Control of ac grid connected DC loads

Abstract

According to the invention it is provided a power supply system comprising: a DC load having a positive terminal, a negative terminal and an inherent capacitance; a power converter being connected to a grid, and configured to control power supply of the DC load through a DC link; the DC link comprising: a first solid-state circuit breaker comprising a solid-state switch being configured to interrupt power supply from the power converter to the DC load; a first inductor having a first terminal connected to the solid-state switch and a second terminal connected to one of the positive terminal and negative terminal of the DC load; at least one freewheeling diode for providing a current path from the other of the positive terminal and negative terminal of the DC load to the first terminal of the inductor. The invention also relates to a renewable energy power installation and a method.

Description

[0001] CONTROL OF AC GRID CONNECTED DC LOADS

[0002] FIELD OF THE INVENTION

[0003] The invention relates to power supply systems for control of grid connected DC loads, and particular to power supply systems comprising DC loads being connected to an AC grid through a power converter.

[0004] BACKGROUND OF THE INVENTION

[0005] Renewable energy sources, such as, e.g., wind turbines may be connected to an electrical AC grid in order to power the grid. Power converters may be utilized in this regard, e.g., in order to convert AC power generated by a wind turbine to a DC voltage to again be converted to an AC voltage that is adapted to the prevailing grid voltage. Similarly, power converters may be utilized to connect other types of renewable power sources, such as solar power systems, hydroelectric power systems, etc. to a grid for powering the grid.

[0006] Use of renewable energy may also be further facilitated using power-to-X conversion and reconversion, where DC loads, such as, e.g., electrolyzer stacks, such as hydrogen electrolyzer stacks, may be utilized to store energy being produced by renewable energy sources for later use. This may be used as a means to alleviate impacts of fluctuating power levels being provided by a renewable energy power source. For example, hydrogen electrolyzer stacks may be connected to a grid through a power converter that converts the grid voltage to a DC voltage powering the hydrogen electrolyzer stack, where the power consumption, and thereby the load that the electrolyzer stack imposes on the grid, is controlled by controlling the DC voltage. The hydrogen electrolyzer stack may be configured to generate hydrogen, where the hydrogen may be used in a fuel cell to generate electricity when further electrical power is needed.

[0007] Systems of this kind may operate at high powers, and may thereby require protective circuits that are capable of interrupting high currents. SUMMARY OF THE INVENTION

[0008] It is an object of the invention to provide a power supply system that may provide for short circuit protection of the DC load. It is a further object of the invention to provide a power supply system that may provide for pre-charging of the DC load.

[0009] According to a first aspect of the invention, it is provided a power supply system comprising: a DC load having a positive terminal, a negative terminal and an inherent capacitance; a power converter being connected to a grid, and configured to control power supply of the DC load through a DC link; the DC link comprising: a first solid-state circuit breaker comprising a solid-state switch being configured to interrupt power supply from the power converter to the DC load; a first inductor having a first terminal connected to the solid-state switch and a second terminal connected to one of the positive terminal and negative terminal of the DC load; at least one freewheeling diode for providing a current path from the other of the positive terminal and negative terminal of the DC load to the first terminal of the inductor.

[0010] There exist different types of renewable energy power systems where, for example, a renewable power source such as a wind turbine generator, or other type of renewable energy source, may be configured to be set up, e.g., as a local electrical grid, i.e., operate in an island, off-grid, mode, where the generated power may be utilized to power loads connected to the system. Such loads may, e.g., comprise DC loads, and a particular kind of DC load that may be utilized in this regard are hydrogen electrolyzer stacks that may be utilized to generate hydrogen gas from electrical power being provided by the renewable energy source. With regard to DC loads, e.g. of the exemplified kind, such loads may also be connected to any kind of AC grid from which power may be drawn to produce hydrogen gas. It is also to be noted that other types of DC loads may be used in similar manners.

[0011] Further with regard in particular to off-grid operation, the one or more renewable energy sources will provide energy, although not at a constant rate but, in general, at a varying power rate due to, inter alia, fluctuations in wind, sun etc. This will provide a varying power supply, where DC loads such as hydrogen electrolyzer stacks can be utilized to alleviate such differences by increasing or decreasing power being consumed from the grid.

[0012] The present invention relates to power supply systems comprising DC loads having an inherent capacitance, and thereby a need for being pre-charged upon startup to prevent excess inrush currents that may be damaging to components of the system and / or trip protective circuits. DC loads are power supplied by a DC voltage, where the DC voltage, in general, is generated from an AC grid voltage through the use of a power converter that converts the AC voltage to a suitable DC voltage. The invention relates to power converters that output voltages, such as, e.g., around 800Vdc to several kVdc depending on the configuration of the power converter and the DC load. The DC loads may further have a high power rating, with the consequence that the DC loads are power supplied by high currents. A high power rating is understood herein as a power of several hundreds kW to multiple of MW.

[0013] This gives rise to challenges when it comes to protection aspects of the system. For example, in case the DC current becomes too high, a need for interrupting, i.e. , breaking, the current may arise to prevent that system components from being damaged. The interruption of a DC current, however, differs substantially from the interruption of an AC current, where the current oftentimes can be interrupted at a voltage zero-crossing, and thereby at a moment in time when instant power is comparatively low. DC currents, on the other hand, do not exhibit this behavior, and hence the full DC current needs to be interrupted. Although there exist DC circuit breakers, such circuit breakers may not exist for the very high currents that may prevail in systems of the disclosed kind. Alternatively, such DC circuit breakers, in case they do exist, may be very costly. As an alternative, a solid-state switch comprising suitable protection may be utilized instead.

[0014] According to the invention, it is provided a power supply system for powering a grid connected DC load having an inherent capacitance that allows for interruption of a DC current, and which also provides for further features such as pre-charging of a DC load having an inherent capacitance in a manner that provides a solution that is both cost-effective and beneficial from a space requirement point of view. The DC load is connected to an AC grid through a power converter that is configured to control power supply of the DC load through a DC link by controlling the voltage on the DC link.

[0015] The DC link comprises a first solid-state circuit breaker comprising a solid-state switch being configured to interrupt power supply from the power converter to the DC load. According to the invention, the power supply system is also adapted for further features such as pre-charging, which are made possible through the use of a first inductor having a first terminal connected to the solid-state switch and a second terminal connected to one of the positive terminal and negative terminal of the DC load. Also, at least one freewheeling diode provides for a current path from the other of the positive terminal and negative terminal of the DC load to the first terminal of the inductor, where the particular design of the power system may depend on, e.g., whether one or more DC loads are connected as will be explained further below.

[0016] Furthermore, as was mentioned, the invention relates to high power systems, and the power converter may be configured to control power supply of the DC load through an output of a DC voltage on the DC link being in the order of at least 650 Vdc, or at least 1100 Vdc, or a voltage exceeding a voltage in the interval 1-10 kVdc. Furthermore, the invention relates to DC loads having a power rating of at least 300 kW, or at least 1 MW. It is to be understood that the DC loads may have a considerable higher power rating than the stated examples.

[0017] As was mentioned above, there exists a need for short circuit protection of AC grid connected DC loads, where such protection, as mentioned, may exhibit difficulties, e.g., in regard of non-existence of suitable DC circuit breakers being capable of interrupting a desired current. According to the invention, it is provided a solution where a solid-state circuit breaker may be utilized in this regard, and where the particular circuitry may allow for a cost-efficient and space efficient solution. Solidstate circuit breakers in comparison to passive components are much faster, and are thereby able to interrupt the current much faster, and are also less bulky in design.

[0018] The solid-state circuit breaker may be used to rapidly interrupt excess currents, to thereby protect system components.

[0019] According to aspects of the invention, the power supply system further comprises means for detecting an overcurrent being supplied to the DC load, the power supply system further being configured to interrupt the overcurrent through opening of the solid-state switch when detecting the overcurrent. That is, the power supply system may be configured to detect when an overcurrent in the power supply of the DC load occurs, where this overcurrent may result for various reasons.

[0020] For example, an overcurrent may occur due to a short-circuit in the DC cables powering the DC load. The overcurrent may also be caused, e.g., by short-circuits in the power converter, and also, e.g., in case someone accidentally leaves a tool in an unfavorable location where the tool may cause short circuits by interconnecting, e.g. busbars or other cabling. Overcurrents may also arise, e.g. due to a fault in the DC load itself. When an overcurrent is detected, the power supply system may rapidly interrupt the overcurrent through the use of the solid- state switch by opening the solid-state switch. This provides for a cost-efficient solution for interrupting the current being provided to the DC load.

[0021] According to aspects of the invention the solid-state switch is any one from: IGBT transistor, IGCT transistor, thyristor. Hence, the invention may be adapted for use with different solid-state switch technologies, where the technology being considered to be most appropriate may be selected for a particular design, although it is to be noted that in particular the use of freewheeling diode according to the invention allows for use of IGBT or IGCT transistors, which in turn may provide for a cost effective and less bulky design in comparison to thyristor designs.

[0022] According to aspects of the invention, the power supply system further comprises means configured to, when applying a voltage to the DC load from a disconnected state, pre-charge the inherent capacitance of the DC load utilizing the solid-state switch of the solid-state circuit breaker, wherein, during pre-charging, the solid- state circuit breaker is configured to successively increase the voltage over the DC load through switching, e.g. PWM switching, utilizing the solid-state switch, wherein, during pre-charging, the solid-state switch, the inductor, the at least one freewheeling diode and inherent capacitance of the DC load is operating as a buck converter.

[0023] That is, the power supply system may provide for not only circuit breaking capabilities, but also for pre-charging of the inherent capacitance of the DC load through switching using the solid-state switch. The freewheeling diode provides for a current path during the periods that when the switch is open during switching to thereby prevent excess voltages from arising.

[0024] Pre-charging in itself is a problem that needs to properly be accounted for when it comes to DC loads having an inherent capacitance. For example, there exists various different manufacturers of DC loads, such as hydrogen electrolyzer stacks, where each design may have its own inherent capacitance and thereby also particular need for proper pre-charging. The invention provides for a solution where pre-charging may be adapted to suit the particular DC load being utilized irrespective of type of super capacitance, and other particular features of the DC load.

[0025] The invention hence has the advantage that the power supply system may be used for various different loads, such as various different kinds of electrolyzer stacks, including alkaline electrolyzer stacks, PEM electrolyzer stacks, solid oxide electrolyzer stacks, etc., while simultaneously the invention does not require a separate pre-charging circuit as is otherwise required according to the prior art.

[0026] According to aspects of the invention, the solid-state circuit breaker, the inductor, the at least one freewheeling diode and the inherent capacitance of the DC load, as was mentioned, form a buck converter connecting the DC load to the power converter. This in particular provides for a power supply system design that may be highly beneficial in regard of, in particular, pre-charging.

[0027] According to aspects of the invention, the solid-state circuit breaker further comprises a protection circuit connecting an output of the solid-state switch to the input of the solid-state switch, the protection circuit being configured to provide for power dissipation when interrupting a current flow from the power converter to the DC load using the solid-state switch. As was mentioned above, DC currents do not exhibit regular zero-crossings, and hence does not exhibit natural power reductions in the transmitted power that may be used to facilitate interruption of the current. The current to be interrupted may be very high, e.g. causing an arc, and protective components connecting the input to the output may be used to provide for power dissipation during the interruption to prevent solid-state switch from single-handedly handle the excess power in order to thereby protect the solid-state switch.

[0028] According to aspects of the invention, the protection circuit connecting the output of the solid-state switch to the input of the solid-state switch comprises a diode providing a current path from the output of the solid-state switch to the input of the solid-state switch, and a resistor-capacitor circuit, and / or a resistor-inductor circuit connecting the output of the solid-state switch to the input of the solid-state switch. In this way the resistor capacitor circuit and / or resistor inductor circuit may provide for the required power dissipation in a manner known per se.

[0029] According to aspects of the invention, the DC load is an electrolyzer stack, and the power converter is configured to control power supply to the electrolyzer stack to thereby control hydrogen production. As discussed above, the invention is suitable for use with this type of DC load that often times inherently exhibit large super capacitor capacitances, and where the invention is suitable for use irrespective of a particular type of electrolyzer stack being used. For example, the electrolyzer stack by be anyone from the types mentioned above, where, e.g., pre-charging can be suitably adapter to the particular load being used.

[0030] According to aspects of the invention, the power converter is configured to supply power to at least two DC loads connected in series. Hence, the power converter may be configured to power more than one DC load, where the DC loads, such as hydrogen electrolyzer stacks may be connected in series. This may be the case, for example, in case the power converter is designed for a particular output voltage, and the DC loads, in turn, are designed for lower DC voltages, so that a desired overall total DC load voltage level is obtained when connecting the DC loads in series. The power supply system may also be configured to pre-charge the series DC loads.

[0031] According to aspects of the invention, the power converter is configured to supply power to two DC loads having an inherent grounding, inherent capacitances and being connected in series. For example, the DC loads may be alkaline electrolyzer stacks that, due to the alkaline solution being utilized in such solutions, may exhibit very low resistance to grounding, and hence be inherently grounded. This also means that there will be a grounding in between the DC loads with a further consequence that the overall DC link may need to be designed with a positive and a negative voltage, i.e., the power converter will output a DC voltage having a positive rail (voltage) and a negative, below zero, rail (voltage) to allow for grounding between the DC loads.

[0032] Furthermore in this regard, the first solid-state circuit breaker may be arranged on the positive rail and configured to control interruption of power to an upstream DC load of the two DC loads. Similar to above, the first inductor may have a first terminal connected to the solid-state switch and a second terminal connected to the positive terminal of the upstream DC load. In addition, a second solid-state circuit breaker having a second solid-state switch may be arranged on the negative rail, and be configured to interrupt power of the downstream DC load. Further, a second inductor may have a first terminal connected to the second solid-state switch, and a second terminal connected to the negative terminal of the downstream DC load. Finally, at least one freewheeling diode is configured to provide a current path from the negative terminal of the downstream DC load to the positive terminal of the upstream DC load through the first and the second inductor.

[0033] This provides for a solution where the two solid-state switches may provide for short-circuit protection of the two DC loads, and where the inductors and in particular the diodes provide for a free-wheeling path for the current in case the solid-state switches are open, such as, e.g., when a fault has occurred, or periodically during pre-charging, so that thereby the arising of excess voltages can be avoided.

[0034] According to aspects of the invention, the DC loads are electrolyzer stacks having an inherent grounding, such as alkaline electrolyzer stacks, wherein the at least one freewheeling diode comprises a first freewheeling diode connecting the first terminal of the second inductor to ground and to a second freewheeling diode providing a connection from the first freewheeling diode to the first terminal of the first inductor. Hence, the same functionality and advantages as has been described is provided for this configuration, too.

[0035] According to aspects of the invention, the grid is an off-grid network, where the power in the off-grid network for powering the electrolyzer stack is provided by at least one renewable energy source, such as at least one wind turbine.

[0036] Consequently, the invention may be used in systems where, e.g., available power may be constantly varying.

[0037] According to a further aspect of the invention, it is provided a renewable energy power installation comprising a renewable energy power source, such as a wind turbine generator, configured to supply power to a grid and a power supply system according to any of the aspects described above, where the renewable energy power installation exhibits the same advantages as has been described above.

[0038] According to aspects of the invention, the electrolyzer stack may form part of a hybrid power plant comprising, e.g., wind turbines and / or solar panels as power sources and additional electrolyzer stacks.

[0039] Further advantageous aspects of the power supply system according to the present invention and further advantages with the aspects of the invention emerge from the detailed description.

[0040] BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Aspects of the invention will be described, by way of example only, with reference to the drawings, in which:

[0042] Fig. 1 illustrates an example of a power supply system according to prior art;

[0043] Fig. 2 illustrates an example of a power supply system according to aspects of the invention; Figs. 3A-3C illustrates examples of solid-state circuit breakers utilizing different types of solid-state switch technologies;

[0044] Fig. 4 illustrates another example of a power supply system according to aspects of the invention.

[0045] DETAILED DESCRIPTION

[0046] The invention will be exemplified in the following for DC loads being constituted by one or more hydrogen electrolyzer stacks comprising an inherent capacitance, such as, for example, alkaline hydrogen electrolyzer stacks. It is to be noted that the invention is equally applicable for any kind of DC loads comprising an inherent capacitance.

[0047] Fig. 1 illustrates a prior art power supply system exemplifying a connection of an electrolyzer stack 101 , such as an alkaline electrolyzer, being connected to a medium voltage AC grid 110 through a line side converter LSC 102. The line side converter 102 is basically an AC to DC converter, e.g., a full bridge converter, for converting the low voltage AC voltage to DC voltage for powering the electrolyzer stack 101 .

[0048] Furthermore, according to the illustrated example, a DC circuit breaker 103 is provided for allowing interruption of power provided to the electrolyzer stack 101 . However, it is to be noted in this regard, as was also mentioned above, that DC circuit breakers may not even be commercially available at the high current ratings that may prevail in systems of the kind illustrated in Fig. 1 , where the required current breaking capability may be in the order of 5 kA or even higher currents to be interrupted.

[0049] DC circuit breakers differ from AC circuit breakers in the regard that the current is more difficult to interrupt. When it comes to AC circuit breakers, as was mentioned, the current can be interrupted at zero-crossings, which substantially facilitates the current interrupting possibilities. There are no such zero-crossings for DC circuit breakers, thereby imposing other requirements on such circuit breakers, and additional components may be required to accomplish the desired interrupting of the current in case this is deemed necessary. But again, such DC circuit breakers may not even be commercially available, and even if so, they may be bulky and space consuming by design, in addition to being costly, in particular in relation to AC circuit breakers.

[0050] According to existing solutions, the load current in the DC link, i.e. the current drawn by the DC load, may be measured, and when it is detected that load current is exceeding some set limit, the PWM (pulse width modulation) of the line side converter may be stopped, and the system may also possibly be disconnected from the grid. If the short circuit level is very high on the DC side, the excess current will also be reflected on the AC side, and, e.g., AC circuit breakers may trip. It is also possible to trigger a tripping of the AC circuit breakers when the detected current exceeds some predetermined limit. Hence the layers of protection may rely mainly on the power electronics itself and AC circuit breakers.

[0051] Fig.1 also illustrates that the connection to the AC grid 110 is carried out through switch gear 109, a medium voltage to low voltage transformer 108, and hence a transformation to a voltage being adapted to the operating voltage of the line side converter 102 and / or the electrolyzer stack 101 . The figure also illustrates an AC circuit breaker 107, which hence may be used in place of a DC circuit breaker to interrupt power supply to the electrolyzer stack 101 , or trip as explained, at least in situations where an overcurrent on the DC side reflected by an overcurrent also on the AC side of the line side converter 102.

[0052] The figure also illustrates a fuse 104 provided for circuit protection, but, similar to DC circuit breakers, such fuses may be costly, and therefore use of such fuses may desirably be avoided. Fuses being capable of handling very high currents may also be difficult to even obtain. The use of such fuses may therefore be limited or avoided. The figure also illustrates a diode 111 to prevent that energy stored in the super capacitor of the electrolyzer stack does not flow back to the converter in case of a fault.

[0053] In addition, Fig. 1 illustrates pre-charging means to provide for pre-charging of the electrolyzer stack. This is illustrated by a resistor 105 to provide for resistive precharging of the electrolyzer stack, where the resistor 105 can be connected by means of a switch 106. Once pre-charging of the electrolyzer is completed, the resistor 105 is disconnected by opening the switch 106, and the DC circuit breaker 103 that this utilized during normal operation is instead closed. The resistor 105 may be adapted to the particular load that is to be pre-charged. However, again, the solution according to Fig. 1 may currently not be available for high-current operation due to lack of suitable DC circuit breakers, and DC circuit breaker 103 may therefore instead consist of another type switch being capable of handling the required currents. However, resistive pre-charging requires a DC circuit breaker. Also, resistive pre-charging gives rise to power dissipation in the resistor and hence losses and undesired heat.

[0054] According to the invention, therefore, a solid-state circuit breaker is used. An example of a power supply system providing power to a DC load such as an electrolyzer stack 201 according to the invention is shown in Fig. 2. The illustrated solution also shows pre-charging capabilities, which will be explained below. The electrolyzer stack 201 may be a hydrogen electrolyzer stack being utilized, e.g., in a local grid I off-grid solution, where, e.g., a wind turbine or one or more other renewable energy power sources may be utilized to power the grid, and where the electrolyzer stack 201 may be used to consume power produced by such renewable energy power sources. In general with regard to such hydrogen electrolyzer stacks 201 , it can be noted that they, inter alia, may comprise an anode, a membrane and a cathode. The hydrogen electrolyzer stack splits water molecules into hydrogen H2 and oxygen O2 through the use of a DC current passing through the stack 201 , where the generated oxygen may be ventilated away at the anode, and the generated hydrogen passes through the membrane to be collected at the cathode and stored, e.g., in tanks for subsequent use. The hydrogen electrolyzer stack 201 consumes power in dependence of the DC voltage level, and hence the power consumption may be controlled through a control of the DC voltage level powering the electrolyzer stack 201 . This hence provides a means for adapting power consumption to fluctuations in the power level produced, e.g., by a wind turbine so that the hydrogen production can be adapted to the currently available power. The power rating of the DC loads may, e.g., be 300 kW or more, such as 1 MW or more.

[0055] The hydrogen electrolyzer stack 201 is hence a power-to-hydrogen gas storage device that can be utilised to convert power generated by, e.g., a fluctuating power source such as a wind turbine when power is available, to allow the energy stored in the generated hydrogen gas to be subsequently used as desired, e.g. by generating electric energy through a fuel cell. As was stated, electrolyzer stacks may inherently by design comprise a large super capacitor that needs to be precharged up on power up of the system in order to avoid excess inrush currents.

[0056] The power supply system 200 that is illustrated in Fig. 2 is a similar to Fig. 1 in terms of connection to AC side components, and is hence connected to medium voltage AC grid 210 through appropriate switchgear 209, transformer 208 and AC circuit breaker 207. It is to be noted, however, that the grid connection may be of various other different designs. The power supply system 200, also similar to Fig.

[0057] 1 , comprises a line side converter 202 that provides power to the electrolyzer stack 201 , and in particular is configured to control power supply to the electrolyzer stack to thereby control hydrogen production.

[0058] Furthermore, in place of a DC circuit breaker, a solid-state circuit breaker 203 comprising a solid-state switch 204 is used instead. The power supply system also comprises a freewheeling diode 205 and an inductor 206.

[0059] The use of a solid-state switch 204 as in Fig. 2 provides for more efficient means to interrupt the current supplied to the electrolyzer stack 201 , and thereby also the power supplied to the electrolyzer stack 201 . Solid state circuit switches are much faster, and are also capable of interrupting the current much faster than DC circuit breakers, and are also much less space consuming. In particular, solid-state switches may be capable of handling the high currents that, e.g. a short circuit may give rise to. The use of a solid-state switch may therefore allow for a costefficient and space efficient solution. Solid-state switches, in comparison to passive components, are much faster, able to interrupt the current much faster and less bulky in design.

[0060] As was discussed, pre-charging is also a problem that needs to be solved when the DC loads comprise inherent capacitances, where such inherent capacitances may function as super capacitors. This is oftentimes the case with electrolyzer stacks. According to the invention, such pre-charging may be accomplished through suitable control of the solid-state switch 204, which may successively increase the voltage over the electrolyzer stack 201 through switching, such as PWM switching, of the solid-state switch 204. However, in order to avoid excess voltages from arising, a freewheeling diode 205 provides a current path from the negative terminal of the electrolyzer stack 201 to the output of the solid-state switch 204. This allows the current to circulate through the diode, the inductor 206 and the electrolyzer stack 201 during the pre-charging switching intervals when the solid-state circuit breaker is open, so that thereby a large voltage overshoot can be prevented. Furthermore, the inductor 206 smoothens the switched output voltage from the solid-state switch 204 during pre-charging. In essence, the solid- state switch 204, freewheeling diode 205 and inductor 206 acts as a buck converter, that is, a step-down converter that decreases the voltage in relation to the upstream side of the solid-state switch 204. This is also schematically illustrated in the figure.

[0061] The power supply system according to the invention thereby provides a solution that is capable of interrupting, e.g., an overcurrent in a more efficient manner, e.g. in terms of cost and bulkiness, in relation to a conventional DC circuit breaker. Furthermore, the invention provides for a system that simultaneously is capable of providing the often times necessary pre-charging of the DC load. It is also to be noted that the design allows the current to circulate through the diode, the inductor 206 and the electrolyzer stack 201 also in situations when the current needs to be interrupted by the solid-state circuit breaker, e.g., due to a fault so that large voltage overshoots can be avoided in situations of this kind also.

[0062] With further regard to Fig. 2, it can be seen that the solid-state circuit breaker 203, in addition to the solid-state switch 204, also comprises passive components connecting the output of the solid-state switch 204 to the input of the solid-state switch 204. Such passive components may comprise an RC circuit and / or an RL circuit and / or other components. According to the illustrated example, the protective circuits comprise an RC circuit in parallel with a diode.

[0063] The protective components are used to provide for a current path from the downstream side of the solid-state switch 204 to the upstream side of the solid- state switch 204, and also provides means for dissipating power when current is interrupted. The reason for this is that when the solid-state switch 204 interrupts an arc there will occur a power dissipation since the current to be interrupted may be very high. The passive components that connect the input to the output may be used to provide for such power dissipation during the interruption to thereby protect the solid-state switch by ensuring that the switch single-handedly need not handle the excess power.

[0064] Furthermore, in Fig. 2 the solid-state switch of the solid-state circuit breaker is schematically indicated by the symbol of a switch. Figs. 3A-C illustrate examples of actual realization of such a solid-state circuit breaker. In Fig. 3A, the solid-state switch is illustrated as being realized by a thyristor, in Fig. 3B the solid-state switch is illustrated as being realized by an IGBT transistor, and in Fig. 3C the solid-state switch is illustrated as being realized by an IGCT transistor. The figures also illustrate protective components, such as passive components, according to the above. However, it is to be understood that the solid-state switch may also be of other designs than of thyristor, IGBT transistor or IGCT design, and that the particular aspects illustrated in Fig. 4 are for illustration purposes only. Various other different designs may be used as is known by the skilled person in the art.

[0065] Fig. 4 illustrates another example of a power supply system 400 according to aspects of the invention. Similar to the above, the power supply system 400 comprises a power converter 402 which may be connected to an AC grid in the same way as in Fig. 2. However, in difference to the embodiment illustrated above, two electrolyzer stacks 401 A and 401 B are now connected in series. There may be various reasons for series connecting electrolyzer stacks. For example, the voltage output by the line side converter 402 may be higher than the operating voltage of the electrolyzer stack. The use of series connected electrolyzer stacks also allows that the line side converter 402 may output a higher DC voltage than would otherwise be possible. For example, the line side converter may be a converter that is utilized also for other applications, such as for connecting a wind turbine to a grid, where the wind turbine may output higher voltages than a single electrolyzer stack may be able to handle. In this way, e.g., the same type of line side converter may be utilized for different applications.

[0066] With regard to the upstream electrolyzer stack 401 A, a solid-state circuit breaker 403A is provided to account for interruption of the current being supplied to the electrolyzer stack 401 A in case this is deemed necessary. Furthermore, freewheeling diodes, in this case two freewheeling diodes 405A and 405B provide for a current path from the negative terminal of electrolyzer stack 401 A (through electrolyzer stack 401 B to the upstream side of the electrolyzer stack 401 A). Furthermore, the electrolyzer stacks according to the present example are alkaline electrolyzer stacks which inherently by design exhibit a very low resistance to ground due to the alkaline solution flowing through the stacks. For this reason the electrolyzer stacks may in fact be grounded, with the result that in reality there will be a grounding in between the series connected electrolyzer stacks 401 A, 401 B as is illustrated in the figure by dotted lines 420. This also means that the electrolyzer stack 401 A will have an operating voltage between a positive voltage, which in this case represents the positive voltage output by the line side converter 402, and ground. The positive voltage, e.g. z UDC, may further be half the overall voltage UDC output by the line side converter 402.

[0067] Similarly, the electrolyzer stack 401 B will operate between ground (i.e. 0 V) and a corresponding negative voltage - Vz UDC. The line side converter may hence be controlled to output an overall DC voltage UDC being centered around zero volts.

[0068] As can be seen in the figure, a second solid-state circuit breaker 403B comprising a solid-state switch 404B is utilized to provide for interruption of the current powering the electrolyzer stack 401 B. The DC circuit breaker 403B is also used in pre-charging of the electrolyzer stack 401 B in the same manner as has been described above where, also similar to the above, an inductor 406B is used to smoothen the PWM voltage resulting from switching of the solid-state switch 404B.

[0069] The solution according to Fig. 4 also allows that the same solution may be utilized for various different types of DC loads, requiring different methods for precharging, since the switching using the solid-state circuit breaker may be adapted to the particular DC load that currently is being used.

[0070] The grid 120 may, as discussed, be a local grid but may, in principle, be any type of grid and may further form part of or be connected to a high voltage wide area transmission line network, e.g. via further transformers.

[0071] The present invention is not limited to the above-described aspects. Instead, the present invention relates to, and encompasses all different aspects being included within the scope of the independent claims.

Claims

CLAIMS1 . A power supply system comprising: a DC load having a positive terminal, a negative terminal and an inherent capacitance; a power converter being connected to a grid, and configured to control power supply of the DC load through a DC link; the DC link comprising: a first solid-state circuit breaker comprising a solid-state switch being configured to interrupt power supply from the power converter to the DC load; a first inductor having a first terminal connected to the solid-state switch and a second terminal connected to one of the positive terminal and negative terminal of the DC load; at least one freewheeling diode for providing a current path from the other of the positive terminal and negative terminal of the DC load to the first terminal of the inductor.

2. A power supply system according to claim 1 , wherein: the solid-state switch is any one from: IGBT transistor, IGCT transistor, thyristor.

3. A power supply system according to any one of the claims 1 -2, wherein the solid-state circuit breaker, the inductor, the at least one freewheeling diode and the inherent capacitance of the DC load form a buck converter connecting the DC load to the power converter.

4. A power supply system according to any one of the claims 1 -3, further comprising: means for detecting an overcurrent being supplied to the DC load, the power supply system further being configured to interrupt the overcurrent through opening of the solid-state switch when detecting theovercurrent.

5. A power supply system according to any one of the claims 1 -4, wherein the solid-state circuit breaker further comprises a protection circuit connecting an output of the solid-state switch to the input of the solid-state switch, the protection circuit being configured to provide for power dissipation when interrupting a current flow from the power converter to the DC load using the solid-state switch.

6. A power supply system according to claim 5, wherein the protection circuit connecting the output of the solid-state switch to the input of the solid-state switch comprises a diode providing a current path from the output of the solid-state switch to the input of the solid-state switch, and a resistorcapacitor circuit, and / or a resistor-inductor circuit connecting the output of the solid-state switch to the input of the solid-state switch.

7. A power supply system according to any one of the claims 1 -6, further comprising: means configured to, when applying a voltage to the DC load from a disconnected state, pre-charge the inherent capacitance of the DC load utilizing the solid-state switch of the solid-state circuit breaker, wherein, during pre-charging, the solid-state circuit breaker is configured to successively increase the voltage over the DC load through switching utilizing the solid-state switch, wherein, during pre-charging, the solid-state switch, the inductor, the at least one freewheeling diode and inherent capacitance of the DC load is operating as a buck converter.

8. A power supply system according to any one of the claims 1 -7, wherein the DC load is an electrolyzer stack, and wherein the power converter is configured to control power supply to the electrolyzer stack to thereby control hydrogen production.

9. A power supply system according to any one of the claims 1 -8, wherein the power converter is configured to supply power to at least two DC loads connected in series, and the power supply system being configured to precharge.

10. A power supply system according to any one of the claims 1-9, wherein the power converter is configured to supply power to two DC loads having an inherent grounding, inherent capacitances and being connected in series, wherein: the first solid-state circuit breaker is arranged on the positive rail and configured to control interruption of power to an upstream DC load of the two DC loads; the first inductor has a first terminal connected to the solid-state switch and a second terminal connected to the positive terminal of the upstream DC load; a second solid-state circuit breaker having a second solid-state switch is arranged on the negative rail and configured to interrupt power of the downstream DC load; a second inductor having a first terminal connected to the second solid-state switch, and a second terminal connected to the negative terminal of the downstream DC load; at least one freewheeling diode providing a current path from the negative terminal of the downstream DC load to the positive terminal of the upstream DC load through the first and the second inductor.11 . A power supply system according to claim 10, wherein: the DC loads are electrolyzer stacks having an inherent grounding, such as alkaline electrolyzer stacks, wherein the at least one freewheeling diode comprises a first freewheeling diode connecting the first terminal of the second inductor to ground and to a second freewheeling diode providing a connection from the first freewheeling diode to the first terminal of the firstinductor.

12. A power supply system according to any one of the claims 1 -11 , the grid being an off-grid network, the power in the off-grid network for powering the electrolyzer stack being provided by at least one renewable energy source, such as at least one wind turbine.

13. A renewable energy power installation comprising a renewable power energy source and a power supply system according to any one of the claims 1 -12.

14. A method for controlling a current in a power supply system comprising: a DC load having a positive terminal, a negative terminal and an inherent capacitance; a power converter being connected to a grid, and configured to control power supply of the DC load through a DC link; the DC link comprising: a first solid-state circuit breaker comprising a solid-state switch being configured to interrupt power supply from the power converter to the DC load; a first inductor having a first terminal connected to the solid-state switch and a second terminal connected to one of the positive terminal and negative terminal of the DC load; the method comprising: providing a current path from the other of the positive terminal and negative terminal of the DC load to the first terminal of the inductor.

15. A method according to claim 14, further comprising, when applying a voltage to the DC load from a disconnected state: pre-charge the inherent capacitance of the DC load utilizing the solid-state switch of the solid-state circuit breaker, wherein, during pre-charging, the solid-state circuit breaker successively increases the voltage over the DC load through switching utilizing the solid-state switch.

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