Method for operating a converter circuit and apparatus for implementing the method

The disclosure specifies a method for operating a converter circuit, the converter circuit having a converter unit with a large number of drivable power semiconductor switches and with a three-phase electrical AC voltage system, in which the drivable power semiconductor switches are driven by means of a drive signal (SA) formed from a control signal ((SR), and the control signal (SR) is formed by adjusting an H-th harmonic component of system currents (iNH) to a system current setpoint value (iNHref), where H=1, 2, 3, . . . . In order to reduce a harmonic component in the system voltages, the system current setpoint value (iNHref) is formed by adjusting an H-th harmonic component of system voltages (uNH) to a predeterminable system voltage setpoint value (uNHref), the control difference (uNHdiff) from the H-th harmonic component of the system voltages (uNH) and the system voltage setpoint value (uNHref) being weighted by a system impedance (yNH) determined with respect to the H-th harmonic component. In addition, an apparatus is disclosed for implementing the method.

RELATED APPLICATION

This application claims priority under 35 U.S.C. §119 to European Patent Application No. 08152989.3 filed in Europe on Mar. 19, 2008, the entire content of which is hereby incorporated by reference in its entirety.

TECHNICAL FIELD

The disclosure relates to the field of power electronics. It is based on a method for operating a converter circuit and on an apparatus for implementing the method.

BACKGROUND INFORMATION

Known converter circuits comprise a multiplicity of drivable power semiconductor switches, which are connected to one another in a known manner in order to switch at least two switching voltage levels. Typically, such a converter circuit is connected to an electrical AC voltage system, which is in particular of three-phase design. Such converter circuits are often used in industrial installations, with the converter circuits being coupled to the power supply system and naturally with further fields of use and possible uses being conceivable.

For the operation of the converter circuit, a control apparatus is provided which has a controller unit for forming a control signal by adjusting an H-th harmonic component of system currents to a system current setpoint value and which is connected to the drivable power semiconductor switches via a drive circuit for forming a drive signal from the control signal, the H-th harmonic component being produced by the converter circuit and generally being H=1, 2, 3, . . . Typical values for H are H=5, 7, 11, 13. The drive signal is therefore used to drive the power semiconductor switches.

The abovementioned method for operating the converter circuit permits adjustment of an H-th harmonic component of the system currents to a system current setpoint value by means of the control apparatus. However, in an electrical AC voltage system, in addition to harmonic components in the system currents, it is also possible for harmonic components to occur in the system voltages, but these cannot be adjusted by means of the known method described above and therefore cannot be reduced.

SUMMARY

A method for operating a converter circuit is disclosed by means of which it is possible to reduce a harmonic component in system voltages of an electrical AC voltage system connected to the converter circuit. A further object of the disclosure is to specify an apparatus with which the method can be implemented in a particularly simple manner.

A method for operating a converter circuit is disclosed, the converter circuit having a converter unit with a large number of drivable power semiconductor switches and with a three-phase electrical AC voltage system, in which the drivable power semiconductor switches are driven by means of a drive signal (SA) formed from a control signal ((SR), and the control signal (SR) is formed by adjusting an H-th harmonic component of system currents (iNH) to a system current setpoint value (iNHref), where H=1, 2, 3, . . . , wherein the system current setpoint value (iNHref) is formed by adjusting an H-th harmonic component of system voltages (uNH) to a predeterminable system voltage setpoint value (uNHref), the control difference (uNHdiff) from the H-th harmonic component of the system voltages (uNH) and the system voltage setpoint value (uNHref) being weighted by a system impedance (yNH) determined with respect to the H-th harmonic component.

An apparatus for implementing a method for operating a converter circuit is disclosed, the converter circuit having a converter unit with a large number of drivable power semiconductor switches and being connected to a three-phase electrical AC voltage system, with a control apparatus, which is used to generate a control signal (SR) and is connected to the drivable power semiconductor switches via a drive circuit for forming a drive signal (SA), the control apparatus (4) having a first controller unit for forming the control signal (SR) by adjusting an H-th harmonic component of system currents (iNH) to a system current setpoint value (iNHref), where H=1, 2, 3, . . . , wherein the control apparatus has a second controller unit for forming the system current setpoint value (iNHref) by adjusting an H-th harmonic component of system voltages (uNH) to a predeterminable system voltage setpoint value (uNHref), the control difference (uNHdiff) from the H-th harmonic component of the system voltages (uNH) and the system voltage setpoint value (uNHref) being weighted by a system impedance (yNH) determined with respect to the H-th harmonic component.

These and further objects, advantages and features of the present disclosure are disclosed in the detailed description below relating to exemplary embodiments of the disclosure in connection with the drawing.

The reference symbols used in the drawing and the significance thereof are listed by way of summary in the list of reference symbols. In principle, identical parts have been provided with identical reference symbols in the figures. The exemplary embodiments described represent, by way of example, the subject matter of the disclosure and do not have a restrictive effect.

DETAILED DESCRIPTION

The converter circuit has a converter unit with a large number of drivable power semiconductor switches and is connected to a three-phase electrical AC voltage system. In the method according to the disclosure for operating the converter circuit, the drivable power semiconductor switches are now driven by means of a drive signal formed from a control signal, and the control signal is formed by adjusting an H-th harmonic component of system currents to a system current setpoint value, where H=1, 2, 3, . . . . According to the disclosure, the system current setpoint value is formed by adjusting an H-th harmonic component of system voltages to a predeterminable system voltage setpoint value, the control difference from the H-th harmonic component of the system voltages and the system voltage setpoint value being weighted by a system impedance determined with respect to the H-th harmonic component. Thus, the corresponding H-th harmonic component of the system voltages directly influences the formation of the system current setpoint value, with the result that the H-th harmonic component in the system voltages can advantageously be reduced to the desired degree. Since the system impedance typically changes over the course of time and this ultimately corresponds to a change in the control path, the controller involved in the adjustment needs to be matched or reset each time with respect to its controller parameter to the changed system impedance. As a result of the control difference being weighted by the system impedance, such matching or resetting of the controller parameters of the controller involved in the adjustment and therefore redesigning of the controller is advantageously superfluous since the system impedance now directly affects the input of the controller and therefore directly affects the adjustment.

The apparatus according to the disclosure for implementing the method for operating the converter circuit has a control apparatus which is used for generating the control signal SR and is connected to the drivable power semiconductor switches via a drive circuit for forming the drive signal, the control apparatus having a first controller unit for forming the control signal by adjusting the H-th harmonic component of the system currents to the system current setpoint value, where H=1, 2, 3, . . . . In accordance with the disclosure, the control apparatus now has a second controller unit for forming the system current setpoint value by adjusting the H-th harmonic component of the system voltages to the predeterminable system voltage setpoint value, the control difference from the H-th harmonic component of the system voltages and the system voltage setpoint value being weighted by the system impedance determined with respect to the H-th harmonic component. The apparatus according to the disclosure for implementing the method for operating the converter circuit can therefore be realized in a very simple and inexpensive manner, since the complexity involved with the circuitry can be kept extremely low and, in addition, only a small number of components is required for the design. The method according to the disclosure can therefore be implemented in a particularly simple manner by means of this apparatus.

FIG. 1shows an exemplary embodiment of an apparatus according to the disclosure for implementing the method according to the disclosure for operating a converter circuit. As shown inFIG. 1, the converter circuit has a converter unit2with a large number of drivable power semiconductor switches and is connected to a three-phase electrical AC voltage system. It should be mentioned that the converter unit2can generally be in the form of any converter unit2for switching ≧2 DC switching voltage levels (multi-level converter circuit). In the method according to the disclosure for operating the converter circuit, the drivable power semiconductor switches are driven by means of a drive signal SAformed from a control signal SR, and the control signal SRis formed by adjusting an H-th harmonic component of system currents iNHto a system current setpoint value iNHref, where H=1, 2, 3, . . . . According to the disclosure, the system current setpoint value iNHrefis formed by adjusting an H-th harmonic component of system voltages uNHto a predeterminable system voltage setpoint value uNHref, the control difference uNHdifffrom the H-th harmonic component of the system voltages uNHand the system voltage setpoint value uNHrefbeing weighted by a system impedance yNHdetermined with respect to the H-th harmonic component. Thus, the corresponding H-th harmonic component of the system voltages uNHdirectly influences the formation of the system current setpoint value iNHref, with the result that the H-th harmonic component in the system voltages can advantageously be reduced to the desired degree. As a result of the control difference uNHdiffbeing weighted by the system impedance yNH, matching or resetting of the controller parameters of the controller involved in the adjustment and therefore redesigning of the controller owing to a system impedance yNHwhich typically changes over time is advantageously superfluous since the system impedance yNHnow directly affects the input of the controller and therefore directly affects the adjustment.

A time profile of the absolute value of an H-th harmonic component of system voltages UNH is illustrated inFIG. 3, which shows a marked reduction in the H-th harmonic component over time t.

Since the system impedance yNHcan typically change over the course of time, with respect to the H-th harmonic component, the absolute value of the system voltage change |ΔuNH| can be monitored in relation to a threshold value uNHTol. Each time the threshold value uNHTolis exceeded, the system impedance yNHis then redetermined. In order to determine the system impedance yNH, for example the present system voltage change ΔuNHwith respect to the H-th harmonic component and a present system current change ΔiNHwith respect to the H-th harmonic component is ascertained, and then the system impedance yNHis determined from this by means of computation. It should be mentioned that other possible ways of determining the system impedance yNH, such as measurement, for example, would also be conceivable.

The H-th harmonic component of the system voltages uNHcan be formed from the Park-Clarke transformation of the system voltages uNd, uNq. uNdand uNqare the corresponding components of the Park-Clarke transformation of the system voltages.

It should be mentioned that the Park-Clarke transformation is generally defined by

x_=(xd+j⁢⁢xq)⁢ⅇj⁢⁢ω⁢⁢t,
wherexis generally a complex variable, xdis the d component of the Park-Clarke transformation of the variablexand xqis the q component of the Park-Clarke transformation of the variablex. Advantageously, not only the fundamental of the complex variablexis transformed in the Park-Clarke transformation, but also all of the harmonic components of the complex variablexwhich arise. The H-th harmonic component is therefore also included and can be extracted by simple filtering.

With respect to the method according to the disclosure, the Park-Clarke transformation of the system voltages uNd, uNqis advantageously formed from the space vector transformation of the system voltages uNα, uNβ, i.e. the system voltages uNa, uNb, uNcare transformed by the space vector transformation.

It should be mentioned that the space vector transformation is defined as follows
x=xα+jxβ
wherexis generally a complex variable, xαis the α component of the space vector transformation of the variablexand xβis the β component of the space vector transformation of the variablex.

The H-th harmonic component of the system currents iNHcan be formed from the Park-Clarke transformation of the system currents iNd, iNq. iNdand iNqare the corresponding components of the Park-Clarke transformation of the system currents. In addition, the Park-Clarke transformation of the system currents iNd, iNqis formed from the space vector transformation of the system currents iNα, iNβ, i.e. the system currents iNa, iNb, iNcare transformed by the space vector transformation.

The adjustment of the H-th harmonic component of the system voltages uNHto the predeterminable system voltage setpoint value uNHrefalready mentioned above can take place in accordance with a proportional/integral characteristic, since said characteristic is characterized by its simplicity. Alternatively, however, it is also conceivable for the adjustment of the H-th harmonic component of the system voltages uNHto the predeterminable system voltage setpoint value uNHrefto take place in accordance with a dead-beat characteristic by means of iteration. In the adjustment in accordance with the dead-beat characteristic by means of iteration, the following formula can be used for the formation:
iNHref=iNHref,old+(uNH−uNHref)·yNH·k,
where the system current setpoint value iNHrefis reformed in each iteration step and iNHref,oldis the system current setpoint value of the preceding iteration step and k is a correction factor, which is advantageously selected to be of the order of magnitude of 0.1 to 1. It should be mentioned, however, that any other control characteristic would also be conceivable.

As has already been mentioned,FIG. 1shows an exemplary embodiment of an apparatus according to the disclosure for implementing the method according to the disclosure for operating a converter circuit. As shown inFIG. 1, the control apparatus4used for generating the control signal SRis connected to the drivable power semiconductor switches of the converter unit2via a drive circuit3for forming the drive signal SA.FIG. 2illustrates an exemplary embodiment of a control apparatus4according to the disclosure, with the control apparatus4having a first controller unit5for forming the control signal SRby adjusting the H-th harmonic component of the system currents iNHto the system current setpoint value NHref, where H=1, 2, 3, . . . . According to the disclosure, the control apparatus4has a second controller unit 6 for forming the system current setpoint value iNHrefby adjusting the H-th harmonic component of the system voltages uNHto the predeterminable system voltage setpoint value uNHref, the control difference uNHdifffrom the H-th harmonic component of system voltages uNHand the system voltage setpoint value uNHrefbeing weighted by the system impedance yNHdetermined with respect to the H-th harmonic component.

The second controller unit6for adjusting the H-th harmonic component of the system voltages uNHto the predeterminable system voltage setpoint value uNHrefcomprises, in accordance withFIG. 2, a controller11, which controller11can be in the form of, for example, a proportional/integral controller with a corresponding characteristic (already mentioned) or in the form of a dead-beat controller with a corresponding characteristic (already mentioned). It should be mentioned, however, that any other controller with its corresponding control characteristic would also be conceivable.

In accordance withFIG. 2, the control apparatus4additionally has a first computation unit7for forming the Park-Clarke transformation of the system voltages uNd, uNqfrom the space vector transformation of the system voltages uNα, uNβand for forming the H-th harmonic component of the system voltages uNHfrom the Park-Clarke transformation of the system voltages uNd, uNq.

In addition, the control apparatus4, in accordance withFIG. 2, has a second computation unit8for forming the space vector transformation of the system voltages uNα, uNβ. In addition, the control apparatus4comprises a third computation unit9for forming the Park-Clarke transformation of the system currents iNd, iNqfrom the space vector transformation of the system currents iNα, iNβand for forming the H-th harmonic component of the system currents iNHfrom the Park-Clarke transformation of the system currents iNd, iN. The control apparatus4furthermore has a fourth computation unit10for forming the space vector transformation of the system currents iNα, iNβ.

The method described in detail above and the associated apparatus for implementing the method causes an H-th harmonic component of the system voltages uNHto be reduced. It goes without saying that, in order to reduce, for example, a plurality of harmonic components of the system voltages, the abovementioned method with the corresponding method steps is implemented separately for each of these harmonic components. As regards the apparatus for implementing the method, in order to reduce the harmonic components, an associated apparatus as described above is therefore required for each of these harmonic components.

All of the steps of the method according to the disclosure can be implemented as software, with said software then being loaded onto a computer system, in particular with a digital signal processor, for example, and can be run on said computer system. In addition, the apparatus according to the disclosure which is described in detail above can also be implemented in a computer system, in particular in a digital signal processor.

Overall, it has been possible to demonstrate that the apparatus according to the disclosure, in particular that shown inFIG. 1andFIG. 2, for implementing the method according to the disclosure for operating the converter circuit can be realized in a very simple and cost-effective manner since the complexity involved with the circuitry is extremely low and in addition only a small number of components is required for the design. The method according to the disclosure can therefore be implemented in a particularly simple manner using this apparatus.