WO2004017505A1

WO2004017505A1 - An installation for transmission of electric power and a method for operation of such an installation

Info

Publication number: WO2004017505A1
Application number: PCT/SE2003/001115
Authority: WO
Inventors: Gunnar Asplund; Bo Bijlenga
Original assignee: Abb Ab
Priority date: 2002-08-16
Filing date: 2003-06-26
Publication date: 2004-02-26
Also published as: AU2003243103A1; SE0202433L; SE0202433D0; SE525546C2

Abstract

An installation for transmission of electric power between a dc-voltage side of a VSC converter (1) and a three-phase ac-voltage network (2), connected to the ac-voltage side of the converter, exhibits a transformerless connection between a phase terminal (19, 19’, 19’’) of the respective phase leg of the converter and the three-phase ac-voltage network. The transformerless connection is formed by connecting the respective phase terminal of the converter to the respective phase of the ac-voltage network through a phase conductor (27-29) wound around an iron core (30) that is common to the other two phase conductors and that is dimensioned to essentially take care of zero-sequence currents generated during the pulse-width modulation of the current valves includes in the converter.

Description

An installation for transmission of electric power and a method for operation of such an installation

FIELD OF THE INVENTION AND BACKGROUND ART

The present invention relates to an installation for transmission of electric power between a dc-voltage side of a VSC converter for conversion of dc voltage into ac voltage and vice versa, and a three-phase ac-voltage network, connected to the ac-voltage side of the converter, according to the preamble to the appended claim 1, as well as to a method for operation of such an installation.

Installations of the above kind may be used in all kinds of situations where dc voltage is to be converted into ac voltage and vice versa, examples of such installations being in stations of high-voltage direct current (HVDC) installations, in which dc voltage is converted into three-phase ac voltage or vice versa, or in so-called back-to-back stations where ac voltage is first converted into dc voltage and this dc voltage then converted into ac voltage, as well as in SVCs (Static Var Compensators) , where the dc-voltage side consists of one or more freely hanging capacitors.

The invention is not limited to any levels of the voltage of the ac-voltage side of the installations or the powers that the installation is capable if transmitting. However, the invention is especially, but not exclusively, directed to medium voltage and high voltage, that is, where the peak voltage of the ac-voltage side of the installations is 10 kV or higher.

Although the omission of transformers between the above- mentioned phase terminals and the three-phase ac-voltage network of installations of this kind leads to a number of problems, which will be discussed in more detail below, because of the absence of galvanic separation between the ac-voltage network and the converter, it is nevertheless desirable to manage without such transformers since they constitute a costly investment in this context.

In hitherto known installations of the kind defined in the introduction, therefore, phase reactors, in other words inductors with an air core, that is, without a core, have been arranged between the respective phase terminal and the respective phase of the three-phase ac-voltage network. These phase reactors are used for transforming said train of pulses on the phase terminal into an essentially sinusoidal phase voltage. However, such a phase reactor is not capable, to any significant extent, to filter away harmonics generated during the pulse-width modulation. It is well known that, in the operation of a VSC converter, harmonics of three different types are generated, namely, positive- sequence, negative-sequence, and zero-sequence harmonics. Out of these, filtering away the positive-sequence and negative-sequence harmonics using harmonic filters connected to the installation presents no major problems. However, harmo- nics of zero-sequence type present greater problems, especially in the case of the combination of a two-level converter and three-phase ac voltage, which results in the total output voltage of the pulses on the ac-voltage side of the converter never being zero, which entails a zero-sequence voltage that gives rise to a zero-sequence current that is closed via the phase reactors of an installation without -a transformer down through a filter and back to capacitors present on the dc-voltage side, and the centres of which are grounded to prevent the zero-sequence current from propaga- ting on the dc-voltage side of the installation. The zero- sequence currents are significant and entail a considerable ripple on the current from the converter, which loads said gate turn-off semiconductor elements of the current valves of the converter in the form of generation of heat, so that the actual useful current therethrough cannot be made as high as if this ripple were not present.

Providing said phase reactors of prior art installations of the kind defined in the introduction with a core for block- ing the zero-sequence voltage is no conceivable alternative, since such a core will be very large if the positive- sequence voltage is to be absorbed due to the fundamental tone being of a positive-sequence nature, which would render such a core unacceptably expensive. In such a case, the magnetic flux in the core will be very great due to the low frequency of the fundamental tone .

An installation of a different kind, but which suffers from the same problem, may also be briefly described here. In the case of an installation for transmission of electric power between a dc-voltage side of a converter with line-commuta- ted semiconductor elements, similar problems arise, but there the problem occurs on the dc-voltage side instead of on the ac-voltage side as in the installation of the kind described in the introduction. US 5,414,612 describes the solution of the problems with currents of zero-sequence type that arise in such a converter, connected to a three-phase ac-voltage network in a transformerless manner, by arranging at least one mutual inductor connected on the dc side of the converter, said mutual inductor having two windings with a high degree of symmetry which are each connected in a respective dc supply line of the converter and which are magnetically well coupled to each other, so that they exhibit a high impedance to so-called ground-mode currents.

SUMMARY OF THE INVENTION

The object of the present invention is to provide an instal- lation for transmission of electric power of the kind defined in the introduction, and a method for operation of such an installation, which considerably reduce the above-mentioned problems of prior art such installations.

The above-mentioned object is achieved according to the invention in that, in such an installation, the respective phase terminal of the converter is connected to the respect- tive phase of the ac-voltage network by a phase conductor wound around an iron core common to the other two phase con- ductors and dimensioned to essentially take care of zero- sequence currents generated during the pulse-width modulation.

Because an iron core common to the three phases is used in this way, the iron core need not be dimensioned to block the positive-sequence and negative-sequence currents, that is, it need not block the fundamental tone which has a positive- sequence characteristic. When the sum of the currents in the phases is zero, which is the case for positive-sequence and negative-sequence currents, the voltage induced in the phases will be zero since the equivalent impedance is zero, and hence the positive sequence and the negative sequence will not sense any impedance from the core. Thus, the positive sequence, that is, inter alia the fundamental tone, does not see the iron but passes the core uninfluenced. On the other hand, the core will exhibit a high impedance to zero-sequence components of the current. However, such zero-sequence components have a considerably higher frequency, so that the core may be given a size that is acceptable from the cost point of view.

One advantage of the fact that the zero-sequence currents in this way are greatly reduced is that the ripple on the cur- rent through the semiconductor elements is lower and in this way the fundamental current itself, that is, the useful current, through the converter may be increased without increasing the load on the semiconductor elements and hence higher power be transmitted.

According to a preferred embodiment of the invention, the control device of the installation is arranged to use a reference ac voltage having the shape of a sine curve, to which is added a third-tone component or a multiple of third-tone components with respect to the fundamental tone of the sine curve for the respective phase during the pulse- width modulation. The reason for this is that so-called third-tone components and multiples thereof have a zero- sequence characteristic and may thus be effectively taken care of by the core common to the three phases, if the core is dimensioned therefor. This implies that so-called third- tone pulse-width modulation (3PWM) may be applied to the converter, which is previously known from, inter alia, PCT/SE02/00066 and may be utilized to give increased useful voltage out from a VSC converter and hence increased power transmissible via the installation. By using such so-called third-tone pulse-width modulation, the peak voltage of the ac voltage on the ac-voltage side of the converter may be caused to become even higher than the voltage between the two poles of the dc-voltage side.

The invention is especially advantageous in those cases where the control device, during the pulse-width modulation, uses a triangular wave, specific for each phase, and by determining the crossing point between the reference ac voltage for the phase and the triangular wave, controls the semiconductor elements of the current valves, so that, for each phase, pulses with a duration between two consecutive said crossing points are supplied at the phase terminal and, in such a case, for each phase terminal the same triangular wave as for the other two phase terminals is used. By "the same" is meant here that it does not only have the same appearance but that the three triangular waves also do not exhibit any mutual time displacement. On the other hand, this definition includes the fact that triangular waves, -one for each phase leg, which are exactly identical and which are not mutually displaced in time, are used for the pulse- width modulation.

According to another preferred embodiment of the invention, the installation comprises a unit configured to enable so- called soft switching of the semiconductor elements of the current valves, that is, so that no high voltages and high currents are combined in the semiconductors of the current valves. By using such soft switching in an installation of this kind, the voltage of the installation may be increased without causing problems with stray capacitances. The invention also relates to a method for operation of an installation according to the appended independent method claims. The advantageous features and the advantages of this method and the methods according to the preferred embodi- ments defined in the other appended method claims should be quite clear from the above discussion of the installation according to the invention.

The invention also relates to a computer program and to a computer-readable medium according to the corresponding appended claims. It is readily realized that the method according to the invention, as defined in the appended set of method claims, is well suited to be carried out by program instructions from a processor which may be influenced by a computer program provided with the program steps in question.

Further advantages and 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 exam- pies, will be described below with reference to the accompanying drawings, wherein:

Figure 1 illustrates a prior art installation with a two- level VSC converter connected to a three-phase ac- voltage network via inductors,

Figure 2 illustrates an installation of the kind shown in Figure 1, designed in accordance with a first embodiment of the invention,

Figure 3 is a side view of an iron core with phase windings, used in the installation according to Figure 2, Figure 4 is a view of the iron core with phase windings according to Figure 3, viewed from another direction,

Figure 5 is a view, corresponding to Figure 4, of an alternative embodiment of the iron core with phase windings, which may be used in the installation according to Figure 2,

Figure 6 illustrates conventional sine-pulse-width modulation for ac voltage for one of the three phases,

Figure 7 illustrates the use of a reference ac-voltage curve in the form of a sine curve, to which a third-tone component is added, for pulse-width modulation of an installation according to Figure 2,

Figure 8 illustrates how the invention may be applied to an installation with a three-level converter, and

Figure 9 illustrates an installation according to the invention, designed to enable so-called soft switching of the semiconductor elements of the current valves.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

A preferred embodiment of the invention will now be described while at the same time referring to Figures 1 and 2, wherein the installations according to these figures differ as regards the connection between the converter 1 and the three-phase ac-voltage network 2. This installation for transmission of electric power is very schematically illustrated, and only those different components that have a direct bearing on the function according to the invention are included on the drawing to facilitate an understanding of the invention. The VSC converter is of a two-level type and comprises three phase legs 3-5 which are connected between two poles 6, 7 of a dc-voltage side of the converter. This dc-voltage side exhibits a grounded centre 8 and capacitors 9, 10, so that, relative to the centre 8,_+U_d/2 lies on pole 6 and -U_d/2 on pole 7, whereby U is the voltage between the poles 6 and 7. The dc-voltage side may be connected to a dc-voltage network for high-voltage direct current (HVDC) , but it could also be connected to another converter in a so-called back-to-back station. As an alternative, one or more freely hanging capacitors could also be arranged between the poles 6 and 7 in a so-called SVC for reactive power compensation.

Each phase leg comprises two series-connected current valves 11-16, each of which comprises a gate turn-off semiconductor element, for example an IGBT (Insulated Gate Bipolar Transistor) or a GTO (Gate Turn-Off Thyristor) , and a rectifier member 18, here in the form of a rectifier diode, connected in anti-parallel therewith. Although only one semiconductor element 17 and one diode 18 per current valve are shown, these may represent a large number of series-connected such elements and diodes, so that a large number of series-connected semiconductor elements of a valve may be turned on and off at the same time to function as one single circuit breaker, whereby the voltage drop across the current valve is distributed on the different series-connected circuit ^•- breakers. The control of the semiconductor elements is performed in conventional manner by pulse-width modulation (PWM) with a pulse frequency of usually 1-4 kHz to achieve a phase voltage with a frequency of, for example, 50 Hz or 60 Hz on the three-phase ac-voltage network 2.

In each phase leg there is a centre, designated phase terminal 19, which divides the phase leg into two identical parts and is connected in a transformerless manner to a phase 20- 22 of the three-phase ac-voltage network 2. In the prior art installation, the respective phase terminal is connected to the respective phase of the three-phase ac-voltage network via its own so-called phase reactor 23-25, whereas the transformerless connection between the phase terminals and the three-phase ac-voltage network has been solved differently according to the invention, which is illustrated in Figure 2 and will be described later on.

The installation also comprises a device 26 for controlling the semiconductor elements 17 of the current valves to generate a said train of pulses with determined amplitudes, more particularly +Ua/2 and -U_d/2, according to a pulse-width mo- dulation pattern on the respective phase terminal, and to use for this pulse-width modulation, for the voltage on the respective phase, a reference ac voltage defining voltage reference values and being displaced by 120 electrical degrees relative to the reference ac voltages used for the other two phases.

Figure 2 illustrates how the respective phase terminal 19, 19', 19'' is connected to the respective phase 20-22 of the ac-voltage network through a phase conductor 27-29 that is wound around an iron core 30 common to the other two phase conductors and being dimensioned to essentially take care of zero-sequence currents generated during the pulse-width mo- dulation. The positive-sequence and negative-sequence components, that is, also the fundamental tone, will not see the iron but will pass it as if it were not there, whereas the zero-sequence components will see a very high impedance and be effectively reduced. Since the iron core does not have to be dimensioned to take care of the fundamental tone, it may be made relatively small. In the event of a dc vol- tage of ±150 kV on the dc-voltage side of the converter, the zero-sequence component of the switching frequency constitutes about 77% of the fundamental tone, which, in case of a modulation index of 90% and a switching frequency of 1 050 Hz during the pulse-width modulation, entails a zero-sequen- ce component of the switching frequency of 104 kV as peak value. For the maximum magnetic flux density in the core, the following applies:

B_max = Umax / (N X W X A) where N is the number of turns of the respective phase conductor around the core, w is the angular frequency of the switching, and A is the cross-section area of the core. If we assume that N is equal to 200 turns and B_max is equal to 1 T, then A = 104 x 10³/ (200 x 2 x π x 1 050 x 1) = 78 x 10^'3,

so that the core will have the dimensions 28 x 28 cm if it is rectangular.

The path length of the iron in the three legs will be approximately 30 m in total, which entails a total core weight of about 18 tons. This typically means a cost for the core of about USD 30,000.

The benefit is that the current load on the semiconductor elements of the converters decreases in the manner described above. It has proved that, when using a core according to Figure 1, the so-called RMS current, which may be said to be a measure of the ripple of the current, will be approximate- ly 4% higher than the fundamental current, whereas an air reactor according to Figure 1, which does not block the zero sequence of the switching frequency, will have an RMS cur- rent that is 16% higher than the fundamental current. Accordingly, it will be possible to increase the fundamental cur- rent by 12% and hence increase the power transmissible by the installation to a corresponding extent without loading the semiconductor elements more. As far as costs are concerned, it may be said that an installation of this kind costs about USD 20 million, so that an increase in power of 12% would give a saving of over USD 2 million, which is to be compared with the additional cost of the core.

Figures 3 and 4 illustrate how the iron core according to the invention exhibits three legs 31-33, around each of which a said phase conductor 27-29 is wound. The iron core legs 31-33 are mutually connected by a yoke 34. In the embodiment according to Figures 3 and 4, the iron core legs are shown in their longitudinal direction arranged next to each other along a straight line, whereas Figure 5 illustrates an alternative embodiment where the iron core legs are seen in their longitudinal direction arranged next to each other while forming corners of a triangle.

Figure 6 illustrates very schematically how the pulse-width modulation is performed for one of the phases by using a reference ac voltage 35, defining voltage reference values, in the form of a sine curve with a peak value essentially corresponding to the voltage between the centre of the dc- voltage side and the respective pole. How a pulse-width modulation of this kind is performed belongs to the state of the art. For each phase, there is utilized a reference ac voltage that is displaced 120 electrical degrees relative to the other two phases. The reference ac voltage has a fre- quency of, for example, 50 or 60 Hz. Across this voltage there is stored, for each phase, one and the same so-called triangular wave 36. The triangular wave has an amplitude of essentially half the dc voltage between the two poles of the dc-voltage side and a frequency that is at least 5 times, preferably 15-45 times, higher than the frequency of the reference ac voltage 35. The control device 26 is designed to control the converter to deliver, on the phase terminal of the phase in question, pulses 38 with a duration between two consecutive crossing points between the triangular wave and the reference ac voltage, whereby these pulses are controlled to be positive if the reference ac voltage lies - above the triangular wave and negative if the situation is the reverse.

As regards harmonics, it is the zero sequences that are difficult. The zero-sequence voltage is defined as the sum of the voltages of the pulses of the three phases at a given moment divided by three, which means that it can never be zero. This zero-sequence voltage will give rise to a zero- sequence current which, however, when arranging the iron core 30, will see a high impedance and be significantly reduced. Figure 7 illustrates how a so-called three-tone component, that is, a tone with a frequency of 150 Hz where the fundamental frequency is 50 Hz, may be added to a sine curve to obtain an alternative reference ac voltage 35' for use dur- ing the pulse-width modulation. The third-tone component may, for example, have a magnitude of about 15% of the fundamental tone. Such an addition of a third-tone component or an optional multiple of third-tone components does not in-, fluence the voltage between the phases. The voltage referen- ce value of the phase-to-phase voltage is thus still sinusoidal. The modulation form that follows from the utilization of such a reference ac voltage gives a higher fundamental voltage out on the ac-voltage side for a given level of the dc voltage between the two dc-voltage poles of the conver- ter, which also increases the efficiency of the converter and reduces the costs thereof, that is, increases the power that may be transmitted by the installation. The third tone also has a zero-sequence characteristic, so this type of pulse-width modulation may be advantageously used in the in- stallation according to the invention with a core common to the three phases. In the case according to the above, this has the following significance for the size of the core: We assume that U_max is equal to 15% of 150 kV is equal to 22.5 kV for the third-tone component. The frequency is 150 Hz.

A equals 104 x 10³ / (200 x 2 x π x 1 050 x 1) + 22.5 x 10-- / (200 x 2 x π x 150) equals 0.2, so that the core will have a cross section of 44 x 44 cm if it is rectangular. This core will then weigh 46 tons and generate a cost of about USD 100,000, but instead the current valves may be reduced by

15% in voltage, which provides lower valve losses and about 15% lower cost of valve and cooling system, which is a saving that exceeds by far the cost of the iron. There is also a possibility of increasing the capacity of the device by 15% as regards transmissible power without changing the composition of the current valves.

Figure 8 illustrates an installation according to an alternative embodiment of the invention, which differs from that according to Figure 2 in that, in this case, instead four current valves are connected in series per phase leg and that a so-called flying capacitor 41 is arranged between a second centre 42 between two converters in one part of the series connection and a corresponding second centre 43 in the other part of the series connection, thus forming a three-level converter. Such a converter with several possible levels entails a finer curve shape and less harmonic content at a given switching frequency of the semiconductor elements or the same quality of the curve shape as in the case of a two-level converter but with a lower switching frequency and hence lower losses of the semiconductor elements .

Finally, Figure 9 illustrates an installation according to a further preferred embodiment of the invention, in which the converter comprises a unit configured to make possible so- called soft-switching of the semiconductor elements of the current valves, that is, such that no high voltages and high currents are combined in the semiconductor elements of the current valves. This is achieved in that each current valve has its own so-called snubber capacitor 39 connected in parallel with said gate turn-off semiconductor elements and in that the unit comprises a resonant circuit 40 for re- charging the snubber capacitor of the current valves in order thus to allow turn-on of the gate turn-off semicon-- ductor elements of the current valves at low voltage across these. How such a resonant circuit may be utilized for so- called soft switching is described in more detail in, for example, PCT/SE02/00697. By utilizing soft switching in this way, it is possible to increase the voltage of the installation without having problems with stray capacitances.

Examples of voltages, which are in no way limiting, possible to be handled by an installation according to the invention are 10 kV-500 kV, preferably between 100 kV and 400 kV between the two dc poles of the converter. As regards the maximum power transmission capacity of such an installation between the dc-voltage side and the ac-voltage network, this power may, for example, exceed 50 MW, preferably exceed 200

MW.

The invention is not, of course, in any way limited to the preferred embodiments described above, but a plurality of possibilities of modifications thereof are obvious to a person skilled in the art, without this person therefore deviating from the basic concept of the invention as defined in the appended claims.

Other shapes of the core, common to the three phase conductors, than those shown in the figures are, of course feasible. The important thing is that the core is common to them.

Claims

1. An installation for transmission of electric power between a dc-voltage side of a VSC converter (1) for conver- sion of dc voltage into ac voltage and vice versa and a three-phase ac-voltage network (2) connected to the ac- voltage side of the converter, wherein the converter comprises three phase legs (3, 5), arranged between two poles (6, 7), one positive and one negative, of the dc-voltage side of the converter, each phase leg having a series connection of at least two current valves (11-16), each comprising a gate turn-off semiconductor element (17) and a rectifier member (18) connected in antiparallel therewith, wherein, in each phase leg, a centre (19), designated phase terminal, that divides the phase leg into two equal parts is connected in a transformerless manner to a phase (20-22) of the three-phase ac-voltage network, wherein the converter comprises a device (26) configured to control the semiconductor elements of the current valves to generate a train of pulses with definite amplitudes according to a pulse-width modulation pattern on the respective phase terminal of the converter, and wherein the device is configured to use for the pulse-width modulation, for the^' voltage on the respec- * tive phase, a reference ac voltage (35) defining voltage reference values and being displaced by 120 electrical degrees relative to the reference ac voltages used for the ... other two phases, characterized in that the respective phase terminal (19, 19', 19'') of the converter is connected to the respective phase (20-22) of the ac-voltage network through a phase conductor (27-29) that is wound around an iron core (30) common to the other two phase conductors and dimensioned to essentially take care of zero-sequence currents generated during the pulse-width modulation.

2. An installation according to claim 1, characterized in that the iron core exhibits three legs (31-33), around each of which a said phase conductor (27-29) is wound, and a yoke (34) mutually connecting the legs.

3. An installation according to claim 2, characterized in that the three iron core legs (31-33) are viewed in their longitudinal direction arranged next to each other along a straight line.

4. An installation according to claim 2, characterized in that the three iron core legs (31-33) are viewed in their longitudinal direction arranged next to each other while forming corners of a triangle.

5. An installation according to any of the preceding claims, characterized in that said device (26) is configured to use a reference ac voltage (35) having the shape of a sine curve for the respective phase during the pulse-width modulation.

6. An installation according to any of claims 1-4, characterized in that said device (26) is configured to use a reference ac voltage (35') having the shape of a sine curve, to which is added a third-tone component or a multi- pie of third-tone components with respect to the fundamental tone of the sine curve for the respective phase during the pulse-width modulation.

7. An installation according to any of the preceding claims, characterized in that the control device (26) is configured to use, during the pulse-width modulation, a triangular wave (36), specific for each phase, and by determining crossing points between the reference ac voltage (35) for the phase and the triangular wave, to control the semiconductor ele- ments (17) of the current valves so that, for each phase, pulses having a duration between two consecutive said crossing points are supplied at the phase terminal, and that the control device is configured to use, for each phase terminal, the same triangular wave as for the other phase termi- nals.

8. An installation according to any of the preceding claims, characterized in that it comprises a unit configured to enable so-called soft switching of the semiconductor elements (17) of the current valves, that is, such that no high voltages and high currents are combined in the semiconductor elements of the current valves.

9. An installation according to claim 8, characterized in that the current valves each have a snubber capacitor (39) connected in antiparallel with said gate turn-off semiconductor elements (17).

10. An installation according to claim 9, characterized in that said unit comprises a resonant circuit (40) for recharging the snubber capacitors (39) of the current valves in order thus to make possible turn-on of the gate turn-off semiconductor elements (17) of the current valves at a low voltage across these.

11. An installation according to any of the preceding claims, characterized in that the converter comprises at least four series-connected said current valves and a second centre (42) between two current valves in one part of the series connection is connected to a corresponding second centre (43), with respect to the phase terminal, of the other part of"^"'the series connection, and that the device (26) is configured to control the semiconductor elements of the current valves such that voltage pulses with at least three different levels are generated in the respective phase terminal .

12. An installation according to claim 11, characterized in that said other two centres are connected to each other via a flying capacitor (41) .

13. An installation according to any of the preceding claims, characterized in that the VSC converter (1) is con- figured to handle a voltage between its two dc-voltage poles of 10 kV - 500 kV, preferably between 100 kV and 400 kV.

14. An installation according to any of the preceding claims, characterized in that it is designed with a maximum power transmission capacity between the dc-voltage side and the ac-voltage network of more than 50 MW, preferably more than 200 MW.

15. An installation according to any of the preceding claims, characterized in that said device (26) is configured to control the semiconductor elements (17) of the current valves to turn on and off with a pulse-width modulation frequency that is at least five times, preferably 15-45 times, higher than the frequency of the fundamental tone of the ac- voltage side of the converter.

16. An installation according to any of the preceding claims, characterized in that said device (26) is configured to use reference ac voltages involving a fundamental frequency of 50 Hz or 60 Hz.

17. An installation according to any of the preceding claims, characterized in that it is designed for trans- mission of electric power between a dc-voltage network for high-voltage direct current (HVDC) , connected to the dc- voltage side of the converter, and a said three-phase ac- voltage network.

18. An installation according to any of claims 1-16, characterized in that it is designed for transmission of .. electric power of an SVC (Static Var Compensator) between at least one capacitor, hanging freely on the dc-voltage side of the converter, and a three-phase ac-voltage network con- nected to the ac-voltage side of the converter.

19. An installation according to any of claims 1-16, characterized in that it exhibits two said VSC converters connected to a common dc-voltage side and connected in said manner to a respective three-phase ac-voltage network in a so-called back-to-back station.

20. A method for operation of an installation for transmission of electric power between a dc-voltage side of a VSC converter (1) for conversion of dc voltage into ac voltage and vice versa and a three-phase ac-voltage network (2) connected to the ac-voltage side of the converter, wherein the converter comprises three phase legs (3-5) , arranged between two poles (6, 7), one positive and one negative, of the dc-voltage side of the converter, each phase leg having a series connection of at least two current valves (11-16), each comprising a gate turn-off semiconductor element (17) and a rectifier member (18) connected in antiparallel there- with, wherein, in each phase leg, a centre (19) , designated phase terminal, that divides the phase leg into two equal parts is connected in a transformerless manner to a phase (20-22) of the three-phase ac-voltage network, in which the semiconductor elements of the current valves are controlled to generate a train of pulses with definite amplitudes according to a pulse-width modulation pattern on the respective phase terminal of the converter by using for the pulse- width modulation, for the voltage on the respective phase, a reference ac voltage (35) defining voltage reference values and being displaced by 120 electrical degrees relative to the reference ac voltages used for the other two phases, characterized in that the current between the respective phase terminals of the converter and the respective phase of the ac-voltage network is conducted around an iron core (30) common to the other two phase terminals and dimensioned to essentially take care of zero-sequence currents generated- during the pulse-width modulation.

21. A method according to claim 20, characterized in that, for the respective phase during the pulse-width modulation, there is used a reference ac voltage (35') having the shape of a sine curve, to which is added a third-tone component or a multiple of third-tone components with respect to the fundamental tone of the sine curve.

22. A method according to claim 20 or 21, characterized in that a triangular wave (36) that is specific for each phase is used during the pulse-width modulation, and by determining the crossing point between the reference ac voltage for the phase and the triangular wave, the semiconductor elements (17) of the current valves are controlled such that, for each phase, pulses with a duration between two consecutive said crossing points are supplied at the phase termi- nal, and that, for each phase terminal, the same triangular wave is used as for the other phase terminals.

23. A method according to any of claims 20-22, characterized in that, during said pulse-width modulation, so-called soft- switching is applied to the semiconductor elements of the current valves, that is, they are controlled so that no high voltages and high currents are combined in the semiconductor elements of the current valves .

24. A computer program that may be loaded directly into the internal memory of a computer and that comprises software code portions for controlling the steps of any of claims 20- 23 when the program is run on the computer.

25. A computer program according to claim 24, provided at least partially via a network, such as the Internet.

26. A computer-readable medium with a program registered thereon, in which the program is designed to bring a compu- ter to control the steps according to any of claims 20-23.