AN APPARATUS FOR CONVERTING ALTERNATING VOLTAGE INTO DIRECT VOLTAGE

Abstract: An apparatus for converting alternating voltage into direct voltage and conversely comprises a VSC-converter arranged between two poles (5, 6), a positive and a negative, of a direct voltage side of the apparatus and having 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, an alternating voltage phase line (20) connected to a first midpoint (18), called phase out-put, of the series connection between two units while dividing the series connection in two equal parts, in which the two poles of the direct voltage side are put on substantially the same voltage but with opposite signs with respect to a zero voltage level of the direct voltage side. The apparatus has also a second midpoint (29) between two said units of one part of the series connection and which is through a flying capacitor (28) connected to a second midpoint of a second part of the series connection corresponding to the second midpoint first mentioned with respect to the phase output. Each second midpoint is connected to ground through a rectifying member (31, 32) each with the rectifying member connected to the second midpoint closest to the positive pole with a conducting direction from ground to the second midpoint and with the rectifying member connected to the second midpoint closest to the negative pole with the conducting direction from the second midpoint to ground.

Description: jo/hj Applicant: ABB AB AN APPARATUS FOR CONVERTING ALTERNATING VOLTAGE INTO DIRECT VOLTAGE FIELD OF THE INVENTION AND PRIOR ART The invention relates to an apparatus for converting alternating voltage into direct voltage and conversely, which comprises a VSC-converter arranged between two poles, a positive and a negative, of a direct voltage side of the apparatus and having a series connection of at least four units each comprising a semi- conductor device and a 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 two equal parts, the two poles of the direct voltage side being put on substantially the same voltage but with different signs with respect to a zero voltage level of the direct voltage side, said apparatus comprising a second midpoint between two said units of one part of the series connection and which is through a flying capacitor connected to a second midpoint of the other part of the series con- nection corresponding to the second midpoint first mentioned with respect to the phase output, and an arrangement for controlling 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. Such apparatuses may be used in all types of situations, where direct voltage is to be converted into 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 normally is 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 SVCs (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 adapted for voltages on the direct voltage side between 10 and 500 kV. An advantage of using such so-called multilevel converters, i.e. converters in which at least three different voltage levels may be "presented" 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 ob- taining 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 in a three level converter may be reduced to about A compared with a two level converter with the same generation of harmonics. An advantage of using so-called flying capacitors 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 switching losses among them takes place, so that in the practice all semiconductor devices have to be dimensioned for being able to take the maximum load that an individual semiconductor device may be exerted to, since otherwise particular regards have to be paid to the design of each individual semiconductor device when controlling them. This results in a very high total cost for the semiconductor devices, since some of them will in most operation situations be heavily overdimensioned. By using flying capacitors instead, such as in the apparatus defined in the introduction, a multilevel 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 extra semiconductor devices. For illuminating the invention and the problems this seeks to solve the case of an apparatus defined in the introduction incorporated in a plant for transmitting electric power in the form of high voltage direct current (HVDC) will be discussed. However, it is pointed out that the invention is not restricted to exactly this application of such an apparatus, but the problems and the solutions described below are also there for other types of applications of such an apparatus. An apparatus of the type defined in the introduction incorporated in a plant for transmitting electric power is already known through for example US 5 737 201 , US 5 706 188 and US 5 940 285. When a fault occurs in an apparatus of this type a considerably higher voltage with respect to said zero level may for different reasons with a short duration occur on said two poles of the direct voltage side of the converter. This may for example take place for a parallel connection of a plurality of said series connections between said two poles for connecting a multiple phase alternating voltage network to the alternating voltage side, such as a three-phase network, accordingly having one said phase output for each phase. If in such a case a one-phase ground fault occurs on the line connecting to one phase output, an over-voltage on the two other phases will charge the direct voltage poles to an upper allowed voltage level thereof, which is defined by an arrangement of any type of protection of the direct voltage side, such as a diverter with a defined protection level. The converter will block, which means that the voltage of the flying capacitor is frozen, but the potential on the phase output will vary depending upon which of said units, i.e. current valve, is conducting for the moment. Would at an instant in question the two units connecting one pole to the phase output conduct and by that said protection level voltage be on the phase output, the voltage across the unit located between said second midpoint and the pole being not connected to the phase output will be twice said protection level minus the voltage across the flying capacitor. Thus, it is a very high voltage that the semiconductor devices in the units located outermost in the series connection have to be able to withstand for a short time in a fault case. It is required that these semiconductor devices are dimensioned in the corresponding degree, and it has then also to be considered that within one unit, which normally consists of a number of semiconductor devices connected in series and also a number of diodes connected in series, an asymmetry in the distribution of the voltage among the different components may exist as a consequence of differences therebetween, such as that they turn-on or turn-off differently quick. Additionally, there is an uncertainty with respect to the voltage across the flying capacitor, and this could in some fault case be lower than it normally is during normal operation. This all together means that the voltage across the so-called outer valves of the converter may be that high that these valves have to be heavily overdimensioned, which usually is achieved by increasing the number of semiconductor devices connected in series therein. This may for example mean that the number has to be increased by about 50% above what is required in normal operation of the converter. This means of course that the costs are increased both with respect to pure component costs as well as the cost for control equipment associated therewith with respect to a need of a lower number of semiconductor devices. SUMMARY OF THE INVENTION The object of the present invention is to provide an apparatus of the type defined in the introduction, which makes it possible to solve the problems discussed above of occurrence of high voltages over said units of the converter in fault cases to a large extent. This object is according to the invention obtained by providing an apparatus of the type defined in the introduction, in which each second midpoint is connected to ground through a rectifying member each with the rectifying member connected to the second midpoint closest to the positive pole with the conducting direction from ground to the second midpoint and with the rectifying member connected to the second midpoint closest to the negative pole with the conducting direction from the second midpoint to ground. When a fault discussed above occurs in the worst position conceivable, i.e. with all the units of one side of the phase output conducting, so that the voltage of one pole of the direct voltage side is put on the voltage of the phase output, the voltage across the opposite outer unit will by this always receive a given value, which substantially corresponds to the po- tential difference between the pole connected thereto and zero, since the connection of ground through the rectifying member, such as a second diode, to the second midpoint will mean that the second midpoint in such a case has a potential being close to zero. Thus, the voltage across the outer unit will then be completely independent of the voltage over the flying capacitor and considerably lower than otherwise would be the case, so that the number of semiconductor devices connected in series of the outer unit may be reduced and by that costs may be saved. This means in the practice that if for example the negative pole is connected to the phase output when the fault occurs and the negative protection voltage level minus the capacitor voltage would in the normal case be on the second midpoint closest to the positive pole, which would mean a comparatively high negative voltage of the second midpoint, a current will now instead flow from ground through the second diode to this second midpoint and then further to the flying capacitor and the diode of the opposite part of the phase output while charging the flying capacitor and keeping the potential of the second midpoint close to ground. According to a preferred embodiment of the invention a voltage limiting member adapted to conduct current if the voltage thereacross exceeds a determined voltage level is connected in series between each of said rectifying members and ground. By arranging such a so-called diverter connected in series with the respective rectifying member it is avoided that this will conduct current in normal operation of the apparatus when a so-called "zero-voltage" is to be put on the phase output and this is not exactly zero, since the voltage between the pole in question and the zero voltage level minus the voltage over the flying capacitor deviates from zero. By arranging such a voltage limiting member said rectifying members may be made very small and no cooling thereof is required, since they will in normal operation of the converter not conduct any current, but only during about half the current period when the converter is provided with voltage or when a fault occurs. According to other preferred embodiments of the invention said voltage level of the voltage limiting member is between 5% and 20% of the voltage across the flying capacitor in normal operation of the apparatus, and more particularly about 10%. Such a voltage level on the voltage limiting member means that it is reliably ensured that the other diodes will not conduct any current in normal operation, but the reduction of the voltage across the outer units (valves) will nevertheless be considerable in the fault case. According to another preferred embodiment of the invention said semiconductor devices are IGBTs, which is advantageous, since these are suitable to connect in series and to control to turn-on and turn-off simultaneously with a high reliability, so that the semiconductor devices connected in series and included in such a unit may function as would they be one single device. According to another preferred embodiment of the invention said direct voltage side of the apparatus 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, but it is also advantageous that the apparatus is designed to be a part of a SVC (Static Var Compensa- tor) with the direct voltage side formed by capacitors hanging freely and the alternating voltage phase line belonging to an alternating voltage phase network. Further advantages as well as advantageous features of the in- vention appear from the following description and the other dependent claims. BRIEF DESCRIPTION OF THE DRAWING With reference to the appended drawing, below follows a description of a preferred embodiment of the invention cited as an example. In the drawing: Fig 1 is a simplified circuit diagram illustrating an apparatus according to a preferred embodiment of the invention. DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION Only that part of the so-called converter apparatus that is connected to one phase of an alternating voltage phase line is shown, 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, 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 apparatus. 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 the potentials + U/2 and - U/2 are in this way provided for the respective pole, in which U is the voltage between the two poles 5, 6. 5 is here the positive pole, while 6 is the negative pole. The units 1 -4 are each formed by 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 represent 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 held by each unit in the blocking state. A first midpoint 18 of the series connection between the two units 2 and 3, to which the phase output 19 is connected, is connected to an alternating voltage phase line 20 through an inductor 21. Said series connection is in this way divided into equal parts with two units 1 , 2 and 3, 4, respectively, of each such part. Furthermore, it is shown how a tuned filter 23 consisting of ca- pacitors, reactors and resistors is connected to the alternating voltage phase line for extinguishing harmonics, which may be created on the alternating voltage phase line 20 as a consequence of the switchings of the valves. A transformer 25 is also arranged for enabling a step-up or a step-down transformation of the level of the voltage out from the converter on the alternating voltage side thereof. Furthermore, it is illustrated how a breaker 26 is adapted to enable connection and disconnection of the alternating voltage network on the alternating voltage side downstream of the converter to and from, respectively, the converter. A second midpoint 27 between two said units of one part of the series connection is through a flying capacitor 28 connected to a second midpoint 29 of the second part of the series connection corresponding to the other second midpoint with respect to the phase output. The apparatus has also an arrangement 30 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 27 and 29, respectively, which for the midpoint 27 means the potential of the pole 6 added by the voltage across the capacitor 28 and for the midpoint 29 the voltage of the pole 5 subtracted by the voltage across the capacitor 28. This arrangement 30 and the arrangement thereof is very simplified illustrated here, and a separate such arrangement would probably in the practice be arranged on high potential at each individual unit and these will then receive control signals from a control arrangement arranged on ground level. The construction of the apparatus described so far is already known, but the new and characterizing features of the converter according to the present invention will now be described. A se- ries connection of a rectifying member in the form of a second diode 31 , 32 and a voltage limiting member in the form of a di- verter 33, 34 is connected with one end thereof to each said second midpoint 27, 29, while the other end of the series connection is connected to ground 35, 36. The second diode of the second midpoint 27 closest to the positive pole 5 has the conducting direction from ground to the second midpoint 27, while the second diode 32 connected to the second midpoint 29 closest to the negative pole 6 has its conducting direction from the second midpoint 29 to ground. The voltage level, at which the two diverters 33, 34 start to conduct, is preferably about 10% of the voltage normally present between the ground point 9 and the respective pole 5, 6 in normal operation of the apparatus, i.e. for example 15 kV, would the pole voltage be 150 kV. The meaning of these features according to the invention for the functions of the apparatus will be explained further below. In normal operation of the converter + U/2 may be obtained on the phase output by making the two units 1 , 2 conducting by turning the semiconductor devices 10 and 1 1 on, while - U/2 may be connected to the phase output by the fact that the ar- rangement 30 makes the two units 3, 4 conducting by turning the semiconductor devices 12, 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 27 to the phase output, which is made by the fact that the ar- rangement 30 ensures that the units 2 and 4 are conducting, or by connecting the second midpoint 29 to the phase output by the fact that the control arrangement ensures that the units 1 and 3 are conducting. Would in such a normal operation the voltage across the flying capacitor 28 not be exactly U/2, the diodes 31 or 32 may be brought to conduct, which is not desired. Would namely for example the voltage of the negative pole 15 be -150 kV, but the voltage across the flying capacitor is 140 kV, a voltage of -10 kV would be obtained at the second midpoint 27 when bringing the units 2 and 4 in conducting state for obtaining a zero voltage on the phase output 19, which would mean that in absence of the diverter 33 the diode 31 would be forward biased with a voltage of -10 kV and conduct current and charge the flying capacitor 28. By the presence of the diverter 33 having a protection level of 15 kV no current will flow. The diverter 34 has the corresponding function with respect to the diode 32. We do now as an example assume that the voltage of the pole conductor 5 in normal operation is 150 kV and of the pole conductor 6 -150 kV. The protection voltage level of these pole conductors is +270 kV and -270 kV, respectively. This means in the practice that the following happens if a ground fault defined in the introduction would occur, i.e. in the case of a plurality of phases of a converter a ground fault occurs on the alternating voltage phase line 20. The over-voltage on the other phases will in such a case charge the two poles 5, 6 to the protection level thereof, which here is +270 kV and -270 kV, respectively. The converter will then block, which means that the voltage across the flying capacitor is frozen. However, the potential of the two second midpoints 27 and 29 will be dependent upon which unit is conducting when the blocking takes place. Would the series connections 31 -36 according to the invention not be there, the voltage across the outer units 1 or 4 of the units in the series connection could be twice the protection level minus the voltage across the capacitor. This would for example mean that at a voltage across the flying capacitor 28 of 150 kV when the nega- tive pole 6 is connected to the phase output, i.e. -270 kV is put thereon, the potential of the second midpoint 27 gets -120 kV, which would mean that a voltage of 390 kV would be across the outer valve 1 . The reality could then be still worse by a lower capacitor voltage then intended in any fault case. The number of semiconductor devices and diodes, respectively, connected in series in these outer valves has in converters of this type already known to be chosen for being able to take such a voltage level, in which it has also to be considered that these devices are not all exactly identical, but the voltage could be asymmetrically distributed thereon. However, by arranging said series connections of a diverter and the second diode between ground and each second midpoint the voltage across the outer valves may be reduced considerably in such a fault case. More exactly, in the fault case just described the diode 31 will start to conduct, if the negative potential of the second midpoint 27 is lower than -15 kV, so that a charging of the flying capacitor 28 takes place, so that the potential of the second midpoint 27 in such a fault case gets exactly -15 kV, which means a voltage across the outer valve 1 of 285 kV, which is to be compared with 390 kV according to above. The corresponding is achieved by the second series connection connected to the second midpoint 29 with respect to the reduction of the maximum voltage across the outer valve 4 closest to the negative pole 6. Another advantage of the arrangement according to the invention of the two ground connections of the two second midpoints 27, 29 through the series connection of a second diode and a diverter is that the flying capacitor 28 will be charged very rap- idly through the diodes 15, 32 or 31 , 16 when the breaker 26 is closed for providing the converter with voltage from the alternating voltage side after operation interruption. This means that the flying capacitor 28 may be charged to the level it is intended to have during normal operation in about half a current period, which in the practice is just as fast as the two capacitors 7 and 8 of the direct voltage side are charged and the semiconductor devices in the outer valves 1 , 4 are by that not exerted to any excessive voltages in connection with the provision of voltage. The invention is of course not in any way restricted to the preferred embodiment described above, but many possibilities to modifications thereof would be apparent to a man with ordinary skill in the art without departing from the basic idea of the invention as defined in the appended claims. It would for example be possible to arrange a resistance in parallel with the respective voltage limiting member 33, 34 as a consequence of the leakage current existing in the second diode 31 and 32, respectively. It could also be advantageous to arrange any member for supervising the second diodes 31 , 32 or overdimensioning them for getting rid of such supervision. In the SVC-case the points 35, 36 and 9 may advantageously be connected to each other and float freely. Although the invention is particularly directed to a so-called three-level converter, it is pointed out that it is not restricted to that number of levels, but it would also be possible that the series connection comprises 2 n units, in which n is an integer > 3, and the converter has then n-1 said second midpoints located in corresponding positions with respect to the phase output on each side thereof and connected to each other through a flying capacitor.

Claims: Claims 1. An apparatus for converting alternating voltage into direct voltage and conversely, which comprises a VSC-converter ar- ranged between two poles (5, 6), a positive and a negative, of a direct voltage side of the apparatus and having a series connection of at least four units (1 -4) each comprising a semiconductor device (10-13) and a diode (14-17) connected in anti-parallel therewith, an alternating voltage phase line (20) connected to a first midpoint (18), called phase output, of the series connection between two units (2, 3) while dividing the series connection in two equal parts, the two poles of the direct voltage side being put on substantially the same voltage but with different signs with respect to a zero voltage level of the direct voltage side, said apparatus comprising a second midpoint (27) between two said units (1 , 2) of one part of the series connection and which is through a flying capacitor (28) connected to a second midpoint (29) of the other part of the series connection corresponding to the sec- ond midpoint first mentioned with respect to the phase output, and an arrangement (30) for controlling 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 each second midpoint (27, 29) is connected to ground through a rectifying member (31 , 32) each with the rectifying member (31 ) connected to the second midpoint (27) closest to the positive pole (5) with the conducting direction from ground to the second midpoint and with a rectifying member (32) connected to the second midpoint (29) closest to the negative pole (6) with the conducting direction from the second midpoint to ground. 2. An apparatus according to claim 1 , characterized in that a voltage limiting member (33, 34) adapted to conduct current when the voltage thereacross exceeds a determined voltage level is connected in series between each said rectifying member (31 , 32) and ground. 3. An apparatus according to claim 2, characterized in that said voltage level of the voltage limiting member (33, 34) is between 5% and 20% of the voltage across the flying capacitor (28) in normal operation of the apparatus. 4. An apparatus according to claim 3, characterized in that said voltage level of the voltage-limiting member (33, 34) is about 10%o of the voltage across the flying capacitor (28) in normal operation of the apparatus. 5. An apparatus according to any of the preceding claims, characterized in that the units (1 -4) of the VSC-converter are adapted to be able to take a transient voltage on the respective pole of the direct voltage side of the apparatus of at least 150%, preferably at least 180% of the normal operation voltage of the respective pole with respect to said zero volt- age level. 6. An apparatus according to claim 5, characterized in that the value of said voltage of the respective pole (5, 6) is about 150 kV in normal operation and about 270 kV in the transient case. 7. An apparatus according to any of the preceding claims, characterized in that said semiconductor devices (10-13) are IGBTs (Insulated Gate Bipolar Transistor). 8. An apparatus according to any of claims 1 -6, characterized in that said semiconductor devices (10-13) are GTOs (Gate Turn-Off thyristor). 9. An apparatus according to the preceding claims, characterized in that said direct voltage side is formed by a direct voltage network (5, 6) for transferring high voltage direct current (HVDC) and the alternating voltage phase line (20) belongs to an alternating voltage phase network. 10. An apparatus according to any of claims 1 -8, characterized in that it is design to be a part of a SVC (Static Var Compensator) with the direct voltage side formed by capacitors hanging freely and the alternating voltage phase line belonging to an alternating voltage phase network.

Inventors: SVENSSON KJELL, SVENSSON, KJELL
Applicants: ABB AB, SVENSSON KJELL, SVENSSON, KJELL
Application year: 2001
Main classification: H02M7
IPC codes: H02M7