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

[0001] This application claims priority under 35 U.S.C. §119 to U.S. Provisional Application 60/646,544 filed in USA on Jan. 25, 2005, and as a continuation application under 35 U.S.C. §120 to PCT/CH2005/000293 filed as an International Application on May 24, 2005, designating the U.S., the entire contents of which are hereby incorporated by reference in their entireties.

[0002] 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.

[0003] 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.

[0004] 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.

[0005] 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.

[0006] 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.

[0007] 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.

[0013] 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.

[0014] FIG. 1 shows one exemplary embodiment of an apparatus for carrying out an exemplary method for operation of a converter circuit. As shown in FIG. 1, the converter circuit has a converter unit 1 with a multiplicity of drivable power semiconductor switches and an LCL filter 3, which is connected to each phase connection 2 of the converter unit 1. Each LCL filter 3 accordingly has a first filter inductance L_fi, a second filter inductance L_fg as well as a filter capacitance C_f, with the first filter inductance L_fi being connected to the associated phase connection 2 of the converter unit 1, to the second filter inductance L_fg and to the filter capacitance C_f. Furthermore, the filter capacitances C_f of the individual LCL filters 3 are connected to one another. Each LCL filter 3 typically has a virtually negligible filter resistance R_f, which is connected in series with the filter capacitance C_f of the associated LCL filter 3 and represents resistive losses in the LCL filter 3. By way of example, the converter unit 1 shown in FIG. 1 is a three-phase unit. It should be mentioned that the converter unit 1 may in general be any form of converter unit 1 for switching of ≧2 switching voltage levels (multi-level converter circuit) with respect to the voltage of a capacitive energy store 9 which is connected to the converter unit 1, with the capacitive energy store 9 then being formed by any desired number of capacitances, which are then connected such that they are matched to the appropriately configured converter circuit element.

[0015] In an exemplary method for operation of the converter circuit, the drivable power semiconductor switches of the conversion unit 1 are 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*₃ are formed from subtraction of damping voltages u_d1, u_d2, u_d3 from reference-phase connection voltages u*i₁, u*i₂, u*i₃, with the damping voltages u_d1, u_d2, u_d3 being formed from filter capacitance currents i_Cf1, i_Cf2, i_Cf3, which are weighted with a variable damping factor K_f, of the LCL filters 3, as illustrated in particular by the following formula.
u_d=K_f·i_Cf

[0016] The damping voltages u_d1, u_d2, u_d3 are thus proportional to the filter capacitance currents i_Cf1, i_Cf2, i_Cf3 and 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 connection 2 of the converter unit 1. This advantageously allows active damping of distortion, that is to say undesirable oscillations, in the filter output currents i_fg1, i_fg2, i_fg3 and filter output voltages u_g1, u_g2, u_g3, 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.

[0017] The damping factor K_f can be set such that the undesirable oscillations of the filter output voltages u_g1, u_g2, u_g3 or of phase-connection voltages, e.g., harmonics, are just not amplified.

[0018] As shown in FIG. 1, an exemplary apparatus for carrying out an exemplary method for operation of a converter circuit for this purpose has a regulation device 4, which is used to produce the reference voltages u*₁, u*₂, u*₃ and is connected via a drive circuit 5 for formation of the drive signal S to the drivable power semiconductor switches. By way of example, the drive circuit 5 has 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 device 4 has a first calculation unit 6 for formation of reference voltages u*₁, u*₂, u*₃ from the subtraction of damping voltages u_d1, u_d2, u_d3 from reference-phase connection voltages u*_i1, u*_i2, u*_i3, with the first calculation unit 6 being supplied with reference-phase connection voltages u*_i1, u*_i2, u*_i3 and, in order to form the damper voltages u_d1, u_d2, u_d3, filter capacitance currents i_Cf1, i_Cf2, i_Cf3 of the LCL filters 3. As shown in FIG. 1, the filter capacitance currents i_Cf1, i_Cf2, i_Cf3 are measured by appropriate measurement devices. Furthermore, the regulation device 4 has a regulation unit 7 for 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.

[0019] It has been found to be advantageous for the filter capacitance currents i_Cf1, i_Cf2, i_Cf3 to be filtered by means of a high-pass filter. This means that the damping voltages u_d1, u_d2, u_d3 are formed only from harmonics of the filter capacitance currents i_Cf1, i_Cf2, i_Cf3, in particular higher-frequency harmonics of the filter capacitance currents i_Cf1, i_Cf2, i_Cf3, and the variable damping factor K_f, so that the active damping can advantageously act only on the harmonics in the filter output currents i_fg1, i_fg2, i_fg3 and filter output voltages u_g1, u_g2, u_g3. High-pass filtering of the filter capacitance currents i_Cf1, i_Cf2, i_Cf3 is carried out by a high-pass filter which is connected between the measurement devices for measurement of the filter capacitance currents i_Cf1, i_Cf2, i_Cf3 and the first calculation unit 6, with the high-pass filter not being shown in FIG. 1, for clarity reasons.

[0020] The reference-phase connection voltages u*_i1, u*_i2, u*_i3 are formed from a d-component of the Park-Clarke transformation (produced by regulation of the dc voltage u_dc of a capacitive energy store 9 which is connected to the converter unit 1 at a dc voltage reference value u*_dc of reference-phase connection currents i*_fid and 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 in FIG. 1, the regulator unit 7 for regulation of the dc voltage u_dc of the capacitive energy store 9 at the dc voltage reference value u*_dc has a first proportional-integral regulator 8, to whose input the difference between the dc voltage (u_dc) of the capacitive energy store 9 and the dc voltage reference value u*_dc is supplied, and at whose output the d-component of the Park-Clarke transformation of the reference-phase connection currents i*_fid is produced.

[0021] The Park-Clarke transformation is in general defined as:
x=(x_d+jx_q)e^jωt
using the variables illustrated in FIG. 1:
ū_g=u_g1+u_g2e^jy+u_g3e^j2y
ī_fi=i_fi1+i_fi2e^jy+i_fi3e^j2y
ī_Cf=i_Cf1+i_Cf2e^jy+i_Cf3e^j2y
ī_fg=i_fg1+i_fg2e^jy+i_fg3e^j2y
where y=2π/3,
where x is a complex variable, x_d is the d-component of the Park-Clarke transformation of the variable x and x_q is the q-component of the Park-Clarke transformation of the variable x All 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 variable x, but also all of the harmonics that occur of the complex variable x.

[0022] The regulation device 4 shown in FIG. 1 has a third calculation unit 16 for formation of the d-component of the Park-Clarke transformation of the filter output voltages u_gd, of the q-component of the Park-Clarke transformation of the filter output voltages u_gq and of the fundamental angle ωt of the filter output voltages u_g1, u_g2, u_g3, with the input side of the third calculation unit 16 being supplied with the filter output voltages u_g1, u_g2, u_g3 of the LCL filters 3. The third calculation unit 16 can be a phase locked loop, in which case the Park-Clarke transformations of the individual variables are carried out using the definitions given above.

[0023] Furthermore, the d-component of the Park-Clarke transformation of the reference-filter output voltages u*_gd is produced by regulation of the d-component of the Park-Clarke transformation of the phase connection current i_fid at the sum of the d-component of the Park-Clarke transformation of the reference-phase connection currents i*_fid and a d-component of the Park-Clarke transformation of at least one harmonic of filter output currents i*_fghd with respect to the fundamental of the filter output currents i_fg1, i_fg2, i_fg3. 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*_gq is produced by regulation of the q-component of the Park-Clarke transformation of the phase connection currents i_fiq at the sum of the q-component of the Park-Clarke transformation of the reference-phase connection currents i*_fiq and a q-component of the Park-Clarke transformation of at least one harmonic of the filter output currents i*_fghq with respect to the fundamental of the filter output currents i_fg1, i_fg2, i_fg3. 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*_fghd i*_fghq represents 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 in FIG. 1, the regulator unit 7 for regulation of the d-component of the Park-Clarke transformation of the phase connection currents i_fid at the sum of the d-component of the Park-Clarke transformation of the reference phase connection currents i*_fid and a d-component of the Park-Clarke transformation of at least one harmonic of the filter output currents i*_fghd with respect to the fundamental of the filter output currents i_fg1, i_fg2, i_fg3 has a second proportional-integral regulator 10 to whose input side the difference between the sum of the d-component of the Park-Clarke transformation of the reference-phase connection currents i*_fid and a d-component of the Park-Clarke transformation of at least one harmonic of the filter output currents i*_fghd with respect to the fundamental of the filter output currents i_fg1, i_fg2, i_fg3 and the d-component of the Park-Clarke transformation of the phase connection currents i_fid are supplied, and on whose output side the d-components of the Park-Clarke transformation of the reference-filter output voltages u*_gd is produced. Furthermore, for regulation of the q-component of the Park-Clark transformation of the phase connection currents i_fiq at the sum of the q-component of the Park-Clarke transformation of the reference phase connection currents i*_fiq and the q-component of the Park-Clarke transformation of at least one harmonic of the filter output currents i*_fghq with respect to the fundamental of the filter output currents i_fg1, i_fg2 i_fg3, the regulator unit 7 has a third proportional-integral regulator 11 to whose input side the difference between the sum of the q-component of the Park-Clarke transformation of the reference phase connection currents i*_fiq and a q-component of the Park-Clarke transformation of at least one harmonic of the filter output currents i*_fghq with respect to the fundamental of the filter output currents i_fg1, i_fg2, i_fg3 and the q-components of the Park-Clarke transformation of the phase connection currents i_fiq are supplied, and on whose output side the q-component of the Park-Clarke transformation of the reference-filter output voltages u*_gq is produced.

[0024] Furthermore, the d-component of the Park-Clarke transformation of the reference-phase connection voltages u*_id is produced by the sum of the d-component of the Park-Clarke transformation of the reference filter output voltages u*_gd and the d-component of the filter output voltages u_gd and 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*_iq is produced by the sum of the q-component of the Park-Clarke transformation of the reference-filter output voltages u*_gq and the q-component of the Park-Clarke transformation of the filter-output voltages u_gq and 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 unit 7 has a first adder 12, 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 u_gd and the d-component of the Park-Clarke transformation of at least one harmonic of the filter output voltages u*_ghd are 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 unit 7 has, as shown in FIG. 1, a second adder 13, 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 u_gq and the q-component of the Park-Clarke transformation of at least one harmonic of the filter output voltages u*_ghq are supplied.

[0025] 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*_fghd with respect to the fundamental of the filter output currents i_fg1, i_fg2, i_fg3, the q-component of the Park-Clarke transformation of at least one harmonic of the filter output currents i*_fghq with respect to the fundamental of the filter output currents i_fg1, i_fg2, i_fg3, the d-component of the Park-Clarke transformation of the at least one harmonic of the reference filter output voltages u*_ghd and the q-component of the Park-Clarke transformation of the at least one harmonic of the reference-filter output voltages u*_ghq the regulation unit 4 has a second calculation unit 15, as shown in FIG. 1. As shown in FIG. 1, the input side of the second calculation unit 15 is supplied with the d-component with the filter output voltages u_gd, the q-component of the filter output voltages u_gq, the phase connection currents i_fi1, i_fi2, i_fi3, the filter capacitance currents i_Cf1, i_Cf2, i_Cf3 and the fundamental angle ωt of the filter output voltages u_g1, u_g2, u_g3. In order to illustrate the formation of the individual variables in the calculation unit 15, FIG. 2 shows one exemplary embodiment of the second calculation unit 15, in which the input variables shown in FIG. 2 are obtained using the following formula:
i_fghd+ji_fghq=i_fihd+ji_fihq−(i_Cfhd+ji_Cfhg)
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 i_fi1, i_fi2, i_fi3 including the associated harmonics, and filter capacitance currents i_Cf1, i_Cf2, i_Cf3 including the associated harmonics. This Park-Clarke Clarke transformation is carried out in particular in the second calculation unit 15, although this is not illustrated in the second calculation unit 15 shown in FIG. 2, for clarity reasons.

[0026] Finally, the reference-phase connection voltages u*_i1, u*_i2, u*_i3 are produced by an inverse Park-Clarke transformation of the d-component of the Park-Clarke transformation of the reference-phase connection voltages u*_id and the q-component of the Park-Clarke transformation of the reference-phase connection voltages u*_iq. As shown in FIG. 1, the regulator unit 7 for this purpose has a calculation unit 14 for formation of the reference-phase connection voltages u*_i1, u*_i2, u*_i3 by inverse Park-Clarke transformation, to whose input side the d-component of the Park-Clarke transformation of the reference-phase connection voltages u*_id and the q-component of the Park-Clarke transformation of the reference-phase connection voltages u*_iq are supplied.

[0027] In order to illustrate an exemplary method of operation of the active damping based on the exemplary method as explained above, FIG. 3 shows a waveform of the filter output currents i_fg1, i_fg2, i_fg3 in which undesirable oscillations in the filter output currents i_fg1, i_fg2, i_fg3 are 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 i_fg1, i_fg2, i_fg3 in FIG. 4 with active damping, and additional active reduction of harmonics using the exemplary method as described above.

[0028] 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.

[0029] Overall, it has been possible to show that the exemplary apparatus, e.g., as shown in FIG. 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.

[0030] It will be appreciated by those skilled in the art that the present invention can be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restricted. The scope of the invention is indicated by the appended claims rather than the foregoing description and all changes that come within the meaning and range and equivalence thereof are intended to be embraced therein.

• [0031] 1 Converter unit
• [0032] 2 Phase connection of the converter unit
• [0033] 3 LCL filter
• [0034] 4 Regulation device
• [0035] 5 Drive circuit
• [0036] 6 First calculation unit of the regulation device
• [0037] 7 Regulator unit
• [0038] 8 First proportional-integral regulator
• [0039] 9 Capacitive energy store
• [0040] 10 Second proportional-integral regulator
• [0041] 11 Third proportional-integral regulator
• [0042] 12 First adder
• [0043] 13 Second adder
• [0044] 14 Calculation unit for the regulator unit
• [0045] 15 Second calculation unit of the regulation device
• [0046] 16 third calculation unit of the regulation device