METHOD FOR EXTENDING A VOLTAGE RANGE OF A RECTIFIER, RECTIFIER FOR CARRYING OUT THE METHOD, AND ELECTROLYSIS SYSTEM

Abstract

A method and related apparatus for extending a DC voltage range of a rectifier circuit for the supply, from an AC grid, of a DC load which is connected to a DC rectifier output of the rectifier circuit, wherein an AC rectifier input of the rectifier circuit is connected via a grid connection point to the AC grid, wherein the rectifier circuit includes an AC/DC converter having an AC input and a DC output, wherein the AC/DC converter includes a converter circuit having semiconductor switches and freewheeling diodes connected in an antiparallel arrangement thereto, wherein an inductance is connected between the AC input of the AC/DC converter and the grid connection point. The method includes setting a desired DC operating voltage U.sub.DOC,soll on the DC output of the AC/DC converter or on the DC rectifier output, or both, by an actuation of semiconductor switches of the AC/DC converter, wherein, when the desired DC operating voltage U.sub.DC,soll lies below a value of an amplitude .Math..sub.4 of an alternating voltage on the AC input of the AC/DC converter, the semiconductor switches of the AC/DC converter are actuated for an exchange of reactive power Q.sub.1(t) with the AC grid, which has a voltage-lowering effect upon the amplitude .Math..sub.4 of the AC voltage at the AC input of the AC/DC converter, such that the amplitude .Math..sub.4 approaches the desired DC operating voltage U.sub.DC,soll, and wherein the exchange of the reactive power Q.sub.1(t) with the AC grid is executed during or shortly before an electrical connection or an electrical isolation of the DC load to or from the rectifier circuit.

Claims

1. A method for extending a DC voltage range of a rectifier circuit for the supply, from an AC grid, of a DC load which is connected to a DC rectifier output of the rectifier circuit, wherein an AC rectifier input of the rectifier circuit is connected via a grid connection point to the AC grid, wherein the rectifier circuit comprises an AC/DC converter having an AC input and a DC output, wherein the AC/DC converter comprises a converter circuit having semiconductor switches and freewheeling diodes connected in an antiparallel arrangement thereto, wherein an inductance is connected between the AC input of the AC/DC converter and the grid connection point, the method comprising: setting a desired DC operating voltage U.sub.DC,Soll on the DC output of the AC/DC converter or on the DC rectifier output, or both, by an actuation of semiconductor switches of the AC/DC converter, wherein, when the desired DC operating voltage U.sub.DC,Soll lies below a value of an amplitude .Math..sub.4 of an alternating (AC) voltage on the AC input of the AC/DC converter, the semiconductor switches of the AC/DC converter are actuated for an exchange of reactive power Q.sub.1(t) with the AC grid, which has a voltage-lowering effect upon the amplitude .Math..sub.4 of the AC voltage at the AC input of the AC/DC converter, such that the amplitude .Math..sub.4 approaches the desired DC operating voltage U.sub.DC,Soll, and wherein the exchange of the reactive power Q.sub.1(t) with the AC grid is executed during or shortly before an electrical connection and/or an electrical isolation of the DC load to or from the rectifier circuit.

2. The method as claimed in claim 1, wherein the reactive power Q.sub.1(t) which is exchanged between the AC/DC converter and the AC grid, to a predominant proportion, is displacement reactive power.

3. The method as claimed in claim 1, wherein the inductance comprises a filter reactor arranged between the AC/DC converter and the AC rectifier input, or a transformer winding on a secondary side of a transformer that is operably coupled to the rectifier circuit, or both, the filter reactor and the transformer winding on the secondary side of the transformer.

4. The method as claimed in claim 1, wherein, if the desired DC operating voltage U.sub.DC,Soll achieves or exceeds a voltage threshold value U.sub.TH, the semiconductor switches of the AC/DC converter are actuated for an exchange of a further reactive power Q.sub.2(t) with the AC grid, such that the exchange of the further reactive power Q.sub.2(t) has a voltage-increasing effect upon the amplitude .Math..sub.4 at the AC input of the AC/DC converter, such that the amplitude .Math..sub.4 approaches the desired DC operating voltage U.sub.DC,soll.

5. The method as claimed in claim 4, wherein the exchange of the further reactive power Q.sub.2(t) comprises determining a reactive power target value, based upon a known voltage variation characteristic u(Q) as a function of a reactive power Q exchanged between the AC input of the AC/DC converter and the grid connection point of the AC grid.

6. The method as claimed in claim 1, wherein the exchange of the reactive power Q.sub.1(t) comprises determining a reactive power target value, based upon a known voltage variation characteristic u(Q) as a function of a reactive power Q exchanged between the AC input of the AC/DC converter and the grid connection point of the AC grid.

7. The method as claimed in claim 4, further comprising: detecting an actual value of a DC voltage U.sub.DC,4 present on the DC output of the AC/DC converter; comparing the detected actual value with the desired DC operating voltage U.sub.DC,soll; and regulating an exchange of the further reactive power Q.sub.2 (t) using a regulating circuit which is connected to a control circuit, such that the actual value of the DC voltage U.sub.DC,4 present on the DC output of the AC/DC converter approaches the desired DC operating voltage U.sub.DC,Soll.

8. The method as claimed in claim 1, further comprising: detecting an actual value of a DC voltage U.sub.DC,4 present on the DC output of the AC/DC converter; comparing the detected actual value with the desired DC operating voltage U.sub.DC,Soll; and regulating an exchange of the reactive power Q.sub.1(t) using a regulating circuit which is connected to the control circuit, such that the actual value of the DC voltage U.sub.DC,4 present on the DC output of the AC/DC converter approaches the desired DC operating voltage U.sub.DC,Soll.

9. The method as claimed in claim 4, wherein the exchange of the reactive power Q.sub.1(t) or the exchange of the further reactive power Q.sub.2(t), or both, generates a variation in the amplitude .Math..sub.4 on the AC input of the AC/DC converter of at least 10% in relation to a nominal value of the amplitude .Math..sub.4.

10. The method as claimed in claim 1 wherein, under specified marginal conditions, and during a state of the AC grid in which an amplitude .Math..sub.7 of the alternating voltage on the AC rectifier input deviates from its nominal value, executing an exchange of a third reactive power Q.sub.3(t) between the AC/DC converter and the AC grid, such that a resulting effect on the amplitude .Math..sub.7 of the alternating voltage on the AC rectifier input counteracts a deviation thereof from its nominal value.

11. The method as claimed in claim 1, wherein the exchange of the reactive power Q.sub.1(t) between the AC/DC converter and the AC grid is only executed when the desired DC operating voltage U.sub.DC,soll, in addition to the value of the amplitude .Math..sub.4, also undershoots an average rectified value of an alternating voltage at the amplitude .Math..sub.4.

12. An actively-controlled rectifier circuit configured to supply a DC load from an AC grid having an AC voltage, comprising: an AC rectifier input comprising a plurality of input terminals configured to connect to the AC grid, and a DC rectifier output comprising two output terminals configured to connect to the DC load, an AC/DC converter comprising an AC-side AC input that is connected to the AC rectifier input, a DC-side DC output that is connected to the DC rectifier output, and a converter circuit arranged between the AC-side AC input and the DC-side DC output, wherein the converter circuit of the AC/DC converter comprises actively controllable semiconductor switches and freewheeling diodes connected in an antiparallel arrangement thereto, and wherein the AC/DC converter is configured for an exchange of reactive power Q.sub.1,2(t) with the AC grid, and wherein the rectifier circuit further comprises a control circuit configured to control the semiconductor switches of the AC/DC converter, wherein the rectifier circuit is configured to: set a desired DC operating voltage U.sub.DC,Soll on the DC-side DC output of the AC/DC converter or on the DC rectifier output, or both, by an actuation of semiconductor switches of the AC/DC converter, wherein, when the desired DC operating voltage U.sub.DC,Soll lies below a value of an amplitude .Math..sub.4 of an alternating voltage on the AC-side AC input of the AC/DC converter, the semiconductor switches of the AC/DC converter are actuated for an exchange of reactive power Q.sub.1(t) with the AC grid, which has a voltage-lowering effect upon the amplitude .Math..sub.4 of the AC voltage at the AC input of the AC/DC converter, such that the amplitude .Math..sub.4 approaches the desired DC operating voltage U.sub.DC,Soll, and wherein the exchange of the reactive power Q.sub.1(t) with the AC grid is executed during or shortly before an electrical connection or an electrical isolation of the DC load to or from the rectifier circuit.

13. The rectifier circuit as claimed in claim 12, wherein the rectifier circuit comprises a regulating circuit configured to detect a DC voltage U.sub.DC,4 present on the DC output of the AC/DC converter or on the DC rectifier output, or both, to compare the detected DC voltage U.sub.DC,4 with the desired DC operating voltage U.sub.DC,soll and, in conjunction with the control circuit, to control the AC/DC converter such that the detected DC voltage U.sub.DC,4 approaches the desired DC operating voltage U.sub.DC,soll.

14. The rectifier circuit as claimed in claim 12, wherein the control circuit comprises a data memory, or is connected to a data memory, which is configured to save therein a voltage variation characteristic u(Q) as a function of a reactive power Q.

15. The rectifier circuit as claimed in claim 12, wherein the rectifier circuit additionally comprises a filter circuit comprising a filter reactor, wherein an impedance of the filter reactor is rated such that, in the event of a nominal current flow I.sub.0 in the filter reactor, a voltage drop of at least 25% relative to the AC voltage U.sub.7 present at the AC rectifier input is generated.

16. An electrolysis system having a rectifier circuit as claimed in claim 12 and an electrolyzer as a DC load, connected to the rectifier circuit on an output side thereof.

17. The electrolysis system as claimed in claim 16, further comprising a transformer that is connected at its secondary side to the AC rectifier input and, at its primary side, is connected to the AC grid via a grid connection point.

18. The electrolysis system as claimed in claim 17, further comprising a reactive power compensating installation for a reduction of any reaction on the AC grid, which is connected to the AC grid at the primary side of the transformer, and functions as a sink for the reactive power Q.sub.1(t) or the further reactive power Q.sub.2(t), or both, exchanged by the AC/DC converter with the AC grid.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0030] The disclosure is represented hereafter with reference to figures. In the figures:

[0031] FIG. 1 shows an embodiment of an electrolysis system according to the disclosure, having a rectifier circuit according to the disclosure;

[0032] FIG. 2 shows an embodiment of a converter circuit of the rectifier circuit according to the disclosure, according to FIG. 1;

[0033] FIG. 3 shows a schematic representation of a temporal characteristic of the method according to the disclosure, according to one embodiment.

DETAILED DESCRIPTION

[0034] FIG. 1 illustrates an embodiment of an electrolysis system 50 according to the disclosure. The electrolysis system 50 comprises an electrolyzer 22, as a DC load 20, a rectifier 1 according to the disclosure and a transformer 32. The transformer 32 is connected, at its primary side 32.P, to an alternating voltage (AC) grid 30 via a grid connection point 31. A secondary side 32.S of the transformer 32 is connected to an AC rectifier input 7 of the rectifier circuit 1. The transformer 32 converts a primary side AC voltage of amplitude .Math..sub.Netz into an AC voltage of amplitude .Math..sub.7, which is present on both the secondary side and on the AC rectifier input 7. A DC rectifier output 8 of the rectifier circuit 1 is connected to an input 21 of the electrolyzer 22.

[0035] The rectifier circuit 1 is an actively controllable rectifier circuit, which is configured to convert the AC voltage present on the input side into a DC voltage which is present on the DC rectifier output 8, in order to supply the DC voltage U.sub.DC,Last to the electrolyzer 22. To this end, the rectifier circuit 1 comprises an AC/DC converter 4 having an AC input 4.1 and a DC output 4.2, which is controlled by a control circuit 9. The AC input 4.1 is connected via a filter circuit 3 with a filter reactor 3.1 and a filter capacitance 3.2, and via an AC isolating circuit 2 to the AC rectifier input 7. The DC output 4.2 is connected via a DC isolating circuit 6 to the DC rectifier output 8. In parallel with the DC output 4.2, an output capacitance 5 is connected for smoothing a DC voltage U.sub.DC,4 which is present on the DC output 4.2. In one embodiment the DC isolating circuit 6 comprises two current paths, which are arranged in parallel with one another. A first current path contains a series-connected arrangement of a precharge resistor and an isolating switch, and is employed for the precharging of the electrolyzer 22. The second current path, which is arranged parallel thereto, contains only a further isolating switch. After precharging, the electrolyzer 22 is operated in its ohmic range, wherein the closed further isolating switch forms a low-impedance electrical connection between the DC output 4.2 of the AC/DC converter 4 and the electrolyzer 22. Both the DC isolating circuit 2 and the AC isolating circuit 6 are actuated by the control circuit 9 of the rectifier circuit 1.

[0036] The rectifier circuit 1 according to the disclosure is configured, by the corresponding actuation of semiconductor switches of the AC/DC converter 4, to exchange reactive power Q.sub.1,2(t), via the transformer 32, with the AC grid 30. A current associated with the reactive power Q.sub.1,2(t) flows via an inductance L which, in the case illustrated in FIG. 1, is formed of filter reactors 3.1 of the filter circuit 3 and windings of the secondary side 32.S of the transformer 32. The reactive power Q.sub.1,2(t), virtually exclusively, but at least to a predominant proportion, is displacement reactive power. The exchange of the reactive power Q.sub.1,2(t), as explained in greater detail with reference to FIG. 2 and FIG. 3, depending upon the type of the reactive power Q.sub.1,2(t), results in a voltage-reducing or voltage-increasing effect on an amplitude .Math..sub.4 of an AC voltage which is applied to the AC input 4.1 of the AC/DC converter 4, by which a DC voltage range of the rectifier circuit 1, particularly of the AC/DC converter 4, is extended. The quantity of reactive power exchanged can be set, on the one hand, by reference to a known voltage variation characteristic u(Q) determined, for example, on a one-off basis, in conjunction with the control circuit 9. To this end, the rectifier circuit 1 can comprise a data memory circuit 11 for the storage of value pairs which reflect the previously determined voltage variation characteristic u(Q). Alternatively or cumulatively, the rectifier circuit 1 can also comprise a regulating device or circuit 10, which is configured to detect the DC voltage U.sub.DC,4 which is present on the DC output 4.2 of the AC/DC converter, and also optionally the AC voltage of amplitude .Math..sub.4 which is present on the AC input 4.1, compare the detected DC voltage U.sub.DC,4 with a desired DC operating voltage U.sub.DC,soll, and transmit a comparison result to the control circuit 9. The control circuit 9, in turn, varies the reactive power Q.sub.1,2(t) exchanged between the AC grid 30 and the AC/DC converter 4 via a corresponding actuation of the semiconductor switches of the AC/DC converter 4, such that the DC voltage U.sub.DC,4 approaches the desired operating voltage U.sub.DC,soll and, insofar as possible, achieves the latter.

[0037] In FIG. 1, the rectifier circuit 1, the transformer unit 32 and the AC grid are exemplarily represented as three-phase components in each case. According to the disclosure, however, it is also possible for each of these to be configured as single-phase components. The control circuit 9 of the rectifier circuit 1 can moreover be connected to a communication circuit (not represented in FIG. 1). In this manner, a synchronously-executed actuation of further reactive power compensating installations, which are connected to the AC grid on the primary side of the transformer, can be initiated and coordinated.

[0038] FIG. 2 shows a more detailed representation of an embodiment of the AC/DC converter 4 according to FIG. 1 which is assigned to, or a component within, the rectifier circuit 1. In the same manner as the rectifier circuit 1 according to FIG. 1, the AD/DC converter 4 is configured in one embodiment as a three-phase AC/DC converter 4, and comprises a converter circuit 40 having a total of three bridge arms 45. Each of the bridge arms 45 comprises two series-connected semiconductor switches 41, each having an antiparallel-connected freewheeling diode 42. The freewheeling diode 42 can be configured as an intrinsic diode of the respective semiconductor switch 41, or as a separate diode. The semiconductor switches 41 can be MOSFET or IGBT semiconductor switches. In accordance with the three-phase configuration of the converter circuit 40, the AC input 4.1 of the AC/DC converter 4 comprises three input terminals, each of which is connected to a connection point 46 of the two semiconductor switches 41 in the bridge arm 45 which is assigned thereto. The DC output 4.2 of the DC/AC converter 4 comprises a positive (+) and a negative (−) output terminal.

[0039] During conversion, the AC/DC converter 4 can transmit active power P(t) from the AC input 4.1 to the DC output 4.2 and, optionally, also in the reverse direction from the DC output to the AC input 4.1. The AC/DC converter 4 is moreover configured to exchange reactive power Q.sub.1,2(t) between the AC input 4.1 of the AC/DC converter 4 and the AC grid 30 connected to the AC input 4.1 (not explicitly represented in FIG. 2). To this end, the semiconductor switches 41 are actuated by the control circuit 9 (not explicitly represented in FIG. 2). Using a corresponding clocking of the semiconductor switches 41, the AC/DC converter 4 is configured to convert the AC voltage which is present on the AC input 4.1 into a DC voltage U.sub.DC,4 on the DC output 4.2. The magnitude of the converted DC voltage, in other words the DC voltage range, can assume values between a minimum U.sub.DC,min and a maximum DC voltage U.sub.DC,max. The minimum DC voltage U.sub.DC,min is downwardly limited, by the freewheeling diodes 42, to a value which—excluding a conducting-state voltage of the freewheeling diodes 42—corresponds to the amplitude .Math..sub.4 of the AC voltage which is present on the AC input 4.2. Using the freewheeling diodes 42, the bridge circuit 43 is configured to generate a DC voltage U.sub.DC,4 on the DC output 4.2 which is greater, but not smaller, at least not significantly smaller than the amplitude .Math..sub.4 of the AC voltage which is applied to the input side. Conversion losses increase, as the ratio of the output-side DC voltage U.sub.DC,4 to the amplitude .Math..sub.4 of the input side AC voltage rises. As the AC/DC converter 4 exchanges reactive power Q.sub.1,2(t) with the AC grid 30 via the inductance L, for example, the filter reactors 3.1 and/or the inductance assigned to the secondary side of the transformer, a resulting voltage-reducing or voltage-increasing effect is executed on the amplitude .Math..sub.4 of the AC voltage which is present on the AC input 4.1. This is described in greater detail with reference to FIG. 3.

[0040] FIG. 2 represents an example two-level converter circuit 40 having two voltage levels. In the context of the disclosure, however, a converter circuit having more than only two voltage levels, for example a three-level or five-level converter circuit, is also possible. Moreover, in the context of the disclosure, it is possible for converter circuit to be configured in the form of a neutral point circuit. An output terminal (−) of the DC output 4.2 can thus be connected to a neutral point tap of a transformer 32, via which the AC/DC converter 4 is connected to the AC grid 30. Alternatively, it can also be connected to a neutral conductor of the AC grid 30.

[0041] FIG. 3 shows a schematic representation of a temporal characteristic of the method according to the disclosure, in an embodiment which can be executed using the regulating circuit 10. Temporal characteristics are plotted for the following: DC voltages U.sub.DC,4 on the DC output 4.2 of the AC/DC converter 4, the amplitude .Math..sub.4 of the AC voltage on the AC input 4.1 of the AC/DC converter 4, and the reactive power Q.sub.1(t) exchanged between the AC/DC converter 4 and the AC grid 30 via the inductance L. In FIG. 3, a positive value for the exchange of the reactive power Q.sub.1 (t) generates a voltage-reducing effect on the amplitude .Math..sub.4 of the AC voltage. Individual temporal characteristics, as illustrated in FIG. 3 next to the vertical coordinate axis, are represented by lines of different types. These temporal characteristics represent an example case which can occur, for example, in the event of the connection of the electrolyzer 22, as a DC load 20, to the actively-controlled rectifier circuit 1.

[0042] The starting point is a state in which the electrolyzer 22 is isolated from the rectifier circuit 1. However, precharging of the electrolyzer 22 has already been executed to the effect that the input-side DC voltage U.sub.DC,Last assumes a value slightly below the critical voltage U.sub.cr, such that no electrolysis reaction proceeds as yet. At times t<t.sub.I, no reactive power Q.sub.1(t) is exchanged initially between the AC/DC converter 4 and the AC grid 30, at t<t.sub.I, Q.sub.1(t)=0. A value for the amplitude .Math..sub.4 of the AC voltage which is present on the AC input 4.1 lies above the critical voltage, in order to generate the lowest possible conversion losses at a high electrolysis reaction speed.

[0043] At time point t.sub.I, it is signaled to the electrolysis system 50 that the rectifier circuit 1 is to be connected to the electrolyzer 22. For the execution of this connection with the most load-free arrangement possible, or at least with a reduced compensating current, a first value of the desired DC operating voltage U.sub.DC,Soll,1, with effect from time point t.sub.I, is likewise set to the currently present DC voltage U.sub.DC,Last on the input of the electrolyzer 22. The first value of the desired DC operating voltage U.sub.DC,Soll,1 is thus lower than the amplitude .Math..sub.4 of the AC voltage which is present on the AC input 4.1 and, as the DC voltage U.sub.DC,4 on the DC output 4.2, excepting the conducting-state voltage of the freewheeling diodes 42, corresponds to the amplitude .Math..sub.4, is also lower than the DC voltage U.sub.DC,4 which is present on the output side. There is thus a relatively large difference ΔU(t.sub.I) between the desired DC operating voltage U.sub.DC,Soll,1 and the DC voltage U.sub.DC,4 which is present on the output side. The regulating circuit 10 detects the DC voltage U.sub.DC,4 which is present on the output side, compares it with the desired DC operating voltage U.sub.DC,Soll,1 and transmits the voltage difference ΔU(t.sub.I) to the control circuit 9. In response thereto, the control circuit 9 actuates the semiconductor switches 41 of the converter circuit 40, in accordance with an increase in the reactive power Q.sub.1(t.sub.I) exchanged with the AC grid 30. The exchange of the reactive power Q.sub.1(t), particularly by way of its associated current flow via the inductance L, generates a voltage-reducing effect on the amplitude .Math..sub.4 of the AC voltage which is present on the AC input 4.1. As a result, the value of the amplitude .Math..sub.4 and the corresponding DC voltage U.sub.DC,4(t) on the AC output 4.2 of the AC/DC converter 4 are reduced. In the time interval between t.sub.I and t.sub.II, the currently present DC voltage U.sub.DC,4(t) is continuously detected by the regulating unit 10, and is compared with the first value for the desired DC operating voltage U.sub.DC,Soll,1. This comparison shows a quantitative reduction in the difference ΔU(t) which, in turn, is communicated to the control circuit 9. The control circuit 9 again actuates the semiconductor switches 41 of the converter circuit 40, with the objective of a further increase in the exchange of the reactive power Q.sub.1(t). An increase in the reactive power Q.sub.1(t), and a consequent reduction of the amplitude .Math..sub.4, and of the DC voltage U.sub.DC,4 on the DC output 4.2 of the AC/DC converter 4, is executed in the time interval between t.sub.I and t.sub.II, until such time as the difference between the DC voltage U.sub.DC,4 on the DC output 4.2 of the AC/DC converter 4 and the first value of the desired DC operating voltage U.sub.DC,Soll,1 disappears. Ultimately, as a result, at time point t.sub.II, the amplitude .Math..sub.4 of the input-side AC voltage, and the DC voltage U.sub.DC,4 on the DC output 4.2 of the AC/DC converter 4, achieve the first value of the desired DC operating voltage U.sub.DC,Soll,1. At time point t.sub.II, the electrolyzer 22 can thus be connected to the rectifier circuit 1 by the closing of the DC isolating circuit 6, in a low-impedance and substantially load-free arrangement.

[0044] With effect from time point t.sub.II, the first value U.sub.DC,Soll,1 of the desired DC operating voltage is replaced by a second value of the desired DC operating voltage U.sub.DC,Soll,2, at which an electrolysis reaction is now to be executed. Thus, in the time interval from t.sub.II to t.sub.IV, a ramped approach of the DC voltage U.sub.DC,4 on the DC output 4.2 of the AC/DC converter 4 to the now applicable second value of the desired DC operating voltage U.sub.DC,Soll,2 is executed. This is accompanied by a likewise ramped reduction in the reactive power Q.sub.1(t) to a value of 0 in the time interval t.sub.II-t.sub.III. With effect from time point t.sub.III, no reactive power Q.sub.1(t) is exchanged further between the AC/DC converter 4 and the AC grid 30, and the amplitude .Math..sub.4 of the AC voltage at the input 4.2 of the AC/DC converter resumes its original value at t=0.

[0045] The ramped characteristics, shown in FIG. 3, for the reactive power Q.sub.1(t) and for the DC voltage U.sub.DC,4 on the DC output 4.2 of the AC/DC converter 4 can also assume a steeper gradient than that represented, and can observe a virtually step-wise temporal variation.

[0046] FIG. 3 represents the method according to the disclosure, for potential execution in an adaptive manner by means of the regulating circuit 10. No detailed knowledge of the voltage variation characteristic u(Q) between the grid connection point 31 of the AC grid 30 and the AC input 4.1 of the AC/DC converter is required. In the context of the disclosure, however, it is also possible for the method to be executed using a known voltage variation characteristic u(Q). By reference to the known voltage variation characteristic u(Q), further to the detection of the DC voltage U.sub.DC,4 present on the DC output 4.2 of the AC/DC converter 4 and the comparison thereof with the first value of the desired DC operating voltage U.sub.DC,soll,1, the corresponding voltage difference ΔU(t.sub.I) is determined. By a comparison of the voltage difference ΔU(t.sub.I) thus determined with the known voltage variation characteristic u(Q), the requisite reactive power Q.sub.1(t) for the setting of the desired DC operating voltage U.sub.DC,Soll,1 can be determined. In response thereto, the control circuit 9 can actuate the semiconductor switches 42 of the AC/DC converter 4 for the exchange of the requisite reactive power Q.sub.1(t). The method according to the disclosure has been described above with reference to the connection of the electrolyzer 22 to the rectifier circuit 1 in the most load-free arrangement possible. Alternatively or cumulatively, however, it can also be executed in conjunction with a load-free isolation of the electrolyzer 22 from the rectifier circuit 1, by the opening of the DC isolating circuit 6. Specifically, by using the short-term exchange of reactive power Q.sub.1(t), the DC voltage U.sub.DC,4 on the DC output of the AC/DC converter 4 which, in the event of the low-impedance connection of the rectifier circuit 1 to the electrolyzer 22, is likewise present on the input 21 thereof, shortly before and during the opening of the DC isolating circuit 6, can be reduced below the critical voltage U.sub.cr which is required for the maintenance of the electrolysis reaction.