FIELD OF INVENTION The present invention generally relates to multilevel converters. More particularly the present invention relates to a multilevel converter configured to convert between alternating current and direct current as well as to a cell that may be provided in a multilevel converter.

BACKGROUND

Multilevel converters are of interest to use in a number of different power transmission environments. They may for instance be used as voltage source

converters in direct current power transmission systems such as high voltage direct current (HVDC) and

alternating current power transmission systems, such as flexible alternating current transmission system

(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 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. Converter elements in such a converter may for instance be of the half-bridge, full-bridge or clamped double cell type. Clamped double cells or double voltage contribution cells are for instance described in WO 2011/067120.

A half-bridge connection in upper and lower arms provides unipolar cell voltage contributions and offers the simplest structure of the chain link converter. This type is described by Marquardt, ' ew Concept for high voltage-Modular multilevel converter' , IEEE 2004 and A. Lesnicar, R. Marquardt, "A new modular voltage source inverter topology", EPE 2003.

However, there is a problem with the half-bridge topology in that the fault current blocking ability in the case of a DC fault, such as a DC pole-to-pole or a DC pole-to-ground fault, is limited.

One way to address this is through the use of full- bridge cells. This is for instance described in WO 2011/012174. Series connection of full-bridge cells offers four quadrant power flows through the energy storage element of the cell capacitor as well as DC fault voltage blocking capability by imposing a reverse voltage. However, the use of full-bridge cells doubles the number of components compared with a half-bridge cell .

One way to reduce the number of components is through the use of director switches in the phase arm together with full bridge cells, where the director switches are essentially used for providing a DC level that is combined with an AC level generated by the full-bridge cells for forming an AC voltage. A converter combining director switches with full-bridge cells is described in WO 2010/149200.

In order to further reduce the number of components in a converter employing director switches and full-bridge cells, WO 2011/124260 further suggests that a number of the full-bridge cells are replaced by half-bridge cells .

This structure is economical. However, it is

problematic in that because there are so few full- bridge cells left, the ability to reduce fault currents caused by DC faults is limited. There is therefore a need to reduce the number of components used in a converter combined with retaining a fair fault current limiting capability.

SUMMARY OF THE INVENTION

The present invention is directed towards enabling to make a reduction of the number of components in a multilevel converter combined with providing a fair fault current limitation capability.

This object is according to a first aspect achieved through a multilevel converter configured to convert between alternating current and direct current. The multilevel converter comprises a phase arm with a number of cells between a DC pole and an AC terminal. The cells comprise at least one single voltage contribution cell and at least one double voltage contribution cell for AC voltage forming and fault current handling operation. The double voltage

contribution cell comprises a first section with a first group of series connected switching units. This first group is connected in parallel with a first energy storage element, where a junction between a first and a second switching unit of the first group forms a first cell connection terminal. The double voltage contribution cell also comprises a second section with a second group of series connected

switching units. This second group is connected in parallel with a second energy storage element, where a junction between a third and a fourth switching unit of the second group forms a second cell connection

terminal. In addition, the double voltage contribution cell further comprises

an interconnecting switch interconnecting the first and the second sections and connected between a positive end of the first energy storage element and a negative end of the second energy storage element.

The above mentioned object is also according to a second aspect of the invention also achieved by a cell for use in a phase arm of a multilevel converter, where the converter converts between alternating current (AC) and direct current (DC) . The cell comprises a first section, a second section an interconnecting switch, a first directional switch and a second directional switch.

The first section has a first group of series connected switching units, which group is connected in parallel with a first energy storage element. A junction between a first and a second switching unit of the first group forms a first cell connection terminal.

The second section has a second group of series

connected switching units, which group is connected in parallel with a second energy storage element. A junction between a third and a fourth switching unit of the second group forms a second cell connection

terminal .

The interconnecting switch interconnects the first and the second sections and is connected between a positive end of the first energy storage element and a negative end of the second energy storage element.

The first directional switch is coupled between the positive end of the first energy storage element and a positive end of the second energy storage element.

Finally, the second directional switch is coupled between a negative end of the first energy storage element and the negative end of the second energy storage element.

The present invention has a number of advantages. It provides or enables the provision of a converter having a low number of components. It furthermore provides low conduction losses because of a low number of components in the conduction path. It also provides a good fault current handling capability.

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 first type of multilevel converter connected between two poles,

fig. 2 schematically shows a variable voltage source used in the multilevel converter of the first type, fig. 3 schematically shows a structure of a full-bridge cell that may be used in the converter,

fig. 4 schematically shows a structure of a first type of double voltage contribution cell that may be used in the converter,

fig. 5 shows various voltages provided in the first type of multilevel converter, and

fig. 6 schematically shows the structure of a second type of double voltage contribution cell that may be used in the first type of converter.

DETAILED DESCRIPTION OF THE INVENTION

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

Fig. 1 shows a block schematic outlining an example of a first type of voltage source converter 10, which may be provided as an interface between a direct current (DC) power system and an alternating current (AC) power system such as an interface between AC and DC power transmission systems. The DC power transmission system may be a High Voltage Direct Current (HVDC) power transmission system and the AC system may be a flexible alternating current transmission system (FACTS) . The voltage source converter 10 is a multilevel converter configured to convert between AC and DC. The example voltage source converter 10 here includes a group of branches in the form of phase legs connected in parallel between two DC poles PI and P2 for connection to the DC transmission system. In the example given here there are three such branches or phase legs PLl, PL2, PL3 in order to enable connection to a three-phase AC transmission system. It should however be realized that as an alternative there may be for instance only two phase legs. Each phase leg PLl, PL2 and PL3 has a first and second end point. In a converter of the type depicted in fig. 1 the first end points of all the phase legs PLl, PL2 PL3 are connected to the first DC pole PI, while the second end points are connected to the second DC pole P2.

Each phase leg PLl, PL2, PL3 of this first type of voltage source converter 10 further includes a lower and upper phase leg half, often denoted phase arm, and at the junction where the phase arms of a phase leg meet, there is provided an AC terminal. In the

exemplifying voltage source converter 10 there is here a first phase leg PLl having an upper phase arm and a lower phase arm, a second phase leg PL2 having an upper phase arm and a lower phase arm and a third phase leg PL3 having an upper phase arm and a lower phase arm. At the junction between the upper and lower phase arms of the first phase leg PLl there is provided a first AC terminal AC1, at the junction between the upper and lower phase arms of the second phase leg PL2 there is provided a second AC terminal AC2 and at the junction between the upper and lower phase arms of the third phase leg PL3 there is provided a third AC terminal

AC3. Each AC terminal AC1, AC2, AC3 is here connected to the corresponding phase leg via a respective

inductor LAC1, LAC2, LAC3. Here each phase arm furthermore includes one current limiting inductor Lul, Lu2, Lu3, Lll, L12, and L13 connected to the

corresponding DC pole PI and P2. Each phase arm

furthermore includes a variable voltage source Uul, Ull, Uu2, U12, Uu3, U13 and a director switch DSul, Dsll, DSu2, DS12, DSu3 and DS13. The variable voltage sources Uul, Ull, Uu2, U12, Uu3, U13 and director switches DSul, Dsll, DSu2, DS12, DSu3 and DS13 are all being controlled by a control unit 12, which control is indicated by dashed arrows. It should also be realized that the inductors are optional.

A variable voltage source is made up of a number of cells, where each cell comprises at least one voltage storage element and switching units for switching the cell, such as pairs of transistors with antiparallel diodes. The converter thus comprises at least one phase arm with a number of cells between a DC pole and an AC terminal. A director switch on the other hand comprises a number of switching units, for instance in the form of a number of pairs of transistors with anti-parallel diodes. In fig. 1 a director switch is shown as only one such pair. The first DC pole PI furthermore has a first potential +DC that may be positive, while the second DC pole P2 has a second potential -DC that may be negative. The first pole PI may therefore also be termed a positive pole, while the second pole P2 may be termed negative pole.

Fig. 2 schematically shows an example of the cells making up a variable voltage source Uul. This variable voltage source Uul comprises a number of series

connected cells. The cells are thus connected in cascade. The cells comprises at least one full-bridge cell and at least one double voltage contribution cell for AC voltage forming and fault current handling operation. There is here a first and a second full- bridge cell FBC1 and FBC2 as well as a first, second and third double voltage contribution cell DVCl, DVC2 and DVC3. It should be realized that the number of cells shown in the figure have been intentionally kept low in order to provide a clearer understanding of the invention. The number of cells of both types could thus be higher and is often significantly higher. It

therefore has to be stressed that the number of cells in a variable voltage source and therefore also in a phase arm may vary. It is often favorable to have many more cells in each phase arm, especially in HVDC applications . Fig. 3 shows a full-bridge cell FBC that may be

provided in the phase arms of the first type of

converter .

The cell FBC is thus a full-bridge converter cell and includes a single energy storage element, here in the form of a capacitor C, which is connected in parallel with a first group of switching units SU1 and SU2. 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 SU1 and SU2 in the first group are connected in series with each other. The first group here includes two switching units SU1 and SU2 (shown as dashed boxes) . These two switching units SU1 and SU2 may be realized in the form of a switching element that may be an IGBT (Insulated Gate Bipolar Transistor) transistor together with an anti-parallel unidirectional conducting element. It is possible that the switching unit is another type of switching unit like a field effect transistor (FET) such as a metal oxide semiconductor field effect transistor (MOSFET) . The switching unit may also have other realizations. It may for instance be a Reverse Conduction IGBT (RC-IGBT) or a Bi-mode Insulated Gate Transistor (BIGT) . In fig. 3 the first switching unit SU1 is therefore provided as a first transistor Tl with a first anti-parallel diode Dl . 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 SU2 is provided as 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 Dl, i.e. conducts current towards the positive end of the energy storage element C. The first switching unit SU1 is furthermore connected to the positive end of the energy storage element C, while the second switching unit SU2 is connected to the negative end of the energy storage element C.

There is also a second group of series-connected switching units SU3 and SU4. 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 SU3 and a fourth switching unit SU4. The third switching unit SU3 is provided as a third transistor T3 with anti-parallel third diode D3. The fourth switching unit SU4 is provided as a fourth transistor T4 with anti- parallel fourth diode D4. This second group of

switching units is provided in a further branch in parallel with the capacitor C. The third switching unit SU3 is furthermore connected to the positive end of the energy storage element C, while the fourth switching unit SU4 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 and are connected between emitter and collector of the transistors in the same way as the first and second switching units SU1 and SU2.

This full-bridge cell FBC comprises a first cell connection terminal TEFBCl and a second cell connection terminal TEFBC2, each providing a connection for the cell to a phase arm of a corresponding phase leg, such as the first phase leg of the first type of voltage source converter. In this full-bridge cell the first cell connection terminal TEFBCl may more particularly provide a connection from a phase arm to the junction between the first and the second switching units SU1 and SU2, while the second cell connection terminal TEFBC2 may provide a connection between the phase arm and a connection point between the third and fourth switching units SU3 and SU4. The junction between the first and second switching units SU1 and SU2 thus provides one cell connection terminal TEFBCl, while the junction between the third and fourth switching units SU3 and SU4 provides a second cell connection terminal TEFBC2. These cell connection terminals TEFBC1 and TEFBC2 thus provide points where the cell FBC can be connected to a phase arm, such as the upper phase arm of a first phase leg of the first type of converter.

Fig. 4 shows a first type of clamped double cell or double voltage contribution cell DVCA that may be used in the converter 10. The cell is designated as a double voltage contribution cell, because it is a cell with the ability to provide two energy storage element voltages for contributing to the forming of an AC voltage on an AC terminal of a phase leg. For the same reason a full-bridge cell is a single voltage contribution cell, since it only employs one energy storage element.

The cell DVCA here comprises a first section SEC1 comprising a first energy storage element CI, here in the form of a first capacitor CI, which is connected in parallel with a first group of switching units. Also this first energy storage element CI 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 first group also here includes two series-connected switching units SW1 and SW2 (shown as dashed boxes) , where the first switching unit SW1 has a first transistor Tl with a first anti-parallel diode Dl, where the diode Dl has a direction of current conduction towards the positive end of the first energy storage element CI. Also the second switching unit SW2 comprises a second transistor T2 with anti-parallel second diode D2 and having the same direction of current conduction as the first diode Dl . The switching units thereby also comprise switching elements with anti-parallel unidirectional conducting elements. The first switching unit SW1 is also here connected to the positive end of the first energy storage element CI, while the second switching unit SW2 is connected to the negative end of the first energy storage element CI . In the cell DVCA there is furthermore a second section SEC2. The second section comprises a second group of switching units connected in series with each other. This second group of switching units is connected in parallel with a second energy storage element C2. The second group also here includes a third switching unit SW3 and a fourth switching unit SW4, where the third switching unit SW3 is provided through a third

transistor T3 with anti-parallel third diode D3 and the fourth switching unit SW4 is provided through a fourth transistor T4 with anti-parallel fourth diode D4. Also this second energy storage element C2 provides a voltage Udm, with advantage the same voltage as the first energy storage element, and therefore has a positive and negative end, where the positive end has a higher potential than the negative end. The fourth switching unit SW4 is in this case connected to the negative end of the second energy storage element C2, while the third switching unit SW3 is connected to the positive end of the second energy storage element C2. The current conducting direction of both diodes D3 and D4 is towards the positive end of the second energy storage element C2. Between the first and second section SEC1 and SEC2 there is furthermore an interconnecting switch IS interconnecting the first and the second sections. This interconnecting switch IS is in the example of fig. 3 also in the form of a fifth transistor T5 with anti- parallel diode D5. The interconnecting switch IS is connected between the positive end of the first energy storage element CI and the negative end of the second energy storage element C2 with the direction of current conduction of the fifth diode D5 being towards the positive end of the first energy storage element CI.

In the example of a double voltage contribution cell shown in fig. 4 there is also a first unidirectional conducting element coupled between the positive end of the first energy storage element CI and the positive end of the second energy storage element C2 as well as a second unidirectional conducting element coupled between the negative end of the first energy storage element CI and the negative end of the second energy storage element C2. The first unidirectional conducting element is here in the form of a sixth diode D6 having a direction of current conduction towards the positive end of the second energy storage element C2. The second unidirectional conducting element is here in the form of a seventh diode D7 having a direction of current conduction towards the negative end of the second energy storage element C2. This cell DVCA comprises a first cell connection terminal TEDVCA1 and a second cell connection terminal TEDVCA2, each providing a connection for the cell to a phase arm. The first cell connection terminal TEDVCA1 provides a connection to the junction between the first and the second switching units SW1 and SW2, while the second cell connection terminal TEDVCA2 provides a connection to the junction between the third and fourth switching units SW3 and SW4. The junction between the first and the second switching units SW1 and SW2 thus provides the first cell connection terminal TEDVCAl and the junction between the third and fourth switching units SW3 and SW4 provide the second cell connection terminal TEDVCA2. In case the cell is to be placed in a positive phase arm, the second cell connection terminal TEDVCA2 faces the first pole and thereby couples the cell to the first pole, while the first cell connection terminal TEDVCAl faces the AC terminal of the phase leg. When being placed in a positive phase arm, the first cell connection terminal TEDVCAl thereby couples the cell to the AC terminal of the phase leg, while the second cell connection terminal TEDVCA2 couples the cell to the first pole. If being connected in the negative phase arm, the first cell connection terminal TEDVCAl faces the second pole and thereby couples the cell to the second pole, while the second cell

connection terminal TEDVCA2 faces the AC terminal of the phase leg. When being placed in a negative phase arm, the first cell connection terminal TEDVCAl thereby couples the cell to the second pole of the phase leg, while the second cell connection terminal couples the cell to the AC terminal of the phase leg. The expression couple or coupling is herein intended to indicate that more components, such as more cells, switching units and inductors, may be connected between two components coupled to each other, while the expression connect or connecting is herein 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.

The cell DVCA has a number of operational states, here four, in order to be employed in the forming of an AC voltage on the AC terminal of a phase leg. When being operated in these operational states the intermediate switch IS is always on. It is thus always conducting in an AC voltage forming operating mode of the double voltage contribution cell. The switching units of the double voltage contribution cell are then controllable to provide four AC voltage contribution states when operated in the voltage forming operating mode.

In order to provide a first state, where the cell DVCA provides a voltage contribution of the first energy storage element CI, the switching elements T2 and T4 of the second and fourth switching units SW1 and SW4 are on together with the switching element T5 of the intermediate switch IS. In order to provide a second state where the cell DVCA provides a voltage contribution of the second energy storage element C2, the switching elements Tl and T3 of the first and third switching units SW1 and SW3 are on together with the switching element T5 of the

intermediate switch IS.

In order to provide a third state where the cell DVCA provides a voltage contribution of both the first and the second energy storage elements CI and C2, the switching elements T2 and T3 of the second and third switching units SW2 and SW3 are on together with the switching element T5 of the intermediate switch IS.

In order to provide a fourth state where the cell DVCA provides a zero voltage contribution, the switching elements Tl and T4 of the first and the fourth

switching units SWl and SW4 are on together with the switching element T5 of the intermediate switch IS.

Finally, with regard to the first type of double voltage contribution cell DVCA there is a fault current operation, for instance due to DC faults, like a pole- to-ground fault or a pole-to-pole fault. In this case all the switching elements Tl, T2, T3 and T4 of both the sections SEC1 and SEC2 as well as the switching element T5 of the intermediate switch IS are turned off. The switching elements T1-T5 are thus turned off if a fault current due to a DC fault runs through the phase arm. In this case a fault current is allowed to run from the first cell connection terminal TEDVCA1 via the first diode Dl of the first switching unit SWl in parallel over the first and second energy storage elements CI and C2 and the diodes D6 and D7 through the fourth diode D4 of the fourth switching unit SW4 and out from the cell DVCA via the second cell connection terminal TEDVCA2. It can in this way be observed that the cell inserts the energy storage elements in the fault current path, which provides a fault current limitation . After having described the basic operation modes of the double voltage contribution cell, the functioning in an environment of the first type of converter will now be described .

The way the converter 10 operates is more particularly shown in fig. 5, where a period of a formed AC voltage V _AC i is shown together with a voltage V _DS provided by the director switches DSul and DSll of the phase leg and the voltage V _SH provided by the controllable sources Uul, Ull. Here the variable voltage source Uul forms a part of the waveform V _SH as the director switch DSul of the upper phase arm is turned on and the director switch DSll of the lower phase arm is switched off and the variable voltage source Ull forms another part of the waveform as the director switch DSll of the lower phase arm is turned on and the director switch DSul of the upper phase arm is turned off. In the first type of converter the director switch DSul of a phase arm, here exemplified by the upper phase arm of the first phase leg PL1, can be considered to form parts of the director switch voltage V _DS and the controllable voltage source Uul may be considered to provide a part of the voltage V _SH -

The control is more particularly provided by the control unit 12, which switches the cells for providing a voltage contribution using the energy storage

elements of the cells to form the voltage V _SH and controls the director switches for forming the voltage V _DS , which together form the AC voltage V _AC i . Control of a cell in a phase arm is more particularly typically done through providing the cell with a control signal directed towards controlling the

contribution of that cell to meeting a reference voltage.

The full-bridge cells are controlled to produce a DC voltage as well as an AC voltage and thus the variable voltage sources Uul and Ull are controlled to produce an AC voltage as well as a DC voltage. The director switches DSul and DS11 are switches with a fundamental switching frequency to connect or block the upper and lower phase arms in positive and negative polarities. Thereby, a pulse wave voltage V _DS is generated by the director switches and a pure AC voltage V _AC i , as shown in figure 5, will appear at the AC terminal AC1 by at the same time inserting and bypassing an appropriate number of cells in each converter arm. The full-bridge cells are able to provide both positive and negative voltages. As can be noted in fig. 5, only 27% of the AC voltage V _SH is below zero and

consequently only 27% percent of the variable voltage source need to be made up of full-bridge cells in order to be able to form the AC waveform V _AC i .

The controllable voltage source according to a prior art realization described in WO 2010/149200 is solely made up of full bridge cells. This means that in this converter full-bridge cells are provided in an amount that is redundant for conversion purposes. It would therefore be beneficial if the number of components is reduced. This means that savings can be made if other types of cells are also used.

In WO 2011/124260 this situation is addressed through employing an arm chain link with mixed half- and full- bridge cells. In this case 27% of the voltage V _SH of the variable voltage source is provided through full- bridge cells while 73% of the voltage V _SH is provided by half-bridge cells. This structure has some

advantages compared with the use of only full-bridge cells in that the number of components are reduced and therefore also the cost of the converter is reduced. Another advantage is that the losses are lowered, since the number of active switching units used in a cell is halved.

However, there is a problem with this structure of WO 2011/124260 and that is that the fault current

limitation capability of the converter has been

significantly reduced compared with when the variable voltage source consists of full-bridge cells.

The reason for this is the following. When there is a DC fault, such as a pole-to-ground fault, the control unit 12 controls the cells and switches to handle the fault. In this case the control unit switches off all the cell switching elements but keeps the director switches turned on. A fault current will then run through the energy storage element of the full-bridge cell, which will help limit the fault current. However, when half-bridge cells are used, these are bypassed and do therefore not contribute to fault current

limitation . Variations of the invention are directed towards improving on the above-mentioned situation.

According to some exemplary embodiments of this

disclosure the variable voltage source Uul comprises double voltage contribution cells instead of some full- bridge cells or instead of half-bridge cells. There is thus a variable voltage source of a phase arm where there is a mixture of single voltage contribution cells and double voltage contribution cells.

The use of double voltage contribution cells has advantages in fault current handling. When there is a fault current caused by a DC fault, the control unit 12 turns off the interconnecting switch IS and the

switches of the first and second sections SEC1 and SEC2. Thereby the energy storage elements CI and C2 are connected into the fault current path and the cell will assist in the reduction of the fault.

In a first variation of the invention, which is also a first aspect of the invention, such a variable voltage source is made up of a mixture of full-bridge cells and cells of the first type of double voltage contribution cell.

In this variation the full-bridge cells may be used to form 27% of the voltage V _SH , where double voltage contribution cells are used for forming the remainder of the voltage VSH, and here 73%.

Compared with the situation in WO 2011/124260, the number of components is slightly increased. There is an increase of half a switching unit per cell in the conducting path. However, at the same time a

significantly improved fault current limitation

capability is obtained. If for instance all the half- bridge cells of WO 2011/124260 are replaced by double voltage contribution cells, then it is possible to obtain significantly improved fault current limitation capability. This means that it is possible to obtain essentially the same advantages of a converter where there is a combination of director switch and solely full-bridge cells but with a significant reduction in the number of components.

As compared with the situation in WO 2010/149200, there is a slightly degraded fault current limitation

capability. However, the number of components used in the converter is significantly decreased. Furthermore, the current conduction losses are significantly reduced, because the number of switching units being passed is lower.

When mixing full-bridge and double voltage contribution cells in this way it is thus possible to obtain a considerable fault current limitation capability even though there are only enough full-bridge cells for forming the 27% of the negative voltage.

It is possible to obtain further component reductions using an alternative double voltage contribution cell.

Fig. 6 shows a second type of clamped double cell or double voltage contribution cell DVCB that may be used in the converter of the first type. The difference between the second and the first types of double voltage contribution cells is that in the second type DVCB there is a first directional switch coupled between the positive end of the first energy storage element CI and the positive end of the second energy storage element C2 and a second directional switch coupled between the negative end of the first energy storage element CI and the negative end of the second energy storage element C2. The first directional switch is here provided in the form of a sixth transistor T6 and the second directional switch is provided in the form of a seventh transistor T7. The sixth transistor T6 is here connected in series with the sixth diode D6 and with advantage between the sixth diode D6 and the positive end of the first energy storage element CI, while the seventh transistor T7 is connected in series with the seventh diode D7 and with advantage between the negative end of the first energy storage element CI and the seventh diode D7. Because of the sixth and seventh diodes D6 and D7, the transistors T6 and T7 do not need any anti-parallel diodes.

The second type of double voltage contribution cell is the subject of a second aspect of the invention. This second type of voltage contribution cell is a cell with increased or enhanced functionality.

The voltage contributing operational states of the second type of double voltage contribution cell DVCB is the same as in the first type. The directional switches T6 and T7 may in this type of operation be turned off. However, the fault current operation of this second type of double voltage contribution cell DVCB is a bit different in that the sixth and seventh transistors T6 and T7 are turned on in this mode of operation. Thereby a fault current is allowed to run from the first cell connection terminal TEDVCB1 via the first diode Dl of the first switching unit SW1 in parallel over the first and second energy storage elements CI and C2 via the transistors T6, T7 and diodes D6 and D7 through the fourth diode D4 of the fourth switching unit SW4 and out from the cell via the second cell connection terminal TEDVCB2.

The directional switches T6 and T7 of the second type of double voltage contribution cell DVCB may also be used as a part of a director switch.

A director switch operation of the cell is obtained when switches T6 and T7 are turned on and off for assisting in the forming of the square wave voltage

V _DS . In this case the double voltage contribution cell DVCB operates as a part of the director switch being switched on or off. The double voltage contribution cell is thereby controllable to provide a director switch operation, where the interconnection switch IS is turned off and the first and second directional switches are controllable for assisting in the

provision of a square wave. With this double voltage contribution cell structure it is possible to reduce the number of director switches significantly because of the double use of the double voltage contribution cell. The number of switching elements in the director switch is thereby reduced by for instance 36% percent of the voltage V _DS , which leads to further component reduction. The director switches in the cells are furthermore not placed in any conduction path in any of voltage forming contribution modes, and therefore this placing does not lead to any additional conduction losses.

The interconnecting switch of the double voltage contribution cell need not be provided in the form of a transistor with anti-parallel diode. It may for

instance be provided as an Integrated Gate-Commutated Thyristor (IGCT) . Furthermore, as it is to be always on in normal operation and only turned off in fault current operation and optionally also in director switch operation, it does not need to be fast.

Therefore it may also be provided in the form of a mechanical switch. Furthermore the use of the above described double voltage contribution cell in multilevel converts is not limited to the type of converter described in fig. 1. It may in fact be combined with full-bridge cells and/or other types of single voltage contribution cells like half-bridge cells in regular multilevel

converters, i.e. in converters that do not employ director switches. In this way it may be possible to obtain a good fault current limitation capability with a low number of components in the current conduction path.

Yet another realization of a multilevel converter is a static VAR compensator. It should also be realized that the second type of double voltage contribution cell is not limited to being combined with other cell types. It is thus possible to provide a converter in which the only types of cells used are double voltage contribution cells of the second type.

The embodiments disclosed hereing provide a number of advantages, such as: a low number of components,

low conduction losses because the number of components in the conduction path is kept low,

good fault current handling capability and

the possibility to optimize the diode Silicon area.

As is known the silicon area depends on the rated voltage and the rated current. As can be seen in the table below, the rated current for the sixth and seventh cell diodes is lowered, which allows the size of the silicon area used for these diodes to be lowered and thereby the size of the component (s) harboring these diodes can be lowered. This in turn leads to a more compact converter.

As can also be seen in the table, a cell comprising the sixth and seventh transistors will, because of the low voltage and current ratings, require a limited extra silicon area compared with a cell without these

transistors. The added functionality is thus obtained with a marginally increased silicon area. The rating of the components in the first and the second type of double voltage contribution cell may the following:

where U _ce ii is the maximum cell voltage and I _arm is the maximum phase arm current.

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 .