Method for operation of a converter circuit, as well as an apparatus for carrying out the method

A method for operation of a converter circuit is specified, wherein the converter circuit has a converter unit with a multiplicity of drivable power semiconductor switches and an LCL filter which is connected to each phase connection of the converter unit, in which method the drivable power semiconductor switches are driven by means of a drive signal which is formed from reference voltages. The reference voltages are formed by subtraction of damping voltages from reference-phase connection voltages, with the damping voltages being formed from filter capacitance currents, weighted with a variable damping factor of the LCL filter. An apparatus for carrying out the method is also specified.

TECHNICAL FIELD

The disclosure relates to the field of power electronics, and is based on a method for operation of a converter circuit, as well as an apparatus for carrying out the method.

BACKGROUND INFORMATION

Known converter circuits have a converter unit with a multiplicity of drivable power semiconductor switches, which are connected in a known manner in order to switch at least two switching voltage levels. An LCL filter is connected to each phase connection of the converter unit. A capacitive energy store is also connected to the converter unit, and is normally formed by one or more capacitors. An apparatus is provided for operation of the converter circuit, which apparatus has a regulation device for production of reference voltages and is connected via a drive circuit for formation of a drive signal from the reference voltages to the drivable power semiconductor switches. The power semiconductor switches are thus driven by means of the drive signal.

The converter circuit mentioned above is subject to the problem but the LCL filters can cause permanent distortion, that is to say undesirable oscillations, in the filter output currents and filter voltages, resulting from resonant oscillations of the LCL filters. In an electrical ac voltage supply system, which is typically connected to the filter outputs, or in an electrical load which is connected to the filter outputs, such distortion can lead to damage or even destruction, and is therefore very undesirable.

SUMMARY

A method is disclosed for operation of a converter circuit, by means of which it is possible to actively damp distortion, caused by LCL filters connected to the converter circuit, in the filter output currents and filter output voltages. An apparatus is disclosed, by means of which the method can be carried out in a particularly simple manner.

The converter circuit has a converter unit with a multiplicity of drivable power semiconductor switches, and an LCL filter which is connected to each phase connection of the converter unit. In an exemplary method for operation of the converter circuit, the drivable power semiconductor switches are now driven by means of a drive signal which is formed from reference voltages. According to the disclosure, the reference voltages are formed from the subtraction of damping voltages from reference-phase connection voltages, with the damping voltages being formed from filter capacitance currents (which are weighted with a variable damping factor) of the LCL filters. The damping voltages are thus proportional to the filter capacitance currents and are then subtracted from the reference-phase connection voltages, which is equivalent to connection of a damping resistance to each phase connection of the converter unit. Distortion, that is to say undesirable oscillations, in the filter output currents and filter output voltages can therefore advantageously be actively damped, so that this type of distortion is greatly reduced and, in the ideal case, is very largely suppressed. A further advantage of the exemplary method is that there is no need to connect any discrete, highly space-consuming damping resistor, which is complex to provide and is therefore expensive, to each phase connection in order to allow the undesirable distortion to be effectively damped.

An exemplary apparatus for carrying out the method for operation of the converter circuit has a regulation device which is used to produce reference voltages and is connected via a drive circuit for formation of a drive signal to the drivable power semiconductor switches. According to the disclosure, the regulation device has a first calculating unit for formation of reference voltages from the subtraction of damping voltages from reference-phase connection voltages, with the first calculation unit being supplied with reference-phase connection voltages and, in order to form the damper voltages, filter capacitance currents of the LCL filters. Furthermore, the regulation device has a regulator unit for production of the reference-phase connection voltages. The exemplary apparatus for carrying out the method for operation of the converter circuit can thus be implemented very easily and cost-effectively, since the circuit complexity can be kept extremely low and, furthermore, only a small number of components are required to construct it. The exemplary method can thus be carried out particularly easily by means of this apparatus.

The reference symbols used in the drawing, and their meanings, are listed in a summarized form in the list of reference symbols. In principle, identical parts are provided with the same reference symbols in the figures. The described exemplary embodiments represent examples of the subject matter of the invention, and have no restrictive effect.

DETAILED DESCRIPTION

FIG. 1shows one exemplary embodiment of an apparatus for carrying out an exemplary method for operation of a converter circuit. As shown inFIG. 1, the converter circuit has a converter unit1with a multiplicity of drivable power semiconductor switches and an LCL filter3, which is connected to each phase connection2of the converter unit1. Each LCL filter3accordingly has a first filter inductance Lfi, a second filter inductance Lfgas well as a filter capacitance Cf, with the first filter inductance Lfibeing connected to the associated phase connection2of the converter unit1, to the second filter inductance Lfgand to the filter capacitance Cf. Furthermore, the filter capacitances Cfof the individual LCL filters3are connected to one another. Each LCL filter3typically has a virtually negligible filter resistance Rf, which is connected in series with the filter capacitance Cfof the associated LCL filter3and represents resistive losses in the LCL filter3. By way of example, the converter unit1shown inFIG. 1is a three-phase unit. It should be mentioned that the converter unit1may in general be any form of converter unit1for switching of ≧2 switching voltage levels (multi-level converter circuit) with respect to the voltage of a capacitive energy store9which is connected to the converter unit1, with the capacitive energy store9then being formed by any desired number of capacitances, which are then connected such that they are matched to the appropriately configured converter circuit element.

In an exemplary method for operation of the converter circuit, the drivable power semiconductor switches of the conversion unit1are now driven by means of a drive signal S which is formed from reference voltages u*1, u*2, u*3. A look-up table is normally used to form the drive signal, in which appropriate drive signals are permanently associated with reference voltage values, or a modulator which is based on pulse-width modulation. According to the disclosure, the reference voltages u*1, u*2, u*3are formed from subtraction of damping voltages ud1, ud2, ud3from reference-phase connection voltages u*i1, u*i2, u*i3, with the damping voltages ud1, ud2, ud3being formed from filter capacitance currents iCf1, iCf2, iCf3, which are weighted with a variable damping factor Kf, of the LCL filters3, as illustrated in particular by the following formula.
ud=Kf·iCf

The damping voltages ud1, ud2, ud3are thus proportional to the filter capacitance currents iCf1, iCf2, iCf3and are then subtracted from the reference-phase connection voltages u*i1, u*i2, u*i3, which corresponds to the connection of a damping resistor to each phase connection2of the converter unit1. This advantageously allows active damping of distortion, that is to say undesirable oscillations, in the filter output currents ifg1, ifg2, ifg3and filter output voltages ug1, ug2, ug3, that this distortion is greatly reduced and, in the ideal case is very largely suppressed. Furthermore, there is no need for connection of discrete, very space-consuming damping resistors, which are complex to implement and are therefore expensive to the respective phase connection in order to allow effective damping of the undesirable distortion.

The damping factor Kfcan be set such that the undesirable oscillations of the filter output voltages ug1, ug2, ug3or of phase-connection voltages, e.g., harmonics, are just not amplified.

As shown inFIG. 1, an exemplary apparatus for carrying out an exemplary method for operation of a converter circuit for this purpose has a regulation device4, which is used to produce the reference voltages u*1, u*2, u*3and is connected via a drive circuit5for formation of the drive signal S to the drivable power semiconductor switches. By way of example, the drive circuit5has a look-up table in which appropriate drive signals are permanently associated with reference-voltage values, or a modulator which is based on pulse-width modulation. According to the disclosure, the regulation device4has a first calculation unit6for formation of reference voltages u*1, u*2, u*3from the subtraction of damping voltages ud1, ud2, ud3from reference-phase connection voltages u*i1, u*i2, u*i3, with the first calculation unit6being supplied with reference-phase connection voltages u*i1, u*i2, u*i3and, in order to form the damper voltages ud1, ud2, ud3, filter capacitance currents iCf1, iCf2, iCf3of the LCL filters3. As shown inFIG. 1, the filter capacitance currents iCf1, iCf2, iCf3are measured by appropriate measurement devices. Furthermore, the regulation device4has a regulation unit7for production of the reference-phase connection voltages u*i1, u*i2, u*i3. The exemplary apparatus for carrying out the method for operation of the converter circuit can accordingly be produced very easily and cost-effectively, since the circuit complexity can be kept extremely low and, furthermore, only a small number of components are required to construct it. This apparatus allows the exemplary method to be carried out particularly easily.

It has been found to be advantageous for the filter capacitance currents iCf1, iCf2, iCf3to be filtered by means of a high-pass filter. This means that the damping voltages ud1, ud2, ud3are formed only from harmonics of the filter capacitance currents iCf1, iCf2, iCf3, in particular higher-frequency harmonics of the filter capacitance currents iCf1, iCf2, iCf3, and the variable damping factor Kf, so that the active damping can advantageously act only on the harmonics in the filter output currents ifg1, ifg2, ifg3and filter output voltages ug1, ug2, ug3. High-pass filtering of the filter capacitance currents iCf1, iCf2, iCf3is carried out by a high-pass filter which is connected between the measurement devices for measurement of the filter capacitance currents iCf1, iCf2, iCf3and the first calculation unit6, with the high-pass filter not being shown inFIG. 1, for clarity reasons.

The reference-phase connection voltages u*i1, u*i2, u*i3are formed from a d-component of the Park-Clarke transformation (produced by regulation of the dc voltage udcof a capacitive energy store9which is connected to the converter unit1at a dc voltage reference value u*dcof reference-phase connection currents i*fidand from a predeterminable q-component of the Park-Clarke transformation of the reference-phase connection currents i*fiq. The regulation can be carried out using a proportional-integral characteristic. As shown inFIG. 1, the regulator unit7for regulation of the dc voltage udcof the capacitive energy store9at the dc voltage reference value u*dchas a first proportional-integral regulator8, to whose input the difference between the dc voltage (udc) of the capacitive energy store9and the dc voltage reference value u*dcis supplied, and at whose output the d-component of the Park-Clarke transformation of the reference-phase connection currents i*fidis produced.

The Park-Clarke transformation is in general defined as:
x=(xd+jxq)ejωt
using the variables illustrated in FIG.1:
ūg=ug1+ug2ejy+ug3ej2y
īfi=ifi1+ifi2ejy+ifi3ej2y
īCf=iCf1+iCf2ejy+iCf3ej2y
īfg=ifg1+ifg2ejy+ifg3ej2y
where y=2π/3,
wherexis 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 variablexAll of the Park-Clarke transformations of variables which have already been mentioned and those which will be mentioned in the following text are produced using the formula quoted above. The Park-Clarke transformation advantageously transforms not only the fundamental of the complex variablex, but also all of the harmonics that occur of the complex variablex.

The regulation device4shown inFIG. 1has a third calculation unit16for formation of the d-component of the Park-Clarke transformation of the filter output voltages ugd, of the q-component of the Park-Clarke transformation of the filter output voltages ugqand of the fundamental angle ωt of the filter output voltages ug1, ug2, ug3, with the input side of the third calculation unit16being supplied with the filter output voltages ug1, ug2, ug3of the LCL filters3. The third calculation unit16can be a phase locked loop, in which case the Park-Clarke transformations of the individual variables are carried out using the definitions given above.

Furthermore, the d-component of the Park-Clarke transformation of the reference-filter output voltages u*gdis produced by regulation of the d-component of the Park-Clarke transformation of the phase connection current ifidat the sum of the d-component of the Park-Clarke transformation of the reference-phase connection currents i*fidand a d-component of the Park-Clarke transformation of at least one harmonic of filter output currents i*fghdwith respect to the fundamental of the filter output currents ifg1, ifg2, ifg3. The regulation can be carried out using a proportional-integral characteristic. Furthermore, the q-component of the Park-Clarke transformation of the reference filter output voltages u*gqis produced by regulation of the q-component of the Park-Clarke transformation of the phase connection currents ifiqat the sum of the q-component of the Park-Clarke transformation of the reference-phase connection currents i*fiqand a q-component of the Park-Clarke transformation of at least one harmonic of the filter output currents i*fghqwith respect to the fundamental of the filter output currents ifg1, ifg2, ifg3. The regulation can be carried out using a proportional-integral characteristic. The index h of the d-component and the q-component of the Park-Clarke transformation of a harmonic of the filter output currents i*fghdi*fghqrepresents the h-th harmonic of these variables, where h=1, 2, 3, . . . The additional variables introduced in the following text with the index h likewise use the index h for the h-th harmonic of the associated variable, h=1, 2, 3, . . . As shown inFIG. 1, the regulator unit7for regulation of the d-component of the Park-Clarke transformation of the phase connection currents ifidat the sum of the d-component of the Park-Clarke transformation of the reference phase connection currents i*fidand a d-component of the Park-Clarke transformation of at least one harmonic of the filter output currents i*fghdwith respect to the fundamental of the filter output currents ifg1, ifg2, ifg3has a second proportional-integral regulator10to whose input side the difference between the sum of the d-component of the Park-Clarke transformation of the reference-phase connection currents i*fidand a d-component of the Park-Clarke transformation of at least one harmonic of the filter output currents i*fghdwith respect to the fundamental of the filter output currents ifg1, ifg2, ifg3and the d-component of the Park-Clarke transformation of the phase connection currents ifidare supplied, and on whose output side the d-components of the Park-Clarke transformation of the reference-filter output voltages u*gdis produced. Furthermore, for regulation of the q-component of the Park-Clark transformation of the phase connection currents ifiqat the sum of the q-component of the Park-Clarke transformation of the reference phase connection currents i*fiqand the q-component of the Park-Clarke transformation of at least one harmonic of the filter output currents i*fghqwith respect to the fundamental of the filter output currents ifg1, ifg2ifg3, the regulator unit7has a third proportional-integral regulator11to whose input side the difference between the sum of the q-component of the Park-Clarke transformation of the reference phase connection currents i*fiqand a q-component of the Park-Clarke transformation of at least one harmonic of the filter output currents i*fghqwith respect to the fundamental of the filter output currents ifg1, ifg2, ifg3and the q-components of the Park-Clarke transformation of the phase connection currents ifiqare supplied, and on whose output side the q-component of the Park-Clarke transformation of the reference-filter output voltages u*gqis produced.

Furthermore, the d-component of the Park-Clarke transformation of the reference-phase connection voltages u*idis produced by the sum of the d-component of the Park-Clarke transformation of the reference filter output voltages u*gdand the d-component of the filter output voltages ugdand a d-component of the Park-Clarke transformation of at least one harmonic of the filter output voltages u*ghd. In addition the q-component of the Park-Clarke transformation of the reference-phase connection voltages u*iqis produced by the sum of the q-component of the Park-Clarke transformation of the reference-filter output voltages u*gqand the q-component of the Park-Clarke transformation of the filter-output voltages ugqand a q-component of the Park-Clarke transformation of at least one harmonic of the filter output voltages u*ghq. In order to produce the d-component of the Park-Clarke transformation of the reference-phase connection voltages u*id, the regulator unit7has a first adder12, to which the d-component of the Park-Clarke transformation of the reference filter output voltages u*gd, the d-component of the filter output voltages ugdand the d-component of the Park-Clarke transformation of at least one harmonic of the filter output voltages u*ghdare supplied. In addition, in order the produce the q-component of the Park-Clarke transformation of the reference phase connection voltages u*iq, the regulator unit7has, as shown inFIG. 1, a second adder13, to which the q-component of the Park-Clarke transformation of the reference-filter output voltages u*gq, the q-component of the Park-Clarke transformation of the filter output voltages ugqand the q-component of the Park-Clarke transformation of at least one harmonic of the filter output voltages u*ghqare supplied.

In order to form the d-component of the Park-Clarke transformation, as has already been mentioned above, of at least one harmonic of the filter output currents i*fghdwith respect to the fundamental of the filter output currents ifg1, ifg2, ifg3, the q-component of the Park-Clarke transformation of at least one harmonic of the filter output currents i*fghqwith respect to the fundamental of the filter output currents ifg1, ifg2, ifg3, the d-component of the Park-Clarke transformation of the at least one harmonic of the reference filter output voltages u*ghdand the q-component of the Park-Clarke transformation of the at least one harmonic of the reference-filter output voltages u*ghqthe regulation unit4has a second calculation unit15, as shown inFIG. 1. As shown inFIG. 1, the input side of the second calculation unit15is supplied with the d-component with the filter output voltages ugd, the q-component of the filter output voltages ugq, the phase connection currents ifi1, ifi2, ifi3, the filter capacitance currents iCf1, iCf2, iCf3and the fundamental angle ωt of the filter output voltages ug1, ug2, ug3. In order to illustrate the formation of the individual variables in the calculation unit15,FIG. 2shows one exemplary embodiment of the second calculation unit15, in which the input variables shown inFIG. 2are obtained using the following formula:
ifghd+jifghq=ifihd+jifihq−(iCfhd+jiCfhg)
with the d-components of the Park-Clarke transformation and the q-components of the Park-Clarke transformation being obtained by applications of the Park-Clarke transformation to the measured phase connection currents ifi1, ifi2, ifi3including the associated harmonics, and filter capacitance currents iCf1, iCf2, iCf3including the associated harmonics. This Park-Clarke Clarke transformation is carried out in particular in the second calculation unit15, although this is not illustrated in the second calculation unit15shown inFIG. 2, for clarity reasons.

Finally, the reference-phase connection voltages u*i1, u*i2, u*i3are produced by an inverse Park-Clarke transformation of the d-component of the Park-Clarke transformation of the reference-phase connection voltages u*idand the q-component of the Park-Clarke transformation of the reference-phase connection voltages u*iq. As shown inFIG. 1, the regulator unit7for this purpose has a calculation unit14for formation of the reference-phase connection voltages u*i1, u*i2, u*i3by inverse Park-Clarke transformation, to whose input side the d-component of the Park-Clarke transformation of the reference-phase connection voltages u*idand the q-component of the Park-Clarke transformation of the reference-phase connection voltages u*iqare supplied.

In order to illustrate an exemplary method of operation of the active damping based on the exemplary method as explained above,FIG. 3shows a waveform of the filter output currents ifg1, ifg2, ifg3in which undesirable oscillations in the filter output currents ifg1, ifg2, ifg3are actively damped, so that this distortion is greatly reduced. A further improvement in the reduction of harmonics is shown in a waveform of the filter output currents ifg1, ifg2, ifg3inFIG. 4with active damping, and additional active reduction of harmonics using the exemplary method as described above.

It should be mentioned that all of the steps of the exemplary method may be implemented in the form of software, which can then be loaded and then run for example on a computer system, in particular with a digital signal processor. The digital delay times which occur in systems such as this, in particular for the calculations, may be in general be taken into account, for example, by addition of an additional term to the fundamental angle ωt in the Park-Clarke transformation. Furthermore, the exemplary apparatus, as described in detail above, can also be implemented in a computer system, in particular in the digital signal processor.

Overall, it has been possible to show that the exemplary apparatus, e.g., as shown inFIG. 1, for carrying out the exemplary method for operation of the converter circuit can be implemented very easily and cost-effectively, since the circuit complexity is extremely low and, furthermore, only a small number of components are required to construct it. This exemplary apparatus therefore makes it possible to carry out the exemplary method particularly easily.

LIST OF REFERENCE SYMBOLS

1Converter unit2Phase connection of the converter unit3LCL filter4Regulation device5Drive circuit6First calculation unit of the regulation device7Regulator unit8First proportional-integral regulator9Capacitive energy store10Second proportional-integral regulator11Third proportional-integral regulator12First adder13Second adder14Calculation unit for the regulator unit15Second calculation unit of the regulation device16third calculation unit of the regulation device