FIELD OF INVENTION

The present invention generally relates to modular multilevel converters. More particularly the present invention relates to a modular multilevel converter cell and a modular multilevel converter comprising such a cell. BACKGROUND

Multilevel converters are of interest to use in a number of different power transmission environments. They may for in stance be used as voltage source converters in direct current power tran smission systems such as high voltage direct current (HVDC) and alternating current power transmission systems, such as flexible alternating current tran smission system (FACTS) . They may also be used as reactive compen sation circuits such as Static VAR compen sators. In order to reduce harmonic distortion in the output of power electronic converters, where the output voltages can assume several discrete levels, so called multilevel converters have been proposed. In particular, converters where a number of cascaded converter cells, each comprising a number of switching units and an energy storage unit in the form of a DC capacitor have been proposed.

Examples of such converters can be found in Marquardt, TSiew Concept for high voltage-Modular multilevel converter', IEEE 2004, A. Lesnicar, R. Marquardt, "A new modular voltage source inverter topology", EPE 2003 , WO 20 10/ 149200 and WO 20 11/ 124260 . Converter elements or cells in such a converter may for instance be of the half-bridge, full-bridge or double cell type. These may be connected in upper and lower phase arms of a phase leg. A converter formed using cells has the advantage of low switching losses. However, the conduction losses are often high.

There is still room for improvement with regard to such converters and then especially with regard to current conduction losses.

SUMMARY OF THE INVENTION

The present invention is directed towards providing a reduction of the current conduction losses in a modular multilevel converter.

This object is according to a first aspect achieved through a multilevel converter cell for providing at least one voltage contribution for assisting in the forming of an alternating current waveform, the cell comprising at least one energy storage element; and

a first branch of series-connected switching units in parallel with the energy storage element, the first branch comprising a first and a second switching unit,

wherein one of the switching units of the first branch that is operative to bypass the energy storage element has a current conduction area that is larger than the current conduction area of the other switching unit of the first branch .

This object is according to a second aspect achieved through a modular multilevel converter for forming an alternating current waveform and comprising:

a phase leg comprising a number of cells, where at least one cell of the phase leg comprises:

at least one energy storage element; and a first branch of series-connected switching units in parallel with an energy storage element, said first branch comprising a first and a second switching unit;

wherein one of the switching units of the first branch that is operative to bypass the energy storage element has a current conduction area that is larger than the current conduction area of the other switching unit of the first branch .

The invention has a number of advantages. Through reducing the conduction losses, the efficiency of the converter is increased. This is furthermore achieved through a limited increase of the cell size and thereby also of the converter size. The reduction of the conduction losses also has the further advantage of relaxing the cooling requirements of the cells. The modified cells also serve to increase the surge current handling capability of the converter. The lower losses also allows for increasing the current/ power capability in the system .

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will in the following be described with reference being made to the accompanying drawings, where fig. 1 schematically shows a cell-based voltage source converter connected between a pole and ground,

fig. 2 schematically shows a full-bridge cell,

fig. 3 schematically shows the structure of a first type of half-bridge cell, fig. 4 schematically shows the structure of a second type of half-bridge cell, fig. 5 schematically shows a half-bridge double cell,

fig. 6 schematically shows a series half bridge cell,

fig. 7 shows a plot of arm voltage and arm current of a phase arm in the converter,

fig. 8 shows plots of total conduction losses and conduction losses through bypass and voltage contributing switching units of a converter, and fig. 9 schematically shows one realization of the first type of half-bridge cell in order to reduce conduction losses, and

fig. 10 plots of total conduction losses and conduction losses through bypass and voltage contributing switching units of a converter with the half-bridge cell realization according to fig. 9.

DETAILED DESCRIPTION OF THE INVENTION

In the following, a detailed description of preferred embodiments of the invention will be given .

Fig. 1 shows one variation of a multilevel converter in the form of a cell based voltage source converter 10 or modular multilevel converter (MMC) . The converter operates to convert between alternating current (AC) and direct current (DC) . The converter 10 in fig. 1 comprises a three-phase bridge made up of a number of phase legs. There are in this case three phase legs. It should however be realized that as an alternative there may be for in stance only two phase legs. There is thus a first phase leg PL1, a second phase leg PL2 and a third phase leg PL3. The phase legs are more particularly connected between a first DC terminal DC1 and a second DC terminal DC2, where the first DC terminal may be connected to a first pole P I of a DC power transmission system, such as a High Voltage Direct Current (HVDC) power transmission system and the second DC terminal DC2 may be connected to ground, where the mid points of the phase legs are connected to corresponding alternating current terminals ACAl, ACB l,

ACC1. A phase leg is in this example divided into two halves, a first upper half and a second lower half, where such a half is also termed a phase arm .

The first DC pole P I furthermore has a first potential Udp that may be positive. The first pole P I may therefore also be termed a positive pole. The

AC terminals ACAl, ACB2, ACC3 may in turn be connected to an AC system, such as a flexible alternating current tran smission system

(FACTS), for in stance via a transformer. A phase arm between the first pole P I and a first AC terminal ACA1, ACB1 and ACC1 may be termed a first phase arm or an upper phase arm, while a phase arm between the first AC terminal ACA1 and ground may be termed a second phase arm or a lower phase arm .

As mentioned above, the type of voltage source converter shown in fig. 1 is only one example of a multilevel converter where the invention may be used. It is for instance possible to use the converter as a reactive

compen sating device, such as a Static VAR Compensator.

The voltage source converter depicted in fig. 1 has an asymmetric monopole configuration . It is thus connected between a pole and ground. As an alternative it may be connected in a symmetric monopole

configuration or a symmetric bipole configuration . In a symmetric monopole configuration the second DC terminal DC2 would be connected to a second pole having a second negative potential that may be as large as the first potential but with the opposite polarity. In a symmetric bipole configuration there would also be a second pole. In the bipole

configuration there would furthermore be a third and a fourth phase arm in the phase leg, where the second and third phase arms would be connected to ground, the first phase arm connected between the positive voltage of the first pole P I and the second phase arm and the fourth phase arm connected between the negative voltage of the second pole and the third phase arm . A first AC terminal of a phase leg would in the symmetric bipole configuration be provided between the first and second phase arms, while a second AC terminal of the same phase leg would be provided between the third and fourth phase arms. The phase arms are furthermore connected to the AC terminals via phase reactors. The phase arms of the voltage source converter 10 in the example in fig. 1 comprise cells. A cell is a unit that may be switched for providing a voltage contribution to the voltage on the corresponding AC terminal. A cell then comprises one or more energy storage elements, for in stance in the form of capacitors, and the cell may be switched to provide a voltage contribution corresponding to the voltage of the energy storage element or a zero voltage contribution . In this case the cell in serts the voltage of the energy storage element. If more than one energy storage element is included in a cell it is possible with even further voltage contributions. When no voltage or a zero voltage is provided by the cell then the energy storage element is bypassed.

The cells are with advantage connected in series or in cascade in a phase arm .

In the example given in fig. 1 there are five series-connected or cascaded cells in each phase arm . Thus the upper phase arm of the first phase leg PL1 includes five cells Clp l, C2p l, C3p l, C4p l and C5p l, while the lower phase arm of the first phase leg PL1 includes five cells Cln 1, C2n 1, C3n 1, C4n l and C5n l. Across the cells of the upper phase arm there is a first phase arm voltage Uvppa and through the upper phase arm there runs a first phase arm current Ivppa. As the upper phase arm is connected to the first pole P I it may also be considered to be a positive phase arm . Across the cells of the lower phase arm there is a second phase arm voltage Uvpna and through the lower phase arm there run s a second phase arm current Ivpna. The upper phase arm is furthermore joined to the AC terminal ACA1 via a first or upper arm reactor Laarm l, while the lower phase arm is joined to the same AC terminal ACA1 via a second or lower arm reactor Laarm2. In a similar fashion the upper phase arm of the second phase leg

PL2 includes five cells Clp2, C2p2, C3p2, C4p2 and C5p2 while the lower phase arm of the second phase leg PL2 includes five cells Cln2, C2n2, C3n2, C4n2 and C5n2. Finally the upper phase arm of the third phase leg PL3 includes five cells Clp3, C2p3 , C3p3, C4p3 and C5p3 while the lower phase arm of the third phase leg PL3 includes five cells Cln3, C2n3 , C3n3,

C4n3 and C5n3. The upper phase arms are furthermore joined to the corresponding AC terminals ACB1 and ACC1 via corresponding first or upper arm reactors Lbarm l and Lcarm l, respectively, while the lower phase arm s are joined to the same AC terminal ACB1 and ACC1 via corresponding second or lower arm reactors Lbarm2 and Lcarm2, respectively. The number of cells provided in fig. 1 is only an example. It therefore has to be stressed that the number of cells in a phase arm may vary. It is often favorable to have many more cells in each phase arm , especially in HVDC applications. A phase arm may for in stance comprise hundreds of cells. There may however also be fewer.

Control of each cell in a phase arm is normally done through providing the cell with a control signal directed towards controlling the contribution of that cell to meeting a reference voltage. The reference voltage may be provided for obtaining an AC waveform on the AC terminal of a phase leg, for in stance a sine wave. In order to control the cells there is therefore a control unit 12.

The control unit 12 is provided for controlling all the phase arms of the converter. However, in order to simplify the figure only the control of the upper phase arm of the first phase leg PL is indicated in fig. 1. The control unit may be implemented through a computer.

The other phase arm s are controlled in a similar manner in order to form output waveform s on the three AC terminals AC1, AC2 and AC3.

There are a number of different cell types that can be used in the converter, such as full-bridge cells, double cells, half-bridge cells and series half bridge cells. Fig. 2 shows a first version of a full-bridge cell FBA.

The cell FBA is thus a full-bridge converter cell and includes an energy storage element, here in the form of a capacitor C, which is connected in parallel with a first group of switching units SI and S2. The energy storage element C provides a voltage Udm, and therefore has a positive and negative end, where the positive end has a higher potential than the negative end. The switching units SI and S2 in the first group are

connected in series with each other, where each switching unit may be realized using a first type of semiconducting element that is a

unidirectional conduction element, such as a diode, and a second type of semiconducting element in the form of a semiconducting element of the turn-off type, such as a transistor like an IGBT (Insulated Gate Bipolar Transistor). The diode may be anti-parallel to the transistor. In fig. 2 the first switching unit SI has a first transistor Tl with a first anti-parallel diode D l. The first diode Dl is connected between the emitter and collector of the transistor Tl and has a direction of conductivity from the emitter to the collector as well as towards the positive end of the energy storage element C. The second switching unit S2 has a second transistor T2 with a second anti-parallel diode D2. The second diode D2 is connected in the same way in relation to the energy storage element C as the first diode D l, i.e. conducts current towards the positive end of the energy storage element C. The first switching unit SI is furthermore connected to the positive end of the energy storage element C, while the second switching unit S2 is connected to the negative end of the energy storage element C.

There is also a second group of series-connected switching units S3 and S4. This second group of switching units is here connected in parallel with the first group as well as with the energy storage element C. The second group includes a third switching unit S3, here provided through a third

transistor T3 with anti-parallel third diode D3 and a fourth switching unit S4, here provided through a fourth transistor T4 with anti-parallel fourth diode D4. The fourth switching unit S4 is furthermore connected to the positive end of the energy storage element C, while the third switching unit

S3 is connected to the negative end of the energy storage element C. Both the diodes D3 and D4 furthermore have a direction of current conduction towards the positive end of the energy storage element C. The switching units S3 and S4 in the second group are thus connected in series with each other. The switching units S3 and S4 may also be denoted cell switches.

This full-bridge cell FBA comprises a first cell connection terminal TEFBAl and a second cell connection terminal TEFBA2, each providing a

connection for the cell to a phase arm of a phase leg of the voltage source converter, such as to the upper phase arm of the first phase leg. In this full- bridge cell the first cell connection terminal TEFBAl more particularly provides a connection from the phase arm to the junction between the first and the second switching units SI and S2, while the second cell connection terminal TEFBA2 provides a connection between the phase arm and a connection point between the third and fourth switching units S3 and S4. The junction between the first and second switching units SI and S2 thus provides one cell connection terminal TEFBAl, while the junction between the third and fourth switching units S3 and S4 provides a second cell connection terminal TEFBA2. These connection terminals TEFBAl and TEFBA2 thus provide points where the cell FBA can be connected to a phase arm of a phase leg. The first cell connection terminal TEFBAl thereby joins a phase arm with the connection point or junction between two of the series-connected switching units of the first group, here the first and second switching units SI and S2, while the second cell connection terminal TEFBA2 joins the upper phase arm with a connection point between two of the series connected switching units of the second group, here between the third and fourth switching units S3 and S4.

The expression couple or coupling is intended to indicate that more components, such as more cells and inductors, may be connected between the pole and the cell, while the expression connect or connecting is intended to indicate a direct connection between two components such as two cells. There is thus no component in-between two components that are connected to each other. Fig. 3 schematically shows a first type of a half-bridge converter cell HBA that may be used in an upper phase arm of a phase leg. This cell has a half- bridge cell structure where there is an energy storage element, here in the form of a capacitor C, which is connected in parallel with a group of switching units. Also this energy storage element C provides a voltage

Udm, and thus also has a positive and negative end, where the positive end has a higher potential than the negative end. The switching units in this group are connected in series with each other. The group here includes a first and a second switching unit SI and S2 (shown as dashed boxes), where each switching unit SI, S2 may be realized in the form of a switching element that may be a transistor like an IGBT together with an anti- parallel unidirectional conduction element, which may be a diode. In fig. 3 there is therefore a first switching unit SI having a first transistor Tl with a first anti-parallel diode D l, where the diode D l has a direction of current conduction towards the positive end of the energy storage element C and a second switching unit S2 connected in series with the first switching unit Dl and having a second transistor T2 with anti-parallel second diode D2, where the diode D2 has the same direction of current conduction as the first diode Dl. The first switching unit SI is connected to the positive end of the energy storage element C, while the second switching unit S2 is connected to the negative end of the energy storage element C.

In order to provide the first type of half-bridge cell HBA based on the half- bridge cell structure, there is a first cell connection terminal TEHBA1 and a second cell connection terminal TEHBA2, each providing a connection for the cell to the upper phase arm of the phase leg of the voltage source converter. In this first type of half-bridge cell the first cell connection terminal TEHBA1 more particularly provides a connection from the upper phase arm to the junction between the first switching unit SI and the capacitor C, while the second cell connection terminal TEHBA2 provides a connection from the upper phase arm to the junction between the first and the second switching units SI and S2. These cell connection terminals thus provide points where the cell can be connected to the upper phase arm . The second cell connection terminal TEHBA2 thus join s the phase arm with the connection point or junction between two of the series-connected switching units of the first group , here the first and second switching units S I and S2, while the first cell connection terminal TEHBA1 join s the upper phase arm with a connection point between the first switching unit S I and the positive end of the capacitor C.

Fig. 4 shows a second type of half-bridge cell HBB for connection in a lower phase arm of a phase leg. This cell has the same type of cell structure as the first type of half-bridge cell. Therefore, it comprises a group of switching units comprising a first and second switching unit S I and S2 connected in the same way as the first and second switching units of the first type of half-bridge cell. However, in this second type of half-bridge cell, the first cell connection terminal TEHBB 1 provides a connection from the lower phase arm to the junction between the first and the second switching units S I and S2, while the second cell connection terminal TEHBB2 provides a connection from the lower phase arm to the junction between the second switching unit S2 and the negative end of the capacitor C.

The half-bridge cell structure can be combined in a number of ways in order to obtain further cell types.

It is for instance possible to obtain a half-bridge double cell HBDC. An example of this cell type is shown in fig. 5.

In this cell a first half bridge cell structure is connected to a second half bridge cell structure so that the negative end of the energy storage element CI of the first half bridge cell structure is connected to the positive end of the energy storage element C2 of the second half bridge cell structure. The first and second switching units of the first cell structure are here also a first and second switching unit S I and S2 of the cell HBDC, while the first and second switching units of the second cell structure are a fifth and sixth switching unit S5 and S6 of the cell. A first cell connection terminal TEDCl is provided at the junction between the first and second switching units S I and S2, while a second cell connection terminal TEDC2 is provided at the junction between the fifth and sixth switching units S5 and S6.

Fig. 6 schematically shows the series connection of two half-bridge cell structure for obtaining a series half bridge cell SHBC.

In this case there is also a first and second half bridge cell structure, where the negative end of the energy storage element CI of the first half-bridge cell structure is connected to the junction between the switching units of the second half bridge cell structure. The first and second switching units of the first cell structure are also here a first and second switching unit S I and S2 of the cell SHBC, while the first and second switching units of the second cell structure are a seventh and eighth switching unit S7 and S8 of the cell. The first cell connection terminal TESHB 1 is in this case provided at the junction between the first and second switching units S I and S2, while the second cell connection terminal TESHB2 is provided at the negative end of the energy storage element C2 of the second cell structure.

As was mentioned earlier, the purpose of a cell is to provide a voltage contribution which is either a voltage of the cell capacitor or a zero voltage, where a half-bridge cell is only able to provide one polarity of the cell capacitor voltage but the full-bridge cell provides two polarities of the cell capacitor voltage.

A cell that gives a zero voltage contribution is in fact bypassed. A switching unit functioning to provide such a bypass may then be termed a bypass switching unit. A switching unit that is operative to bypass the energy storage element, i.e. to make the cell give a zero voltage contribution , is thus termed a bypass switching unit. Furthermore, in a half bridge cell one switching unit function s as a bypass switching unit, while in a full-bridge cell two switching units function as bypass switching units because the turning on of these switching units is used for bypassing the cell.

It can be seen that in a half-bridge cell the switching unit that is connected between the two connection terminals is a bypass switching unit. In a full- bridge cell either the two upper switching units of the two branches or the two lower switching units of the two branches operate as bypass switching units. Therefore a bypass switching unit may in the case of a full-bridge cell be an assigned bypass switching unit, where the cell control may be set to only use assigned bypass switching units when controlling the cell to be bypassed. A switching unit that is not used as a bypass switching unit may be termed a voltage contributing switching unit, since it is solely used for in serting the cell voltage into a phase arm when being turned on . It should here also be realized that in the full-bridge case an assigned bypass switching unit may in fact also be used in the provision of a voltage contribution of the cell. However, a voltage contributing switching unit should not be used for bypass operation . Only assigned bypass switching units should be used for bypass operation . It can thus be seen that in fig. 2 either the first and the fourth switching units SI and S4 are bypass switching units or the second and third switching units S2 and S3 are bypass switching units.

In a similar manner it can be seen that in fig. 3 , the first switching unit S I is a bypass switching unit and the second switching unit S2 is a voltage contributing switching unit, while in fig. 4 the second switching unit S2 is a bypass switching unit and the first switching unit S I is a voltage

contributing switching unit. In fig. 5 the second and fifth switching units S2 and S5 are bypass switching units and the first and sixth switching units S I and S6 are voltage contributing switching units, while in fig. 6 the second and eighth switching units S2 and S8 are bypass switching units and the first and seventh switching units S I and S7 are voltage contributing switching units.

The invention is concerned with allowing conduction losses to be reduced in a voltage source converter. As will be shown now with reference being made to fig. 7 and 8 , these losses are closely related to the bypass switching units.

Fig. 7 shows a phase arm current Ivppa and an insertion index Ninspa, where the in sertion index is an index indicating the number of inserted cells. It therefore also corresponds to a phase arm voltage Vvppa of a converter with phase legs comprising half-bridge cells.

Fig. 8 shows the total conduction loss Ptot, the conduction loss Pbp of bypass switching units and the conduction loss Pvc of voltage contributing switching units of a converter using half-bridge cells.

The MMC offers low switching losses thanks to the modular and multilevel structure. However, conduction losses still remain high . In fact the conduction losses often make up more than 70 % of the total loss of the converter. Conduction loss (Ploss_ c) is the function of RMS (Root Mean Square) current L-ms, average current Iavg and the semiconductor on-state resistance R _on as shown below:

Ploss_c ⁼ (VT * ) where VT is the semiconductor threshold voltage.

Since the conduction loss is related to the square of the RMS current Irms, it can be seen that, the higher the RMS current is, the higher is the

conduction loss. In for in stance a half bridge cell of the second type as shown in fig. 4, the conduct of each switching unit depends on both arm current direction and whether the cell capacitor is to be inserted or bypassed. According to these definition s, the average and RMS currents of a converter are defined as follows:

where, lul I is the current passing through the cells and Inn/ 1 is the in sertion index of the arms.

Looking at the insertion index Nmspa corresponding to the upper arm voltage Uvppa and current Iv _PP a shown in fig. 7, it can be seen that the voltage is low when the current is high . Thereby a lot of the cells are bypassed at high current levels. In fact, it is clear that all cells in the arm are bypassed for 50 % of the time and more importantly, when the current is at the maximum level a majority of the cells in both arms are bypassed.

Therefore the bypass switching unit encounters a higher average and RMS current than a voltage contribution switching unit as shown in Figure 8 . From this it can also be gathered that the influence of the current conduction losses are higher in the bypass switching units than in the voltage contributing switching units. As shown , this results in that the contribution of the bypass switching unit in a half bridge MMC is almost 70 -80 % of the total conduction losses. Therefore, if a bypass switching unit (with lower Ron) is selected, it is possible to obtain a substantial decrease in the conduction losses in the MMC converter. One way to reduce the losses is through increasing the current conduction areas of the switching units. If this is only done for the bypass switching units then it is possible to obtain a substantial reduction of conduction losses with a limited converter size increase. It is thus possible to increase the current conduction area of the semiconducting elements that are used in the bypass switching unit of a cell in order to reduce the current conduction losses. Through leaving the voltage contributing switching units unchanged, the converter size increase is at the same time limited. A limited converter size increase can thus be combined with a substantial lowering of the conduction losses. The bypass switching unit of a branch thus has a current conduction area that is larger than the current conduction area of the other switching unit of the branch.

One way in which this may be achieved is through arranging more semiconducting elements in parallel in one switching unit than in the other switching unit in order to obtain the bypass switching unit. The increasing of the number of semiconducting elements may involve connecting more semiconducting elements of the turn-off type in parallel and/ or more unidirectional semiconducting elements in parallel. There are thus more parallel semiconducting elements in the bypass semiconducting unit than in the voltage contributing switching unit. It can thereby be seen that each switching unit of a branch comprises a number of parallel semiconducting elements and that the bypass switching unit of the branch comprises more parallel semiconducting elements than the other switching unit of the branch.

It is for instance possible that only the number of parallel diodes is higher in the bypass switching unit than in the voltage contributing switching unit. It is thus possible that a bypass switching unit of a branch comprises more parallel semiconducting elements of the first type than the other switching unit of the branch. In this case the number of parallel transistors may be the same. This is of advantage if the converter is to operate as a rectifier. If the converter is to operate as an inverter it is on the other hand possible that only the number of transistors is higher in the bypass switching unit than in the voltage contributing switching unit. It is thus possible that a bypass switching unit of a branch comprises more parallel semiconducting elements of the second type than the other switching unit of the branch. In this case the number of diodes may be the same in both types of switching units. Naturally it is also possible that there are both more transistors and diodes in the bypass switching unit of the branch than in the other switching unit of the branch .

Furthermore, it is possible that switching units are provided or formed as modules comprising submodules with parallel semiconducting elements. A submodule may then comprise only transistors, only diodes or a

combination of diodes and transistors. It is as an example possible that the bypass semiconducting unit comprises a module having six submodules, while the voltage contributing switching unit comprises a module only having four submodules. It can thus be seen that the switching units may be formed as modules comprising a group of parallel submodules implementing the semiconducting elements of the first and second types, where the bypass switching unit of a branch comprises more submodules than the other switching unit of the branch.

Fig. 9 shows an example of how the first type of half-bride cell may be realized using modules comprising a number of submodules. Here it can be seen that the first switching unit SI is realized through a first module MA having six submodules SMA1, SMA2, SMA3, SMA4 SMA5 and SMA6, while the second switching unit S2 is realized through a second module

MB only having four submodules SMB1, SMB2, SMB3 and SMB4. Now, if the first four submodules SMA1, SMA2, SMA3 and SMA4 of the first switching unit SI are identical to the submodules SMB1, SMB2, SMB3 and SMB4 of the second switching unit S2, it can be seen that there will be more semiconducting elements in the first switching unit S I, either transistors, diodes or both , than in the second switching unit S2. Fig. 10 shows the conduction losses Pbp in the bypass switching units, the conduction losses Pvc in the voltage contributing switching units and the total conduction losses Ptot for an MMC converter employing modified half-bridge cells, i.e. half-bridge cells where a bypass switching unit has a larger current conduction area than the voltage contributing switching unit with which it is connected in series. It can be seen that as compared with fig. 8 , the conduction losses Pbp in the bypass switching units and the total conduction losses Ptot are significantly lower than when regular half- bridge cells are used. It is possible to make the same type of change to the full-bridge cell in fig. 2, the second type of half-bridge cell in fig. 4, the double cell in fig. 5 and the series half bridge cell in fig. 6.

In the case of the full-bridge cell in fig. 2, two of the cells connected to the same end of the energy storage element C, either the positive or the negative end, will act as bypass switching units and consequently have more semiconducting elements than the other two switching units. It is for instance possible to provide the first and fourth switching units S I and S4 as modules comprising six submodules and the second and third switching units S2 and S3 as modules limited to four submodules each . Alternatively, the second and third switching units S2 and S3 are bypass switching units, in which case their modules may each comprise six submodules, while the first and the fourth switching units S I and S4 are made up of modules only having four submodules each .

In the same manner the second switching unit S2 of the second type of half-bridge cell in fig. 4 may comprise a module made up of six submodules, while the first switching unit S I may be realized through a module only having four submodules.

Similarly, in the double cell in fig. 5 the first and sixth switching units S I and S6 may each be realized through a module only having four

submodules, while the second and fifth switching units S2 and S5 may each be realized through a module comprising six submodules.

Finally in the series half-bridge cell of fig. 6, the first and seventh switching units SI and S7 would each be realized through a module only having four submodules, while the second and eighth switching units S2 and S8 would each be realized through a module comprising six submodules.

The invention has a number of advantages. Through reducing the conduction losses the efficiency of the converter may be increased. This is furthermore achieved through a limited increase of the cell size and thereby also of the converter size. The reduction of the conduction losses also has the further advantage of relaxing the cooling requirements of the cells. Through the use of fixed sized modules with submodules an easily expandable standardized system can be used requiring little or no design changes. The modified cells also serve to increase the surge current handling capability of the converter.

It should be realized that it is possible to perform further variations in addition to those already described. The number of submodules in a module is not limited to four and six. These numbers were only given as examples because they represent currently existing module realizations. Furthermore, the transistor is not limited to an IGBT. It may for in stance be an Junction Field Effect Tran sistor (J FET) or a SiC Metal- Oxide- Semiconductor Field-Effect Transistor (MOSFET) instead. The

semiconducting device of the turn-off type is not limited to tran sistors. It may for instance also be an Integrated Gate-Commutated Thyristor (IGCT) . From the foregoing discussion it is evident that the present invention can be varied in a multitude of ways. It shall consequently be realized that the present invention is only to be limited by the following claims.