WO2020007516A1

WO2020007516A1 - Improved modular multilevel converter

Info

Publication number: WO2020007516A1
Application number: PCT/EP2019/060495
Authority: WO
Inventors: Yuhei OKAZAKI; Kalle ILVES; Panagiotis Bakas; Anshuman Shukla
Original assignee: Abb Schweiz Ag
Priority date: 2018-07-06
Filing date: 2019-04-24
Publication date: 2020-01-09
Also published as: GB2590211B; SE1850857A1; GB202100235D0; SE542175C2; GB2590211A

Abstract

A voltage source converter comprises a number of converter modules, one for each phase of an AC waveshape to be generated, connected between two DC terminals and each comprising a first and a second DC connection point (DCCP1, DCCP2), a string of director switches (S1, S2), the midpoint of which provides a third connection point (CP3), and waveshaper branches (WSB1, WSB2) connected in parallel with each other as well as to the string, each waveshaper branch (WSB1, WSB2) comprising submodules (SMA1, SMA2, SMA3, SMA4, SMB1, SMB2, SMB3, SMB4) and being connected to an AC connection point (ACCP1, ACCP2) provided for the branch, wherein at least one of the AC connection point and the third connection point is provided for connection to a corresponding phase of an AC link, the first and second waveshaper branches produce two similar waveshapes for the AC link and the switches change the way the waveshapes are applied to connection points.

Description

IMPROVED MODULAR MULTILEVEL CONVERTER

FIELD OF INVENTION The present invention relates to a voltage source converter. BACKGROUND

The thyristor-based load commutated converter (LCC) has been employed for high-power High Voltage Direct Current (HVDC) systems. One of the drawbacks of such a converter is its lack of black start capability and independent active and reactive power controllability.

A modular multilevel converter (MMC) is one type of voltage source converter that has been accepted to be an alternative of the LCC in lower power range, e.g., active power up to 3 GW in bipole system. This is because of the fact that the MMC can overcome the mentioned drawbacks in the LCC. In order to replace the LCC with the MMC, the MMC needs to handle as much current as the LCC is capable of, so that the MMC can cover the entire power range of the LCC.

A hybrid topology combining switches using thyristors and submodules with active power devices has been proposed in order to reduce the footprint of the MMC. An active power device is typically a power device that is controllable to both being turned on an off, and may be a transistor.

This hybrid topology employs a string of director switches connected between two Direct Current (DC) poles and a waveshaper branch comprising multilevel submodules connected to this string of director switches. In this hybrid topology the director switches are used for directivity and the multilevel submodules for waveshaping.

This hybrid structure can be used in a parallel configuration, i.e. in three parallel phase legs. EP 2999105 discloses one such hybrid modular multilevel converter (MMC) having three parallel phase legs, where each phase leg comprises a string of director switches realized as anti-parallel thyristors connected in series with submodules.

US 2014/0092661 discloses another hybrid converter with parallel phase legs, where a phase leg has a first string comprising a plurality of controllable semiconductor switches, a first connecting node and a second connecting node. Furthermore, the leg includes a second string operatively coupled to the first string via the first connecting node and the second connecting node, where the second string includes submodules.

The use of director switches and branches of submodules have also been employed in a series MMC (SMMC). This is described in WO 2016/177398 where a number of converter modules are connected in series between two DC terminals and each converter module comprises a string of director switches connected in parallel with a string of submodules. However, in a hybrid topology employing thyristors as director switches, the hybrid topology cannot maximize the thyristor-current capability in terms of its peak/average/rms current because the transmission power is limited by a peak/rms current rating of the submodule switches that are realized as active power devices.

There are a number of further problems associated with the hybrid topology:

Maximize thyristor current: The thyristor current in both peak and rms can be close to twice as high as those of the active power devices such as

Bimode Insulated Gate Transistors or Insulated Gate Transistors

(BIGT/IGBT). It leads to underutilization of thyristors in hybrid topologies. For the MMC consisting of only the active power devices, the power is limited by almost half of that of the LCC.

Reduce cost/MVA (Mega Volt Ampere): The MMC can be used for an alternative of LCC in terms of the power transmission capability if two MMCs or two arms are connected in parallel. However, it doubles the required number of power devices, leading to the same cost/MVA to the single MMC. Reduce DC voltage stress on transformer: The series converter

configuration such as SMMC needs multiple single-phase transformer withstanding the DC-link voltage. In order to achieve the power rating of the SMMC as high as the LCC (to GW), the DC-link voltage has to be increased, leading to the DC voltage level of currently available

transformers being exceeded.

There is therefore a need for an improved MMC converter structure addressing one or more of the above mentioned problems. SUMMARY OF THE INVENTION

The present invention is directed towards obtaining an improved modular multilevel converter where at least some of the above-mentioned issues are overcome.

This object is according to a first aspect of the present invention achieved through a voltage source converter having a first and a second direct current, DC, terminal for connection to a DC voltage and comprising: a number of converter modules, one for each phase of an alternating current, AC, waveshape to be generated, the converter modules being connected between the DC terminals and each comprising

a first and a second DC connection point for connection between the first and second DC terminals, a first string of director switches comprising at least two director switches, the midpoint of which string provides a third connection point, and a first and a second waveshaper branch connected in parallel with each other as well as connected to the first string of director switches, each waveshaper branch comprising a number of submodules and being connected to an AC connection point provided for the branch,

wherein at least one of the AC connection point and the third connection point is a connection point for connection to a corresponding phase of an AC link, the first and second waveshaper branches are controllable to produce two similar waveshapes for the AC link and the director switches are controllable to change the way the waveshapes are applied to some of the connection points.

The converter modules may be connected in series between the DC terminals using the first and second DC connection points. As an alternative, the converter modules may be connected in parallel between the DC terminals using the first and second DC connection points

The director switches may be thyristor switches, for instance realized using pairs of anti-parallel thyristors. The director switches may also be controllable to change the polarity of the waveshape provided by the waveshaper branches.

For each of the first and second waveshaper branches there may be least one first inductor connected between the associated AC connection point of the waveshaper branch and a first point of the first string leading to the first DC connection point and at least one second inductor connected between the associated AC connection point and a second point of the first string leading to the second DC connection point.

Each waveshaper branch may furthermore have a first and a second end. In one variation at least one end of each waveshaper branch is connected to the corresponding point of the first string via the corresponding at least one inductor. The ends being connected in this way may be the same ends of the waveshaper branches. This means that the first ends of both waveshaper branches may be connected to the first point of the first string via the at least one first inductor. It is additionally or instead possible that the second ends are connected to the second point of the first string via the at least one second inductor. he first end of the first waveshaper branch may additionally be connected to the first point of the first string via a separate inductor, the first end of the second waveshaper branch may be connected to the first point of the first string via a separate inductor, the second end of the first waveshaper branch may be connected to the second point of the first string via a separate inductor and the second end of the second waveshaper branch is connected to the second point of the first string via a separate inductor.

Alternatively the at least one first inductor may be a first common inductor connected between the waveshaper branches and having a midpoint with a connection leading to the first point of the first string and the at least one second inductor may be a second common inductor connected between the waveshaper branches and having a midpoint with a connection leading to the second point of the first string. According to the latter alternative it is furthermore possible that the first ends of the first and second waveshaper branches are connected to the first point of the first string via a midpoint of a first common inductor. It is additionally or instead possible that the second ends of the first and second waveshaper branches are connected to the second point of the first string via a midpoint of the second common inductor.

Each waveshaper branch may additionally comprise a first and a second chain link with submodules. In this case it is furthermore possible that a first end of the first chain link of the first waveshaper branch is directly connected to the first point of the string and a second end of the first chain link of the first waveshaper branch is connected to the AC connection point of the first waveshaper branch via a first separate inductor, a first end of the first chain link of the second waveshaper branch is directly connected to the first point of the string, a second end of the first chain link of the second waveshaper branch is connected to the AC connection point of the second waveshaper branch via another first separate inductor, a first end of the second chain link of the first waveshaper branch is directly connected to the second point of the string, a second end of the second chain link of the first waveshaper branch is connected to the AC

connection point of the first waveshaper branch via a second separate inductor, a first end of the second chain link of the second waveshaper branch is directly connected to the second point of the string and a second end of the second chain link of the second waveshaper branch is connected to the AC connection point of the second waveshaper branch via another second separate inductor.

The first and second waveshaper branches may furthermore be connected in parallel with the first and second switches.

Each waveshaper branch may furthermore comprise an upper waveshaper arm comprising submodules, a lower waveshaper arm comprising submodules and an intermediate arm between the lower and upper waveshaper arms, where the intermediate arm is connected in parallel with a second string of switches, wherein the AC connection point associated with a waveshaper branch is provided at a midpoint of the second string of switches. The second string of switches may thereby be connected between a first and a second junction of a waveshaper branch, where the first junction is a junction between the upper and intermediate arm and the second junction is a junction between the intermediate and lower arm. The intermediate arms may comprise submodules. As an alternative they may each comprise a capacitor in series with a bypass switch.

It is furthermore possible that the AC connection point associated with a waveshaper branch is a first AC connection point that is common to or shared by the first and second waveshaper branches.

A first AC connection point may alternatively be provided for the first waveshaper branch and a second AC connection point may be provided for the second waveshaper branch. The first AC connection point may for instance be provided at a midpoint of the first waveshaper branch while the second AC connection point may be provided at a midpoint of the second waveshaper branch. The first and second AC connection points may also be connected to the same conductor for the phase of the AC link for forming an AC voltage of the converter module, while the third connection point may be an AC connection point connected to another conductor for the phase of the AC link.

The first and second AC connection points of the first and second waveshaper branches may be interconnected. As an alternative the first AC connection point of the first waveshaper branch may be connected to a secondary winding of a first transformer and the second AC connection point of the second waveshaper branch may be connected to a secondary winding of a second transformer, where the primary windings of these two transformers are connected in parallel to the corresponding phase of the

AC link.

The first point of the first string may be placed at a junction between the first switch and the first DC connection point and the second point of the first string may be placed at a junction between the second switch and the second DC connection point. The present invention has a number of advantages. It allows an increase of the current through the first string of switches to twice the size compared with the current through a waveshaper branch, which improves the efficiency of a string if thyristors are used. The current capability of the whole system is thereby also raised through only doubling the number of components in the submodules, but retaining the number of director switch components. Another advantage is that the number of redundant submodules may be reduced compared with the use of redundant parallel submodules for a single waveshaper branch. The replacement of an LCC HVDC system with the MMC using the above-described converter module offers several further advantages such as black start capability,

independent active/reactive power capability, and high reliability due to forced commutations.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will in the following be described with reference being made to the accompanying drawings, where fig. l schematically shows a first type of voltage source converter with first, second and third stacked converter modules,

fig. 2 schematically shows a second type of voltage source converter with first, second and third converter modules connected in parallel with each other,

fig. 3 schematically shows a first type of converter module that may be used in both the first and second types of converters,

fig. 4A and 4B schematically show two types of submodules that may be used in the converter module,

fig. 5 schematically shows a second type of converter module that may be used in both the first and second types of converters,

fig. 6 schematically shows a third type of converter module that may be used in both the first and second types of converters,

fig. 7 schematically shows a variation of the third type of converter module, fig. 8 schematically shows the second type of converter module connected to a first and a second transformer,

fig. 9 schematically shows a fourth type of converter module,

fig. to schematically shows a fifth type of converter module, and fig. n shows one realization of the second type of converter where the fifth type of converter module is used with first and second transformers.

DETAILED DESCRIPTION OF THE INVENTION In the following, a detailed description of preferred embodiments of the invention will be given.

Fig. l shows a first type of modular multilevel converter (MMC) in the form of a series modular multilevel converter (SMMC) 10A. The converter IOA converts between Direct Current (DC) and Alternating Current (AC) and may with advantage be provided as an interface between a High Voltage Direct Current (HVDC) network, such as an Ultra High Voltage Direct Current (UHVDC) network, and an AC network, where the connection to the AC network is made via an AC link. In fig. l there is shown a first and a second conductor ACLAi and ACIA.2 of the AC link provided for a first phase A, a first and a second conductor ACLBi and ACLB2 of the AC link provided for a second phase B and a first and a second conductor ACLCi and ACLC2 of the AC link provided for a third phase C.

More particularly, the converter ioA comprises a number of stacked converter modules, one for each phase of an AC waveshape to be generated for a corresponding phase of the AC link. The converter modules are also connected in series between a first and a second DC terminal DCi and DC2 of the converter 10a for connection to a DC voltage Ud.

The converter modules are thus connected in series between a first and a second DC terminal DCi and DC2 and this connection is done using first and second DC connection points DCCPi and DCCP2. In fig. 1 three such modules 12, 14, 16 are shown, where the first module 12 is provided for the first phase A, a second module 14 is provided for the second phase B and a third module 16 is provided for the third phase C. The DC terminals DCi and DC2 are each connected to a corresponding (DC) pole Pi and P2, where a first pole Pi has a first voltage +UDC/2 and a second pole P2 has a second voltage -UDC/2. It should be realized that as an alternative one of the DC terminals may be connected to ground instead. Each module has the first and the second DC connection point DCCPi and DCCP2 for connection between the two DC terminals DCi and DC2 and at least one

AC connection point-. In the example shown in fig. 1 there are two AC connection points ACCPi and ACCP2.

The first module 12 thus has a first DC connection point DCCPi connected to the first DC terminal DCi and a second DC connection point DCCP2 connected to the first DC connection point DCCPi of the second module 14, which second module 14 in turn has a second DC connection point DCCP2 connected to a first DC connection point DCCPi of the third module 16, which third module i6has a second DC connection point DCCP2 connected to the second DC terminal DC2. There is also a third connection point CP3, which in the first type of converter may be an AC connection point or a DC connection point.

The first converter module 12 transmits power PA to or from the first phase of the AC link, the second converter module 14 transmits power PB to or from the second phase of the AC link and the third converter module 16 transmits power PC to or from the third phase of the AC link.

In order to enable this, the first module 12 has a first and a second AC connection point ACCPi and ACCP2 and the third connection point CP3 that may also act as an AC connection point for connection to the corresponding phase of the AC link. Thereby at least one of the AC connection points and the third connection point is a connection point for connection to the corresponding phase of the AC link, which in this case is the first phase A of the AC link. In this type of converter at least two, and if the third connection point is an AC connection point perhaps all three, of the AC connection points may be connected to the phase of the AC link. If two AC connection points are connected to the AC link, it is furthermore possible that they are connected to two different conductors. It is for example possible that the third connection point CP3 is an AC connection point connected to the first conductor ACLAi and the first AC connection point ACCPi is connected to the second conductor ACLA2 of the AC link provided for the first phase. When three AC connection points are used two may be connected to the same conductor. It is for instance possible that the first and second AC connection points ACCPi and ACCP2 are connected to the second conductor ACLA2 when the third connection point CP3 is an AC connection point connected to the first conductor ACLAi. Put differently, it is possible that the first and second AC connection points ACCPi and ACCP2 are connected to the same conductor of the AC link provided for the phase, while the third connection point CP3 may be an AC connection point connected to another conductor of the AC link provided for the phase.

In a similar manner the second module 14 has a first and a second AC connection point ACCPi, and ACCP2 and a third connection point CP3 that may also act as an AC connection point, where at least one of the AC connection points and the third connection point is a connection point for connection to a corresponding phase of the AC link, which in this case is the second phase B of the AC link. Also here at least two and perhaps all three of the AC connection points may be connected to the phase of the AC link, where if two AC connection points are connected to the AC link, it is possible that they are connected to two different conductors and if three AC connection points are used, two are connected to the same conductor.

These connections may be made in the same way as was described above in relation to the first converter module 12. In a similar manner the third module 16 has a first and a second AC connection point ACCPi, ACCP2 and a third connection point CP3 that may also act as an AC connection point, where at least one of the AC connection points and the third connection points is a connection point for connection to a corresponding phase of the AC link, which in this case is the third phase C of the AC link. Also here at least two and perhaps all three of the AC connection points may be connected to the phase of the AC link, where if two AC connection points are connected to the AC link, it is possible that they are connected to two different conductors and if three AC connection points are used, two are connected to the same conductor. These connections may be made in the same way as was described above in relation to the first converter module 12.

Between the first DC terminal DCi and the first pole Pi there is also connected an optional smoothing reactor. In the figure there is also shown a control unit 18 set to control the different converter modules 12, 14 and 16, which control involves the forming of an AC voltage, which AC voltage is provided by the converter module using at least two of the first and second AC connection points and the third connection point.

Fig. 2 schematically shows a second type of converter 10B where instead the three modules 12, 14 and 16 are connected in parallel between the two DC terminals DCi and DC2 using the first and second DC connection points DCCPi and DCCP2.

In this type of converter the first DC connection point DCCPi of all modules 12, 14 and 16 is connected to the first DC terminal DCi, while the second DC connection point DCCP2 of all modules is connected to the second DC terminal DC2. The third connection point CP3 may in this case be either a DC connection point or an AC connection point.

In this case at least one of the AC connection points or the third

connection points of each of the modules, such as the first AC connection point ACCPi or the third connection point CP3, may be provided for connection to the corresponding phase of the AC link. This connection point may more particularly be connected to a conductor of the

corresponding phase. As an alternative two connection points may be connected to the phase. The first and second AC connection points ACCPi and ACCP2 or the first AC connection point ACCPi and the third connection point CP3 of a converter module may be connected to the phase. As an example, the first and the second AC connection points ACCPi and ACCP2 may be connected to the same conductor of the phase and the first AC connection point ACCPi and the third connection point

CP3 may be connected to separate conductors of the phase. If the third connection point is an AC connection point and all three AC connection points are used, the first and the second AC connection points ACCPi and ACCP2 may be connected to the same conductor of the phase while the third connection point CP3 may be connected to another conductor of the phase.

The structure of a first variation of the first converter module 12A, which is also the structure of a first type of converter module, is shown in fig. 3.

There is in this converter module 12A a first string of director switches connected between the first and second DC connection points DCCPi and DCCP2, where the first string comprises at least two director switches; a first director switch Si and a second director switch S2. The third connection point CP3, which in this case forms a first AC terminal of the converter module 12A for connection to a first conductor of a

corresponding phase of the AC link, is provided at the midpoint of the first string of switches. It can thereby be seen that the first switch Si is connected between the first DC connection point DCCPi and the third connection point CP3, while the second switch S2 is connected between the third connection point CP3 and the second DC connection point DCCP2. There is also a first and a second waveshaper branch WSBi and WSB2 connected in parallel with each other and with the first and second director switches of the first string of director switches. These waveshaper branches WSBi and WSB2 are more particularly connected between a first and a second connection point SPi and SP2 of the first string, where in this first type of converter module the first string connection point SPi is provided or placed at a junction between the first switch Si and the first DC connection point DCCPi and the second string connection point SP2 is provided or placed at a junction between the second switch S2 and the second DC connection point DCCP2. Each waveshaper branch is also connected to an AC connection point provided for the branch, which may be a common or a separate connection point. In the example in fig. 3 there are two separate AC connection points; a first and a second AC connection point ACCPi and ACCP2.

The first and second waveshaper branches WSBi and WSB2 also both have a first and a second end, where the first end of the first waveshaper branch WSBi is connected to the first string connection point SPi via a separate inductor LiA and the second end of the first waveshaper branch WSBi is connected to the second string connection point SP2 via a separate inductor L2A. In a similar manner the first end of the second waveshaper branch WSB2 is connected to the first string connection point SPi via a separate inductor LiB and the second end of the second waveshaper branch WSB2 is connected to the second string connection point SP2 via a separate inductor L2B.

The first waveshaper branch WSBi comprises a first and a second chain link CLiA and CL2A connected between the first and second ends, where each chainlink comprises a number of submodules. As an example each chainlink is shown as comprising two submodules. The first chainlink

CLiA thus comprises a first and second submodule SMAi and SMA2, while the second chainlink CL2A comprises a third and a fourth submodule SMA3 and SMA4. The first AC connection point ACCPi is connected to the junction between the two chainlinks CLiA and CL2A, which in this example is the junction between the second and third submodules SMA2 and SMA3, which is also a midpoint of the waveshaper branch WSBi. In a similar manner, the second waveshaper branch WSB2 comprises a first and a second chain link CLiB and CL2B connected between the first and second ends, where each chainlink comprises a number of

submodules. As an example each chainlink is shown as comprising two submodules. The first chainlink CLiB thus comprises a first and second submodule SMBi and SMB2, while the second chainlink CL2B comprises a third and a fourth submodule SMB3 and SMB4. The second AC connection point ACCP2 is connected to the junction between the two chainlinks CLiB and CL2B, which in this example is the junction between the second and third submodules SMB2 and SMB3. It can thereby be seen that the AC connection point is provided at the midpoint of the second waveshaper branch WSB2.

It can thereby also be seen that for each of the first and second waveshaper branches WSBi and WSB2 there is at least one first inductor connected between the associated AC connection point and the first string connection point SPi that leads to the first DC connection point DCCPi and at least one second inductor connected between the associated AC connection point and the second string connection point SP2 that leads to the second DC connection point DCCP2. It is furthermore possible that at least one end of each waveshaper branch is connected to the corresponding connection point of the first string via the corresponding at least one inductor. It is in this case possible that the first ends of both waveshaper branches WSBi and WSB2 are connected to the first string connection point SPi via the at least one first inductor. It is also or instead possible that the second ends of both waveshaper branches WSBi and WSB2 are connected to the second string connection point SP2 via the at least one second inductor. In the example of fig. 3, the at least one first inductor comprises the separate inductors LiA and LiB and the at least one second inductor comprises the separate inductors L2A and L2B. Moreover, in the example of fig. 3, the first ends of the first and second waveshaper branches are connected to the first string connection point via at least one first inductor, in this example the separate inductors LiA and L2A and the second ends of the first and second waveshaper branches are connected to the second string connection point via at least one second inductor, in the form of separate inductors L2A and L2B.

It can furthermore be seen that the AC connection point associated with the first waveshaper branch WSBi is the first AC connection point ACCPi and the AC connection point associated with the second waveshaper branch is the second AC connection point ACCP2.

The first and second AC connection points ACCPi and ACCP2 may be interconnected and thereby they may be connected to a conductor of the corresponding phase of the AC link, such as the second conductor of the phase. However, they may also, as will be seen later on, be connected to this conductor via separate transformers. An alternative inductor realization is the following:

A first end of the first chain link CLiA of the first waveshaper branch WSBi is directly connected to the first string connection point SPi, a second end of the first chain link CLiA of the first waveshaper branch WSBi is connected to the first AC connection point ACCPi via a first separate inductor, a first end of the first chain link CLiB of the second waveshaper branch WSB2 is directly connected to the first string connection point SPi, a second end of the first chain link CLiB of the second waveshaper branch WSB2 is connected to the second AC connection point ACCP2 via another first separate inductor, a first end of the second chain link CL2A of the first waveshaper branch WSBi is directly connected to the second string connection point SP2, a second end of the second chain link CL2A of the first waveshaper branch WSBi is connected to the first AC connection point ACCPi via a second separate inductor, a first end of the second chain link CL2B of the second waveshaper branch WSB2 is directly connected to the second string connection point SP2 and a second end of the second chain link CL2B of the second waveshaper branch WSB2 is connected to the second AC connection point ACCP2 via a second separate inductor.

The chain links are used for forming a number of discrete voltage levels of an AC waveshape. Therefore each chain link may comprise more submodules than two.

The submodules may, as can be seen in fig. 4a, be realized as a first type of submodule SMTi having unipolar voltage contribution capability, here exemplified by a half-bridge submodules, where two switches are connected in parallel with an energy storage element, for instance realized as a capacitor. One submodule terminal is then provided at the junction between the two switches and the other submodule terminal is provided at a junction between one of the switches and the energy storage element.

The half-bridge submodule is configured to either provide a zero voltage or a unipolar voltage corresponding to the voltage across the submodule capacitor.

As can be seen in fig. 4b, the submodules may as an alternative be realized as a second type of submodule SMT2 having a bipolar voltage contribution capability. Here exemplified by a full-bridge submodule where there are two strings of series connected switches connected in parallel with the energy storage element, Here one submodule terminal is provided at the midpoint of one of the strings, while the other submodule terminal is provided at the midpoint of the other string. The full-bridge submodule has a zero and bipolar voltage contribution capability corresponding to the voltage across the capacitor. The director switches are bidirectional switches and may be realized as either anti-parallel self-commutated components or active power devices, such as two transistors, like Insulated Gate Bipolar Transistors (IGBTs), Bimode Insulated Gate Transistors (BIGTs) or Metal-Oxide- Semiconductor Field-Effect Transistor (MOSFETs), or Integrated Gate- Commutated Thyristors (IGCTS), using a self-commutated circuit with anti-parallel circuit-commutated components, such as an IGBT or IGCT together with a diode or thyristor, or as anti-parallel circuit commutated components, such as two anti-parallel thyristors or a thyristor with anti- parallel diode. It should be realized that the use of anti-parallel thyristors may be preferred. A self-commutated component is here a component that may be directly turned off through receiving a control signal in order to stop conducting current, while a circuit-commutated component is a component needing an applied negative voltage to stop conducting current, for instance through the use of a dedicated circuit. A thyristor is an example of one type of circuit commutated component, it can be seen that this type of circuit commutated component also has the ability of being directly turned on through receiving a control signal in addition to requiring an applied external negative voltage for being turned off.

Moreover, a string may be realized through a number of series-connected component combinations of the types described above. In the following a switch formed through a self-commutated component with anti-parallel self-commutated component, such as a pair of anti-parallel IGBTs, or as a self-commutated component with anti-parallel circuit-commutated component, such as an IGBT with anti-parallel diode, will be termed a self- commutated switch, while a switch with two anti-parallel circuit- commutated components, such as a pair of anti-parallel thyristors will be termed a circuit-commutated switch. As can be seen in fig. 3 the director switches of the first string are circuit commuted switches. The submodule switches may also be realized through the use of self- commutated or circuit-commutated switches or as combinations of such switches. It may here also be mentioned that in case the director switches are realized in the form of circuit-commutated switches, then a full-bridge submodule in any chain link of a waveshaper branch may be used for turning off a switch. One way of operating the first converter module 12 will now be described.

In this variation, the third connection point CP3 is a third AC connection point and forms a first AC terminal of the converter module 12A and the first and second AC connection points ACCP2 and ACCP3 are

interconnected for forming a second AC terminal of the converter module 12A in order for an AC waveshape to be formed between the two AC terminals.

In operation the control unit 18 controls a converter module 12A for forming a first part of a waveshape in a half cycle or first half-period and for forming a second part of the waveshape in a second half cycle or half- period, which waveshape is formed between the two AC terminals.

Therefore in the first half period in the first converter module 12A, the first switch Si is turned on and the second switch S2 turned off. Thereby the first chain link CLiA of the first waveshaper branch WSBi comprising the submodules SMAi and SMA2 is connected between the first AC connection point ACCPi and the third connection point CP3 via the inductor LiA and thereby these submodules SMAi and SMA2 are also connected between the two AC terminals. In a similar manner the first chain link CLiB of the second waveshaper branch WSB2 comprising the submodules SMBi and

SMB2 are connected between the second AC connection point ACCP2 and the third connection point CP3 via the inductor LiB and thereby these submodules SMBi and SMB2 are also connected between the two AC terminals. Thereby the submodules SMAi, SMA2, SMBi and SMB2 of the two first chain links CLiA and CLiB are also controlled to form a half period or half cycle of two separate but similar waveshapes being combined into a half period or half cycle of an output waveshape via the inductors. Thereby the submodules SMAi, SMA2, SMBi, SMB2 of the first chain links CLiA and CLiB can be controlled by the control unit 18 to produce the first half period of the output waveshape.

In the second half period the first switch Si is turned off and the second switch S2 is turned on. Thereby the second chain link CL2A of the first waveshaper branch WSBi comprising the third and the fourth submodules SMA3 and SMA4 is connected between the first AC connection point ACCPi and the third connection point CP3 via the inductor L2A. In a similar manner the second chain link CL2B of the second waveshaper branch WSB2 comprising the submodules SMB3 and SMB4 is connected between the second AC connection point ACCP2 and the third connection point CP3 via the inductor L2B. Thereby the two chain links CL2A and CL2B are also controlled to form a half period or half cycle of two separate but similar waveshapes being combined into a half period or half cycle of an output waveshape via the inductors. Thereby the submodules SMA3, SMA4, SMB3, SMB4 of the second chain links CL2A and CL2B of the first and second waveshaper branches WSBi and WSB2 can be controlled by the control unit 18 to produce the second half period of the output waveshape.

It can thus be seen that the control unit 18 controls the first and second switches Si and S2 of the first string to alternatingly connect the first and second chainlinks CLiA, CLiB, CL2A and CL2B of the first and second waveshaper branches WSBi and WSB2 between the two AC terminals. The control unit 18 more particularly controls the two waveshaper branches to form the same waveshape. Thereby the first and second waveshaper branches WSBi and WSB2 are controlled to produce two similar waveshapes for the same phase of the AC link and the director switches Si and S2 are controlled to change the way the waveshapes are applied to some of the connection points, which in the first type of converter is to change the way the waveshapes are applied to the first and second AC connection points ACCPi and ACCP2 and the third connection point CP3. In the above-mentioned operation of the director switches Si and S2 the operation more particularly involves changing the polarity of the waveshapes provided by the waveshaper branches WSBi and WSB2.

The second and third converter modules perform the same type of operation for the other two phases. A converter employing this converter module realization has a number of advantages. It allows a current of twice the size of the current through the first string of director switches compared with the current through a waveshaper branch, which improves the utilization of a string if thyristors are used. The current capability of the converter is thereby also raised through only doubling the number of components in the submodules, such as doubling the number of transistors like IGBT/BIGT, but retaining the number of director switch components, that may be thyristors, leading to reduced cost/MVA (Mega Volt Ampere). The inductors LiA, LiB, L2A and L2B balance the current flowing through each wavehaper branch. They thus provide circulating current control to achieve current sharing between the parallel waveshaper branches. This energy balancing capability may be advantageous in low-power operation. Through the introduction of the inductors between the waveshaper branches the switching accuracy requirement is also relaxed. As only one chain link of a waveshaper branch is used at a time for forming a waveshape, the non-used chainlink may be employed for other purposes such as reducing DC-link harmonics. Another advantage is that the number of redundant submodules may be reduced compared with the use of redundant parallel submodules for a single waveshaper branch. A parallel submodule configuration requires twice as high number of redundant submodules as the parallel waveshaper branch configuration due to the following reasons:

One submodule failure in the parallel submodule configuration causes two submodules to be bypassed, leading to increased redundant submodule requirements.

One submodule failure in the parallel waveshaper branch configuration does not affect to the parallel waveshaper branch, which reduces the number of required redundant submodules.

As a result, the parallel waveshaper branch configuration can reduce the number of redundant submodules compared to the parallel submodule configuration.

The first type of converter employing the above described converter module can enhance transmission of power from parallel arm MMC without increasing switching component costs. This circuit configuration is thus a cost-effective alternative to a load commutated converter used for LCC HVDC system. The replacement of a LCC HVDC system with the

SMMC using the above-described converter module also offers several further advantages such as black start capability, independent

active/reactive power capability, and high reliability due to forced commutations.

There are a number of variations that may be made of the converter module in fig. 3. Fig 5 shows the first converter module 12B according to a second type. This converter module 12B has essentially the same realization as the first type. However, in this case there are no separate inductors between a waveshaper end and a string connection point.

Instead center tapped inductors are used. There is here a first common inductor Li interconnecting the first end of the first waveshaper branch WSBi with the first end of the second waveshaper branch WSB2, where a midpoint of this first inductor Li is connected to the first string connection point SPi between the first switch Si and the first DC connection point DCCPi. It can thereby be seen that the first common inductor Li is connected between the waveshaper branches and has a midpoint leading to the first string connection point SPi. There is also a second common inductor L2 interconnecting the second end of the first waveshaper branch WSBi with the second end of the second waveshaper branch WSB2, where a midpoint of this second inductor L2 is connected to the second string connection point SP2 between the second switch S2 and the second DC connection point DCCP2. It can thereby be seen that the second common inductor L2 is connected between the waveshaper branches and has a midpoint leading to the second string connection point SP2. Put differently, the first ends of the first and second waveshaper branches

WSBi and WSB2 are connected to the first string connection point via a midpoint in the first common inductor Li and the second ends of the first and second waveshaper branches WSBi and WSB2 are connected to the second string connection point via a midpoint in the second common inductor L2.

This reduces the number of required inductors. On the other hand, the center tapped inductor can be used only to regulate circulating current which is sufficient to achieve current sharing between the two waveshaper branches.

Another possible variation of the first converter module 12C according to a third type is shown in fig. 6, which has the same type of first string realization and use of the first a center-tapped inductor Li as in fig. 5. However, the waveshaper branches are realized in a different way. The first waveshaper branch WSBi in this case comprises a first or upper waveshaper arm uaa or upper chainlink with submodules SMAi and SMA2 and second or lower waveshaper arm laa or lower chainlink with submodules SMA3 and SMA4. There is additionally and intermediate arm iaa or intermediate chain link with submodules SMAIi and SMAI2. The first end of the first waveshaper branch at the first submodule SMAi is also a first end of the upper waveshaper arm uaa, while a second end of the upper waveshaper arm is formed after the last submodule of the arm, which is after the second submodule SMA2. The second end of the first waveshaper branch WSBi, i.e. after the last submodule SMA4 of the waveshaper branch, is also the second end of the lower waveshaper arm laa, while a first end of the lower waveshaper arm laa is provided before the first submodule of lower waveshaper arm, which in this case is before the third submodule SMA3. A first end of the intermediate arm iaa is formed before the first submodule of the arm, which in this case is before the first intermediate submodule SMAIi, while a second end of the intermediate arm iaa is formed after the last submodule of the arm, which in this case is after the second intermediate submodule SMAI2 .

In a similar manner, the second waveshaper branch WSB2 comprises the first or upper waveshaper arm uab or upper chainlink with submodules SMBi and SMB2 and second or lower waveshaper arm lab or lower chainlink with submodules SMB3 and SMB4 as well as an intermediate arm iab or intermediate chain link with submodules SMBIi and SMBI2. The first end of the second waveshaper branch WSB2 at the first submodule SMBi is also a first end of the upper waveshaper arm uab, while a second end of the upper waveshaper arm uab is formed after the last submodule of the arm, which is after the second submodule SMB2.

The second end of the second waveshaper branch WSB2, i.e. after the last submodule SMB4, is also the second end of the lower waveshaper arm lab, while a first end of the lower waveshaper arm lab is provided before the first submodule of the arm, which in this case is before the third submodule SMB3. A first end of the intermediate arm iab is formed before the first submodule of the arm, which in this case is before the first intermediate submodule SMBIi, while a second end of the intermediate arm iab is formed after the last submodule of the arm, which in this case is after the second intermediate submodule SMBI2.

It can thereby be seen that each waveshaper branch comprises an upper waveshaper arm, a lower waveshaper arm and an intermediate arm between the upper and lower waveshaper arms.

Moreover, as can be seen in fig. 6 there is a second string of switches, for instance bidirectional switches, which second string is at a first end connected to the second ends of the upper arms uaa and uab of both the first and the second waveshaper branches WSBi and WSB2, while a second end of the second string of switches is connected to the second ends of the intermediate arms iaa and iab of both the waveshaper branches WSBi and WSB2. The second string comprises a third and a fourth switch S3 and S4 and the midpoint between the switches of the second string also forms the first AC connection point ACCPi, where it can also be seen that the third and fourth switches S3 and S4 are provided on opposite sides of this midpoint. It can furthermore be seen that in this type of module 12C, the first AC connection point ACCPi is associated with both the waveshaper branches WSBi and WSB2. The AC connection point associated with a waveshaper branch is thereby provided at the midpoint of the second string of switches connected between a first and a second junction of the waveshaper branch, where the first junction is a junction between the upper and intermediate arm and the second junction is a junction between the intermediate and lower arm.

Moreover, it can also be seen that there is an intermediate inductor LI connected between the first ends of the intermediate arms iaa and iab in the first and second waveshaper branches WSBi and WSB2, the center point of which inductor LI is connected to the second ends of both the upper arms uaa and uab of the waveshaper branches WSBi and WSB2. It can also be seen that the second inductor L.2 is connected between the first ends of the lower arms laa and lab of the first and second waveshaper branches WSBi and WSB2, where the center point of the second inductor L2 is connected to the second ends of the intermediate branches iaa and iab of both the first and second waveshaper branches WSBi and WSB2. It can finally be seen that in this case the second ends of both the waveshaper branches are directly connected to the second string connection point, i.e. directly to the second connection point of the first string of switches, which is here provided at the junction between the second switch S2 and the second DC connection point DCCP2.

The operation of this type of converter module is the following.

The converter module 12C also forms an AC waveshape between the first AC connection point ACCPi and the third connection point CP3, which connection points thus forms two AC terminals.

In the forming of this waveshape the director switches Si and S2 again provides the direction or polarity of the wave and the wavehshaper branches WSBi and WSB2 the shapes through suitable control of the submodules in the arms. It is thereby possible to for example form a sine wave on a pair of AC terminals.

When the first switch Si is on and used to form a connection between the first AC connection point ACCPi and the third connection point CP3, also the fourth switch S4 is on. Thereby the upper and intermediate arms uaa, uab, iaa, and iab of both waveshaper arms WSBi and WSB2 are connected in parallel between the AC terminals formed by the first AC connection point ACCPi and the third connection point CP3 and controlled to form the first half period of the waveshape. When the second switch S2 is on and used to form a connection between the first AC connection point ACCPi and the third connection point CP3, also the third switch S3 is on. Thereby the lower and intermediate arms laa, lab, iaa and iab of both waveshaper arms WSBi and WSB2 are connected in parallel between the AC connection terminals ACCPi and ACCP3 and controlled to form the second half period of the waveshape. In this way there is an improved submodule usage in that the submodules in the intermediate branch are used for forming waveshapes in both half- periods, which increases the efficiency of the submodule usage as well as allows the number of submodules to be reduced. There are a number of variations that can be made of this concept of separating the waveshaper branches into upper, lower and intermediate arms.

As can be seen in fig. 6, there is no second AC connection point but instead the first AC connection point ACCPi is shared by the first and second waveshaper branches WSBi and WSB2. It should be realized that it is possible to provide separate AC connections for the branches also in this type of waveshaper branch realization. In this case the second string of switches comprising the third and fourth switch S3 and S4 would only be connected between the submodules of the first waveshaper branch WSBi. The second string would thus have its first end connected to a junction between the second end of the upper arm uaa and first end of the intermediate arm iaa and its second end connected to the junction between the second end of the intermediate arm iaa and the first end of the lower arm laa. The second AC connection point would in this case be provided at a midpoint of a third string of switches comprising at least two switches, which third string is only connected between the submodules of the second waveshaper branch WSB2. The third string would thus have a first end connected to a junction between the second end of the upper arm uab and a first end of the intermediate arm iab and a second end connected to the junction between the second end of the intermediate arm iab and the first end of the lower arm lab. In such a variation, there would be no intermediate inductor, the second ends of the upper arms uaa and uab would lack interconnection, the first and second ends of the intermediate arms iaa and iab would lack interconnections and the first end of the lower branches would lack interconnections. The second inductor L2 would also be connected between the second ends of the wavehaper branches WSBi and WSB2 and the second string connection point. Another variation can be seen in fig. 7, where each intermediate arm comprises a common capacitor CCA and CCB in series with a bypass switch BPSWA and BPSWB.

In this case the control unit 18 controls the switches S3, S4 to make the common capacitor CC contribute to the voltage level of an AC waveshape being formed. This means that during a first half-period the fourth switch S4 is on, which makes the common capacitors CCA and CCB contribute to the forming of the waveshape in this half period, while during the second half-period the third switch S3 is on, which makes the common capacitors CCA and CCB contribute to the forming of the waveshape in this half period.

During, the above described control, the bypass switch BPSWA or BPSWB is used to selectively bypass the common capacitor CCA or CCB, which is accomplished through turning off the bypass switches BPSWA or BPSWB while turning on both S3 and S4.

The common capacitor CCA and CCB of a waveshaper branch is thus shared between the upper arm VAp and the lower arm Van. This leads to a reduction of the number of submodules being used.

It is naturally also possible to change the converter module 12D in fig. 7 to obtain the second AC connection point. Another possibility that may be used in different converter module realizations is to have the two waveshaper branches connected to two different transformers. The first and the second AC connections points ACCPi and ACCP2 may thus be connected to two different transformers and may more particularly be connected to secondary windings of a first and a second transformer, respectively.

One example of this is shown in fig. 8, which shows the second type of converter module 12B, where the secondary winding of a first transformer

TRi is connected between the first AC connection point ACCPi and the third connection point CP3, while a secondary winding of a second transformer TR2 is connected between the second AC connection point ACCP2 and the third connection point CP3. The primary windings of these transformers TRi and TR2 are connected in parallel to the corresponding phase of the AC link.

The use of parallel transformer to each phase converter leads to reduced DC-voltage stress to the transformer as compared with an SMMC under the given power and current to the submodule switches. It can save the cost for the DC-voltage on the transformers. This parallel transformer arrangement is applicable equally to any kinds of hybrid converters to utilize the current carrying capability of the director switches. There are a number of further variations that are possible especially with regard to providing converter modules connected in parallel between two DC poles.

A first such variation of a first converter module 12E is shown in fig. 9. Fig. 9 thus shows a fourth type of converter module 12E. In this case the converter module comprises a first and second parallel waveshaper branch WSBi and WSB2, the first ends of which are interconnected by a first inductor Li and the second ends of which are interconnected by a second inductor L2.

In this case the center point of the first inductor Li is connected to the first DC connection point DCCPi, while the center point of the second inductor L2 is connected to a first end of a first string of switches comprising a first and a second switch Si and S2, where the second end of the string is connected to a midpoint of a third inductor L3 interconnecting the first ends of a third and a fourth waveshaper branch WSB3 and WSB4, the second ends of which are interconnected by a fourth inductor L4, the center point of which fourth inductor L4 is finally connected to the second DC connection point DCCP2.

When this converter module is used in the second type of converter, the first and second AC connection points ACCPi and ACCP2 of the first and second waveshaper branches WSBi and WSB2 are not used, which is indicated through dashed lines. Corresponding AC connection points of the third and fourth wavehaper branches WSB3 and WSB4 are also unused.

The only connection point that is used as an AC connection point is the third connection point CP3 which is provided at the midpoint of the first string of switches. As before the switches are used for directivity and the waveshaper branches for forming waveshapes.

Fig. 10 shows the first converter module 12F according to a fifth type having basically the same realization as the second type in fig. 5. The difference here is that there are two further switches in the first string of switches. There is an upper switch Su connected between the first DC connection point DCCPi and the first switch Si and a lower switch Sl connected between the second switch S2 and the second DC connection point DCCP2. Moreover, the first string connection point SPi is provided at the junction between the first and upper switches Si and Su, while the second string connection point SP2 is provided at the junction between the second and the lower switches S2 and Sl. In this type of converter module 12F the third connection point CP3 is a DC connection point. This type of converter module is with advantage used in a converter of the type shown in fig. 2.

This type of module may have a different type of operation. In a first state the upper and the second switch Su and S2 may be closed, with the first and the lower switches Si and Sl being open for providing a DC current path from the first DC connection point DCCPi to the third connection point CP3. In a second state the first and lower switches Si and Sl may be closed with the upper and second switches Su and S2 being open for providing a DC current path from the third connection point CP3 to the second DC connection point DCCP2. In a third state, the first and second switches Si and S2 are closed for providing a free-wheeling current flow path through the first and second switches of the first string and the two waveshaper branches, while the upper and lower switches Su and Sl are open.

Another converter realization that may be used is shown in fig. 11. This converter is of the second type, i.e. with three parallel converter blocks 12F, 14F and 16F. Moreover, it can be seen that converter module type used is the fifth type shown in fig. 10 employing upper and lower switches in the first string of switches. It can be seen that this type of converter module is also combined with transformers, where the secondary winding of a first transformer is connected to the first AC connection point of a first waveshaper branch and the secondary winding of a second transformer is connected to the second AC connection point of a second waveshaper branch. However, as the converter is of the second type, the third connection point is not used as an AC terminal. The transformers may be single-phase transformers, where the first ends of three secondary windings TRAi, TRBi and TRCi of three first transformers are connected to the first AC connection points of the three modules 12F, 14F and 16F, with the second ends of the secondary windings being interconnected, for instance in a Y connection In a similar manner the first ends of three secondary windings TRA2, TRB2 and TRC2 of three second transformers are connected to the second AC connection points of the three modules 12F, 14F and 16F, with the second ends of the secondary windings being interconnected, for instance in a Y connection. In this converter also the three third connection points of the converter modules

12F, 14F and 16F are interconnected. As an alternative, it is possible to use three-phase transformers.

Although a Y connection of windings was described above, it is possible also with a delta connection.

Similar to the SMMC case, the thyristor current can be maximized, leading to increased power with less number of series connected converter modules per pole.

It should be realized that the third, fourth and fifth converter module types may be modified so that each waveshaper branch is equipped with separate inductors instead of sharing a center-tapped inductor with the other waveshaper branch.

It should also be realized that all converter modules having both a first and a second AC connection point may be connected to a first and a second transformer. The control unit may be realized in the form of discrete components, such as Field-Programmable Gate Arrays (FPGAs) and Application-Specific Integrated Circuits (ASICs). However, it may also be implemented in the form of a processor with accompanying program memory comprising computer program code that performs the desired control functionality when being run on the processor. A computer program product carrying this code can be provided as a data carrier such as one or more CD ROM discs or one or more memory sticks carrying the computer program code, which performs the above-described control functionality when being loaded into a processor performing the role of control unit of the voltage source converter.

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.

Claims

1. A voltage source converter (10A; 10B) having a first and a second direct current, DC, terminal (DCi, DC2) for connection to a DC voltage and comprising:

a number of converter modules (12, 14, 16), one for each phase of an alternating current, AC, waveshape to be generated, the converter modules being connected between the DC terminals and each comprising

a first and a second DC connection point (DCCPi, DCCP2) for connection between the first and second DC terminals (DCi,

DC2),

a first string of director switches (Si, S2; Si, S2, Su, Sl) comprising at least two director switches (Si, S2), the midpoint of which string provides a third connection point (CP3), and

a first and a second waveshaper branch (WSBi, WSB2) connected in parallel with each other as well as connected to the first string of director switches, each waveshaper branch (WSBi, WSB2) comprising a number of submodules (SMAi, SMA2, SMA3, SMA4, SMBi, SMB2, SMB3, SMB4) and being connected to an AC connection point (ACCPi; ACCPi, ACCP2) provided for the branch,

2. The voltage source converter (10A; 10B) according to claim 1, wherein for each waveshaper branch (WSBi, WSB2) there is at least one first inductor (LiA, LiB; Li) connected between the associated AC connection point and a first point (SPi) of the first string leading to the first DC connection point (DCCPi) and at least one second inductor (L2A, L2B; L2) connected between the associated AC connection point and a second point (SP2) of the first string leading to the second DC connection point (DCCP2).

3. The voltage source converter (10A; 10B) according to claim 2, wherein each waveshaper branch comprises a first and second end, where at least one end of each waveshaper branch (WSBi, WSB2) is connected to the corresponding point of the first string via the corresponding at least one inductor.

4. The voltage source converter (10A; 10B) according to claim 3, wherein the first end of the first waveshaper branch (WSBi) is connected to the first point of the string via a separate inductor (LiA), the first end of the second waveshaper branch (WSB2) is connected to the first point of the string via a separate inductor (LiB), the second end of the first waveshaper branch (WSB2) is connected to the second point of the string via a separate inductor (L2A) and the second end of the second

waveshaper branch (WSB2) is connected to the second point of the phase arm via a separate inductor (L2B).

5. The voltage source converter (10A; 10B) according to claim 2 or 3, wherein the at least one first inductor is a first common inductor (Li) connected between the waveshaper branches (WSBi, WSB2) and having a midpoint with a connection leading to the first point (SPi) of the first string and the at least one second inductor is a second common inductor (L2) connected between the waveshaper branches (WSBi, WSB2) and having a midpoint with a connection leading to the second point (SP2) of the first string.

6. The voltage source converter (10A; 10B) according to any previous claim, wherein the first and second waveshaper branches (WSBi, WSB2) are connected in parallel with the first and second switches (Si,

S2).

7. The voltage source converter (10A; 10B) according to any previous claim, wherein each waveshaper branch (WSBi, WSB2) comprises an upper waveshaper arm (uaa, uab) comprising submodules, a lower waveshaper arm (laa, lab) comprising submodules and an intermediate arm (iaa, iab) between the lower and upper waveshaper arms, the intermediate arm being connected in parallel with a second string of switches (S3, S4), wherein the AC connection point (ACCPi) associated with a waveshaper branch is provided at a midpoint of the second string of switches.

8. The voltage source converter (10A; 10B) according to claim 7, wherein the intermediate arms (iaa, iab) comprise submodules (SMAIi,

SMAI2, SMBIi, SMBI2).

9. The voltage source converter (10A; 10B) according to claim 7, wherein each intermediate arm comprises a capacitor (CCA, CCB) in series with a bypass switch (BPSWA, BPSWB).

10. The voltage source converter (10A; 10B) according to any previous claim, wherein a first AC connection point (ACCPi) is provided for the first waveshaper branch and a second AC connection point

(ACCP2) is provided for the second waveshaper branch.

11. The voltage source converter (10A; 10B) according to claim 10, wherein the first and second AC connection points (ACCP11, ACCP2) of the first and second waveshaper branches are interconnected.

12. The voltage source converter (10A; 10B) according to claim 10, wherein the first AC connection point (ACCPi) of the first waveshaper branch (WSBi) is connected to a secondary winding of a first transformer (TRi; TRAi) and the second AC connection point (ACCP2) of the second waveshaper branch (WSB2) is connected to a secondary winding of a second transformer (TR2; TRA2), where the primary windings of these two transformers are connected in parallel to the corresponding phase of the AC link.

13. The voltage source converter (10A; 10B) according to any previous claim, wherein the first point (SPi) of the first string is placed at a junction between the first switch (Si) and the first DC connection point (DCCPi) and the second point (SP2) of the first string is placed at a junction between the second switch (S2) and the second DC connection point (DCCP2).

14. The voltage source converter (10 A) according to any previous claim, wherein the converter modules (12, 14, 16) are connected in series between the DC terminals (DCi, DC2) using the first and second DC connection points (DCCPi, DCCP2).

15. The voltage source converter (10B) according to any of claims 1 - 13, wherein the converter modules (12, 14, 16) are connected in parallel between the DC terminals (DCi, DC2) using the first and second DC connection points (DCCPi, DCCP2).