WO2003065559A1

WO2003065559A1 - An apparatus for voltage conversion and a method for control thereof

Info

Publication number: WO2003065559A1
Application number: PCT/SE2003/000113
Authority: WO
Inventors: Georgios Demetriades
Original assignee: Abb Ab
Priority date: 2002-01-28
Filing date: 2003-01-23
Publication date: 2003-08-07
Also published as: EP1470632A1; SE0200223D0; SE0200223L; SE521211C2

Abstract

An apparatus for converting direct voltage of a first input side into direct voltage of a second output side has a first converter (1) located on the input side, a second converter (13) located on the output side and a transformer (9) arranged therebetween. A unit (14) is adapted to control semiconductor devices of turn-off type of the converters to vary the duty cycle of the voltage delivered on the alternating voltage side of the respective converter for controlling the appearance of the current through an inductance (12) associated with the transformer.

Description

An apparatus for voltage conversion and a method for control thereof

FIELD OF THE INVENTION

The present invention relates to an apparatus for converting direct voltage of a first input side into direct voltage of a second output side of the apparatus for transferring electric power between said sides, in which the apparatus comprises: a first converter adapted to form said first side through the input thereof so as to receive a direct voltage from a direct voltage source and which comprises a bridge configuration of the current valves having semiconductor devices of turn-off type, rectifying means connected in anti-parallel therewith and snubber members for soft switching of the semiconductor devices, a transformer with a primary winding, a secondary winding and an inductance, said primary winding being connected to the output of the first converter, and a second converter connected to the secondary winding of the transformer through the input thereof and with said second side of the apparatus through the output thereof and which comprises a bridge configuration of the current valves having semiconductor devices of turn-off type, rectifying means connected in anti-parallel therewith and snubber members for soft switching of the semiconductor devices,

in which the semiconductor devices of turn-off type of the first converter are controlled to convert the direct voltage of the di- rect voltage source into an alternating voltage on the output thereof according to a soft switching mode and the second converter to convert an alternating voltage on the input thereof from the secondary winding of the transformer into a direct voltage on the output thereof on said second side of the apparatus according to a soft switching mode, in which the semiconductor devices of the two converters are controlled so that a phase shift of the voltages across the primary and secondary windings of the transformer is achieved so as to obtain a desired transfer of electric power between said two sides and a desired level of the direct voltage on said second side, as well as a method for controlling such an apparatus.

The invention is not restricted to any levels of the voltage of the two sides of the apparatus or any levels of the power that may be transferred between the sides.

Furthermore, the converters may have any bridge configurations whatsoever able to convert a direct voltage on one side into an alternating voltage on another side in the form of direct voltage pulses. Thus, the converters may have an arbitrary number of so-called legs with the current valves connected in series, for example be half bridges (one leg) or full bridges (two legs) so as to be able to deliver direct voltage pulses of differing levels on the alternating voltage side thereof.

The invention comprises the possibility to transfer power in both directions, but in most applications power will probably be transferred in only one direction, such as from one side of the apparatus connected to a generator of electric power to one side of the apparatus connected to a load (consumer of electric power). In which direction the power through the apparatus flows may be determined by selecting the phase shift between the direct voltage pulses of the two converters delivered to the two windings of the transformer, and the power flows from the converter leading with respect to the direct voltage pulses generated thereby. Thus, "input" and "output" as used in the claims are also to be given a very broad interpretation and they may just as well be considered as "output" and "input", respectively, instead depending upon which reference is applied, for example if the power flow direction prevailing may decide the choice of words. The converter feeding electric power is normally called inverter, and the converter receiving electric power is said to work as rectifier.

The inductance of the transformer may be the own leakage inductance of the transformer, but it could also be formed by an inductor associated with the transformer, or more exactly then be formed by the sum of the leakage inductance and the inductance of said inductor.

Even if an apparatus of this type is not restricted to any particular frequencies for the switching of the semiconductor device of turn-off type by the converters this is, however, primarily adapted for comparatively high switching frequencies, i.e. in the kHz-region, since it is for such high frequencies that it is important to have said snubber members for being able to control the semiconductor devices according to a soft switching mode, so as to keep the switching losses down on an acceptable level. Such a high frequency makes it possible to design the trans- former considerably smaller than should the frequency be lower, which means great savings of costs.

The function of an apparatus of this type may be summarized as follows: direct voltage on one side of one of the converters is converted into direct voltage on the other side of the converter with a desired voltage and/or current level. The input direct voltage is provided to a bridge configuration converting the direct voltage into an alternating voltage and delivering this to a primary winding of a transformer and the alternating voltage output from the transformer on the secondary winding thereof is pro- vided to another bridge configuration delivering a rectified voltage to a load.

The meaning of using an apparatus of exactly this type is the possibility to obtain a short-circuit protection and also other protections through the galvanic separation existing between the two sides of the apparatus. It is at the same time possible to obtain both step up and step down transformation of the level of the direct voltage from one side to the other, and the direct volt- age level on the output side may be selected to be nearly independent of the level of the direct voltage on the input side of the apparatus. This is particularly interesting for apparatuses which generate electric power and are subjected to varying conditions, but where it is still a requirement to keep the level of the direct voltage out on the load side of the apparatus constant. For example when connecting a wind power generator to the first side of the apparatus it may through appropriate control of the semiconductor devices of the converters be ensured that the level of the direct voltage on the second side of the apparatus is kept constant all the time in spite of the fact that the wind conditions and thereby the power that may be generated by the wind power generator may vary dramatically over time.

PRIOR ART

An apparatus of the type defined in the introduction is already known through for example US 5 027 264. An apparatus according to one of the embodiments shown in that publication is schematically shown in the appended Fig 1. The apparatus has a first converter 1 adapted to form a first side 2 of the apparatus through the input thereof so as to receive a direct voltage from a direct voltage source indicated at 3 and which comprises a bridge configuration in the form of a so called half bridge, i.e. with only one leg, with current valves 4, 5 having semiconductor devices 6 of turn-off type, such as for example IGBTs (Insulated Gate Bipolar Transistor), rectifying means 7 connected in anti- parallel therewith, such as rectifying diodes, and snubber members 8 in the form of a capacitor for soft switching of the semiconductor devices. A transformer 9 with a primary winding 10, secondary winding 11 and a leakage inductance 12 schemati- cally indicated is through the primary winding connected to the output of the first converter and through the secondary winding connected to the input of a second converter 13, which here has the same design as the first converter. Furthermore, a unit schematically indicated at 14 is adapted to control the semicon- ductor devices of turn-off type of the converters so as to control the first converter to convert the direct voltage of the direct voltage source into an alternating voltage on the output thereof according to a soft switching mode and the second converter to convert an alternating voltage on the input thereof from the sec- ondary winding of the transformer to a direct voltage on the output thereof on said second side of the apparatus according to a soft switching mode. The unit is then adapted to control the semiconductor devices of the two converters so as to provide a phase shift Φ (see now also Fig 2) of the voltages Vi and V₂ de- livered to the primary and secondary windings, respectively, of the transformer so as to obtain a desired transfer of electric power between said two sides and a desired level of the direct voltage on the second side. The power transfer from one side to the other will reach a maximum if the phase shift is 90 electrical degrees. If we assume that Vi represents the voltage pulses obtained from the first converter and V₂ the voltage pulses on the alternating voltage side of the second converter the transfer of power will in this case take place from the first to the second converter. In the case of two half bridges the voltage provided on the alternating voltage side of the respective converter will be able to assume two different levels being equally high but having opposite signs. This voltage delivered in this way across the inductor of the transformer and constituted by the sum of the voltage pulses in Fig 2 results in a current I through the induc- tance of the transformer having the appearance versus time t shown in Fig 3. The magnitude of the harmonics created gets considerable and means several problems. Examples of such problems are uncertain levels of zero currents in the three phase system, heating and reduction of the life time of transformers and induction motors and also so-called "pollutions" in the form of electromagnetic disturbances of surrounding systems. Costly filters are required for this sake. Should the current through the inductance of the transformer be closer to a sine- shape the problems of harmonics would be reduced considerably. An embodiment in which each converter has a bridge con- figuration with three phase legs, so that there is in total six phase legs, is for this reason suggested in said US patent 5 027 264. The problems of harmonics may hereby be reduced considerably, but the manufacturing costs for the very apparatus gets instead considerably higher and the apparatus gets more complicated with respect to construction and control.

SUMMARY OF THE INVENTION

The object of the present invention is to provide an apparatus of the type defined in the introduction, which is at least in certain respects improved with respect to such previously known apparatuses described above.

This object is according to the invention obtained for such an apparatus by designing the control unit to control the semiconductor devices of the two converters to vary the duty cycle of the voltage delivered on the output of the respective converter for controlling the appearance of the current through said inductance. By varying the duty cycle it gets possible to obtain a cur- rent through the inductance being close to a sine-shape even if a small number of legs are used in the bridges of the converters. It is then not necessary that the duty cycle of the two converters is varied in the same way, but this may take place completely independent of each other, so that if the duty cycle is reduced in one of the converters this could be kept constant or increased in the other converter or be reduced to another proportion in the other converter. Thus, a given curve shape of the currents through the inductance may be obtained by utilizing fewer legs of the bridge configurations of the converters and manufacturing costs may thereby be saved and a higher reliability of the func- tion of the apparatus may be obtained.

According to a preferred embodiment of the invention the control unit is adapted to control the semiconductor devices of at least one of the converters with a constant frequency, and it is ad- vantageous to control the semiconductor devices of both converters with a constant frequency. Such an operation with constant frequency is favourable by the fact that passive components connected to the apparatus gets simpler to the construction thereof and thereby less costly. Such passive components may be inductors, capacitors and transformers, which may be optimized for operation with a certain switching frequency of the semiconductor devices. Thus, by keeping the switching frequency constant but varying the duty cycle of the alternating voltage side of the converters the power flow between the two sides of the apparatus may be controlled with respect to magnitude and direction.

According to another preferred embodiment of the invention the units are adapted to vary the frequency by which the semicon- ductor devices of the converters are controlled to switch for at least one of the converters. By not only varying the duty cycle but also the frequency the turn-on and turn-off conditions of the semiconductor devices and thereby the switching losses may be controlled by combining frequency and duty cycle in a desired way, but said passive components get in return more complicated to their construction, since they have to operate at different frequencies, than in the case of a constant frequency.

According to another preferred embodiment of the invention the unit is adapted to control the semiconductor devices of at least one of the converters to carry out additional commutations between regular commutations when there is a desire to reduce the current through said inductance for delivering voltage pulses on the output of the converter, which are oppositely directed with respect to a voltage pulse that would have been delivered at a determined instant in absence of the additional commutations.

According to another preferred embodiment of the invention the unit is adapted to control the semiconductor devices of one of the converters according to a pulse width modulation pattern for obtaining a more sine-like shape of the current through the inductance in the region of said maximum voltage during periods of a maximum voltage across said inductance formed by voltage pulses from the two converters. By proceeding in this way a sine-like curve shape may be imitated even more and the generation of harmonics may be further reduced.

According to another preferred embodiment of the invention one of the converters has a bridge configuration in the form of a so- called half bridge, i.e. with a leg with current valves connected in series, interconnecting two direct voltage poles and having a midpoint forming the output of the bridge, and the other converter has a bridge configuration in the form of a full bridge, i.e. with two legs connected in parallel, interconnecting two direct voltage poles and each having a series connection of current valves and an output midpoint. By varying said duty cycle of such an apparatus with in total three legs it is possible to obtain a curve shape being just as good, i.e. just as close to a sine- curve shape, as in the embodiment according to the US patent mentioned above having six legs and a constant duty cycle.

According to another preferred embodiment of the invention said first side of the apparatus is adapted to be connected to a wind power generator, and the advantages of such an apparatus ap- pear from the discussion above. According to a further preferred embodiment of the invention the apparatus is designed to be arranged in a plant for transmission or distribution of high voltage direct current (HVDC) so as to control the magnitude and the direction of flows of electric power and/or step up or step down the voltage. This is a very suitable application for an apparatus of the type according to the invention.

It is advantageous if the apparatus is designed for converting direct voltage between 1 kV and 500 kV, but also other voltage levels are conceivable. ,

The invention also relates to a method for controlling an apparatus as mentioned above and the advantages of this method and methods according to embodiments of the invention defined in the dependent claims appear from the discussion above of the apparatus according to the invention.

The invention also relates to computer program product and a computer readable medium according to the corresponding appended claims. It is easy to understand that the method according to the invention defined in the appended set of method claims is well suited to be carried out through program instructions from a processor that may be influenced by a computer program provided with the programs steps in question.

Further advantages as well as advantageous features of the invention appear from the following description and the other dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

With reference to the appended drawings, below follows a description of preferred embodiments of the invention cited as examples. In the drawings:

Fig 1 is simplified circuit diagram of an apparatus of the type shown in the US patent mentioned above and to which also the present invention is applicable,

Fig 2 schematically illustrates the appearance of the voltage pulses on the alternating voltage sides generated in the respective converter in an apparatus according to Fig 1 and said US patent,

Fig 3 schematically illustrates the development of the current I through the inductance of the transformer in the apparatus according to Fig 1 in accordance with said US patent versus time t,

Fig 4 is a simplified circuit diagram of an apparatus according to a preferred embodiment of the invention,

Fig 5 schematically illustrates a possible appearance of the voltage U across the inductance of the transformer of the apparatus in Fig 4 when controlling the apparatus according to the method according to the invention,

Fig 6 illustrates the appearance of the current I through said inductance versus time t at a voltage according to Fig 5,

Fig 7 is a view corresponding to Figs 5 and 6 illustrating voltage and current across and through, respectively, said inductance for a control of the semiconductor devices of the apparatus being somewhat modified with respect to the control according to Fig 5,

Figs 8 and 9 illustrate two embodiments of the method according to the invention differing somewhat from that illustrated through Fig 5, Fig 10 very schematically illustrates the basic principle of a preferred embodiment of the invention with a constant switching frequency of the semiconductor devices but a varied duty cycle, and

Fig 1 1 illustrates the basic principle of another embodiment of the invention with variations of both the duty cycle and the frequency.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Fig 4 schematically illustrates the construction of an apparatus according to a preferred embodiment of the invention, and the parts having correspondence in the apparatus according to Fig 1 are provided with the same reference numerals, and it is here refrained from describing them further. It is illustrated how a wind power generator 15 adapted to generate an alternating voltage is through a rectifier 16 connected to the input side 2 of the first converter 1 of the apparatus according to the invention so as to feed a direct voltage thereto. A load 18, i.e. consumer of electric power, is schematically indicated on the output side 17 of the second converter 13. This embodiment of the invention differs from the apparatus according to Fig 1 with respect to the second converter by the fact that this converter has two legs 19, 20 with current valves connected in series, which each has a semiconductor device 6 of turn-off type, a rectifying means 17 connected in anti-parallel therewith as well as a snubber mem- ber 8 in the form of a capacitor. The two midpoints 21 , 22 of the legs are connected to one end of the secondary winding 1 1 of the transformer each. The control unit 14 of this apparatus is adapted to control the semiconductor devices of turn-off type according to a soft switching mode, which means that a semi- conductor device is turned on when the current flows through the diode connected in anti-parallel therewith, so that the turn- ing on takes place with a low voltage across the semiconductor device, and the semiconductor device is turned off while charging a snubber capacitor 8, so that the current therethrough gets low before the voltage across the semiconductor device gets high. The switching losses may hereby be kept on a low level even if the semiconductor devices are switched with a high frequency. This frequency is depending upon the power and may typically for high powers in the MW-range be some kHz and for lower powers in the KW-region above 10 kHz.

The control unit 14 is also adapted to control the semiconductor devices to vary the duty cycle of the current delivered on the output of the respective converter, i.e. on the alternating voltage side thereof, for controlling the appearance of the current through the inductance 12. By such a variation of the duty cycle the voltage U across the inductance 12 having the appearance shown in Fig 5 may be obtained. The duty cycle may then be varied in one of the converters completely independently of the duty cycle of the other converter for controlling the magnitude and the direction of the energy flow between the two converters. The switching frequency of the semiconductor devices is then advantageously constant, so that passive components of the apparatus or in connection thereto, such as inductors, capacitors and transformers, may be optimized for a determined switching frequency. By creating the appearance of the voltage across the inductance 12 shown in Fig 5 in this way the current I through the inductance will get the appearance shown in Fig 6, i.e. a shape being very close to a sine shape. This means that the size of harmonics may be kept on a very low level and de- mands on filters for removing harmonics may thereby be lowered. The size of the inductance 12 is determined once and for all when constructing the apparatus and decides the maximum current I allowed to flow therethrough.

An alternative way of controlling the semiconductor devices of the apparatus according to Fig 4 is illustrated in Fig 7, which re- suits in a somewhat different shape of the voltage U across the inductance 12 and thereby a somewhat different shape of the current I through the inductance. However, a strongly reduced size of the harmonics with respect to the control illustrated in Fig 2 is obtained also here. The duty cycle of the converter of the apparatus has in Fig 7 been changed with respect to the control according to Fig 5 so as to change the magnitude of the power flow between the two converters.

The principle of a method for controlling the apparatus according to Fig 4 according to another preferred embodiment of the invention is schematically illustrated in Fig 8, and this differs from the one illustrated through Fig 5 by the fact that during periods of a maximum voltage across said inductance formed by voltage pulses from the two converters the semiconductor devices of one of the converters are controlled according to a pulse width modulation pattern for obtaining a more sine-like curve shape of the current through the inductance in the region of said maximum voltage U^ and -U-i , respectively.

Fig 9 illustrates a further variation of the control according to Fig 5, in which the semiconductor devices of at least one of the converters are controlled to carry out additional commutations between the regular commutations (illustrated at E) when there is a desire to reduce the current through the inductance so as to deliver voltage pulses on the output of the converter being oppositely directed with respect to the voltage pulse that would have been delivered at a determined instant in absence of the additional commutations.

It is very schematically illustrated in Fig 10 how the duty cycle may be changed over time t when controlling the semiconductor devices of one converter of the apparatus according to the invention with a constant frequency. The duty cycle (duty cycle) is defined as the quotient of the time period (such as b^ or b₂) and the time of one period T, i.e. the period of time separating the times for delivering a voltage pulse.

Fig 1 1 shows the principles of controlling the apparatus accord- ing to the invention according to a method according to another preferred embodiment of the invention in which the duty cycle as well as the frequency of the switchings are changed over time, so that here for example T-i is not the same as T₂. However, this embodiment put somewhat higher demands on the passive com- ponents of the apparatus, since different frequencies occur.

The invention is of course not in any way restricted to the preferred embodiments described above, but many possibilities to modifications thereof would be apparent to a person with skill in the art without departing from the basic idea of the invention as defined in the appended claims.

As already mentioned, the number of phase legs of the two converters may for example be another than shown in Fig 4, and an apparatus according to the invention may for example have two legs, such as illustrated in Fig 1 , and one such embodiment could for example be suitable for DC/DC-conversion in X-ray devices.

Claims

1. A method for controlling an apparatus for converting a direct voltage on a first input side into a direct voltage on a second output side of the apparatus for transferring electric power between said sides, said apparatus comprising a first converter (1 ) adapted to form said first side (2) through the input thereof so as to receive a direct voltage from a direct voltage source (13) there and which comprises a bridge configuration of the current valves (4, 5) having semiconductor devices (6) of turn- off type, rectifying means (7) connected in anti-parallel therewith and snubber members (8) for soft switching of the semiconductor devices, a transformer (9) with a primary winding (10), a secondary winding (1 1 ) and an inductance (12), said primary winding being connected to the output of the first converter, and a second converter (13) connected through the input thereof to the secondary winding of the transformer and through the output thereof to said second side (17) of the apparatus and which comprises a bridge configuration of current valves having semi- conductor devices of turn-off type, rectifying means connected in anti-parallel therewith and snubber members for soft switching of the semiconductor devices,

in which the semiconductor devices of turn-off type of the first converter are controlled to convert the direct voltage of the direct voltage source into an alternating voltage on the output thereof according to a soft switching mode and the second converter to convert an alternating voltage on the input thereof from the secondary winding of the transformer into a direct voltage on the output thereof on said second side of the apparatus according to a soft switching mode, in which the semiconductor devices of the two converters are controlled so that a phase shift of the voltages across the primary and secondary windings of the transformer is achieved so as to obtain a desired transfer of electric power between said two sides and a desired level of the direct voltage on said second side, characterized in that the semiconductor devices of the two converters are controlled so that the duty cycle of the voltage delivered on the output of the respective converter is varied for controlling the appearance of the current through said inductance.

2. A method according to claim 1 , characterized in that the semiconductor devices of at least one of the converters are controlled with a constant frequency.

3. A method according to claim 1 , characterized in that the semiconductor devices of the two converters are controlled with a constant frequency.

4. A method according to claim 1 , characterized in that the semiconductor devices in at least one of the converters are controlled to switch with a variable frequency for variation of the frequency for controlling the appearance of the current through said inductance.

5. A method according to claim 4, characterized in that the variation of the frequency is carried out for both converters.

6. A method according to claim 4 or 5, characterized in that upon a desire to reduce the current through said inductance the semiconductor devices of at least one of the converters are controlled to carry out additional commutations between regular commutations so as to deliver voltage pulses on the output of the converter being oppositely directed with respect to the voltage pulse to be delivered at a determined instant in absence of the additional commutations.

7. A method according to claim 4 or 5, characterized in that during periods of a maximum voltage across said inductance formed by voltage pulses from the two converters semiconductor devices of one converter are controlled according to a pulse width modulation pattern for obtaining a more sine-like curve shape of the current through the inductance in the region of said maximum voltage.

8. A method according to any of the preceding claims, charac- terized in that said variation of the controlling of the semiconductor devices of the converters is carried out with the aim to obtain a sine-like curve shape of the current through said inductance.

9. A method according to claim 1 , characterized in that the duty cycle of the converters is controlled so as to control the direction and the magnitude of the flow of electric power between said two sides.

10. A method according to any of the preceding claims, characterized in that the control is carried out for an apparatus having a bridge configuration for both converters formed by a so called half bridge, i.e. a leg with the current valves connected in series interconnecting two direct voltage poles and a midpoint forming the output of the bridge.

11 . A method according to any of claims 1-9, characterized in that the control is carried out on an apparatus in which one of the converters has a bridge configuration in the form of a so called half bridge, i.e. with a leg having current valves connected in series interconnecting two direct voltage poles and a midpoint forming the output of the bridge, and in which the other converter has a bridge configuration in the form of a full bridge, i.e. with two legs connected in parallel, interconnecting two di- rect voltage poles and each having a series connection of current valves and an output midpoint.

12. A method according to claim 11 , characterized in that the control is carried out on an apparatus having the two output midpoints (21 , 22) of said full bridge connected to one end each of one (1 1 ) of said transformer windings.

13. A method according to any of the preceding claims, characterized in that the semiconductor devices of turn-off type of the converter are controlled to switch with a frequency exceeding 1 kHz, preferably between 1 kHz and 20 kHz.

14. A method according to any of the preceding claims, characterized in that a direct voltage emanating from an arrangement for generating electric power is fed to said first side.

15. A method according to claim 14, characterized in that said electric power is generated by a wind power generator.

16. An apparatus for converting direct voltage of a first input side (2) into direct voltage of a second output side (17) of the apparatus for transferring electric power between said sides, in which the apparatus comprises:

a first converter (1 ) adapted to form said first side through an input thereof so as to there receive a direct voltage from a direct voltage source (3) and which comprises a bridge configuration of current valves (4, 5) having semiconductor devices (6) of turn-off type, rectifying means (7) connected in anti-parallel therewith and snubber members (8) for soft switching of the semiconductor devices,

a transformer (9) with a primary winding (10), a secondary winding (1 1 ) and an inductance (12), said primary winding being connected to the output of the first converter,

a second converter (13) connected through the input thereof to the secondary winding of the transformer and through the output thereof to said second side of the apparatus and which comprises a bridge configuration of current valves having semi- conductor devices of turn-off type, rectifying means connected in anti-parallel therewith and snubber members for soft switching of the semiconductor devices, and

a unit (14) adapted to control the semiconductor devices of turn- off type of the converters so as to control the first converter to convert the direct voltage of the direct voltage source into an alternating voltage on the output thereof according to a soft switching mode and the second converter to convert an alternating voltage on the input thereof from the secondary winding of the transformer into a direct voltage on the output thereof on said second side of the apparatus according to a soft switching mode, said unit being adapted to control the semiconductor devices of the two converters so as to obtain a phase shift of the voltages across the primary and the secondary windings of the transformer for obtaining a desired transfer of electric power between said two sides and a desired level of the direct voltage on said second side,

characterized in that the control unit (14) is adapted to control the semiconductor devices of the two converters to vary the duty cycle of the voltage delivered on the output of the respective converter for controlling the appearance of the current through said inductance (12).

17. An apparatus according to claim 16, characterized in that the control unit (14) is adapted to control the semiconductor devices (6) of at least one of the converters with a constant frequency.

18. An apparatus according to claim 16, characterized in that the unit (14) is adapted to control the semiconductor devices (6) of the two converters with a constant frequency.

19. An apparatus according to claim 16, characterized in that the unit (14) is adapted to vary the frequency through which the semiconductor devices of the converters are controlled to switch in at least one of the converters.

20. An apparatus according to claim 19, characterized in that the unit (14) is adapted to carry out said frequency variation for both converters (1 , 13).

21. An apparatus according to claim 19 or 20, characterized in that the unit (14) is adapted, upon a desire to reduce the current through said inductance (12), to control the semiconductor devices (6) of at least one of the converters to carry out additional commutations between regular commutations so as to deliver voltage pulses on the output of the converter being oppositely directed with respect to the voltage pulse that would have been delivered at a determined instant in absence of the additional commutations.

22. An apparatus according to claim 19 or 20, characterized in that the unit (14) is adapted to control the semiconductor de- vices (6) of one of the converters according to a pulse width modulation pattern during periods of a maximum voltage across said inductance (12) formed by the voltage pulses from the two converters for obtaining a more sine-like curve shape of the current through the inductance in the region of said maximum volt- age.

23. An apparatus according to any of claims 16-22, characterized in that the unit (14) is adapted to produce said variation of the control of the semiconductor devices of the converters with the aim to obtain a sine-like curve shape of the current through said inductance (12).

24. An apparatus according to claim 16, characterized in that the unit (14) is adapted to control the duty cycle of the con- verters so as to control the direction and the magnitude of the flow of electric power between said two sides (2, 17).

25. An apparatus according to any of claims 16-24, characterized in that the bridge configuration of the two converters is formed by a so called half bridge, i.e. a leg with the current valves connected in series interconnecting two direct voltage poles and having a midpoint forming the output of the bridge.

26. An apparatus according to any of claims 16-24, characterized in that one of the converters (1 ) has a bridge configura- tion in the form of a so-called half bridge, i.e. a leg with current valves (4, 5) connected in series interconnecting two direct voltage poles and having a midpoint forming the output of the bridge, and that the other converter (13) has a bridge configuration in the form of a full bridge, i.e. with two legs connected in parallel, interconnecting two direct voltage poles and each having a series connection of current valves and an output midpoint.

27. An apparatus according to claim 26, characterized in that the two output midpoints (21 , 22) of said full bridge are connected to an end each of one (11 ) of said transformer windings.

28. An apparatus according to any of claims 16-27, characterized in that the unit (14) is adapted to control the semicon- ductor devices (6) of turn-off type of the converters to switch with a frequency exceeding 1 kHz, preferably between 1 kHz and 20 kHz.

29. An apparatus according to any of claims 16-28, charac- terized in that said first side (2) of the apparatus is designed to be connected to an arrangement (15) for generating electric power.

30. An apparatus according to claim 29, characterized in that said first side (2) of the apparatus is designed to be connected to a wind power generator (15).

31 . An apparatus according to any of claims 16-30, characterized in that said second side (17) of the apparatus is designed to be connected to a consumer (18) of electric power.

32. An apparatus according to any of claims 16-31 , characterized in that it is adapted to be arranged in a plant for transmission or distribution of high voltage direct the current (HVDC) so as to control the magnitude and the direction of flows of electric power and/or step up or step down the voltage.

33. An apparatus according to any of claims 16-32, characterized in that it is designed for converting direct voltage between 1 kV and 500 kV.

34. A computer program product adapted to be loaded directly into the internal memory of a computer and comprising software code portions for instructing a processor to carry out the steps according to any of claims 1 -15 when the product is run on a computer.

35. A computer program product according to claim 34 provided at least partially over a network as the Internet.

36. A computer readable medium having a program recorded thereon and designed to make a computer control the steps according to any of claims 1 -15.