EP4143955A1 - Method for extending a dc voltage range of a rectifier, rectifier for carrying out the method, and electrolysis system - Google Patents

Abstract

The application describes a method for extending a DC voltage range of a rectifier (1) for supplying a DC load (20), which is connected to a DC rectifier output (8) of the rectifier (1), from an AC supply system (30), which is connected to the rectifier (1), - wherein an AC/DC converter (4) of the rectifier (1) comprises a converter circuit (40) with semiconductor switches (41) and freewheeling diodes (42) connected in antiparallel with said semiconductor switches, - wherein an inductance L is arranged between the AC/DC converter (4) and a supply system connection point (31) via which the rectifier is connected to the AC supply system (30), comprising the steps of: - setting a desired DC operating voltage UDC,Soll at the DC output (4.2) of the AC/DC converter (4) and/or at the DC rectifier output (8) by actuating the semiconductor switches (41) of the AC/DC converter (4), - wherein, when the desired DC operating voltage UDC,Soll lies below a value of an amplitude Û4 of an AC voltage at the AC input (4.1) of the AC/DC converter (4), the semiconductor switches (41) of the AC/DC converter (4) are actuated to exchange a reactive power Q1(t) with the AC supply system (30), which reactive power has a voltage-lowering effect on the amplitude Û4 of the AC voltage. The application further comprises a rectifier (1) for carrying out the method, and an electrolysis system (50) comprising a rectifier (1) of this kind.

Description

METHOD FOR EXTENDING A DC VOLTAGE RANGE OF A RECTIFIER, RECTIFIER FOR CARRYING OUT THE METHOD AND ELECTRO LYSIS PLANT

Technical field of the invention

The invention relates to a method for expanding a DC voltage range of a rectifier, a rectifier for carrying out the method and an electrolysis system with such a rectifier.

State of the art

Hydrogen can be produced by an electrolysis reaction in which water is broken down into its constituents, hydrogen and oxygen, in an electrolyser. A speed of the electrolysis reaction is set via a DC voltage applied to an input of the electrolyzer. The DC voltage is usually generated by a rectifier, which is connected on the input side to an alternating voltage (AC) network and on the output side to the electrolyzer as a DC load. Actively controlled single-stage rectifiers are often used as rectifiers. In this case, a converter circuit of an AC / DC converter assigned to the rectifier comprises a plurality of semiconductor switches, each of which has a freewheeling diode connected in anti-parallel to the semiconductor switches. The anti-parallel connected freewheeling diodes result in a minimum DC voltage at the DC output of the AC / DC converter being limited to a value of an amplitude of an AC voltage present on the input side of the AC / DC converter. In other words, the minimum DC voltage at the DC output of the AC / DC converter corresponds to the amplitude of the AC voltage applied to the AC / DC converter on the input side. This applies at least to a predominantly capacitive DC load at the DC output of the AC / DC converter, which has a smoothing effect on the DC voltage, so that a voltage ripple at the DC output is negligible. As the ohmic component of the DC load increases, the voltage ripple at the DC output of the AC / DC converter becomes more apparent and the minimum DC voltage shifts to slightly smaller values. In this case it is usually sufficient to describe the minimum DC voltage at the DC output with the so-called rectification value. The rectified value corresponds to the arithmetic mean value of the rectified DC voltage and is dependent on the topology of the rectifier used in each case, in particular its AC / DC converter.

Conventional electrolysers usually have a characteristic current-voltage characteristic (I-U characteristic). The I-U characteristic can be divided into two areas. If the DC voltages present at the input of the electrolyzer are below a critical voltage Ucr, there is still no electrolysis reaction and therefore no steady flow of current either. Rather, the electrolyser shows a predominantly capacitive behavior here, which is caused by the formation of double layers in the electrolysis cells. Only when the critical voltage Ucr is exceeded does an electrolysis reaction occur, the speed of which increases with increasing DC voltage. A stationary current flows in this area, which drives the electrolysis reaction forward and the electrolyser mainly behaves like an ohmic load. The maximum permitted DC voltage ÜDc.max is limited by a nominal power or component properties of the electrolyzer. The values for the critical voltage Ucr and the maximum permitted DC voltage Uoc.max are dependent on the type of electrolyzer and therefore usually different from type to type.

In general, it is desirable to use the rectifier to continuously adjust and operate the electrolyser in an entire operating range for the DC voltage applied to the input, but at least in a range just below the critical voltage Ucr up to the maximum permitted DC voltage Uoc.max to be able to. In addition, it is desirable to generate high reaction speeds and the associated high DC voltages at the input of the electrolyzer with the lowest possible conversion losses of the rectifier. In order to generate a DC voltage at the DC rectifier output below the critical voltage Ucr with the single-stage rectifiers, the amplitude of the alternating voltage at the AC input of the converter circuit can be set to a correspondingly low value, e.g. by means of a transformer. However, this then results in high conversion losses if high DC voltages are required at the DC rectifier output during operation of the electrolyzer.

From the prior art, for example from the document WO 2013 160486 A2, a method for voltage correction at a junction point of an AC network is known, via which a regenerative generation plant is connected to the AC network connected is. If an amplitude of the alternating voltage deviates from its assigned nominal value, reactive power is exchanged between the regenerative generation plant and the AC network. The exchanged reactive power counteracts the deviation in a grid-supporting manner and pursues the goal of minimizing the deviation.

The document DE 103 03 710 A1 discloses a method for regulating a self-commutated mains converter with a DC voltage output in the event of mains overvoltage. In the method, depending on a determined value of an occurring network overvoltage, a setpoint value for a reactive component of a network current is specified in such a way that an actual value of a modulation level of the self-commutated network converter is reduced.

Object of the invention

The invention is based on the object of specifying a method for expanding a DC voltage range of an actively controlled, in particular a single-stage rectifier, with which a DC voltage at the DC output of the single-stage rectifier can be set to values just below a critical voltage Ucr , even if the critical voltage falls below a nominal value of the amplitude of the alternating voltage. It should also be possible to set high DC voltages at the DC rectifier output with the lowest possible conversion losses. The method should be able to be carried out as simply and inexpensively as possible. It is also an object of the invention to provide a rectifier suitable for carrying out the method, as well as an electrolysis system with such a rectifier.

solution

In a method of the type mentioned at the outset, the object is achieved according to the invention with the features of independent claim 1. The object of showing a rectifier suitable for carrying out the method is achieved according to the invention with the features of independent claim 10. The object of showing an electrolysis system with such a rectifier is achieved according to the invention with the features of independent claim 14. Advantageous variants of the method are in claims 2 to 9, advantageous Embodiments of the rectifier listed in claims 11 to 13. Advantageous embodiments of the electrolysis system are given in claims 15 and 16.

Description of the invention

The method according to the invention aims to expand a DC voltage range of a rectifier for supplying a DC load connected to a DC rectifier output from an alternating voltage (AC) network, with an AC rectifier input of the rectifier connected to the AC network via a network connection point is. In this case, the rectifier has an AC / DC converter with an AC input and a DC output, which comprises a converter circuit with semiconductor switches and free-wheeling diodes connected in anti-parallel thereto. The AC input of the AC / DC converter is connected to the AC rectifier input, if necessary via an AC isolating unit, and the DC output of the AC / DC converter is connected to the DC rectifier output, possibly via a DC isolating unit. An inductance L is arranged between the AC input of the AC / DC converter and the grid connection point. The procedure consists of the following steps:

Setting a desired DC operating voltage UDC.SOII at the DC output of the AC / DC converter and / or at the DC rectifier output by activating the semiconductor switch of the AC / DC converter, whereby if the desired DC operating voltage UDC.SOII is below a The value of the amplitude LU of an alternating voltage is at the AC input of the AC / DC converter, the semiconductor switches of the AC / DC converter are controlled to exchange reactive power Qi (t) with the AC network in such a way that the exchange of reactive power Qi ( t) has a voltage-lowering effect on the amplitude LU at the AC input of the AC / DC converter - and thus also at the AC input of the converter circuit. Due to the voltage-lowering effect, the amplitude LU of the alternating voltage is brought closer to the desired DC operating voltage UDC.SOII. The amplitude LU approximates the desired DC operating voltage UDC.SOII in such a way that the desired DC operating voltage UDC.SOII is also achieved, provided that an allowed exchange of reactive power with the AC network, for example due to a specification by an operator of the AC network that does not limit the approach. The reactive power Qi (t) is exchanged in connection with the AC grid with an electrical connection and / or in connection with an electrical disconnection of the DC load from the rectifier.

The term “in connection with” is to be interpreted in the sense of “during and / or shortly before”. The rectifier can in particular be a single-stage rectifier that is free of a DC / DC converter arranged between the AC / DC converter and the DC rectifier output. On the AC side, the rectifier can only have one phase connection, but alternatively also have several phase connections. Since the AC input of the AC / DC converter corresponds to the AC input of the converter circuit, but is at least connected to it with low impedance, the amplitude LU present at the AC input of the AC / DC converter also corresponds to that amplitude at the AC -Input of the converter circuit is present.

The amplitude LU of the AC voltage applied to the AC input of the AC / DC converter, as well as the amplitude LU of the AC voltage applied to the AC rectifier input of the rectifier, can each be the amplitude of that prevailing on a phase conductor AC voltage relative to a center or star point of a transformer, i.e. the amplitude of the star voltage act. This is particularly the case when the converter circuit of the AC / DC converter is designed as a midpoint circuit. In this case, an output connection of the DC output can be connected to the center point of the transformer or a neutral conductor of the AC network. Alternatively, the amplitude LU of the AC voltage present at the AC input of the AC / DC converter can also be the amplitude of an alternating voltage prevailing between two phase conductors of the AC network, i.e. the amplitude of the linked voltage. The same also applies to the amplitude LU of the AC voltage applied to the AC rectifier input of the rectifier. This is particularly the case when the converter circuit of the AC / DC converter is designed as a so-called bridge circuit. In this case, a transformer with a center tap is not required at all, as a current only flows between the phase conductors of the AC network. For example, in a three-phase AC network, the amplitudes of the star voltage and the linked voltage are linked to one another via the linkage factor L / 3. The method according to the invention uses the knowledge that an exchange of reactive power between the AC / DC converter and the AC network via the inductance L arranged between them results in a change in the amplitude LU of the AC present at the AC input of the AC / DC converter -Tension causes. Specifically, the amplitude LU of the AC voltage can be reduced by exchanging a type of reactive power, for example inductive reactive power, while it is increased by exchanging the complementary type of reactive power, for example capacitive reactive power. In the context of the invention, the exchange of the reactive power Qi (t) is deliberately used to change the amplitude LU of the AC voltage, resulting in the DC voltage at the DC output of the converter circuit and, with the DC isolating unit closed, also at the DC rectifier output influence. If, for example, a DC operating voltage UDC.SOII is to be set at the DC rectifier output - and thus also at the DC output of the AC / DC converter - that is smaller than the amplitude LU of the AC voltage at the AC input of the AC / DC Converter, there is a temporary exchange of reactive power Qi (t) with the AC network. A current assigned to the reactive power Qi (t) flows through the inductance L and has a voltage-reducing effect on the amplitude LU of the AC voltage. In this way, the amplitude LU of the AC voltage applied to the AC / DC converter is reduced. The reduced amplitude LU also generates a reduced value of the DC voltage at the DC output of the converter circuit and thus at the DC rectifier output via the free-wheeling diodes of the converter circuit. Via the amount of exchanged reactive power Qi (t), a percentage decrease or percentage increase in the amplitude U4 relative to an amplitude U4 present at the beginning - ie before the exchange of reactive power - can be set. The temporary exchange of the reactive power Qi (t) now takes place in such a way that the amplitude Ü4 at the AC input of the AC / DC converter, and thus the DC voltage at the DC output of the AC / DC converter, corresponds to the desired DC operating voltage UDC.SOII is approximated, and - depending on the maximum possible or permitted exchange of reactive power Qi (t) - the desired DC operating voltage UDC.SOII is also achieved.

In contrast to the known prior art, the exchange of the reactive power Qi (t) between the AC / DC converter and the AC network does not take place with the aim of preventing an existing deviation in an amplitude of the AC voltage counteract this and thus support the AC grid. In contrast to the prior art, the exchange of the reactive power Qi (t) pursues the goal here of deliberately removing the amplitude LU from its nominal value, at least temporarily. The deliberately induced deviation of the amplitude from its nominal value is used according to the invention to operate the DC load, in particular an electrolyzer, at least for a short time with a low DC voltage at its input that would otherwise not be reached. The short-term change in the DC voltage, in this case the lowering of the DC voltage, is therefore the actual goal of the exchange of the reactive power Qi (t). In this way, a connection or disconnection of the DC load, in particular the electrolyzer, from the rectifier with as little load as possible and therefore gentle operation of the disconnection units and also of the electrolyzer is made possible.

According to the invention, the reactive power Qi (t) is exchanged during and / or shortly before connecting the DC load to the rectifier. It is possible, but not absolutely necessary, for the reactive power Qi (t) to be exchanged with the AC grid even after, in particular shortly after, the electrical connection of the DC load to the rectifier. Specifically, in the case of an electrolyzer as a DC load, the voltage at the input of the electrolyzer can be reduced to just below the critical voltage Ucr. As a result, the connection, as well as the disconnection of the electrolyzer from the rectifier, can be carried out with as little load as possible and as gently as possible for the respective separation unit. The electrolysis reaction can also be started and / or ended in a gentle and controlled manner via a change in the exchanged reactive power Qi (t). Specifically, when the electrolysis reaction starts, the exchanged reactive power Qi (t) can decrease, while a converted active power P (t) increases at the same time. Correspondingly, when the electrolysis reaction ends, the converted active power can decrease, while at the same time the exchanged reactive power increases in order to further reduce the DC voltage at the DC output of the AC / DC converter. Furthermore, it is possible, although not absolutely necessary, for the reactive power Qi (t) to be exchanged with the AC grid even after, in particular shortly after, the DC load has been disconnected from the rectifier. For example, the exchange of the reactive power Qi (t) with the AC network can in particular decrease continuously and in a controlled manner, for example decrease in a ramp-shaped manner in a controlled manner. In this way, there can be an abrupt change in the reactive power exchanged with the AC grid Qi (t) and any undesirable effects it may have on the AC grid can be prevented.

The inductance L is, at least to a certain extent, usually a component of the rectifier anyway. Specifically, in the rectifiers in question, a filter is provided between the AC rectifier input and the AC input of the AC / DC converter for damping high-frequency interference signals, which filter comprises one or more filter chokes. The one or more filter chokes can often be used without, but at least with only a slight adaptation, as part of the inductance L via which the reactive power Qi (t) is exchanged with the AC network. Additional flardware expenditure is therefore often not required at all, at least only to a small extent. With regard to the semiconductor switch of the converter circuit, no flardware adaptation is required for the exchange of reactive power Qi (t), but rather only a software adaptation of its clocking method. Overall, this results in a relatively inexpensive and inexpensive adaptation of a conventional rectifier in order to carry out the method according to the invention.

In an advantageous variant of the method, the reactive power Qi (t) that is exchanged between the AC / DC converter and the AC network has almost exclusively, but at least a predominant proportion, a displacement reactive power. Accordingly, it contains no or only an unavoidable proportion of distortion reactive power. This ensures that a desired sinusoidal shape of the alternating voltage is maintained even when the reactive power Qi (t) is exchanged.

In a further variant of the method, it is possible for the reactive power Qi (t) to be exchanged between the AC / DC converter and the AC network only when the desired DC operating voltage UDC.SOII is below the value by a certain difference The value of the amplitude Ü4 lies. For example, the reactive power can be exchanged when the desired DC operating voltage UDC.SOII, in addition to the value of the amplitude U4, also falls below the rectified value of the alternating voltage with the amplitude U4. In this way, with a DC load with a predominantly ohmic component, an unnecessary Exchange of the reactive power Qi (t) and any associated undesired network perturbation can be reduced.

According to a further variant of the method, the exchange of reactive power Qi (t) can be used not only for a decrease, but also for an increase in the amplitude U4 of the AC voltage. In the latter case, during a certain operating situation of the DC load, for example when the desired DC operating voltage UDC.SOII reaches or exceeds a voltage threshold value UTH, the semiconductor switches of the AC / DC converter can be used to exchange a further reactive power Q2 (t) with the AC network are controlled so that the exchange of the further reactive power Q2 (t) has a voltage-increasing effect on the amplitude LU at the AC input of the converter circuit. In this way, here too, the amplitude LU can be approximated to the desired DC operating voltage UDC.SOII. The further reactive power Q2 (t) can be a complementary type of reactive power Qi (t). In other words, if the reactive power Qi (t) is inductive reactive power, the further reactive power Q ₂ (t) can be capacitive reactive power and vice versa. In this case, the AC / DC converter has to increase the DC voltage applied to the AC output of the AC / DC converter at the same time as the AC voltage is rectified. Overall, this can lead to a reduction in the conversion losses of the AC / DC converter.

The exchange of the reactive power Qi (t) and / or the further reactive power Q2 (t) can determine a reactive power setpoint based on a known voltage change characteristic u (Q) as a function of the between the AC input of the AC / DC converter and the Network connection point of the AC network include exchanged reactive power Q. Specifically, the known voltage change characteristic u (Q) dependency can be determined, for example, once via a measurement and stored in a data memory connected to a control unit of the rectifier. As an alternative to this, it is possible for the reactive power Qi (t) and / or the further reactive power Q2 (t) to be exchanged in each case adaptively and by means of a control unit connected to the control unit. An actual value for the DC voltage UDC, 4 present at the AC output of the AC / DC converter can be detected, the detected actual value with the desired DC operating voltage UDC.SOII can be compared, and the exchange of the respective reactive power Q-1,2 (t) can be regulated in such a way that the actual value of the desired DC operating voltage UDC.SOII is approximated. The control unit can comprise a proportional controller, an integral controller and / or a differential controller.

Regardless of whether the reactive power Qi (t) and / or the further reactive power Q2 (t) are exchanged by means of a known voltage change characteristic u (Q) or adaptively by means of a control unit, the reactive power Qi (t) and / or the exchange of the further reactive power Q2 (t) a change in the amplitude Ü4 at the AC input of the AC / DC converter of at least 10%, preferably of at least 20%, particularly preferably of at least 25% based on a nominal value of the amplitude Ü4 to generate. The amount of the exchanged reactive power Qi, 2 (t), which is necessary to generate the corresponding change in the amplitude U4, is dependent on the value of the inductance L between the AC / DC converter and the grid connection point. The nominal value of the amplitude Ü4 is to be understood here as that value of the amplitude Ü4 that is present at the AC input of the AC / DC converter when there is no exchange of reactive power Qi, 2 (t) between the AC / DC converter and the AC grid.

In an advantageous variant of the method, under specified framework conditions, an exchange of reactive power between the AC / DC converter and the AC network can take place, for example for the purpose of voltage maintenance. Specifically, during a state of the AC network in which the amplitude U7 of the AC voltage at the AC rectifier input deviates from its nominal value, a third reactive power Cb (t) can be exchanged between the AC / DC converter and the AC network, which, depending on the quality of the third reactive power Q3 (t), causes a voltage-lowering or voltage-increasing effect on the amplitude U7 of the AC voltage. For the purpose of maintaining the voltage, the third reactive power Q3 (t) is selected in such a way that an effect on the amplitude U7 results which is opposite to the deviation from its nominal value. The specified framework conditions can include a ripple control signal from and / or a contractual agreement with an operator of the AC network. According to one embodiment of the method, the inductance can comprise a filter choke arranged between the AC / DC converter and the AC rectifier input, via which the reactive power Qi (t) and / or the further reactive power Q2 (t) is exchanged with the AC network . Alternatively or cumulatively, the inductance can comprise a transformer winding on a secondary side of a transformer assigned to the rectifier. This is particularly the case when the rectifier is connected to the AC network via a transformer, the rectifier being connected to the secondary side of the transformer and the AC network to the primary side of the transformer. In such a configuration, at least one other system suitable for reactive power compensation can also be connected to the AC network, i.e. on the primary side of the transformer, which acts as a sink for the reactive power Qi ( t) and / or further reactive power Q2 (t) is effective. The system suitable for reactive power compensation can be controlled in a coordinated manner with the rectifier, so that its exchange of reactive power with the AC network takes place at the same time. Since the further system acts as a sink for the reactive power exchanged by the AC / DC converter with the AC network, a network feedback effect of the exchanged reactive power on the AC network can be eliminated, or at least reduced. An operator of the AC network therefore does not need to have any of his other systems in place to carry out reactive power compensation if necessary. A permitted proportion of reactive power exchanged with the AC grid can thus be increased if necessary.

A rectifier according to the invention is formed by an actively controlled rectifier which is designed to supply a DC load from an AC network having an AC voltage. The rectifier comprises: an AC rectifier input with several input connections for connecting the AC network, as well as a DC rectifier output with two output connections for connecting the DC load, and an AC / DC converter, which has an AC connected to the AC rectifier input -Input, a DC output connected to the DC rectifier output, and a converter circuit arranged between the AC input and the DC output. The converter circuit of the AC / DC converter has actively controllable semiconductor switches and free-wheeling diodes connected in anti-parallel to this. In addition to the rectification function, the AC / DC converter is still there designed and set up to exchange reactive power Qi, 2 (t) with the AC grid. The rectifier also includes a control unit for controlling the AC / DC converter, in particular its semiconductor switch. The rectifier is characterized in that it is designed and set up to carry out the method according to the invention.

The plurality of input connections of the rectifier can only include a phase connection and a neutral conductor connection. As an alternative to this, however, they can also contain several phase connections and no or one neutral conductor connection. The advantages already mentioned in connection with the method result.

According to an advantageous embodiment, the rectifier can have a control unit which, in conjunction with the control unit, is designed and set up to set the reactive power Qi (t) exchanged with the AC network, and possibly also the further reactive power Q2 (t), so that the DC -Voltage at the DC output of the AC / DC converter approximates the desired DC operating voltage UDC.SOII - if possible, until the DC operating voltage UDC.SOII is reached. Specifically, the control unit can be designed and set up to detect a DC voltage UDC, 4 ZU present at the DC output of the AC / DC converter and / or a DC voltage UDC, 4 with the desired one at the DC rectifier output Compare the DC operating voltage UDC.SOII and, in conjunction with the control unit, control the AC / DC converter so that the detected DC voltage UDC, 4 approximates the desired DC operating voltage UDC.SOII and the desired DC operating voltage UDC.SOII achieved if possible. In this way, the AC / DC converter is able to react in an adaptive manner to a current voltage change characteristic u (Q) between the AC input of the AC / DC converter and the grid connection point without having to determine this beforehand and, if necessary, save it to have to. Alternatively, it is also possible to determine the voltage change characteristic u (Q) as a function of the reactive power Q beforehand and the reactive power Qi (t) to be exchanged with the AC network and / or further reactive power Q2 (t) according to the determined voltage change characteristic u (Q ). For this purpose, the control unit of the rectifier can have a data memory or with a Be connected to a data memory which is set up to store determined value pairs which reflect the previously determined voltage change characteristic u (Q).

In an advantageous embodiment, the rectifier can have a filter unit with a filter choke or a plurality of filter chokes. The at least one filter choke can be arranged between the AC input of the AC / DC converter and the AC rectifier input. It is thus at least part of the inductance via which the reactive power Qi (t) and / or the further reactive power Q2 (t) is exchanged with the AC network. Advantageously, an impedance of the filter choke can be dimensioned such that, with a nominal current Io of the rectifier flowing through the filter choke, a voltage drop of at least 25%, preferably of at least 35%, particularly preferably of at least 45%, relative to the AC voltage applied to the AC rectifier input. Tension is caused. Because the impedance, and thus also the inductance L of the filter choke, is dimensioned in this way, there is, on the one hand, a particularly effective voltage-lowering effect on the amplitude when the reactive power Qi, 2 (t) is exchanged, as well as an additional current limitation in the event of a short circuit on the DC side. The additional current limitation minimizes the risk that the freewheeling diodes of the converter circuit will be damaged in the event of a short circuit in the DC load.

An electrolysis system according to the invention comprises a rectifier according to the invention and an electrolyzer connected on the output side to the rectifier as a DC load. The electrolysis system can additionally comprise a transformer, which is connected with its secondary side to the AC rectifier input and with its primary side via the network connection point to the AC network. If the electrolysis system has a transformer, it can also include a system suitable for reactive power compensation for reducing network feedback. The system on the primary side of the transformer suitable for reactive power compensation is connected to the AC network and acts as a sink for the reactive power Qi (t) and / or further reactive power Q2 (t) exchanged by the AC / DC converter with the AC network ). The advantages already mentioned in connection with the method result here as well. Brief description of the figures

The invention is illustrated below with the aid of figures. From these show

1 shows an embodiment of an electrolysis system according to the invention with a rectifier according to the invention;

FIG. 2 shows an embodiment of a converter circuit of the rectifier according to the invention from FIG. 1;

3 shows a time profile of the method according to the invention according to an embodiment in a schematic manner.

Fiqurenbeschreibunq

An embodiment of an electrolysis system 50 according to the invention is illustrated in FIG. 1. The electrolysis system 50 includes an electrolyzer 22 as a DC load 20, a rectifier 1 according to the invention and a transformer 32. The transformer 32 is connected with its primary side 32. P via a network connection point 31 to an alternating voltage (AC) network 30. A secondary side 32. S of the transformer 32 is connected to an AC rectifier input 7 of the rectifier 1. The transformer 32 converts an AC voltage present on the primary side with the amplitude ÜNetz into an AC voltage with the amplitude Ü7 present both on the secondary side and at the AC rectifier input 7. A DC rectifier output 8 of the rectifier 1 is connected to an input 21 of the electrolyzer 22.

The rectifier 1 is an actively controllable rectifier which is designed to convert the AC voltage present on the input side into a DC voltage present at a DC rectifier output 8 in order to supply the electrolyzer 22 with the DC voltage UDc.Last supply. In addition, the rectifier 1 comprises an AC / DC converter 4 with an AC input 4.1 and a DC output 4.2, which is controlled via a control unit 9. The AC input 4.1 is connected to a filter choke 3.1 and a filter capacitance 3.2 via a filter unit 3 and to the AC rectifier input 7 via an AC isolating unit 2. The DC output 4.2 is connected to the DC rectifier output 8 via a DC isolating unit 6. Parallel to the DC output 4.2 is a Output capacitance 5 for smoothing a DC voltage UDC, 4 present at the DC output 4.2. The DC disconnection unit 6 has two current paths arranged parallel to one another. A first current path contains a series connection of a precharge resistor and an isolating switch and is used to precharge the electrolyzer 22. The second current path, which is arranged in parallel therewith, contains only one further isolating switch. After the precharge has taken place, the electrolyzer 22 is operated in its ohmic range, the closed further disconnector providing a low-impedance electrical connection between the DC output 4.2 of the AC / DC converter 4 and the electrolyzer 22. Both the DC disconnection unit 2 and the AC disconnection unit 6 are controlled by the control unit 9 of the rectifier 1.

The rectifier 1 according to the invention is designed to exchange reactive power Qi, 2 (t) via the transformer 32 with the AC network 30 by appropriately activating semiconductor switches of the AC / DC converter 4. A current assigned to the reactive power Qi, 2 (t) flows via an inductance L which, in the case illustrated in FIG. The reactive power Qi, 2 (t) is almost exclusively, but at least for the most part, displacement reactive power. The exchange of reactive power Qi, 2 (t), as also explained in more detail in connection with FIGS. 2 and 3, depending on the type of reactive power Qi, 2 (t), has a voltage-lowering or voltage-increasing effect on an amplitude LU of the AC -Input 4.1 of the AC / DC converter 4 applied AC voltage, by means of which a DC voltage range of the rectifier 1, in particular of the AC / DC converter 4, is expanded. The amount of exchanged reactive power can on the one hand be set via a known, for example one-time determined voltage change characteristic u (Q) in connection with the control unit 9. For this purpose, the rectifier 1 can have a data memory 11 for storing pairs of values which reflect the previously determined voltage change characteristic u (Q). Alternatively or cumulatively, the rectifier 1 can also have a control unit 10 which is designed to measure the DC voltage UDC, 4 present at the DC output 4.2 of the AC / DC converter 4.2, and possibly also the AC voltage present at the AC input 4.1 of the amplitude LU to detect and to compare the detected DC voltage UDC, 4 with a desired DC operating voltage UDC.SOII, and a To transmit the result of the comparison to the control unit 9. The control unit 9 in turn varies the reactive power Qi, 2 (t) exchanged between the AC network 30 and the AC / DC converter 4 via a corresponding control of the semiconductor switches of the AC / DC converter 4 so that the DC voltage UDC, 4 the desired operating voltage UDC.SOII is approximated and this is achieved as far as possible.

In Fig. 1, the rectifier 1, the transformer unit 32 and the AC network are each shown as an example in three phases. According to the invention, however, it is also possible for these components to be of single-phase design. The control unit 9 of the rectifier 1 can also be connected to a communication unit (not shown in FIG. 1). In this way, a synchronous control of other systems for reactive power compensation, which are connected to the AC network on the primary side of the transformer, can be initiated and coordinated.

FIG. 2 shows an embodiment of the AC / DC converter 4 from FIG. 1 assigned to the rectifier 1 in greater detail. Like the rectifier 1 from FIG. 1, the AC / DC converter 4 is designed as a three-phase AC / DC converter 4 and comprises a converter circuit 40 with a total of three bridge branches 45. Each of the bridge branches 45 has two series-connected semiconductor switches 41 a freewheeling diode 42 connected in anti-parallel. The freewheeling diode 42 can be designed 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. Corresponding to the three-phase configuration of the converter circuit 40, the AC input 4.1 of the AC / DC converter 4 comprises three input connections which are each connected to a connection point 46 of the two semiconductor switches 41 of the bridge arm 45 assigned to them. The DC output 4.2 of the DC / AC converter 4 comprises a positive (+) and a negative output connection (-).

During the conversion, the AC / DC converter 4 can transport an active power P (t) from the AC input 4.1 to the DC output 4.2, possibly also in the opposite direction from the DC output to the AC input 4.1. In addition, the AC / DC converter 4 is designed, a reactive power Qi, 2 (t) between the AC input 4.1 of the AC / DC converter 4 and the AC network 30 connected to the AC input 4.1 (not explicitly shown in FIG. 2). For this purpose, the semiconductor switches 41 are controlled by the control unit 9 (not explicitly shown in FIG. 2). Via a corresponding clocking of the semiconductor switches 41, the AC / DC converter 4 is able to convert the AC voltage present at the AC input 4.1 into a DC voltage UDC, 4 at the DC output 4.2. The level of the converted DC voltage, in other words the DC voltage range, can assume values between a minimum ÜDc.min and a maximum DC voltage ÜDc.max. The minimum DC voltage ÜDc.min is limited downwards via the freewheeling diodes 42 to a value which - apart from a forward voltage of the freewheeling diodes 42 - corresponds to the amplitude Ü4 of the AC voltage present at the AC input 4.2. Due to the freewheeling diodes 42, the bridge circuit 43 is thus able to generate a DC voltage UDC, 4 at the DC output 4.2, which is larger, but not smaller, at least not significantly smaller than the amplitude U4 of the AC voltage applied on the input side is. The conversion losses increase with an increasing ratio of the DC voltage UDC, 4 to the amplitude U4 of the AC voltage applied to the input. Since the AC / DC converter 4 now exchanges reactive power Qi, 2 (t) with the AC network 30 via the inductance L, for example the filter chokes 3.1 and / or the inductance Qi, 2 (t) assigned to the secondary side of the transformer, a voltage-reducing or voltage-increasing effect results the amplitude Ü4 of the AC voltage present at the AC input 4.1. This is explained in more detail in FIG. 3.

In FIG. 2, a two-stage converter circuit 40 with two voltage stages is shown as an example. In the context of the invention, however, a converter circuit with more than just two voltage levels, for example a three-stage or a five-stage converter circuit, is also possible. It is also possible within the scope of the invention for the converter circuit to be designed as a midpoint circuit. Here, an output connection (-) of the DC output 4.2 can be connected to a center tap of a transformer 32, via which the AC / DC converter 4 is connected to the AC network 30. Alternatively, it can also be connected to a neutral conductor of the AC network 30.

FIG. 3 schematically shows a time profile of the method according to the invention in one embodiment as it can be carried out with the control unit 10. The time curves of: the DC voltages UDC, 4 at the DC output 4.2 of the AC / DC converter 4, the amplitude Ü4 of the AC voltage at the AC input 4.1 of the AC / DC converter 4 and that between the AC are shown here / DC converter 4 and the AC network 30 via the inductance L exchanged reactive power Qi (t). In this case, in FIG. 3, a positive value of the exchanged reactive power Qi (t) produces a voltage-lowering effect on the amplitude LU of the AC voltage. As illustrated in FIG. 3 next to the vertical coordinate axis, the individual time courses are shown in different line forms. As an example, the time courses reflect a case such as can occur, for example, when the electrolyzer 22 is connected as a DC load 20 to the actively controlled rectifier 1.

The starting point is a state in which the electrolyzer 22 is separated from the rectifier 1. However, the electrolyser 22 has already been precharged in such a way that its input-side DC voltage UDc.Last corresponds to a value just below the critical voltage Ucr, so that no electrolysis reaction takes place yet. For times t <ti, initially no reactive power Qi (t) is exchanged between the AC / DC converter 4 and the AC network 30, ie for t <ti Qi (t) = 0. A value of the amplitude LU of the am AC voltage applied to AC input 4.1 is above the critical voltage in order to generate the lowest possible conversion losses at high speed of the electrolysis reaction.

At time t 1, it is now signaled to the electrolysis system 50 that the rectifier 1 is to be connected to the electrolyser 22. In order to make the connection as load-free as possible, but at least with a reduced compensating current, a first value of the desired DC operating voltage UDC.SOIU is also set to the currently available DC voltage Uüc.Last at the input of the electrolyzer 22 from time ti. This means that the first value of the desired DC operating voltage UDC, SOII, I is smaller than the amplitude LU of the AC voltage present at the AC input 4.1 and, since the DC voltage UDC, 4 at the DC output 4.2 is down to a forward voltage of the Freewheeling diode 42 corresponds to the amplitude LU, also smaller than the DC voltage UDC applied on the output side, 4. There is thus a relatively large difference AU (ti) between the desired DC operating voltage UDC, SOII, I and the DC voltage UDC, 4 present on the output side. The control unit 10 detects the DC voltage UDC, 4 present on the output side, compares it with the desired DC operating voltage UDC.SOII.I and transmits the Differential voltage AU (ti) of the control unit 9. In response to this, the control unit 9 controls the semiconductor switches 41 of the converter circuit 40 in accordance with an increase in the reactive power Qi (ti) exchanged with the AC network 30. The exchanged reactive power Qi (t), in particular the current assigned to it and flowing through the inductance L, produces a voltage-lowering effect on the amplitude LU of the AC voltage present at the AC input 4.1. As a result, the value of the amplitude LU and the corresponding DC voltage UDc, 4 (t) at the AC output 4.2 of the AC / DC converter 4 decrease. In the period between ti and tu, the currently present DC voltage UDc, 4 (t) is now continuously detected by the control unit 10 and compared with the first value of the desired DC operating voltage UDC.SOIU. The comparison results in a decrease in the amount of the difference AU (t), which in turn is communicated to the control unit 9. The control unit 9 again controls the semiconductor switches 41 of the converter circuit 40 with the aim of further increasing the exchanged reactive power Qi (t). An increase in the reactive power Qi (t), and consequently a decrease in the amplitude LU, as well as the DC voltage UDC, 4 at the DC output 4.2 of the AC / DC converter 4 takes place in the period between ti and tu until the The difference between the DC voltage UDC, 4 at the DC output 4.2 of the AC / DC converter 4 and the first value of the desired DC operating voltage UDC.SOII.I disappears. This ultimately leads to the amplitude LU of the AC voltage on the input side, as well as the DC voltage UDC, 4 at the DC output 4.2 of the AC / DC converter 4 being the first value of the desired DC operating voltage UDC.SOII at the instant tu .I achieved. Therefore, the electrolyser 22 can be connected to the rectifier 1 with low impedance and as load-free as possible at the time tu by closing the DC isolating unit 6.

From the point in time tu, the first value UDC.SOII.I of the desired DC operating voltage is replaced by a second value of the desired DC operating voltage UDC, SOII, 2, at which an electrolysis reaction is now to take place. Therefore, in the period tu to tiv, the DC voltage UDC, 4 at the DC output 4.2 of the AC / DC converter 4 approaches the now valid second value of the desired DC operating voltage UDC, SOII, 2 in a ramp-like manner. This is accompanied by a likewise ramp-like decrease in the reactive power Qi (t) to the value 0 in the time period tii-tm. From the point in time tm, no more reactive power Qi (t) is exchanged between the AC / DC converter 4 and the AC network 30 and the amplitude LU of the AC The voltage at the AC input 4.2 of the AC / DC converter resumes its original value at t = 0.

The ramp-like curves of the reactive power Qi (t) as well as the DC voltage UDC, 4 at the DC output 4.2 of the AC / DC converter 4 illustrated in FIG. 3 can also be steeper than shown and have an almost step-like change over time.

FIG. 3 shows the method according to the invention as it can be carried out in an adaptive manner by means of the control unit 10. No detailed knowledge of the voltage change characteristic u (Q) between network connection point 31 of AC network 30 and AC input 4.1 of the AC / DC converter is required here. In the context of the invention, however, it is also possible to carry out the method with a known voltage change characteristic u (Q). Using the known voltage change characteristic u (Q), after the DC voltage UDC, 4 present at the DC output 4.2 of the AC / DC converter 4 has been detected and compared with the first values of the desired DC operating voltage UDC.SOIU, the corresponding differential voltage AU (ti) can be determined. By comparing the determined differential voltage AU (ti) with the known voltage change characteristic u (Q), the reactive power Qi (t) required to set the desired DC operating voltage UDC.SOIU can be determined. In response to this, the control unit 9 can control the semiconductor switches 42 of the AC / DC converter 4 in order to exchange the required reactive power Qi (t). The method according to the invention was explained above using the example of connecting the electrolyzer 22 to the rectifier 1 with as little load as possible. Alternatively or cumulatively, however, it can also take place in connection with a load-free disconnection of the electrolyzer 22 from the rectifier 1 by opening the DC disconnection unit 6. Specifically, via the temporary exchange of reactive power Qi (t), the DC voltage UDC, 4 at the DC output of the AC / DC converter 4, which is also applied to the input 21 of the rectifier 1 with a low-impedance connection to the electrolyzer 22, shortly before and during the opening of the DC disconnection unit 6 are lowered below the critical voltage Ucr, which is necessary to maintain the electrolysis reaction. Reference list

Rectifier

AC disconnection unit

Filter unit

Filter choke

Filter capacity

AC / DC converter

AC input (of the AC / DC converter)

DC output (of the AC / DC converter)

Output capacity (of the AC / DC converter)

DC separation unit

AC rectifier input

DC rectifier output

Control unit

Control unit

Data storage

DC load

Input (of the DC load)

Electrolyzer

Alternating voltage network (AC) network

Grid connection point

transformer

Primary side

Secondary side converter circuit Semiconductor switch Freewheeling diode Input (of the converter circuit) Output (of the converter circuit) 45 bridge branch

46 connection point

Qi (t), Q2 (t) reactive power P (t) active power

ÜNetz, Ü 7, Ü4 Amplitude

UDC, 4 DC voltage

UDC.SOII DC voltage

UDC.min, UDC.max DC voltage

UTH voltage threshold

Claims

Claims 1. Method for expanding a DC voltage range of a rectifier (1) for supplying a DC load (20) connected to a DC rectifier output (8) of the rectifier (1) from an AC rectifier input via a network connection point (31) (7) of the rectifier (1) connected AC network (30) wherein the rectifier (1) has an AC / DC converter (4) with an AC input (4.1) and a DC output (4.2), which has a Converter circuit (40) with semiconductor switches (41) and free-wheeling diodes (42) connected in anti-parallel thereto, an inductance L being arranged between the AC input (4.1) of the AC / DC converter (4) and the network connection point (31) the steps: Set a desired DC operating voltage UDC.SOII at the DC output (4.2) of the AC / DC converter (4) and / or at the DC rectifier output (8) by activating the semiconductor switch (41) of the AC / DC converter (4 ), with the semiconductor switch (41) of the AC / DC converter if the desired DC operating voltage UDC.SOII is below a value of an amplitude Ü4 of an alternating voltage at the AC input (4.1) of the AC / DC converter (4) (4) to exchange a reactive power Qi (t) with the AC network (30), which has a voltage-lowering effect on the amplitude LU of the AC voltage at the AC input (4.1) of the AC / DC converter (4), so that the amplitude LU approximates the desired DC operating voltage UDC.SOII and achieves this as far as possible, and - the reactive power Qi (t) being exchanged with the AC network (30) during and / or shortly before an electrical connection and / or electrical disconnection of the DC load (20) from the rectifier (1). 2. The method according to claim 1, wherein the exchanged reactive power Qi (t) between the AC / DC converter (4) and the AC network (30) has a predominant proportion of a displacement reactive power. 3. The method according to claim 1 or 2, wherein the inductance is arranged between the AC / DC converter (4) and the AC rectifier input (7) Filter choke (3.1) and / or a transformer winding of a secondary side (32. S) of a transformer (32) assigned to the rectifier (1). 4. The method according to any one of the preceding claims, wherein when the desired DC operating voltage UDC.SOII reaches or exceeds a voltage threshold value UTH, the semiconductor switch (41) of the AC / DC converter (4) in such a way to exchange a further reactive power Ü2 ( t) are controlled with the AC network (30) so that the exchange of the additional reactive power Ü2 (t) increases the voltage on the amplitude LU of the AC voltage present at the AC input (4.1) of the AC / DC converter (4) so that the amplitude LU approximates the desired DC operating voltage UDC.SOII. 5. The method according to any one of claims 1 to 4, wherein the exchange of reactive power Qi (t) and / or the further reactive power Ü2 (t) a determination of a reactive power setpoint based on a known voltage change characteristic u (Q) depending on the exchanged Reactive power Q between the AC input (4.1) of the AC / DC converter (4) and the grid connection point (31) of the AC grid (30). 6. The method according to any one of claims 1 to 4, wherein an actual value of the DC voltage UDC, 4 present at the DC output (4.2) of the AC / DC converter (4) is detected, the actual value with the desired DC operating voltage UDC.SOII is compared, and the exchange of the respective reactive power Qi, 2 (t) is regulated by means of a control unit (10) connected to the control unit (9) so that the actual value of the desired DC operating voltage UDC.SOII is approximated. 7. The method according to any one of the preceding claims, wherein the exchange of the reactive power Qi (t) and / or the exchange of the further reactive power Ü2 (t) a change in the amplitude LU at the AC input (4.1) of the AC / DC converter (4 ) of at least 10%, preferably of at least 20%, particularly preferably of at least 25%, based on a nominal value of the amplitude LU. 8. The method according to any one of the preceding claims, wherein under predetermined framework conditions and during a state of the AC network (30) in which the amplitude LU of the AC voltage at the AC rectifier input (7) deviates from its nominal value, an exchange of a third reactive power Cb (t) takes place between the AC / DC converter (4) and the AC network (30), so that an effect on the amplitude U7 results which is the opposite of the deviation from its nominal value. 9. The method according to any one of the preceding claims, wherein the exchange of the reactive power Qi (t) between the AC / DC converter (4) and the AC network (30) only takes place when the desired DC operating voltage UDC.SOII in addition to the value of the amplitude LU also falls below the rectified value of the alternating voltage with the amplitude LU. 10. Actively controlled rectifier (1) for supplying a DC load (20) from an AC network (30) having an AC voltage, comprising: - an AC rectifier input (7) with several input connections for connecting the AC network (30), and a DC rectifier output (8) with two output connections for connecting the DC load (20), - an AC / DC converter (4), which has an AC input (4.1) on the AC side connected to the AC rectifier input (7), and a DC output (4.2 ), and a converter circuit (40) arranged between the AC input (4.1) and the DC output (4.2), - the converter circuit (40) of the AC / DC converter (4) comprising actively controllable semiconductor switches (41) and free-wheeling diodes (42) connected in anti-parallel thereto, and - wherein the AC / DC converter (4) is designed to exchange reactive power Qi, 2 (t) with the AC network (30), and wherein the rectifier (1) furthermore has a control unit (9) for controlling the AC / DC converter (4), in particular its semiconductor switch (41), characterized in that the rectifier (1) is designed and set up to carry out the method according to one of the preceding claims. 11. Rectifier (1) according to claim 10, characterized in that the rectifier (1) has a control unit (10) which is designed and set up, one at the DC output (4.2) of the AC / DC converter (4) and / or detect a DC voltage UDC, 4 ZU applied to the DC rectifier output (8), which Compare the detected DC voltage UDC, 4 with a desired DC operating voltage UDC.SOII and, in conjunction with the control unit (9), control the AC / DC converter (4) so that the detected DC voltage UDC, 4 of the desired DC operating voltage UDC.SOII is approximated and this is achieved as far as possible. 12. Rectifier (1) according to claim 10 or 11, characterized in that the Control unit (9) has a data memory (11) or is connected to a data memory (11) which is used to store a Voltage change characteristic u (Q) as a function of the reactive power Q is set up. 13. Rectifier (1) according to one of claims 10 to 12, characterized in that the rectifier (1) additionally has a filter unit (3) with a filter choke (3.1), an impedance of the filter choke (3.1) being dimensioned such that with a nominal current Io flowing through the filter choke (3.1), a voltage drop of at least 25%, preferably of at least 35%, particularly preferably of at least 45%, is caused relative to the AC voltage II7 applied to the AC rectifier input (7). 14. Electrolysis system (50) with a rectifier (1) according to one of claims 10 to 13 and an electrolyzer (22) connected on the output side to the rectifier (1) as a DC load (20). 15. Electrolysis system (50) according to claim 14, additionally comprising a transformer (32), which with its secondary side (32nd S) with the AC rectifier input (7) and with its primary side (32nd P) via the grid connection point (31) is connected to the AC grid (30). 16. Electrolysis system (50) according to claim 15, further comprising a system suitable for reactive power compensation for reducing network feedback, which is connected to the AC network (30) on the primary side (32nd P) of the transformer (32) and which acts as a sink for the reactive power Qi (t) exchanged by the AC / DC converter (4) with the AC network (30) and / or further reactive power U2 (t).