SE1650845A1 - Modular multilevel converter and cell for reducing current conduction losses - Google Patents

Abstract

lO 24 ABSTRACT A multilevel converter for forming an alternating current Waveformincludes at least one cell (HBA) comprising at least one energy storageelement (C) and a first branch of series-connected switching units inparallel With the energy storage element (C). The first branch comprises afirst and a second switching unit (Sl, S2), Where one (Sl) ofthe switchingunits that is operative to bypass the energy storage element (C) has acurrent conduction area that is larger than the current conduction area ofthe other switching unit (S2). This cell structure has the advantage ofreducing the conduction losses through the cell With a limited increase of the cell size. Fig. 9

Description

MODULAR MULTILEVEL CONVERTER AND CELL FOR REDUCINGCURRENT CONDUCTION LOSSES 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 ofinterest to use in a number of different powertransmission environments. They may for instance be used as voltagesource converters in direct current power transmission systems such ashigh voltage direct current (HVDC) and alternating current powertransmission systems, such as flexible alternating current transmissionsystem (FACTS). They may also be used as reactive compensation circuits such as Static VAR compensators.

In order to reduce harmonic distortion in the output of power electronicconverters, 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 anumber 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, “New Concept forhigh voltage-Modular multilevel converterfl IEEE 2004, A. Lesnicar, R.Marquardt, “A new modular voltage source inverter topology”, EPE 2003, WO 2010/ 149200 and WO 20 ll/ 124260. lO Converter elements or cells in such a converter may for instance be of thehalf-bridge, full-bridge or double cell type. These may be connected in upper and lower phase arms ofa phase leg.

A converter formed using cells has the advantage ofloW 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 multilevelconverter cell for providing at least one voltage contribution for assistingin 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 an theenergy storage element, the first branch comprising a first and a secondswitching unit, Wherein one ofthe switching units of the first branch that is operative tobypass the energy storage element has a current conduction area that islarger than the current conduction area ofthe other switching unit of the first branch.

This object is according to a second aspect achieved through a modularmultilevel converter for forming an alternating current waveform andcomprisin g: a phase leg comprising a number ofcells, Where at least one cell ofthephase leg comprises: at least one energy storage element; and lO a first branch of series-connected switching units in parallel With an energystorage element, said first branch comprising a first and a secondswitching unit; wherein one ofthe switching units of the first branch that is operative tobypass the energy storage element has a current conduction area that islarger than the current conduction area of the other switching unit of the first branch.

The invention has a number of advantages. Through reducing theconduction losses, the efficiency of the converter is increased. This isfurthermore achieved through a limited increase of the cell size andthereby also of the converter size. The reduction of the conduction lossesalso has the further advantage of relaxing the cooling requirements of thecells. The modified cells also serve to increase the surge current handling capability of the converter.

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 connectedbetween a pole and ground, fig. 2 schematically shows a full-bridge cell, fig. 3 schematically shows the structure of a first type ofhalf-bridge cell,fig. 4 schem atically shows the structure of a second type ofhalf-bridge cell,fig. 5 schematically shows a half-bridge double cell, fig. 6 schem atically shows a series half bridge cell, fig. 7 shows a plot of arm voltage and arm current ofa phase arm in theconverter, fig. 8 shows plots oftotal conduction losses and conduction losses through bypass and voltage contributing switchíng units of a converter, and fig. 9 schem atically shows one realization of the first type ofhalf-bridgecell in order to reduce conduction losses, and fig. 10 plots oftotal conduction losses and conduction losses throughbypass 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.

Pig. 1 shows one variation of a multilevel converter in the form of a cellbased voltage source converter 10 or modular multilevel converter (MMC).The converter operates to convert between alternating current (AC) anddirect current (DC). The converter 10 in fig. lcomprises a three-phasebridge made up of a number ofphase legs. There are in this case threephase legs. It should however be realized that as an alternative there maybe for instance only two phase legs. There is thus a first phase leg PL1, asecond phase leg PL2 and a third phase leg PL3. The phase legs are moreparticularly connected between a first DC terminal DCl and a second DCterminal DC2, where the first DC terminal may be connected to a first poleP1 of a DC power transmission system, such as a High Voltage DirectCurrent (HVDC) power transmission system and the second DC terminalDC2 may be connected to ground, where the mid points ofthe phase legsare connected to corresponding alternating current terminals ACAl, ACB 1,ACC1. Aphase 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 P1 furthermore has a first potential Udp that may bepositive. The first pole Pl may therefore also be termed a positive pole. TheAC terminals ACA1, ACB2, ACC3 may in turn be connected to an ACsystem, such as a flexible alternating current transmission system (FACTS), for instance via a transformer. Aphase arm between the first lO pole Pl and a first AC terminal ACAl, ACBl and ACCl may be termed afirst phase arm or an upper phase arm, while a phase arm between the firstAC terminal ACAl 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. l isonly one example of a multilevel converter where the invention may beused. It is for instance possible to use the converter as a reactive compensating device, such as a Static VAR Compensator.

The voltage source converter depicted in fig. lhas an asymmetricmonopole configuration. It is thus connected between a pole and ground.As an alternative it may be connected in a symmetric monopoleconfiguration or a symmetric bipole configuration. In a symmetricmonopole configuration the second DC terminal DC2 would be connectedto a second pole having a second negative potential that may be as large asthe first potential but with the opposite polarity. In a symmetric bipoleconfiguration there would also be a second pole. In the bipoleconfiguration there would furthermore be a third and a fourth phase armin the phase leg, where the second and third phase arms would beconnected to ground, the first phase arm connected between the positivevoltage of the first pole Pl and the second phase arm and the fourth phasearm connected between the negative voltage of the second pole and thethird phase arm. A first AC terminal ofa phase leg would in the symmetricbipole configuration be provided between the first and second phase arms,while a second AC terminal ofthe same phase leg would be providedbetween 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. lcomprise cells. A cell is a unit that may be switched for providing a voltagecontribution to the voltage on the corresponding AC terminal. Acell then comprises one or more energy storage elements, for instance in the form of lO capacitors, and the cell may be switched to provide a voltage contributioncorresponding to the voltage of the energy storage element or a zerovoltage contribution. In this case the cell inserts the voltage of the energystorage element. Ifmore than one energy storage element is included in acell it is possible with even further voltage contributions. When no voltageor 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 afm.

In the example given in fig. lthere are five series-connected or cascadedcells in each phase arm. Thus the upper phase arm of the first phase legPL1 includes five cells Clp 1, C2p 1, C3pl, C4pl and C5p 1, while the lowerphase arm of the first phase leg PLl includes five cells Clnl, C2n 1, C3n1,C4nl and C5n1. Across the cells ofthe upper phase arm there is a firstphase arm voltage Uvppa and through the upper phase arm there runs afirst phase arm current Ivppa. As the upper phase arm is connected to thefirst pole Pl it may also be considered to be a positive phase arm. Acrossthe cells of the lower phase arm there is a second phase arm voltage Uvpnaand through the lower phase arm there runs a second phase arm currentIvpna. The upper phase arm is furthermore joined to the AC terminalACAl via a first or upper arm reactor Laarm 1, while the lower phase arm isjoined to the same AC terminal ACA1 via a second or lower arm reactorLaarm2. In a similar fashion the upper phase arm of the second phase legPL2 includes five cells C1p2, C2p2, C3p2, C4p2 and C5p2 while the lowerphase arm of the second phase leg PL2 includes five cells C1n2, C2n2,C3n2, C4n2 and C5n2. Finally the upper phase arm of the third phase legPL3 includes five cells Clp3, C2p3, C3p3, C4p3 and C5p3 while the lowerphase arm of the third phase leg PL3 includes five cells Cln3, C2n3, C3n3,C4n3 and C5n3. The upper phase arms are furthermore joined to thecorresponding AC terminals ACB1 and ACC1 via corresponding first or upper arm reactors Lbarml and Lcarm l, respectively, while the lower lO phase arms are joined to the same AC terminal ACB1 and ACC1 viacorresponding 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 oftenfavorable to have many more cells in each phase arm, especially in HVDCapplications. Aphase arm may for instance comprise hundreds ofcells.

There may however also be fewer.

Control of each cell in a phase arm is normally done through providing thecell with a control signal directed towards controlling the contribution ofthat cell to meeting a reference voltage. The reference voltage may beprovided for obtaining an AC waveform on the AC terminal ofa phase leg,for instance 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 theconverter. However, in order to simplify the figure only the control of theupper 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 arms are controlled in a similar manner in order to form output waveforms on the three AC terminals AC1, AC2 and AC3.

There are a number of different cell types that can be used in theconverter, such as full-bridge cells, double cells half-bridge cells and serieshalf 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 ofa capacitor C, which is connected in lO parallel with a first group of switching units Sl and S2. The energy storageelement C provides a voltage Udm, and therefore has a positive andnegative end, where the positive end has a higher potential than thenegative end. The switching units Sl and S2 in the first group areconnected in series with each other, where each switching unit may berealized using a first type of semiconducting element that is aunidirectional conduction element, such as a diode, and a second type ofsemiconducting element in the form ofa semiconducting element of theturn-off type, such as a transistor like an IGBT (Insulated Gate BipolarTransistor). The diode may be anti-parallel to the transistor. In fig. 2 thefirst switching unit Sl has a first transistor Tl with a first anti-paralleldiode Dl. The first diode Dl is connected between the emitter and collectorof the transistor Tl and has a direction of conductivity from the emitter tothe collector as well as towards the positive end of the energy storageelement C. The second switching unit S2 has a second transistor T2 with asecond anti-parallel diode D2. The second diode D2 is connected in thesame way in relation to the energy storage element C as the first diode Dl,i.e. conducts current towards the positive end of the energy storageelement C. The first switching unit Sl is furthermore connected to thepositive end ofthe 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 thefirst group as well as with the energy storage element C. The second groupincludes a third switching unit S3, here provided through a thirdtransistor T3 with anti-parallel third diode D3 and a fourth switching unitS4, here provided through a fourth transistor T4 with anti-parallel fourthdiode D4. The fourth switching unit S4 is furthermore connected to thepositive end ofthe energy storage element C, while the third switching unitS3 is connected to the negative end of the energy storage element C. Boththe diodes D3 and D4 furthermore have a direction of current conduction towards the positive end of the energy storage element C. The switching lO 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 TEFBAland a second cell connection terminal TEFBA2, each providing aconnection for the cell to a phase arm of a phase leg of the voltage sourceconverter, such as to the upper phase arm ofthe first phase leg. In this full-bridge cell the first cell connection terminal TEFBAl more particularlyprovides a connection from the phase arm to the junction between the firstand the second switching units S1 and S2, while the second cell connectionterminal TEFBA2 provides a connection between the phase arm and aconnection point between the third and fourth switching units S3 and S4.The junction between the first and second switching units Sl and S2 thusprovides one cell connection terminal TEFBAl, while thejunction betweenthe third and fourth switching units S3 and S4 provides a second cellconnection terminal TEFBA2. These connection terminals TEFBAl andTEFBA2 thus provide points where the cell FBA can be connected to aphase arm of a phase leg. The first cell connection terminal TEFBAltherebyjoins a phase arm with the connection point or junction betweentwo ofthe series-connected switching units ofthe first group, here the firstand second switching units Sl and S2, while the second cell connectionterminal TEFBA2 joins the upper phase arm with a connection pointbetween 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 morecomponents, such as more cells and inductors, may be connected betweenthe pole and the cell, while the expression connect or connecting isintended to indicate a direct connection between two components such astwo cells. There is thus no component in-between two components that are connected to each other. lO lO Fig. 3 schematically shows a first type of a half-bridge converter cell HBAthat may be used in an upper phase arm ofa phase leg. This cell has a half-bridge cell structure where there is an energy storage element, here in theform ofa capacitor C, which is connected in parallel with a group ofswitching units. Also this energy storage element C provides a voltageUdm, and thus also has a positive and negative end, where the positive endhas a higher potential than the negative end. The switching units in thisgroup are connected in series with each other. The group here includes afirst and a second switching unit S1 and S2 (shown as dashed boxes),where each switching unit S1, S2 may be realized in the form ofa switchingelement that may be a transistor like an IGBT together With an anti-parallel unidirectional conduction element, which may be a diode. In fig. 3there is therefore a first switching unit Sl having a first transistor Tl with afirst anti-parallel diode Dl, where the diode Dl has a direction of currentconduction towards the positive end of the energy storage element C and asecond switching unit S2 connected in series with the first switching unitDl and having a second transistor T2 with anti-parallel second diode D2,where the diode D2 has the same direction of current conduction as thefirst diode Dl. The first switching unit Sl is connected to the positive endof the energy storage element C, while the second switching unit S2 is connected to the negative end ofthe energy storage element C.

In order to provide the first type ofhalf-bridge cell HBA based on the half-bridge cell structure, there is a first cell connection terminal TEHBAl anda second cell connection terminal TEHBA2, each providing a connectionfor the cell to the upper phase arm of the phase leg of the voltage sourceconverter. In this first type of half-bridge cell the first cell connectionterminal TEHBAl more particularly provides a connection from the upperphase arm to the junction between the first switching unit Sl and thecapacitor C, while the second cell connection terminal TEHBA2 provides aconnection from the upper phase arm to thejunction between the first andthe second switching units S1 and S2. These cell connection terminals thus provide points where the cell can be connected to the upper phase arm. lO ll The second cell connection terminal TEHBA2 thus joins the phase arm with the connection point or junction between two of the series-connectedswitching units of the first group, here the first and second switching unitsS1 and S2, while the first cell connection terminal TEHBA1joins the upperphase arm with a connection point between the first switching unit S1 and the positive end of the capacitor C.

Fig. 4 shows a second type of half-bridge cell HBB for connection in alower phase arm ofa phase leg. This cell has the same type of cell structureas the first type ofhalf-bridge cell. Therefore, it comprises a group ofswitching units comprising a first and second switching unit S1 and S2connected in the same way as the first and second switching units ofthefirst type ofhalf-bridge cell. However, in this second type of half-bridgecell, the first cell connection terminal TEHBB1 provides a connection fromthe lower phase arm to thejunction between the first and the secondswitching units S1 and S2, while the second cell connection terminalTEHBB2 provides a connection from the lower phase arm to thejunctionbetween the second switching unit S2 and the negative end of the capacitorC.

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 halfbridge cell structure so that the negative end of the energy storage elementCl ofthe first halfbridge cell structure is connected to the positive end ofthe energy storage element C2 of the second half bridge cell structure. Thefirst and second switching units of the first cell structure are here also afirst and second switching unit S1 and S2 of the cell HBDC, while the first and second switching units of the second cell structure are a fifth and sixth lO l2 switching unit S5 and S6 of the cell. A first cell connection terminal TEDClis provided at the junction between the first and second switching units S1and 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 schem atically 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, wherethe negative end of the energy storage element Cl of the first half-bridgecell structure is connected to the junction between the switching units ofthe second halfbridge cell structure. The first and second switching unitsof the first cell structure are also here a first and second switching unit S1and S2 of the cell SHBC, while the first and second switching units of thesecond cell structure are a seventh and eighth switching unit S7 and S8 ofthe cell. The first cell connection terminal TESHBl is in this case providedat thejunction between the first and second switching units Sl and S2,while the second cell connection terminal TESHB2 is provided at the negative end ofthe energy storage element C2 of the second cell structure.

As was mentioned earlier, the purpose ofa cell is to provide a voltagecontribution 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 cellcapacitor voltage but the full-bridge cell provides two polarities if the cell cap acitor voltage.

A cell that gives a zero voltage contribution is in fact bypassed. A switchingunit functioning to provide such a bypass may then be termed a bypassswitching unit. A switching unit that is operative to bypass the energystorage element, i.e. to make the cell give a zero voltage contribution, isthus termed a bypass switching unit. Furthermore, in a half bridge cell one switching unit functions as a bypass switching unit, while in a full-bridge lO 13 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 connectedbetween 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 thetwo lower switching units of the two branches operate as bypass switchingunits. Therefore a bypass switching unit may in the case ofa full-bridgecell be an assigned bypass switching unit, where the cell control may be setto only use assigned bypass switching units when controlling the cell to bebypassed. A switching unit that is not used as a bypass switching unit maybe termed a voltage contributing switching unit, since it is solely used forinserting the cell voltage into a phase arm when being turned on. It shouldhere also be realized that in the full-bridge case an assigned bypassswitching unit may in fact also be used in the provision ofa voltagecontribution ofthe cell. However, a voltage contributing switching unitshould 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 switchingunits S1 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 S1is a bypass switching unit and the second switching unit S2 is a voltagecontributing switching unit, while in fig. 4 the second switching unit S2 is abypass switching unit and the first switching unit S1 is a voltage contributing switching unit.

In fig. 5 the second and fifth switching units S2 and S5 are bypassswitching units and the first and sixth switching units Sl and S6 are voltage contributing switching units, while in fig. 6 the second and eighth lO 14 switching units S2 and S8 are bypass switching units and the first and seventh switching units Sl and S7 are voltage contributing switching units.

The invention is concerned with allowing conduction losses to be reducedin a voltage source converter. As will be shown now with reference beingmade 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 insertion index is an index idnciating the number of insertedcell. 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 ofbypass switching units and the conduction loss Pvc of voltage contributing switching units of a converter using half-bridge cells.

MMC offers low switching losses thanks to the modular and multilevelstructure. However, conduction losses still remain high. In fact theconduction losses often make up more than 70% of the total loss of theconverter. Conduction loss (Ploss_ c) is the function of RMS (Root MeanSquare) current Ifms, average current Iavg and the semiconductor on-state resistance Ron as shown in below: Plüss; I ÜIT X Iavg) "l" (Ron X Ir^zn52)where VT is the semiconductor threshold voltage.

As the conduction loss is related to the square of the RMS current Ifms, itcan be seen that, the higher the RMS current is, the higher is the conduction loss. lO In for instance a half bridge cell ofthe second type as shown in fig. 4, eachswitching unit conducts depends on both arm current direction andinserting/ bypassing of cell capacitor. According to these definitions, the average and RMS currents ofa converter are defined as follows: 2 i r Irins-ïgfš : :í Itt/l X šnufâ~e TIatrg-*uji 2 í Iujl X šnztftÜ where, Iu/ l is the current passing through the cells and Inu/ l is the insertion index ofthe arms.

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

Therefore the bypass switching unit encounters a higher average and RMScurrent than a voltage contribution switching unit as shown in Figure 8.From this it can also be gathered that the influence ofthe currentconduction losses are higher in the bypass switching units than in thevoltage contributing switching units As shown, this results in thecontribution ofthe bypass switching unit to HB-MMC is almost 70-80% of the total conduction losses. lO 16 Therefore, ifa bypass sWitching unit (With lower Ron) is selected, it ispossible to obtain a substantial decrease in the conduction losses in MMC COI1 VCIICI.

One Way to reduce the losses is through increasing the current conductionareas of the switching units. If this is only done for the bypass sWitchingunits then it is possible to obtain a substantial reduction of conductionlosses With a limited converter size increase. It is thus possible to increasethe current conduction area of the semiconducting elements that are usedin the bypass switching unit ofa cell in order to reduce the currentconduction losses. Through leaving the voltage contributing sWitchingunits unchanged, the converter size increase is at the same time limited. Alimited converter size increase can thus be combined with a substantialloWering of the conduction losses. The bypass switching unit ofa branchthus has a current conduction area that is larger than the current conduction area ofthe other switching unit of the branch.

One Way in Which this may be achieved is through there being moresemiconducting elements in parallel in one switching unit than the othersWitching unit in order to obtain the bypass sWitching unit. The increasingof the number of semiconducting elements may involve connecting moresemiconducting elements of the turn-offtype in parallel and/ or moreunidirectional semiconducting elements in parallel. There are thus moreparallel semiconducting elements in the bypass semiconducting unit thanin the voltage contributing sWitching unit. It can thereby be seen that eachsWitching unit of a branch comprises a number ofparallel semiconductingelements and the bypass switching unit of the branch comprises moreparallel semiconducting elements than the other switching unit of the branch.

It is for instance possible that only the number ofparallel diodes is higherin the bypass sWitching unit than in the voltage contributing switching unit. It is thus possible that a bypass switching unit ofa branch comprises lO l7 more parallel semiconducting elements of the first type than the otherswitching unit ofthe branch. In this case the number of parallel transistorsmay be the same. This is of advantage if the converter is to operate as arectifier. If the converter is to operate as an inverter it is on the other handpossible that only the number of transistors is higher in the bypassswitching unit than in the voltage contributing switching unit. It is thuspossible that a bypass switching unit ofa branch comprises more parallelsemiconducting elements of the second type than the other switching unitof the branch. In this case the number of diodes may be the same in bothtypes of switching units. Naturally it is also possible that there are both more transistors and diodes.

Furthermore, it is possible that switching units are provided or formed asmodules comprising submodules with parallel semiconducting elements. Asubmodule may then comprise only transistors, only diodes or acombination of diodes and transistors. It is as an example possible that thebypass semiconducting unit comprises a module having six submodules,while the voltage contributing switching unit comprises a module onlycomprising four submodules. It can thus be seen that the switching unitsmay be formed as modules comprising a group ofparallel submodulesimplementing 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 ofhalf-bride cell may berealized using modules comprising a number of submodules. Here it canbe seen that the first switching unit S1 is realized through a first moduleMAhaving six submodules SMAl, SMAZ, SMA3, SMA4 SMA5 and SMA6,while the second switching unit S2 is realized through a second moduleMB only having four submodules SMBl, SMB2, SMB3 and SMB4. Now, ifthe first four submodules SMAl, SMA2, SMAS and SMA4 ofthe firstswitching unit S1 are identical to the submodules SMB1, SMB2, SMB3 andSMB4 of the second switching unit S2, it can be seen that there will be 18 more semiconducting elements in the first switching unit S1, either transistors, diodes or both, than in the second switching unit S2.

Fig. 10 shows the conduction losses Pbp in the bypass switching units, theconduction losses Pvc in the voltage contributing switching units and thetotal conduction losses Ptot for an MMC converter employing modifiedhalf-bridge cells, i.e. half-bridge cells where a bypass switching unit has alarger current conduction area than the voltage contributing switching unitwith which it is connected in series. It can be seen that as compared withfig. 8, the conduction losses Pbp in the bypass switching units and the totalconduction 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 andthe series half bridge cell in fig. 6.

In the case ofthe full-bridge cell in fig. 2, two of the cells connected to thesame end of the energy storage element C, either the positive or thenegative end, will act as bypass switching units and consequently havemore semiconducting elements than the other two switching units. It is forinstance possible to provide the first and fourth switching units S1 and S4as modules comprising six submodules and the second and third switchingunits 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 thefirst and the fourth switching units S1 and S4 are made up of modules only comprising four submodules each.

In the same manner the second switching unit S2 of the second type ofhalf-bridge cell in fig. 4 may comprise a module made up of sixsubmodules, while the first switching unit S1 may be realized through a module only comprising four submodules. lO l9 Similarly, in the double cell in fig. 5 the first and sixth switching units Sland S6 may each be realized through a module only comprising foursubmodules, 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 switchingunits Sl and S7 Would each be realized through a module only comprisingfour 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 theconduction losses the efficiency of the converter may be increased. This isfurthermore achieved through a limited increase of the cell size andthereby also of the converter size. The reduction of the conduction lossesalso has the further advantage of relaxing the cooling requirements of thecells. Through the use of fixed sized modules With submodules an easilyexpandable standardized system can be used requiring little or no designchanges. 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 inaddition to those already described. The number of submodules in amodule is not limited to four and six. These numbers Were only given asexamples because they represent currently existing module realizations.Furthermore, the transistor is not limited to an IGBT. It may for instancebe an Junction Filet Effect Transistor (J FET) or a SiC Metal- Oxide-Semiconductor Field-Effect Transistor (MOSFET) instead. Thesemiconducting device of the turn-off type is not limited to transistors. Itmay for instance also be an Integrated Gate-Commutated Thyristor (IGCT).

From the foregoing discussion it is evident that the present invention canbe varied in a multitude of ways. It shall consequently be realized that the present invention is only to be limited by the following claims.

Claims (12)

1. Amultilevel converter cell (FBA; HBA; HBB; HBDC; SHBC) forproviding at least one voltage contribution (Udm) for assisting in theforming of an alternating current Waveform, the cell comprising at least one energy storage element (C; Cl, C2); and a first branch of series-connected switching units in parallel With theenergy storage element, said first branch comprising a first and a secondsWitching unit (S1, S2); Wherein one ofthe sWitching units (S1) of the first branch that is operativeto bypass the energy storage element (C) has a current conduction areathat is larger than the current conduction area ofthe other switching unit (S2) of the first branch.

2. The cell according to claim 1, Wherein each sWitching unit (S1, S2) of thefirst branch comprises a number ofparallel semiconducting elements andthe sWitching unit (S1) operative to bypass the energy storage element (C)comprises more parallel semiconducting elements than the other switching unit (S2) of the first branch.

3. The cell according to claim 2, Wherein a switching unit comprises a firsttype of semiconducting element in the form ofa unidirectional conductionelement (D1,D2) and a second type of semiconducting element (T1, T2) in the form ofa semiconducting element ofthe turn-off type.

4. The cell according to claim 3, Wherein the switching unit (S1) ofthe firstbranch operative to bypass the energy storage element (C) comprises moreparallel semiconducting elements ofthe first type than the other switching unit (S2) of the first branch.

5. The cell according to claim 3 or 4, Wherein the sWitching unit ofthe first branch (S1) operative to bypass the energy storage element (C) comprises lO 22 more parallel semiconducting elements of the second type than the other switching unit (S2) of the first branch.

6. The cell according to any of claims 3 - 5, wherein the switching units areformed as modules (MA, MB) comprising a group ofparallel submodules(SMAl, SMAZ, SMA3, SMA4, SMA5, SMA6, SMBl, SMBZ, SMB3, SMB4)implementing the semiconducting elements of the first and second types,wherein the switching unit (Sl) of the first branch being operative tobypass the energy storage element (C) comprises more submodules(SMAl, SMA2, SMA3, SMA4, SMAS, SMA6) than the other switching unit(SMBl, SMBZ, SMBB, SMB4) ofthe first branch.

7. The cell according to any of claims 1- 6, wherein there is oneconnection terminal (TEHBAZ) at the junction between the first andsecond switching units (Sl, S2) and another connection terminal(TEHBAI) at a junction between a switching unit (Sl) and the energystorage element (C), Where the switching unit (Sl) operative to bypass theenergy storage element is connected between the two connection termin als.

8. The cell according to any of claims l- 6, wherein the cell comprises asecond branch with a third switching unit (S3) connected in series with afourth switching unit (S4), said second branch being connected in parallelwith the energy storage element (C), and where one of the switching unitsof the second branch that is operative to bypass the energy storage elementhas a current conduction area that is larger than the current conduction area ofthe other switching unit ofthe second branch.

9. The cell according to claim 8, wherein the energy storage element hastwo ends and the switching units operative to bypass the energy storageelement ofthe first and second branches are connected to the same end of the energy storage element. lO 23

10. The cell according to claim 8 or 9, wherein there is one connectionterminal (TEFBAl) at the junction between the first and second switchingunits (S1, S2) and another connection terminal (TEFBA2) at ajunctionbetween the third and fourth switching units (S3, S4).

11. ll. A modular multilevel converter (10) for forming an alternatingcurrent waveform and comprising a phase leg (PLl) comprising a number of cells (Clp l, C2p 1, C3pl, C4p 1,C5p l, Cln l, C2n 1, C3nl, C4nl, C5nl), where at least one cell (HBA) of thephase leg comprises: at least one energy storage element (C; Cl, C2); and a first branch of series-connected switching units in parallel with an energystorage element (C), said first branch comprising a first and a secondswitching unit (S1, S2); wherein one (Sl) of the switching units of the first branch that is operativeto bypass the energy storage element (C) has a current conduction areathat is larger than the current conduction area of the other switching unit (S2) of the first branch.

12. The multilevel converter according to claim ll, wherein the phase leghas a first and a second phase arm, said first phase arm being connectedbetween a first DC terminal (DCl) and a first AC terminal (ACAl) and thesecond phase arm being connected between the first AC terminal (ACAl)and a second DC terminal (DC2), each phase arm comprising said at least one cell (HBA) according to claim ll.