WO2002013364A1

WO2002013364A1 - A method and a device for providing a vsc-converter with voltage

Info

Publication number: WO2002013364A1
Application number: PCT/SE2001/001057
Authority: WO
Inventors: Bo Bijlenga; Kjell Svensson; Gunnar Asplund; Mats Hyttinen; Lars-Erik Juhlin
Original assignee: Abb Ab
Priority date: 2000-08-10
Filing date: 2001-05-15
Publication date: 2002-02-14
Also published as: EP1307959A1; SE517063C2; SE0002861D0; SE0002861L

Abstract

An apparatus for providing a VSC-converter with a voltage has a series connection of at least four units (1-4) arranged between two poles (5, 6) of a direct voltage side of the converter, each said unit comprising a semiconductor device of turn-off type and a rectifying diode connected in anti parallel therewith, an alternating voltage phase line (19) connected to a first midpoint, called phase output, of the series connection between two units while dividing the series connection in two equal parts, in which the converter comprises a second midpoint (22, 24) between two said units of one part of the series connection through a flying capacitor (23) connected to a second midpoint of the other part of the series connection corresponding with respect to the phase output, comprises means for connecting an alternating voltage to the phase output (18) for providing the converter with voltage through provision of voltage of the direct voltage side thereof and an arrangement adapted to co-ordinate the provision of voltage of the direct voltage side of the converter and the charging of the flying capacitor, so that the capacitor is charged to a substantial proportion of full charging before the direct voltage side receives full voltage between the poles thereof.

Description

A METHOD AND A DEVICE FOP PROVIDING A VSC-CONVERTER TTH VOLTAGE.

FI ELD OF THE INVENTION AN D PRIOR ART

The present invention relates to an apparatus for providing a VSC-converter for converting a direct voltage into an alternating voltage and conversely in a plant for transmitting electric power with voltage, said converter comprising a series connection of at least four units arranged on a direct voltage side of the converter between two poles, one being positive and one negative after voltage have been provided thereto, each said unit comprising a semiconductor device of turn-off type and a rectifying diode connected in anti-parallel therewith, an alternating voltage phase line connected to a first midpoint, called phase output, of the series connection between two units while dividing the series connection in to two equal parts, said two poles of the direct voltage side being intended to be put on substantially the same voltage but with different signs with respect to a zero voltage level of the direct voltage side after said voltage providing, said converter comprising a second midpoint between two said units of one part of the series connection being through a flying capacitor connected to a second midpoint of the other part of the series connection and corresponding thereto with respect to the phase output, and a member being , once the converter has been provided with voltage, adapted ^"to control the semiconductor devices of the units to generate a train of pulses with determined amplitudes according to a pulse width modulation pattern on the phase output of the converter, as well as a method according to the preamble of the appended independent method claim. Such apparatuses may be used in all types of situations, when a direct voltage is to be converted into an alternating voltage and conversely, in which examples of such uses are in stations of HVDC-plants (high voltage direct current), in which direct volt- age is normally converted into a three-phase alternating voltage or conversely or in so-called back-to-back-stations, where the alternating voltage is firstly converted into direct voltage and then this into alternating voltage as well as in SVCs (Static Var Compensator), were the direct voltage side consists of one or a plurality of capacitors hanging freely.

The invention is not restricted to any voltage or power levels, but it is particularly directed to the provision of VSC-converters for voltages on the direct voltage side between 10 kV and 500 kV with voltage.

An advantage of using so-called multiple level converters, i.e. converters for which at least three different voltage levels may be "delivered" on said phase output, with respect to so-called two level bridges, is that the semiconductor devices of said units may be switched with a considerably lower frequency for obtaining an alternating voltage on the alternating voltage phase line of a determined frequency and quality, so that the losses in the converter may be reduced considerably. More exactly, the switching frequency of the semiconductor devices may through a three level converter under said conditions be reduced to about 50%. An advantage of using so-called capacitors for obtaining additional voltage levels on the phase output besides the voltage level of the two poles of the direct voltage side with respect to a use of so-called clamping diodes is primarily that the semiconductor devices in the later case have to be controlled- in such a way that an uneven distribution of the switching losses there- among occurs, so that in the practise all semiconductor devices have to be dimensioned for being able to take the maximum load to which an individual semiconductor device may be excerted, since otherwise particular regard has to be paid to the design of each individual semiconductor device when controlling them. This makes the total cost for the semiconductor devices very high, since some of them will be heavily over-dimensioned in most operation situations. By using so-called flying capacitors in stead, as is the case here, a multiple level converter with the possibility to a more even load on the semiconductor devices with respect to switching losses may be obtained without using expensive so-called clamping diodes or additional semiconductor devices.

However, a problem when using VSC-converters with flying capacitors according to above is that when starting them, i.e. when the converter is provided with voltage, the load on the outer of said units, i.e. those located between the respective second midpoint and the respective direct voltage pole, tends to be initially too high. Irrespectively of providing the voltage from the alternating voltage side through connection of an alternating voltage source to the phase output or from the direct voltage side through connection of a direct voltage to said poles, full voltage will nearly immediately be applied between the two poles of the direct voltage side and this will in all essential degree be taken by said two outer units shouldn't no particular measures be taken. It will namely take a comparatively long time, in the order of possibly several minutes, before the flying capacitor has been charged, since this will be charged slowly through only the weak current existing through the voltage dividers connected in parallel with the different units. This means that the outer units, i.e. the rectifying diodes and semiconductor devices included therein, such as for example IGBTs or GTOs, have to be dimensioned to be able to take this peak voltage and also somewhat higher voltages -that may occur across the outer- units as a consequence of over-voltage transients during the provision of voltage. This means that they have to be heavily over-dimensioned with respect to voltage withstand capability, which removes a part of the advantage of using a VSC-converter of this type for converting direct voltage into alternating voltage and conversely.

SUMMARY OF THE INVENTION

The object of the present invention is to provide an apparatus i i u ci i i i€ ii i u i iuc ty pe uci u i cu H I ti ic n ui ϋuu υUυi i , vv i i i i i makes it possible to a large extent deal with the problems mentioned above when providing a VSC-converter of said type with voltage.

This object is according to the invention obtained as regards the apparatus by providing such an apparatus with means for connection of an alternating voltage to said phase output for pro- viding the converter with voltage through providing the direct voltage side thereof with voltage and an arrangement adapted to co-ordinate the provision of voltage of the direct voltage side of the converter and the charging of the flying capacitor as a consequence of the connection of the alternating voltage to the phase output by accomplishing charging of the capacitor to a substantial proportion of full charging thereof before the direct voltage side receives full voltage between the poles thereof corresponding to the voltage it is intended to have in operation of the converter once provided with voltage.

By carrying out the provision of the alternating voltage side of the converter with voltage and co-ordinating the provision of the direct voltage side of the converter with voltage and the charging of the flying capacitor in this way, it is ensured that the high peak voltages mentioned above of said outer units (current valves) at the voltage provision may not occur any longer, since it is ensured that full voltage is not obtained over the two poles of the direct voltage side until a considerable voltage has been built up over the flying capacitor and by that between said two second midpoints. This means that the dimensioning of the outer units may be lowered and by that considerable costs are saved. It is here pointed out that it is already known through US 5 668 71 1 to provide a VSC-converter of this type, i.e. which has a flying capacitor, with voltage from the direct voltage side, but the provision of voltage described in that publication does not deal with the problem mentioned above solved by the present invention.

According to a preferred embodiment of the invention the ar- rangement comprises members being after said connection of an alternating voltage to the phase output adapted to make the increase of the voltage between the two poles of the direct voltage side obtained therethrough dependent upon the voltage over the capacitor, so that it is efficiently ensured that no high peak voltages are applied to said outer units during the provision of voltage.

According to another preferred embodiment of the invention the arrangement has members being after said connection of an al- ternating voltage to the phase output adapted to retard the increase of voltage between the two poles of the direct voltage side taking place as a result of said connection with respect to the case of connecting the alternating voltage phase line to the phase output without any influence on the converter. By retard- ing the voltage increase over the two poles of the direct voltage side it will take longer time until full voltage is reached therebetween and the flying capacitor will by this until this takes places be charged to a higher level than otherwise. "Without any influence" is here to be interpreted as if a comparison is made with a case that for the provision of voltage the alternating voltage phase line is connected to the converter in the way it is to be connected thereto in the later operation of the converter once provided with voltage and it is then waited and the operation of the converter is not started until it is entirely provided with volt- age. Furthermore, all connections to and within the converter are during the provision of voltage the same as they will be during the operation.

According to another preferred embodiment of the invention the arrangement comprises members adapted to increase the speed of the charging of the flying capacitor with respect to a connection of the alternating voltage phase line to the phase output without any influence on the converter. By increasing the speed of the charging of the flying capacitor in this way this will have a higher voltage built up thereover when the full voltage is obtained over the two poles of the direct voltage side, so that the voltage over the outer units then get lower than without such members for increasing the charging speed.

It is then particularly advantageous if the two embodiments of the invention last mentioned are combined, so that the speed of the charging of the flying capacitor is increased at the same time as the increase of the voltage between the two poles of the direct voltage side is retarded, so that both a comparatively fast provision of voltage, so that the converter is quickly arriving to normal operation conditions, and an elimination of each risk of excessive peak voltages over said outer units during the provision of voltage may be obtained.

According to another preferred embodiment of the invention, which constitutes a further development of the embodiment last mentioned, said control member is adapted to control turn-on of the semiconductor device of the unit/the units between at least one second midpoint and the direct voltage pole closes thereto during the provision of voltage through connection of an alternating voltage to the phase output when the alternating voltage on the phase output is in such a phase position that a current charging the flying capacitor is generated through the voltage between said second midpoint and the phase output. By starting to immediately during the provision of voltage turn any of said outer units on the time for charging the flying capacitor may be shortened considerably. Another advantage of this embodiment is that no extra equipment is required for obtaining a faster charging of the flying capacitor, but the control members existing for controlling the semiconductor devices of the units in normal operation of the converter may be utilised for this. This design of the control member for a quicker charging of the flying capacitor should, however, be combined with other measures retarding the voltage increase over said outer units, since otherwise the outer unit in question has to be switched (at least be turned-off) at a high voltage (half the voltage between the two poles of the direct voltage side minus the voltage over the flying capacitor).

According to another preferred embodiment of the invention, which constitutes a further development of the embodiment last mentioned, the control member is adapted to control turn-on of the semiconductor device alternatingly of the unit/the units between a second midpoint and one direct voltage pole and of the unit/the units between the opposite second midpoint and the other direct voltage pole during the provision of voltage through connection of an alternating voltage to the phase output during a half period of said alternating voltage each in such a way that a current charging the flying capacitor is generated at each such turn-on. It is then particularly advantageous to design the con- trol member to accomplish a turning on of the semiconductor device of the unit/the units between a said second midpoint and the plus pole of the direct voltage side when the alternating voltage of the phase output is positive and turn the semiconductor device of the unit/the units between the opposite second midpoint and the negative direct voltage pole on when the alternating voltage of the phase output is negative. The voltage to be switched by the outer units made by this is kept down and by that high peak voltages over these units are avoided.

According to another preferred embodiment of the invention the apparatus comprises members adapted to accomplish a switch- ing of the voltage divider provided with a resistance and connected in parallel with each of said units between a state during said provision of voltage with a first resistance value and a state in normal operation of the converter after completed provision of voltage with a substantially higher resistance value, so that the current to the flying capacitor for charging thereof is increased during the provision of voltage with respect to the case if there would be a normal operation resistance of the voltage dividers during the provision of voltage. The time for charging the flying capacitor may by this be shortened considerably, so that a full provision of voltage of the converter may be obtained in a comparatively short time without any risk of occurrence of excessively high peak voltages over the outer units during the provision of voltage.

According to another preferred embodiment of the invention the apparatus comprises members adapted to connect one terminal of the flying capacitor to the plus pole of the direct voltage side and the other terminal of the flying capacitor to ground while breaking the connection between the two terminals of the flying capacitor and said second midpoint after connecting said alternating voltage to the phase output during the provision of voltage, and said members are adapted to switch over said two terminals to be connected to said two second midpoints and open the connection to the plus pole and ground, respectively, after completed provision of voltage. It is by this ensured that the flying capacitor is not switched in between said two second midpoints until after the voltage provision has been carried out, when the flying capacitor has been charged to a substantial voltage level, so that during the very provision of voltage all units-connected in series will share the voltage between the two poles of the direct voltage side and the outer units will not be overloaded.

According to another preferred embodiment of the invention a series connection of an extra capacitor and a rectifying diode is connected between the plus pole of the direct voltage side and the second midpoint next thereto as well as the minus pole of the direct voltage side and the second midpoint located next thereto with the rectifying diode of the series connection con- nected to the plus pole of the direct voltage side with a conducting direction from the plus pole to the second midpoint and the rectified diode of the series connection connected to the minus pole of the direct voltage side with the conducting direction from the second midpoint to the minus pole for fast charging of the flying capacitor through current from the plus pole through the extra capacitor to the flying capacitor or from said phase output to the positive terminal of the flying capacitor and then through the second extra capacitor to the minus pole of the direct voltage side. Said extra capacitor has then preferably a ca- pacitance being somewhat lower than the capacitance of the flying capacitor, and a provision of this arrangement will result in charging the flying capacitor in few periods of the alternating voltage, i.e. with fractions of a second, after connecting the alternating voltage source to the phase output, which may be compared with a couple of minutes, which could be required if the provision of voltage have taken place in the way known before.

According to another preferred embodiment of the invention said means for connecting an alternating voltage to the phase output comprises a first breaker between said phase output and an alternating voltage source and a resistance between the breakers and the phase output connected in series therewith having such a value that it is adapted to substantially retard the provision of voltage of the direct voltage side when closing said breaker, and the apparatus comprises also a second breaker connected in parallel with said resistance and when the voltage over the flying capacitor has reached a predetermined level adapted to be closed and by that make the connection between the alternating voltage source and the phase output to bypass the resistance. It will through this for sure take longer time before the direct volt- age side receives full voltage, but the charging process of the flying capacitor gets more similar to the charging process of the capacitors located on the direct voltage side between the two poles thereof, and upon occurrence of any fault, such as a short circuit, and by that a fault current at said phase output it gets also possible to open the first breaker and by that disconnect said alternating voltage source before any higher voltage may be built up over the two poles of the direct voltage side and by that the risks for damaging components as a consequence of such a fault may be reduced considerably. Thus, the equipment may in this way survive a switching in of the converter when a fault is present. It is pointed out that "alternating voltage source" usually is an alternating voltage network, such as a feeding three-phase alternating voltage network.

According to another preferred embodiment of the invention said means for connecting an alternating voltage to the phase output comprises a series connection of a capacitor and a second breaker connected in parallel with a first breaker for connection of the phase output to an alternating voltage side, and members are during said provision of voltage adapted to first close the second breaker and by that connect the alternating voltage source through the capacitor to the phase output and when a certain voltage level has been built up over the flying capacitor control the first breaker to close and connect the alternating voltage source directly to the phase output. This embodiment has a similar function as the embodiment lastly discussed.

According to another embodiment of the invention the apparatus has a resistance connected in series with the flying capacitor between one of said second midpoint and the one terminal of the flying capacitor and a breaker connected in parallel therewith, and members are adapted to control said breaker to open during the provision of voltage of the converter for building up a voltage substantially just as fast over all units connected in series between the two poles of the direct voltage side and closing the breaker for bypassing the resistance after charging of the flying capacitor above a predetermined value. The different units will by this be charged substantially equally fast, i.e. receive substantially the same voltage increase, while the flying ca- pacitor is charged through said resistance, so that no said peak voltage will occur across the outer units during the provision of voltage. When the voltage across the flying capacitor is sufficiently high the resistance may be disconnected without any risk of overloading the outer units as a consequence of the voltage provision procedure.

According to another preferred embodiment of the invention the apparatus comprises a low voltage source connected to a resonance circuit adapted to convert the low voltage of the source to and by that generate a substantially higher voltage, and said means for connection of an alternating voltage to the phase output comprises a breaker adapted to connect the low voltage

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According to another preferred embodiment of the invention, which constitutes a further development of the embodiment last mentioned, said connecting means comprises a first breaker adapted to be open during the voltage provision and to be closed after completed voltage provision while opening the breaker between the phase output and the low voltage source for connecting the phase output to an alternating voltage source for operation of the converter. By using the low voltage source for the voltage provision and said alternating voltage source for operation of the converter after that is connected to the phase output, it will be possible to check that everything is okay with the circuitry, and said first breaker may then be closed and the alternating voltage source be connected to the phase output with a clean conscience.

According to another preferred embodiment of the invention said first breaker is adapted to be closed when the voltage over the flying capacitor has exceeded a predetermined level, and ac- cording to a further development thereof said level is half the voltage over the flying capacitor once the converter has been provided with voltage. It is by this avoided that a feeding alternating voltage network is connected to the converter before it has been ensured that a considerable part of the voltage on the direct voltage side thereof will be taken by said inner units.

According to another preferred embodiment of the invention the apparatus comprises members adapted to detect possibly fault currents at the phase output of the converter and control the first breaker to connect the alternating voltage source to said phase output if no faults may be detected through said members. It is by this according to above ensured that the alternating voltage source will not at all be switched in to the converter before it has been assured that no faults are present in the cir- cuitry.

It should be mentioned that the embodiments with said low voltage source and the resonance circuit may very well be combined with other embodiments, such as the one relating to the turning on of the outer units during the provision of voltage for obtaining a quicker charging of the flying capacitor^ since a control of these units in such a case may take place simpler and safer, since the direct voltage source may for example be brought to feed in a voltage being half the normal operation voltage between the two poles of the direct voltage side during operation of the converter to said phase output, so that the outer units does not have to be turned-off at an excessively high voltage during charging of the flying capacitor. There are of course also a possibility to charge the flying capacitor in a conventional slow way while using said low voltage source to a certain level and then switch in the regular alternating voltage source, such as an alternating voltage network to the converter.

The invention also relates to methods according to the appended method claims, and the advantages thereof appear without any doubt from the discussion mentioned above of apparatuses according to preferred embodiments of the invention.

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

BRIEF DESCRIPTION OF THE DRAWINGS

With reference to the appended drawings, below follows a de- scription of preferred embodiments of the invention cited as examples.

In the drawings;

Fig 1 is a simplified circuit diagram illustrating the construction of a VSC-converter provided with a flying capacitor of the type for which the present invention aims to improve the provision of voltage,

Fig 2-4 show very schematically different conceivable alternatives according to the invention to design the part of the apparatus within the dashed box in fig 1 , ig 5 illustrates very schematically a part of an apparatus according to another preferred embodiment of the invention,

ig 6 illustrates schematically a part of a converter according to fig 1 provided with an apparatus according to another preferred embodiment of the invention,

Fig 7 and 8 are views corresponding to fig 1 of apparatuses according to further preferred embodiments of the invention, and

Fig 9 illustrates schematically an apparatus according to a still further preferred embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Only the part of a VSC-converter which during the operation thereof is intended to be connected to an alternating voltage source in the form of an alternating voltage phase line is shown in fig 1 , in which the number of phases is normally three, but it is also possible that fig 1 shows the entire converter, when this is connected to a one-phase alternating voltage network. The VSC-converter has four units 1 -4, usually called transistor valves or alternatively thyristor valves, connected in series between the two poles 5, 6 of a direct voltage side of the con- verter. Two capacitors 7, 8 connected in series are arranged between said two poles, and a point 9 therebetween is usually connected to ground, so that in this way the potentials + U/2 and - U/2, respectively, are provided at the respective pole, in which U is the voltage between the two poles 5, 6. The units 1 -4 are each made of a semiconductor device 10-13, of turn-off type, such as an IGBT or a GTO, and a rectifying diode 14-17 connected in parallel therewith, a so-called freewheeling diode. Although only one IGBT or GTO per unit is shown this may stand for an amount of IGBTs or GTOs connected in series and controlled simultaneously, which also is the case, since a comparatively high number of such semiconductor devices are required for holding the voltage to be hold by each unit in the blocking state thereof.

A first midpoint 18 of the series connection between the two units 2 and 3, which constitutes the phase output of the converter, is connected to an alternating voltage phase line 19, through an inductor 20 and a first breaker 21. Said series con- nection is in this way divided into two equal parts with two units 1 , 2 and 3, 4, respectively, of each such part.

A second midpoint 22 between two said units of one part of the series connection is through a flying capacitor 23 connected to a second midpoint 24 corresponding thereto with respect to the phase output of the other part of the series connection.

The apparatus has also a member 25 adapted to control the different semiconductor devices of the units 1-4 and by that en- sure that said phase output is connected to and receives the same potential as the pole 5, the pole 6 or any of said second midpoints 22, 24, which for the midpoint 24 means the potential of the pole 5 minus the voltage over the capacitor 23 and for the midpoint 22, the voltage of the pole 6 plus the voltage over the capacitor 23. This member 25 and the arrangement thereof is here represented very simplifiedly, and a separate such member should in the practise be arranged on high potential for each individual unit and these receive control signals from a control arrangement arranged on ground level. It is also illustrated how different measuring members 26-29 are arranged to measure electric values of different parts of the converter, primarily the magnitude and direction of currents, upon which the control of the converter may be based, espe- cially the provision of voltage thereof, to which the present invention is directed and which will now be discussed.

If the VSC-converter is provided with voltage from the alternating voltage side and then the first breaker 21 is simply closed without taking any particular measures the two capacitors 7 and 8 will almost immediately be entirely charged and full voltage be established between the two poles 5 and 6 of the direct voltage side, but the flying capacitor 23 will only be charged very slowly through the leak current, usually in the order of 50 mA, existing in the voltage dividers (one thereof is very schematically shown in fig 5) which connected in parallel with the respective unit for obtaining a good voltage division between the units during operation of the converter. This means then that the two outer valves 1 , 4 will alone have to take substantially the entire volt- age at the beginning of the provision of voltage, and it may take several minutes before the flying capacitor 23 is entirely charged.

A possibility to approach this problem is to design the control member 25 in the way previously described for alternatingly turn the two outer valves on during the voltage provision for producing a charging current between one of said second midpoints 22, 24 and the phase output 18. The unit 4 may for example be turned on, so that a current charging the flying capacitor 23 flows through the phase output 18, through the diode 15, to the flying capacitor 23 and through the unit 4 to the minus pole 6. Alternatively, the unit 1 may be turned on, so that a current for charging the capacitor 23 flows from the plus pole 5, through the unit 1 , to the flying capacitor 23, further to the second midpoint 24 and through the diode 16 to the phase output 18. The semiconductor devices 10 and 13 are then preferably controlled to turn on so that the maximum voltage load on the units is reduced, more exactly by only turning the semiconductor device 10 on when the alternating voltage on the phase output 18 is in a positive half period and turning the semiconductor device 14 off only when the alternating voltage of the phase output 18 is within a negative half period. It is then possible to control the time for the turn-on within the respective half period so that exactly the charging current and by that the charging speed of the flying capacitor 23 desired is obtained.

An alternative to the embodiment just described is illustrated in fig 2 and this differs from the other embodiments of the invention by the fact that the voltage provision here takes place from the direct voltage side, and in this embodiment a breaker 30 is adapted to connect the phase output 18 to ground during the voltage provision, and the control member 25 is adapted to control turning on of the semiconductor device of the unit 1 or 4 during the voltage provision for generating a current charging the flying capacitor through the voltage difference between the respective second midpoint 22, 24 and the phase output 18. Once the voltage provision is completed the breaker 30 is opened and the breaker 21 is closed.

Fig 3 illustrates very schematically how an apparatus for voltage provision of a VSC-converter of this type may be designed. This apparatus comprises in series with the first breaker 21 between this and the phase output 18 a resistance 31 with a comparatively high resistance value, more exactly with such a value that it is adapted to substantially retard the voltage provision of the direct voltage side when connecting the breaker 21 . It is by this possible to discover possible faults before full voltage is reached on the direct voltage side and by that the first breaker 21 may be opened without occurrence of damage. The flying capacitor has also enough time to be charged to a higher level before full voltage is established on the direct voltage side. A second breaker 32 is connected in parallel with the resistance 31 and adapted to close when the voltage over the flying capacitor reaches a predetermined level and by that make the connection between the alternating voltage source and the phase output to bypass the resistance.

The series connection of a capacitor 33 and a second breaker 34 is in a corresponding way arranged in parallel with the said first breaker 21 for connecting the phase output 18 to an alternating voltage source 19. Members, such as the control member 25, are adapted to first close the second breaker 34 at said provision of voltage and by that connect the alternating voltage source through the capacitor to the phase output and when a certain voltage level has been built up over the flying capacitor control the first breaker 21 to close and connect the alternating voltage source directly to the phase output. The function and the advantages of this embodiment are similar to those of the embodiment according to fig 3.

The converter has as already mentioned before a voltage divider with a resistance connected in parallel with each of the units 1 -4, and this is very schematically illustrated through the resistance 35 in fig 5 for one unit. According to a preferred embodiment of the invention the apparatus comprises members 36 adapted to cause a switching of the voltage divider 37 between a state during said voltage provision with a first resistance value and a state at normal operation of the converter after completed voltage provision with a substantially higher resistance value, which means that the leakage current through the voltage divider will be considerably higher during the voltage provision than otherwise would be the case at the higher resistance value. The leakage current may for example be 0,5-5 A, i.e. an increase by a factor 10-100 with respect to normal operation of the converter. The speed of the charging of the flying capacitor is by that increased. It would then also be possible to design the voltage divider in the way shown in fig 5 with a breaker 38 for shifting between the two positions for optional short-circuiting one part of the resistance 35. However, it would also be possible to design the resistance 35 as a voltage divider being in principal infinitely variable for an optimum control of the charging current of the flying capacitor.

An apparatus according to another preferred embodiment of the invention is schematically illustrated in fig 6, in which only the two inner units 2, 3 of the converter are shown. A resistance 39 is in this embodiment connected in series with a flying capacitor 23 between one of said second midpoints 24 and one terminal 40 of the flying capacitor. A breaker 41 is connected in parallel with the resistance 39. Members, such as the control member 25, are adapted to control the breaker 41 to open when the converter is provided with voltage for building up a voltage sub- stantially just as fast over all the units connected in series between the two poles of the direct voltage side and closing the breaker 41 for bypassing the resistance after charging the flying capacitor above a predetermined value. By arranging such a comparatively considerable resistance 39 in series with the ca- pacitor 23 between the two midpoints 24 and 22 at the provision of voltage a considerable voltage will rapidly be built up also over the two units 2, 3.

An apparatus for providing a VSC-converter with voltage ac- cording to another preferred embodiment of the invention is illustrated in fig 7, in which two switching devices 42, 43 being very schematically indicated are shown. The switching device 42 is adapted to be able to connect the one terminal 44 of the flying capacitor 23 either to the second midpoint 22 or to the plus pole 5 of the direct voltage side, while the other switching device 43 is adapted to be able to connect the other terminal 40 of the flying capacitor 23 either to the second midpoint 24 or ground (the point 9). The two switching devices are, after connection of the alternating voltage to the phase output at the provision of voltage, adapted to connect one terminal 44 of the flying capacitor to the plus pole 5 of the direct voltage side and the other terminal 40 of the capacitor to ground 9, such as illustrated in fig 7, while breaking the connection between the two terminals of the flying capacitor and said second midpoints 22, 24. The flying capacitor will through this at the provision of voltage be charged very fast at the same time as the provision of voltage of the four units 1-4 gets symmetric without any increased stresses on any of the units. The switching devices 42, 43 are also adapted to switch over the two terminals 44, 40 of the flying capacitor 23 after the provision of voltage has been carried out to be connected to the two second midpoints 22 and 24, respectively, and open the connection between these and the plus pole and ground, respectively.

It is illustrated in fig 8 what an apparatus for providing a VSC- converter of the type described with voltage according to another preferred embodiment of the invention may look like. In this apparatus a series connection of an extra capacitor 45, resistance 46 as well as rectifying diode 47 is connected between the respective direct voltage pole 5, 6 and one terminal of the flying capacitor 23. The rectifying diodes 47 are so directed that the one between the plus pole of the direct voltage side and the flying capacitor has its conducting direction towards the flying capacitor, while the other diode has its conducting direction away from the flying capacitor towards the minus pole of the di- rect voltage side. The capacitors 45 are somewhat smaller than the flying capacitor 23. When closing the breaker 21 the flying capacitor will depending upon the current direction in the phase output 18 either be charged through a current flowing from the phase output 18, through the diode 15, to the flying capacitor 23 and further through the series connection of the diode 47', the resistance 46' and the extra capacitor 45', to the minus pole 6, or at a current in the phase output 18 away from the breaker 21 through a current through the extra capacitor 45, the resistance 46, the diode 47, the capacitor 23, the diode 16 to the phase output 18. The flying capacitor 23 will by this be charged entirely within only a couple of periods of the alternating voltage connected to the phase output 18. The resistances 46, 46' are therefor limiting the charging current, for example to 250 A, but it would also be possible to omit them should the diodes 47, 47' be chosen so that they can take sufficiently high currents.

The construction of an apparatus for provision of voltage according to a further, very preferred embodiment of the invention is illustrated in fig 9. Some additional details of the surrounding equipment of the converter have been shown here with respect to the other figures. More exactly, all three phase lines 19, 19', 19" of a three phase alternating voltage line and first breakers 21 , 21 ', 21 " for connecting them to a phase output of a phase leg each of a VSC-converter have been shown, while only one phase leg of the converter is shown in the figure. Furthermore, it is illustrated how a transformer 48 is arranged between the alternating voltage line 19 and the phase output 18, and such a transformer may very well also be arranged in the other embodiments, although it is not shown there. The transformer has primary windings 49 being in delta-connection, while the secon- dary windings 50 are in Y-connection. Furthermore, a low voltage source 51 , which for example may be adapted to generate a voltage in the order of 100-200 V, is connectable to the star point 52 of the Y-connection through a breaker 53 with a variable resistance 54 and a variable inductance 55 arranged there- between. The other end of the low voltage source is grounded. An additional breaker 56 is arranged between ground and the filters 57 belonging to the alternating voltage side of the converter for eliminating harmonics generated on the alternating voltage side as a consequence of the high frequency switchings of the semiconductor devices of the units 1 -4 at said pulse width modulation. The filters are capacitive for the fundamental tone. Finally, a still further breaker 58 is arranged between ground and the midpoint 9 between the two capacitors of the direct voltage side. The function of this apparatus is as follows:

When the converter is to be provided with voltage the breaker 53 is closed, while the breakers 21 are kept open. For an SVC the breaker 58 will be kept open during the voltage provision, but for the HVDC-application there is no breaker 58 but the point 9 is all the time grounded, also during the provision of voltage. If the neutral conductors of the filters are not grounded the breaker 56 is also kept close. The Q-value of the alternating voltage filters 57 is to be high at the fundamental frequency of the alternating voltage. The Q-value of the zero sequence impedance of the transformer has also to be high. By regulating the inductance 55 a zero sequence resonance circuit having a high Q-value may by this be formed, and this circuit consists of the alternating voltage filters 57 capacitive for the fundamental tone, the zero sequence impedance of the transformer as well as the additional inductance 55. This means when connecting the low voltage source 51 that it is possible to charge the filters with a series sequence voltage being much higher than the volt- age of the low voltage source 51 by adjusting the amplitude of the low voltage source, the resonance frequency of the circuit and the Q-value of the circuit, so that the amplitude of the filter voltage may be controlled. The voltage at the connection to the phase output 18 gets then Q x U₅₁ , in which Q is the figure of merit of the circuit and U₅₁ is the voltage from the source 51 . The voltage on the phase output may be regulated either by changing the figure of merit Q with the resistance 54 or by controlling the amplitude of the source 51 . Thus, a sufficiently high voltage for providing the direct voltage side of the converter with voltage is obtained at least until obtaining a considerable proportion, such as for example half the full voltage it has to have at normal operation of the converter at the same time as the flying capacitor 21 may be provided with voltage. It is very schematically illustrated how members 59 are adapted to detect possible faults, such as short-circuits in the circuitry of the converter or in the direct voltage network, so that this is done be- fore any high voltage is connected to the phase output through the alternating voltage network 19. The filter voltage is preferably in this way increased to about 50 % of the level of the full voltage to be present on the alternating voltage side during op- eration of the converter. When the units 1 and 4 have received sufficient voltage and may function these are preferably turned on according to the procedure described before for accelerating the charging of the flying capacitor 23. This is preferably done alternatingly for balancing the charging of the two capacitors 7, 8 which in this way are charged to about 50 % of the full voltage. It is possible to block all units or valves 1-4 and increase the filter voltage further to said full voltage, so that then also the full voltage will be applied between the direct voltage poles over the capacitors 7 and 8. Finally, it is advantageous to remarkably reduce the Q-value by increasing the value of the variable resistance 54 for discharging the alternating voltage filters 57 before the breakers 53, 56 and 58 are opened. The breaker 21 is then closed and the alternating voitage network is switched in to the phase output 18 of the converter before the capacitors 7, 8 and 23 have been discharged.

It is in principal possible to charge also the transmission cables on the direct voltage side of the converter and another converter station of a plant for transmitting electric power through high voltage direct current, while the flying capacitor in the second station is charged through the voltage dividers of the valves or the units.

It would also be possible to ground the zero point of the trans- former when connecting the alternating voltage network through ^{■■ ""} closing the breakers 21 , this could especially be made in an absence of an additional low voltage source 51 , so that the converter has not been charged before through any such additional voltage source before connection of the alternating voltage net- work. If a ground fault would exist in one of the phases the voltage of the direct voltage poles would otherwise be about 2 times as high as normally. Thus, the voltage between the two poles of the direct voltage side would also be two times higher as the normal value. Since the flying capacitor 23 is not charged the two outer units 1 and 4 would then be loaded with this high pole- to-pole-voltage. This problem may be solved by grounding the zero-point of the transformer until the flying capacitor has been charged to a normal voltage.

The invention is of course not in any way restricted to the pre- ferred embodiments described above, but many possibilities to modifications thereof would be apparent to a man skilled in the art, without departing from the basic idea of the invention as defined in the appended claims. Thus, a person with ordinary skill in the art would be able to suggest other ways to co-ordinate the provision of the direct voltage side of the converter with voltage and charging the flying capacitor so as to ensure that the capacitor is charged to a substantial proportion of full charging thereof before the direct voltage side is allowed to receive full voltage between the poles thereof. Finally, it is pointed out that, as already indicated above, it is well possible and may be often also desirable to combine two or more of the embodiments described above with each other.

Claims

Claims:

1 . A method for providing a VSC-converter for converting a direct voltage into an alternating voltage and conversely in a plant for transmitting electric power with voltage, said converter comprising a series connection of at least four units (1 -4) arranged on a direct voltage side of the converter between two poles (5, 6), one being positive and one negative after voltage have been provided thereto, each said unit comprising a semiconductor device of turn-off type and a rectifying diode connected in anti-parallel therewith, an alternating voltage phase line (19) connected to a first midpoint (18), called phase output, of the series connection between two units while dividing the series connection into two equal parts, said two poles of the direct voltage side being intended to be put on substantially the same voltage but with different signs with respect to a zero voltage level of the direct voltage side after said voltage providing, said converter comprising a second midpoint (22) between two said units of one part of the series connection being through a flying capacitor (23) connected to a second midpoint (24) of the other part of the series connection and corresponding thereto with respect to the phase output, and a member (25) being, once the converter has been provided with voltage, adapted to control the semiconductor devices of the unit to generate a train of pulses with determined amplitudes according to a pulse width modulation pattern on the phase output of the converter, characterized in that the provision of the converter with voltage is carried out by providing the direct volt- age side with voltage through connection of an alternating voltage to said phase output (18), and that the provision of the direct voltage side of the converter with voltage and the charging of the flying capacitor are co-ordinated so that the capacitor is charged to a substantial proportion of full charg- ing thereof before the direct voltage side is allowed to receive full voltage between the poles thereof corresponding to the voltage it is intended to have in operation of the converter once provided with voltage.

2. A method according to claim 1 , characterized in that after said connection of an alternating voltage to the phase output

(18) the increase of the voltage between the two poles (5, 6) of the direct voltage side obtained thereby is made dependent upon the voltage over the capacitor.

3. A method according to claim 1 or 2, characterized in that after said connection of an alternating voltage to the phase output (18) the increase of voltage between the two poles (5, 6) of the direct voltage side taking place as a result of said connection is retarded with respect to the case of connecting the alternating voltage phase line (19) to the phase output

(18) without any influence upon the converter.

4. A method according to any of the preceding claims, characterized in that the speed of the charging of the flying ca- pacitor (23) is increased with respect to a connection of the alternating voltage phase line (19) to the phase output (18) without any influence on the converter.

5. A method according to claim 4, characterized in that during provision of voltage through connection of an alternating voltage to the phase output the turn-on of the semiconductor device (10, 13) of the unit/the units between at least one said midpoint (22, 24) and the direct voltage pole (5, 6) closest thereto is controlled when the alternating voltage on the phase output is in such a phase position that a current charging the flying capacitor (23) is generated through the voltage between said second midpoint and the phase output.

6. A method according to claim 5, characterized in that during the voltage provision through connection of an alternating voltage to the phase output during a half period of said alter- nating voltage each the turn-on of the semiconductor device is alternatingly controlled in the unit/the units (1 ) between a second midpoint (22) and one direct voltage pole (5) and in the unit/the units (4) between the opposite second midpoint (24) and the other direct voltage pole (6) in such a way that a current charging the flying capacitor (23) is generated through each such turn-on.

7. A method according to claim 6, characterized in that a turn- on of the semiconductor device (10) of the unit/the units between a said second midpoint (22) and the plus pole (5) of the direct voltage side is accomplished when the alternating voltage of the phase output is positive and the semiconductor device (13) of the unit/the units between the opposite second midpoint (24) and the negative direct voltage pole (6) is turned on when the alternating voltage of the phase output is negative.

8. A method according to claim 7, characterized in that the turn-on times of the semiconductor device (10, 13) of the respective unit/the units is selected during the half period of the alternating voltage in question for controlling the charging speed of the flying capacitor (23).

9. A method according to claim 4, in which the converter has a voltage divider (37) having a resistance (35) connected in parallel with each of said units, characterized in that a switching of the voltage divider between a state during said provision of voltage with a first resistance value and a state in normal operation of the converter after completed provision of voltage with a considerably higher resistance value is accomplished, so that the current to the flying capacitor (23) for charging thereof is increased during the provision of voltage with respect to the case if there had been a normal op- eration resistance of the voltage dividers during the provision of voltage.

0. A method according to claim 4, characterized in that after connection of said alternating voltage to the phase output (18) during the provision of voltage one terminal (44) of the flying capacitor (23) is connected to the plus pole (5) of the direct voltage side and the other terminal (40) of the flying capacitor is connected to ground (9) while interrupting any connection between the two terminals of the flying capacitor and said second midpoints (22, 24) and that after completed provision of voltage said two terminals are switched to be connected to said two second midpoints and the connection to the plus pole and ground, respectively, is opened .

1 . A method according to claim 3, characterized in that when connecting an alternating voltage to the phase output for said provision of voltage this is made through a resistance (31 ) having such a value that it is adapted to substantially retard the provision of voltage of the direct voltage side, and that when the voltage over the flying capacitor has reached a predetermined level the connection between the alternating voltage source (19) and the phase output (18) is made to bypass said resistance.

12. A method according to claim 3, characterized in that when connecting an alternating voltage to the phase output for said provision of voltage this is made firstly through a capacitor (33) between the alternating voltage source and the phase output, and that when a certain voltage level has been built up over the flying capacitor (23) the alternating voltage source is directly connected to the phase output while bypassing the capacitor between the alternating voltage source and the phase output.

13. A method according to claim 3, characterized in that in said provision of voltage of the converter a resistance (39) is switched in in series with the flying capacitor (23) between one of said second midpoints (24) and one terminal (40) of the flying capacitor for building up a voltage substantially just as fast over all units connected in series between the two poles of the direct voltage side, and that after charging the flying capacitor above a predetermined value said one terminal of the capacitor is directly connected to the second midpoint in question while bypassing the resistance.

14. A method according to any of the preceding claims, charac- terized in that a low voltage source (51 ) connected to a resonance circuit adapted to convert the low voltage of the source to and by that generate a substantially higher voltage is connected to said phase output (18) for the provision of voltage.

15. A method according to claim 14, characterized in that a first breaker (21 ) is kept open during the provision of voltage and is closed after completed provision of voltage while opening a connection between the phase output (18) and the low voltage source (51 ) for connecting the phase output to an alternating voltage source (19) for operation of the converter.

16. A method according to claim 15, characterized in that the first breaker (21 ) is closed when the voltage over the flying capacitor has exceeded a predetermined level.

17. A. method according to claim 15 or 16, characterized in that the current of the phase output (18) of the converter is measured during the provision of voltage for detecting possi- bly fault currents, and that the first breaker (21 ) is closed for connecting the alternating voltage source to said phase output if no faults may be detected through said current measurement.

18. An apparatus for providing a VSC-converter for converting a direct voltage into an alternating voltage and conversely in a plant for transmitting electric power with voltage, said converter comprising a series connection of at least four units (1-4) arranged on a direct voltage side of the converter between two poles (5, 6), one being positive and one negative after voltage have been provided thereto, each said unit comprising a semiconductor device of turn-off type (10-13) and a rectifying diode (14-17) connected in anti-parallel therewith, an alternating voltage phase line (19) connected to a first midpoint (18), called phase output, of the series con- nection between two units while dividing the series connection into two equal parts, said two poles of the direct voltage side being intended to be put on substantially the same voltage but with different signs with respect to a zero voltage level of the direct voltage side after said voltage providing, said converter comprising a second midpoint (22) between two said units of one part of the series connection being through a flying capacitor (23) connected to a second midpoint (24) of the other part of the series connection corresponding thereto with respect to the phase output, and a member (25) being, once the converter has been provided with voltage, adapted to control the semiconductor devices of the units to generate a train of pulses with determined amplitudes according to a pulse width modulation pattern on the phase output of the converter, characterized in that it com- prises means for connection of an alternating voltage to said phase output for providing the converter with voltage through providing the direct voltage side thereof with voltage and an arrangement adapted to co-ordinate the provision of voltage of the direct voltage side of the converter and the charging of the flying capacitor (23) as a consequence of the connection of the alternating voltage to the phase output (18) by accomplishing charging of the capacitor to a substantial proportion of full charging thereof before the direct voltage side receives full voltage between the poles thereof corresponding to the voltage it is intended to have in operation of the converter once provided with voltage.

19. An apparatus according to claim 18, characterized in that the arrangement comprises members being after said connection of an alternating voltage to the phase output adapted to make the increase of the voltage between the two poles

(5, 6) of the direct voltage side obtained therethrough dependent upon the voltage over the capacitor (23).

20. An apparatus according to claim 18 or 19, characterized in that the arrangement has members being after said connection of an alternating voltage to the phase output adapted to retard the increase of voltage between the two poles (5, 6) of the direct voltage side taking place as a result of said connection with respect to the case of connecting the alternating voltage phase line to the phase output without any influence on the converter.

21 . An apparatus according to any of claims 18-20, characterized in that the arrangement comprises members adapted to increase the speed of the charging of the flying capacitor with respect to a connection of the alternating voltage phase line to the phase output without any influence on the converter.

22. An apparatus according to claim 21 , characterized in that said control member (25) is adapted to control turn-on of the semiconductor device (10, 13) of the unit/the units (1 , 4) between at least one second midpoint (22, 24) and the direct voltage pole (5, 6) closest thereto during the provision of voltage through connection of an alternating voltage to the phase output when the alternating voltage on the phase output is in such a phase position that a current charging the flying capacitor is generated through the voltage between said second midpoint and the phase output.

3. An apparatus according to claim 22, characterized in that the control member is adapted to control turn-on of the semiconductor device alternatingly of the unit/the units between a second midpoint (22) and one direct voltage pole (5) and of the unit/the units between the opposite second midpoint (24) and the other direct voltage pole (6) during the provision of voltage through connection of an alternating voltage to the phase output during a half period of said alternating voltage each in such a way that a current charging the flying capaci- tor is generated at each such turn-on.

24. An apparatus according to claim 23, characterized in that the control member is adapted to accomplish turn-on of the semiconductor device (10) of the unit/the units between a said second midpoint (22) and the plus pole (5) of the direct voltage side when the alternating voltage of the phase output is positive and turn the semiconductor device (13) of the unit/the units between the opposite second midpoint (24) and the negative direct voltage pole (6) on when the alternating voltage of the phase output (18) is negative.

25. An apparatus according to claim 24, characterized in that the control member (25) is adapted to select turn-on times of the semiconductor device of the respective unit/units during the half period of the alternating voltage in question for controlling the charging speed of the flying capacitor.

26. An apparatus according to claim 21 , in which the converter has a voltage divider (37) with a resistance (35) connected in parallel with each of said units, characterized in that it comprises members (36) adapted to accomplish a switching of the voltage divider between a state during said provision of voltage with a first resistance value and a state in normal operation of the converter after completed provision of volt- age with a substantially higher resistance value, so that the current to the flying capacitor for charging thereof is in- creased during the provision of voltage with respect to the case if there would be a normal operation resistance of the voltage dividers during the provision of voltage.

27. An apparatus according to claim 26, characterized in that said members (36, 38) for switching the voltage divider (37) is adapted to provide a possibility to an infinite variation of the resistance value of the voltage divider for controlling the charging of the flying capacitor.

28. An apparatus according to claim 21 , characterized in that it comprises members (42, 43) adapted to connect one terminal (44) of the flying capacitor to the plus pole (5) of the direct voltage side and the other terminal (40) of the flying capaci- tor to ground while breaking the connection between the two terminals of the flying capacitor and said second midpoint (22, 24) after connecting said alternating voltage to the phase output during the provision of voltage, and that said members are adapted to switch over said two terminals to be connected to said two second midpoints and open the connection to the plus pole and ground, respectively, after completed provision of voltage.

29. An apparatus according to claim 21 , characterized in that a series connection of an extra capacitor (45) and a rectifying diode (47) is connected between a plus pole (5) of the direct voltage side and the second midpoint (22) next thereto and the minus pole (6) of the direct voltage side and the second midpoint (24) located next thereto with the rectifying diode of the series connection connected to the minus pole of the direct voltage side with a conducting direction from the plus pole to the second midpoint and a rectifying diode of the series connection connected to the minus pole of the direct voltage side with the conducting direction from the second midpoint to the minus pole for fast charging of the flying capacitor (23) through current from the plus pole through the extra capacitor to the flying capacitor or from said phase output to the positive terminal of the flying capacitor and then through the second extra capacitor to the minus pole of the direct voltage side.

30. An apparatus according to claim 29, characterized in that each series connection of rectifying members and extra capacitors has also a resistance (46) connected in series therewith.

31. An apparatus according to claim 20, characterized in that said means for connection of an alternating voltage to the phase output comprises a first breaker (21 ) between said phase output (18) and an alternating voltage source (19) and a resistance (31 ) connected between the breaker and the phase output in series therewith having such a value that it is adapted to substantially retard the provision of voltage of the direct voltage side when closing said breaker, and that it comprises a second breaker (32) connected in parallel with said resistance and when the voltage over the flying capacitor has reached a predetermined level adapted to close and by that switch the connection between the alternating voltage source and the phase output to bypass the resistance.

32. An apparatus according to claim 20, characterized in that said means for connection of an alternating voltage to the phase output comprises a series connection of a capacitor (33) and a second breaker (34) connected in parallel with a first breaker (21 ) for connection of the phase output to an alternating voltage side, and that members are during said provision of voltage adapted to first close the second breaker and by that connect the alternating voltage source through the capacitor to the phase output and when a certain voltage level has been built up over the flying capacitor control the first breaker to close and connect the alternating voltage source directly to the phase output.

3. An apparatus according to claim 20, characterized in that it has a resistance (39) connected in series with the flying capacitor (23) between one of said second midpoints (24) and the one terminal (40) of the flying capacitor and a breaker

(41 ) connected in parallel therewith, and that members are adapted to control said breaker to open during the provision of voltage of the converter for building up a voltage substantially just as fast over all units connected in series between the two poles of the direct voltage side and closing the breaker for bypassing the resistance after charging the flying capacitor above a predetermined value.

34. An apparatus according to any of claims 18-33, character- ized in that it comprises a low voltage source (51 ) connected to a resonance circuit adapted to convert the low voltage of the source to and by that generate a substantially higher voltage, and that said means for connection of an alternating voltage to the phase output comprises a breaker (53) adapted to connect the low voltage source through the resonance circuit to the phase output (18) for the provision of voltage.

35. An apparatus according to claim 34, characterized in that said connecting means comprises a first breaker (21 ) adapted to be open during the voltage provision and to be closed after completed voltage provision while opening the breaker (53) between the phase output and the low voltage source for connecting the phase output to an alternating voltage source for operation of the converter.

36. An apparatus according to claim 35, characterized in that said first breaker (21 ) is adapted to be closed when the voltage over the flying capacitor (23) has exceeded a predeter- mined level.

37. An apparatus according to claim 36, characterized in that said first breaker (21 ) is adapted to be closed when the voltage over the flying capacitor (23) has exceeded half the voltage thereover once the converter has been provided with voltage.

38. An apparatus according to any of claims 35-37, characterized in that it comprises members (59) adapted to detect possibly fault currents of the phase output (18) of the con- verter and to control the first breaker (21 ) to connect the alternating voltage source to said phase output if no faults may be detected through said members.

39. An apparatus according to any of claims 34-38, character- ized in that the resonance circuit comprises a transformer

(48) with one side in Y-connection and connectable to the low voltage source (51 ) through a variable resistance (54) and a variable inductance (55), and that this transformer side is connected to the phase output (18).

40. An apparatus according to claim 39, characterized in that the other side of the transformer is in delta-connection and connected to the alternating voltage source (19) through one first breaker (21 ) per phase.

41. An apparatus according to any of claims 18-40, characterized in that it is adapted for providing a converter comprising a series connection of 2n said units with voltage, n being an integer > 3, and (2n-2)/2 couples of said second midpoints located in corresponding positions with respect to the phase output on opposite sides thereof are connected to each other through a flying capacitor.

42. An apparatus according to any of claims 18-41 , character- ized in that said semiconductor devices are (10-13) IGBTs

(Insulated Gate Bipolar Transistor).

43. An apparatus according to any of claims 18-41 , characterized in that said semiconductor devices (10-13) are GTOs (Gate Turn-off Thyristor).

44. An apparatus according to any of claims 18-43, characterized in that it is adapted for providing a VSC-converter with voltage through connection to an alternating voltage network with a voltage of between 10 kV and 300 kV.

45. An apparatus according to any of claims 18-43, characterized in that it is adapted for providing a VSC-converter connected to a direct voltage network for high voltage direct current (HVDC) with voltage.

46. An apparatus according to any of claims 18-44, characterized in that it is adapted for providing a VSC-converter of a SVC (Static Var Compensator) with voltage.

47. An apparatus for providing a VSC-converter for converting a direct voltage into an alternating voltage and conversely in a plant for transmitting electric power with voltage, said converter comprising a series connection of at least four units (1 -4) arranged on a direct voltage side of the converter be- tween two poles (5, 6), one being positive and one negative after voltage have been provided thereto, each said unit comprising a semiconductor device (10-13) of turn-off type and a rectifying diode (14-17) connected in anti-parallel therewith, an alternating voltage phase line (19) connected to a first midpoint (18), called phase output, of the series connection between two units while dividing the series connection into two equal parts, said two poles of the direct voltage side being intended to be put on substantially the same voltage but with different signs with respect to a zero voltage level of the direct voltage side after said voltage providing, said converter comprising a second midpoint (22) between two said units of one part of the series connection being through a flying capacitor (23) connected to a second midpoint (24) of the other part of the series connection corresponding thereto with respect to the phase output, and a member (25) being, once the converter has been provided with voltage, adapted to control the semiconductor devices of the units to generate a train of pulses with determined amplitudes according to a pulse width modulation pattern on the phase output of the converter, in which the converter is pro- vided with voltage from the direct voltage side thereof by connecting a direct voltage on the direct voltage side of the converter to said phase output, characterized in that it comprises members (30) adapted to connect said phase output (18) to ground during the voltage provision of the converter, that said control member (25) is adapted to control turn-on of the semiconductor device of the unit/the units between at least one second midpoint (22, 24) and the direct voltage pole next thereto during the provision of voltage for generating a current charging the flying capacitor (23) through volt- age differences between said second midpoint and the phase output, and that said member (30) is adapted to disconnect the phase output from the connection to ground when the provision of the converter with voltage is completed.