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 included in a SVC (Static Var Compensator) with a direct voltage side formed by one or a plurality of capacitors hanging freely, said apparatus comprising a series connection of at least four current valves arranged between two poles, one positive and one negative, of said direct voltage side, said valves each comprising a semi- conductor device of turn-off type and a rectifying member con- nected in anti-parallel therewith, an alternating voltage phase line connected to a first midpoint, called phase output, of the series connection between two current valves while dividing the series connection into equal parts, the 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, said apparatus comprising a second mid- point between two said current valves of one part of the series connection, which is through a flying capacitor connected to a second midpoint of the other part of the series connection corresponding thereto with respect to the phase output, and an arrangement for controlling the semiconductor devices of the current valves to generate a train of pulses with determined amplitudes according to a pulse width modulation pattern on the

phase output of the apparatus by alternatingly connecting the phase output to at least the plus pole and the minus pole of the direct voltage side and each of said second midpoints by making the current valve/current valves between another second midpoint and the direct voltage pole closest thereto and the current valve/current valves between the second midpoint in question and the phase output to conduct, so as to give the phase output a voltage level corresponding to the sum of the voltage of said adjacent direct voltage pole and the voltage across the flying capacitor, as well as a method for control of such-an apparatus.

Thus, the invention is directed to a voltage stiff converter appa- ratus adapted only for transferring reactive power and being ad- vantageously arranged along high voltage alternating voltage lines for achieving reactive power compensation.

The invention is not restricted to any levels of the voltage of the alternating voltage side of the apparatus, of the reactive power the converter apparatus is able to transfer or the number of phases of the alternating voltage side of the apparatus, and it may accordingly very well be designed for the one-phase case, for example for feeding railway vehicles.

However, the invention is particularly, but not exclusively, di- rected to intermediate and high voltage, i. e. where the peak voltage on the alternating voltage side of the apparatus is 10 kV or higher.

It is pointed out that the invention is not in any way restricted to the presence of only one said flying capacitor and one couple of midpoints associated therewith, but the number of these may principally be arbitrary.

The apparatus defined in the introduction is a so called multi- level-converter, since it may on said phase output deliver at

least three different phase potentials. An advantage of using such so called multi-level-converters with respect to so called two level bridges, *is that the semiconductor devices of the cur- rent valves may be switched with a considerably lower fre- quency for obtaining an alternating voltage of a determined fre- quency and quality on the alternating voltage phase line, so that the losses of the converter apparatus may be reduced consid- erably. More exactly, the switching frequency of the semicon- ductor devices in a three level converter may under said condi- tions be reduced to about 1/2. An advantage of using so called flying capacitors for obtaining further voltage levels of the phase output besides the voltage level of the two poles of the direct voltage side with respect to a use of a so called clamping diode is primarily that the semiconductor devices in the latter case have to be controlled in such a way that a non-uniform distribu- tion of switching losses on them will occur, so that in the prac- tice all semiconductor devices have to be dimensioned for being able to take the maximum load to which an individual semicon- ductor device may be subjected, since otherwise particular considerations have to be taken to the design of each individual semiconductor device when controlling them. This makes the total cost for the semiconductor devices very high, since some of them will in most operation situations be heavily overdimen- sioned. By instead using flying capacitors, such as in the appa- ratus defined in the introduction, a multiple-level-converter with a possibility to a more uniform load on the semiconductor de- vices with respect to switching losses may be obtained without using expensive so called clamping diodes or additional semi- conductor devices. An apparatus of the type defined in the in- troduction is already known through US-5 737 201, even if this apparatus is intended to transfer active power. There are of course always aimed at improving converter apparatuses of this type, inter alia with respect to the switching losses created in the semiconductor devices of the current valves, and this is also valid in the-case of using an apparatus according to said US patent in an SVC.

SUMMARY OF THE INVENTION The object of the present invention is to provide a converter ap- paratus of the type defined in the introduction as well as a method for control thereof, which in at least some respect re- sults in an improved function of such an apparatus with respect to such apparatuses already known, especially enables a reduc- tion of the total switching losses of the semiconductor devices included in the current valves.

This-object is according to the invention obtained by designing the arrangement of such an apparatus to control the semicon- ductor devices of the inner current valves between the two sec- ond midpoints to be turned on and turned off with a pulse width modulation frequency being at least an order of magnitude higher than the fundamental frequency of the alternating voltage of said alternating voltage phase line and to control the semi- conductor devices of the outer current valves between the re- spective second midpoint and the direct voltage pole closest thereto to be turned on and turned off with a frequency being substantially lower than said pulse width modulation frequency and within or in the proximity of the frequency range one or a few times said fundamental frequency.

By in this way only switching the inner current valves with the pulse width modulation frequency, while the outer are switched with a considerably lower frequency, the total switching losses in the converter apparatus may be reduced considerably with respect to the apparatus according to the prior art, in which all current valves are switched with the pulse width modulation fre- quency. Thus, the switching losses get considerably lower in the semiconductor devices of the two outer current valves than would they have been switched with the full pulse width modula- tion frequency. Another advantage of a lower frequency of the switching of-the outer current valves is that in the case of a se- ries connection of semiconductor devices, which is necessary

when handling high voltages, this is easier at low switching fre- quencies, since an unbalance with respect to the voltage held by different semiconductor devices connected in series will then get smaller, so that it is possible to arrange semiconductor devices having a smaller margin between the voltage they may maxi- mally withstand and the average voltage per semiconductor de- vice in the current valve. This means that either fewer semicon- ductor devices having a determined ability to withstand voltage or semiconductor devices having a lower ability to withstand voltage than otherwise may be arranged in the two outer current valves and thereby costs may be saved.

According to preferred embodiments of the invention the appa- ratus is adapted to control the semiconductor devices of the outer current valves to be turned on and turned off with a fre- quency being substantially equal to said fundamental frequency, said fundamental frequency being in the order of 40-70 Hz, preferably 50 Hz or 60 Hz, and the arrangement is adapted to turn on and turn off the semiconductor devices of the inner cur- rent valves with a frequency being 15-45 times said fundamental frequency, which usually means 1-2 kHz.

According to another preferred embodiment of the invention the semiconductor devices and the rectifying members of said cur- rent valves are designed so that the time average of the voltage across the flying capacitor will be a factor 0.2-0. 5 times the voltage between the two poles of the direct voltage side, in which it is advantageous that said. factor is less than 0.5. If this factor differs from 0.5 the use of one single flying capacitor and four said current valves would in fact result in a possibility to obtain four different levels of the potential, while in an apparatus according to US 5 737 201 six current valves connected in se- ries and two flying capacitors are required for obtaining four levels, which means a considerable saving of costs with respect to components as well as switching losses with respect to such an apparatus already known, should there be a desire to obtain

four levels instead of three. This is also something desirable, since the switching losses typically decreases with an increasing number of levels. Thus, different voltages will be applied across the outer current valves with respect to across the inner current valves, which is not the case in apparatuses already known.

More exactly, this means that when said factor is less than 0.5 higher voltages will be applied across the outer current valves than across the inner ones, which is advantageous from the switching loss point of view, since the voltage will then be higher where the frequency is low, while it is lower where the frequency is high. Thus, other types of semiconductor devices may be arranged in the outer current valves than in the inner ones, such as devices being adapted to take high voltages and have low conduction losses rather than low switching losses. As an alternative identical semiconductor devices connected in se- ries are arranged in all current valves, but a different number thereof in the outer than in the inner.

Another advantage of the fact that a higher voltage is applied across the outer current valves than across the inner ones in the blocking state is that the semiconductor devices of the outer current valves do not have to be dimensioned just as large with respect to the ability to withstand voltage so as to withstand the stresses when connecting the converter apparatus to voltage from the alternating voltage side. The clamping capacitors of the direct voltage side hanging freely will namely when connecting the apparatus to voltage be charged before the flying capacitor.

The flying capacitor will thereafter be charged to the nominal voltage thereof thanks to the voltage dividers normally arranged in parallel with the semiconductor devices of turn-off type in the different current valves. The entire voltage between the two di- rect voltage poles will then during a short period of time be ap- plied across one or the other of the outer current valves before the flying capacitor has been charged. Thanks to the fact that the nominal voltage of the flying capacitor is dimensioned so that it constitutes less than 0.5 of the total voltage between the

direct voltage poles, the outer current valves will when con- necting the apparatus to voltage be subjected to voltage being closer to the nominal voltage thereof than otherwise. For in- stance in the case of a factor of 1/3 the voltage to which they are subjected is in the order 3/2=1,5 times higher than the nominal voltage thereof, which these current valves normally may take without the need of taking any particular measures in order to restrict the voltage across the valves.

According to a preferred embodiment of the invention said factor is 1/3. The fact is that the four voltage levels obtainable on the phase output will then be uniformously distributed between the positive and the negative pole voltage, which results in a lower voltage switched, i. e. a lower pulse height step, which reduces the stresses on equipments such as reactors and transformers, connected thereto.

According to another preferred embodiment of the invention the apparatus has a unit adapted to enable so called soft-switching of the semiconductor devices of the inner current valves, i. e. so that no high voltages and high currents are combined in the semiconductor devices in these two valves. It is very advantageous to arrange such a unit exactly where the frequencies are high, since this has the greatest impact on the switching losses. Otherwise expressed, it is chosen to have high frequencies where the losses per switching are very low. Thus, the maximum revenue of the additional costs resulting from such a unit is obtained in this way. A device of this type gets extremely low switching losses, since the outer current valves switch according to the hard-switching principle with a low frequency and the inner current valves switch according to the soft-switching principle with a high frequency. This embodiment is particularly advantageous in combination with a selection of said factor as being less than 0.5, so that lower voltages have to be handled-by the inner current valves and thereby also by the

unit, which thereby may be made smaller and manufactured to a lower cost.

According to another preferred embodiment of the invention the two inner current valves have a snubber capacitor each connected in parallel with said semiconductor device of turn-off type and the unit comprises a resonance circuit for recharging the snubber capacitors of the current valves so as to thereby enable turning on of the semiconductor devices of turn-off type of the current valves at a low voltage thereacross. This is an ad- vantageous way to obtain so called soft-switching of the inner current valves, in which'a preferred embodiment has a reso- nance circuit being made of an ARCP-circuit (Auxiliary Resonant Commutation Pole).

According to another preferred embodiment of the invention also the two outer valves have a snubber member each, for example a snubber capacitor or a RC-circuit, connected in parallel with said semiconductor devices of turn-off type, so that the time de- rivative of the voltage across the semiconductor devices of turn- off type is restricted when switching the outer current valves and thereby the capacitive currents in a transformer associated therewith are limited.

According to another preferred embodiment of the invention said arrangement is adapted to control the semiconductor devices of the current valves and thereby the current valves according to a voltage set value for said phase voltage, and when the voltage set value is located between Udc/2 and (1-k) x Udc/2, corre- sponding above a first level, to alternatively make the two inner current valves conducting and continuously keep the outer current valve closest to the positive direct voltage pole con- ducting and the other outer current valve blocked, when the cur- rent set value is located between-UdJ2 and (-1+k) X Udc/2, cor- responding, to a second level, alternatively make the two inner current valves conducting and continuously keep the current

valve closest to the positive direct voltage pole blocked and the opposite outer current valve conducting, and when the current set value is located between said two levels to alternatively make one outer current valve and the inner current valve located on the opposite side of the phase output with respect thereto conducting and at the same time the other outer current valve and the other inner current valve to block and conversely, Udc being the voltage between said two direct voltage poles and k being said factor. A switching with almost fundamental fre- quency of the current valves and with pulse width modulation frequency of the inner ones is in this way obtained.

According to a preferred embodiment of the invention, which constitutes a further development of the embodiment last men- tioned, the arrangement is adapted to control the current valves according to a voltage set value for said phase voltage with the shape of a sine curve having a third tone component or a multi- ple of third tone components with respect to the fundamental tone of the sine curve added thereto for prolonging the period of time during which the voltage set value is located above said first level and below said second level and the two outer current valves may be located in a fixed position and do not have to be switched. The flanks of the voltage set value are in this way made steeper and the period of time during which the outer cur- rent valves have to be switched is hereby made shorter. It gets possible to reduce this period of time to a value being in the same order of magnitude or lower than the period of time for the switching frequency by which the inner valves operate, so that a fundamental tone commutation may be used for the two outer current valves without any negative consequences in the form of an increased content of harmonics occurring in associated net- works.

The invention also relates to a method for the control of a con- verter apparatus in accordance with the above according to the appended independent method claim.

Advantages of this method and of embodiments of this method defined in the appended dependent claims appear without any doubt from the discussion above of preferred embodiments of the converter apparatus according to the invention.

The invention also relates to a computer program product as well as a computer readable medium according to the corre- sponding 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 pro- gram instructions from a processor which may be influenced by a computer program provided with the program steps in question.

Further advantages as well as advantageous features of the in- vention appear from the other dependent 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 of a converter apparatus ac- cording to a first preferred embodiment of the invention, Fig 2 is a view corresponding to Fig 1 of a converter apparatus according to a second preferred embodiment of the invention, Fig 3 is a view corresponding to Fig 1 of a converter apparatus according to a third preferred embodiment of the invention, which is formed by a minor modification of the converter appa- ratus according to Fig 2,

Figs 4 and 5 illustrate a voltage set value with sine shape and a voltage set value in the form of a sine curve having a third tone component added thereto, respectively, for the voltage between the phase output and an imagined zero voltage level on the di- rect voltage side of the converter apparatus, which is used for pulse width modulation of the converter apparatus, and Fig 6 illustrates schematically what a pulse width modulation pattern based on the voltage set value according to Fig 1 may look like for a converter apparatus according to Fig 1.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION Only the part of the converter apparatus 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 con- verter apparatus is a so-called VSC-converter (Voltage Source Converter), which has four current valves 1-4 connected in se- ries between the two poles 5,6, positive and negative, respec- tively, of a direct voltage side of the apparatus. A so-called coupling capacitor 7 is arranged between the two poles. The voltage between the two poles defined therethrough gets Udc, in which the potentials for the poles get +Udc/2 and-Udc/2, respec- tively. When there are more than one phase these have the di- rect voltage side and the capacitor 7 in common, but they have for the rest a VSC-converter according to Fig 1 each.

The current valves 1-4 are each constituted by a semiconductor device 8-11 of turn-off type, such as an IGBT, GTO or IGCT, and a rectifying member connected in anti-parallel therewith in the form of a rectifying diode 12-15. Although only one semicon- ductor device of turn-off type is shown per current valve this may stand for an amount of semiconductor devices connected in

series and simultaneously controlled, which also is the case, since a comparatively high number of such semiconductor devices connected in series are required for holding the voltage to be held by each current valve in the blocked state.

A first midpoint 16, which constitutes the phase output of the converter, is connected to an alternating voltage line 19 through an inductor 20. Said series connection is in this way divided into two equal parts with two current valves 1,2 and 3,4, respec- tively, of each such part.

A second midpoint 21 between two said current valves of one part of the series connection is through a flying capacitor 22 connected to a second midpoint 23 of the other part of the se- ries connection corresponding to the first one with respect to the phase output.

The apparatus has also an arrangement 24 adapted to control the different semiconductor devices of the current valves 1-4 and thereby 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 21,23, which for the midpoint 21 means the potential of the pole 6 having the voltage across the ca- pacitor 24 added thereto and the midpoint 23 the voltage of the pole 5 having the voltage across the capacitor 22 subtracted therefrom. The arrangement 24 and the connecticn thereof is very schematically shown here and a separate such ar- rangement should in the practice be arranged on high potential at each individual current valve and these will receive control signals from a control arrangement arranged on ground level.

The current valves are advantageously so designed that the time average of the voltage across the flying capacitor 22 will be a factor 0.2-0.5 the voltage between the two poles of the direct voltage side. This means that when this factor is less than 0.5 a higher voltage will be taken by the outer current valves 1,4 than

the inner ones in the blocking state at the same time as four different levels are obtainable for the potential of the phase out- put 16. As mentioned, it is particularly advantageous that k=1/3, since this means that the levels Ut,/2, Udc/6,-UaC/6 and-Udc/2 may be obtained on the phase output, i. e. levels being uniformly distributed, so that there will be in average the smallest possible difference between the selected voltage level of a pulse and the voltage set value of a voltage set value curve. However, in quite particular cases a voltage set value curve may have such an appearance that it is most advantageous to have such levels being ununiformly distributed, since it results in lower switching losses exactly there.

It is shown in Figs 4 and 5 how in the case of k = 1/3 the differ- ent possible levels on the phase output are located with respect to a voltage set value curve in the form of a sine curve and a sine curve having a third tone component with respect to the fundamental tone of the sine curve added thereto, respectively.

Thus, the different current valves have here different voltage blocking capability.

The arrangement is adapted to control the semiconductor device of the current valves in the following way for obtaining a voltage according to the voltage set value curve in question (which the arrangement 24 gets as an input signal) on the phase line 19 : the outer valves are controlled to turn off and turn on with a fre- quency close to the fundamental frequency of the alternating voltage of the alternating voltage phase line 19, which typically may be 50 Hz or 60 Hz, while the two inner current valves 2,3 are controlled to turn on and turn off with a pulse width modula- tion pattern being at least an order of magnitude higher than the fundamental frequency, preferably 1 kHz-2 kHz. Reference is also made to Fig 6. This means in the practice that the ar- rangement 24 is, when the voltage set value is located between Udc/2 and (1-k) x Udc/2, which corresponds to a value above a first level 25, adapted to alternatively make the two inner current

valves 2,3 conducting and continuously keep the outer current valve 1 closest to the positive direct voltage pole conducting and the other outer current valve 4 blocking. When the voltage set value is located between-Udc/2 and (-1+k) x Udc/2, which corresponds to a value below a second level 26, the control ar- rangement makes alternatively the two inner current valves 2,3 conducting and keeps the current valve 1 located closest to the positive direct voltage pole blocking and the opposite outer cur- rent valve 4 conducting. When the voltage set value is located between the levels 25 and 26 the control arrangement makes one outer current valve 1 and the inner current valve 3 located on the opposite side of'the phase output with respect thereto alternatively conducting and at the same time the other outer current valve 4 and the other inner current valve 2 blocking and conversely. Thus, the two outer current valves 1,4 only switch within the time interval 27, which is located between the intersection points of the voltage set value curve with the two levels 25,26. By adding a third tone component or a multiple of third components to the sine curve for obtaining a voltage set value curve this period of time may be shortened without changing the voltage between the phases.

Since the outer current valves 1,4 switch with a low frequency, close to said fundamental frequency, the switching losses in these current valves will be very low.

A converter apparatus according to. a very preferred embodiment of the invention is shown in Fig 2, and this differs from the one according to Fig 1 by the fact that here a unit 28 for so called soft-switching of the inner current valves 2, 3 is arranged. This unit is formed by snubber capacitors 29,30 connected in paral- lel with the respective current valve as well as a resonance cir- cuit for recharging these snubber capacitors for enabling turning on of the semiconductor devices of turn-off type of the current valves at aslow voltage thereacross. The resonant circuit is con- stituted by an ARCP-circuit (ARCP = Auxiliary Resonant Com-

mutation Pole). How a circuit of this type functions may be con- sidered to belong to general knowledge, and reference is here made to inter alia US 5 047 913. An ARCP-circuit comprises more exactly an auxiliary valve 31 comprising two auxiliary valve circuits 32,33 connected in series, which each comprises a semiconductor device 34 of turn-off type, such as an IGBT or a GTO, and a rectifying member 35 connected in anti-parallel therewith in the form of a diode, such as a free wheeling diode.

The semiconductor devices 34 of turn-off type of the two auxil- iary valve circuits are arranged in opposite polarity with respect to each other. The ARCP-circuit comprises also an inductor 36 connected in series with the auxiliary valve circuits. This auxil- iary valve 31 constitutes a bi-directional valve, which may be brought to conduct in one or the other direction. It appears that the flying capacitor is here divided into two capacitors 37,38, which together hold a voltage according to the factor k men- tioned before times the voltage between the poles 5,6. The function of this resonant circuit may be very briefly described.

When for example the inner valve 3 conducts and the current flows from the phase output to the valve and this is controlled to turn off the current flowing into the phase output from the phase line is transferred directly to the two snubber capacitors 29 and 30 and the voltage increases slowly across the current valve 3, so that the current through the semiconductor device will man- age to get low before the voltage gets high and thereby the switching loss gets low. When the current direction with respect to the phase output is the same and instead the diode 13 in the inner current valve 2 conducts and this shall be turned off the semiconductor device 34 in the auxiliary valve circuit 32 is turned on. The load current inwards towards the phase output from the phase line is more and more transferred to go through the inductor 36 having a large inductance and the current therethrough rises linearly. When the current through the diode 13 has reached zero, i. e. the entire load current flows through the inductor-36, the voltage of the phase output 16 will describe a sine function and swing over to get the same potential as the

second midpoint 23, so that the semiconductor device 10 in the inner current valve 3 then may be turned on at zero voltage thereacross.

By using a resonant circuit of ARCP-type for the current valves 2,3 being switched with the regular pulse width modulation fre- quency, the switching losses of the converter apparatus may be reduced further, at the same time as the cost for the resonance circuit may be made low, should the auxiliary valve circuits 32, 33 only need to handle a low voltage, which is the case when k is less than 0.5, such as 1/3, since the voltage to be handled thereby gets only Udc/6.

The outer current valves 1 and 4 will in the practice switch with a fundamental tone frequency at normal stable SVC-operation, i. e. when the SVC is connected to an alternating voltage net- work with a sine shaped voltage and with a demand upon the converter apparatus to deliver a certain reactive power to or from the connecting network. However, during transient condi- tions it-is conceivable that the voltage set value will a longer pe- riod of time be present in the voltage interval between the levels 25 and 26 (see Figs 4 and 5). It is then possible to let the valves 1 and 2 switch several times per period. However, as an alter- native a higher content of harmonics may be transiently ac- cepted. The voltage regulation of the flying capacitor gets sim- ple thanks to the fact that the converter apparatus only operates with reactive power. As seen over half a period of the funda- mental tone of the phase current. the charging of the capacitor will be equal to the discharging thereof, so that the capacitor voltage is naturally self-balancing.

A converter apparatus according to a third preferred embodi- ment of the invention is illustrated in Fig 3 and this differs from the one according to Fig 2 by having snubber capacitors 39,40 connected in parallel also with the two outer current valves 1,4, so that the time derivative of the voltage is restricted and

thereby the capacitive currents in a connected transformer are limited in connection with switching of the outer current valves 1,4. These snubber capacitors could also be replaced by any other snubber member, such as a RC-circuit. These additional snubber members 39,40 are advantageous when that part of the converter (the inner valves) which operates with the pulse width modulation frequency is designed as a resonance con- verter, for example of ARCP-type, since the voltage derivative on the phase output then gets very low, which may enable a di- rect connection of the converter apparatus to a transformer, without intermediate reactors or filters. A restriction of capaci- tive currents in the connected transformer is in this case achieved as mentioned through the members 39,40.

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 skill in the art without departing from the basic idea of the invention as defined in the appended claims.

As already mentioned, other types of units for achieving"soft- switching"of the inner current valves than an ARCP-circuit may be used, and a person with skill in the art is aware of these pos- sibilities.