Modular switching cell

A modular switching cell of a high voltage direct current power converter has a modular switching cell that includes a base module, which has: a first switching unit; a second switching unit; a first capacitor; and a second capacitor. The first switching unit, the second switching unit, the first capacitor, and the second capacitor are mounted on a chassis. The base module is configured to receive at least three different busbar sets, each of the busbar sets having a plurality of busbars for interconnecting the first switching unit, the second switching unit, the first capacitor, and the second capacitor to form one of: two parallel half bridge circuits between a first cell terminal and a second cell terminal; two serial half bridge circuits between the first cell terminal and the second cell terminal; or a full bridge circuit between the first cell terminal and the second cell terminal.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National Phase application under 35 U.S.C. § 371 of International Application No. PCT/EP2020/073922, filed on Aug. 27, 2020, and claims benefit to European Patent Application No. EP 19197834.5, filed on Sep. 17, 2019. The International Application was published in English on Mar. 25, 2021 as WO 2021/052730 A1 under PCT Article 21(2).

FIELD

The invention relates to a modular switching cell.

BACKGROUND

A Multi-level Modular Converter (MMC) design is the standard approach within the power converter industry to realise High Voltage Direct Current (HVDC) converters for use in power transmission. They are being widely adopted, too, for use in medium-voltage (MV) drive applications, including power distribution applications such as in a static synchronous compensator (STATCOM).

An MMC converter consists of multiple cells connected in series to form a phase arm; one or two of these may be used per phase, depending on the application. Traditional MMC cells are half-bridge or full-bridge. Half-bridge cells block voltage in one direction, while full-bridge cells block voltage in both directions. The MMC cells usually include insulated gate bipolar transistors (IGBTs).

In a family of production converters, there may be a range of overall converter current and voltage ratings, to suit a wide range of applications. Accommodating different voltage ratings is straightforward given that many cells are placed in series to achieve the required voltage rating; however, the current rating of the converter typically falls within the current rating of a single cell. Therefore to produce a range of converter current ratings either requires a range of different cell designs, each with a different current rating, or underutilization of the cell current rating for low-current converter designs resulting in a high cost per kW.

SUMMARY

In an embodiment, the present disclosure provides a modular switching cell of a high voltage direct current power converter, which includes a base module that has: a first switching unit; a second switching unit; a first capacitor; and a second capacitor. The first switching unit, the second switching unit, the first capacitor, and the second capacitor are mounted on a chassis. The base module is configured to receive at least three different busbar sets, each of the busbar sets having a plurality of busbars for interconnecting the first switching unit, the second switching unit, the first capacitor, and the second capacitor to form one of: two parallel half bridge circuits between a first cell terminal and a second cell terminal; two serial half bridge circuits between the first cell terminal and the second cell terminal; or a full bridge circuit between the first cell terminal and the second cell terminal.

DETAILED DESCRIPTION

A problem with the above-discussed state of the art is that the cell design is highly optimized for a particular topology and current rating. The optimization for a particular current rating affects, for example, the choice of IGBT/diode module, cell capacitor and bypass switch. The current rating is often given as share of a predetermined current value. For example, a fully-rated cell can carry the whole predetermined current. A half-rated cell can only carry half of the predetermined current. While a half-rated cell may theoretically result in half the size, the constituent component availability may preclude this, e.g. if the IGBT module is only available to suit the full cell rating. Even where half-rated cell components are available, each half-rated cell will still require the same quantity of control components, i.e. cell controller, current/voltage sensors, cell power supply, etc., as the fully-rated cell and a similar cost and volume of mechanical components. The rack-support structure for multiple cells will also be different, because of the different size. Therefore a half-rated cell will result in significantly more than half the cost and volume of the fully-rated cell, and hence the resulting MMC converter will be more expensive (€/kW) and less power dense (kW/m3). Furthermore, the half-rated cell will require a separate design, with associated engineering costs.

This lack of flexibility is also apparent if a full-bridge cell is required. Unless half-rated cell components are available, then a half-rated full-bridge cell will similarly require a separate design, with associated engineering costs.

Therefore, a problem solved is finding a switching cell that is cheap, has a high power density and can be used for different topologies and current ratings. Aspects of the present disclosure solve this problem.

The present disclosure relates to a modular switching cell. The switching cell may include a base module, which can be configured by connecting terminals of the base module via pre-shaped busbars in different ways in order to realize different cell module types. The present disclosure further relates to a system including the modular switching cell and respective busbars. Finally, the present disclosure relates to a method for manufacturing a modular switching cell.

Particularly, the present disclosure describes a common cell design, which uses the same basic mechanical design for either a fully-rated cell or two series half-rated cells, with the latter resulting in the same cost per unit power rating and power density as the fully-rated cell. These could share a mainly common design, simplifying the design effort and resulting in a lower cost system.

The present disclosure provides a modular switching cell of a high voltage direct current power converter, which comprises a base module. The base module has a first switching unit, a second switching unit, a first capacitor, and a second capacitor. These elements, i.e. the first switching unit, the second switching unit, the first capacitor, and the second capacitor are all mounted on a chassis. In this way, the base module allows for different interconnection of the elements in order to arrive at different designs. The first switching unit and the second switching unit preferably include IGBTs.

The base module is adapted to receive at least three different busbar sets, each including a plurality of busbars for interconnecting the first switching unit, the second switching unit, the first capacitor, and the second capacitor. Each of the busbar sets allows a different design of the base module. In particular, one of the busbar sets allows forming two parallel half bridge circuits between a first cell terminal and a second cell terminal. In addition, another one of the busbar sets allows forming two serial half bridge circuits between the first cell terminal and the second cell terminal. Yet another one of the busbar sets allows forming a full bridge circuit between the first cell terminal and the second cell terminal. Therefore, each busbar set interconnects the above described elements of the base module in a different manner. Hence, the base module can be used for different cell types, particularly for different current ratings and topologies. Therefore, the modular switching cell is cheap and allows high power densities.

Embodiments of the invention provide a common cell design, which uses the same basic mechanical design, i.e. the base module, for either a fully-rated cell or two series of half-rated cells, with the latter resulting in the same cost per unit power rating and power density as the fully rated cell. These cell types share a manly common design and thereby simplifying the design effort and resulting in a lower cost system.

In a preferred embodiment, the first switching unit has a first AC voltage terminal, a first negative DC terminal, a first positive DC terminal, a first switch between the first AC voltage terminal and the first negative DC terminal, and a second switch between the first AC voltage terminal and the first positive DC terminal. In this way, the first switching unit is particularly formed as half bridge module. Further, the second switching unit, in a preferred embodiment, has a second AC voltage terminal, a second negative DC terminal, a second positive DC terminal, a third switch between the second AC voltage terminal and the second negative DC terminal, and a fourth switch between the second AC voltage terminal and the second positive DC terminal. Again, the second switching unit is particularly formed as half bridge module. The base module is adapted to receive each busbar set such that the busbars of the respective busbar set interconnect all the terminals of the first switching unit and the second switching unit and the first capacitor and the second capacitor. In this way, particularly the first AC voltage terminal, the first negative DC terminal, the first positive DC terminal, the second AC voltage terminal, the second negative DC terminal, the second positive DC terminal, the first capacitor, and the second capacitor are interconnected. The busbars are provided such that they form one of the two parallel half bridge circuits, the two serial half bridge circuits, and the full bridge circuit. Therefore, only the busbars have to be different in case a different circuit has to be provided. Therefore, one basis module can be used for implementing a wide variety of power converters.

In a preferred embodiment, at least the first negative DC terminal is electrically connected to the chassis. The chassis preferably functions as chassis ground. Depending on the cell design defined by the busbar sets, the second positive DC terminal or the second negative DC terminal are preferably also connected to the chassis. It is preferable that all switching units share the same chassis ground, such that particularly only a single chassis ground potential exists in the modular switching cell.

In a preferred embodiment, the first switch and the second switch of the first switching unit and the third switch and the fourth switch of the second switching unit all include an electronic switch. The electronic switch particularly comprises an insulated-gate bipolar transistor (IGBT), and a diode which are wired in parallel to each other. Instead of IGBTs, also other electronic switches like integrated gate-commutated thyristors (IGCT), gate turn-off thyristors (GTO-Thyristors), metal oxide semiconductor field-effect transistors (MOSFETs), high-electron-mobility transistors (HEMT) or bipolar junction transistors (BJT) can be used. All these elements can be combined in different manners, i.e. via different busbar sets, in order to realize different modular switching cells and thus power converters.

The modular switching cell, in a preferred embodiment, includes a single control unit. The control unit is adapted to switch the first switch and the second switch of the first switching unit and the third switch and the fourth switch of the second switching unit. In this way, the power input to the modular switching cell can undergo optimal power conversion. The converted power is output. Thereby, the whole control of all the switches is preferably controlled by the single control unit.

In preferred embodiment, the first switching unit includes a first sub-switching unit and a second sub-switching unit. In this case, the first switching unit and the second switching unit are not half bridge modules (e.g. half-bridge IGBTs) but rather are single switch modules (e.g. IGBTs). The first sub-switching unit preferably includes the first AC voltage terminal, the first negative DC terminal, and the first switch. The second sub-switching unit preferably includes the first AC voltage terminal, the first positive DC terminal, and the second switch. Thus, the interconnection of the first switching element is still possible via the busbar sets, wherein the two sub-switching units have to be connected separately. In the same way, the second switching unit preferably includes a third sub-switching unit and a fourth sub-switching unit. The third sub-switching unit has the second AC voltage terminal, the second negative DC terminal, and the third switch. The fourth sub-switching unit on the other side has the second AC voltage terminal, the second positive DC terminal, and, the fourth switch. Thus, the second switching unit also includes two sub-switching units, which are to be connected separately via the busbars.

It is further preferred that a heat sink is provided. The heat sink is particularly provided to dissipated heat of more than one switching unit. Preferably, the first switching unit, the second switching unit, the first capacitor, and the second capacitor are provided with a common heat sink. Therefore, the power density of the cell is improved and the costs for providing the cell is reduced.

The modular switching cell, in an embodiment, has a bypass switch provided between the first cell terminal and the second cell terminal. The bypass switch is adapted to bypass the switching cell, by shortcutting the first cell terminal and the second cell terminal. In case a converter is set up of the modular switching cells as described above, the bypass switch allows to not use the respective switching cell, e.g. in case of damage of the switching cell. Thus, failure of one switching cell does not necessarily lead to a failure of the whole converter. In an alternative embodiment, the bypass switch is omitted and the components of the switching cell are adapted to shortcut the first cell terminal and the second cell terminal in case of component failure.

In another advantageous embodiment, the first switching unit, the second switching unit, the first capacitor, and the second capacitor are electrically connectable via the busbar sets all from the same side. Thus, the busbar set only has to be arranged on one side of the base module to interconnect all the components. Therefore, different cell designs, particularly the cell designs as described above, can be provided in a simplified manner.

An embodiment of the present invention provides a system, including the modular switching cell—as described above—and at least two of a first busbar set, and a second busbar set, and a third busbar set. Therefore, the system can be configured to provide different cell designs. A converter can preferably be set up from several of these systems. In the system, the first capacitor is provided between a first capacitor terminal and a second capacitor terminal and the second capacitor is provided between a third capacitor terminal and a fourth capacitor terminal. All these capacitor terminal can be connected via the busbars of the busbar set to connect the switching units and the capacitors. With the different busbar sets, different designs can be provided, wherein these different busbar sets are now described:

The first busbar set includes a first busbar, a second busbar, and a third busbar. The first busbar electrically connects the first negative DC terminal and the second negative DC terminal and the second capacitor terminal and the fourth capacitor terminal. The second busbar electrically connects the first positive DC terminal and the second positive DC terminal and the first capacitor terminal and the third capacitor terminal. Finally, the first AC voltage terminal and the second AC voltage terminal are electrically connected via the third busbar. In this cell design, the first busbar is the second cell terminal and the third busbar is the first cell terminal. Therefore, the first busbar set preferably defines a fully rated half-bridge cell. This means that the switching units are half-rated and are connected in parallel via the first busbar set. Hence, the cell design is a half-bridge, while each switching unit particularly carries half the current flowing through each cell.

In an additional or alternative embodiment, the second busbar set includes a fourth busbar, a fifth busbar, a sixth busbar, a seventh busbar, and an eighth busbar. The fourth busbar is electrically connected to the first AC voltage terminal. The fifth busbar electrically connects the first positive DC terminal and the first capacitor terminal. The second negative DC terminal and the fourth capacitor terminal are electrically connected via the sixth busbar. The seventh busbar electrically connects the first negative DC terminal and the second positive DC terminal and the second capacitor terminal and the third capacitor terminal. Finally, the eighth busbar is electrically connected to the second AC voltage terminal. According to this cell design, the fourth busbar is the first cell terminal and the eighth busbar is the second cell terminal. In this cell design, a half-rated dual cell is provided. This means that the two switching units are connected in series such that the cell fulfils the function of two half-rated cells.

In another additional or alternative embodiment, the third busbar set includes the second busbar as described before, the fourth busbar as described before, the eighth busbar as described before, and a ninth busbar. The ninth busbar electrically connects the first negative DC terminal and the second negative DC terminal and the second capacitor terminal and the fourth capacitor terminal. This cell design realizes a half-rated full-bridge. Therefore, the switching units are coupled such that they form a full-bridge. Particularly, as described above, each of the switching unit is a half bridge modules, such that these half bridge modules are combined by the third busbar set to form a full-bridge.

Hence, from the same base module, i.e. from the same switching units and capacitors, at least three different cell designs can be produced only by combining the base module with one of the first busbar set and second busbar set and third busbar set. This allows an increased flexibility in providing different cell types and/or converters.

The busbars of each busbar set advantageously are laminated busbars. By providing laminated busbars, the required space for interconnecting the switching units and capacitors is reduced. In addition, the electrical properties are enhanced, e.g. the stray inductance is reduced.

Each busbar set includes busbars that define the first cell terminal and the second cell terminal arranged for electrically connecting the switching cell with other components, particularly with other switching cells of the power converter. Therefore, in order to set up a converter, each cell is connected via the respective busbars forming the cell terminals.

An embodiment of the present invention provides a high voltage direct current power converter. The converter includes a three phase voltage input and, for each phase of the three phase voltage input, a plurality of switching cells as described above. The switching cells are connected in series. Therefore, the power converter can be provided with flexible and inexpensive switching cells as described above, wherein the switching cells particularly have a high energy density.

An embodiment of the present invention provides a method of manufacturing a modular switching cell. The method comprises the following steps: On the one side, a base module is provided by mounting a first switching unit, a second switching unit, a first capacitor, and a second capacitor on a chassis. This base module is adapted to receive different busbar sets in order to realize different cell designs. Hence, at least two different busbar sets are provided. Each set includes a plurality of busbars for interconnecting the first switching unit, the second switching unit, the first capacitor, and the second capacitor. The busbars of each busbar set are adapted to form one of two parallel half bridge circuits between a first cell terminal and a second cell terminal, and two serial half bridge circuits between the first cell terminal and the second cell terminal, and a full bridge circuit between the first cell terminal and the second cell terminal. Thus, each busbar set allows another cell design by interconnecting the elements of the base module in different manners. As a last step, one of the busbar sets is mounted on the base element. By mounting the respective busbar set on the base element, a switching cell is provided which has one of the designs listed above. Since any one of the bus bar sets can be mounted on the base module, it is a simple and flexible way of providing different switching cell types.

FIG.1is a schematic view of a high voltage direct current converter20, particularly for use in power transmission. The high voltage direct current power converter20includes a three phase voltage input600and is adapted to output direct voltage. For each phase of the three phase voltage input600, a plurality of switching cells1are connected in series. A phase output voltage300can thus be divided into a plurality of cell voltages400of the single switching cells1as shown inFIG.2. By respective switching of the switching cells1, the phase output voltage300can be converted to direct voltage.

FIGS.3and4show different cell types of the switching cells1. All switching cells1are provided between a first cell terminal100and a second cell terminal200. InFIG.3, a half bridge is shown, whileFIG.4shows a full bridge. While the half bridge can block the current in one direction only, the full bridge can block the current in two directions. The half bridge design shown inFIG.3therefore is a switching cell1using only one single first switching unit2. The switching cell1according to the full bridge design shown inFIG.4on the other side has a first switching unit2and a second switching unit3. In this embodiment, the first switching unit2and the second switching unit3are half bridge elements. Therefore, the two half bridge elements of the first switching unit2and the second switching unit3are combined to form a full bridge inFIG.4.

Each of the switching units2,3therefore has two IGBTs80and two diodes90. One IGBT80and one diode90are connected in parallel and the two sets of parallel IGBT80and diode90are connected in series. In this way, the above-described half bridge element is formed.

Each switching cell1further includes at least a first capacitor4. The first capacitor4is provided in parallel to the switching units2,3. Thus, the switching cells1divide the phase output voltage300in several cell voltages400, between the first cell terminal100and second cell terminal200of the respective switching cell1, such that the switching cells1can be used to convert the phase output voltage300to direct voltage.

A bypass switch19may be provided to allow current to bypass the switching cell1in case of a cell failure and thus keep the converter20operating. This may be realized using a combination of thyristor, mechanical contactor or fast-acting pyrotechnic device. The bypass switch19may not be required if the semiconductor switch used in the cell fail to short, e.g. specially-designed press-pack IGBTs or integrated gate commutated thyristors (IGCTs).

FIG.5is a schematic view of the switching cell1according to an embodiment of the invention. The switching cell1includes a first switching unit2and a second switching unit3as shown inFIGS.3and4as well as a first capacitor4and a second capacitor5.FIG.5shows the first switching unit2, the second switching unit3, the first capacitor4and the second capacitor5being mounted on a common chassis15. The two switching units2,3also share the same heat sink17, such that the switching cell1only has a single inlet17aand outlet17bfor cooling fluid. The switching cell1also has a common control unit16, which particularly drives the IGBTs80of the switching units2,3. Therefore, the switching cell1has a higher energy density compared with two single cells only having one switching unit. In addition, since all the components are provided on the same chassis15, the chassis15functions as common chassis ground.

The switching cell1does not have a fixed cell design. The switching cell1can rather be configured to have different cell designs such that a flexibility in providing a converter20with different demands is given. To allow different cell designs, the switching cell1has a base module18, which includes the first switching unit2, the second switching unit3, the first capacitor4and the second capacitor5. The base module18is also adapted to receive different busbar sets. Each busbar set connects the elements of the base module18in a different manner to realize different cell designs.

Preferably, the busbars of the busbar sets contact the elements of the base module18all from the same side. Therefore, mounting the busbar sets and electrically connecting the first switching unit2, the second switching unit3, the first capacitor4and the second capacitor5is simplified.

In order to be electrically contacted by the busbar sets, the first switching unit2has a first AC voltage terminal21, a first negative DC terminal22, and a first positive DC terminal23. A first switch is provided between the first AC voltage terminal21and the first negative DC terminal22, and a second switch between the first AC voltage terminal21and the first positive DC terminal23, wherein the two switches consist of the above described parallel IGBT80and diode90.

In the same way, the second switching unit3has a second AC voltage terminal31, a second negative DC terminal32, and second positive DC terminal33. A third switch is provided between the second AC voltage terminal31and the second negative DC terminal32, and a fourth switch between the second AC voltage terminal31and the second positive DC terminal33. Again, the two switches, i.e. the third switch and the fourth switch, consists of the above described parallel IGBT80and diode90.

The first capacitor4is provided between a first capacitor terminal41and a second capacitor terminal42and the second capacitor5is provided between a third capacitor terminal51and a fourth capacitor terminal52. All the terminals described above are preferably on the same side of the switching cell1to ensure that the switching cell1can be electrically contacted via the busbar sets from a single side.

Starting from such a basic design, the switching cell1can be set to different cell designs. Particularly, the two switching units3,4can be paralleled to increase the current rating or to form a full bridge. The switching units3,4can also be put in series to form a dual switching cell consisting of two conventional switching cells. In the following, three different designs are shown which are all realized by different busbar sets with the same base module18.

InFIGS.7and8, two parallel half bridge circuits between the first cell terminal100and the second cell terminal200are shown.FIG.7schematically shows the design of the switching cell1, whileFIG.8shows a schematic wire diagram. InFIGS.9and10, two serial half bridge circuits between the first cell terminal100and the second cell terminal200is shown.FIG.9schematically shows the design of the switching cell1, whileFIG.10shows a schematic wire diagram. A full bridge circuit between the first cell terminal100and the second cell terminal200is finally shown inFIGS.11and12.FIG.11schematically shows the design of the switching cell1, whileFIG.12shows a schematic wire diagram.

In the design according toFIGS.7and8, a first busbar set including a first busbar6, a second busbar7, and a third busbar8is used to interconnect the first AC voltage terminal21, the first negative DC terminal22, the first positive DC terminal23, the second AC voltage terminal31, the second negative DC terminal32, the second positive DC terminal33, the first capacitor terminal41, the second capacitor terminal42, the third capacitor terminal51, and the fourth capacitor terminal52.

The first negative DC terminal22and the second negative DC terminal32and the second capacitor terminal42and the fourth capacitor terminal52are electrically connected via the first busbar6. The first positive DC terminal23and the second positive DC terminal33and the first capacitor terminal41and the third capacitor terminal51are electrically connected via the second busbar7. The third busbar8electrically connects the first AC voltage terminal21and the second AC voltage terminal31. In this design, the first busbar6is the second cell terminal200and the third busbar8is the first cell terminal100.

As particularly shown inFIG.8, the first switching unit2, the second switching unit3as well as the first capacitor4and the second capacitor5are connected in parallel to each other. Therefore, the switching cell1is a fully-rated half bridge since the parallel connection of components doubles the current the switching cell can carry. A common chassis ground is provided via the chassis15at the first negative DC terminal22and the second negative terminal23, which are identical in this design.

The same base module18can be provided with a different second busbar set shown inFIGS.9and10to form a half-rated dual cell. The second busbar set includes a fourth busbar9, a fifth busbar10, a sixth busbar11, a seventh busbar12, and an eighth busbar13, which interconnect the first AC voltage terminal21, the first negative DC terminal22, the first positive DC terminal23, the second AC voltage terminal31, the second negative DC terminal32, the second positive DC terminal33, the first capacitor terminal41, the second capacitor terminal42, the third capacitor terminal51, and the fourth capacitor terminal52.

The first AC voltage terminal21is electrically connected to the fourth busbar9. The first positive DC terminal23and the first capacitor terminal41are electrically connected via the fifth busbar10. The second negative DC terminal32and the fourth capacitor terminal52are electrically connected via the sixth busbar11. The seventh busbar12electrically connects the first negative DC terminal22and the second positive DC terminal33and the second capacitor terminal42and the third capacitor terminal51. Finally, the second AC voltage terminal31is electrically connected to the eighth busbar13. In this cell design, the fourth busbar9is the first cell terminal100and the eighth busbar13is the second cell terminal200. Thus, the first switching unit2and the first capacitor4form a half-rated half bridge which is connected in series to another half-rated half bridge formed from the second switching unit3and the second capacitor5. Particularly, the two half-rated half bridges are formed as mirror of each other such that they can share the same power supply and controller. A common chassis ground is provided by chassis15at the first negative DC terminal22and the second positive DC terminal which are identical in this design.

In the design ofFIGS.9and10, the cell outputs the double cell voltage500, which is twice the cell voltage400. Thus, with the same components, either a fully-rated half bridge or two half-rated half bridges can be provided. Hence, based on the respective needs of the converter20, the same base module18can be provided as different cell topologies.

Further, in the design ofFIGS.9and10, two options apply to the bypass switch19. Either one bypass switch may be used for each of the two half bridges, with the two bypass switches connected in series and the mid-point between them also connected to the mid-point between the two half bridges, or one bypass switch may be used for the whole switching cell1. In case two bypass switches are used, each one needs the same voltage rating as for the single switching cell1, while using a single bypass switch for the whole switching cell1requires a higher voltage rating since the bypass switch19has to carry the double cell voltage500which is twice the cell voltage400.

FIGS.11and12finally show a half-rated full bridge design. In this design, the two switching units2,3, which are formed as half bridge modules, are combined to set up a full bridge. To realize such a design, a third busbar set including the above-described second busbar7, the above-described fourth busbar9, the above-described eighth busbar13, and an additional ninth busbar14. The ninth busbar14electrically connects the first negative DC terminal22and the second negative DC terminal32and the second capacitor terminal42and the fourth capacitor terminal52. The chassis15provides a common chassis ground at the first negative DC terminal22and the second negative DC terminal32.

The busbars of each busbar set advantageously are laminated busbars. This particularly helps to reduce the stray inductance between the IGBTs80and the capacitors4,5to ensure optimal switching of the switching cells1.

Hence, in addition to the fully-rated half bridge and the dual half-rated half bridges, the base module18can be provided with another busbar set to form a half-rated full bridge. Thus there is a wide variety of different cell options which can be provided from one single base module18.

FIG.13shows the same cell design of the switching cell1as shown inFIG.7, wherein a different first switching unit2and second switching unit3are used. In the above described embodiments, the first switching unit2and the second switching unit3are described as half bridge modules. As shown inFIG.13, another setup using single switch modules can also be adopted.

Thus, the first switching unit2includes a first sub-switching unit2aand a second sub-switching unit2b. The first sub-switching unit2ahas the first AC voltage terminal21, the first negative DC terminal22, and the first switch provided between the first AC voltage terminal21and the first negative DC terminal22. The second sub-switching unit2balso has the first AC voltage terminal21and further includes the first positive DC terminal23, and the second switch provided between the first AC voltage terminal21and the first positive DC terminal23. In an analogous manner, the second switching unit3includes a third sub-switching unit3aand a fourth sub-switching unit3b. The third sub-switching unit3ahas the second AC voltage terminal31, the second negative DC terminal32, and the third switch, which is provided between the second AC voltage terminal31and the second negative DC terminal31. The fourth sub-switching unit3bhas the second AC voltage terminal31, the second positive DC terminal33, and the fourth switch, which is provided between the second AC voltage terminal31and the second positive DC terminal33. Thus the sub-switching units2a,2b,3a,3bhave to be contacted separately via the first busbar6, the second busbar7, and the third busbar8, wherein the cell design remains the same as shown inFIG.7. The setup of the switching units2,3, i.e. the usage of the sub-switching units2a,2b,3a,3bcan be applied to any other cell design, particularly to the designs shown inFIGS.9and11.

FIG.14finally shows a further design option of the switching units2,3. In this design option, the switching units2,3are provided as multi-level bridge legs with three levels. To realize such a design, an additional diode90is included in each switching unit2,3.

The switching cell1as described above can be used for different purposes, while all switching cells1share the same base components. Thus, always the same hardware can be adopted for providing a variety of different switching cells1for a convertor20. The switching cells1are of lower costs since only one set of control components, e.g. the control unit16and the heat sink17, has to be provided. The switching cells1are such that a maximum component re-use and design re-use is possible. Further, particularly in the case of the dual cell including two half-rated half bridges, a compact cell design is provided. Since the component count and the number of coolant and electrical connections are reduced, the reliability is increased.

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