at a time.

[0133] Note, further, that embodiments of the invention may have additional method steps, for example a step of controlling the AC-to-DC conversion arrangement according to a temporary pre-charge mode, in which the DC power is gradually increased. In these embodiments, the AC-to-DC conversion arrangement is typically first controlled according to the temporary pre-charge mode, then according to the standby mode, and then according to the production mode. Such a sequence of steps may be employed to commence operation of an electrolysis facility. Further, prior to performing these steps, the AC-to-DC conversion arrangement may be controlled according to a termination mode, in which conversion of AC power into DC power is terminated.

[0134] Fig. 4 illustrates DC voltages 26 in the standby mode 15a-c and in the production mode 16a-c according to an embodiment of the invention. The illustration is shown via a coordinate system having a vertical voltage axis 14 and a horizontal time axis 13.

[0135] In more detail, the figure provides an example of how a converter controller arrangement may switch between the converter control modes during operation of an electrolysis power converter system, and how the DC voltage may develop during the converter control modes and during switching between these modes.

[0136] In the coordinate system, the electrolysis voltage 17 is indicated by a horizontal line. A DC voltage lying below this line in the coordinate system does thus not result in substantial decomposition of water, whereas a DC voltage lying above this line in the coordinate system does result in substantial decomposition of water.

[0137] The durations in which the converter controller arrangement controls (the AC-to-DC conversion arrangement) according to the standby mode 15a-c and according to the production mode 16a-c are indicated by horizontal arrows below the time axis 13. In the exemplary illustration, there are three periods of control according to the standby mode 15a-c, which are referred to as a first standby mode 15a, a second standby mode 15b, and a third standby mode 15c. Further, there are three periods of control according to the production mode 16a-c, which are referred to as a first production mode 16a, a second production mode 16b, and a third production mode 16c. [0138] In the illustration, the DC voltage 26 is also shown relatively to the electrolysis voltage 17 and in association with each of the production modes 16a-c and the standby modes 15a-c. Vertical dotted lines indicate the DC voltage which each of the modes 15a-c,16a-c are associated with.

[0139] Initially, at the left-most point on the illustrated time axis 13, the converter controller arrangement controls according to the (first) standby mode 15a. Accordingly, the DC voltage 26 at this time is below the electrolysis voltage 17. Coinciding with the initiation of the (first) production mode 16a, the DC voltage 26 rises above the electrolysis voltage 17. The DC voltage 26 then remains at a constant value above the electrolysis while control according to the (first) production mode 16a is being performed. As this production mode 16a stops, the (second) standby mode 15b is initiated. Once again, while in this standby mode 15b, the electrolysis voltage 26 lies below the electrolysis voltage 17. Upon initiation of the (second) production mode 16b, the DC voltage 26 increases above the electrolysis voltage 17. While performing control according to this production mode 16b, the DC voltage 26 is continuously being adjusted in level. Resultingly, the actual electrolysis production is also continuously being adjusted. This production mode 16b stops as the (third) standby mode 15c is started, which once again lowers the DC voltage 26 below the electrolysis voltage 17. The DC voltage 26 remains below the electrolysis voltage 17 until the (third) production mode 16c is commenced. While in this production mode 16c, the DC voltage 26 lies above the electrolysis voltage while being adjusted in a stepwise manner.

[0140] Fig. 4 illustrates how the DC voltage 26 may behave when switching between converter control modes 15a-c,16a-c. But in addition, fig. 4 also illustrates various examples of how the DC voltage behaves while in the production mode 16a-c.

[0141] Note that the electrolysis voltage 17 may typically be temperature dependent, which results in the electrolysis voltage not being constant in time, as otherwise indicated in fig. 4. Some embodiments comprise compensating control which takes into account such temperature dependency while controlling the DC voltage. [0142] Fig. 5 illustrates an electrolysis power converter system 1 comprising a transformer 20 and a DC-to-DC converter 21 according to an embodiment of the invention.

[0143] In comparison with the illustration in fig. 1, the AC-to-DC conversion arrangement 4 comprises a transformer 20 and a DC-to-DC converter 21, in addition to the AC-to-DC switch converter 6. The transformer 20 is connected between the AC electrical power grid 5 and the AC-to-DC switch converter 6. It converts the AC power of the AC electrical power grid 5, for example from a first AC voltage (of the AC electrical power grid 5) to a second AC voltage (provided, e.g., to the AC-to-DC switch converter 6). Such a conversion may for example ensure that the AC voltage is at a level which is efficiently and safely useable by the AC-to-DC switch converter 6.

[0144] In this particular embodiment, the transformer 20 is a passive transformer with three-phase input and three-phase output. However, note that embodiments are not limited to such transformers.

[0145] The DC-to-DC converter 21 is connected between the AC-to-DC switch converter 6 and the electrolytic cell arrangement 2. It converts the DC power of the AC-to-DC switch converter 6, for example from a first DC voltage (of the AC-to-DC switch converter 6) into a second DC voltage (provided to the electrolytic cell arrangement 2).

[0146] Note that, in this embodiment, the converter controller arrangement 7 is connected to both the AC-to-DC switch converter 6 and the DC-to-DC converter 21. The converter controller arrangement 7 is configured to control the AC-to-DC conversion arrangement 4 according to at least two converter control mode by controlling the DC-to-DC converter 21, and, optionally, also by controlling the AC- to-DC switch converter 6. For example, the converter controller arrangement may alter a conversion ratio of the DC-to-DC converter 21 to switch between the at least two converter control modes.

[0147] Generally, in embodiments of the invention, the converter controller arrangement 7 may control an AC-to-AC converter, the AC-to-DC switch converter, a DC-to-DC converter, or any combination thereof, to facilitate the at least two converter control modes and to facilitate switching between these at least two converter control modes.

[0148] Fig. 6 illustrates an electrolysis power converter system 1 comprising a precharge circuit system 22 according to an embodiment of the invention. Such a precharge circuit system serves as one approach for implementing a temporary pre-charge mode in practice. This mode is particularly useful for commencing production in the electrolysis power converter system 1. Note that fig. 7 illustrates an example of DC voltages to commence operation of an electrolytic cell arrangement, which may, for example, be realized using the embodiment illustrated in fig. 6.

[0149] The illustration of fig. 6 further shows a transformer 20 connected to the AC electrical power grid 5. In the illustrated embodiment, the AC power provided by the AC electrical power grid 5 is three-phased electrical power which is drawn, in the illustration, via single lines for the sake of simplicity.

[0150] The pre-charge circuit system 22 comprises a first pre-charge circuit path 23a and a second pre-charge circuit path 23b. These paths 23 a, 23b parallelly connect the AC-to-DC switch converter 6 to the AC electrical power grid 5 (via the transformer 20).

[0151] The first pre-charge circuit path 23a comprises a decoupling switch 27, and a dampening element 24. The second pre-charge circuit path 23b comprises a circuit breaker 43. The circuit breaker 43 may be a standalone breaker such as a main breaker capable of allowing current to be conducted by the second pre-charge circuit path 23b.

[0152] The circuit breaker 43 permits decoupling of AC power from the AC-to-DC switch converter 6, such that AC power cannot be provided to the AC-to-DC switch converter 6 without passing through a dampening element 24. Instead, it may pass through the first pre-charge circuit path 23a, comprising the dampening element 24.

[0153] When the decoupling switch 27 is in a coupling state (i.e. closed state) while the circuit breaker 43 is in a decoupling state (i.e. open state) the dampening element reduces the AC voltage provided to the AC-to-DC switch converter 6. The dampening element 24 may for example be a resistive element, for example a voltage-controlled resistor.

[0154] Whenever, the AC-to-DC conversion arrangement 4 is controlled according to the standby mode, the circuit breaker 43 is in a coupling state (i.e. closed state), such that the AC voltage provided to the AC-to-DC switch converter 6 is not reduced via the dampening element 24. Here, it does not typically matter whether the decoupling switch 27 is open or closed.

[0155] Prior to commencing operation of the electrolysis power converter system, the decoupling switch 27 and circuit breaker 43 are typically in an open state, such that no AC power is provided to the AC-to-DC switch converter. Then, the decoupling switch 27 is closed to provide a voltage which is reduced due to the dampening element. Eventually, the circuit breaker 43is closed to provide an unreduced AC voltage to the AC-to-DC switch converter. When the circuit breaker 43 is closed, the decoupling switch 27 is opened. At this point, the converter controller arrangement 7 can switch between the standby mode and the production mode based on, e.g., the circumstances, such as based on excess power in the AC electrical power grid 5.

[0156] In practice, the circuit elements of the pre-charge circuit system are controlled by the converter controller arrangement 7. For example, the converter controller arrangement 7 may provide control signals/gate signals to the decoupling switch 27 and circuit breaker 43.

[0157] Fig. 7 illustrates DC voltages 26 to commence operation of an electrolytic cell arrangement according to an embodiment of the invention.

[0158] The example in fig. 7 shows how a converter controller arrangement may switch between converter control modes 15,16,18,19 to provide DC power 26 to the electrolytic cell arrangement to commence operation of the electrolytic cell arrangement. Commencing operation as illustrated in fig. 7 may for example be performed in practice by the embodiment illustrated in fig. 6. [0159] As previously, the illustration of fig. 7 is shown via a coordinate system having a vertical voltage axis 14 and a horizontal time axis 13. And again, the electrolysis voltage 17 is indicated by a horizontal line, where a DC voltage lying below this line does not result in substantial decomposition of water, whereas a DC voltage lying above this line in the coordinate system does result in substantial decomposition of water (in electrolysis cells of the electrolytic cell arrangement).

[0160] The durations in which the converter controller arrangement controls according to the various modes 15,16,18,19 are indicated by horizontal arrows below the time axis 13. Further, the extend of these modes is also indicated by the vertical dotted lines and is their resulting DC voltage 26 shown relatively to the electrolysis voltage 17.

[0161] Initially, the converter controller arrangement controls according to the termination mode 19, in which no AC power is converted into DC power. Accordingly, the DC voltage 26 illustrated initially in fig. 7 is approximately zero.

[0162] Next, the converter controller arrangement controls according to the temporary pre-charge mode 18. During the temporary pre-charge mode 18, the DC voltage 26 is lower than when controlling the AC-to-DC conversion arrangement according to said standby mode.

[0163] In the illustrated example, the DC voltage 26 is gradually increased, for example via active control of the AC power provided to the AC-to-DC switch converter.

[0164] The DC voltage 26 keeps rising until the AC-to-DC conversion arrangement is controlled according to the standby mode 15. Here, the DC voltage 26 lies at a constant level below the electrolysis voltage 17.

[0165] After a duration in the standby mode 15, the AC-to-DC conversion arrangement is controlled according to the production mode 16, in which AC power is converted such that the DC voltage is larger than the electrolysis voltage, and, accordingly, substantial decomposition of water occurs. [0166] The described sequence of steps in relation to commencing operation of an electrolytic cell arrangement may advantageously ensure a safe and efficient startup of an electrolysis facility.

[0167] Note that the embodiment and a pre-charge circuit system 22 of fig. 6 is merely one of several ways of implementing a pre-charge circuit in practice. In another embodiment, a thyristor-based AC-to-DC switch converter is used, which is configured to provide a gradually increasing DC voltage below the electrolysis voltage, for example as exemplified in Fig. 7. In another embodiment, the AC-to-DC conversion arrangement comprises a DC-to-DC converter and an AC-to-DC switch converter, such as an IGBT-based AC-to-DC switch boost converter, in which the DC- to-DC converter is configured to provide a gradually increasing DC voltage below the electrolysis voltage, for example as exemplified in Fig. 7.

[0168] Fig. 8 illustrates a qualitative example of current as a function of voltage in relation to electrolysis of water.

[0169] The illustration is shown via a coordinate system having a vertical current axis 28 and a horizontal voltage axis 14. It represents a qualitative example of current which runs through an electrolysis cell as a function of the applied voltage.

[0170] The exemplary illustration shows a total current 29, which is based on a background current 30 and an electrolysis current 31. The electrolysis current 31 is responsible for decomposition of water, whereas the background current 30 does not result in substantial decomposition of water.

[0171] The background current 30 increases linearly, starting from the origin of the coordinate system. The electrolysis current 31 also increases linearly, but only above a lower voltage threshold. This voltage threshold may correspond to the reversible voltage, also referred to as the reversible potential. The slope of the electrolysis current is far greater than the slope of the background current.

[0172] The figure exemplifies the concept of a lower threshold above which substantial decomposition of water occurs in an electrolytic cell, and further exemplifies how small currents may nevertheless be conducted below this threshold, which is of relevance to the invention.

[0173] Fig. 9 illustrates a qualitative example of voltage as a function of current density with a stacked plot of various contribution to that voltage in relation to electrolysis of water.

[0174] The illustration is shown via a coordinate system having a vertical voltage axis 14 and a horizontal current density axis 32.

[0175] The illustration is a stacked plot in the sense that various contributions to a given voltage are combined in the plot such that the vertical positions of the curves corresponds to sums. The actual individual contributions may be obtained by evaluating the differences between the plotted curves.

[0176] The uppermost curve (electrode ohmic loss 37) thus corresponds to a sum of all the contributions to the voltage. This upper curve may then be used as model upon which current and/or voltage may be regulated during production in electrolysis facilities.

[0177] The bottommost contribution to the voltage is the reversible potential 33, which is largely independent on current density. The next illustrated contribution to the voltage is the electrolyte ohmic loss 34 The next illustrated contribution to the voltage is the oxygen overvoltage 35. The next illustrated contribution to the voltage is the hydrogen overvoltage 36. The final illustrated contribution to the voltage is the electrode ohmic loss 37. Overvoltage may be considered as a loss due to resistance by the chemical reaction rate. Ohmic loss is mainly caused by electric resistance of the electrolyte and the electrical resistance of circuity such as electrodes. Both overvoltages and ohmic losses tend to increase with increasing current density.

[0178] In embodiments of the invention, the DC voltage preferably lies in the linear regime of the upper curve (electrode ohmic loss 37) while controlling according to the production mode. This regime may correspond to voltages from approximately 1.8 V to approximately 2.2 V. [0179] Fig. 10 illustrates a qualitative example of temperature-dependency of voltages in relation to electrolysis of water.

[0180] The illustration is shown via a coordinate system having a vertical voltage axis 14 and a horizontal temperature axis 38.

[0181] The reversible potential 33 tends to decrease with increasing temperature 38. It defines a region of no substantial decomposition 40 which lies below the reversible potential 33.

[0182] Since electrolysis is an endothermic process, the decomposition of water typically absorbs thermal energy. However, the difference between the reversible voltage 33 and the actual voltage across an electrolysis cell is converted into heat. The voltage at which the heat absorbed by the decomposition equals the heat generated due to the voltage difference may be referred to as the thermoneutral voltage 39. At this voltage, there is no heat generation or absorption, at least ideally. The thermoneutral voltage 39 and the reversible voltage 33 determines an endothermic region 41 in which the electrolysis absorbs more heat than it generates. An exothermic region lies above the thermoneutral voltage 39 in which the electrolysis generates more heat than it absorbs.

[0183] Often, it is preferable to perform industrial electrolysis at or near the thermoneutral voltage 39, to ensure heat balance.

[0184] The figure exemplifies that electrolysis is temperature dependent, which is of relevance to the invention.

[0185] From the above, it is now clear that the invention relates to an electrolysis power converter system and a method for controlling an AC-to-DC conversion arrangement to provide power to an electrolytic cell arrangement. By having complementary standby and production modes, by which an AC-to-DC conversion arrangement can be controlled, electrolysis facilities may be efficiently operated under the variable circumstances provided by renewable energy production. [0186] The invention has been exemplified above with the purpose of illustration rather than limitation with reference to specific examples of methods and electrolysis power converter systems. Details such as a specific method and system structures have been provided in order to understand embodiments of the invention. Note that detailed descriptions of well-known systems, devices, circuits, and methods have been omitted so as to not obscure the description of the invention with unnecessary details. It should be understood that the invention is not limited to the particular examples described above and a person skilled in the art can also implement the invention in other embodiments without these specific details. As such, the invention may be designed and altered in a multitude of varieties within the scope of the invention as specified in the claims.

LIST OF REFERENCE SIGNS:

1 electrolysis power converter system

2 electrolytic cell arrangement

3 electrolytic cell

4 AC-to-DC conversion arrangement

5 AC electrical power grid

6 AC-to-DC switch converter

7 converter controller arrangement

8 DC connection

9 AC phase input

10 switching arrangement

11 active switch unit

12 passive rectification unit

13 time axis

14 voltage axis

15 standby mode

16 production mode

17 el ectroly si s voltage

18 temporary pre-charge mode

19 termination mode

20 transformer

21 DC-to-DC converter

22 pre-charge circuit system

23 pre-charge circuit path

24 dampening element

25 converter leg

26 DC voltage

27 decoupling switch

28 current axis

29 total current

30 background current

31 electrolysis current

32 current density axis

33 reversible potential

34 electrolyte ohmic loss

35 oxygen overvoltage

36 hydrogen overvoltage

37 electrode ohmic loss

38 temperature axis

39 thermoneutral voltage

40 region of no substantial decomposition

41 endothermi c regi on

42 exothermic region

43 circuit breaker

SI -S3 method steps