WO2002063757A1

WO2002063757A1 - A converter apparatus and a method for control thereof

Info

Publication number: WO2002063757A1
Application number: PCT/SE2002/000056
Authority: WO
Inventors: Johan Lindberg
Original assignee: Abb Ab
Priority date: 2001-02-05
Filing date: 2002-01-15
Publication date: 2002-08-15
Also published as: SE0100331D0; EP1364451A1; SE0100331L; SE520840C2

Abstract

In an apparatus for converting alternating voltage to direct voltage and conversely with a series connection of at least four units (1-4) each comprising a semiconductor device (10-13) of turn-off type and a diode (14-17) connected in anti-parallel therewith between two poles of a diode voltage side, a flying capacitor (25) is connected between two outer midpoints (24, 26) between two units of the series connection, while the midpoint of the series connection is connected to an alternating voltage phase line (19). An arrangement (27) is adapted to control the units (1-4) for transferring the apparatus between a main operation mode and an alternative operation mode depending upon function parameters of the apparatus. The semiconductor devices of the outer units (1, 4) are then kept continuously turned on in the alternative operation mode and only semiconductor devices of the inner units (2, 3) are then switched. The control arrangement (27) controls the voltage of the pulses on the phase output (18) of the apparatus to be lower in the alternative operation mode.

Description

A converter apparatus and a method for control thereof

FIELD OF THE INVENTION AND PRIOR ART

The present invention relates to an apparatus for converting al- ternating voltage into direct voltage and conversely, which comprises a series connection of at least four units arranged between two poles, one positive and one negative, of a direct voltage side of the apparatus, each unit comprising a semiconductor device of turn-off type and a diode connected in anti-parallel therewith, an alternating voltage phase line connected to a first midpoint, called phase output, on the series connection between two units while dividing the series connection in two equal parts, said two poles of the direct voltage side being put on substantially the same voltage but with opposite signs with respect to a zero voltage level of the direct voltage side, in which it comprises a second midpoint between two said units of one part of the series connection connected through a flying capacitor to a second midpoint corresponding thereto with respect to the phase output of the other part of the series connection, and an arrangement for controlling the semiconductor devices of the units, which in a main operation mode of the apparatus is adapted to control the semiconductor devices to generate a train of pulses with determined amplitudes according to a pulse width modulation pattern on the phase output of the apparatus by al- ternatively connecting the phase output to at least the plus pole of the direct voltage side, the minus pole of the direct voltage side and each of said second 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 phase output conducting, so as to give the phase output a voltage level corresponding to a sum of the voltage of said direct voltage pole closest thereto and the voltage across the flying capacitor, as well as a method for controlling such an apparatus.

Such apparatuses may be used in all kinds of situations, where direct voltage is to be converted into alternating voltage and conversely, in which examples of such uses are in stations in HVDC-plants (High Voltage Direct Current), in which the direct voltage 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 this is then converted into alternating voltage, as well as in SVC:s (Static Var Compensator), where 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.

It is pointed out that the invention is not at all restricted to a case of only one said flying capacitor and one couple of second midpoints associated therewith, but the number thereof may be arbitrary, so that in said main operation mode 3, 4, 5 and so on different voltage levels of the pulses on the phase output are conceivable. An advantage of using such so-called multiple level converters, i.e. converters in which at least three different voltage levels may be "laid out" on said phase output, with respect to so-called two level bridges, is that 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 apparatus may be reduced considerably. More exactly, the switching frequency of the semiconductor devices may in a three-level converter under said conditions be reduced to about 1 /3. An advantage of using so-called flying ca- pacitors for obtaining further 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 latter case have to be controlled in such a way that an uneven distribution of the switching losses among them takes place, so that in the practise all semiconductor devices have to be dimensioned for being able to take the maximum load that an individual semiconductor device may be subjected to, since otherwise particular considerations to the design of each individual semiconductor device are to be taken 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 instead using flying capacitors, such as in the apparatus defined in the introduction, a multiple level converter with a 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.

Apparatuses of the type defined in the introduction are already known through inter alia US 5 737 201 , US 5 706 188 and US 5 940 285.

There is of course always a desire to improve converter apparatuses of this type, i.e. with respect to the switching losses caused in the semiconductor devices of the units (current valves). Especially when during certain periods of time powers of a level considerably lower than the nominal power for which the converter apparatus is dimensioned to be able to transfer are transferred through the converter apparatus between the di- rect voltage side and the alternating voltage side thereof, the switching losses in the semiconductor devices of such appara- tuses already known tend to be noticeable, since their part of the total losses in the converter apparatus then gets considerably larger. There is also a desire to otherwise improve the function of a converter apparatus of this type.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a converter apparatus of the type defined in the introduction and a method for control thereof, which at least in some respect result in an improved function of such an apparatus with respect to such apparatuses already known.

This object is according to the invention obtained by the fact that such an apparatus also comprises members adapted to measure one or more operation parameters of the apparatus able to indicate if a second, alternative operation mode of the apparatus is suitable, that the apparatus comprises means adapted to enable adjustment of the level of the alternating voltage on the phase output, and that the control arrangement is adapted to transfer the apparatus to a second, alternative operation mode when it is established through the parameters measured by the measuring members that an alternative operation mode is suitable by:

a) controlling the semiconductor devices of the unit/units between the respective said second midpoint and the direct voltage pole closest thereto to be continuously turned on for continuous connection of the respective second midpoint to the potential of the direct voltage pole, b) controlling said means to reduce the level of the alternating voltage on the phase output with respect to the level in the main operation mode to be at the most in that proportion to the level in the main operation mode that corresponds to the proportion of units of the series connection located between said two second midpoints continuously connected to the direct voltage poles to the total number of units of the series connection, and c) controlling the semiconductor devices of the units lo- cated between said two second midpoints continuously connected to the direct voltage poles so that a train of pulses is generated with the different levels possible through a temporary converter formed in this operation mode through the units between the two second mid- points connected to the respective direct voltage potential.

By in this way changing the operation mode of the converter apparatus when the conditions enables this considerable advan- tages may be obtained. However, it is pointed out that normally, i.e. when the conditions are such for which the converter apparatus is dimensioned, this is operated according to the main operation mode. In the alternative operation mode there is a possibility to reduce the losses, since the way to proceed means that the voltage/current relationship is changed in the switching instant with respect to the main operation mode. In the alternative operation mode, the current level is maintained, but the voltage is reduced with respect to the main operation mode, in which the current is reduced but the voltage is maintained for the same power. This is made possible by ensuring through said means that the voltage level on the phase output is reduced in the way mentioned with respect to the main operation mode, so that the semiconductor device of these units do not receive higher voltages thereacross in the blocking state of the respective unit than in the main operation mode. The main advantage obtained by transferring the apparatus to the alternative operation mode and results in use of the alternative operation mode for stabilising the transmission with respect to networks connected thereto is that the flying capacitor, which now will be directly connected between the two direct voltage poles, will function as a so-called decoupling capacitor, so that the capacitance thereof is added to the capacitance of the decoupling capacitors located on the direct voltage side. Since the capacitance of the flying capacitor is normally large with respect to the decoupling capacitors on the direct voltage side, normally 2-4 times as large, the total energy amount on the direct voltage side of the apparatus, i.e. the energy storage, will increase remarkably, which increases the stability of the entire system, since the relationship (energy stored on the direct voltage side)/(power handled by the apparatus) is increased.

It is in itself so that when changing from the main operation mode to the second operation mode the number of possible voltage levels on the phase output will be reduced and by that a somewhat higher switching frequency will be necessary for the semiconductor devices still switched, but this will be outweighed by the fact that no switchings have to take place in the outer units any longer.

According to a preferred embodiment of the invention said means for adjusting the level of the voltage on the phase output comprises a tap changer connected to a transformer arranged between the phase output and the alternating voltage phase line, and the arrangement is adapted to control the tap changer to reduce the voltage on the phase output in the alternative op- eration mode with respect to the main operation mode by controlling the tap changer. A transformer is normally present, and the tap changer may also be so, so as to give the voltage on the alternating voltage line a desired level with respect to the voltage on the direct voltage side, so that the adjustment of the level of the voltage on the phase output may take place by very simple means by simply controlling the tap changer in a required way when the operation mode of the apparatus is to be changed. However, would the alternating voltage phase line be connected to a passive or dead alternating voltage network, it would then also be possible to adjust the level of the alternating voltage on the phase output indirectly by adjusting the level of the direct voltage between the two poles on the direct voltage side of the apparatus.

According to another preferred embodiment of the invention said members are adapted to measure the power that is transferred and/or is to be transferred through the apparatus between the direct voltage side and the alternating voltage side thereof, and the arrangement is adapted to transfer the apparatus to the second, alternative operation mode when this power is at the most in that proportion to the nominal power transferable by the apparatus in the main operation mode that corresponds to the proportion of units of the series connection located between said two second midpoints continuously connected to the direct voltage poles according to a) to the total number of units of the se- ries connection. A transfer of the apparatus to the second operation mode is by this made possible as soon as the power to be transferred by the apparatus sinks below the level enabling this operation mode, and it is at the same time ensured that the current through the apparatus may not be higher than the maxi- mum allowed nominal level thereof in the main operation mode. Would the voltage on the phase output and by that between the two poles of the direct voltage side be lowered, which for sure is necessary if the outer units shall be turned on all the time, without in any way control that the power to be transferred through the apparatus is reduced in a corresponding degree, the current through the apparatus could get higher than allowed. A further embodiment of the invention is more explicitly intended to ensure this, in which the arrangement and said means are adapted to co-operate so that the intensity of the current through the converter apparatus when changing between the main operation mode and the alternative operation mode is kept equal to or lower than the maximum allowed nominal level thereof in the main operation mode.

This means in the practise that in another preferred embodiment of the invention, which has only two said second midpoints and is adapted to function as a 3-level converter in the main operation mode, in which the arrangement is adapted to in c) of the second alternative operation mode control the semiconductor devices of the units located between the two second midpoints as a 2-level converter, so that a train of alternatively positive and negative pulses is generated on the phase output, the power has to sink to be at the most the half of the nominal power for which the entire converter is designed in the main operation mode before the converter apparatus may be transferred to the alternative operation mode, since the voltage on the phase output has to be reduced to at the most the half of the nominal voltage, under the condition that all units are identical. It would of course also be possible to reduce the voltage more, for example to 1/3, but the power to be transferred may then not be higher than 1/3 of the nominal power of the converter apparatus in the main operation mode.

According to another preferred embodiment of the invention the apparatus has a series connection of 2n units, in which n is an integer > 3, and (n-1 ) couples of said second midpoints located in corresponding positions with respect to the phase output on the opposite sides thereof are connected to each other through a flying capacitor, and the arrangement is adapted to in a) in the second, alternative operation mode of the apparatus control the semiconductor devices of the unit/units between a second midpoint of any of the couples of second midpoints and the respective direct voltage pole to be continuously turned on for continuously connecting this second midpoint and also the opposite second midpoint belonging to the same couple to the di- rect voltage potential closest thereto. It is then possible that for example a converter apparatus being a 4-level converter in the main operation mode is transferred to an alternative operation mode in the form of a 3-level converter by connecting the outer couple of second midpoints to the respective direct voltage pole or to a 2-level converter by connecting the inner couple of second midpoints to the respective direct voltage potential. It would then be possible that said members, which for example measure the power to be transferred by the converter apparatus, are used for determining which of the two alternative operation modes shall be selected. This means that it would also be possible to alternate between the two second alternative operation modes, i.e. between the 3-level converter case and the 2-level converter case depending upon the magnitude of the power to be transferred. However, the apparatus will normally be operated as a 4-level converter, since it is dimensioned therefor.

The invention also relates to a method according to the appended method patent claims, and the features and advantages thereof appear without any doubt from the above discussion of corresponding preferred embodiments of the apparatus accord- ing to the invention.

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

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

BRIEF DESCRIPTION OF THE DRAWINGS

With reference to the appended drawings, below follows a description of preferred embodiments of the invention cited as examples. In the drawings: Fig 1 is a simplified circuit diagram illustrating an apparatus according to a first preferred embodiment of the invention,

Fig 2 is an extremely simplified graph illustrating the appearance of voltage pulses on the phase output of the apparatus according to fig 1 in two different operation modes thereof,

Fig 3 is a view corresponding to fig 1 of an apparatus according to a second preferred embodiment of the invention, and

Fig 4 is a graph corresponding to that of fig 2 illustrating possible voltage pulses on the phase output of the apparatus according to fig 3 in three different possible operation modes of the apparatus.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Only that part of the converter apparatus that is connected to one phase 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 this constitutes the entire converter apparatus, when this is connected to a one phase alternating voltage network. The converter apparatus is a so-called VSC-converter (Voltage Source Converter), which has four units 1-4, usually called transistor valves or alternatively thyristor valves, con- nected in series between the two poles 5, 6 of a direct voltage side of the apparatus. Two so-called decoupling capacitors 7, 8 connected in series are arranged between said two poles, and a midpoint 9 between these is usually connected to ground, so that in this way the potentials +U/2 and -U/2, respectively, are provided on 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 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 represent an amount of IGBTs or GTOs connected in series and simultaneously controlled, which is also the case, since a comparatively high number of such semiconductor devices are required for holding the voltage to held by each unit in the blocking state thereof.

A first midpoint 18, which constitutes the phase output of the converter, is connected to an alternating voltage phase line 19, through a filter in the form of an inductor 20 and a capacitor 21 as well as through a transformer 22, to which a tap changer 23 is connected and controllable in a way to be explained further below. The series connection is in this way divided into two equal parts with two units 1 , 2 and 3, 4, respectively, in each such part.

A second midpoint 24 between two said units of one part of the series connection is through a flying capacitor 25 connected to another midpoint 26 of the other part of the series connection corresponding thereto with respect to the phase output.

The apparatus has also an arrangement 27 adapted to control the different semiconductor devices of the units 1 -4 and by that ensure 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 to which the voltage across the capacitor 25 is added and for the midpoint 26 the voltage of the pole 5 from which the voltage across the capacitor 25 is subtracted. This is what the arrangement is adapted to do in the main operation mode of the apparatus, which corresponds to how a converter apparatus of this type is normally controlled. The arrangement 27 and the provision thereof is here shown very simplified, and one separate such arrangement would in the practise be arranged on high potential at each individual unit and these will receive control signals from a control arrangement located on ground level.

The apparatus has also members 28 very schematically indicated adapted to measure the power transferred by the apparatus between the direct voltage side and the alternating voltage side thereof. These members comprises normally a measurement of voltage and current as well as a calculation of the power from the values so measured. Information about the power transferred is then sent from the members 28 to the control arrangement 27.

It will here not be dealt with how a pulse width modulation in a conversion of a converter apparatus according to fig 1 normally takes place, since it is for this sake made reference to for example the american patents mentioned in the introduction, but the specific features of the present invention will hereinafter be described, namely that the converter apparatus is transferred between different operation modes meaning different numbers of possible levels of the voltage pulses on the phase output depending upon different operation conditions of the apparatus.

When it is detected by the member 28, which also could comprise a control arrangement located on ground level and giving orders with respect to the power to be transferred, that the power to be transferred between the direct voltage side and the alternating voltage side of the converter is < half the nominal power for which the converter is designed, the control arrangement 27 transfers the converter to a second alternative operation mode after receiving information about this. However, such a transfer only takes place if it is judged that this alternative op- eration mode may be maintained during a comparatively long period of time, such as at least in the order of 15 minutes, and no continuous switching between the two operation modes takes place, since it takes at least a few seconds before the operation mode change is completely carried out. The control arrangement 27 now controls the tap changer 23 to change the voltage level of the phase output to half the level it had in the regular operation mode, since the voltage level on the alternating voltage phase line 19 is set and may not be changed. The arrangement 27 simultaneously controls all semiconductor devices in the units 1 and 4 to be turned on and be maintained continuously turned on in this second operation mode, so that the second midpoints 24, 26 are continuously connected to the respective direct voltage pole 5, 6. This means that the two capacitors 7, 8 will start to be discharged so that the voltage between the direct voltage poles 5, 6 gets the same as across the flying capacitor 25, i.e. U/2.

The flying capacitor 25 will now also be a decoupling capacitor and the energy stored by the decoupling capacitors on the direct voltage side will by that increase remarkably with respect to the power transferred through the converter apparatus, so that the stability of the system is considerably increased.

Furthermore, the control arrangement 27 begins to control the semiconductor devices 1 1 , 12 of the two units 2, 3 as a conven- tional two-level converter. Thus, the semiconductor devices in the units 1 and 4 are not switched during this second alternative operation mode. If the power is half the nominal power the intensity of the current through the apparatus will be the same as the nominal current intensity in the main operation mode, but if it is lower the current intensity will decrease. However, it will be higher than if the converter would have been operated according to the main operation mode at the same level of the power transferred.

It is very schematically shown in fig 2 what happens with the pulses on the phase output 18 at the change from one operation mode to the other. The main operation mode is illustrated to the left during that part of the current period during which the voltage pulses are positive. The amplitude of the pulses is here U/2 and 0, respectively, while it is shown to the right how the pulses with an amplitude of U/4 and -U/4 are generated on the phase output in the alternative operation mode.

It is illustrated in fig 3 how the invention may be applied on a converter having more levels, more exactly a 4-level converter. Thus, the number of units of a converter apparatus according to the invention is 2n, in which n is an integer > 2, and in such a converter the possible levels on the phase output are n+1 . There are n-1 couples of said second midpoints located in corresponding positions with respect to the phase output on oppo- site sides thereof and connected to each other through a flying capacitor. Thus, in the case shown in fig 3 two couples 29, 30 and 31 , 32 of second midpoints having a flying capacitor 33 and 34, respectively, each are there. In a converter apparatus of this type the levels +U/2, +U/6, -U/6 and -U/2 are obtained on the phase output 18 when the converter is operated according to the regular main operation mode. The two intermediate levels may then be obtained in different ways. +U/6 may for example be obtained on the phase output either by making the units 35, 36 and 38 conducting or by making the units 40, 36 and 37 con- ducting. The corresponding reasoning is valid for obtaining - U/6.

When it is recognised that during a not neglectable period of time a considerably lower power is to be transferred between the direct voltage side and the alternating voltage side of the converter apparatus than the nominal power of the apparatus, the control arrangement 27 controls the tap changer 23 and the semiconductor devices of the units in the corresponding way that has been described for the embodiment according to fig 1 for obtaining an alternative operation mode. Two different alternative operation modes may then be selected depending upon how low the power to be transferred is with respect to said nominal power. Is this lower than 2/3 but higher than 1/3 of the nominal power, the semiconductor devices of the two outer valves 35, 40 are brought to be continuously turned on, so that the second midpoints 29, 30 are connected to the direct voltage poles 5 and 6, respectively, and the flying capacitor 33 become a decoupling capacitor. The tap changer 23 is then simultaneously controlled to change the voltage on the phase output 18 to be 2/3 of the voltage in the main operation mode, so that the voltage across the different units 36-39 is not increased at the change from one operation mode to the other. The units 36-39 are then operated as a three-level converter.

Would the power to be transferred be lower than 1 /3 of the nominal power, the semiconductor devices of the valves 36-39 may also be controlled to be continuously turned on, so that the two midpoints 31 , 32 are maintained continuously connected to the respective direct voltage pole 5, 6. The control arrangement 27 controls in this case the tap changer 23 to reduce the voltage on the phase output 18 to be 1 /3 of the voltage in the main operation mode. The semiconductor devices of the units 37 and 38 are then controlled as a two-level converter. As soon as the conditions change, i.e. the level of the power to be transferred through the apparatus become another, the operation mode is of course changed. It is for example returned to the main operation mode if a higher power again is to be transferred through the apparatus.

The difference between the three operation modes of the con- verter apparatus according to fig 3 is schematically illustrated in fig 4, in which to the left the voltage pulses on the phase output 18 in the main operation mode as a four-level converter are shown during a part when the pulses are positive. Thus, the pulses have here a magnitude of U/2 and U/6. In the alternative operation mode with the second midpoints 29, 30 continuously connected to the direct voltage poles the three levels illustrated in the middle of fig 4 may be obtained for the voltage on the phase output 18. Thus, either U/3, zero or -U/3 are here obtained. It is shown to the left that in the two-level converter operation pulses of U/6 and -U/6 are alternatively obtained on the phase output 18.

The invention is of course not in any way restricted to the preferred embodiments described above, but many possibilities to modifications thereof would be apparent to a person skilled in the art without departing from the basic idea of the invention such as defined in the appended claims. As already mentioned, in the case of a passive alternating voltage network connected to the alternating voltage side of the converter apparatus the voltage on the phase output 18 could indirectly be adjusted by adjusting the voltage between the two poles of the direct voltage side from the direct voltage side.

The patent claim definition that the apparatus has measuring members is to be given a very broad sense and covers also the case in which power orders are given from a control arrangement, for example on ground level, and this information is sent further to the arrangement of the apparatus according to the invention. The members are then in a way constituted by the control arrangement transmitting and having knowledge about the power order.

Claims

1 . An apparatus for converting alternating voltage into direct voltage and conversely, which comprises a series connec- tion of at least four units (1 -4, 35-40) arranged between two poles (5, 6), one positive and one negative, of a direct voltage side of the apparatus, each unit comprising a semiconductor device (10-13) of turn-off type and a diode (14-17) connected in anti-parallel therewith, an alternating voltage phase line (19) connected to a first midpoint, called phase output (18), on the series connection between two units while dividing the series connection in two equal parts, said two poles of the direct voltage side being put on substantially the same voltage but with opposite signs with respect to a zero voltage level of the direct voltage side, in which it comprises a second midpoint (24, 29, 31 ) between two said units of one part of the series connection connected through a flying capacitor (25, 33, 34) to a second midpoint (26, 30, 32) corresponding thereto with respect to the phase output of the other part of the series connection, and an arrangement (27) for controlling the semiconductor devices of the units, which in a main operation mode of the apparatus is adapted to control the semiconductor devices to generate a train of pulses with determined amplitudes according to a pulse width modulation pattern on the phase output of the apparatus by alternatively connecting the phase output to at least the plus pole of the direct voltage side, the minus pole of the direct voltage side and each of said second 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 phase output conducting, so as to give the phase output a voltage level corresponding to a sum of the voltage of said direct voltage pole closest thereto and the voltage across the fly- ing capacitor, characterized in that the apparatus also comprises members (28) adapted to measure one or more operation parameters of the apparatus able to indicate if a second, alternative operation mode of the apparatus is suitable, that the apparatus comprises means (23) adapted to enable adjustment of the level of the alternating voltage on the phase output (18), and that the control arrangement is adapted to transfer the apparatus to a second, alternative operation mode when it is established through the parameters measured by the measuring members that an alternative operation mode is suitable by:

a) controlling the semiconductor devices of the unit/units between the respective said second midpoint and the direct voltage pole closest thereto to be continuously turned on for continuous connection of the respective second midpoint to the potential of the direct voltage pole, b) controlling said means (23) to reduce the level of the alternating voltage on the phase output with respect to the level in the main operation mode to be at the most in that proportion to the level in the main operation mode that corresponds to the proportion of units of the series connection located between said two second midpoints continuously connected to the direct voltage poles to the total number of units of the series connection, and c) controlling the semiconductor devices of the units located between said two second midpoints continuously connected to the direct voltage poles so that a train of pulses is generated with the different levels possible through a temporary converter formed in this operation mode through the units between the two second midpoints connected to the respective direct voltage potential.

An apparatus according to claim 1 , characterized in that said means (23) for adjusting the level of the voltage of the phase output comprises a tap changer (23) connected to a transformer (22) arranged between the phase output (18) and the alternating voltage phase line (19), and that the arrangement (27) is adapted to control the tap changer to re- duce the voltage of the phase output in the alternative operation mode with respect to the main operation mode by controlling the tap changer.

3. An apparatus according to claim 1 , characterized in that the alternating voltage phase line (19) is intended to be connected to a passive alternating voltage network, and that said means is adapted to adjust the level of the alternating voltage of the phase output by adjusting the level of the direct voltage between the two poles (5, 6) on the direct voltage side of the apparatus.

4. An apparatus according to any of the preceding claims, characterized in that said members (28) are adapted to measure the power that is transferred and/or is to be transferred through the apparatus between the direct voltage side and the alternating voltage side thereof, and that the arrangement (27) is adapted to transfer the apparatus to the second, alternative operation mode when this power is at the most in that proportion to the nominal power transfer- able by the apparatus in the main operation mode that corresponds to the proportion of units of the series connection located between said two second midpoints continuously connected to the direct voltage poles according to a) to the total number of units of the series connection.

5. An apparatus according to any of the preceding claims, characterized in that the arrangement (27) and said means (23) are adapted to co-operate so that the intensity of the current through the converter apparatus at a transfer be- tween the main operation mode and the alternative opera- tion mode is kept equal to or lower than the maximum allowed nominal level thereof in the main operation mode.

6. An apparatus according to any of the preceding claims, characterized in that it has only two said second midpoints

(24, 26) and is adapted to function as a three-level converter in the main operation mode, and that the arrangement (27) is adapted to in c) in the second, alternative operation mode control the semiconductor devices of the units (2, 3) located between the two second midpoints as a two- level converter, so that a train of alternatively positive and negative pulses is generated on the phase output.

7. An apparatus according to claim 6, characterized in that said means (23) are adapted to in b) in the alternative operation mode control the level of the alternating voltage of the phase output (18) to decrease to the half of the level in the main operation mode.

8. An apparatus according to any of claims 1 -5, characterized in that it has a series connection of 2n units, in which n is an integer > 3, that (n-1 ) couples of said second midpoints (29-32) located in corresponding positions with respect to the phase output (18) on the opposite sides thereof are connected to each other through a flying capacitor (33, 34), and that the arrangement is adapted to in a) in the second, alternative operation mode of the apparatus control the semiconductor devices of the unit/units between a second midpoint of any of the couples of second midpoints (29-32) and the respective direct voltage pole (5, 6) to be continuously turned on for continuously connecting this second midpoint and also the opposite second midpoint belonging to the same couple to the direct voltage potential closest thereto.

. An apparatus according claim 8, characterized in that n=3, that the arrangement is adapted to in a) of the alternative operation mode control the semiconductor devices of the unit/units (35, 40) between the respective outer second midpoint (29, 30) located closest to the respective direct voltage pole and the direct voltage pole to be continuously turned on, and that the arrangement is adapted to in c) control the semiconductor devices of the units (36, 39) located between these two outer second midpoints as a three-level converter, so that a train of positive, negative and zero-level pulses is generated on the phase output.

10. An apparatus according to claim 9, characterized in that the arrangement is adapted to control said means (23) to in b) in the alternative operation mode adjust the level of the alternating voltage on the phase output (18) to decrease to 2/3 of the level in the main operation mode.

1 1 . An apparatus according to claim 8, characterized in that n=3, that the arrangement is adapted to control the semiconductor devices of the units (35, 36, 39, 40) between the two inner second midpoints (31 , 32) located closest to the phase output (18) and the respective direct voltage pole (5, 6) in the alternative operation mode to be continuously turned on, and that the arrangement is adapted to in c) control the semiconductor devices of the units (37, 38) located between the two inner second midpoints as a two- level converter, so that a train of alternatively positive and negative pulses is generated on the phase output.

12. An apparatus according to claim 1 1 , characterized in that the arrangement is adapted to in b) in the alternative operation mode control said means (23) to adjust the level of the alternating voltage of the phase output (18) to be 1/3 of the level in the main operation mode.

13. An apparatus according any of the preceding claims, characterized in that said semiconductor devices (10-13) are IGBTs (Insulated Gate Bipolar Transistor).

14. An apparatus according to any of claims 1 -12, characterized in that said semiconductor devices (10-13) are GTOs (Gate Turn-Off thyristor).

15. An apparatus according any of the preceding claims, char- acterized in that said direct voltage side is formed by a direct voltage network for transmitting High Voltage Direct Current (HVDC) and the alternating voltage phase line belongs to an alternating voltage phase network.

16. An apparatus according any of claims 1 -14, characterized in that it is a part of a SVC (Static Var Compensator) with a direct voltage side formed by capacitors hanging freely and the alternating voltage line belonging to an alternating voltage network.

17. A method for controlling an apparatus for converting alternating voltage into direct voltage and conversely, which comprises a series connection of at least four units (1 -4) arranged between two poles (5, 6), one positive and one negative, of a direct voltage side of the apparatus, each unit comprising a semiconductor device (10-13) of turn-off type and a diode (14-17) connected in anti-parallel therewith, an alternating voltage phase line (19) connected to a first midpoint, called phase output (18), of the series connection between two units while dividing the series connection in two equal parts, said two poles of the direct voltage side being put on substantially the same voltage but with opposite signs with respect to a zero voltage level of the direct voltage side, in which it comprises a second midpoint (24, 29, 31 ) between two said units of one part of the series connection connected through a flying capacitor (25, 33, 34) to a second midpoint (26, 30, 32) corresponding thereto with respect to the phase output of the other part of the series connection, and an arrangement (27) for controlling the semiconductor devices of the units, which in a main opera- tion mode of the apparatus is adapted to control the semiconductor devices to generate a train of pulses with determined amplitudes according to a pulse width modulation pattern on the phase output of the apparatus by alternatively connecting the phase output to at least the plus pole of the direct voltage side, the minus pole of the direct voltage side and each of said second 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 phase output conduct- ing, so as to give the phase output a voltage level corresponding to a sum of the voltage of said direct voltage pole closest thereto and the voltage across the flying capacitor, characterized in that the arrangement transfers the apparatus to a second, alternative operation mode when there is a desire to do so by:

a) controlling the semiconductor devices of the unit/units between the respective said second midpoint and the direct voltage pole closest thereto to be continuously turned on for continuous connection of the respective second midpoint to the potential of the direct voltage pole, b) controlling the level of the alternating voltage of the phase output to decrease with respect to the level in the main operation mode to be at the most in that proportion to the level in the main operation mode that corresponds to the proportion of units of the series connection located between said two second midpoints continuously connected to the direct voltage poles to the total number of units of the series connection, and c) controlling the semiconductor devices of the units located between said two second midpoints continuously connected to the direct voltage poles so that a train of pulses is generated with the different levels possible through a temporary converter formed in this operation mode through the units between the two second midpoints connected to the respective direct voltage potential.

18. A method according to claim 17, characterized in that in the alternative operation mode in b) the arrangement (27) controls the level of the alternating voltage on the phase output (18) by controlling a tap changer (23) connected to a transformer (22) arranged between the phase output and the alternating voltage phase line (19) so as to reduce the voltage on the phase output in the alternative operation mode with respect to the main operation mode.

19. A method according to claim 17 or 18, characterized in that the arrangement (27) transfers the apparatus to the second, alternative operation mode when the power to be transferred through the apparatus between the direct voltage side and the alternating voltage side thereof is at the most that proportion of the power nominally transferable by the apparatus in the main operation mode that corresponds to the proportion of units of the series connection located between said two second midpoints continuously connected to the direct voltage pole of the total number of units of the series connection.

20. A method according to any of claims 17-19, characterized in that the arrangement controls the converter apparatus when changing between the main operation mode and the alternative operation mode to keep the intensity of the cur- rent through the apparatus equal to or lower than the maxi- mum allowed nominal level thereof in the main operation mode.

21 . A method according to any of claims 17-20, characterized in that it is an apparatus having only two said second midpoints (24, 26) and functioning as a three-level converter in the main operation mode that is controlled, and that the arrangement (27) in the second, alternative operation mode in c) controls the semiconductor devices of the units (2, 3) lo- cated between the two second midpoints as a two-level converter, so that a train of alternatively positive and negative pulses are generated on the phase output.

22. A method according to claim 21 , characterized in that in d) the arrangement controls the level of the alternating voltage on the phase output (18) to decrease to half the level in the main operation mode.

23. A method according to any of claims 17-20, characterized in that it is an apparatus having a series connection of 2n units that is controlled, in which n is an integer >3, that (n- 1 ) couples of said second midpoints (29-32) located in corresponding positions with respect to the phase output (18) on opposite sides thereof are connected to each other through a flying capacitor (33, 34), and that in the second, alternative operation mode of the apparatus in a) the semiconductor devices of the unit/units between a second midpoint of any of the couples of second midpoints (29-32) and the respective direct voltage pole (5, 6) are controlled to be continuously turned on for continuously connecting this second midpoint and also the opposite second midpoint belonging to the same couple to the direct voltage pole potential closest thereto.

24. A method according to claim 23, characterized in that n=3, and that in the alternative operation mode in a) the arrangement controls the semiconductor devices of the unit/units (35-40) between the respective outer second midpoint (29, 30) located closest to the respective direct voltage pole and the direct voltage pole to be continuously turned on, and that the arrangement in c) controls the semiconductor devices of the units (36-39) located between these two outer second midpoints as a three-level converter, so that a train of positive, negative and zero- level pulses is generated on the phase output.

25. A method according to claim 24, characterized in that the arrangement in the alternative operation mode in b) controls the level of alternating voltage on the phase output (18) to decrease to 2/3 of the level in the main operation mode.

26. A method according to claim 23, characterized in that n=3, and that in the alternative operation mode the arrangement controls the semiconductor devices of the units (35, 36, 39, 40) between the two inner second midpoints (31 , 32) lo- cated closest to the phase output (18) and the respective direct voltage pole (5, 6) to be continuously turned on, and that in c) the arrangement controls the semiconductor units (37, 38) located between the two inner second midpoints as a two-level converter, so that a train of alternatively positive and negative pulses is generated on the phase output.

27. A method according to claim 26, characterized in that the arrangement in the alternative operation mode in b) controls the level of the alternating voltage on the phase output (18) to be 1 /3 of the level in the main operation mode.

28. A computer program directly loadable into the internal memory of a digital computer, which comprises software code portions for controlling the steps according to any of claims 17-27 when the program is run on a computer.

29. A computer program according to claim 28 provided at least partially over a network as the Internet.

30. A computer readable medium having a program recorded thereon designed to make a computer control the steps according to any of claims 17-27.