CN110635744A

CN110635744A - Converter system

Info

Publication number: CN110635744A
Application number: CN201910890311.2A
Authority: CN
Inventors: 范宝龙
Original assignee: Jiangsu Longyang Power Equipment Co Ltd
Current assignee: Jiangsu Longyang Power Equipment Co Ltd
Priority date: 2019-09-20
Filing date: 2019-09-20
Publication date: 2019-12-31

Abstract

A converter system (10) comprises a plurality of converter cells (26), each converter cell (26) being adapted to convert an AC cell input voltage into a cell output voltage and a transformer (18). Having a plurality of secondary windings (22), each secondary winding (22) 5 being connected with one converter cell (26) and providing an AC cell input voltage of one converter cell (26), wherein the secondary windings (22) are arranged in at least two groups (G1, G2, G3) and the converter cells (26) connected to one group (G1, G2, G3) are connected in series, wherein the transformer (18) is designed such that the secondary windings (22) of different groups (G1, G2, G3) provide AC cell input voltages which are phase shifted 10 with respect to each other such that higher harmonics (26) generated by the converter cells cancel each other out, and wherein the secondary windings (22) of one group (G1, G2, G3) provide AC cell input voltages with at least two different phase shifts (θ 1, θ 2, θ 3, θ 4, θ 5, θ 6).

Description

Converter system

Technical Field

The present invention relates to the field of harmonic cancellation in electrical converter systems. In particular, the invention relates to a converter system with a special transformer for minimizing input harmonics, in particular when applying redundancy of converter cell stages.

Background

One particular type of converter system is the so-called cascaded H-bridge converter system, in which converter cells connected in series at their output terminals are connected at their input terminals to the secondary winding of a transformer. For example, such converter systems are used in subsea drives, wherein the converter system may be arranged subsea at sea, e.g. near a pump or the like. Such subsea systems require high reliability and long maintenance times (up to 5 to 7 years), so the use of miniaturization and redundancy of components may be one of the key issues.

When a multi-pulse transformer is used in such a cascaded H-bridge converter system, the transformer has several secondary windings providing different phase shifts, which may allow low current Total Harmonic Distortion (THD) values to be obtained.

For example, US5,625,545 and EP0913918a2 show converter systems in which converter cells of one phase are connected to a secondary winding, providing a different phase shift for each converter cell.

However, developing a multi-pulse transformer that produces a well-defined phase shift angle between the multiple three-phase systems provided on the secondary windings of the transformer may require the application of complex winding strategies (Zig-zag, Poligon, etc.). This winding strategy may result in different secondary-related short circuit impedances due to different coupling factors between the primary and secondary windings and coupling between the secondary windings. This difference in coupling factor and equivalent short circuit impedance of each secondary winding can result in ineffective harmonic cancellation on the primary side of the transformer, different DC link voltages associated with each DC link, and different harmonic currents of the DC link capacitors. These problems are therefore overcome, and become more complex as the number of secondary windings increases.

Even with a large number of converter cells and corresponding transformer secondary windings, current harmonic cancellation may be practically limited by the transformer design (coupling factor between windings, turns difference, etc.).

In addition, in the event of a failure of one of the converter cells, when redundancy of the converter cell stages is provided, the input of the converter cell may be disconnected from its corresponding transformer secondary and the output of the converter cell may be short circuited. In this case, the converter operation can continue even under nominal conditions. Also, even if the converter unit is fully operational, a unit standby strategy (N +0 operation) may be applied to minimize the total losses of the converter system and to increase the reliability of the converter system. In the case of one or more defective converter cells disconnected from the transformer, however, the number of active secondary windings of the multi-pulse transformer is reduced compared to full operation, and thus harmonic cancellation may be affected,

EP2587658a2 discloses a converter system comprising: a transformer having a plurality of secondary windings, each secondary winding being connected to one of the converter units and providing the AC unit input voltage of one of the converter units. The secondary windings are arranged in at least two groups, and the converter cells connected to one group are connected in series.

At US6,229,722B1, a universal converter system is disclosed.

Further, EP2782240a2 discloses a multilevel inverter and battery, wherein a battery switch circuit is selectively disconnected from the battery output and a bypass is closed to connect first and second battery output terminals to selectively bypass a power stage. A multi-level inverter having an optional AC input switch to selectively disconnect the AC input from the battery switch circuit during bypass.

Disclosure of Invention

It is an object of the present invention to provide a simple and robust converter system having a low THD even in case of failure of one or more converter cells and/or providing converter cell level redundancy.

This object is achieved by the subject matter of the independent claims. Further exemplary embodiments are apparent from the dependent claims and the following description.

Converter system technical field the present invention relates to a converter system, e.g. based on a cascaded H-bridge converter, connected to a multi-pulse transformer. The converter system may be

Adapted to operate subsea, may for example comprise a housing that is subjected to subsea pressure and/or is sealed against sea water. The converter system may be a medium voltage system, which may for example be adapted to handle voltages larger than 1 kV.

According to an embodiment of the invention, the converter system comprises a plurality of converter units, each converter unit being adapted to convert an AC unit input voltage into a unit output voltage, and a transformer having a plurality of secondary windings, each secondary winding being connected to one converter unit and providing the AC unit input voltage of one converter unit. Wherein the transformers are designed such that the secondary windings of the different groups provide AC unit input voltages that are phase shifted with respect to each other such that higher harmonics generated by the converter units cancel each other out.

It has to be understood that since each secondary winding is associated with one converter cell, the set of secondary windings can also be considered as a set of converter cells. These secondary winding/converter cell groups may be used to provide phases of the output voltage of the converter system, or may be connected in series to provide one output phase.

Since the converter system comprises secondary windings providing phase shifted AC converter cell voltages for different groups, the THD of the overall system can be reduced. In addition, to keep the transformer complexity down, each group of converter cells may be associated with only one phase shift or only two phase shifts relative to the other groups. Also in this case, the THD of the converter system can be reduced even in the case where the redundant unit is disconnected from the transformer.

According to one embodiment of the invention, in case of K ≧ 3 converter cells per group, the secondary windings of one group provide the AC cell input voltage with at most two different phase shifts, in order to minimize the transformer complexity despite the higher number or phase. Gear shifting may be applied.

According to an embodiment of the invention, each group of converter cells comprises at least one redundant converter cell. One or more or all converter cells in a group may be redundant, i.e. may be disconnected or connected to the group, while the converter system is still operational. Redundancy may be applied at the cell level.

In particular, the phase shift angle between different transformer secondary windings of different sets may be selected to ensure a sufficient transformer input current THD when the converter system is operated with all converter units.

Redundancy is provided at the cell level (N +1 operation), but when it is necessary to apply redundancy concepts/strategies to some non-working and/or malfunctioning converter cells over some different driving phases (N +0 operation). Furthermore, by using one or more secondary windings of a group having the same phase shift angle, the design complexity of the transformer can be minimized. This may also minimize the asymmetry between the secondary windings of the transformer in terms of short-circuit impedance.

Thus, different redundancy concepts can be applied to the converter system. For example, 1,2,3, etc. may consider each set of redundant converter cells (N +1 operation, N +2 operation, N +3 operation, etc.).

Furthermore, it is also possible to use all converter cells for normal operation (N +0 operation) to save losses in the converter and minimize stress/increase the reliability of the converter. In this case, the THD can be kept at a low level even when one or more converter units fail.

Especially for subsea drives with such converter systems, where reliability may be a critical issue, the converter system may provide redundancy of the entire rectifying-inverting stage when using modular converter units (e.g. with N +1 operational redundancy applications). At the cellular level). These modular converter units may comprise medium voltage components.

The transformer may have a primary winding and a common magnetic core for the primary and secondary windings. Since the input of the transformer at the primary winding and the output of the secondary winding may be multi-phase (in particular three-phase), the primary winding and/or the secondary winding may each have a plurality of single-phase windings.

According to an embodiment of the invention, the system comprises M groups of secondary windings, M being an integer, and the phase shift between the AC unit input voltages of different groups is 607M. In this case, the groups provide the phase of the output of the converter system, M may typically be 3 in the case of M =3, the phase shift between the AC unit input voltages of the different groups may be 20 °.

It has to be understood that in this case, when there is also a phase shift between the AC unit input voltages of a group, there may also be a phase shift between the groups other than 607M.

According to an embodiment of the invention, the number of different phase shifts is a multiple of M. When counting all possible phase shifts (independent of the group) provided by the secondary winding, there may be M, 2M, 3M, etc. Phase shifts the total number of phase shifts may be M in the case of only one phase shift per group, the number of phase shifts may be 2M in the case of two phase shifts per group.

The secondary windings of a group may have the same phase shift. As mentioned above, each group may have only one phase shift. In this case, the AC output voltages of a group are all in phase.

According to an embodiment of the invention, the phase shift between the AC unit input voltages of a set of secondary windings is 30 °. With two different phase shifts per group, there may be two types of secondary windings per group that provide AC unit input voltages that are 30 ° phase shifted between the two types.

According to an embodiment of the invention, each group comprises two, three or four secondary windings and converter cells. For medium voltage semiconductors, only a small number of converter cells per group may be used to generate the output voltage required by the medium voltage converter. Provided is a system. However, depending on the output voltage, a larger number of secondary windings and converter cells may also be applied.

In summary, the secondary windings are arranged over M sets of secondary windings (e.g., number of inverter phases M ≧ 3), where each set of secondary windings contains K secondary windings (e.g., number of converter cells K ≧ 2 per converter stage)). The number X of phase shift angles θ may be selected as a number Y (X = Y × M) in the M groups, wherein the number of phase shift angles will be assigned to each of the M secondary groups. The winding is Y (Y =1 or Y = 2). The phase shift between the secondary windings of a group may be

And the offset between adjacent secondary winding groups may be θ 1= 607M.

According to an embodiment of the invention, the converter cells of a group are connected in series such that two directly connected converter cells have the same phase shift in their AC cell input voltage. For example, in case of three converter cells per group, two adjacent converter cells may not have a phase shift, while the third converter cell is phase shifted with respect to the two adjacent converter cells. In case of four converter cells per group, there may be two pairs of adjacent converter cells, each pair of converter cells being not phase shifted, but the pairs being phase shifted.

According to an embodiment of the invention, the converter cells of a group are connected in series such that two directly connected converter cells have different phase shifts in their AC cell input voltages. For example, the converter cells of different phase shifts may alternate with respect to each other.

According to an embodiment of the invention, each group of series connected converter cells provides a phase for the converter output voltage, i.e. one end of the group may provide the output of the converter system. Furthermore, the series-connected groups of converter cells may be star-connected or delta-connected with their other end.

According to an embodiment of the invention, the series-connected groups of converter cells are connected in series to provide one output phase. It is also possible that the converter system has only one output phase AC or DC, which is generated by all converter units of all groups connected in series.

According to an embodiment of the invention, each converter cell comprises a bypass switch, for example for and/or bypassing the converter cell when the converter system fails. To provide redundancy, the converter system may disconnect each bypass converter cell from its group.

According to an embodiment of the invention, the AC unit input voltage is a multi-phase (e.g. three-phase) voltage. That is, each secondary winding may include a single winding for each phase.

Further, the AC battery output voltage may be a single phase voltage, and the converter output voltage may be a multi-phase (e.g., three-phase) voltage. The multi-phase voltage from the secondary winding may be converted by the respective converter unit into a single-phase voltage (of different frequency), which is added to one phase of the converter output voltage.

According to embodiments of the invention, each converter unit may comprise a (6-pulse) rectifier for rectifying the AC unit input voltage, each converter unit may comprise a DC-link and/or each converter unit may comprise an (H-bridge) inverter. For providing an AC battery output voltage. The converter unit may be an indirect converter unit, wherein the rectifier is connected to the secondary winding and the output may be an inverter connected in series. These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.

Drawings

Fig. 1 schematically shows a converter system according to an embodiment of the invention;

fig. 2 schematically shows a converter cell of the converter system of fig. 1;

fig. 3 schematically shows a converter system according to another embodiment of the invention;

FIG. 4 schematically illustrates a converter system according to another embodiment of the invention;

FIG. 5 schematically illustrates a converter system according to another embodiment of the invention;

FIG. 6 schematically illustrates a converter system according to another embodiment of the invention;

FIG. 7 schematically illustrates a converter system according to another embodiment of the invention;

fig. 8 schematically shows a converter system according to another embodiment of the invention.

In the figure: 10 converter systems, 12 inputs, 14 outputs, 16 loads, 18 transformers, 20 primary windings, 22 secondary windings, 24 secondary windings for outputs, 26 converter cells, 28 converter cell outputs, 30 cell modules, 32 cell controllers, 34 rectifiers, 36 dc links, 38 inverters, 40 disconnect switches, 42 bypass switches, a, B, C output phases, Gi, G2, G3 groups, 01,02,03,04,05,06 phase shifts.

Detailed Description

Fig. 1 shows a converter system 10 in the form of a cascaded H-bridge converter that can be used for subsea drives, for example to provide secondary loads, such as pumps and compressors, without regeneration requirements.

The converter system comprises a three-phase input 12 connectable to a power grid and a three-phase output 14 having phases a, B, C, which may be connected to an electrical load 16, such as an electric motor.

The input 12 is connected to a transformer 18 having a primary winding 20 and a plurality of secondary windings 22, each secondary winding 22 having a three-phase output 24 that provides an AC unit input voltage to a converter unit 26. For each phase a, B, C, the secondary winding 22 and the converter cells 26 are grouped into three groups Gi, G2, G3. The converter cells 26 of each group are connected in series with their single phase outputs 28 and with each group Gi. The G2, G3 of the series connected converter cells 26 provide the phases a, B, C of the output 14 of the converter system 10 at one end. At the other end, the groups Gi, G2, G3 of converter cells are star-connected.

Fig. 2 shows a converter unit 26, components of the converter unit 26 may be arranged on a converter unit module 30, and the converter unit module 30 may further comprise control components 32, such as local unit controllers, gate units and/or fibre optic connections.

To convert the AC battery input voltage from the respective secondary winding 22 into an output current, each converter unit 26 includes a passive 6-pulse rectifier 34, a DC link 36 and an inverter 38 with an H-bridge. The diodes of the rectifier 34 and the IGBTs of the inverter 38 may be medium voltage semiconductors (e.g., rated at 4.5 kV).

Furthermore, to bypass the converter cells 26, each converter cell 26 may comprise a switch 40 for disconnecting the converter cell 26 from the corresponding secondary winding 22 and a bypass switch 42 for bypassing the converter cell 22 at its output 28.

The converter system 10 based on cascaded H-bridge converters using medium voltage semiconductors is considered a good compromise in terms of complexity, reliability and performance of the subsea drive. For output voltages in the range of 6.6kV, the converter system 10 may comprise 4 converter cells 26 per phase.

To avoid and/or minimize the above-mentioned problems of multi-pulse transformers, the transformer 18 is designed in a special way when applying redundancy at the converter cell level, the secondary winding asymmetry and the current THD increase. The secondary windings 22 provide phase shifts θ 1 to θ β in the AC battery input voltage, where there is a phase shift between groups Gi, G2, G3 of the secondary windings 22 and a phase shift between secondary windings within one group Gi, G2, G3.

It has to be noted that the absolute phase shifts θ 1 to θ β can be provided with respect to the non-shifted (three-phase) voltage, wherein the relative phase shift between the two secondary windings 22 can be calculated as the corresponding difference. An absolute phase shift. All of the absolute phase shifts θ 1 to θ β and the relative phase shift may be provided as phase shift angles, for example, in degrees.

In the case of fig. 1, the number of secondary windings is 12 for all output phases (in the case of N +1 operation), and 9 of them may be operable if N +0 operation is applied in all phases. Redundant operations may be applied in the following cases:

one battery fails and the converter system 10 operates without the converter unit 26. Thus, the converter output voltage capability with 11 operating converter cells 26 can be maintained to the maximum.

After a failure of one unit, another converter unit 26 in a different output phase a, B, C fails and the converter system 10 operates without both converter units 26. Thus, a converter output voltage capability with 10 operating converter cells 26 may be preserved. And max.

One converter cell 26 per phase a, B, C fails and the converter system 10 operates without these three converter cells 26. Thus, the converter can maintain operation.

One converter cell 26 fails and therefore all phases a, B, C are rearranged to work with the same number of converter cells 26 per phase (3 converter cells 26 per phase a, B).

3 converter cells 26 per phase a, B, C are applied, wherein one converter cell 26 per phase a, B, C is configured in standby mode) in order to minimize converter power losses, corresponding additional thermal stresses and to improve the overall reliability of the converter.

Since the 12 secondary windings 22 all have different phase shifts, ideally up to 72 pulses can be applied. However, in this case actual harmonic cancellation is not easily achieved. Furthermore, this would mean an increase in the complexity of the transformer design and manufacture to actually provide the 12 phase shifts required between the 12 three-phase secondary windings. In addition, operation with only at most 9 converter cells 26 in case of N +0 operation may have an impact on the harmonic cancellation of such a transformer 18.

Thus, the arrangement and design of the 12 secondary windings 22 of the transformer 18 takes into account the possible operation of N +1 (12 converter cells) and N +0 (down to 9 converter cells 26) so that in any operating mode the current THD remains below the required value at the transformer input 12. In particular, consider the N +0 mode of a non-working converter cell 26 with any configuration in all converter phases a, B, C. It should be noted that in terms of THD, the case with 9 converter cells 26 is worse than the case with 10 and 11 converter cells 26. Therefore, in the following, only the case with 9 converter cells 26 is considered.

Fig. 1 shows a solution in which 12 secondary windings 22 are arranged with 6 phase shifts θ x as follows:

the 4 converter cells 26 of each phase a, B, C are provided by 4 three-phase secondary windings 22, the 3 secondary windings 22 being arranged with only 2 respective phase shift angles (θ 1 and θ 2) as if they would provide a 2-time 12-pulse rectifier, (Θ 1- Θ 2=30 ° being equivalent to the classical DY configuration of a 12-pulse transformer, which means that for each phase a, B, C only two phase shifts =30 ° between the 4 respective secondary windings 22 are required when each two secondary windings 22 have the same phase shift angle (Θ 1- Θ 2= Θ 3- Θ 4= Θ 5- Θ 6).

Secondary windings 22 having the same phase shift angle may actually be damaged along with the corresponding insulation between the wires to make secondary windings 22 as identical as possible to minimize winding asymmetry.

The phase shift between the 4 secondary windings 22 associated with one phase a, B, C relative to the 4 secondary windings 22 associated with the other phase a, B, C corresponds to the angle of the transformer 18 for the desired 18 pulse rectifier. (θ ι = -20 °, θ 3=0 °, θ 5=20 °).

With this configuration of the secondary windings 22 and from the perspective of the current THD, in N +1 operation, the 4 secondary windings 22 associated with each phase a, B, C appear as transformers for a 12-pulse rectifier, and the 12 secondary windings 22 act as transformers for a 36-pulse rectifier. In the case of N +0 operation, the 3 working secondary windings 22 of each phase a, B, C do not provide equivalent operation to the transformer for a 12-pulse rectifier, but provide improved operation of a 6-pulse rectifier with reduced 5 and 0. The 7 th harmonic. For all three phases a, B, C, the behavior of the current THD at the input 12 from the converter system 10 is always that of the transformer for the 18-pulse rectifier, regardless of the converter cell configuration employed (such as a random bypass converter cell).

In addition, the phase shifts θ 1 to θ β of the 12 secondary windings 22 are selected to minimize the number or desired phase shift angle between different 3-phase systems, which can help simplify the mechanical design of the windings, thereby minimizing short circuits. The impedance between them is asymmetric.

Fig. 1 shows only an example of how the phase shifts θ 1 to θ β are arranged to achieve the desired THD when operating in the N +1 and N +0 modes of operation.

Fig. 3 to 8 below show other configurations of the converter system 10 in which the above-described principles are applied. Note that the connection between the secondary winding 22 and the converter cell 26 is omitted for clarity. In each secondary winding 22, a corresponding absolute phase shift θ x is depicted. Each converter cell 26 also shows the corresponding absolute phase shift θ x and the group Gi, G2, G3 to which it belongs.

In fig. 3, a configuration of a converter cell 26 with four, three and two secondary windings 22 and each phase a, B, C (or group Gi, G2, G3) is shown, with two different phase shifts. A phase (or group). Note that in fig. 3, the converter cells 26 belonging to different phase shifts are connected in series in an alternating manner, whereas the converter cells 26 belonging to the same phase shift in fig. 1 are adjacent in the series.

The number of phase shifts can be further reduced by having only one phase shift per phase a, B, C or group Gi, G2, G3. In fig. 4, 6,8 configurations of converter cells 26 with four, three and two secondary windings 22 and each phase a, B, C (or group Gi, G2, G3) are shown, with only one phase shift per phase (or group).

It has to be noted that the above discussion also applies to the groups Gi, G2, G3 of secondary windings 22 and converter cells 26, which are not directly related to the output phases a, B, C of the converter system 10. For example. Fig. 7 shows that the groups Gi, G2, G3 are connected in series to form a single phase output 14 fig. 8 shows that the groups Gi, G2, G3 may be delta connected. In fig. 1,3,4,5 and 6, the groups Gi, G2, G3 are star-connected at one end to form phases a, B, C.

In general, the following rules apply to all configurations shown in fig. 1 and 3 to 8:

the secondary windings 22 are grouped (e.g., Gi, G2, G3) into secondary windings 22 that provide series-connected converter cells 26. For M output phases (M =3 for three phases a, B, C), there will be M sets of secondary windings 22.

Each group contains K ≧ 2 converter cells 26, (K being even or odd), so at least redundancy N +1 at the level of each group of cells (higher redundancy levels, N +1, N +2, etc. may also be applied, when there are a corresponding number K of converter cells 26).

The number of phase shifts θ X is selected as a multiple Y of M groups (X = Y × M). For M =3, the number of phase shifts X is considered to be effectively limited to a maximum of X =6 in order to minimize the complexity of the transformer design and/or to minimize the short circuit impedance asymmetry between the different secondary stages.

Thus, the maximum number X of phase shifts θ X per secondary winding 22 may be practically limited to Y =1,2, although in theory Y may be a higher integer value, as long as the transformer complexity and its meaning is affordable.

The number of phase shifts θ x assigned to each of the M groups of secondary windings 22 is Y the difference in phase shift (displacement) between the secondary windings 22 of each group is θ γ = 607Y. If Y is 1, θ γ =60 ° but only one phase shift, then θ γ will not actually apply in this case.

The phase shift between adjacent secondary winding groups 22 is chosen such that in case of loss of redundant converter cells 26 (N +0 redundancy applied at the cell level), the entire rectifier (rectifier-based) converter system 10 of fig. 1) behaves as at least a M x 6 pulsed rectifier. For M =3, the overall rectifier will behave in the worst case as an 18-pulse rectifier (at least as a 24-pulse rectifier for M = 4). Therefore, the phase shift difference between adjacent secondary winding groups is θ χ = 607M.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive, the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art and practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single processor or controller or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims shall not be construed as limiting the scope.

Claims

1. A converter system (10) comprising: a plurality of converter cells (26), each converter cell (26) being adapted to convert an AC cell input voltage into a cell output voltage, a transformer (18) having a plurality of secondary windings (22), each secondary winding (22) being connected with one converter cell (26) and providing an AC cell input voltage of one converter cell (26), wherein the secondary windings (22) are arranged in at least two groups (Gi, G2, G3) and connected in series to the converter cells (26) of one group (Gi, G2, G3), wherein the transformer (18) is designed such that the secondary windings (22) of different groups (Gi, G2, G3) provide AC cell input voltages phase shifted with respect to each other such that higher harmonics resulting therefrom, the converter cells (26) cancel each other out, wherein the secondary windings (22) of one group (GI, G2, G3) provide AC input cells with at least two equal phase shifts (Θ 1), 02,03, θ voltage 4, 9S, θ β).

2. The converter system (10) of claim 1 wherein the secondary winding (22) of one group (Gi, G2, G3) provides an AC cell input voltage-displacement (Θ 1,02,03, θ 4, 9S, θ β) having at most two different phases in the case where K ≧ 3 converter cells (26) per group (Gi, G2, G3).

3. The converter system (10) according to claim 1 or 2, wherein each group (Gi, G2, G3) of converter cells (26) comprises at least one redundant converter cell.

4. The converter system (10) of any one of the preceding claims 1 to 3, wherein the converter system (10) comprises M groups (Gi, G2, G3) of secondary windings (22), M being an integer and the phase shift between the AC cell input voltages of the different groups being 607M.

5. The converter system (10) of claim 4, wherein the number of different phase shifts is a multiple of M.

6. The converter system (10) according to any of claims 2 to 5, wherein the phase shift between the AC cell input voltages of the secondary windings of a group (Gi, G2, G3) is 30 °.

7. The converter system (10) of one of the preceding claims, wherein each group (Gi, G2, G3) comprises two, three, four or more secondary windings (22) and converter cells (26).

8. The converter system (10) of one of the preceding claims, wherein the converter cells (26) of a group (Gi, G2, G3) are connected in series such that two directly connected converter cells (26) have the same phase shift in their AC cell input voltages.

9. The converter system (10) of one of the preceding claims, wherein the converter cells (26) of a group (Gi, G2, G3) are connected in series such that two directly connected converter cells (26) have different phase shifts in their AC cell input voltages.

10. The converter system (10) of one of the preceding claims, wherein each group (Gi, G2, G3) of series-connected converter cells (26) provides a phase (A, B, C) for the converter output voltage, and/or wherein the groups (Gi, G2, G3) of series-connected converter cells (26) are star-connected or delta-connected.