WO2015128455A2

WO2015128455A2 - Three-phase to three-phase ac converter

Info

Publication number: WO2015128455A2
Application number: PCT/EP2015/054118
Authority: WO
Inventors: Jürgen Steinke
Original assignee: Abb Technology Ag
Priority date: 2014-02-28
Filing date: 2015-02-27
Publication date: 2015-09-03
Also published as: WO2015128455A3

Abstract

A three-phase to three-phase AC converter(34) comprises three primary side phase legs (20a, 20b, 20c), three secondary side phase legs (24a, 24b, 24c) and at least nine single-phase AC-to-AC converter cells (26), each converter cell (26) comprising a distinct galvanic isolation. Three converter cells (26) of the nine converter cells (26) are connected in series in each primary side phase leg(20a, 20b, 20c) and three converter cells (26) of the nine converter cells (26) are connected in series in each secondary side phase leg(24a, 24b, 24c), such that three converter cells (26) connected to one primary side phase leg(20a, 20b, 20c) are connected to each of the secondary side phase legs (24a, 24b, 24c).

Description

DESCRIPTION

Three-phase to three-phase AC converter

FIELD OF THE INVENTION

The invention relates to the field of power conversion. In particular, the invention relates to a three-phase to three-phase AC converter.

BACKGROUND OF THE INVENTION

More and more power converters are required to connect three-phase loads,either machines or local distribution grids, to three-phase grids with the requirement to include, besides frequency and/or voltage control on the load side, a galvanic isolation, i.e. voltage level adaptation between their sides with load and grid voltage both in the voltage range of more than 10 kV. At the same time, the restricted space and/or high costs for 50 Hz or 60 Hz transformers ask for more compact and cheaper solutions.

The conventional solution today for connecting a high voltage three-phase grid supply to a high voltage three-phase load, which requires to be supplied by a voltage system with another combination of frequency and voltage that the grid can supply, is to install two three- phase transformers and a power converter in-between. Two transformers are usually required because the typical converter based on available semiconductors cannot handle more than max. 6.9 kV.

In order to offer a more compact solution, modular converters have been proposed including subassemblies that are supplied via a galvanic isolation and can be arranged in series to increase the voltage. A common solution is feeding the subassemblies, often called "cells", via 50 Hz resp. 60 Hz transformers, usually realized by a multi-winding transformer. Due to the very complex transformers, these kinds of products are usually only applied for load voltages up to 13.9 kV.

However, three-phase systems usually have their greatest advantage if used symmetrically. Typical three-phase loads like high power electrical machines are all built fully symmetrical. Also the supplying grids are designed for a symmetrical supply for high power loads. In normal operation, a three-phase grid provides a symmetrical three-phase voltage system. Nevertheless, each three-phase system is composed of three single-phase sub-systems. In a symmetrical three-phase system, the three single-phase sub-systems are providing equal voltages and currents, just 120° phase shifted. The instantaneous power in each single-phase sub-system is pulsating with double of the fundamental system frequency due to multiplication of two sinusoidal functions. But the sum of the three power values is constant at each instant of time in steady state.

In the conventional solution, with a transformer on the primary and the secondary side, each phase of the converter transfers a pulsating power with double of the related fundamental frequency to the DC link, but due to the electrical parallel connection of the three phases, these three power values are added directly at the DC link terminals, i.e. their sum does not include any pulsating power component related to the fundamental converter operating frequency. The second converter on the secondary side connected to the DC link thus does not see any effect with respect to the fundamental frequency of the first converter connected to the primary side, i.e. with respect to their fundamental operating frequencies both converters are perfectly decoupled.

Converters composed by multiple single-phase cells as a rule cannot profit directly from the advantages of a symmetrical three-phase system. Each phase of the load is fed by series connected single-phase converter cells. Each converter cell contributes electrical power to the load, which,as explained above, is pulsating with double of the fundamental load frequency. Within the converter cell, this pulsating power is partially or completely transferred to the feeding grid, depending on the design of the converter cell. Due to the finite energy storage capacity of the converter cell's DC link, the converter cell DC link voltage varies with double of the load frequency. I.e. even for a fully symmetrical supply, the input power of the converter cell will include a component with double of the load fundamental frequency. At the primary side of the converter, the total power of all cells is seen, which is again constant, i.e. the grid is decoupled from the load. But of course the sum of the ratings of the secondary three-phase systems could be high, because especially in case the load is operated at a low frequency, the high peak load for each winding system may require a higher level of design power.

Fig. 1 shows a more general solution, which in theory may be used for higher voltages and potentially eliminates the multi-winding 50 Hz or 60 Hz transformer. When applying the modular solution shown in Fig. 1, which in literature is known as Solid State transformer or Power Electronic transformer, also the supply side is split into single-phase sub-systems. I.e. this converter cannot utilize the advantages of the three-phase system any more with respect to its internal design. Assuming that the converter is operated with currents and voltages at the three-phase terminals that are almost sinusoidal with the dedicated fundamental frequency, each converter cell is fed by two power components that typically have two different frequencies, just with an equal average power balancing the average energy content of the converter cell. The DC link of the converter cell now has to handle the sum of the two pulsating power components from the average transferred power. As long as load and grid frequency are different and not synchronized, the peak difference could not only be equal to the average power like in the conventional converter, but even two times the average power, because both power components might feed for a certain time almost synchronously power into the dc link, resulting in an almost double amplitude of the resulting voltage variation of the dc-link capacitor compared to the situation where only one of the sources feeds a pulsating power while the other would feed a constant power equal to the average power of the pulsating source, but opposite in sign.

A further modular converter is known from US 2010 0327793 Al . It discloses a three- phase to three-phase AC converter interconnecting the three primary side with the three secondary side phase legs by means of six converter cells. Each of these convert cells is connected at one side exclusively to one of the phase legs and on its other side to a respective winding of a common multi-winding transformer.

DESCRIPTION OF THE INVENTION

It is an objective of the invention to provide an electrical converter that is adapted to handle high voltages, that is at least reliable, modular, allows a simple, cost-effective construction and/or easy maintenance and/or repair, that is easy to control and that has a high power rating.

This objective is achieved by the subject-matter of the independent claim. Further exemplary embodiments are evident from the dependent claims and the following description.

The invention relates to a three-phase to three-phase AC converter. In other words, the converter is adapted for connecting three-phase electrical grid with a three-phase load, such as an electrical machine or a further electrical grid. In particular, the converter is adapted for processing high voltages, for example above 10 kV, and/or high currents, for example above 100 A. According to an embodiment of the invention, the converter comprises three primary side phase legs, three secondary side phase legs and at least nine single-phase AC-to-AC converter cells, each converter cell comprising a distinct galvanic isolation. In each primary side phase leg at least a group of three converter cells of the at least nine converter cells is connected in series such that within these groups each converter cell is connected to a differen phase leg of the secondary side. Consequently, also in each secondary side phase leg, there is at least a group of three converter cells of the at least nine converter cells connected in series such that within these groups each converter cell is connected to a different phase leg of the primary side.

The word "distinct" is used with the meaning individual, particular to the structure to which it belongs, physically and constructively distinguished of others of its kind. It is used as "distinct to others of its kind", but does not imply "distinct to others of different kind". A converter cell with a distinct galvanic isolation, for example, describes an individual galvanic isolation, particular to this convert cell, the galvanic isolation being physical and constructive distinguished of other galvanic isolations. "A converter cell with a distinct galvanic isolation" does, however, not imply that the galvanic isolation is physical and constructive distinguished from the converter cell itself.

It has to be understood that the converter cells are connected in series with respect to their primary and secondary side, i.e. their inputs are connected in series.

The converter is adapted to interconnect a first grid or load on its primary side with a second grid or load on its secondary side. Each side comprises three phase legs and each phase leg on one side is connected via a converter cell, comprising a distinct galvanic isolation, with a phase leg on the other side. On each side, at least three converter cells are connected in series in each phase.

This matrix-like, symmetric configuration of converter cells results also in a symmetric distribution of the double-frequency power components transferred from a symmetric load/grid on the secondary side to the grid/source on the primary side. These power components substantially cancel each other and the converter is easy to control and may have a higher power rating.

In addition, the matrix-like structure, with converter cells with a distinct galvanic isolation allows simple and cost-effective construction as it does not impose any layout constraints among the converter cells. The matrix-like structure, with converter cells with distinct galvanic isolation further enables easy maintenance and repair. In case of failure of a transformer, the failing converter cell can be replaced without any interference with other converter cells.

According to an embodiment of the invention, the phase legs on the primary side and/or on the secondary side may be star-connected or delta-connected. In particular, the three primary side phase legs may be star-connected and the three secondary side phase legs may be star-connected. The three primary side phase legs may be delta-connected and the three secondary side phase legs may be star-connected. The three primary side phase legs may be star-connected and the three secondary side phase legs may be delta-connected. Also, the three primary side phase legs may be delta-connected and the three secondary side phase legs may be delta-connected.

In a star-connection, three phase legs with each having at least three series-connected converter cells are on one side connected to a respective phase conductor of the load or grid and are connected on their other side with a star-point.

In a delta-connection, three phase legs with each having at least three series-connected converter cells are connected on to each other in a way to form a ring and each connection point between two legs is connected to one of the three phase conductors of the load or gird. According to an embodiment of the invention, each converter cell comprises a distinct, galvanically isolating transformer. The isolation of each converter cell may be provided by a transformer. For example, each converter cell may comprise two single-phase AC-to-AC converters interconnected via the transformer.

According to an embodiment of the invention, the distinct galvanic isolating transformer is a single-phase transformer.

According to an embodiment of the invention, the transformer is adapted for being operated with a frequency higher, for example at least twice or five times as high as the frequency in the primary side phase legs and/or secondary side phase legs. Such a transformer may be substantially smaller, cheaper und lighter than a transformer adapted for being operated with the grid frequency.

The transformer may be a medium frequency transformer. Medium frequency may be a frequency higher than the grid frequency, but in the range that power semiconductors can cover, which may be for high power applications below 100 kHz. It is also possible that each of the at least nine converter cells is replaced with two or more converter cells. In such a configuration, the converter may be adapted to convert higher voltages.

According to an embodiment of the invention, the converter comprises 9N single-phase AC-to-AC converter cells, N being a natural number. In each primary side phase leg N groups of three converter cells of the 9N converter cells are connected in series, such that within each of these groups each converter cell is connected to a different phase leg of the secondary side. The matrix-like structure, with 9N converter cells with a distinct galvanic isolation enables the creation of new groups fulfilling the group requirement - three cells connected in series in the phase leg of one of the three-phase system and each of these cells being connected to a different phase leg of the other three-phase system - out of functioning cells of groups in which a failure occurred. In case of a high number of redundant groups, this may improve the reliability of the converter substantially.

According to an embodiment of the invention, each converter cell comprises a primary side AC-to-DC converter, a secondary side DC-to-AC converter and a DC-to-DC converter with a distinct galvanic isolation interconnecting the primary side AC-to-DC converter with the secondary side DC-to-AC converter. The DC-to-DC converter comprises a distincttrans former for providing the galvanic isolation.

According to an embodiment of the invention, each converter cell further comprises a primary side DC link interconnected between the primary side AC-to-DC converter and the DC-to-DC converter, and/or a secondary side DC link interconnected between the secondary side DC-to-AC converter and the DC-to-DC converter. Due to the symmetric arrangement of the converter cells, the DC link capacitors may be smaller dimensioned as in the unsymmetrical cases mentioned above.

According to an embodiment of the invention, the DC-to-DC converter is a resonant converter, for example may comprise capacitors connected in series with the inductors of a transformer.

According to an embodiment of the invention, the converter further comprises an additional three-phase to three-phase AC converter cell with a galvanic isolation, the additional converter cell interconnecting the three primary side phase legs with the three secondary side legs. In such a case, the primary side phase legs and the secondary side phase legs may be star-connected via the inputs of the additional converter cell. According to an embodiment of the invention, the additional converter cell comprises a primary side three-phase AC-to-DC converter, a secondary side three-phase DC-to-AC converter, a DC-to-DC converter with a galvanic isolation interconnecting the primary side AC-to-DC converter with the secondary side DC-to-AC converter. The DC-to-DC converter may be designed like the DC-to-DC converter of the other converter cells.

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject-matter of the invention will be explained in more detail in the following text with reference to exemplary embodiments which are illustrated in the attached drawings.

Fig. 1 schematically shows a converter.

Fig. 2 shows a diagram with an example for a DC link voltage of a converter cell of the converter of Fig. 1.

Fig. 3 schematically shows a converter according to an embodiment of the invention.

Fig. 4 shows a diagram with an example for a sum of the DC link voltage of converter cells of the converter of Fig. 3.

Fig. 5 schematically shows a converter according to a further embodiment of the invention.

Fig. 6 schematically shows a converter cell for a converter according to an embodiment of the invention.

Fig. 7 schematically shows a converter according to a further embodiment of the invention.

Fig. 8 schematically shows an additional converter cell for the converter of Fig. 7.

The reference symbols used in the drawings, and their meanings, are listed in summary form in the list of reference symbols. In principle, identical parts are provided with the same reference symbols in the figures. DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Fig. 1 shows a converter 10 that interconnects a three-phase electrical grid 12 with a three- phase electrical load 14. A three-phase to three-phase transformer 16 may be interconnected between the electrical grid 12 and the converter 10.

In its primary side 18, the converter has three primary side phase legs 20a, 20b, 20c and on its secondary side 22, the converter 10 has three secondary side phase legs 24a, 24b, 24c.

The phase legs 20a, 20b, 20c are connected with the phase legs 24a, 24b 24c with six converter cells 26, each of which comprises a single-phase AC-to-AC converter. The converter cells are arranged such that two converter cells 26 are connected in series to one phase leg 20a, 20b, 20c of the primary side 18 and the same two converter cells 26 are connected in series to a phase leg 24a, 24b, 24c of the secondary side.

Each converter cell 26 comprises a first AC-to-DC converter 28, a galvanic isolating DC- to-DC converter 30 and a second DC-to-AC converter 32.

Fig. 2 shows an example for a possible DC link voltage in a converter cell of the converter 10, here having 50 Hz on the primary side 18 and 60 Hz at the secondary side. The diagram of Fig. 2 shows a normalized voltage over the time in seconds.

Important for controlling the converter 10 is that the currents especially in the grid 12 exactly follow the reference values. I.e. the voltages between the phase legs 20a, 20b, 20c and the phase legs 24a, 24b, 24c should not include any component of double of the load or the grid frequency that can be seen in the DC link of a converter cell 26. A common strategy to achieve this is to adapt the modulation index to the DC link voltage in order to obtain exactly the requested reference voltage at the specific phase leg 20a, 20b, 20c, 24a, 24b, 24c.

Due to the fact that the peak power that the DC link capacitors either have to store or have to deliver is double compared to a conventional solution as mentioned in the beginning, either the capacitance installed per converter cell 26 has to be double or, for equal installed capacitance, double of the peak deviation of the DC link voltage from an average value has to be considered. In order to provide the reference voltage at the phase legs 20a, 20b, 20c, 24a, 24b, 24c, the allowed value of the maximal reference is therefore reduced because of the reduced available voltage if using the same type of semiconductors like in a conventional multi-cell converter as mentioned in the beginning.

Fig. 3 shows a converter 34 with a symmetric, matrix-like arrangement of converter cells 26. The converter 34 comprises nine single-phase AC-to-AC converter cells 26. Three converter cells 26 are connected in series in each primary side phase leg 20a, 20b, 20c and three converter cells of the nine converter cells 26 are connected in series in each secondary side phase leg 24a, 24b, 24c, such that three converter cells connected to one primary side phase leg 20a, 20b, 20c are each connected to a different phase leg of the secondary side 24a, 24b, 24c.

As an example, the three converter cells connected in series to the phase leg 20c are connected to the different phase legs 24a, 24b and 24c, respectively.

Looking at the power pulsation within three converter cells included in three different phases 24a, 24b, 24c for example at the load 14, the power functions are phase shifted by 120° with respect to fundamental frequency or 240° with respect to pulsation frequency. Since the three converter cells 26 are connected to three different phase legs either at the primary side or the secondary side, the pulsating components of the power functions cancel each other.

Fig. 4 shows an example for the summarized DC link voltages of three series connected cells 26 that are connected to the three different phase legs at the other side of the converter 34. The second harmonic voltage ripple resulting from the other side is canceled and only the second harmonic voltage ripple caused by the load 14 that is connected to this part of the converter 34 remains.

Fig. 4 is a diagram analogous to Fig. 2 and assumes the same load 14 like in the example shown in Fig. 2 with exactly the same individual cell voltages. Fig. 4 shows the sum of three converter cell DC link voltages connected in series at 50 Hz terminals.

With respect to the converter 10 of Fig. 1, the converter of Fig. 3 provides two new major advantages.

Firstly, the converter 34 has an increased power rating of each converter cell 26 due to better utilization of the voltage ratings of the components. With respect to the example of Fig. 2 and Fig. 4, if the modulation has to be adapted to the voltage curve shown in Fig. 2, not more than 96.6% of the nominal voltage of the converter cell 26 can be utilized to guarantee a linear control behavior of the converter 10. This assumes a strategy to transfer balanced power between all converter cells 26 of one phase. Taking three converter cells 26 as a group and utilizing a common modulation index adaptation, 98.3% of the nominal voltage can be utilized. In this case, the converter cell 26 with the lowest voltage contributes in absolute values less than the converter cell 26 with the maximum voltage to the output voltage, but in total the three converter cells 26 provide exactly the foreseen portion of the output voltage. Secondly, the converter 34 allows a decoupling of the control algorithms for the two three-phase AC voltages of the grid and the load. The adaptation of the modulation index for the grid related converter cell switching elements 28 can be synchronized to the grid frequency while the adaptation of the modulation index for the load related converter cell switching elements 32 can be synchronized to the load frequency. No knowledge of the frequency of the load 14 for controlling the grid related switching elements 28 of the cells and no knowledge of the frequency of grid 12 for controlling the load related switching elements 32 of the cells of the converter 34 is required, i.e. control algorithms of the primary side converters 28 and secondary side converters 32 may be decoupled, which makes them simpler and more modular. In case they were implemented on two physically separated control hardware devices, no fast signal connection between the two may be required for transferring the momentary values of both frequencies.

A further advantage is that unsymmetrical voltages in grid 12 do not result in unbalanced voltages in the load 14 and vice versa.

In the converters 10 and 34 shown in Fig. 1 and Fig. 3, the primary side phase legs 20a, 20b, 20c are star-connected and the secondary side phase legs are star-connected. However, other designs with either a delta-connection on the primary side 18 and/or secondary side 22 are possible.

Fig. 5 shows a converter 34 with a delta-connected primary side. The series connected converter cells 26 are connected into a ring, wherein the grid 12 is connected to the points between each pair of series connected converter cells 26.

Fig. 6 shows a circuit diagram of converter cell 26. The AC-to-DC converters 28, 32 may be H-bridges composed of semiconductor switches. However, also a passive rectifier for one of them is possible in case energy transfer is only required in one direction.

The DC-to-DC converter 30 comprises a medium frequency single-phase transformer 36 which inductors are series connected with a capacitor and connected to a half-bridge on both sides. The DC-to-DC converter 30 may be seen as a resonant converter. Any kind of DC-to- DC converter 40 including an isolating transformer 36 may be used here, for example also a dual active bridge converter. Due to the distinct galvanic isolation by the single-phase transformer 36, the AC inputs on both sides of the converter cell 26 can be freely used, i.e. either series or parallel connected to other converter cells 26. Combined with the single- phase transformer ratio, a wide range of input and output voltage may be covered. The primary side AC -to-DC converter 28 is connected via a first DC link 38 to the DC- to-DC converter 30, which is connected via a second DC link 40 to the DC-to-AC converter 32.

Fig. 6 shows turn-off thyristors as switching devices. Other switching devices like IGBTs, MOSFETs, etc. may also be used.

Fig. 7 shows a converter 34 having the same components as the converter 34 of Fig. 3 but comprising an additional converter cell 42 that replaces the star-point connections shown in Fig. 3 interconnecting the phase arrangements related to the legs 20a, 20b, 20c and 24a, 24b, 24c.

The star-point connection in each of both sides 18, 22 is done via the converter cell 42. Assuming a symmetrical load and a symmetrical modulation of the converter cell 42, the power transferred from each side is constant in time in steady state, no fundamental related power pulsation is seen. Therefore, both sides are with respect to both fundamental frequencies completely decoupled.

This additional star-point connecting converter cell 42 may be used in two alternative ways, which potentially also may be combined.

Firstly, it may be used as a power and voltage booster. The maximum three-phase AC input voltage at both sides 18, 22 may be increased and with this also the power transferred. The additional voltage is what a typical two-level inverter can deliver, which is for the phase- to-phase voltage maximally equal to the DC link voltage divided by square-root of three, assuming sinusoidal modulation.

Secondly, the additional converter cell 42 may provide redundancy with minimal hardware effort. The converter cell 42 may offer redundancy in case of up to one converter cell 26 in each phase fails. In this case, the failed converter cell 26 has to be shorted by an appropriate device. By proper modulation of the star-point connecting converter cell 42, the missing cell voltage may be exactly replaced without influence on the phase-to-phase voltage of load or grid. In this case, the converter cell 42 may transfer the related single- phase power that otherwise would have been transferred by the shorted converter cell 26.

It is worth to point out that the converter topology affects the power rating requirements of the additional converter 42. The higher the power through put capability of the failing structure due to a failure within a converter cell, the higher the power rating requirements on the additional converter 42. In case of a failure of a converter cell 26 with a distinct galvanic isolation, only the particular converter cell 26 is affected while other converter cells 26, even the ones within the same group, may continue to operate. Therefore, with converter cells 26 with a distinct galvanic isolation the power rating of the additional converter 42 may be chosen according the power ratings of a single converter cell 26 and redundancy requirements. Otherwise, such a redundancy may also be achieved by adding three complete converter cells 26, which, however, would include a much higher number of semiconductors.

The converter cell 42 comprises a first AC-to-DC converter 28', a galvanic isolating DC- to-DC converter 30 and a second DC-to-AC converter 32'.

Fig. 8 shows a circuit diagram of an additional converter cell 42. The additional converter cell may be very similar to the converter cell 26 shown in Fig. 6. Only, the first AC-to-DC converters 28', and the second DC-to-AC converter 32' are three-phase converters.

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 practising 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 fulfil 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 should not be construed as limiting the scope.

LIST OF REFERENCE SYMBOLS

10 converter

12 electrical grid

14 load

16 transformer

18 primary side

20a, 20b, 20c primary side phase legs

22 secondary side

24a, 24b, 24c secondary side phase legs 26 converter cell

28, 28' AC-to-DC converter

30 DC-to-DC converter

32, 32' DC-to-AC converter

34 converter

36 transformer

38 first DC link

40 second DC link

42 additional converter cell

Claims

1. A three-phase to three-phase AC converter (34), comprising:

three primary side phase legs (20a, 20b, 20c);

three secondary side phase legs (24a, 24b, 24c);

at least nine single-phase AC-to-AC converter cells (26), characterized in that each converter cell (26) comprises a distinct galvanic isolation;

wherein in each primary side phase leg (20a, 20b, 20c) at least a group of three converter cells (26) of the at least nine converter cells (26) is connected in series such that within these groups each converter cell (26) is connected to a different phase leg of the secondary side (24a, 24b, 24c).

2. The converter (34) of claim 1,

wherein the three primary side phase legs (20a, 20b, 20c) are star-connected and the three secondary side phase legs (24a, 24b, 24c) are star-connected; or

wherein the three primary side phase legs (20a, 20b, 20c) are delta-connected and the three secondary side phase legs (24a, 24b, 24c) are star-connected; or

wherein the three primary side phase legs (20a, 20b, 20c) are star-connected and the three secondary side phase legs (24a, 24b, 24c) are delta-connected; or

wherein the three primary side phase legs (20a, 20b, 20c) are delta-connected and the three secondary side phase legs (24a, 24b, 24c) are delta-connected.

3. The converter (34) of claim 1 or 2,

where in each converter cell (26) the galvanic isolation is implemented as a transformer (36).

4. The converter (34) of claim 3,

where the transformer (36) is a single-phase transformer.

5. The converter (34) of claim 3 or 4, wherein the transformer (36) is adapted for being operated with a frequency at least twice as high as the frequency in the first side phase legs (20a, 20b, 20c) and/or second side phase legs (24a, 24b, 24c).

6. The converter (34) of one of the preceding claims,

wherein the converter comprises 9N single-phase AC-to-AC converter cells (26), N being a natural number;

wherein in each primary side phase leg N groups of three converter cells (26) of the 9N converter cells (26) are connected in series such that within each of these groups each converter cell (26) is connected to a different phase leg of the secondary side (24a, 24b, 24c).

7. The converter (34) of one of the preceding claims, wherein each converter cell (26) comprises:

a primary side AC-to-DC converter (28);

a secondary side DC-to-AC converter (32);

a DC-to-DC converter (30) with the distinct galvanic isolation interconnecting the primary side AC-to-DC converter (28) with the secondary side DC-to-AC converter (32).

8. The converter (34) of claim 7, wherein each converter cell further comprises: a primary side DC link (38) interconnected between the primary side AC-to-DC converter (28) and the DC-to-DC converter (30); and/or

a secondary side DC link (40) interconnected between the secondary side DC-to- AC converter (32) and the DC-to-DC converter (30).

9. The converter (34) of claim 7 or 8,

wherein the DC-to-DC converter (30) is a resonant converter.

10. The converter (34) of one of the preceding claims, further comprising:

an additional three-phase to three-phase AC converter cell (42) with a galvanic isolation, the additional converter cell (42) interconnecting the three primary side phase legs (20a, 20b, 20c) with the three secondary side legs (24a, 24b, 24c).

11. The converter (34) of claim 10, wherein the additional converter cell (42) comprises:

a primary side three-phase AC-to-DC converter (28');

a secondary side three-phase DC-to-AC converter (32');

a DC-to-DC converter (30) with a galvanic isolation interconnecting the primary side AC-to-DC converter (28') with the secondary side DC-to-AC converter (32').