Method and device for controlling the potential position of DC conductors

Abstract

The application relates to a method and a device (10) for regulating the potential position of a first and a second DC conductor (DC+, DC-), wherein the DC conductors (DC+, DC-) are provided for supplying an electrolyzer (14) with electrical power and the electrical power can be transferred from an AC network (12) to the DC conductors (DC+, DC-) via a rectifier (20), wherein in a first operating mode the potential position of the first and the second DC conductor (DC+, DC-) is regulated such that both DC conductors (DC+, DC-) have the same polarity with respect to ground potential (GND). The application further relates to a DC network (30) comprising an electrolyzer (14), such a device (10) and the first and second DC conductors (DC+, DC-).

Description

Technical FieldThe application relates to a method and a device for regulating the potential position of direct current (DC) conductors and to a DC grid which has such a device and an electrolyser.BackgroundElectrolyzers can be supplied with electrical DC power via DC connections for carrying out the electrolysis. The electrical DC power can be obtained from an AC grid and made available to the electrolyser via a rectifier and via DC conductors. The potential difference between the DC conductors corresponds to the DC voltage present at the electrolyser. The potential position of the respective electrical potentials of the DC conductors with respect to ground depends here on the type of the AC network and its grounding and on the type of the rectifier and in particular on whether it has a galvanic isolation or not.It has been found during the operation of electrolysers that an increasing current flows out of the electrolyser to earth over the course of the operating period, i.e. the insulation resistance of the electrolyser decreases. For safety reasons, the operation of the electrolyser must be interrupted if the insulation resistance falls below a predetermined limit value or the earth current exceeds a corresponding limit value. Therefore, there is a desire to counteract such a gradual reduction in the insulation resistance already during ongoing operation of the electrolyser, or to increase the insulation resistance again above the limit value without a maintenance intervention having to take place, for example by replacing parts of the electrolyser.OverviewA first and a second DC conductor are provided for supplying an electrolyser with electrical power. The electrical power can be transmitted to the DC conductors via a rectifier from an AC mains (alternating current / alternating voltage).In a method for regulating the potential position of the first and second DC conductors, the potential position of the first and second DC conductors is regulated in a first operating mode such that both DC conductors have the same polarity with respect to ground potential.A device for regulating the potential position of the first and second DC conductors is designed to regulate the potential position of the first and second DC conductors in a first operating mode such that both DC conductors have the same polarity with respect to ground potential.The described regulation makes it possible to specifically influence the potential position of the DC conductors. In particular, the described control can reduce a so-called potential-induced degradation (PID) in the case of electrolyzers, in particular in the case of electrolyzers which have a plurality of electrolysis stacks. In the PID effect, the insulation properties of the electrolyser can be impaired due to the potential position of the DC conductors by migration of ions in the electrolyser. By specifically influencing the potential position of the DC conductors, this ion migration can be reduced or reversed, and the PID effect can thus be reduced or reversed.The potential position of the DC conductors is adjusted, for example, by the power electronics of the device such that the potential of both DC conductors has the same polarity with respect to ground. This can reduce or avoid the PID effect, for example.In one embodiment of the method, the potential position in the first operating mode is regulated in such a way that a regeneration of the electrolyser is effected. Depending on the structure of the electrolyser, it may be necessary for both DC conductors to have a positive or a negative potential with respect to earth in order to effect regeneration. In one embodiment, the device controls the potential position with the corresponding polarity.During the regeneration of the electrolyser, the PID effect, in particular the undesired migration of the ions or a chemical reaction effected thereby, is at least partially reversed again. For this purpose, the potential position of the DC conductors is selected in a targeted manner such that the reversal effect occurs. In particular, the potentials of the DC conductors can be set as close as possible to one another at a minimum distance from the potential ground.In one embodiment of the method and / or of the apparatus, the first operating mode has a first sub-operating mode in which the potential difference between the first and second DC conductors is controlled such that the voltage present at the electrolyser is below a threshold voltage. In this embodiment, the voltage applied to the electrolyser is thus below the threshold voltage which can be selected, for example with regard to the characteristic curve of the electrolyser, such that the electrolyser assumes a predefined working range thereof.The threshold voltage can depend on a threshold voltage of the electrolyser and correspond in particular to the threshold voltage. The starting voltage of the electrolyser denotes that voltage from which the electrolysis process starts in the electrolyser. The threshold voltage can also be referred to as the decomposition voltage. The threshold voltage can lie in particular in a range between 10% and 50% of the threshold voltage. In this embodiment, the position of the potentials of the DC conductors is thus selected such that the voltage applied to the electrolyser is so low that the electrolyser does not carry out any electrolysis. The potential position can be regulated in such a way that other desired processes proceed at the electrolyser, for example the regeneration process, without significant electrolysis taking place during this time and the power required for this having to be provided.In an embodiment of the method and / or the device, the DC conductors are disconnected from the AC network in the first sub-operating mode. This is particularly advantageous if the AC network has a fixed earth reference, which is the case, for example, with a TN network. It can then be provided to draw the electrical power required for the regulation of the potential position from sources other than the AC grid, for example from an auxiliary power supply which provides power with a potential position of the supply voltage which is independent of the potentials of the AC grid and in particular galvanically isolated therefrom. The auxiliary power supply may draw the electrical power from, for example, a battery and / or an electrical generator. However, the auxiliary power supply can also have a galvanically separated connection to the AC grid, for example via a transformer, so that a power extraction of the auxiliary power supply from the AC grid is nevertheless not excluded.In one embodiment of the method and / or the device, the first operating mode has a second sub-operating mode. The first and the second operating mode can be assumed alternatively. In the second sub-operating mode of the first operating mode, the electrolyser is supplied with electrical power via the rectifier from the AC grid in order to carry out an electrolysis process, wherein in the second sub-operating mode a potential difference between the first and second DC conductors is regulated as a function of the electrolysis process. This second sub-operating mode can be provided in particular in order to supply the electrolyser with electrical power for carrying out the electrolysis from the AC grid. At the same time, the potential control makes it possible to reduce the PID effect and / or to regenerate the electrolyser.In embodiments, in the second sub-operating mode, the potential difference between the first and second DC conductors may depend on an operating voltage of the electrolyser. In particular, the potential difference can be regulated in such a way that a voltage is present at the electrolyser which corresponds to the operating voltage of the electrolyser. In this embodiment, the position of the potentials of the DC conductors is thus selected such that the voltage applied to the electrolyser is large enough for the electrolyser to be able to carry out the electrolysis. The operating voltage of the electrolyser is above the threshold voltage and depends, inter alia, on the type of electrolyser.In embodiments of the method and / or the apparatus, a second operating mode is provided in which the first or the second DC conductor is grounded, wherein the electrolyser is supplied with electrical power from the AC grid via the rectifier for carrying out the electrolysis process. In the second operating mode, the potential difference between the first and second DC conductors is regulated as a function of the electrolysis process. This second operating mode can be provided in particular in order to supply the electrolyser with electrical power for carrying out the electrolysis from the AC grid. In contrast to the first operating mode, in which the potentials of the two DC conductors are controlled in such a way that they have the same polarity with respect to ground, that is to say both lie on the same side of ground, it is also possible at times in the second operating mode for the potentials of the two DC conductors to have different polarity, that is to say lie on different sides of ground.In embodiments, in the second operating mode, the potential difference between the first and second DC conductors corresponds to the operating voltage of the electrolyser. In this embodiment, the position of the potentials of the DC conductors is thus selected such that the voltage applied to the electrolyser is large enough for the electrolyser to be able to carry out the electrolysis.In one embodiment of the method, the control of the potential position takes place via the clocking of a semiconductor switch which is arranged between the first DC conductor and the ground potential or between the second DC conductor and the ground potential. The clocking can be effected, for example, by the described apparatus. By connecting one of the DC conductors to ground via the semiconductor switch, which can be clocked, a so-called soft, current-carrying connection of one of the DC conductors to ground is made possible. The current can be controlled by means of the clocking and the potential position can thereby be regulated. The semiconductor switch can therefore be used to realize what is known as "soft rounding".In one embodiment of the method, a ground current flowing via the semiconductor switch is monitored. The ground current is the current that flows between the DC conductor to which the semiconductor switch is connected and ground. The monitoring takes place, for example, via the described device. The monitoring can be carried out in particular by detecting the earth current by means of a current meter A, wherein the current meter A can be arranged in the apparatus. In a computing device of the apparatus, the magnitude of the earth current can then be determined and a reaction can be triggered if the earth current becomes too large. This allows a protective concept for grounding to be realized, which can react to supercritical ground currents and thus ground faults.The respective potential of each of the two DC conductors can have a predefined minimum distance with respect to ground potential. It can be provided that this minimum distance is maintained in each of the operating modes described. The minimum distance is adjusted in particular by regulating the potential positions. Maintaining the minimum distance ensures that a regeneration progress is achieved in a predefined duration or that degeneration is counteracted to a sufficient extent.A DC grid has the electrolyser, the apparatus described and the first and second DC conductors. Optionally, the DC grid has the auxiliary power supply for supplying the device with electrical power as required. A need to supply the device with electrical power may be present, for example, when the DC grid is disconnected from the AC grid and no electrical power can be obtained from the AC grid. The requirement for electrical power for regulating the potential position can then be met, for example, by the auxiliary power supply.List of FiguresExemplary embodiments of this application are explained and described in greater detail below with reference to the figures. They show FIG. 1 schematically shows a DC grid with a device for regulating the potential position, FIG. 2 schematically shows a first exemplary embodiment of a potential layer of DC conductors with an exemplary ground current profile, FIG. 3 schematically shows a second exemplary embodiment of a potential layer of DC conductors, FIG. 4 schematically shows a third exemplary embodiment of a potential layer of DC conductors, FIG. 5 schematically shows a fourth exemplary embodiment of a potential layer of DC conductors, FIG. 6 schematically shows a fifth exemplary embodiment of a potential layer of DC conductors.The same reference numerals are used in the figures for the same or similar elements. Representations in the figures may not be to scale.DESCRIPTION OF THE FIGURESFIG. 1 schematically shows a device 10 for regulating the potential position of a first DC conductor DC+ and of a second DC conductor DC-. Via a rectifier 20, electrical AC power from an AC grid 12 can be converted into electrical DC power and transmitted to the DC conductors DC+, DC-.A DC grid 30 comprises the device 10, the DC conductors DC+, DC, DC switches 18, a fuse 24 and an electrolyser 14. The electrolyser 14 has a plurality of so-called electrolysis stacks 16 connected in series. Each electrolysis stack 16 has at least one electrolysis cell in which the desired electrolysis process takes place. Electrolysis may be, for example, electrolysis of water to produce hydrogen and oxygen. The electrolyser 14 is supplied with electrical DC power via the DC conductors DC+, DC-, which is obtained from the AC grid 12 via the rectifier 20. The AC network 12 can be a public supply network, for example. The AC grid 12 may be single-phase or polyphase. The AC grid 12 may be grounded or ungrounded.In a first operating mode, device 10 controls the potential position of the two DC conductors DC+, DC- in such a way that both have the same polarity with respect to ground potential GND. The device is therefore designed to carry out a method which, in a first operating mode, regulates the potential position of the two DC conductors DC+, DC- in such a way that both have the same polarity with respect to ground potential GND. For this purpose, the device 10 has a semiconductor switch 22, which is clocked by the device. The semiconductor switch connects either the first DC conductor DC+ or the second DC conductor DC- to ground potential GND. This connection is clocked, i.e. according to a duty cycle 26, the semiconductor switch 22 is opened and closed, so that the flowing current can be adjusted by the duty cycle 26 of the clock. The semiconductor switch 22 can be clocked in such a way that the potential position of the two DC conductors DC+, DC- with respect to one another and in relation to ground potential GND can be set in a targeted manner.At the same time, the flowing current is optionally detected via an ammeter A. The device can detect the current flowing via the semiconductor switch via the ammeter A and, if necessary, open the DC switches, for example, in order to disconnect the electrolyser 14 from the rectifier 20. The DC grid 30 is additionally protected against large ground currents via a fuse 24.A second operating mode is executed alternatively to the first operating mode. The operation of the DC network 30 may transition between the first operating mode and the second operating mode. In the second operating mode, the potentials of the first and second DC conductors DC+, DC- are regulated such that the electrolysis process can be carried out in the electrolyser 14. Thus, electrical power is transferred from the AC grid 12 via the DC conductors DC+, DC- into the electrolyser 14 for carrying out the electrolysis. The voltage which is applied to the electrolyser 14 and which corresponds to the potential difference between the first DC conductor DC+ and the second DC conductor DC- is greater than a threshold voltage of the electrolyser 14 and corresponds, for example, to an operating voltage of the electrolyser 14.If the DC grid 30 with the electrolyser 14 is operated on an AC grid 12 which is designed as a TN grid, then in the second operating mode the voltage of the first DC conductor DC+ and of the second DC conductor DC- is symmetrically connected with respect to ground potential GND if the DC grid 30 is connected via the rectifier 20 to the AC grid 12 which is designed, for example, as a TN grid, for example as the AC grid 12 which is grounded at the neutral point. As a result, the second DC conductor DC- to ground GND has a negative voltage. This negative voltage may cause negatively charged ions to drain to earth, thereby degrading the electrolyser 14 due to a PID effect and possibly greatly reducing its lifetime. If, as shown in FIG. 1, a plurality of electrolysis stacks 16 are connected in series to form a string, this effect can once again be significantly enhanced.Even if the electrolyser 14 is connected via a rectifier to a galvanically isolated AC mains 12, for example an IT mains, a PID effect can be observed. Here too, in the second operating mode, a voltage can result between the two DC conductors DC+, DC- if their potential is symmetrical or approximately symmetrical with respect to ground potential GND. The PID effect can thus occur in different types of AC networks 12, TN network or IT network.The described device 10 and the described method provide the advantage that in the first operating mode such a PID effect can be reduced, reduced or even reversed.FIG. 2 shows, by way of example, a further exemplary embodiment for the potential position of first DC conductor DC+ and second DC conductor DC- in the second operating mode. The second operating mode can be assumed alternately with the first operating mode. The following describes how the method and the device 10 can act as ground current monitoring in the second operating mode.In this exemplary embodiment, the DC grid 30 is supplied via the rectifier 20 from a galvanically isolated AC grid 12, for example an IT grid. One of the DC conductors, in the example shown the second DC conductor DC-, is grounded. The first DC conductor is regulated to a potential position such that the voltage between the conductors is high enough to operate the electrolysis in the electrolyser 14.By grounding the one DC conductor, here the second DC conductor DC-, the PID effect can be reduced or avoided. Which of the two DC conductors DC+, DC- is grounded may depend, for example, on the type of rectifier 20 used.The middle graph of FIG. 2 shows a possible profile of the duty cycle 26 of the timing of the semiconductor switch 22, and a possible profile of the ground current 28 is shown in the lower graph of FIG. 2.By means of the method and the device for regulating the potential position, the potential position is set as shown in the upper graph of FIG. 2 by clocking the semiconductor switch 22. In order to maintain the potential position, the timing must be adjusted over time, for example due to a fault, and the ground current 28 rises, for example due to the fault. If the ground current 28 reaches the first threshold 32, the clocking is stopped and, for example, the electrolyser 14 is disconnected from the rectifier 20 by opening the DC switches 18. If the ground current 28 continues to increase, for example due to the severity of the fault, the fuse 24 is triggered, for example, when the threshold 34 is reached.FIG. 3 shows a first exemplary embodiment of a first sub-operating mode of the first operating mode. The potential for ground GND and the potential corresponding to a maximum DC voltage 36 for the DC grid 30 are shown.The potential of the first DC conductor DC+ and the second DC conductor DC- are regulated by the device 10 so that they are both positive with respect to the ground potential GND. Both have thus a positive polarity with respect to ground potential GND.The potential difference between the first DC conductor DC+ and the second DC conductor DC- is regulated in such a way that the voltage present at the electrolyser 14 is below a threshold voltage. The threshold voltage is below a threshold voltage of the electrolyser 14, so that in the first sub-operating mode no electrolysis takes place in the electrolyser 14. The threshold voltage can be, for example, in a range between 10% and 50% of the threshold voltage.Since no electrolysis takes place in the electrolyser 14 in the first sub-operating mode, the DC conductors DC+, DC- of the DC grid 30 can be disconnected from the AC grid 12 in the first sub-operating mode. This also enables the first sub-operating mode to be performed regardless of the type of AC network 12. The first sub-operating mode is thus possible for different types of AC network 12, i.e. for example a TN network or an IT network.If the DC grid 30 is disconnected from the AC grid in the first sub-operating mode, the electrical power used for regulating the potential position can be obtained from an auxiliary power supply 21 of the DC grid 30.FIG. 4 shows a second exemplary embodiment of the first sub-operating mode of the first operating mode.The potential of the first DC conductor DC+ and the second DC conductor DC- are controlled by the device 10 so that they are both negative with respect to the ground potential GND. Both have thus a negative polarity with respect to ground GND.The potential difference between the first DC conductor DC+ and the second DC conductor DC is regulated in such a way that the voltage present at the electrolyser 14 is below the threshold voltage. The threshold voltage is below a threshold voltage of the electrolyser 14, so that in the first sub-operating mode no electrolysis takes place in the electrolyser 14. The threshold voltage can be, for example, in a range between 10% and 50% of the threshold voltage.Since no electrolysis takes place in the electrolyser 14 in the first sub-operating mode, the DC conductors DC+, DC- of the DC grid 30 can also be disconnected from the AC grid 12 in the first sub-operating mode. This also enables the first sub-operating mode to be performed regardless of the type of AC network 12. The first sub-operating mode is thus possible for different types of AC network 12, i.e. for example a TN network or an IT network.If the DC grid 30 is disconnected from the AC grid in the first sub-operating mode, the electrical power used for regulating the potential position can be obtained from an auxiliary power supply 21 of the DC grid 30.FIG. 5 shows a first exemplary embodiment of the second sub-operating mode of the first operating mode. In the second sub-operating mode, the electrolyser 14 is supplied with electrical power from the AC grid 12 via the rectifier 20 in order to carry out the electrolysis process in the electrolyser 14.The potential of the first DC conductor DC+ and the second DC conductor DC- are regulated by the device 10 so that they are both positive with respect to the ground potential GND. Both have thus a positive polarity with respect to ground potential GND.The potential difference between the first DC conductor DC+ and the second DC conductor DC- in the second sub-operating mode is controlled such that, in the second sub-operating mode, the potential difference between the first DC conductor DC+ and the second DC conductor DC- corresponds to an operating voltage of the electrolyser 14. At least the potential difference between the first DC conductor DC+ and the second DC conductor DC- approximately corresponds to the operating voltage of the electrolyser 14 in such a way that the electrolysis process can take place in the electrolyser 14.FIG. 6 shows a second exemplary embodiment of the second sub-operating mode of the first operating mode. In the second sub-operating mode, the electrolyser 14 is supplied with electrical power from the AC grid 12 via the rectifier 20 in order to carry out the electrolysis process in the electrolyser 14.The potential of the first DC conductor DC+ and the second DC conductor DC- are controlled by the device 10 so that they are both negative with respect to the ground potential GND. Both have thus a negative polarity with respect to ground potential GND.The potential difference between the first DC conductor DC+ and the second DC conductor DC- in the second sub-operating mode is controlled such that, in the second sub-operating mode, the potential difference between the first DC conductor DC+ and the second DC conductor DC- corresponds to an operating voltage of the electrolyser 14. At least the potential difference between the first DC conductor DC+ and the second DC conductor DC- approximately corresponds to the operating voltage of the electrolyser 14 in such a way that the electrolysis process can take place in the electrolyser 14.Reference numerals denote reference numerals10 Device 12 AC grid 14 electrolyser 16 electrolysis stack 18 DC switch 20 rectifier 21 auxiliary power supply 22 clocked semiconductor switch 24 fuse 26 duty cycle semiconductor switch 28 earth current 30 DC grid 32 first threshold 34 second threshold 36 maximum DC voltage A ammeter DC+, DC-DC conductor GND earth potential

Claims

Method for regulating the potential position of a first and a second DC conductor (DC+, DC-), wherein the DC conductors (DC+, DC-) are provided for supplying an electrolyser (14) with electrical power and the electrical power can be transmitted via a rectifier (20) from an AC grid (12) to the DC conductors (DC+, DC-), wherein, in a first operating mode, the potential position of the first and the second DC conductor (DC+, DC-) is regulated such that both DC conductors (DC+, DC-) have the same polarity with respect to ground potential (GND).Method according to claim 1, wherein the potential position in the first operating mode is controlled in such a way that a regeneration of the electrolyser (14) is effected.Method according to Claim 1 or 2, wherein the first operating mode has a first sub-operating mode in which the potential difference between the first and second DC conductors (DC+, DC-) is regulated such that the voltage present at the electrolyser (14) is below a threshold voltage.Method according to claim 3, wherein the threshold voltage depends on a threshold voltage of the electrolyser (14), wherein the threshold voltage is in particular in a range between 10% and 50% of the threshold voltage.Method according to claim 3 or 4, wherein the DC conductors (DC+, DC-) are disconnected from the AC network (12) in the first sub-operating mode.Method according to one of Claims 3 to 5, wherein the electrical power used for regulating the potential position is obtained from an auxiliary power supply (21).Method according to one of the preceding claims, wherein the first operating mode has a second sub-operating mode in which the electrolyser (14) is supplied with electrical power from the AC grid (12) via the rectifier (20) in order to carry out an electrolysis process, wherein, in the second sub-operating mode, a potential difference between the first and second DC conductors (DC+, DC-) is regulated as a function of the electrolysis process.Method according to claim 7, wherein in the second sub-operating mode the potential difference between the first and second DC conductors (DC+, DC-) depends on an operating voltage of the electrolyser.Method according to one of the preceding claims, wherein a second operating mode is provided in which the first or the second DC conductor (DC+, DC-) is grounded, wherein the electrolyser (14) is supplied via the rectifier (20) with electrical power from the AC grid (30) for carrying out the electrolysis process, and wherein, in the second operating mode, the potential difference between the first and the second DC conductor (DC+, DC-) is regulated as a function of the electrolysis process.Method according to one of the preceding claims, wherein the control of the potential position takes place via the clocking of a semiconductor switch (22) which is arranged between the first DC conductor (DC+) and the ground potential (GND) or between the second DC conductor (DC-) and the ground potential (GND).The method of claim 10, wherein a ground current (28) flowing across the semiconductor switch (22) is monitored.Method according to one of the preceding claims, wherein the respective potential of each of the two DC conductors (DC+, DC-) has a minimum distance with respect to ground potential (GND).Device (10) for regulating the potential position of a first and a second DC conductor (DC+, DC-), wherein the DC conductors (DC+, DC-) are provided for supplying an electrolyser (14) with electrical power and the electrical power can be transmitted via a rectifier (20) from an AC grid (12) to the DC conductors (DC+, DC-), wherein the device (10) is designed, in a first operating mode, to regulate the potential position of the first and the second DC conductor (DC+, DC-) such that both DC conductors (DC+, DC-) have the same polarity with respect to ground potential (GND).Device according to claim 13, wherein the device (10) is configured to regulate the potential position in the first operating mode such that a regeneration of the electrolyser (14) is effected.Device according to Claim 13 or 14, wherein the device (10) is designed to clock a semiconductor switch (22), which is arranged between the first DC conductor (DC+) and the ground potential (GND) or between the second DC conductor (DC-) and the ground potential (GND), for regulating the potential position.Device according to one of Claims 13 to 15, wherein the device (10) is designed to monitor a ground current (28) flowing via the semiconductor switch (22).A DC mains (30) comprising an electrolyser (14), an apparatus (10) according to any one of claims 13 to 16 and the first and second DC conductors (DC+, DC-).DC grid (30) according to Claim 17, further comprising an auxiliary power supply (21) for supplying the device (10) with electrical power as required.

Images