FIELD OF THE INVENTION AND PRIOR ART The invention relates to a device for conversion of alternating voltage into direct voltage and conversely, which comprises a series connection of at least four units arranged on a direct voltage side of the device between two poles thereof, one posi- tive and one negative, each of said units comprising a semicon- ductor device of turn-off type and a diode connected in anti-par- allel therewith, an alternating voltage phase line connected to a first midpoint, called phase output, of the series connection be- tween two units while dividing the series connection into two equal parts, said two poles of the direct voltage side being pro- vided with substantially the same voltage but with different signs with respect to a zero voltage level of the direct voltage side, said device comprising a second midpoint between two said units of one of the parts of the series connection through a fly- ing capacitor connected to a corresponding second midpoint of the other part of the series connection with respect to the phase output, and an arrangement for controlling the semiconductor devices of the units to generate a train of pulses having deter- mined amplitudes according to a pulse width modulation pattern on the phase output of the device by alternatingly connecting the alternating voltage phase line to at least the plus pole of the direct voltage side, the minus pole thereof and each of said sec- ond midpoints by making the unit/units between another second

midpoint and the direct voltage pole closest thereto and the unit/units between the second midpoint in question and the first midpoint conducting, so as to give the phase output a voltage level corresponding to a sum of the voltage of said closest direct voltage pole and the voltage across the flying capacitor, as well as a method for controlling such a device.

Such devices may be used in all kinds of situations, in which 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 a direct voltage and this is then converted into an alternating voltage, as well as in SVC: s (Static Var Compensator), in which the direct voltage side consists of one or more capacitors hanging freely.

The invention is not restricted to any voltage or power levels, but it is particularly suited for voltages on the direct voltage side between 10 and 500 kV.

An advantage of using such so called multi level converters, i. e. converters in which at least three different voltage levels may be "delivered"to 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 ob- taining an alternating voltage on the alternating voltage phase line having a determined frequency and quality, so that the losses of the converter device may be reduced considerably.

More exactly, the switching frequency of the semiconductor de- vices in a three level converter may under said conditions be reduced to about 1/3. An advantage of using so called flying ca- pacitors for obtaining additional voltage levels of the phase out- put besides the voltage level of the two poles of the direct volt- age side with respect to a use of so called clamping diodes, is

primarily that the semiconductor devices in the latter case have to be controlled in such a way that there will be a non-uniform distribution of switching losses thereon, so that in the practice all semiconductor devices have to be dimensioned for being able to take the maximum load possibly occurring for a separate semiconductor device, since otherwise particular considerations have to be taken to the design of each individual semiconductor device when controlling them. This makes the total cost of the semiconductor devices very high, since some of them will in most operation situations be heavily over-dimensioned. By using flying capacitors instead, such as in the device defined in the introduction, a multi level converter with the possibility to a more even load on the semiconductor devices with respect to switch- ing losses is obtained without using expensive so called clamp- ing diodes or additional semiconductor devices.

A device of the type defined in the introduction is already known through US 5 737 201. The different semiconductor devices are in this device controlled with a fixed frequency according to a fixed pulse width modulation pattern, in which said units con- nected in series are seen as a number of two level bridges added to each other. Such a fixed switching pattern with a fixed switching frequency functions well under the condition that the conditions are constant, but things may in the practice look dif- ferent. The load on said alternating voltage phase line may for example suddenly be changed remarkably or other irregularities may occur, which means that especially the levels reached by the voltage on the phase output when connecting any of said second midpoints may deviate substantially from the desired value by the fact that said flying capacitor has been charged or discharged too much. This means that the capacitor has to be over-dimensioned for being able to take such asymmetrical loads. There is also a risk that the converter has to be blocked during a period of time when such variations occur. Further- more, disturbances, such as harmonics, are generated on the alternating voltage phase line, should the voltage across said

flying capacitor deviate from a desired substantially constant level, Devices of the type defined in the introduction are also known through US 5 706 188 and US 5 940 285, in which the current in said alternating voltage phase line is measured and conducting intervals for the different semiconductor devices and their posi- tion in time are made dependent upon the detected current. This is made for avoiding the problems of the control according to a fixed switching pattern of the device firstly discussed, but this control seems to be unnecessarily complicated and it is neither sufficiently flexible and adaptable to varying conditions.

SUMMARY OF THE INVENTION The object of the present invention is to provide a device and a method of the type defined in the introduction, which makes it possible to benefit from the advantages of such a device by simple means while simultaneously solving said problems of previously known such devices and methods to a large extent.

This object is according to the invention obtained by providing such a device, which comprises members adapted to measure the direct voltage between the two poles of the direct voltage side, members adapted to measure the voltage across said fly- ing capacitor, members adapted to detect the current direction in said alternating voltage phase line and means adapted to compare the values measured for the direct voltage and the voltage across the capacitor, and the arrangement is, when connecting any of said second midpoints to the phase output for establishing a predetermined voltage level therefor, adapted to make the choice of second midpoint connected to the phase output and thereby which of said units are to be made conduct- ing dependent upon said comparison and the current direction in the alternating voltage phase line so as to cause charging or discharging of the capacitor depending upon this comparison, as

well as a method according to the appended independent method claim.

The invention differs fundamentally from the devices and the methods for conversion already known, in which the semicon- ductor devices of the device are controlled with a fixed fre- quency, by instead making the control dependent upon the volt- age across the capacitor, i. e. the charging degree thereof ; with respect to the direct voltage side between the two poles of the direct voltage side, so that when there is a possibility to obtain a predetermined voltage level on said phase output by switching the semiconductor device in different ways the way being most appropriate in the given situation may be utilized. This means a high flexibility of the conversion method, so that it may be adapted to varying conditions of the converter device and lines connected thereto, for obtaining a desired level of the voltage across the capacitor and avoiding disturbances generated as a consequence of the conversion all the time. This means a pos- sibility to continuously consider the operation conditions pre- vailing to an optimum.

According to a preferred embodiment of the invention the ar- rangement is, when a relation of the capacitor voltage : the direct voltage resulting from said comparison is above a first limit level, adapted to chose the following connection of one of said second midpoints to the phase output for establishing said pre- determined voltage level aiming at discharging the flying ca- pacitor at this connection and, when a relation of the capacitor voltage : the direct voltage is below a second limit level results, to choose the following connection of one of said second mid- points to the phase output for establishing the predetermined voltage level aiming at charging the-flying capacitor at this con- nection. By proceeding in this way when choosing said second midpoint for connection to the phase output for establishing said predetermined voltage level the voltage across the flying ca- pacitor may continuously be controlled towards a desired level

located between said two limit levels independently of occur- rence of variations of the electric conditions of the converter de- vice.

According to another preferred embodiment of the invention, which constitutes a further development of the embodiment last mentioned, the arrangement is, when relations of the capacitor voltage : the direct voltage resulting from said comparison is lo- cated between said first and second limit levels, adapted to control said units so that said second midpoints corresponding to the same predetermined voltage level are connected to the phase output just as often when a predetermined voltage level corresponding to such a connection is to be established thereon.

By having a predetermined control frequency with respect to the connection of said second midpoints to the phase output as long as the capacitor voltage : the direct voltage is located between said limit levels, it is possible both to simplify the control of the arrangement of the semiconductor devices and to efficiently keep said relation between said two limit levels without any re- quirement of any control measure at substantially constant op- eration conditions, and control measures only have to be taken when the operation conditions are changed considerably. In the case of two second midpoints corresponding to the same pre- determined voltage level of the phase output these are accord- ingly connected thereto every second time.

According to another preferred embodiment of the invention the device comprises members adapted to predict the direction of the current in the alternating voltage phase line the next time one of said second midpoints is to be connected to the phase output for establishing said predetermined voltage level, and the arrangement is adapted to make said choice dependent upon said predicted current direction in the alternating voltage phase line. By such a prediction the probability is increased that the current direction in the alternating voltage phase line is really causing the charging and discharging, respectively, of said fly-

ing capacitor asked for when connecting a said second midpoint to the phase output. Such a prediction is important, since it may be difficult to obtain such a time resolution that the current di- rection may be measured exactly in connection with the connec- tion of a said second midpoint to the phase output and this con- nection may be made dependent upon the measurement result obtained. However, such a prediction may find a remedy thereto.

According to another preferred embodiment of the invention the arrangement is, when a predetermined third level is exceeded or a predetermined fourth level of the relation of the capacitor volt- age: the direct voltage is underpassed, when connecting one of said second midpoints to the phase output, adapted to control said units to switch over to immediately connect another second midpoint to the phase output instead. The flying capacitor may hereby be protected, so that it may be made smaller and/or a blocking of the converter device as a consequence of a too large deviation of the relation the capacitor voltage : the direct voltage from the value desired may be avoided.

According to another preferred embodiment of the invention the arrangement is, when a change of the direction of the current in the alternating voltage phase line has been measured, adapted to cause an extra commutation by replacing the one of second midpoints then connected to or going to be connected to the phase output by another second midpoint. The result of the control of the converter device may hereby be improved further, since an optimum choice of the current direction through the ca- pacitor may be made on both sides of said direction change and it is avoided that"the wrong"midpoint is connected to the phase output when the current direction is changes.

According to another preferred embodiment of the invention the arrangement is adapted to control the semiconductor devices of the units according to a pulse width modulation pattern for gen- erating a voltage containing a fundamental tone having substan-

tially the same frequency as the alternating voltage of said al- ternating voltage phase line.

According to another preferred embodiment of the invention the number of said units is four and said predetermined voltage level is substantially zero, and the arrangement is adapted to make said choice between making the outer unit of one part with respect to said phase output and the inner unit of the other part of the series connection conducting or conversely. The device and the method according to the invention are particularly well suited for such a so called three-level converter device, since this will only have one flying capacitor and said choice may then all the time be made so that the voltage across this capacitor is kept at a desired, substantially constant level. The control gets more complicated when more levels than three and thereby more than one flying capacitor are present, since in some cases the control with respect to the connection of second midpoints has to be made so that a charging or discharging of the capaci- tor judged to have the worst state takes place while considering considerably more parameters than in the three-level case.

However, under certain conditions such an embodiment having more than three possible levels of the voltage on the phase out- put could be well suited.

Further advantages as well as advantageous features of the in- vention 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 ex- amples.

In the drawings:

Fig 1 is a simplified circuit diagram illustrating a device accord- ing to a first preferred embodiment of the invention, Fig 2 illustrates how a three-level converter device of the type shown in Fig 1 is conventionally controlled through a sinusoidal pulse width modulation, Fig 3 is a graph illustrating how different voltages and currents are changed over time when controlling the converter device ac- cording to Fig 1 according to the invention, Figs 4a and 4b are graphs illustrating how the current through the flying capacitor of the device according to Fig 1 is controlled depending upon measured values of the relation capacitor volt- age: direct voltage, and Fig 5 is a simplified view corresponding to Fig 1 of a converter device according to a second preferred embodiment of the in- vention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION Only the part of the converter device being connected to one phase of an alternating voltage phase line is shown in Fig 1, whereas the number of phases is normally three, but it is also possible that this constitutes the entire converter device, when this is connected to a one-phase alternating voltage network.

The converter device is a so called VSC-converter, which 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 device. Two capacitors 7,8 con- nected 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 pro-

vided 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 anti-parallel therewith, a so called free- wheeling diode. Although only one IGBT or GTO per unit is shown, this may stand for a plurality of IGBT : s or GTO: s- con- nected in series and controlled simultaneously, which is also the case, since a comparatively high number of such semiconductor devices is required for taking the voltage to be taken by each unit in the blocking state.

A first midpoint 58 of the series connection between the two units 2 and 3, which constitutes the phase output of the con- verter, is connected to an alternating voltage phase line 18 through an inductor 19. Said series connection is in this way di- vided into two equal parts having two units 1,2 and 3,4, re- spectively, of each such part. The alternating voltage phase line is summarized through the symbols for an impedance 20 and an alternating voltage generator 21.

A second midpoint 24 between two said units of one of the parts of the series connection is through a flying capacitor 25 con- nected to a corresponding, with respect to the phase output, second midpoint 26 of the other part of the series connection.

The device has also an arrangement 27 adapted to control the different semiconductor devices of the units 1-4 and thereby 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 24,26, which for the midpoint 24 means the potential of the pole 6 with a voltage across the capacitor 25 added thereto and for the midpoint 26 the voltage of the pole 5 minus the voltage across the capacitor 25. This arrangement 27 and the provision thereof is here shown in a very simplified manner,

and a separate such arrangement may probably in the practice be arranged on high potential at each individual unit and these receive control signals from a control arrangement located on ground level.

The device has further members 28 schematically indicated and adapted to measure the direct voltage between the two poles of the direct voltage side (here indirectly by measuring the direct voltage across the capacitor 7 and thereby half said direct volt- age) and members 29 adapted to measure the voltage across said flying capacitor 25. Moreover, there are members 30 adapted to detect the current direction in the alternating voltage phase line. Said members 28-30 are adapted to send informa- tion about their measurement results to the arrangement 27, with which means 31 are associated for comparing the values measured for the direct voltage and the voltage across the ca- pacitor. The arrangement 27 is adapted to make the control of the semiconductor devices of the units 1-4 dependent upon said comparison of the direct voltage and the voltage across the ca- pacitor as well as the current direction detected in the alternat- ing voltage phase line in a way to be explained below. The de- vice may also be provided with members 62 for measuring the current through the capacitor.

It is schematically illustrated in Fig 2 how a pulse width modula- tion in a conversion of a converter device according to Fig 1 typically takes place. Two specific triangle waves 32,33 with an amplitude made dependent upon the direct voltage and adjusted to be substantially half of the direct voltage between the two poles of the direct voltage side are applied so that a triangle wave 32 varies between a zero level 34 and a level corre- sponding to the positive half of the direct voltage, while the other triangle wave varies between the zero level and the nega- tive half of the direct voltage. The frequency of the triangle waves is a multiple of the frequency of a desired reference alter- nating voltage 35 on the alternating voltage phase line 18, which

for example may have a frequency of 50 Hz, while the frequency of the triangle waves may be about 1 kHz. The pulse width modulation is carried out so that the intersecting points between the reference alternating voltage and any of the two triangle waves are determined, so that the pulses 36 with a duration between two consecutive such intersecting points and with an amplitude of substantially half said direct voltage, positive if the reference alternating voltage is located above the positive tri- angle wave, negative if it is located below the negative triangle wave and substantially on zero level if the reference alternating voltage is located therebetween, is delivered on said phase out- put.

+U/2 may be obtained on the phase output by making the two units 1,2 conducting by turning the semiconductor devices 10 and 11 on, while-U/2 may be connected to the phase output when the arrangement 27 makes the two units 3,4 conducting by turning the semiconductor devices 12 and 13 on. However, a voltage being substantially zero may be obtained on the phase output in two different ways, namely either by connecting the second midpoint 24 to the phase output, which is made by en- suring through the arrangement 27 that the units 2 and 4 are conducting or by connecting the second midpoint 26 to the phase output by ensuring through the control arrangement that the units 1 and 3 are conducting. Exactly the presence of a pos- sibility to choose between these two alternatives when there is a desire to have a zero voltage on the phase output and making this choice dependent upon the voltage across the flying ca- pacitor in relation to the direct voltage of the direct voltage side of the converter device and the direction of the current of the alternating voltage phase line 18 for obtaining a charging or dis- charging of the flying capacitor 25, is what the present invention is based upon and this will now be explained more in detail while making reference to Fig 3.

It is illustrated more in detail in Fig 3 how a converter device according to the invention and according to Fig 1 is controlled and how this influences the different voltages and currents of the device over the time t. A sampled, i. e. a step-wisely chang- ing, reference voltage 35 is drawn here, and when this inter- sects a triangle wave changes of the potential of the phase out- put according to the way to proceed described above take place.

The line 37 shows the development of the voltage put on said phase output, the line 38 the current of the alternating voltage phase line, the line 39 the current through the flying capacitor 25 and the line 40 the voltage across the flying capacitor, in which this is compared with half the direct voltage 41 between the poles 5 and 6. The line 64 shows the voltage between the point P and the point 9 in Fig 1.

It appears that the first time a"zero pulse"42 is to be delivered on the phase output this is made while charging the flying ca- pacitor, i. e. by turning the semiconductor device 10 in the unit 1 on and let the phase current go from the flying capacitor to the phase output through the diode 16 in the unit 3. When then the next zero pulse 43 is to be delivered this is achieved according to the second alternative according to a procedure already de- termined in advance to alternate between the two possibilities offered. This means that the flying capacitor 25 is discharged (see the current curve 39) by turning the semiconductor device 11 of the unit 2 on and the phase current then flows through the diode 17 of the unit 4, the flying capacitor 25 and the semicon- ductor device 11 of the unit 2. A comparison between the volt- age 40 across the flying capacitor and the level 41 this should have, i. e. half the voltage between the poles 5,6, is simultane- ously carried out by the means 31, and it is then determined that the voltage across the capacitor is too high, so that the next time a zero pulse 44 is to be delivered on the phase output a control procedure is carried out, so that this zero pulse is ob- tained in the same way as the just preceding zero pulse and

thereby a continued discharging of the flying capacitor and thereby an approaching of the voltage 40 thereacross to the op- timum level 41 takes place in this state.

The control principle according to the invention is very sche- matically illustrated in Figs 4a and 4b, and it is illustrated how the current 39 through the flying capacitor is developed over time t depending upon the magnitude of the voltage 40 across the flying capacitor with respect to an upper, first limit level 60 and a lower, second limit level 61. When the voltage 40 is lo- cated between said two limit levels a"zero pulse"is accom- plished every second time on the phase output by controlling the semiconductor devices so that the capacitor is charged and every second time so that it is discharged, in which the direction of the phase current measured by the member 30 has of course to be considered when determining which valves 1-4 are to be opened. However, when the voltage 40 exceeds the upper limit level 60, the arrangement 27 chooses the consecutive connec- tion of a said second midpoint to the phase output so that the flying capacitor is discharged, independent of whether such dis- charging was on the line or not, and the reverse condition pre- vails when underpassing the lower limit level 61. It is shown in Fig 4a how the capacitor is discharged several times in a row because the voltage 40 is located below the level 61.

A predetermined third level 62 and a predetermined fourth level 63 of the relation of the capacitor votage: the direct voltage are also drawn, which are there for avoiding that this relation will deviate too much from the desired relation. More exactly, the arrangement is, when the third level 62 is exceeded when con- necting one of the second midpoints to the phase output, adapted to control said units to switch over to immediately connect another second midpoint to the phase output instead.

The corresponding is valid when underpassing the fourth level 63, and it is illustrated in Fig 4b how such an immediate switch- over takes place at the time T1.

The invention is particularly well suited for three-level convert- ers according to Fig 1, but it is applicable to all types of multi- level converters, and what it may then look like is schematically illustrated in Fig 5 in the form of a four-level converter. Thus, the number of units of a converter device according to the in- vention is 2n, in which n is an integer > 2, and n+1 possible lev- els at the phase output is achieved therefor. There are (n-2)/2 couples of said second midpoints located in corresponding posi- tions with respect to the phase output on opposite sides thereof and connected to each other through a flying capacitor in such a converter device. Thus, in the case shown in Fig 5 there are two couples 44,45 and 46,47 of second midpoints having each a flying capacitor 48 and 49, respectively. The levels +U/2, +U/6,- U/6 and-U/2 may be obtained on the phase output of the alter- nating voltage phase line 18 in a converter device of this type, in which the two intermediate levels may be obtained in different ways, and it is here that the control according to the invention is introduced by measuring the voltage across the two capacitors (members 50,51), the voltage between the two poles of the di- rect voltage side and the direction of the phase current in the alternating voltage phase line 18 and compare the voltage levels measured across the capacitors with desired reference voltage levels so as to choose said control so that preferably the ca- pacitor being with respect to percentage most far away from the desired voltage level is charged or discharged when delivering the voltage pulse in question on the phase output. +U/6 may for example be obtained on the phase output either by making the units 52,53 and 55 conducting or by making the units 57,53 and 54 conducting.-U/6 may in the same way be achieved ei- ther by making the units 56,57 and 54 conducting or the units 52,56 and 55 conducting. Accordingly, there is each time a choice between discharging one of the flying capacitors or charging the other flying capacitor. Thus, it is here not two mid- points belonging to the same couple that correspond to the same predetermined voltage level on the phase output.

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

Even if it is shown above that the converter device is applied where a substantially sinusoidal current is produced on an alter- nating voltage phase line, it could very well be applied where the current has a totally different appearance and sometimes may vary very heavily. The design according to the invention of the converter device makes it possible also to manage such situations, by using only one of two possible states at a given desired voltage level out on the phase output in a series of switchings, when the current changes heavily, so as to bring the voltage over the flying capacitor in question back to the desired level. Nothing like that is possible in the devices already known with controlling the valves included therein with fixed frequen- cies.

The different measurements of voltages and currents of the de- vice according to the invention may of course be made in an- other way and on other locations than shown in the Figures, which are only there for illustrating the principle according to the invention. The direct voltage between the poles may for example just as well be measured directly and not by measuring the volt- age across a capacitor and multiply it by 2.

It would also be conceivable to have a certain phase shift be- tween the two triangle waves in the embodiment according to Figs 1 and 2 with respect to each other in very special applica- tions of the invention.