CLUSTERS

FIELD OF INVENTION

The present invention relates to a modular multilevel converter as well as to a method and computer program product for controlling cells in the modular multilevel converter. BACKGROUND

Modular 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 (DC) power transmission systems such as high voltage direct current (HVDC) and alternating current (AC) power transmission systems, such as flexible alternating current transmission system (FACTS).

In these converters a number of cascaded cells, each comprising a number of switches and an energy storage element providing a cell voltage, where a cell may be controlled to assist in the forming of a wave shape in a phase arm through inserting or withdrawing the cell voltage into and from the phase arm. At the same time the voltage levels that these converters are to provide have increased, for instance in order to obtain an increased power transfer capability with the same or lower converter losses. Voltage levels of 8oo kV and above are known to have been used in for instance ultra high voltage direct current (UHVDC) system.

The obvious way of enhancing the transmission capability is through simply stacking up a multitude of cells onto each other. However, when this is done further problems may arise. It may be difficult to modulate a voltage synthesizing the multitude of cells with reasonably low-switching frequency. It may be difficult to achieve a sufficient insertion time for each cell in order to achieve proper switching actions. It may be difficult to obtain a control structure capable of handling a multitude of input/output (I/O) signals.

The invention is provided for addressing one or more of these problems. SUMMARY OF THE INVENTION

The present invention is directed towards providing a sufficient cell insertion time for a modular multilevel converter having a multitude of cells.

This object is according to a first aspect achieved through a modular multilevel converter comprising at least one phase arm with a number of series-connected cells, where the cells are configured for providing at least one voltage contribution that assists in the forming of an alternating current waveform and where the cells of a phase arm are organized in clusters, the converter further comprising a converter control structure comprising a control objective control unit, at least one cluster selecting unit and a plurality of cluster control units, one for each cluster, wherein the control objective control unit is configured to determine a number of cells of a phase arm needed to be used during a current time interval for meeting a control objective and to inform the cluster selecting unit about the number,

the cluster selecting unit is configured to

obtain, from every cluster control unit, a cluster voltage measure,

select at least one first cluster based on the obtained cluster voltage measures using an inter-cluster priority scheme, and order a first cluster control unit to control cells of the selected first cluster for allowing the control objective to be met, and the first cluster control unit is configured to select cells of the first cluster and to control the selected cells to change conduction state in the current time interval according to an intra-cluster cell priority scheme.

This object is according to a second aspect achieved through a method of controlling cells in a modular multilevel converter comprising at least one phase arm with a number of series-connected cells, the cells being configured for providing at least one voltage contribution that assists in the forming of an alternating current waveform and where the cells of a phase arm are organized in clusters, the method being performed in a converter control structure and comprising:

determining a number of cells of a phase arm needed to be used during a current time interval for meeting a control objective,

obtaining, from every cluster, a cluster voltage measure,

selecting at least one first cluster based on the obtained cluster voltage measures using an inter-cluster priority scheme, and

selecting cells of the first cluster and controlling the selected cells to change conduction state in the current time interval according to an intra- cluster cell priority scheme for allowing the converter control structure to meet the control objective.

The object is according to a third aspect achieved through a computer program product for controlling cells in a modular multilevel converter comprising at least one phase arm with a number of series-connected cells, the cells being configured for providing at least one voltage contribution that assists in the forming of an alternating current waveform and where the cells of a phase arm are organized in clusters, the computer program product comprising a data carrier with computer program code configured to cause a converter control structure to

determine a number of cells of a phase arm needed to be used during a current time interval for meeting a control objective, obtain, from every cluster, a cluster voltage measure, select at least one first cluster based on the obtained cluster voltage measures using an inter-cluster priority scheme, and

select cells of the first cluster and control the selected cells to change conduction state in the current time interval according to an intra-cluster cell priority scheme for allowing the converter control structure to meet the control objective.

The invention has a number of advantages. The distributed control structure offers to reduce I/O requirements and calculation tasks for individual control units. The converter could achieve low losses, small three-phase ac-filter requirements, independent reactive power control, and black start capability. Moreover, it may be possible to increase the number of cells per arm without increasing control structure capability, control complexity, and without lowering the capacitor-voltage balancing capability. By using the cluster concept, factory assembly is possible to a greater extent. Assembly of clusters can also be made locally where the converter is to be installed, to comply with such requirements. Clusters can be made to fit the max suitable size and weight for shipping (a container e.g.). Furthermore, factory testing and burn-in of larger units, in the form of clusters is also possible, speeding up site work and commissioning and reducing the probability of infant failures. Maintenance and repair can be made more easily since entire cluster can be kept as a spare, and easily be replaced.

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 modular multilevel voltage source converter connected between a pole and ground and comprising cells grouped into clusters, fig. 2 schematically shows a converter control structure provided for the voltage source converter and controlling cell clusters,

fig. 3 schematically shows the variation of a cluster voltage measure and cell voltages between a first and second upper voltage level and a first and second lower voltage level,

fig. 4 shows a flow chart of a number of method steps being performed by a control objective control unit of the converter control structure, fig. 5 shows a flow chart of a number of method steps being performed by a cluster selecting unit of the converter control structure,

fig. 6 shows a number of method step being performed by a cluster control unit of the converter control structure,

fig. 7 schematically shows a first way of controlling the cells of a cluster, fig. 8 schematically shows a second way of controlling the cells of a cluster, fig. 9 schematically shows a third way of controlling the cells of a cluster, and

fig. 10 schematically shows a computer program product computer program medium comprising computer program code for implementing at least parts of the control structure. DETAILED DESCRIPTION OF THE INVENTION

In the following, a detailed description of preferred embodiments of the invention will be given. Fig. l shows a simplified variation of a multilevel converter in the form of a cell based voltage source converter 10 or modular multilevel converter (MMC). The converter operates to convert between alternating current (AC) and direct current (DC). The converter 10 in fig. l comprises a three- phase bridge made up of a number of phase legs. There are in this case three phase legs. It should however be realized that as an alternative there may be for instance only two phase legs. There is thus a first phase leg PLi, a second phase leg PL2 and a third phase leg PL3. The phase legs are more particularly connected between a first DC terminal DCi and a second DC terminal DC2, where the first DC terminal may be connected to a first pole Pi of a DC power transmission system, such as a High Voltage Direct Current (HVDC) power transmission system and the second DC terminal DC2 may be connected to ground, where the mid points of the phase legs are connected to corresponding alternating current terminals ACA, ACB, ACC. A phase leg is in this example divided into two halves, a first upper half and a second lower half, where such a half is also termed a phase arm.

The first DC pole Pi furthermore has a first potential Udp that may be positive. The first pole Pi may therefore also be termed a positive pole. The AC terminals ACA, ACB, ACC may in turn be connected to an AC system, such as a flexible alternating current transmission system (FACTS), for instance via a transformer. A phase arm between the first pole Pi and a first AC terminal ACA, ACB and ACC may be termed a first phase arm or an upper phase arm, while a phase arm between the first AC terminal and ground may be termed a second phase arm or a lower phase arm.

The HVDC system may more particularly be an Ultra High Voltage Direct Current System (UHVDC) operating at 800 kV and above.

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

The voltage source converter 10 depicted in fig. 1 has an asymmetric monopole configuration. It is thus connected between a pole and ground. As an alternative it may be connected in a symmetric monopole

configuration or a symmetric bipole configuration. In a symmetric monopole configuration the second DC terminal DC2 would be connected to a second pole having a second negative potential that may be as large as the first potential but with the opposite polarity. In a symmetric bipole configuration there would also be a second pole. In the bipole configuration, there would furthermore be a third and a fourth phase arm in the phase leg, where the second and third phase arms would be connected to ground, the first phase arm connected between the positive voltage of the first pole Pi and the second phase arm and the fourth phase arm connected between the negative voltage of the second pole and the third phase arm. A first AC terminal of a phase leg would in the symmetric bipole configuration be provided between the first and second phase arms, while a second AC terminal of the same phase leg would be provided between the third and fourth phase arms. The phase arms are furthermore connected to the AC terminals via phase reactors.

The phase arms of the voltage source converter 10 in the example in fig. ι comprise cells. A cell is a unit that may be switched for providing a voltage contribution to a voltage being formed on the corresponding AC terminal. A cell then comprises one or more energy storage elements, for instance in the form of capacitors, in parallel with one or two branches with switches, where the switches may be switched to start to contribute to the forming of a wave shape through inserting a voltage contribution corresponding to the voltage of the energy storage element. When a cell is to stop

contributing to the forming of the wave shape it may likewise be

withdrawn through operation of the switches.

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

In the simplified example given in fig. l there are six series-connected or cascaded cells in each phase arm. Thus the upper phase arm of the first phase leg PLi includes six cells Cipi, C2pi, C3pi, C4pi, Cspi and C6pi, while the lower phase arm of the first phase leg PLi includes six cells Cini, C2ni, C3ni, C- i, Csni and C6ni. Across the cells of the upper phase arm there is a first phase arm voltage Uvppa and through the upper phase arm there runs a first phase arm current Ivppa. As the upper phase arm is connected to the first pole Pi it may also be considered to be a positive phase arm. Across the cells of the lower phase arm there is a second phase arm voltage Uvpna and through the lower phase arm there runs a second phase arm current Ivpna. The upper phase arm is furthermore joined to the AC terminal ACA via a first or upper arm reactor Laarmi, while the lower phase arm is joined to the same AC terminal ACA via a second or lower arm reactor Laarm2. In a similar fashion the upper phase arm of the second phase leg PL2 includes six cells Cip2, C2p2, C3p2, C4p2, Csp2 and C6p2 while the lower phase arm of the second phase leg PL2 includes six cells Cm2, C2n2, C3n2, C- 2, Csn2 and C6n2. Finally the upper phase arm of the third phase leg PL3 includes six cells Cip3, C2p3, C3P3, C4P3, C5P3 and C6p3, while the lower phase arm of the third phase leg PL3 includes six cells Cm3, C2n3, C3n3, - 3, Csn3 and C6n3. The upper phase arms are furthermore joined to the corresponding AC terminals ACB and ACC via corresponding first or upper arm reactors Lbarmi and Lcarmi, respectively, while the lower phase arms are joined to the same AC terminal ACB and ACC via corresponding second or lower arm reactors Lbarm2 and Lcarm2, respectively.

The number of cells provided in fig. 1 is only an example used to show the principle according to which cells are organized. It therefore has to be stressed that the number of cells in a phase arm may be considerably higher, such as for instance a thousand.

In order to improve the way such a high number of cells is handled, the cells are grouped or organized into clusters and each cluster may comprise a fixed number of phase arm cells physically located close to each other. In fig. 1 only the clusters in the upper phase are indicated. In this example the cells Cini and C2ni form a first cluster CLi, the cells, C3ni and C- i a second cluster CL2 and the cells Csni and C6ni a third cluster CL3. It has to be stressed that a cluster may comprise more than two cells. One exemplifying alternative number given later is 10.

The same type of clustering is made also in the other phase arms. Control of the cell in a phase arm is normally done through providing the cell with a control signal directed towards controlling the contribution of that cell to meeting a voltage reference, where the voltage reference may be set to provide a wave shape that meets a control objective, such as to obtain a certain power transfer like an active or reactive power transfer. The cells may thus be controlled for achieving a variety of goals, where the shape of a waveform may be one. In order to control the cells there is provided a converter control structure. The converter control structure for a phase arm comprises a control objective control unit 12, one or more cluster selecting units 14 and a number of cluster control unis 16a, 16B, 16C, one for each cluster. There is thus a first cluster control unit 16A for controlling the cells Cipi and C2pi in the first cluster CLi, a second cluster control unit 16B for controlling the cells C3pi and C4pi in the second cluster CL2 and a third cluster control unit 16C for controlling the cells C5pi and C601 in the third cluster CL3. The cells Cipi, C2pi, C3pi, C4pi, C5pi and C6pi report cell voltages, currents and temperatures, to the corresponding cluster control unit 16A, 16B and 16C and receive control signals from this cluster control unit 16A, 16B and 16C, which is indicated through bidirectional dashed arrows. The cluster control units 16A, 16B and 16C also communicate with the cluster selecting unit 14, which communication is also indicated with dashed bidirectional arrows. The cluster selecting unit 14 in turn communicates with the control objective control unit 12, which is also indicated with a bidirectional dashed arrow. The communication may be provided via communication channels, for instance optical communication channels.

As described above the shown control structure is provided for one phase arm. A similar control structure may be provided for each phase arm.

In this case it is possible that the control objective control unit 12 is common for two cluster selecting units 14, where one cluster selecting unit is provided for the upper phase arm and the other for the lower phase arm. As an alternative it is possible that there is only one cluster selecting unit for the whole phase leg, which may especially be advantageous in case full bridge cells are used. The same type of structure may furthermore be provided for all phase legs. The control objective control unit 12 may in this case also be common for all phase legs. Alternatively there may be provided one control objective control unit per phase leg.

The control structure may be implemented through one or more

computers. It may as an example also be implemented through the use of one or more Field-Programmable Gate Arrays (FPGAs).

In the control structure the cluster control units 16A, 16B, 16C are typically provided close to the clusters of cells, with the other units located on a distance from the cells. The cluster control units 16A, 16B, 16C may be connected to the cluster selecting units via cables, for instance via optical fibers.

Fig. 2 schematically shows the control structure for a phase leg, where there is a common control objective control unit 12, two cluster selecting units 14A and 14B, one for each phase arm, and a number of cluster control units, where the only cluster control units shown are the cluster control units of the upper phase arm and then only the first and last cluster control units 16A and 16J. Furthermore in this example there are 1000 cells in each phase arm, where each cluster comprises 10 cells. In order to simplify the figure only the first and tenth cell of the first cluster and the thousandth cell (of the tenth cluster) are shown. Furthermore of the control provided by the cluster control units 16A and 16J only the control performed by the first cluster control unit 16A associated with the first cluster CLi is indicated. All this is only done for simplifying understanding of the various aspects of the invention.

In the figure the control objective control unit 12 is shown as obtaining control values P ^* , Q ^" that are desired active and reactive power values as well as a desired cell capacitor value V _c ^* , i.e. a desired cell voltage value of the energy storage elements. The control objective control unit 12 is also shown as providing the first cluster selecting unit 14A with an upper voltage index or phase arm voltage reference n _p and the second cluster selecting unit 14B with a lower voltage index or phase arm voltage reference n _N .

The control objective control unit 14 is also shown as receiving an average upper phase arm voltage v _cp from the first cluster selecting unit 14A and an average lower phase arm voltage v _CN from the second cluster selecting unit 14B. The first cluster selecting unit 14A is furthermore shown as providing a cluster index to the first cluster control unit 16A and as receiving an average cell voltage level v _cl as well as information indicating potentially excluded cells from the first cluster control unit 16A. The information indicating potentially excluded cells is here in the form of a maximum allowed index « and a minimum allowed index n - . The first cluster selecting unit 14A is also shown as sending a cluster index n _m to the last cluster control unit 14J of the upper phase arm. The first cluster control unit 16A is in turn shown as sending control signals S _l and S ₁₀ to the first and tenth cells of the first cluster CLi as well as obtaining cell voltage measurement v _cl and v _cl0 from these cells.

As can be seen in fig. 2, the converter is supposed to comprise a high number of cells, such as a thousand cells. This will lead to problems such as a difficulty in modulating a voltage with a reasonably low-switching frequency, difficulty in achieving a sufficient insertion time for each cell in order to achieve proper switching actions and a difficulty in obtaining a control structure capable of handling thousands of input/output (I/O) signals. Of these problems the insertion time may be the most serious.

In order to address this a control strategy being performed by the converter control structure is suggested.

How this control according to the control strategy may be carried out will now be described with reference also being made to fig. 3, 4, 5 and 6, where fig. 3 shows cell voltage variations, fig. 4 shows a flow chart of a number of method steps being performed by the control objective control unit 12, fig. 5 shows a flow chart of a number of method steps being performed by the first cluster selecting unit 14A and fig. 6 shows method steps being performed by a selected cluster control unit, for instance the first cluster control unit 16 A. As can be understood from fig. 3 the cell voltages are not static, but they vary. The capacitor of a cell being inserted in a phase arm is typically either charged or discharged depending on the direction of current through the phase arm, where a positive current typically charges the cell capacitor and a negative current discharges the cell capacitor. The cell voltages are furthermore allowed to vary between a first upper voltage level ViUL and a first lower voltage level ViLL, where a cell that has reached the first upper voltage level ViUL is not allowed to be charged and a cell that has reached the first lower voltage level ViLL is not allowed to be discharged. This means that a cell that is at the first upper voltage level ViUL is excluded from being inserted for positive arm currents, while a cell that is at the first lower voltage level ViLL is excluded from being inserted for negative arm currents. A cell that has a cell voltage that is close to the first upper voltage level may then then have a cell voltage variation VCMAX, while a cell having a cell voltage that is close to the first lower voltage level ViLL may have a cell voltage variation VCMIN.

Furthermore, this principle can be followed also on the cluster level. A cluster control unit 16 can obtain the cell voltages of the cells and form a cluster voltage measure, such as a cell voltage average VCAVE, which cluster voltage measure is allowed to vary between a second upper voltage level V2UL and a second lower voltage level V2LL, where the second upper voltage level V23UL with advantage is lower than the first upper voltage level ViUL and the second lower voltage level V2LL is higher than the first lower voltage level ViLL. The cluster voltage measure may be used for guidance in selecting clusters.

Therefore, each cell control unit obtains the cell voltages of the cells in the cluster and performs three activities. The first is to compare the individual cell voltages with the first upper voltage level ViUL in order to determine if any are at or above it and the second is to compare the individual cell voltages with the first lower voltage level ViLL in order to determine if any are at or below it. In this way a determination is made about which cells that are to be excluded in case the cluster is to be used for controlling cells with positive arm current and negative arm current, respectively. The determination of potentially excluded cells may be made through determining the previously mentioned maximum cluster index and minimum cluster index and provide these indexes to the first cluster selecting unit 14A. The values of these indexes would be equal to the total number of cluster cells and zero cluster cells in case none of the cells are excluded. However, in case any cell had reached the first upper voltage level ViUL, then the maximum cluster index would be decreased by an amount corresponding to a nominal cell voltage. In a similar manner a cell having reached or being below the first lower voltage level ViLL would lead to the minimum cluster index being increased with an amount corresponding to a nominal cell voltage. The maximum and minimum cluster indexes are then reported to the cluster selecting unit 14A. It can in this way be seen that information about possibly excluded cells in respect of both current directions are determined and reported to the cluster selecting unit, step 32. It should here be realized that other ways of notification of potentially excluded cells may be used, such as directly giving a number of excluded cells for the first current arm direction and another number of excluded cells for the opposite current direction.

The third activity performed by each cell control unit is to combine the values of the cell voltages in order to obtain the previously mentioned cluster voltage measure, which cluster voltage measure then represents the voltage level of the cluster. This could be obtained as an average of the cell voltages v _cl or a mean cell voltage. Another possibility is to use a sum. As an alternative it is possible to determine a median of the cell voltages. The cluster voltage measure is then also reported to first cluster selecting unit 14A. Thereby also the cluster voltage measure is determined and reported to the first cluster selecting unit 14A, step 34.

Now the control being carried out by the control structure will be described in some more detail in relation to the upper phase arm of the first phase leg.

The control objective control unit 12 first determines a voltage to be provided in the upper phase arm for achieving the control objective in a current time interval, step 18. In this example the control objective is to match the desired active power P ^* the desired reactive power Q ^" and the desired balanced cell voltage V _c ^* . This control is merely an example of a control objective and is as such known. The capacitor voltage control may more particularly be an open loop control or a closed loop control.

The active and reactive power control may be based on conventional measurements of active and reactive power (not shown), while the cell voltage balancing may be made based on the measured cell voltage averages voltage v _cp and v _CN in the upper and lower phase arms. Based on this data the control objective control unit 12 then determines a voltage reference n _p for the upper phase arm and a voltage reference n _N for the lower phase arm in a fashion that is known per se. The upper phase arm voltage reference n _p in essence stipulates the voltages or the number of cells in the upper phase arm that are needed to contribute to meeting the reference in the current time interval, i.e. in order to meet the control objective. Based on how many cells that were needed in a previous time interval, it is then possible to know how many cells that are to be inserted or withdrawn in the current time interval.

The reference n _p is then sent to the first cluster selecting unit 14A and thereby the first cluster selecting unit 14A is informed of the cell voltages that are to be controlled, step 20.

As the control may be used for forming a wave shape, some cells may thus already be inserted in the phase arm and therefore the cluster selecting unit knows how many additional cells need to be inserted or how many of the already inserted cells need to be withdrawn or removed.

The first cluster selecting unit 14A thus receives the voltage reference, i.e. the voltage insertion information about how many cells are needed in the current time interval for meeting the control objective, from the control objective control unit 12, step 22. It also receives the information about the average cell voltages and indications relating to potentially excluded cells in the form of the maximum and minimum cluster indexes « and n - from each cluster control unit, step 24.

The cluster selecting unit 14A also has knowledge about the direction of the phase arm current. Based on the voltage reference, current direction and an inter-cluster priority scheme linked to the phase arm current direction, the cluster selecting unit 14A then selects a cluster to be inserted or withdrawn, step 26. It here typically selects the cluster having the highest or lowest average cell voltage for insertion or withdrawal, where in case the current is a positive phase arm current that charges cells, the cluster having the lowest cell voltage average may be selected for insertion or the cluster having the highest cell voltage average may be selected for withdrawal and in case the current is a negative arm current discharging cells then the cluster having the highest cell voltage average may be selected for insertion or the cluster having the lowest cell voltage average may be selected for withdrawal.

Thereafter the cluster selecting unit 14A orders the selected cluster control unit to control cells of the selected first cluster for allowing the control objective to be met and in a way that avoids using excluded cells, step 28., which cluster control unit as an example may be the first cluster control unit 16A. This ordering may be done through sending a cluster index to the cluster control unit 16A that is modified. The cells of a cluster are typically to be operated as one entity. All the cells, apart for the excluded cells, may typically to be inserted or withdrawn in the current time interval. This means that if no cells are to be excluded then the cluster index/? ^* would correspond to the inserting of all the cells in the cluster. However, when there are cells that are to be excluded, i.e. not used, then this cluster index could be lowered with an amount corresponding to the number of excluded cells. This means that the ordering of the first cluster control unit to control cells may comprise an order to omit excluded cells from this control. Moreover, if no cells are excluded it is possible that no other cluster needs to be involved in cell insertion or withdrawal in order to reach the desired voltage level. However, in case one or more cells are excluded in a cluster, then it is very well possible that the level cannot be obtained. Therefore in order to reach the desired voltage level, the cluster selecting unit may order another cluster control unit to control cells needed to reach the desired number of cells of the phase arm, for instance in order to compensate for the voltage of the excluded cells, step 30. This means that if for instance two further cells need to be inserted, the cluster selecting unit 14A orders the other cluster control unit to insert two cells and if two cells need to be withdrawn, the cluster selecting unit orders the other cluster control unit to withdraw two cells (that have previously been inserted with the selected cluster). This means that the excluded cells may be compensated by another cluster. The cluster associated with this other cluster control unit may be a cluster having a lower priority than the first cluster, for instance the next cluster in the priority order. It may also be a special dedicated cell compensation cluster.

Put differently, the cluster control unit thus inserts the cells at the voltage limitation irrespective of the original reference voltage at the cluster selecting unit. Therefore, the actual insertion index may no longer be the same. The reference insertion index for the cluster control unit may be corrected at the cluster selecting unit in such a way that the actual insertion index can be as close as possible to the reference voltage. This correction can be done by adding or subtracting an extra insertion index to or from the reference voltage.

Here it may also be mentioned that it is possible that no change of the cluster index is made by the cluster selecting unit, but that the cluster control unit itself makes the adaptation based on the number of excluded cells. However, the cluster selecting unit may still have to compensate for the excluded cells.

Here it may furthermore mentioned that it is also possible that the needed voltage change requires more cells than there are cells in the cluster. In this case it is possible that a further cluster control unit is ordered to control one or more cells even though none of the cells of the selected cluster are potentially excluded. The selected cluster control unit, which in the given example may be the first cluster control unit 16A, then continues and selects cells to be operated and thereafter controls the selected cells to change conduction state, i.e. to be inserted into or withdrawn from the phase arm, in the current time interval, where the insertion/withdrawal may be carried out according to an intra-cluster cell priority scheme linked to the arm current direction, step 36. It thus selects the cells that are within the first upper or first lower voltage level and excludes cells that are at, above or below the voltage levels and controls the others. It is for instance possible that the selected cells are handled in a priority order defined by the individual cell voltages. The cell with the highest cell voltage may be inserted first when there is a negative phase arm current, while the cell with the lowest cell voltage may be inserted first when there is a positive phase arm current. In a similar manner the cell with the lowest cell voltage may be withdrawn first when there is a negative phase arm current, while the cell with the highest cell voltage may be withdrawn first when there is a positive phase arm current.

It is also possible that the priority order of the cells is organized at zero voltage crossings. Thereby, the cluster selecting unit as well as each cluster control unit may change priorities at current zero crossings.

It can be seen in fig. 3, that the cluster selecting unit obtains intra-cluster balancing through regulating the cluster voltage measure, for instance the average capacitor voltage, of a cluster to be within a certain band. A cluster control unit in turn obtains inner-cluster balancing that regulates individual capacitor voltages in a cluster to be within another band, where each band can be designed according to the acceptable capacitor voltage. The way cells in a cluster are inserted or withdrawn in the current time interval can furthermore be performed according to a few schemes. It is for instance possible that a two-level scheme is used so that all cells are inserted or withdrawn at the same time, that a multistep scheme is used where the cells are inserted sequentially in steps or that a random level scheme is used where first group of cells are inserted at a first time followed by a second group of cells being inserted at a second time. The first situation is shown in fig. 7, where a change of 20 kV is made simultaneously for the first cluster CLi of ten cells, each having a cell voltage of 2.5 kV. The second situation is shown in fig. 8 where a change from zero to 20 kV is made in a number of equal sized steps, where there is one step per cell. The third situation is shown in fig. 9 where a change from zero to 20 kV is made using random levels, here in the form of two steps, both of 10 kV for the first cluster CLi. Half of the cells may in this case be used for the first step and half for the second step. Here it may be mentioned that the voltage level 2.5 kV, which is also an example, may be used in order to provide a margin of error for the operation. It is furthermore possible that the first scheme shown in fig. 7 is used when the cell voltages are balanced and the second and third when the cell voltages are unbalanced. The selection of scheme may thus be made based on if the individual cell voltages are balanced or not. The choice of scheme may also be determined according to the required total harmonic distortion (THD) for three-phase converter voltages, and required time for calculation and communication.

The invention has a number of advantages.

It can be seen that the cluster can be seen as a variable voltage source, where the harmonic canceling occurs inside the cluster. That is, the modulation and cell voltage balancing occurs inside the cluster whereby significant simplifications and benefits are made possible.

The distributed control structure offers to reduce I/O requirements and calculation tasks for individual controllers. Thereby it is possible to replace the currently-installed current-source based thyristor converter rated at several GW with the MMC-based converter. The controller could achieve low losses, small three-phase ac-filter requirements, independent reactive power control, and black start capability. Moreover, it may be possible to increase the number of cells per arm without increasing control structure capability, control complexity, and without lowering the capacitor-voltage balancing capability.

By using the cluster concept, factory assembly is possible to a greater extent. Assembly of clusters can also be made locally where the equipment is to be installed, to comply with such requirements. Clusters can be made to fit the max suitable size and weight for shipping (a container e.g.).

Furthermore, factory testing and burn -in of larger units, in the form of clusters is also possible, speeding up site work and commissioning and reducing the probability of infant failures.

Maintenance and repair can be made more easily since entire cluster can be kept as a spare, and easily be replaced thanks to the use of simple interfaces for cooling and control signals. Given the simple interfaces, even automated (by a robot) exchange of clusters during operation (hot swapping) is conceivable. In this way maintenance outages could be avoided, thus lowering, or even reversing, the gap in reliability compared to conventional AC-based grid technology.

In a preferable embodiment the cluster control units may be provided at an electrical potential close to the cells in the cluster. This way fiber links interfaces control of the switches as well as for collecting signals measurements does not have to be made for the full voltage. Functions of the cluster control unit may include modulation, cell capacitor voltage balancing and protection features. Thus at least some low-latency communication may be kept within the cluster. The auxiliary power supply required for the cluster may as an example be taken from the capacitor voltage of one or more of the cells. Other options include power-over-fiber or microturbine in a cooling water circuit. Only one bidirectional communication channel may be required to a cluster control unit. Therefore two or more, redundant, optical fibers, each capable of handling the communication needs of a cluster, may be routed along different physical paths to each cluster control unit without excessive cost. This way fault tolerance can be increased resulting in improved reliability. In total, this amounts to much less cost of optic fiber

communication links and increased reliability.

By enforcing standard interfaces (cooling medium, control, electrical) interoperability clusters from different manufacturers can be achieved.

A further option is to have the clusters internally cooled and insulated by alternative media than air, e.g. oil, SF6 gas, or other superior insulant or cooling medium. This could pave the way for a much more compact design.

According to a further variation a cluster can include cells with different topology and/or semiconductor technology. One example is the use of a combination of say 90% half-bridge cells equipped with HV gate commutated thyristors (GCTs) and 10% full-bridge cells equipped with SiC MOSFETS of lower blocking voltage. In such a configuration the GCTs can operate at fundamental frequency, whereas the SiC MOSFETs operate at a higher frequency. This way the harmonic requirements can be fulfilled although most semiconductors operate at fundamental frequency. The result will be that use of HV semiconductors is enabled, and that the overall semiconductor losses are reduced.

A cluster may furthermore be connected in parallel with a thyristor switch. A cluster that his to be bypassed, i.e. where all cells are to provide a zero voltage contribution of the waveform, may then be bypassed by the thyristor switch. This has the advantage of reducing the conduction losses in the converter. In the description above half-bridge cells were assumed. However, it is equally as well possible to use full-bridge cells.

As was mentioned above, the control structure may be realized in the form of discrete components. However, it may also be implemented in the form of one or more processor with accompanying program memories comprising computer program code that performs the desired control functionality when being run on a processor. A computer program product carrying such 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 waveform analyzer functionality when being loaded into a waveform analyzer. One such data carrier in the form of a CD ROM disk 38 carrying computer program code 40 is shown in fig. 10.

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.