阅读说明：本技术 具有电网换相的变流器的变流器装置和用于启动其的方法 (Converter arrangement with a grid-commutated converter and method for starting the converter arrangement ) 是由 U.哈夫曼 S.哈默 V.胡森尼瑟 S.西格特 于 2021-05-17 设计创作，主要内容包括：本发明涉及一种变流器装置(1),其包括电网换相的变流器(2),该变流器具有交流电压接头,该交流电压接头能够经由至少一个相线与交流电网连接,其中,该变流器装置还包括至少一个开关模块支路(31),该开关模块支路串联地布置在至少一个相线中,并且该开关模块支路包括开关模块(341,342)的串联电路,在开关模块的接头处分别能够生成其总和为支路电压的双极性电压。本发明的特征在于,以与开关模块支路并联连接的方式布置桥接支路(33),在该桥接支路中布置有至少一个开关设备(37),其中该开关设备包括反并联连接的可接通的半导体开关(36,42)。本发明还涉及一种用于启动变流器装置的方法。(The invention relates to a converter arrangement (1) comprising a network-commutated converter (2) having an ac voltage connection which can be connected to an ac network via at least one phase line, wherein the converter arrangement further comprises at least one switching module branch (31) which is arranged in series in the at least one phase line and which comprises a series circuit of switching modules (341, 342) at whose connections in each case a bipolar voltage can be generated which sums up to a branch voltage. The invention is characterized in that a bridge branch (33) is arranged in parallel connection with the switching module branch, in which bridge branch at least one switching device (37) is arranged, wherein the switching device comprises an antiparallel-connected switchable semiconductor switch (36, 42). The invention also relates to a method for starting a converter device.)

1. Converter arrangement (1) comprising a grid-commutated converter (2) having an ac voltage connection (14-16) which can be connected to an ac grid (5) via at least one phase line (21-23), wherein the converter arrangement further comprises at least one switching module branch (31) which is arranged in series in the at least one phase line (21-23) and which comprises a series circuit of switching modules (341, 342) at whose connections (X1, X2) a bipolar voltage can be generated in each case which sums up to a branch voltage (Ue),

it is characterized in that the preparation method is characterized in that,

a bridge branch (33) is arranged in parallel connection with the switching module branch (31), in which bridge branch at least one switching device (37) is arranged, wherein the switching device (37) comprises an antiparallel-connected switchable semiconductor switch (36, 42).

2. The current transformer arrangement (1) of claim 1,

the converter (2) has n-phase alternating voltage terminals (14-16) which can be connected to an alternating current network via n phase lines (21-23), wherein a switching module branch (31) is arranged in series in each phase line, wherein a series circuit of switching modules (341, 342) is arranged in each switching module branch (31), and a bridge branch (33) is arranged in parallel to each of the switching module branches, each having a switching device with an antiparallel switchable semiconductor switch (36, 42).

3. The converter device (1) of any one of the preceding claims,

wherein the switch modules (341, 342) are full bridge switch modules.

4. The converter device (1) of any one of the preceding claims,

the phase change current converter (2) for power grid conversion is a thyristor-based current converter.

5. The converter device (1) of any one of the preceding claims,

wherein a mechanical bypass switch (35) is arranged in parallel connection with the switch module branch (31) and with the bridge branch (33).

6. The converter device (1) of any one of the preceding claims,

wherein a first inductance (40) is arranged in the switching module branch (31).

7. The converter device (1) of any one of the preceding claims,

wherein a second inductance (39) is arranged in the bridge branch (33).

8. The converter device (1) of any one of the preceding claims,

wherein the number Ah of antiparallel semiconductor switches (36, 42) in the bridge branch (33) is established: ah < ═ As < ═ 3 Ah, where As denotes the number of switch modules in the associated switch module branch.

9. The converter device (1) of any one of the preceding claims,

wherein the converter arrangement (1) comprises a central control unit which is designed to switch on the semiconductor switches in the bridge branch (33) when a predetermined condition exists.

10. The current transformer arrangement (1) of any one of claims 1 to 9,

wherein the semiconductor switches (36, 42) in the bridge branch (33) are designed to be switched on automatically when a predetermined condition exists.

11. The converter device (1) of any one of the preceding claims,

wherein the converter arrangement (1) further comprises a controllable transformer (26) arranged between the at least one switching module branch (31) and the converter (2).

12. Method for starting a converter device (1) according to one of the preceding claims, in which method

-turning off the switching modules in the switching branch;

-turning on the semiconductor switches in the bridge branch with a predetermined delay; and

commutating the branch current from the switching branch to the bridging branch by turning on the semiconductor switch with a delay.

13. The method of claim 12, wherein the first and second light sources are selected from the group consisting of,

the switching module is switched as a function of the switching module voltage and the current direction of the branch current, so that the energy store of the switching module is charged up to a predefined voltage level.

14. The method according to claim 12 or 13,

wherein a mechanical bypass switch (35) arranged in parallel with the bridging branch (33) is opened by switching on semiconductor switches (36, 42) in the bridging branch (33) such that the branch current is commutated to the bridging branch (33).

Technical Field

The invention relates to a converter arrangement comprising a mains-commutated converter having an ac voltage connection which can be connected to an ac power supply system via at least one phase line, wherein the converter arrangement further comprises at least one switching module branch which is arranged in series in the at least one phase line and which comprises a series circuit of switching modules, in each case bipolar voltages of which the sum is a branch voltage being able to be generated at the connection of the switching modules.

Background

Grid commutated converters are known from the prior art. Grid-commutated converters are characterized in particular by the fact that the switching of the converter valves (also referred to as commutation) is effected via the connected ac grid. The semiconductor switches used in grid commutated converters are usually passive elements (such as diodes) or semiconductor switches (e.g. thyristors) that can be switched on but not actively switched off. The advantages of grid commutated converters are mainly their robustness, reliability, relative simplicity in handling and control and the possibility of designing a grid commutated converter for particularly high voltages.

In some applications, grid commutated converters are used in conjunction with poorly performing or unstable ac grids. In these applications, situations may arise where: due to transients, the ac voltage (or voltage time area) provided by the ac grid is less than the ac voltage required for commutation of the converter. Such transients may be, for example, the switching of an ac voltage filter or the change in the electrical power drawn from the ac power grid (so-called voltage dips). This may lead to commutation faults and other disturbances in operation, or even to an interruption of operation. A further challenge associated with grid commutated converters is their high reactive power requirements. Fundamental reactive power compensation of about half the active power is usually required. This also disadvantageously increases the overall device area.

In order to improve the voltage stability in the case of a poorly performing grid, a grid commutated converter can be combined with a parallel compensation. In the case of very poor ac grid performance, the use of rotary phase shifters may additionally be considered.

From the article "Hybrid Topologies for Series and Shunt Compensation of the Line-regulated Converter" by Bakas et al, an arrangement with a Converter for grid commutation and a controllable Series capacitance is known from IEEE 2016 ". The series capacitance is realized here by a full-bridge switching module connected in series in the phase line. In the known arrangement, a "passive" use of full-bridge switching modules is provided in particular, wherein these are used only for the supply of the fundamental voltage. The full-bridge switching module is characterized in particular in that bipolar voltages, namely a positive switching module voltage and a negative switching module voltage, can be generated at its terminals. The magnitude of the switching module voltage substantially corresponds to the energy store voltage formed at the energy store of the full-bridge switching module. The network impedance between the converter and the connection point can advantageously be compensated for by means of the controllable series capacitance.

Disclosure of Invention

The object of the invention is therefore to provide a converter arrangement of the type which is as efficient and reliable as possible.

According to the invention, this object is achieved in a converter arrangement of this type by: a bridge branch is arranged in parallel with the switching module branch, in which bridge branch at least one switching device is arranged, wherein the switching device comprises a switchable semiconductor switch connected in anti-parallel. In particular, the switchable semiconductor switches are controllable, i.e., can be switched on, for example, by means of a suitable control device, so that the switchable semiconductor switches can be actively switched into the conductive state by means of measures at the respective semiconductor switch. The semiconductor switches are connected in anti-parallel if, in the presence of a significant forward conduction direction or blocking direction, the semiconductor switches are oriented oppositely with respect to one another in the respective semiconductor switch pair.

An advantage of the invention is that the protective semiconductor of the switching device allows the switching module to be dimensioned for a lower voltage range, which is usually predefined by steady-state operation. An oversizing of the switching module to achieve a sufficiently high reverse voltage in the event of a fault can thereby be avoided. This is necessary in terms of efficiency and cost, since a large number of series switches, i.e. the number of switch modules used in a switch module branch, implies high transmission losses and high investment.

A further advantage of the invention is that the switching device can be used for controlled precharging of the energy store of the switching module. In other words, the switching module is used on the premise that the energy storage device is charged. By means of the switching devices in the bridge branch, the energy store can be charged from the ac power supply system. In this way it is possible to dispense with the provision of a separate pre-charging device.

By means of the switching devices in the bridging branch, the switching module branch can be bridged without interrupting the load current. This results in advantages for operating the converter device, for example for performing maintenance work independently of the main transmission, and for flexibly switching the device on and off in accordance with further operating requirements.

For the bridge branch, it is suitable to use a semiconductor switch having a higher current carrying capacity than that of the semiconductor switches that can be switched off (for example, transistors such as IGBTs or the like) that may be used in the switching module. The high short-circuit currents that occur can be suppressed by means of a fast current commutation to the bridge branch.

The charging state of the energy store of the switching module can be monitored indirectly from the difference of the voltages measured at the two outer connections of one or more switching module branches, which voltages can be labeled with U1 and U2. This is advantageous when precharging the switching module, since the voltage build-up of the energy store (usually a capacitor) is derived directly from the measured total voltage (branch voltage) as the difference between U1 and U2. This method is more accurate than methods based on measuring the (relatively small) current flowing through the branch of the switch module. The difference U1-U2 may be used as an input value to coordinate at overvoltage. In steady-state operation, the possibility is obtained of using the measured voltages U1 and U2 as input variables for setting the operating point of the switching module branch.

If the converter is connected to a three-phase or multi-phase ac power network, it may be advantageous to provide an arrangement of parallel branches with a switch module branch and a bridge branch for each of the network phases. Correspondingly, the converter has an n-phase ac voltage terminal which can be connected to an ac power supply system via n phase lines, wherein switching module branches are arranged in series in each phase line, wherein a series circuit of switching modules is arranged in each switching module branch, and a bridge branch is arranged in parallel with each of the switching module branches, each having a switching device with an antiparallel switchable semiconductor switch.

Preferably, the switch module is a full bridge switch module. However, further variants (partly known from the prior art) are also conceivable here, such as also half-bridge switching modules in the form of single or multiple parallel connections. Furthermore, further bipolar switching modules are known from the prior art, which are suitable for the application, in particular those which are able to generate more than one positive voltage and/or more than one negative voltage at their connections.

A particularly reliable variant of the invention provides that the network commutated converter is a thyristor-based converter. A thyristor-based converter may have a three-phase bridge circuit with six phase legs (e.g., in a 6-pulse configuration). Each phase leg extends between one of the dc voltage poles of the converter and one of the ac voltage connections. A series circuit of thyristors is arranged in each phase leg. The number of thyristors in the phase leg is determined by the desired design of the converter. In particular, the converter arrangement can be designed to a voltage of more than 100kV, preferably more than 500kV, independently of the choice of power semiconductor switches of the converter. In case of a converter applying grid commutation in the HVDC (high voltage direct current transmission) range, two 6-pulse converters connected in series on the direct current side are preferably operated in a configuration of 12 pulses total. The characteristics described below for the 6-pulse converter can likewise always be transferred to the 12-pulse converter.

According to one embodiment of the invention, the mechanical bypass switch is arranged in parallel connection with the switch module branch and with the bridge branch.

According to one embodiment of the invention, a first inductance is arranged in the switching module branch. In the event of a fault, the first inductance advantageously limits the current rise to the rate/steepness of rise allowed for the protection switch module. The upper limit of the rise rate is defined by the duration required by the protection system until the fault current is commutated into the bridge branch.

According to a further embodiment of the invention, a second inductance is arranged in the bridging branch. The second inductance together with the first inductance limits the current rise during commutation between the switching module branch and the bridge branch to the maximum permitted for the semiconductors of the switching device. Since the first inductance is resistant to both the rise in the fault current and the rise in the current during commutation to the bridge branch, it can be used for optimization in determining the first inductance and the second inductance. The first and/or second inductance may for example be realized as at least one suitable choke.

Suitably, for the number Ah of anti-parallel semiconductor switches in the bridge branch, the following holds: ah < ═ As < ═ 3 Ah, where As denotes the number of switch modules in the associated switch module branch. With this number, on the one hand over-dimensioning is avoided and on the other hand reliable protection is provided.

Suitably, the converter arrangement comprises a central control unit which is designed to switch on the semiconductor switches in the bridge branch when a predetermined condition exists. The bridge protection of the switch module branch or the switch module is therefore predefined by the defined voltage threshold. In the event of a voltage threshold being reached, the semiconductor of the switching device is triggered by means of the central control unit and the switching module is suitably switched off/blocked at the same time.

According to one embodiment of the invention, the semiconductor switches in the bridge branches are designed to be switched on automatically or independently when predetermined conditions exist, i.e. without communication with the central control unit. The selection of the number of series switches of the anti-parallel semiconductor switches in the bridge branch relative to the number of switching modules in the switching module branch can be coordinated such that the automatic triggering unit integrated into the switching device triggers or causes the semiconductor switches of the switching device to be switched on if a maximum permissible applied switching module voltage is reached. The advantage of this variant is that the integrated triggering of the semiconductor switch ensures protection of the switch module even without a central control unit. A possible criterion for this design is that the maximum allowed total voltage (branch voltage) causes the semiconductor switches in the bridging branch to reach the BOD threshold ("Break Over Diode") threshold.

Preferably, the converter arrangement further comprises a controllable transformer arranged between the at least one switching module branch and the converter. Within the scope of the invention, a controllable transformer is in particular a transformer with a controllable transformation ratio. By means of the controllable transformer, voltage amplitude variations (in particular increases) caused by the additional series voltage applied to the switching module branch can advantageously be compensated. In this way, additional voltage loading of the converter or its valves (e.g., thyristor valves) can be prevented. Furthermore, the converter operating point (dc voltage, dc current, firing angle, overlap angle) can advantageously be kept independent of the operation of the switching module. Furthermore, by using a controllable transformer, it is possible to extend an existing grid commutated converter ("upgrade") with a switching module branch.

The invention further relates to a method for starting or for starting a converter arrangement according to the invention. The starting or starting of the converter device is usually carried out after an interruption of the operation of the converter device, for example due to an internal or external fault.

The object of the present invention is to provide a method which makes it possible to start a converter device as reliably as possible.

According to the invention, this technical problem is solved in such a method by: switching off the switch module in the switch branch; turning on the semiconductor switches in the bridging branch with a predetermined delay; and commutating the branch current from the switching branch to the bridging branch by delayed turn-on of the semiconductor switch.

Suitably, the semiconductor switches in the bridge branch are switched on at a defined point in time after the zero crossing of the current, wherein the defined point in time is characterized by a delay time between the zero crossing of the current and the switching on of the semiconductor switches; selecting a delay time depending on the magnitude of the current, wherein the delay time is selected such that the greater the current, the smaller the delay time (in this case the delay time may also be zero), the branch current is commutated from the bridge branch to the switching branch by controlling the semiconductor switches; after the current zero crossing of the branch current, the branch current is commutated from the switching branch to the bridge branch by a delayed switching-on of the semiconductor switch.

In this case, the switching modules are preferably switched (independently) as a function of the switching module voltage and the current direction of the branch current, so that the energy store of the switching module is charged up to a predefined voltage level. In particular, the semiconductor switches in the bridge branch can thus be used for precharging the energy store. This method advantageously enables the converter device or the switching module branch to be started even when the converter device is already in operation, i.e. the load current flows at the usual operating level.

Preferably, the mechanical bypass switch arranged in parallel with the bridge branch is switched off, i.e. cut off for the current flow, by switching on the semiconductor switch in the bridge branch, so that the branch current is commutated to the bridge branch.

A preferred variant of the method can be described as follows. In a first method step, the switching module branch is bridged by means of a mechanical bypass switch, so that the load current flows through the mechanical bypass switch. In a second method step, a fast bridge is switched by means of a switching device (for example, an antiparallel thyristor) in the bridge branch by means of one or more suitable switching signals. In a third method step, the mechanical bypass switch is opened; the load current is commutated to the bridge branch; in this case, the switching module is blocked and its energy store is discharged. In a fourth method step, the turn-on signals to the switchable semiconductor switches are interrupted, so that these switchable semiconductor switches are switched off or switched off at the zero crossing of the current; the load current commutates to the switch module branch and charges the energy storage (e.g., capacitor); the duration of the charging process is determined as a function of the magnitude of the load current and the magnitude of the capacitance and the desired target voltage to which the energy store is to be charged. After a predetermined time, the semiconductor switches in the bridge branch are switched on again and the current is commutated again to the bridge branch; the charging process is interrupted for the blocked switching module. If the switch module does not report to the central control unit after a defined time, as appropriate, it can be assumed that the precharging has not been adequately performed in the past; in this case, the above-described method steps may be repeated. In this case, the blocking time of the semiconductor switches in the bridge branch can be reduced with each repetition, for example to half. Suitably, after a sufficiently high energy storage voltage has been reached and/or a sufficient number of switching modules have been fed back, a clocked operation of the switching modules is transferred, in which a bipolar voltage or an off state is generated by switching the semiconductor switches of the switching modules. The flow of clock-triggered operations may proceed as follows. When the semiconductor switches in the bridge branch are switched on, all the switching modules are switched to the off state. The semiconductor switches in the bridge branches are switched to the off state (without a switching-on pulse, the semiconductor switches are switched off at the zero crossing of the current). By switching the individual switching modules (or their semiconductor switches) to the "positive output voltage" or "negative output voltage" state, the energy store reaches the voltage range required for normal operation. After a sufficient number of switching modules are available for normal operation, a transition is made to normal operation and the original function of the switching modules is carried out.

A further embodiment of the precharging consists in that, instead of the fourth method step, the semiconductor switches in the bridge branch are controlled with a phase angle change, i.e. a delay angle of triggering of the semiconductor switches (for example thyristors) that can be switched on is used, so that only a (small) fraction of the current half-oscillation is commutated to the switching module and the energy store of the switching module is charged. This deformation is advantageous in particular in the case of large charging currents. For this purpose, a small trigger delay angle is used, so that the current flows first through the switching module starting from the zero crossing of the current and commutates to the switching module with a characteristic delayed by only a few degrees. This enables the switching module to be charged with a defined charging current. The effective value of the current can be controlled by triggering the delay angle. The load current can be kept small as long as the operating principle of the load allows, for example, in such a way that the semiconductor switches in the bridge branch are no-load on the grid and therefore only no-load current flows. The lowest load, for example, for which the charging current of the wiring of the blocked thyristor valve contributes in addition to the HVDC converter transformer in no load, is determined by the thyristor converter with connected HVDC.

Drawings

The invention is further elucidated below on the basis of the embodiments of fig. 1 to 4.

Fig. 1 shows an exemplary embodiment of a converter arrangement according to the present invention in a schematic illustration;

fig. 2 shows an embodiment of an arrangement of parallel branches for a converter arrangement according to the invention in a schematic diagram;

fig. 3 shows a first vector diagram of a branch current and a branch voltage for a branch of a switching module in a schematic diagram;

fig. 4 shows a second vector diagram for the branch current and the branch voltage of a branch of a switching module in a schematic diagram.

Detailed Description

Fig. 1 shows a converter arrangement 1 which is connected to a three-phase ac power grid 5 at a grid connection point 4. The converter arrangement 1 comprises a grid commutated converter 2. The converter 2 has a dc voltage side, which is connected to a dc network or a dc line 3. The ac voltage side of the converter 2 is arranged with a controllable transformer 26 comprising a step switch. The converter 2 comprises six converter limbs or converter valves 6 to 11, which each extend between one of the dc voltage poles 12 or 13 of the converter 2 and one of the three ac voltage connections 14 to 16. A series circuit of thyristors 17 is arranged in each of the converter arms 6-11. The converter 2 is connected to the ac power grid 5 via three phase lines 21 to 23 by means of ac voltage connections 14 to 16.

The converter arrangement 1 further comprises a first switching module branch in the first arrangement of the parallel branches 18, a second switching module branch in the second arrangement of the parallel branches 19 and a third switching module branch in the third arrangement of the parallel branches 20. The first branching arrangement 18 leads in series into a first phase line 21, the second branching arrangement 19 leads in series into a second phase line 22, and the third branching arrangement 20 leads in series into a third phase line 23. The three phase lines 21-23 extend between a connection point 25 to a transformer 26 and the grid connection point 4. In the example shown in fig. 1, the three branch arrangements 18-20 are constructed in the same way, but this need not generally be the case. The structure of the arrangement of the parallel branches 18-20 and the structure of the switch module branches are discussed in more detail in fig. 2 below.

The voltage dropped across the switch branch is marked Uc. The line-to-ground voltage on the converter side is labeled U1, and correspondingly the line-to-ground voltage on the grid side is labeled Unet. The branches 18 to 20 serve to compensate the grid impedance Xnetz and/or the converter-side impedance Xc and to stabilize the connection voltage Uprim at the connection point 25, in order to ensure a stable and reliable operation of the converter device 1 and in particular of the converter 2. For this purpose, the converter device 1 has a central control unit 24, which is designed to regulate the control of the switching module branches or to initiate the control of the semiconductor switches used there. By means of the controllable transformer 26, the connection voltage Uprim is converted into the output voltage Usec, so that the amplitude of this output voltage is reduced.

In fig. 2 is shown an arrangement of parallel branches which can be used as one or more of the branches 18-20 in the converter arrangement of fig. 1. The bridging branch 33 is arranged in parallel connection with the switching module branch 31 and the bypass switch 35 is arranged in a further parallel connection. The switching module branch 31 comprises a series circuit 34 of switching modules 341, 342, which are full-bridge switching modules known from the prior art (only two switching modules 341, 342 are shown in the figure, although the number thereof can in principle be arbitrary and adapted to the respective application). Each full-bridge switching module comprises its own energy store 38 in the form of a storage capacitor, and a semiconductor switch 41 in the form of, for example, an IGBT, which can be switched on and off. Here, a freewheeling diode is connected in anti-parallel to each IGBT. A bipolar voltage can be generated at the connection terminals of each full-bridge switching module. Furthermore, a first inductance 40 is arranged in the switching module branch 31.

The bridging branch 33 comprises a switching device 37. The switching device 37 has a first switchable semiconductor switch 36 in the form of a thyristor and a second switchable semiconductor switch 42 also in the form of a thyristor. The two semiconductor switches 36 and 42 are turned on in opposite directions in the forward direction. In this sense, semiconductor switches 36 and 42 are connected in anti-parallel. The bridge branch 33 further comprises a second inductance 39. The further inductance of the arrangement is marked with reference numeral 32. The difference between the voltages U1 and U2 corresponds to the branch voltage developed on the switch module branch.

A vector diagram 50 is shown in fig. 3. The vector diagram 50 is a voltage-current diagram for a rectifier operating condition of a converter device, which corresponds, for example, to the converter device 1 of fig. 1. The primary side voltage Uprim on the primary side of a controllable transformer (e.g. transformer 26 of fig. 1) and the secondary side voltage Usec on the secondary side of the transformer are shown in the vector diagram 50. The primary-side voltage Uprim corresponds here to the connection voltage at the connection point between the switching module branch and the transformer. It can be seen that the branch voltage UFB applied to the switching module branch is phase shifted by pi/2 with respect to the primary side current iprim on the primary side of the transformer. At the same time, the primary side current iprim is offset by an angle relative to the network voltage Unet of the ac network connected to the converter deviceIt can also be seen that the primary-side voltage Uprim is composed of the network voltage Unet and the branch voltage UFB. The secondary-side voltage Usec is in phase with the primary-side voltage Uprim, but has a reduced amplitude (by means of a transformer). Vector diagram 50 also illustrates that a reference frame for the branch current iprim is selected for the regulation of the branch voltage UFB. In this case, a branch current iprim flowing through one or more switching module branches corresponds to the network currentinet. In the case shown in fig. 3, the network voltage uet leads the primary-side voltage Uprim by an angle

A vector diagram 60 is shown in fig. 4. The vector diagram 60 is a voltage-current diagram for an inverter operating state of a converter device, which corresponds, for example, to the converter device 1 of fig. 1. The primary side voltage Uprim on the primary side of the controllable transformer (e.g. transformer 26 of fig. 1) and the secondary side voltage Usec on the secondary side of the transformer are shown in the vector diagram 60. The primary-side voltage Uprim corresponds here to the connection voltage at the connection point between the switching module branch and the transformer. It can be seen that the branch voltage UFB applied to the switching module branch is phase shifted by pi/2 with respect to the primary side current iprim on the primary side of the transformer. At the same time, the primary side current iprim is offset by an angle relative to the network voltage Unet of the ac network connected to the converter deviceIt can also be seen that the primary-side voltage Uprim is composed of the network voltage Unet and the branch voltage UFB. The secondary-side voltage Usec is in phase with the primary-side voltage Uprim, but has a reduced amplitude (by means of a transformer). Vector diagram 60 also illustrates that a reference frame for the branch current iprim is selected for the regulation of the branch voltage UFB. In this case, a branch current iprim flowing through one or more switching module branches corresponds to the network current inet. In the case shown in fig. 4, the network voltage Unet lags behind the primary-side voltage Uprim by an angle