WO2004082115A1

WO2004082115A1 - A vsc converter and a method

Info

Publication number: WO2004082115A1
Application number: PCT/SE2004/000205
Authority: WO
Inventors: Mats Hyttinen
Original assignee: Abb Technology Ltd
Priority date: 2003-03-14
Filing date: 2004-02-16
Publication date: 2004-09-23
Also published as: SE0300704L; SE0300704D0

Abstract

A VSC converter exhibits, for each phase (25), at least two series-connected phase legs (1', 1'). Phase terminals (12', 12') belonging to phase legs for the same phase are designed to be simultaneously connected to the phase. All the phase legs of the converter are connected in series and the opposite ends of this series connection, which are formed from an outer end of a respective outer phase leg in the series connection, are each intended to be connected to a pole conductor (19, 20) of a dc-voltage network.

Description

A VSC converter and _a method

FIELD OF THE INVENTION AND BACKGROUND ART

The present invention relates to a VSC converter (Voltage Source Converter) and a method for converting dc voltage into ac voltage and vice versa by operation of a VSC converter according to the preambles to the appended independent claims .

A VSC converter of the above kind, which is connected between a dc-voltage network and an ac voltage network and is subjected to forced commutation for transmitting electric power between the thus voltage-source dc-voltage network, in the present case for high-voltage direct current, and ac- voltage networks connected thereto, offers a plurality of considerable advantages in relation to the use of line- commutated CSCs (current source converters) in HVDC (High- Voltage Direct Current) , of which it may be mentioned that the consumption of active and reactive power may be controlled independently of each other and that there is no risk of commutating errors in the converter and hence no risk of commutating errors being transferred between different HVDC links, as may occur in line-commutated CSCs. In addition, there is a possibility of feeding a weak ac-power network or a network without any generation of its own (a dead ac-voltage network). There are also other advantages.

The invention is not limited to this application, but the converter may just as well be intended for conversion in an

SVC, in which case the dc-voltage network is replaced by a dc intermediate link. The term "network" should also be construed in a very broad sense, and it does not have to be a question of such networks in the true sense of the word. The voltages on the dc-voltage side of the converter are advantageously high, 10-400 kV, preferably 50-400 kV.

When transmitting dc voltage on a dc-voltage network connected to the converter, it is desirable to have as high a vol- tage as possible, since the transmission losses decrease with increasing voltage. At the same time, the number of semiconductor elements with turn-off means [hereinafter referred to as "turn-off semiconductor elements"] and the • number of rectifier members (rectifying diodes) should be kept at a relatively low level to reduce the costs thereof and the arrangement for control thereof. This then means that relatively high voltages may occur across each individual semiconductor element when transmitting high powers, since the power is proportional to the voltage. This in combination with the fact that the semiconductor elements of prior art converters of this kind are controlled with relatively high frequencies, of the order of magnitude of 1 kHz - 4 kHz, to form an acceptable, essentially sinusoidal shape of the curve of said ac-voltage network, leads to considerable losses and to requirements for relatively large and costly filters, among other things for protecting a transformer which may possibly be arranged between the converter and the ac-voltage network.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a VSC converter and a method of the initially defined kind, which make possible a solution to the above-mentioned problems associated with the prior art.

This object is achieved according to the invention by the provision of such a converter, which for each said phase exhibits at least two series-connected said phase legs. The phase terminals associated with phase legs for the same phase are designed to be simultaneously connected to said phase, and all the phase legs of the converter are series- connected and the opposite ends of this series connection, which are formed from an outer end of a respective outer phase leg in the series connection, are each intended to be connected to a pole conductor of a dc-voltage network. By arranging several phase legs for the same phase in this way, and series-connecting them, and at the same time connecting the phase legs to the phase in question, they will cooperate and individually contribute to the voltage conversion. This mplies a possibility of adapting the design and the control of the semiconductor elements of the different phase legs such that the switching losses are reduced and the requirements for filters for removing' harmonics on the ac-voltage side are reduced.

According to a preferred embodiment of the invention, a first one of said at least two series-connected phase legs belonging to the same phase is designed to maintain a first higher voltage across its respective said filters in the blocking state thereof than a corresponding second voltage of a second one of the phase legs. By having a differently high voltage across the phase legs belonging to the same phase, it is possible to achieve a desired curve shape of the voltage of said phase by adapting the control of the respective phase leg such that the losses and the harmonic content will have a favourable appearance. The different blocking voltage levels are advantageously obtained in that the number of series-connected semiconductor elements and series-connected rectifier members are, for each phase, higher in the first of said phase legs than in the second phase leg. It has proved to be suitable that the first, higher voltage is 2-10 times, advantageously 2-6 times, and preferably 3-5 times, the second voltage.

According to a preferred embodiment of the invention, the converter comprises a device adapted to control said turn- off semiconductor elements to be turned on and off to generate a train of pulses with definite amplitudes according to a pulse-width modulation pattern to said phase via a combination of pulses with definite amplitudes on said phase terminals, and said device is adapted to control the semi- conductor elements of the valves in at least one of the phase legs of the respective phase according to a switching pattern which differs from the switching pattern according to which the device is adapted to control the semiconductor elements of the valves in the other phase legs of the phase in question. It is then, of course, possible to have different switching patterns for all the phase legs belonging to the same phase. By a different switching pattern is meant that the switching occurs in different ways, but this does not necessarily mean a difference in switching frequency; the length of the conduction intervals of different semiconductor elements may, for example, be varied in different ways. By having different switching patterns for the semiconductor elements in semiconductor elements belonging to the same phase and included in different valves, the converter may be controlled in a manner considered to be most appropriate according to the conditions prevailing, for example for keeping the losses at as low a level as possible and/or for distributing the load on the different semicon- ductor elements in the most favourable way, for example uniformly between them.

According to a preferred embodiment of the invention, the device is adapted to bring about said different switching pattern by a difference in frequency for the control of said semiconductor elements. In this context, the device is adapted, according to another preferred embodiment of the invention, to control the semiconductor elements of the valves in said first phase leg to be turned on and off with a lower frequency than the semiconductor elements of the valves in the second phase leg. By using a higher switching frequency where the voltage is low, and a lower switching frequency where the voltage is high, the losses in the converter may be considerably reduced. It is also possible that a transformer connected between the converter and the ac- voltage network may withstand the pulses from the converter without any intermediate filter, or at any rate with considerably smaller, and hence also less costly, filters than in prior art converters of a corresponding kind.

According to a preferred embodiment of the invention, which is a development of the embodiment described above, the device is adapted to control the semiconductor elements of the first phase leg with essentially the same frequency as the fundamental tone of the ac voltage on said ac-voltage network and the semiconductor elements of the second phase leg with a pulse-width modulation frequency that is at least one order of magnitude higher than said fundamental frequency, advantageously 15-50 and preferably 20-40 times higher than said fundamental frequency. Thus, by only switching the semiconductor elements in one phase leg with fundamental frequency and the phase leg with a considerably lower voltage across it with the pulse-width modulation frequency, it will be possible to considerably reduce the switching losses in relation to a switching of one single phase leg with the entire voltage across it with said pulse-width modulation frequency.

According to another preferred embodiment of the invention, said phase legs belonging to the same phase have the same composition and their current valves are arranged to maintain essentially the same voltage in their blocking state, and said device is adapted to switch over a current valve in one phase leg and a current valve in the other phase leg every other time in order to achieve a given voltage pulse to said phase. By reducing the frequency, with which the semiconductor elements are switched, to half the value, the losses may be reduced, in spite of the fact that both phase legs have the same composition.

According to another preferred embodiment of the invention, the converter comprises means for disconnecting one phase leg of a said phase and continuing the operation of the converter with one or more second phase legs belonging to said phase for service of one or more parts of said one phase leg and/or when the requirements for power transmission capacity of the converter are low. It this way it will be possible to carry out service on the converter while in- tact operation thereof is in progress, in the event that this approach is chosen when a relatively low power is to be transmitted via the converter. In the above-mentioned case of a first phase leg with a higher voltage, that phase leg in each phase may then be disconnected and only the second phase with a lower voltage, but which is designed for higher switching frequencies, be controlled while still achieving a desired appearance of the voltage curve of the phase in question. In the case of each phase of the converter, seve- ral identical phase legs are coordinated, said means may also achieve redundancy, and it is theoretically possible to operate the converter with one phase leg and having the other phase of the same phase as a "spare" phase leg.

According to another preferred embodiment of the invention, the converter is designed for connection to a polyphase ac- voltage network, such as a three-phase ac-voltage network, where it exhibits at least six series-connected said phase legs .

In the polyphase case, the phase legs of the different phases will thus be series-connected, which is the object of the invention disclosed in WO 00/62409. This implies that each current valve at a given voltage of the dc-voltage side will have a lower voltage to maintain in its blocking state than in conventional VSC converters, where the different phase legs are connected in parallel between the two pole conductors of the dc-voltage side, such that a smaller number of series-connected turn-off semiconductor elements and rectifier members, or such designed for lower voltages and hence less expensive, may be used in each current valve to achieve the voltage in question. Another way of expressing it is to state that, at a given set of turn-off semiconductor elements and rectifier members of the converter, and hence a given cost of these, a higher voltage may be achieved on the dc-voltage side by a said series connection instead of a parallel connection, such that the cost per volt is reduced. Series connection means that the components must be of a high-current type instead of a low-current type, and the former type is always cheaper.

According to other preferred embodiments of the invention, each phase leg may have an NPC connection or each phase leg is formed of two mutually parallel-connected series connec- tions of two current valves each, that is, a so-called H- bridge.

According to a preferred embodiment of the invention, for each phase a transformer is connected to said at least two series-connected phase legs between the phase terminals thereof and the associated ac-voltage network phase. In this way, the invention may be realized in a simple manner. It is then advantageous that the respective transformer exhibits a separate primary winding for each said phase leg, and that the primary winding is adapted to cooperate with a common secondary winding connected to the ac-voltage network phase.

The invention also relates to a method and a use of a VSC converter according to the invention for conversion of dc voltage into ac voltage and vice versa, and the advantages of such a method and such a use should be quite clear from the above discussion.

Additional advantages and other advantageous features of the invention will be clear from the following description and the other dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention, mentioned as examples, will be described below with reference to the accompanying drawings, wherein:

Figure 1 is a schematic circuit diagram illustrating the composition of a prior art VSC converter in the form of a so-called 6-pulse bridge,

Figure 2 is a view, corresponding to that of Figure 1, of a VSC converter according to a first preferred embodiment of the invention, Figure 3 is a view, corresponding to that of Figure 2 , of a VSC converter according to a second preferred embodiment of the invention,

Figure 4 is a diagram schematically illustrating the principle of control of a VSC converter according to a preferred embodiment of the invention,

Figure 5 is a view, corresponding to that of Figure 2, of a VSC converter according to a third preferred embodiment of the invention,

Figure 6 is a view, corresponding to that of Figure 2, of a VSC converter according to a fourth preferred embodiment of the invention,

Figure 7 is a view, corresponding to that of Figure 2, of a VSC converter according to a fifth preferred embodiment of the invention, and

Figure 8 is a view, corresponding to that of Figure 2, of a VSC converter according to a sixth preferred embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

Figure 1 schematically shows a prior art VSC converter with a so-called 6-pulse bridge which exhibits three phase legs 1-3 with two series-connected current valves 4-9 each, each being composed of a plurality of series-connected turn-off semiconductor elements 10 and a plurality of rectifier members, series-connected in anti-parallel therewith, in the form of so-called freewheeling diodes 11. In the figure, the series connection of semiconductor elements and diodes, respectively, is commonly denoted by one single symbol therefor, although in practice they must occur in a relatively large number to be able to maintain the high voltage, certainly in the order of magnitude of several 100 kV, which they commonly have to maintain in the blocking state of the valve. A centre on the respective phase legs between said valves is designed to form a phase terminal 12-14 and, via a phase reactor 15-17, be connected to a phase of an ac-vol- tage network. All the semiconductor elements in a valve are intended to be turned on and off simultaneously, and they are preferably GBTs, since these can be reliably turned on and off simultaneously, via signals from a schematically indicated control device 18, so that for one phase the se- iconductor elements in the first valve 4 are conducting when a positive potential is desired on the phase terminal 12 and the semiconductor elements in the second current valve 5 of the phase leg are conducting when a negative potential is desired on the phase terminal 12. By control- ling the power semiconductor elements according to a fixed pulse-width modulation pattern (PWM) , the dc voltage across a capacitor 21, connected between the two pole conductors 19, 20 on the dc-voltage side of the converter, may be used for generating a voltage on the phase terminal 12, the fun- damental component of which constitutes an ac voltage with the desired amplitude, frequency and phase position. If, for example, a voltage of 100 kV is desired between the two pole conductors 19, 20 of the dc-voltage network, then the semiconductor elements and the freewheeling diodes, respective- ly, of each current valve must be able to maintain this voltage together without failing, so that, for example in the case where all the semiconductor elements may maintain 5 kV, at least 40 (only half the voltage can be utilized) such elements are required per valve. However, if occasionally low powers are to be transmitted on the dc-voltage network, then the valves become oversized, that is, the voltage handling capacity thereof is not utilized since the voltage drops with the power to be transmitted.

Figure 2 schematically illustrates a VSC converter according to a first preferred embodiment of the invention. Parts corresponding to those of Figure 1 are provided with the same reference numerals. This converter differs from that shown in Figure 1 in that the three phase legs 1, 2, 3 are series-connected, and one transformer 22-24 per phase is arranged. This principle is previously known from the above- mentioned WO 00/62409. The novelty of the present invention, however, is that each phase 25-27 exhibits two series- connected phase legs, such as 1 ' , 1" for the phase 25. The two phase terminals belonging to the same phase leg, such as 12' and 12", for the same phase are designed to be simultaneously connected to said phase, and this is done so that the respective transformer exhibits one separate primary winding 28-33 for each phase, and the primary windings are adapted to cooperate with a common secondary winding 34-36 connected to the ac-voltage network phase 25-27. In the embodiment shown in Figure 2, each phase leg is formed from a so-called 2-pulse bridge. Although the various current val- ves are indicated in Figure 2 with the same symbol 37, they may very well be designed differently, and this is normally also most advantageous . It is preferable that out of the two series-connected phase legs, such as 1' and 1", belonging to the same phase, a first one, such as 1', is designed to maintain a first higher voltage across its respective said valves in the blocking state thereof than a corresponding second voltage of the second one, such as 1", of the phase legs. To this end, the control device 18 may be arranged to control the current valves of the first phase leg, that is, the one with a higher blocked voltage, with a considerably lower frequency than the frequency with which the current valves in the second phase leg is controlled.

Figure 3 schematically illustrates that the invention may be applied in the same way to an ac-voltage network with one phase 25 as with several phases.

Figure 4 illustrates very simplified, through a voltage (U) - time(t) diagram, how the control of the current valves of the two phase legs belonging to a phase may be performed according to the invention. According to one example, the current valves of the phase leg with the highest blocking voltage are controlled to be turned on and off with the fundamental frequency, that is, the frequency that the ac- voltage on the ac-voltage network will have, usually 50 Hz. This desired voltage is illustrated in Figure 4 by a sine curve 38. The voltage pulses which are generated in this way on the primary winding, such as 28, belonging to the first phase leg are illustrated by a dashed line 39.

At the same time, the current valves of the second phase leg, such as 1", are controlled to switch over with a pulse- width modulation frequency which is advantageously 15-50 times higher than the fundamental frequency, usually about 1-4 kHz. This is illustrated by the lower but more-high- frequency pulses 40 in Figure 4. Figure 4 only serves to explain the invention, and in practice the small pulses probably have a considerably higher frequency relative to the large ones than what is shown in this figure.

By thus controlling the valves with a high blocking voltage but a low frequency and the valves with a low blocking voltage but a high frequency, the losses may be reduced consi- derably. At the same time, the pulses will be considerably more agreeable to the transformers 22-24, such that possibly no intermediate filters (inductors and capacitors) are needed between the converter and the transformer for protecting the transformer, as is illustrated in Figure 2. In any case, the invention permits such filters to be dimensioned smaller, hence making them less costly.

In addition to the above-mentioned advantages, the series connection of the phase legs shown in Figure 2, in relation to conventional parallel connection, results in the additional advantage that, for the same number of components, that is turn-off semiconductors and freewheeling diodes, per current valve, a voltage three times higher may be handled and hence a voltage three times higher be achieved on the dc-voltage side. Alternatively, one-third as many components may be used to manage a certain voltage. In the case discussed above, for example, this means that 14 instead of 40 turn-off semiconductor elements are required per current valve in order for these to be able to maintain 100 kV to¬

ll gether. Further, in the case illustrated in Figure 2, one of the pole conductors 20 of the dc-voltage side is connected to ground, whereas the other 19 are connected to high voltage, thus achieving so-called monopolar operation which permits a low-voltage inexpensive cable 41 to be used for the return current. Utilizing monopolar operation entails an additional multiplication of the voltage of the high-voltage pole conductor 19 relative to ground by a factor 2 in relation to bipolar operation.

Figure 5 schematically illustrates part of a VSC converter according to a third preferred embodiment of the invention, which differs from that of Figure 2 in that each phase leg here has an NPC connection, that is, four series-connected current valves 42-45, whereby a point on the phase leg between the two inner valves 43, 44 in the series connection forms the phase terminal 12, and a series connection of two so-called clamping diodes 46, 47, directed in the same direction with respect to the series connection of the recti- fier members, is connected between, on the one hand, a point

48 between one outer valve in the series connection and the nearest inner valve and, on the other hand, a point 49 between the other outer valve in the series connection and the nearest inner valve. A centre 50 between the two clamping diodes is connected to a zero potential defined by capacitors 51, 52 series-connected in parallel with the phase leg. A transformer 22 is connected in the same way to the respective phase as in the embodiment according to Figure 2. How an NPC, that is, a three-level valve of this kind is controlled is common knowledge among those skilled in the art. One advantage of this embodiment in relation to the one according to Figure 2 is that a control of the semiconductor elements in an NPC connection does not have to take place with a frequency just as high as in a common two-pulse bridge according to Figure 2, so the switching losses may be kept low.

Figure 6 illustrates a VSC converter according to a fourth preferred embodiment of the invention, which differs from that according to Figure 2 above all in that each phase leg is formed from a parallel connection in an H-bridge 53, 54, each of which comprises a series connection of two current valves. Two such phase legs are series-connected here accor- ding to the same principle as is shown in Figure 2. Further, a winding of the transformer 22 in question is here connected to the converter with the first end connected to a centre 55 between the two current valves of one series connection of the current valves and the other end connected to a centre 56 of the other series connection of current valves. One advantage of this embodiment in relation to the one according to Figure 2 is that the current-handling capacity is doubled here, permitting higher powers to be transmitted and the losses to be reduced at a given transmitted power. However, more components, that is semiconductor elements and freewheeling diodes, than in the embodiment according to Figure 2 are required.

Figure 7 illustrates a converter according to a fifth pre- ferred embodiment of the invention, which is a variant of the embodiment shown in Figure 2. More specifically, all so- called first phase legs, that is, those intended for a higher blocking voltage, are arranged in series one after the other in one half of the series connection of phase legs, whereas the other phase legs intended for a lower blocking voltage are arranged in series one after the other at the bottom of Figure 7 between the centre 57 and the pole conductor 20. The converter also exhibits means, in the form of a change-over switch 58 and a bypass 59, for disconnecting the first phase leg in each phase and continuing the operation of the converter with only the second phase leg of each phase, for example for service of parts belonging to the upper half of the series connection and/or when the requirements for power transmission capacity of the converter are low. The low level of the voltage pulses provided through the second phase legs may very well be sufficient for achieving the desired voltage at low power requirements. It is, of course, also possible to design the converter according to Figure 7 with identical phase legs and to achieve a form of redundancy through the arrangement of the changeover switch 58 and the bypass 59.

Figure 8 illustrates still another, sixth preferred embodi- ment of the invention, which differs from the one according to Figure 2 in that here three phase legs per phase are connected in series, such that the transformer 22 here has three primary windings. In this case, the three phase legs could be designed such that the level of the voltage pulses decreases in the primary windings in a downward direction in Figure 8, whereas the conditions are the reversed for the switching frequencies of valves connected thereto.

As already mentioned in the introductory part of the de- scription, it is also possible, for each phase, to series- connect two phase legs with the same composition and to coordinate the primary windings of the two phase legs by a so- called Y-delta connection of the transformer, and control the valves of the respective phase leg such that the phase valves of the first phase leg switch every other time and those of the second phase leg every other time, such that the switching frequency of each phase leg becomes f/2 with a pulse-width modulation frequency of f .

The invention is not, of course, in any way limited to the preferred embodiments described above, but a number of possibilities of modifications thereof will be obvious to a person skilled in the art without such a person deviating from the fundamental concept of the invention as defined in the appended claims.

For example, it would be entirely possible to design the converters according to the invention for bipolar operation if desired.

Instead of IGBTs, other turn-off semiconductor elements could be used, for example GTOs (Gate Turn-Off thyristors) The switching pattern of different phase legs belonging to the same phase could also differ from each other in other ways than with respect to frequency, as mentioned in the introductory part of the description.

Claims

1. A VSC converter method for conversion of dc voltage into ac voltage, and vice versa, and which exhibits at least one phase leg (1-3) with at least two series-connected current valves (4-9) , each one comprising at least one turn-off semiconductor element (10) and a rectifier member (11) connected in anti-parallel therewith, wherein a centre of the phase leg between said valves is designed to form a phase terminal (12-14) and be connected to a phase (25-27) of an ac-voltage network, characterized in that the converter for each said phase exhibits at least two series-connected said phase legs (1', 1"), that said phase terminals (12', 12") belonging to phase legs for the same phase are designed to be simultaneously connected to said phase, and that all the phase legs of the converter are connected in series and the opposite ends of this series connection, which are formed from an outer end of a respective outer phase leg in the series connection, are each intended to be connected to a pole conductor (19, 20) of a dc-voltage network.

2. A converter according to claim 1, characterized in that said at least two series-connected phase legs (1', 1") belonging to the same phase comprise a first one (1') designed to maintain a first higher voltage across its respective said valves in the blocking state thereof than a corresponding second voltage of a second one.(l") of the phase legs.

3. A converter according to claim 2, characterized in that each current valve comprises a plurality of series-connected turn-off semiconductor elements (10) and a plurality of series-connected rectifier members (11) , and that the number of series-connected semiconductor elements and series-connected rectifier members is, for each phase, higher in the first one (1') of said phase legs than in the second phase leg (1") for maintaining a higher voltage in the blocking state of the valves in the first phase leg than in the second one.

4. A converter according to claim 2 or 3 , characterized in that the first, higher voltage is 2-10 times, advantageously 2-6 times, and preferably 3-5 times said second voltage.

5. A converter according to any of the preceding claims, characterized in that it comprises a device (18) adapted to control said turn-off semiconductor elements (10) to be turned on and off in order to generate a train of pulses with definite amplitudes according to a pulse-width modu- lation pattern to said phase via a combination of pulses with definite amplitudes of said phase terminals, and that said device is adapted to control the semiconductor elements of the valves in at least one of the phase legs of the respective phase according to a switching pattern that differs from the switching pattern according to which the device is adapted to control the semiconductor elements of the valves in the other phase legs of the phase in question.

6. A converter according to claim 5, characterised in that the device is adapted to achieve said different switching pattern by a difference in frequency for control of said semiconductor elements.

7. A converter according to any of claims 2-4 and claim 6, characterised in that said device is adapted to control the semiconductor elements of the valves in said first phase leg (1') to be turned on and off with a lower frequency than the semiconductor elements of the valves in the second phase leg (1") •

8. A converter according to claim 7, characterized in that the device (18) is adapted to control the semiconductor elements of the first phase leg (1') with essentially the same frequency as the fundamental tone of the ac voltage of said ac-voltage network and the semiconductor elements of the second phase leg (1") with a pulse-width modulation frequency that is at least one order of magnitude higher than said fundamental frequency, advantageously 15-50 and preferably 20-40 times higher than said fundamental frequency.

9. A converter according to claim 1, characterized in that said phase legs belonging to the same phase have the same composition and their current valves are adapted to maintain essentially the same voltage in their blocking state, that the converter comprises a device (18) adapted to control the semiconductor elements of the current valves to be turned off and on in order to generate a train of pulses with definite amplitudes according to a pulse-width modulation pattern on the same phase terminal, and that the device is adap- ted to switch over one current valve in one phase leg and one current valve in the other phase every other time to achieve a given voltage pulse to said phase.

10. A converter according to any of the preceding claims, characterized in that it comprises means (58, 59) for disconnecting one phase leg of a said phase and continuing the operation of the converter with one or more other phase legs belonging to said phase for service of one or more parts of said one phase leg and/or when the requirements for power transmission capacity of the converter are low.

11. A converter according to claims 7 and 10, characterized in that said means are adapted to disconnect the first phase leg (1') .

12. A converter according to any of the preceding claims, characterized in that, for each said phase (25) , it exhibits two series-connected said phase legs (1', 1").

13. A converter according to claim 8 and any of claims 1-11, characterized in that, for each said phase (25) , it exhibits three series-connected said phase legs (1', 1", 1" ' ) , that a third one of the phase legs is adapted to maintain a lower third voltage across its^' respective said valves in the blocking state thereof than said first and second voltages, and that said device is adapted to control the semiconductor elements of the valves in the third phase leg { ! " ' ) to turn on and off with a frequency that is higher than that of the second phase leg.

14. A converter according to any of the preceding claims, characterized in that it is designed for connection to a single-phase ac-voltage network.

15. A converter according to any of claims 1-13, characterized in that it is designed for connection to a three-phase ac-voltage network and exhibits at least six series-connected said phase legs.

16. A converter according to any of the preceding claims, characterized in that each phase leg exhibits two series- connected current valves .

17. A converter according to any of the preceding claims, characterized in that each phase leg has an NPC connection, that is, four series-connected current valves (42-45), wherein a point on the phase leg between the two inner valves in the series connection forms said phase terminal (12), and a series connection of two so-called clamping rectifier members (46, 47), directed in the same direction as said rectifier member with respect to said series connection, is connected between, on the one hand, a point (48) between one outer valve in the series connection and the nearest inner valve and, on the other hand, a point (49) between the other outer valve in the series connection and the nearest inner valve with a centre (50) between the two clamping rectifier members connected to a zero potential defined by capacitors series-connected in parallel with the phase leg.

18. A converter according to any of claims 1-15, characterized in that each phase leg is formed from two mutually parallel-connected series connections (53, 54) of two current valves each, that is, a so-called H-bridge.

19. A converter according to any of the preceding claims, characterized in that, for each phase, a transformer (22-24) is connected to said at least two series-connected phase legs between the phase- terminals thereof and the associated ac-voltage network phase (25-27) .

20. A converter according to claim 19, characterized in that the respective transformer exhibits a separate primary winding (28-33) for each said phase leg, and that the primary windings are adapted to cooperate with a common secondary winding (34-36) connected to the ac-voltage network phase (25-27) .

21. A converter according to claim 20, characterized in that the respective primary winding (28-33) is connected with a first end to the phase terminal of the phase leg in question and a second end to a centre between two capacitors (51, 52) series-connected in parallel with the current valves of the phase leg.

22. A converter according to claims 18 and 20, characterised in that, in each phase leg, a primary winding of the trans- former is connected to the converter with a first end connected to a centre (55) between the two current valves of one series connection of the current valve and the second end is connected to the centre (56) between the two current valves of the other series connection of current valves .

23. A converter according to any of the preceding claims, characterized in that it is connected to a dc-voltage network for transmission of high-voltage direct current (HVDC) .

24. A converter according to any of the preceding claims, characterized in that it is designed to deliver on the dc- voltage network a high voltage within the interval 10-400 kV, preferably 50-400 kV.

25. A converter according to any of the preceding claims, characterized in that said turn-off semiconductor elements (10) are in the form of bipolar transistors with an insulated gate (IGBT = Insulated Gate Bipolar Transistor) .

26. A method for conversion of dc voltage into ac voltage, and vice versa, by operation of a VSC converter which exhibits at least one phase leg (1) with at least two series- connected current valves, each one comprising at least one turn-off semiconductor element and a rectifier member connected in anti-parallel therewith, wherein a centre of the phase leg between said valves is designed to form a phase terminal (12) and be connected to a phase (25) of an ac- voltage network, wherein the converter for each said phase exhibits at least two series-connected said phase legs (1', 1"), wherein said phase terminals (12', 12") belonging to phase legs for the same phase are designed to be simultaneously connected to said phase, and wherein all the phase legs of the converter are series-connected and the opposite 5 ends of said series connection, which are formed from an outer end of a respective outer phase leg in the series connection, are each intended to be connected to a pole conductor (19, 20) of a dc-voltage network, characterised in that said turn-off semiconductor elements (10) are con- 0 trolled to be turned on and off to generate a train of pulses with definite amplitudes according to a pulse-width modulation pattern to said phase via a combination of pulses with definite amplitudes on said phase terminals.

^'5 27. A method according to claim 26, wherein said at least two series-connected phase legs belonging to the same phase are a first one (1') designed to maintain a first higher voltage across its respective said valves in the blocking state thereof than a corresponding second voltage of a se- 0 cond one (1") of the phase legs, characterized in that the semiconductor elements of the valves in said first phase leg are controlled to switch over with a lower frequency than the semiconductor elements of the valves in the second phase leg. 5

28. Use of a VSC converter according to any of claims 1-25 for conversion of dc voltage into ac voltage and vice versa,