SE521243C2 - Converter device and method for controlling such - Google Patents

Abstract

A converter apparatus for converting direct voltage into alternative voltage and conversely comprises a first VSC-converter (6) in cascade connection with at least one second VSC-converter (5). It has also a unit adapted to control the semiconductor devices of the converters and thereby the converter apparatus to generate a phase voltage being constituted by the sum of the voltages generated in said first and second VSC-converters. The first VSC-converter has between the positive and negative pole (7, 8) thereof a direct voltage being substantially higher than the direct voltage of the second VSC-converter (5) between its positive and negative pole.

Description

35. . . . , fl 521 243 as in SVCs (Static Var Compensator), where the DC side consists of one or more freely hanging capacitors.

The invention is not limited to any levels of the voltage of the AC side of the device, the effects that the converter device is able to transmit or the number of phases the AC side of the device has, and thus it may well be designed to generate a single-phase AC voltage, e.g. for example for track feeding of rail vehicles.

However, the invention is particularly, but not exclusively, focused on medium and high voltage, i.e. where the peak voltage on the AC side of the device is 10 kV or higher.

By the cascade connection of at least two VSC inverters of a device of this kind, the device enables an achievement of relatively many different levels on its AC voltage side, which in turn means that relatively fine waveforms of the AC voltage out of the device can be obtained without the invertible semiconductor elements included in the inverters must be switched at particularly high frequencies. Expressed in another way, it becomes possible to obtain a certain quality of the alternating voltage from the device through a lower switching frequency of the controllable semiconductor elements in a pulse width modulation pattern for controlling them than if, for example, a two-level converter were to be used. This thus means lower losses in the inverter device. If you instead use the same switching frequency as with a two-level converter, the curve shape of the alternating voltage can instead be made significantly better.

Previously known inverter devices of this kind have been used in single-phase configuration and also in three-phase configuration in that three mentioned cascade connections are connected in either A- or Y-connection. Fig. 1 illustrates what a previously known such device in three-phase configuration with three mentioned cascade couplings connected in Y-coupling looks like. Each phase 1, 2, 3 has n cascaded single-phase inverters 4, 5, 6, each with two between two and 10 15 20 25 30 35, a. n - f 521 243 inverters belonging to DC poles 7, 8 parallel-connected branches 9, 10 of two series-connected current valves 11-14 comprising a controllable (extinguishable) semiconductor element 15 and an anti-parallel rectifier means 16, such as a rectifier diode, with the center of a branch At one end, the cascade coupling of the single-phase inverters at a phase reactor 19 is connected to the alternating voltage phase 1, while at the other end the cascade coupling is connected. closed to a point 20 in a Y-joint common with the other phases. The DC poles of the respective VSC inverters receive voltage via a DC voltage source illustrated in the form of a capacitor 21-23. In this case, the “direct voltage pole” in the patent claims states that there is some type of direct voltage source connected to or included in the inverter, which is expressed by “one or more means for capacitive energy storage”.

The different DC voltage sources 21-23 supply the same voltage to the respective VSC inverters, and in this way it is possible to achieve 2n + 1 different levels of the voltage on the alternating voltage side of the respective cascade connection. Thus, in the case of three interconnected inverters 7 different levels of the voltage can be achieved, namely -3U, -2U, -U, 0, + U, + 2U and + 3U, where U is the voltage level between said DC voltage poles of the respective VSC inverters. . So many levels enable a "fine" waveform of the AC voltage without using a high switching frequency of the controllable semiconductor elements 15. Incidentally, the figure shows a controllable semiconductor element and a diode drawn per current valve, but these are intended to stand as symbols of a possibly larger number of series-connected such elements arranged to function as a single, i.e. the controllable semiconductor elements connected in series in such a current valve are intended to be controlled simultaneously in order to function as a single such element.

A device of the initially defined type is previously known from, for example, U.S. Patent 5,673,189. especially with regard to the possibilities of achieving an improved curve shape of the voltage on the AC side while using a given number of cascaded VSC inverters and to reduce the switch losses and if possible also the cost of components included in the converter device.

SUMMARY OF THE INVENTION The object of the present invention is to provide a converter device of the type initially defined, which is capable of fulfilling the just mentioned wishes to a not insignificant degree.

This is achieved according to the invention in that in such a converter device the first VSC converter has a direct voltage between its positive and negative poles which is substantially higher than the direct voltage of the second VSC converter between its positive and negative poles.

The new approach to using different levels of voltage between the DC terminals of the VSC inverters opens up completely new possibilities for optimizing the function of such an inverter device. The number of possible levels of the voltage out on said AC voltage side can be increased, so that fewer VSC inverters are needed to achieve an AC voltage of a given quality or with a certain number of VSC inverters an improved curve shape of the AC voltage can be achieved. Thus, in this way, costs can be saved, either by using fewer components of the converter device, or by less expensive filters to filter out disturbances of said alternating voltage. Due to the different level of the voltages between the DC poles of the first and the second VSC inverters, it is also possible to refine the methods for controlling the inverters and, for example, to adjust the frequency with which the different inverters are controlled to the level of said DC voltage. reduce the switch losses and thereby increase the efficiency of the inverter device. The overall efficiency of the converter device can be increased and its harmonic content improved. It is also possible to reduce the step between the voltage levels of said phase voltage and, if desired, to minimize the number of switches of the inverter with the highest voltage between its poles. These possibilities offered by a drive device according to appended claim 1 form the basis for a number of preferred embodiments of the invention defined in appended dependent claims.

According to a preferred embodiment of the invention, the current valves of the first VSC inverter have several semiconductor elements connected in series. Because the first VSC converter has a significantly higher DC voltage between its two poles, it is desirable and also necessary in many applications, especially when using the device in stations in transmission systems, for each current valve to have several semiconductor semiconductor elements connected in order for these together shall be able to maintain the voltage that the valve must maintain when it is blocked. On the other hand, it is conceivable that the one or the other VSC inverters have only one, or fewer, series-connected semiconductor elements in their current valves than the first VSC inverter. Thus, the VSC inverters can be designed in different ways in terms of the number of semiconductor elements, and possibly also in terms of the properties of each individual semiconductor element, so that the existing substantially different level of the DC voltage between the poles of the inverters can be utilized optimally.

According to another preferred embodiment of the invention, the device has several mentioned second, mutually cascaded VSC inverters and all VSC inverters of the cascading coupling have mutually different voltages between their positive and negative poles. As a result, the number of possible levels of the voltage out on the AC side of the inverter device for a given number of VSC inverters of said cascade coupling can be markedly increased in relation to previously known such inverters with the above-mentioned advantages as a result. ,. . According to another preferred embodiment of the invention, the voltage LJ between the DC poles of the vSCeomarts can be expressed as U = kUO, where UD is a given voltage level, k = a '° *, a is a positive numbers separate from 1, p are> 0 and x is equal to the serial number of the respective VSC inverter -1 when assigning the inverters serial numbers from 1 and up. In this way, a stepwise change of the direct voltage level of the respective VSC inverters is achieved and possibilities of, by adding these in a suitable manner, achieving a number of different levels of the total voltage out on the alternating voltage side of the inverter device.

In this case, in a preferred embodiment, p = 1 and a = 2, so that a binary-weighted converter device is achieved with the direct voltages U0, 1 / 2U0, 1 / 4U0, 1 / 8U0, 1 / 16U0 and so on. This means that with m VSC inverters of the cascade coupling, 2 ('“+ 1) -1 levels can be achieved, which is to be compared with 2m + 1 levels of prior art. This means in the case of m = 3 that 15 different levels can be achieved instead of 7. These levels range from -7 / 4U0 to + 7 / 4U0 in steps of 'A UO.

According to another highly preferred embodiment of the invention, the first VSC converter is arranged to handle a substantially higher apparent power than said second VSC converter. Through this design of the device, the first VSC converter can be controlled in a different way than the second VSC converter for achieving a variety of objects, which appear, among other things, from further preferred embodiments of the invention discussed below. For example, relative to prior art devices of this type, the switching frequency of the first VSC inverter with high apparent power can be reduced to achieve lower switch losses and thus higher efficiency of that inverter, while the second VSC inverter with a significantly lower apparent power can be switched. with a higher frequency to achieve the desired waveform of the AC voltage on the AC side of the device.

. . . | According to another preferred embodiment of the invention, which constitutes a further development of the latter embodiment, the ratio between the apparent power / that of the first VSC converter handled by the first VSC converter is the inverter handled the apparent power 0, 10-1.0, said ratio being preferably 0.30-1.0 when the device is designed for SVC operation and 0, 10-0.30 when the device is designed for transmission of active power between its DC and AC side. In the case of SVC operation, i.e. in the case of transmission of reactive power, then it may be advantageous that the ratio is high, because the higher this ratio is, the more contribution the other VSC converter will make to the total apparent power of the device, and the lower switching frequency can be used on the first VSC drive with high apparent power. When active power passes the converter device, however, the second VSC converter can usually not be used to increase the overall apparent power of the system, but it is then mainly used to compensate for various disturbing harmonics generated by the first VSC converter, in which case it is advantageous to allow the apparent power of the other VSC drive to be significantly lower.

According to another preferred embodiment of the invention, the device has a first VSC converter in the form of a three-phase converter with three phase legs with controllable semiconductor elements between its two DC poles, a phase terminal of each phase leg is connected on its AC voltage side to a phase line. the said cascade couplings with said first VSC converter common to the cascade couplings, a second VSC converter of each cascade coupling is connected at one end opposite its AC voltage side to said phase terminal of each phase leg of the first VSC converter and the second VSCs. the rectifiers are formed by H-bridges with two branches of controllable semiconductor elements, a first of which is connected to a phase leg of the first VSC converter and a second connected to the AC voltage side of the device. This embodiment has the advantage that three possible levels can be obtained at the voltage that the other VSC inverter adds to the voltage from the Å. . | - ß 10 15 20 25 30 35 521 243 first VSC inverter, so that in the case of a second VSC inverter per cascade 2x3 = 6 different levels can be obtained on the voltage pulses out on the AC side of the device. According to an alternative embodiment, which is similar to the previous one, in addition to each other VSC converter being connected with its DC side to a phase leg of the fourth VSC converter via a potential midway between the potential of this converter's two DC poles of the inverter DC link and connected to the AC side of the device via a branch of controllable semiconductor devices, the total switch losses of the inverter device increase slightly, as the second VSC inverter can only provide two different levels, so that in the case of a second VSC inverter per cascade the number of possible voltage levels will be four, but instead the advantage is achieved that the number of current valves of the inverter device that must be controlled will be lower and thus costs for components included therein can be saved.

According to another preferred embodiment, the device has a first VSC converter in the form of a three-phase converter with three phase legs with controllable semiconductor elements between its two DC voltage poles, a phase socket of each phase leg is connected on its alternating voltage side to a phase line, this being achieved in that each phase leg is connected to its own secondary winding of a transformer, the other end of the secondary winding is connected to a phase leg of a second VSC inverter in the form of a three-phase inverter, and the transformer has three primary windings, each connected to said phase line of the AC side of the device. An advantage of this embodiment is that only two DC intermediate links are required, one for the first VSC converter and one for the second VSC converter, although we are talking here about three phases, which simplifies the control of the converter device. Furthermore, the fact that the DC intermediate capacitors for both inverters are common to all three phases means that the size of the DC intermediate capacitors can be chosen relatively small, which reduces the costs of the inverter device. . | . According to a preferred embodiment of the invention, which constitutes a further development of the latter embodiment, the direct voltage side of the first VSC converter is connected to a network for transmitting active power between the direct voltage side of the device and the alternating voltage side, wherein the DC side of the first VSC converter is preferably connected to an HVDC transmission system. In such a case, it is particularly advantageous if an additional second three-phase VSC converter is connected to the second VSC inverter side with the midpoints of its phase legs connected to separate phase lines of an alternating voltage network for supplying power into and out of, respectively. said second VSC drive. As a result, the second VSC converter, which has a significantly lower voltage between its DC poles than the first VSC converter, can handle both active and reactive power, since said further second three-phase VSC converter with its alternating voltage mains connection allows the capacitors of the second VSC inverter can be kept charged at the desired level and not be discharged or overcharged when transmitting active power via the other VSC inverter. Consequently, for a given level of the DC voltage of the DC network, the voltage on the AC side of the device can be regulated upwards or downwards by controlling the supply of power towards and out of the second VSC converter via said AC network if desired. If now the first VSC inverter has a significantly higher apparent power than the second VSC inverter and the high apparent inverter is connected to a transmission network for HVDC or functions as a back-to-back inverter, then the inverter with low apparent power is used partly to reduce harmonics generated by the inverter with high apparent power, and partly to generate fundamental tones.

In this way, the high apparent power converter can use a pulse width modulation method with a very low switching frequency and with a fixed ratio between alternating voltage and direct voltage, while the low apparent power converter is used for harmonic compensation, but also for reactive power compensation 10 15 20 25 30 35 521 243 10 and / or for rapid adjustment of the total fundamental voltage of the inverter device on said AC voltage side.

According to another preferred embodiment of the invention, each phase legs of the first VSC inverter are connected to a separate phase line of the AC voltage side of the device, and the device has at least two other VSC inverters, each with a connection to a DC voltage pole of the first VSC. the inverter and a second connection to a pole conductor of a direct voltage network. This way of connecting the VSC inverters to each other is particularly suitable in the case of HVDC, where the first VSC inverter on the DC side is connected to an alternating voltage transmission network via reactors and filters without any intermediate transformer.

According to another preferred embodiment of the invention, the device has at least one dc / dc converter with a high-frequency transformer connected with one side to said second VSC converter and with its other side to a device for supplying power towards and out from said second VSC inverter. This arrangement enables wider application possibilities for the second VSC inverter, both for contributions to reactive power compensation and transmission of active power in a manner similar to the embodiment discussed above with an additional three-phase VSC inverter connected to an AC mains.

According to another preferred embodiment of the invention, the unit is arranged to control the semiconductor elements of said VSC inverter according to a pulse width modulation pattern with a frequency which is lower the higher the DC voltage between the DC voltage poles of the VSC inverter. In practice, this means that the VSC inverters of the device with the lowest apparent power are switched with a higher frequency than inverters with a higher apparent power, so that the total switch losses can be kept down and different types of semiconductor elements can be used in different inverters for optimal adaptation. to the intended switching frequency-. . f t - I 10 15 20 25 30 35 521 243 11 sen. It is pointed out that here VSC converters of the same device are meant and that in those with higher voltage between their direct voltage poles a lower frequency is controlled than those with lower corresponding voltage. However, in a device with a converter with 5 kV between its poles, this can very well be controlled with a lower frequency than a converter with 20 kV between the poles of another device.

According to another preferred embodiment of the invention, said unit is arranged to control the first VSC converter with a determined fundamental frequency and the second VSC converters with a frequency which is substantially higher, preferably a multiple of the fundamental frequency. This keeps the total switch losses of the inverter device at a very low level.

According to another preferred embodiment of the invention, the unit is arranged to, in order to achieve said phase voltage, keep the first VSC inverter in fixed switching positions for as long as possible and during these times control the semiconductor elements of the second VSC inverters to alternately add different voltages. - voltages to the voltage from the first VSC converter according to a pulse width modulation pattern. In this case, it is particularly advantageous if the unit is arranged to control the VSC inverters according to a voltage setpoint for said phase voltage in the form of a sine curve added with a thirteenth component or a multiple of third tone components with respect to the fundamental tone of the sine curve for extending said time. The VSC inverter can be in a fixed position and does not need to be switched. Such an addition of a third tone component, or an optional multiple of third tone components, does not affect the voltage between the phases, which will thus have the desired shape, as is known per se, but the number of switches of the VSC converter with high apparent effect can be further reduced and thus the losses are reduced. The increase in efficiency often means in practice that the apparent power of the inverter device can be increased due to a lower thermal load of components included therein. According to another preferred embodiment of the invention, said unit is arranged to control the second VSC inverters to add the voltage to the voltage from the first VSC converter to compensate for low frequency voltage harmonics generated as a result of the first VSC converter is arranged to be in a fixed position for large parts of the period of the fundamental tone voltage on the alternating voltage side of the device.

According to yet another preferred embodiment of the invention, the device is designed for SVC operation, i.e. for a reactive power compensation, and the unit is arranged to control the semiconductor elements of the other VSC inverters to generate voltage pulses with a fundamental tone which is offset relative to the current. through the 90 degree electrical converter and controlling the first VSC converter with the same ratio of voltage fundamental to current through that converter to add the contribution of the first and second VSC inverters to a reactive power compensation. Advantages of using the other VSC inverters in this way are clear from the discussion above.

According to another preferred embodiment of the invention, the device is designed for transmitting active power between its DC and AC side, and said unit is arranged to control the semiconductor elements of the other VSC inverters for compensating for harmonics generated as a result of the operation of the first VSC converter without making any contribution to the transmission of active power.

According to another preferred embodiment of the invention, said unit is arranged to control only the semiconductor elements of two of the phase legs of the first VSC inverter at a time during parts of the period of voltage of the inverter and at the same time have the AC phase side connection of the third phase The DC interface of the VSC converter and switching between the three phase legs with respect to said connection to one of the poles at the transition between said period parts for applying a so-called deadband PWM to this VSC converter , and the unit is arranged to simultaneously control the VSC inverters according to a voltage setpoint for said phase voltage in the form of a sine curve added with a zero-sequence component or zero-sequence components, for example a third-tone component or a multiple of third-tone components. The advantage of such a deadband PWM is above all that the switching frequency of the first VSC converter, which is preferably arranged to handle a high apparent power, can then be reduced to 2/3, since the phases only need to switch during 2/3 of the fundamental tone. period. The disadvantage is that zero-sequence components of a third-tone character or multiples of third tones need to be added to the voltage setpoint of all phases, which does not affect the phase-phase voltage but rather the voltage between phase and earth. Thanks to the introduction of other VSC inverters with low apparent power, however, these zero-sequence components can be compensated away, which leads to the introduction of deadband PWM on inverters with high apparent power without this leading to any negative consequences for the voltage on the phase line relatively soil.

The invention also relates to a method for controlling a converter device as above, in which the semiconductor elements of said VSC converter are controlled according to a pulse width modulation pattern with a frequency which is the lower the higher the DC voltage between the DC poles of the VSC converter. is. The advantages of this method and of embodiments of the method defined in the appended dependent claims are apparent with all the desired clarity from the above discussion of preferred embodiments of the converter device according to the invention.

The invention also relates to a computer program product and a computer-readable medium according to the corresponding appended claims. It is readily appreciated that the method of the invention defined in the appended set of method patent claims is well suited to be performed by program instructions from a processor operable by a computer program provided with the program step in question.

Additional advantages and advantageous features of the invention appear from the other dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS As exemplified preferred embodiments of the invention are described below with reference to the accompanying drawings, in which: Fig. 1 is a simplified circuit diagram of a converter device according to a preferred embodiment of the invention, Fig. 2 is a corresponding view of a transducer device according to a second preferred embodiment of the invention, Figs. Fig. 2, which is used for pulse width modulation of the converter device, Fig. 5 schematically illustrates what a pulse width modulation pattern based on the voltage setpoint according to Fig. 3 can look like for a converter device according to Fig. 2, Fig. 6 is a Fig. 2 corresponding view of a converter device according to a three The preferred embodiment of the invention, Fig. 7 is a drive device according to a fourth preferred embodiment of the invention, which constitutes a variant of the drive device according to Fig. 2, 10 15 20 25 30 35 521 243 15 Fig. 8 is a Fig. 2 corresponding view of a converter device according to a fifth preferred embodiment of the invention, Fig. 9 is a Fig. 8 corresponding view of a converter device which constitutes a variant of that shown in Fig. 8, Fig. 10 is a Fig. 8 corresponding view of a converter device according to a further variant in the converter device of Fig. 8, Fig. 11 is a Fig. 2 corresponding view of a converter device according to an eighth preferred embodiment of the invention, and Fig. 12 is finally a Fig. 2 corresponding view of a converter device according to a ninth preferred embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION Fig. 1 illustrates a drive device with a general construction which is known per se and described above with three cascade connections, one for each phase of the AC network, interconnected according to a Y in the common point 20. What is new with the present invention, however, is that the voltage levels of the different DC voltage sources 21, 22, 23 are different.

More specifically, in a preferred embodiment, the level of the VSC inverter 4 is 2 ° U0, in the subsequent 2'1U0 and so on by 2 '* U0 of the last VSC inverter 6 with x = the serial number of the respective VSC inverter of cascade - clutch -1. For the sake of simplicity, we can now assume that each cascade connection has only three single-phase inverters. Then the inverter 4 between its connections 24 and 25 can supply the voltage -U0 / 4, 0 or + U0 / 4 depending on the condition of the current valves 11-14. The same applies to the single-phase inverter 5 which can supply 0.1 / 2U0 or -1 / 2U0 between the connection 24 and the connection 26 to the subsequent single-phase inverter 6. For that single-phase inverter, the levels 0, U0 and -U0 apply in turn. By adding these different combinations, 15 different levels can be achieved: -7 / 4U0 (-4/4 -2/4 -1/4), -6/4 (-4/4 - 2/4 +0), -5/4 (-4/4 -1/4 +0), -4/4 (-4/4 + O +0), -3/4 (-4/4 + O +1/4 or -1/4 -2/4 +0) and so on to + 7 / 4U0. The advantages of a device of this kind have been mentioned above.

Should 3 instead of 2 be used as the base, 3 "'possible levels could be achieved at each cascade AC voltage side, where m is the number of single-phase inverters within each cascade. For example, in the case of 3 single-phase inverters in each cascade at such 27 different levels is achieved from -13 / 9U0 to + 13 / 9U0 in increments of 1 / 9U0. extinguishable semiconductor elements of the single-phase converter 4 with the lowest voltage between their DC poles and the frequency of the control of the single-phase converter semiconductor elements are increased in the direction from the single-phase converter 6 to the single-phase converter 4 to achieve a desired pulse width modulation pattern (PWM).

In this case, for example, IGBTs (linsulated Gate Bipolar Transistor) could be used for higher switching frequencies and GTOs (Gate Turn-Off thyristors) for lower switching frequencies. The unit 27 for controlling the respective single-phase converter, i.e. its power semiconductor element 15, is designed to achieve this.

Fig. 2 shows a converter device according to a second preferred embodiment of the invention, in which the first VSC converter 6 here appears in the form of a three-phase converter with three phase legs 28-30 with controllable semiconductor elements between its two direct voltage poles (see far right in the figure). A phase socket of each phase leg is connected on its alternating voltage side to a phase line 1-3. This is done via a second VSC converter 5, 5 ', 5 "for each phase line, the second VSC converter being formed by an H-bridge with two branches 31, 32 of controllable semiconductor elements, one of which is connected to a phase legs of the first VSC converter and a second to the AC side of the device. We assume here that the voltage between the DC voltage coils 7, 8 of the first VSC inverter is U, and then the voltage of the DC intermediate voltage of the other VSC inverters is 33 k x U, where k is substantially lower than 1, preferably 0.05-0.5. This means that the apparent power of the second VSC inverters is low in relation to the apparent power of the first VSC inverter, when the same phase current l passes the first and the second inverters. The series-connected VSC inverters 5, 5 ”, 5” are controlled according to a pulse width modulation pattern, whereby they generate an alternating voltage between their input and output. The voltage between the input and the output can assume three discrete levels, namely kx U, 0 or -kx U. The voltage on the phase lines 1, 2, 3 relative to the center point 34, which is defined as the phase voltage, can assume a total of 2 x 3 = 6 levels, compared with 2 levels for the case of using only the first inverter 6. In this embodiment, the first VSC inverter has three connection points 35-37 on its alternating voltage side, while the second VSC inverter has two connection points 38, 39 for each phase.

In the case of reactive power compensation, no active exchange takes place with the surrounding alternating voltage network other than to cover the losses that the converter device loads on the network. This means that in the case of reactive power compensation, the second VSC inverter connected in series in each phase can be controlled to generate a fundamental voltage which is 90 electrical degrees phase-shifted in relation to the fundamental tone of the phase current, just like the first VSC inverter.

In this way, the inverter with low apparent power can be controlled to make a contribution to the overall reactive power of the inverter device. Furthermore, the second VSC inverter connected in series in each phase can be controlled to compensate for the low-frequency voltage harmonics generated as a result of the first VSC inverter not switching during large parts of the fundamental tone period. The harmonics this is a question of are primarily fifth and seventh tones, eleventh and thirteenth tones, but also higher tones. In the case of reactive power, the factor k can be chosen freely, preferably in the range 0.15-0.5. For example, if k is selected to be 1/3, the six voltage levels will be uniform 521 243; «. »_. 18 distributed, which can be particularly advantageous. The higher the number selected, the larger the contribution that the series-connected second VSC inverters make to the total apparent power of the inverter device, and the lower switching frequency can be used on the first high apparent power VSC inverter. Fig. 3 illustrates the use of a sine curve 40 as a voltage setpoint for the phase voltage of the converter device according to Fig. 2 intended to form the basis for the pulse width modulation of the VSC converter included therein. The voltage levels U and - U, respectively, which can be achieved between the center point 34 and the alternating voltage-side connection point 35-37 of the respective phases are marked, as well as the possible additions that can be made via control of the other VSC inverters around each level, so that 41 corresponds to (1 / 2 + k) U, 42 (1/2-k) U, 43 (-1 / 2 + k) U and 44 (-1 / 2-k) U. As long as the voltage setpoint is between levels 41 and 42, it is sufficient for the second VSC inverter with low apparent power to be connected in series in each phase. For the first VSC inverter with high apparent power, its phase socket 35-37 during this time period can be connected to the positive pole 7. Correspondingly, when the voltage setpoint is between levels 43 and 44, then the phase socket for the first VSC the inverter connected to the negative pole and the pulse width modulation switching is performed by the second VSC inverter connected in series in phase with low apparent power. It is then only for the rest of the time, see arrow 45, that the first VSC inverter with high apparent power must be switched. This means that the number of switches that the “large” inverter with high apparent power must perform during a period of the fundamental tone can be greatly reduced, with lower switch losses as a result. Typically, the switching frequency of the second VSC converter is in the region of 1-3 kHz. Fig. 4 shows an alternative possibility for designing the voltage setpoint underlying the pulse width modulation of a converter device according to Fig. 2. In this case, a converter device according to Fig. 2 has a The third-tone component, which here is about 20% of the fundamental tone, has been added to the voltage setpoint in all phases. Such addition of a third tone component or any multiple of third tone components does not affect the voltage between the phases. The voltage setpoint of the phase-to-phase voltage is thus still sinusoidal. This pulse width modulation method can advantageously be combined with the use of a second VSC converter with low apparent power connected in series in each phase. It can be seen that the voltage setpoint of the phase voltage now has steeper edges, so that the time (arrow 45) that the voltage setpoint is between levels 42 and 43 is shortened in comparison with the corresponding time period of the control according to Fig. 3. The number of PWM switches the inverter with high apparent power must perform during a period of the fundamental tone, which usually has a frequency of about 50 Hz, can thereby be further reduced, which increases the efficiency of the inverter. Furthermore, this modulation form gives a higher fundamental tone voltage out on the AC voltage side for a given level of the DC voltage of the first VSC inverter, which also increases the efficiency of the inverter device and lowers its cost.

Fig. 5 illustrates in the case reactive power how the different voltage pulses on the alternating voltage side of the device according to Fig. 2 can look like when using the control form which has been discussed with reference to Fig. 3.

The converter device according to Fig. 2 is particularly well suited for reactive power compensation, but it can also be used for transmitting active power in the manner described below.

If instead active power passes the first VSC inverter with high apparent power (for example in the case of HVDC or back-to-back application), then the second VSC inverters connected in series in each phase cannot be used in the same way to increase the system. the total apparent effect of the ning. Namely, these inverters cannot contribute to the active power of the inverter device, as this would result in either the DC voltage capacitor of each other VSC inverter being either charged or discharged. However, the series-connected other VSC converter converters can in this case be controlled to compensate for voltage components of, for example, fifth and seventh tones, eleventh and thirteenth tones and higher harmonics generated by the converter with high apparent power as above. In the case of active power, the factor k is advantageously selected to a low number, for example 5-15%, as it is normally sufficient to add a small voltage component in series with the voltage from the large, first VSC converter to generate and compensate remove the above harmonics.

Thus, in the case of active power, it is assumed that both the first VSC converter and the smaller second VSC converter connected in series in each phase work with pulse width modulation. The larger number of available levels means that for a given requirement that the inverter should not generate more than a given amount of harmonics out on connecting networks 1-3, the switching frequency of the first VSC inverter 6 can be reduced with high apparent power.

Fig. 6 illustrates a converter device which differs from that of Fig. 2 only in that the other VSC converters, instead of being formed by H-bridges, are connected with their DC voltage side to a phase leg of the first VSC converter via a a potential substantially between the potential of both inverters' DC voltage poles lies the potential of the inverter's DC intermediate, which here has two capacitors, and connected to the AC side of the device via a branch of controllable semiconductor elements. By arranging such a half bridge, only two voltage levels, namely kU / 2 and -kU / 2, respectively, can be added to the voltage of the first VSC inverter to achieve the phase voltage. Thus, only four levels can be achieved on the phase voltage of the respective phase line 1-3 relative to the center point 34. The advantage of this embodiment is that the number of valves that need to be controlled decreases. The unit 27 can use the same control procedure as that indicated above. Fig. 7 illustrates a further development of the embodiment according to Fig. 2, which differs from it in that each cascade coupling has two other VSC inverters, so that here it is achieved that if, for example, k = 1/3 is selected for the other VSC inverters 5, 5 ', 5 "and k = 1/6 is selected for the other VSC inverters 4, 4' and 4", then a total of 13 possible levels, which are evenly distributed between + U and -U with step U / 6. Should instead k = 1/3 and 1/9, respectively, a full 18 levels are obtained, which are evenly distributed between + 17 / 18U and -17 / 18U with step 1 / 9U. The increased number of levels achieved in this way can be used to switch the inverter current valves with a lower frequency to achieve a given curve shape and thus reduce the switch losses or switch the valves with unchanged frequency and achieve an improved curve shape with less harmonic content. Fig. 8 shows a device according to a further variant of the invention, which is very suitable when the inverter device is connected to a connecting network 1-3 via a transformer 47. Here the first VSC inverter is connected on its alternating voltage side with each phase pin to a separate secondary winding 48-50 of the transformer, and the other end of the secondary winding is connected to a phase leg of a second VSC converter in the form of a three-phase converter. The transformer further has three primary windings 51-53, each connected to each of the mentioned phase lines 1-3 at the alternating voltage side of the device. Thus, the Y-coupled transformer on the secondary side has been provided in phases with an additional bushing in the neutral point 54 of the transformer, to which the second VSC converter with low apparent power is connected. In this connection, four different levels are obtained per phase, but it is pointed out that this embodiment can be freely varied with other embodiments according to the invention if more levels are desired. An advantage of this embodiment is that it contains only two DC intermediates, which simplifies the control of the converter device, and that the DC intermediate capacitors for both VSC inverters are common to all three phases, which means that the size of the DC intermediate capacitors can be chosen relatively small. 15 20 25 30 35 ». . . ~ 1. 521 243 ,, -w- 22 which reduces the cost of the inverter. Here, the phase voltage is above the secondary winding of the transformer. Fig. 9 illustrates a variant of the embodiment according to Fig. 8, which differs from this in that the first VSC converter 6 with high apparent power on its DC voltage side is connected to a transmission system for HVDC or alternatively directly to a similar station for a reverse to-back transmission, as indicated by the cables 55, 56. Since the voltage between the DC coils of the first VSC converter is now assumed to be high, reactors 57 and filters 58 have also been placed between this high-voltage converter and the transformer 47, in order to avoid the transformer exposed to high voltage derivatives relative to earth.

A further modification of the embodiment according to Fig. 8 is shown in Fig. 10 and this differs from the embodiment according to Fig. 9 in that on the DC voltage side of the second VSC converter 5 a further three-phase VSC converter 76 is connected with the center points and its phase legs connected. to each phase line of an alternating voltage network 60 for supplying power in towards and out of said second VSC inverter 5 with lower apparent power.

In this way, the converter 5 with low apparent power can be used partly to reduce harmonics generated by the converter 6 with high apparent power, and partly to generate fundamental tones. In this way, the high apparent power converter 6 can use a pulse width modulation pattern with a very low switching frequency and with a fixed ratio between alternating voltage and direct voltage, while the low apparent power converter 5 is used both for harmonic compensation and for reactive power compensation and / or fast. adjusting the total fundamental voltage of the inverter device on the AC voltage side.

Fig. 11 illustrates a converter device according to a further preferred embodiment of the invention, in which each phase leg of the first VSC converter 6 is connected to a separate phase line 1-3 of the alternating voltage side of the device and two other VSC converters 5, 5 are connected on the one hand to each DC voltage pole of the first VSC converter and on the other hand to a pole conductor of a DC network. This embodiment of the invention is particularly suitable in the case of HVDC, where the first VSC inverter on the AC side is connected to an AC transmission network 1-3 via reactors 58 and filters 59 without any intermediate transformer. The other VSC inverters with low apparent power are preferably controlled synchronously with a pulse width modulation pattern, so that both either add or subtract a voltage kU to the respective pole voltage relative to ground. They can also be connected so that the pole voltage that the converter 6 has with a high apparent power becomes identical to the voltage across the respective direct voltage capacitor relative to earth. The current passing through the two VSC inverters 5, 5 'with low apparent power is essentially a direct current. The voltage they generate is a pure AC voltage without any DC component. Because they switch synchronously, they will generate a zero-sequence voltage that is found in all phases on the AC side. The first VSC inverter 6 here has three connection points on its alternating voltage side and two 72, 73 on its direct voltage side, while the respective VSC inverter 5 has two connection points 74, 75.

The phase voltage for one phase is between 34 and 1.

The converter device according to this embodiment is well suited for the use of so-called deadband PWM. In this case, for a given part of the period of the voltage fundamental tone, only two of the three phases of the first VSC converter 6 are allowed to switch with their PWM pattern with high apparent power, while the third phase is connected to either DC voltage pole 7, 8. For example. one phase can be left connected to one DC voltage pole below 60 electrical degrees for the fundamental tone period, then the pole is switched below 120 electrical degrees, and then the pole below 60 electrical degrees is connected to the opposite pole, whereupon allows the pole to switch again below the remaining 120 electrical degrees. The advantage of deadband PWM is, as stated above, above all that the switching frequency of the VSC inverter with high apparent power can then be reduced to 2/3, since the phases only need to switch during 2/3 of the period of the fundamental tone voltage. The disadvantage, on the other hand, is that zero-sequence components of a third-tone character or multiples of third-tones need to be added to the voltage setpoint of all phases, which, however, does not affect the phase-phase voltage but rather the voltage between phase and earth. However, thanks to the introduction of the two synchronously controlled VSC inverters 5, 5 'with low apparent power, these zero-sequence components can be compensated away, which leads to the introduction of deadband PWM on the VSC inverter 6 with high apparent power without this leading to some negative consequences for the voltage on each phase line 1-3 relative to earth. In this case, a typical value of the factor k can be about 15-20%. Even larger values of the factor k can be used. This can be valuable, for example, if the VSC inverter with high apparent power is connected directly, ie without a transformer, to an AC transmission network that is impedance grounded. In the case of single-phase earth faults, for example, a zero-sequence component then arises on the alternating voltage side, among other things of a fundamental tone character. In such a case, the VSC inverters with low apparent power can, provided that the factor k is selected sufficiently large, compensate for this zero-sequence component and the inverter device can transmit power regardless of whether single-phase faults occur in connecting networks or not.

Finally, Fig. 12 illustrates a device according to a further preferred embodiment of the invention, in which the other VSC inverters can also contribute to the transmission of active power in that their direct voltage side can exchange energy with a further alternating voltage network 61 via a dc / dc converter 62. This embodiment has a dc / dc converter 62 with a high frequency transformer 63 connected with one side to a second VSC converter 5 with its other side to a device (61) for supplying power into towards and out of the said VSC converter. More specifically, the device has a dc / dc converter common to all phase lines 1-3 with a said transformer with three secondary windings 64-66 connected to a converter part 10 connected to each other VSC converter 10 15 20 25 521 243 25 67-69 and a primary winding 70 connected to a single converter part 71. connected to said device, whereby the additional network 61 can supply power to or draw power from the other VSC inverters 5 with low apparent power, so that they can function in a similar manner as the the second VSC converter 5 of the embodiment of Fig. 10.

The invention is of course not in any way limited to the preferred embodiments described above, but a number of possibilities for modifications thereof should be obvious to a person skilled in the art, without this departing from the basic idea of the invention as defined. in the appended patent claims.

For example, the last described embodiment could be varied by arranging a separate transformer / phase. However, it is advantageous to use a multi-winding transformer according to Fig. 12, as in this way the number of primary windings can be reduced.

"Adding voltage" is to give in this dissertation the meaning of also covering the addition of negative voltages, ie a subtraction of a positive voltage.

The described converter devices are preferably designed to handle phase voltages between 5 kV and 500 kV, although other voltage levels are conceivable.

Claims (56)

10 15 20 25 30 35 521 243 26 Patent claims A converter device for converting direct voltage to alternating voltage or vice versa, comprising a first VSC converter (6) cascaded with at least one second VSC converter (4, 5), each said VSC converter of the device containing a direct voltage intermediate (21-23, 33) with a positive and negative pole and one or more means for capacitive energy storage, partly current valves (11-14) with controllable semiconductor elements, the device comprising a unit (27) arranged to control the semiconductor elements to generate voltages between the respective The connection points of VSC inverters are mutually separated in steps of the magnitude of the DC voltage between the positive and negative poles of the DC DC intermediate, and wherein the unit is arranged to control said semiconductor element and thus the inverter device to generate a phase voltage constituted by the sum of said first second VSC inverter, characterized in that the first VSC inverter n has a direct voltage between its positive and negative poles that is significantly higher than the direct voltage of the other VSC inverter between its positive and negative poles. Device according to claim 1, characterized in that said unit is arranged to control the semiconductor elements to utilize the ratio of the direct voltages of the first and second VSC inverters to advantageously generate a said phase voltage with several available voltage levels. Device according to claim 1, characterized in that said unit is arranged to control the semiconductor elements to utilize the ratio of the DC voltages of the first and second VSC inverters to increase the overall efficiency of the inverter device. 10 15 20 25 30 35 521 243 27 4. A device according to claim 1, characterized in that said unit is arranged to control the semiconductor elements to utilize the ratio of the DC voltages of the first and second I / SC inverters to improve the harmonic content of the inverter device. Device according to claim 1, characterized in that said unit is arranged to control the semiconductor elements to utilize the ratio of the DC voltages of the first and second VSC inverters to reduce the step between different possible voltage levels of said phase voltage. Device according to claim 1, characterized in that said unit is arranged to control the semiconductor elements to utilize the ratio of the DC voltages of the first and second VSC inverters to minimize the number of switches of the VSC inverter (6) with the highest voltage between its poles. Device according to one of Claims 1 to 6, characterized in that the current valves of the first VSC inverter have several semiconductor elements (15) connected in series. Device according to any one of claims 1-7, characterized in that it has several said second, cascaded VSC converters (4, 5). Device according to Claim 8, characterized in that all the VSC inverters (4-6) of the cascade coupling have mutually different voltages between their positive and negative poles. Device according to claim 9, characterized in that the voltage U between the DC poles of the VSC inverters can be expressed as U = kU0, where UO is a given voltage level, k = a "" ", a is a positive number different from 1, where p is> 0 and x are equal to the serial number of the respective VSC inverter -1 when assigning the inverters serial numbers from 1 and up. Device according to claim 10, characterized in that p = 1. . ~ s, Q - ~ 10 15 20 25 30 35 521 243 28 Device according to claim 10 or 11, characterized in that a is an integer 2 2. Device according to claim 12, characterized in that a = 2. Device according to claim 12, characterized in that a = 3. Device according to any one of the preceding claims, characterized in that the first VSC converter (6) is arranged to handle a substantially higher apparent power than said second VSC converter (4, 6). Device according to claim 15, characterized in that the ratio between the apparent power handled by the respective VSC inverter (5) / the apparent power handled by the first VSC inverter is 0.10-1. Device according to claim 16, characterized in that it is designed for SVC operation, and that said ratio is 0.30-1. Device according to claim 16, characterized in that it is designed for transmitting active power between its direct and alternating voltage side, and that said ratio is 0.10-0.30. Device according to any one of the preceding claims, characterized in that it comprises several said cascade couplings, which are connected at one end opposite the alternating voltage side of the respective cascade coupling in a common point (20). Device according to claim 19, characterized in that three said cascade connections are interconnected in said common point (20) to form a Y-connection of three phases of the DC voltage side of the inverter device. Converter device according to one of Claims 1 to 18, characterized in that it has a first VSC converter (6) in the form of a three-phase converter with three phase legs. (28-30) with controllable semiconductor elements between their two DC poles, and that a phase socket of each phase leg is connected to a phase line on its AC voltage side (1-3). Device according to claim 21, characterized in that it has three said cascade couplings with said first VSC converter (6) common to the cascade couplings, and that a second VSC converter (5, 5 ', 5 ") of each cascade coupling is connected at one end opposite its AC voltage side to said phase socket of each phase leg (28-30) of the first VSC converter. Device according to claim 22, characterized in that said second VSC inverters are formed by H-bridges with two branches (31, 32) of controllable semiconductor elements, a first of which is connected to a phase leg of the first VSC the drive and a second are connected to the AC side of the device. Device according to claim 22, characterized in that each second VSC converter is connected either with its DC side to a phase leg of the first VSC converter via a potential of the converter's DC voltage lying substantially at a potential substantially between the potential of this converter's two DC poles and connected to the AC side of the device via a branch of controllable semiconductor elements or via said branch connected to said phase leg of the first VSC inverter and with its intermediate potential to the AC side of the device. Device according to claim 21, characterized in that the first VSC converter (6) is connected on its alternating voltage side with each phase leg to a separate secondary winding (48-50) of a transformer (47), that the second end of the secondary winding is connected to a phase leg of a second VSC converter (5) in the form of a three-phase converter, and that the transformer has three transducers. «.-. v fl 521 243 30 primary windings (51-53), each connected to a separate phase line (1-3) of the alternating voltage side of the device. Device according to claim 25, characterized in that the DC voltage side of the first VSC converter is connected to at least one free-hanging capacitor for SVC (Static Var Compensator) operation of the device. Device according to claim 25, characterized in that the direct voltage side of the first VSC converter is connected to a network (55, 56) for transmitting active power between the direct voltage side of the device and the alternating voltage side. Device according to claim 27, characterized in that the DC voltage side of the first VSC converter is connected to an HVDC (High Voltage Direct Current) transmission system_ Device according to claim 27, characterized in that the direct voltage side of the first VSC converter (6) is connected via a direct voltage intermediate to a VSC converter with an opposite alternating voltage side connected to an alternating voltage network for back-to-back transmission between the AC mains and the AC side of the device. Device according to claim 27, characterized in that to the DC voltage side of the second VSC converter (5) a further three-phase VSC converter (4) is connected with the midpoints of its phase legs connected to each phase line of an alternating voltage network (60) for supply of power towards and out of said second VSC inverter. Device according to one of Claims 27 to 30, characterized in that filters (59) are arranged on the alternating voltage side of the first VSC inverter (6) to filter out harmonics generated during the switching of the semiconductor elements of the inverters. 10 15 20 25 30 35 521 243 31 Device according to claim 21, characterized in that each phase leg (28-30) of the first VSC converter (6) is connected to a separate phase line (1-3) of the growth voltage side of the device, and that the device has at least two other VSCs. inverters (5, 5 '), each with a connection to a DC terminal of the first VSC inverter and a connection to a pole conductor of a DC network (55, 56). Device according to claim 32, characterized in that said second VSC inverters (5, 5 ') are formed by H-bridges with two branches of controllable semiconductor elements, a first of which is connected to a direct voltage pole of the first VSC the drive (6) and a second one are connected to a said pole conductor. Device according to claim 22, characterized in that it has at least one dc / dc converter (62) with a high-frequency transformer (63) connected with one side to said second VSC converter (5) and with its other side to a device (61) for supplying power into and out of said second VSC inverter. Device according to claim 34, characterized in that it has a separately mentioned dc / dc converter for each phase line for power exchange with associated other VSC converters. Device according to claim 34, characterized in that it has a dc / dc converter (62) common to all phase lines (1-3) with said transformer with three sub-secondary windings (64-66) connected to each converter part (67-69) connected to the respective other VSC inverters and a primary winding (70) connected to a single converter part (71) connected to said device. Device according to any one of the preceding claims, characterized in that the unit (27) is arranged to control the semiconductor elements of said VSC converter according to a pulse width modulation pattern with a frequency which is the lower the higher the DC voltage. 243 32 the voltage between the DC poles of the VSC inverter in question is. Device according to claim 37, characterized in that said unit (27) is arranged to control the first VSC converter (6) with a determined fundamental tone frequency and the second VSC inverters (4, 5) with a frequency which is significantly higher, preferably a multiple of the fundamental frequency. Device according to any one of claims 21-38, characterized in that said unit (27) is arranged to hold the first VSC converter (6) in fixed switching positions for as long times as possible and during these times in order to achieve said phase voltage. controlling the semiconductor elements of the second VSC inverters (4, 5) to alternately add different voltages to the voltage from the first VSC inverter according to a pulse width modulation pattern. Device according to claim 39, characterized in that said unit (27) is arranged to control the VSC inverters according to a voltage setpoint (40) for said phase voltage in the form of a sinusoidal curve added with a thirteenth component or a multiple of thirteenth components. with respect to the fundamental tone of the sine curve for extending said time the first VSC converter may be in a fixed position and not have to be switched. Device according to any one of claims 21-40, characterized in that said unit is arranged to control the second VSC inverters (4, 5) to add voltages to the voltage from the first VSC inverter (6) to compensate for low-frequency voltage harmonics generated as a result of the first VSC converter being arranged to be in a fixed position for large parts of the period of the fundamental tone voltage on the AC voltage side of the device. Device according to any one of claims 21-27 or 34-41, characterized in that it is designed for SVC operation, i.e. for a reactive power compensation, and that said unit (27) is arranged to control the semiconductor elements of the second VSC inverters (4, 5) to generate voltage pulses with a fundamental tone offset relative to the current through the inverters by 90 electric degrees and to control the first VSC inverter (6) with the same ratio between the fundamental fundamental tone and the current through that inverter for adding the first and second VSC inverters' contribution to reactive power compensation. Device according to any one of claims 21-25 or 27-41, characterized in that it is designed for transmitting active power between its DC and AC voltage side, and that said unit (27) is arranged to control the semiconductor elements of the the second VSC inverters (4, 5) for compensating for harmonics generated as a result of the operation of the first VSC inverter (6) without making any contribution to the transmission of active power. Device according to claim 32 or 33, characterized in that said unit (27) is arranged to control said second VSC converter (5, 5 ') to switch synchronously. Device according to any one of claims 21-31 or 34-44, characterized in that said unit (27) is arranged to control only the semiconductor elements of two of the phase legs (28-30) of the first VSC converter at a time during parts of the period of the voltage fundamental tone of the inverter and at the same time have the alternating voltage side connection of the third phase leg connected to one of the poles (7, 8) of the first VSC inverter DC and switch between the three phase legs with respect to said connection to one of the poles at transition between said period parts for applying a so-called deadband PWM to this VSC converter, and that said unit (27) is arranged to simultaneously control the VSC inverters according to a voltage setpoint for said phase voltage in the form of a sine curve added with a zero-sequence component or zero-sequence components, for example a third-tone component or a multiple of third-tone components. 10 15 20 25 30 35 521 243 34 A method for controlling a converter device according to claim 1, characterized in that the semiconductor elements of said VSC converter are controlled according to a pulse width modulation pattern with a frequency which is lower the higher the DC voltage between the DC poles of the VSC converter. Method according to claim 46, characterized in that the first VSC converter (6) is controlled at a certain fundamental frequency and the second VSC converters (4, 5) are controlled at a frequency which is substantially higher, preferably a multiple of the fundamental frequency. late. A method according to claim 46 or 47 for controlling a converter device which otherwise has a first VSC converter (6) in the form of a three-phase converter with three phase legs (28-30) with controllable semiconductor elements between its two direct voltage poles and with a phase socket of each phase leg on its alternating voltage side connected to a phase line (1-3), characterized in that to achieve said phase voltage the first VSC converter is kept in fixed switching positions for as long as possible and during these times the semiconductor elements of the other The VSC inverters (4, 5) to alternately add different voltages to the voltage from the first VSC inverter according to a pulse width modulation pattern. A method according to claim 48, characterized in that the VSC inverters are controlled according to a voltage setpoint (40) for said phase voltage in the form of a sine curve added by a third tone component or a multiple of third tone components with respect to the fundamental tone of the sine curve to extend said time the first VSC inverter can be in a fixed position and does not have to be switched. Method according to claim 48 or 49, characterized in that the second VSC inverters (4, 5) are controlled to add voltages to the voltage from the first VSC inverter to compensate for low-frequency voltage harmonics generated as a result of The first VSC inverter (6) is arranged to be in a fixed position for large parts of the period of the fundamental voltage on the AC voltage side of the device. A method according to any one of claims 46-50, wherein the device is designed for SVC operation, i.e. for a reactive power compensation, characterized in that the semiconductor elements of the other VSC inverters (4, 5) are controlled to generate voltage pulses with a fundamental tone offset relative to the current through the converter by 90 electrical degrees, and that the first VSC converter (6) is controlled with the same ratio of voltage fundamental tone to the current through that converter for adding the first and the second The VSC inverters' contribution to reactive power compensation. A method according to claims 46-50, wherein the device is designed for transmitting active power between its DC and AC voltage side, characterized in that the semiconductor elements of the other VSC inverters (4, 5) are controlled to compensate for harmonics generated to resulting from the operation of the first VSC drive (6) without any contribution to the transmission of active power. A method according to claim 46, which is intended to be applied to a drive device with a first VSC drive (6) in the form of a three-phase drive with three phase legs with controllable semiconductor elements between its two DC poles and in which a phase socket of each phase leg is on its alternating voltage side connected to a phase line (1-3), characterized in that during parts of the period of the voltage fundamental tone of the first VSC inverter only the semiconductor elements of two of this inverter's phase legs (28-30) are controlled at a time to switch and at the same time the alternating voltage side connection of the third phase leg is kept connected to one of the poles (7, 8) of the DC voltage converter of the first VSC converter, whereby it is switched between the three phase legs with respect to said connection to one of the poles at transition between said period parts, for application of a so-called dead-band PWM on this VSC inverter, and that the VSC inverters are simultaneously controlled according to a voltage; is (40) for said phase voltage in the form of a sine curve added with a zero-sequence component or zero-sequence components, i.e. a third-tone component or a zero-type of third-tone components. A computer program product intended to be loaded directly into the internal memory of a computer and comprising software code portions for instructing a processor to perform the steps of any of claims 46-53 when the product is run on a computer. The computer program product of claim 54 provided at least in part over a network such as the Internet. A computer-readable medium having a program registered thereon intended to cause a computer to control the steps of any of claims 46-53.