SELF-GUIDED INVERTER AND ITS OPERATION

Description

[0001] The invention relates to a self-commutated inverter which is supplied from a DC circuit and has electronic switches for generating a three-phase output voltage and a three-phase output filter connected downstream of the electronic switches and coupled to the DC circuit.

[0002] Self-commutated inverters, especially fast-switching inverters, often incorporate an output filter to limit high voltage slew rates that occur during switching and / or to eliminate the need for shielded cables. A specific type of output filter feeds interference back into a DC circuit supplying the inverter.

[0003] Inverter output filters often have a pronounced resonant frequency. To achieve the highest possible damping in the region of the inverter's switching frequency and its multiples, the components of an output filter are typically selected so that the output filter's resonant frequency lies between a maximum fundamental output frequency and the switching frequency. The filter's output currents can be used for current control. Using the filter's output currents can favorably influence the current control behavior compared to using the filter's input currents.

[0004] By connecting the output filter to the DC circuit, portions of the filter's input currents can flow directly into the DC circuit. These so-called common-mode currents are not detected by current control on the output side and therefore cannot be regulated. Even a classic dq current control with current measurement after the inverter would not detect these currents. Considering the currents after a dq0 transformation, a significant zero-sequence component is observed in this configuration, in addition to the actual d and q components. The output filter's resonant frequency is also visible in the common-mode circuit and leads to oscillations in the filter input currents, which stress the filter components of the output filter and the inverter's electronic switches.

[0005] An output filter coupled to the DC circuit therefore requires measures to dampen these oscillations. One approach is to introduce damping resistors to reduce the amplitude peaking in the region of the resonant frequency. However, these damping resistors require additional installation space and generate losses that must be dissipated.

[0006] In addition, active electronic circuits can be used to dampen oscillations. However, this alternative is very complex. Another solution to circumvent this problem is to change the filter topology. For example, if the connection to the DC circuit is disconnected, common-mode currents can no longer flow due to the principle of common-mode operation, but the filtering effect for common-mode disturbances is also lost. By using additional components, such as a common-mode inductor or additional capacitors, the filter transfer function can be designed differently for common-mode and so-called differential-mode signals, and thus resonant oscillations in the common-mode system can also be reduced. However, this solution also requires additional components and additional installation space.

[0007] US 2013 / 329471 A1 discloses a method and a device for damping zero-system currents.

[0008] US 2016 / 0156291 A1 discloses a bidirectional high-frequency drive configured to connect a power supply network and an electric machine. The high-frequency drive comprises inductors, each configured to connect to respective phase outputs of the electric machine, circuit breakers connected to the inductors, and a controller connected to the circuit breakers that generates control signals based on an operating state and a predetermined operating state of the electric machine.

[0009] CN 105 553 309 A discloses a three-level T-type inverter and a method for controlling its midpoint voltage. In the inverter, a connection point of an upper bus capacitor and a lower bus capacitor on a DC side of the inverter is short-circuited to a neutral line on an AC side. The method for controlling the midpoint voltage is based on dq-axis decoupling control.

[0010] EP 3 058 646 A1 discloses a method for controlling a power converter connected to an electrical load or an electrical energy source via a filter. The method determines output signals comprising currents and / or voltages measured in the filter. At least one of the output signals is algorithmically filtered by applying a signal filter to the output signal in order to amplify the output signal at a resonant frequency of the filter.

[0011] The invention is based on the objective of suppressing oscillations of filter input currents in a self-commutated inverter with an output filter coupled to a DC circuit supplying the inverter, without using additional components.

[0012] The problem is solved according to the invention by a method with the features of claim 1 and a self-commutated inverter with the features of claim 5. Advantageous embodiments of the invention are the subject of the dependent claims.

[0013] In the inventive method for operating a self-commutated inverter, which is supplied from a DC circuit and has electronic switches for generating a three-phase output voltage and a three-phase output filter connected downstream of the electronic switches and coupled to the DC circuit, the electronic switches are controlled by a space vector modulation in which a zero-system voltage of filter input voltages of the output filter is regulated as a function of a target voltage space vector of the space vector modulation and filter input currents of the output filter in such a way that oscillations in a zero system of the filter input currents are suppressed.

[0014] The invention thus provides for the targeted suppression of oscillations in the zero-sequence phase of the filter input currents of the output filter by modifying a space vector modulation used to control the electronic switches. When using conventional space vector modulation, the zero-sequence phase of the inverter voltage changes depending on the modulation level and the angular position of the voltage. These changes in the zero-sequence phase lead to common-mode excitation of the output filter and oscillations of the filter input currents. Therefore, by selectively influencing the zero-sequence voltage of the filter input voltages of the output filter through a modification of the space vector modulation, oscillations in the zero-sequence phase of the filter input currents can be suppressed.The inventive method thus reduces oscillations of the filter input currents and therefore loads on the output filter and the electronic switches of the inverter, without using further passive components, such as damping resistors, additional inductors or capacitors, or active electronic circuits.

[0015] The invention also provides that the zero-sequence voltage of the filter input voltages is controlled by a zero-sequence shift, which defines a time ratio of the durations during which one of the zero-sequence space vectors is applied during each clock cycle of the space vector modulation. This feature of the invention utilizes the fact that there are multiple zero-sequence space vectors and that the zero-sequence voltage of the filter input voltages can be influenced by the time ratio of the durations during which one of the zero-sequence space vectors is applied during each clock cycle of the space vector modulation.

[0016] A further embodiment of the aforementioned invention provides that the zero-sequence shift comprises two shift components, wherein a first shift component is determined as a function of the target voltage space vector of the space vector modulation, and the second shift component is formed as a function of the filter input currents of the output filter. For example, the zero-sequence shift is formed by adding the two shift components. The first shift component defines, for example, a time-duration ratio with which the target voltage space vector can be realized without zero-sequence distortion. The second shift component induces a zero-sequence voltage in the filter input voltages, which dampens oscillations in the zero-sequence of the filter input currents.The second displacement component is determined, for example, using a proportional (P) controller as a function of a total current calculated from the filter input currents of the output filter. For instance, the filter input currents of the output filter are measured, and the total current is calculated by adding the measured filter input currents.

[0017] The aforementioned embodiments of the invention make it possible to avoid excitation of common-mode oscillations as far as possible through the first displacement component of the zero-sequence shift and to dampen oscillations in filter input currents through the second displacement component. Avoiding excitation of common-mode oscillations by a zero-sequence-free realization of the target voltage space vector is primarily possible at relatively low modulation intensities, that is, at small ratios of the target voltage to the DC voltage of the DC circuit. However, such zero-sequence-corrected space vector modulation is not effective at higher modulation intensities and cannot dampen slowly decaying oscillations in the zero-sequence of the filter input currents.The second displacement component of the zero-system displacement, which depends on the filter input currents, serves to dampen these oscillations and dampens oscillations in the zero system of the filter input currents by means of a zero-system displacement that counteracts the oscillations.

[0018] A further embodiment of the invention provides that the target voltage space vector of the space vector modulation is formed as a function of the filter output currents of the output filter. The invention provides that the space vector modulation controls the filter output currents and simultaneously suppresses oscillations in a zero-sequence system of the filter input currents.

[0019] A self-commutated inverter according to the invention is powered from a DC circuit and comprises electronic switches for generating a three-phase output voltage, a three-phase output filter connected downstream of the electronic switches and coupled to the DC circuit, and a control unit. The control unit is configured to control the electronic switches with a space vector modulation, in which a zero-sequence voltage of the filter input voltages of the output filter is regulated as a function of a target voltage space vector of the space vector modulation and filter input currents of the output filter such that oscillations in a zero-sequence system of the filter input currents are suppressed. The inverter can be implemented in a two-level or multi-level topology. The advantages of an inverter according to the invention correspond to the advantages of the method according to the invention already mentioned above.Embodiments of the inverter according to the invention correspond to embodiments of the method according to the invention.

[0020] In particular, one embodiment of the inverter according to the invention provides that the control unit is configured to regulate the zero-sequence voltage of the filter input voltages by means of a zero-sequence shift that defines a time ratio of the durations in which one of the zero-voltage space vectors is applied during each clock cycle of the space vector modulation. For example, the control unit includes a space vector modulator configured to determine a first shift component of the zero-sequence shift as a function of the target voltage space vector of the space vector modulation. Furthermore, the control unit includes, for example, a proportional (P) controller configured to generate a second shift component of the zero-sequence shift as a function of a sum current formed from the filter input currents of the output filter.

[0021] Another embodiment of the inverter according to the invention provides that the control unit is designed to form a target voltage space vector of the space vector modulation as a function of filter output currents of the output filter.

[0022] Another embodiment of the inverter according to the invention provides a first measuring device for measuring the filter input currents and / or a second measuring device for measuring filter output currents of the output filter.

[0023] The properties, features, and advantages of this invention described above, as well as the manner in which they are achieved, will become clearer and more readily understandable in connection with the following description of exemplary embodiments, which are explained in more detail in conjunction with the drawings. These drawings show: FIG 1 shows a circuit diagram of a self-commutated inverter, FIG 2 shows a block diagram of a self-commutated inverter.

[0024] Corresponding parts are marked with the same reference symbols in the figures.

[0025] Figure 1 Figure 1 shows a circuit diagram of a self-commutated inverter 1. The inverter 1 is powered by a DC circuit 3. The inverter 1 has electronic switches S1, S2, and S3 for generating a three-phase output voltage, a three-phase output filter 5, and a control unit 7 for current regulation. The inverter 1 and the DC circuit 3 can be components of a converter that also includes a rectifier connected to the inverter 1 via the DC circuit 3.

[0026] Each electronic switch S1, S2, S3 switches a phase of the output voltage of inverter 1 and has a first switching state in which it connects the phase to a first DC potential Z+ of the DC circuit 3, and a second switching state in which it connects the phase to a second DC potential Z- of the DC circuit 3. The potential difference between the DC potentials Z+ and Z- is a DC voltage Uz of the DC circuit 3. The electronic switches S1, S2, S3 are in Figure 1 only schematically represented and, for example, each configured as a half-bridge with two semiconductor switches 9. Each semiconductor switch 9 is configured, for example, as an IGBT (bipolar transistor with insulated gate electrode) or as a MOSFET (metal-oxide-semiconductor field-effect transistor).

[0027] The output filter 5 is connected downstream of the electronic switches S1, S2, S3 and has one filter inductor 11 and two filter capacitors 13, 15 for each phase of the output voltage. The filter inductor 11 of a phase is connected to the electronic switch S1, S2, S3 of that phase. The output filter 5 is connected to the DC circuit 3 via the filter capacitors 13, 15. The first filter capacitor 13 of each phase is connected to the first DC potential Z+ of the DC circuit 3, and the second filter capacitor 15 of each phase is connected to the second DC potential Z- of the DC circuit 3.

[0028] Between the electronic switch S1, S2, S3 of a phase and the output filter 5, a filter input current i1_in, i2_in, i3_in of that phase flows. Downstream of the output filter 5, i.e., on the output side, filter output currents i1_out, i2_out, i3_out flow, each forming an output current of the inverter 1 for one phase. Common-mode currents of the output filter 5 can flow through the filter capacitors 13, 15.

[0029] The control unit 7 controls the electronic switches S1, S2, S3 with control signals C1, C2, C3. The control signals C1, C2, C3 are generated by the control unit 7 using a space vector modulation, in which a zero-sequence voltage of the filter input voltages of the output filter 5 is regulated as a function of a target voltage space vector u α,β_s of the space vector modulation and the filter input currents i1_in, i2_in, i3 in, as described in more detail below, such that oscillations in a zero-sequence of the filter input currents i1_in, i2_in, i3_in are suppressed, whereby the target voltage space vector µ α,β_s is generated as a function of the filter output currents i1_out, i2_out, i3_out. The filter input currents i1_in, i2_in, i3 in are detected by means of a first measuring device 17. The filter output currents i1_out, i2_out, i3_out are recorded by means of a second measuring device 19.

[0030] Figure 2 shows a block diagram of the in Figure 1The illustrated inverter 1, wherein the output of the inverter 1 is connected to a three-phase electric motor 21, such that the filter output currents i1_out, i2_out, i3_out of the inverter 1 are the motor currents of the electric motor 21. The electric motor 21 is designed as a rotating electric machine with a rotor and a stator and has an angular position sensor that detects a rotor angle φ, which indicates the position of the rotor relative to the stator. Alternatively, the electric motor 21 can also be designed as a linear motor with a stator and a rotor that is linearly movable relative to the stator and has a position sensor that indicates the rotor position relative to the stator. The rotor position then replaces the rotor angle φ.

[0031] The control unit 7 is configured to execute the inventive method described below. For this purpose, the control unit 7 comprises an adder 23, a P controller 25, a PI controller 27, a space vector modulator 29, and two transformers 31, 33.

[0032] The actual values ​​of the filter output currents i1_out, i2_out, i3_out and the actual value of the rotor angle φ, acquired by the second measuring device 19, are fed to a first transformer 31. The first transformer 31 determines an actual current space vector id,q_out of the filter output currents i1_out, i2_out, i3_out from these actual values ​​by means of a dq transformation in a dq coordinate system rotating with the rotor.

[0033] A differential current vector id,q_delta is calculated from the actual current vector id,q_out and a target current vector id,q_s by subtracting the actual current vector id,q_out from the target current vector id,q_s. The differential current vector id,q_delta represents the control deviation of the current regulation of the filter output currents i1_out, i2_out, i3_out.

[0034] The PI controller 27 forms a dq target voltage space vector ud,q_s from the differential current space vector id,q_delta, which describes a target voltage of the inverter 1 in a dq coordinate system that counteracts the control deviation.

[0035] The second transformer 33 transforms the dq target voltage space vector ud,q_s into a target voltage space vector u α,β_s for space vector modulation by transforming the dq target voltage space vector ud,q_s into a stator-fixed αβ coordinate system using the rotor angle φ.

[0036] The space vector modulator 29 generates the control signals C1, C2, C3 from the target voltage space vector u α,β_s and a zero-sequence shift µ. These signals control the electronic switches S1, S2, S3. The zero-sequence shift µ defines a time ratio of the durations during which each of the zero-voltage space vectors is applied during a clock cycle of the space vector modulation. In the case of two zero-voltage space vectors, the zero-sequence shift µ defines, for example, the quotient of the first duration during which a first zero-voltage space vector is applied during a clock cycle and the second duration during which the second zero-voltage space vector is applied during the same clock cycle.

[0037] The zero-system shift µ is formed additively according to µ = µ₁ + µ₂ from two shift components µ₁ and µ₂. A first shift component µ₁ is generated by the space vector modulator 29 as a function of the target voltage space vector uα,β_s and defines a time-duration ratio with which the target voltage space vector uα,β_s can be realized without a zero system.

[0038] The second displacement component µ2 is generated by the adder 23 and the proportional controller 25 as a function of the filter input currents i1_in, i2_in, i3_in such that it produces a zero-sequence voltage of the filter input voltages, which dampens oscillations in the zero-sequence of the filter input currents i1_in, i2_in, i3_in. For this purpose, the adder 23 adds the actual values ​​of the filter input currents i1_in, i2_in, i3_in, acquired by the first measuring device 17, to form a total current i0_in. The second displacement component µ2 is then determined by the proportional controller 25 from this total current i0_in.

[0039] Instead of measuring the filter input currents i1_in, i2_in, i3_in and the filter output currents i1_out, i2_out, i3_out with measuring devices 17 and 19, it is also possible to measure only the filter input currents i1_in, i2_in, i3_in and calculate the filter output currents i1_out, i2_out, i3_out. Alternatively, it is possible to measure only the filter output currents i1_out, i2_out, i3_out and calculate the filter input currents i1_in, i2_in, i3_in.

[0040] Although the invention has been further illustrated and described in detail by means of preferred embodiments, the invention is not limited by the disclosed examples and other variations can be derived by the person skilled in the art without leaving the scope of protection of the invention, as defined by the attached claims.

Claims

1. Method for operating a self-commutated inverter (1), which is supplied from a direct voltage circuit (3) and electronic switches (S1, S2, S3) for generating a three-phase output voltage and a three-phase output filter (5) arranged downstream of the electronic switches (S1, S2, S3) and coupled to the direct voltage circuit (3), so that common mode portions of filter input currents (i1_in, i2_in, i3_in) of the output filter (5) can flow into the direct voltage circuit (3), wherein - the electronic switches (S1, S2, S3) are controlled in an open-loop manner by space vector modulation, in which - filter output currents (i1_out, i2_out, i3_out) of the output filter (5) are controlled in a closed-loop manner and a zero-phase-sequence system voltage of filter input voltages of the output filter (5) is controlled in a closed-loop manner as a function of a target voltage space vector (uα,β_s) of the space vector modulation and the filter input currents (i1_in, i2_in, 13_in) of the output filter (5) so that oscillations in a zero-phase-sequence system of the filter input currents (11_in, i2_in, i3_in) are suppressed, - the zero-phase-sequence system voltage of the filter input voltages is controlled in a closed-loop manner by a zero-phase-sequence system shift (µ), which defines a duration ratio of durations, in which one of the zero-phase-sequence voltage space vectors is applied in each case during a clock period of the space vector modulation, characterised in that the zero-phase-sequence system shift (µ) has two shift portions (µ1, µ2) , wherein a first shift portion (µ1) is determined as a function of the target voltage space vector (uα,β_s) of the space vector modulation and the second shift portion (µ2) is formed as a function of the filter input currents (i1_in, 12_in, 13_in) of the output filter (5), - the zero-phase-sequence system shift (µ) is formed by adding the two shift portions (µ1, µ2), - the first shift portion (µ1) defines a duration ratio, with which the target voltage space vector (uα,β_s) can be realised free of a zero-phase-sequence system and - a zero-phase-sequence system voltage of the filter input voltages is effected by the second shift portion (µ2) and dampens oscillations in the zero-phase-sequence system of the filter input currents (i1_in, i2_in, i3_in).

2. Method according to claim 1, wherein the second shift portion is formed by means of a P controller (25) as a function of a total current (i0_in), which is formed from the filter input currents (i1_in, i2_in, i3_in) of the output filter (5).

3. Method according to claim 2, wherein the filter input currents (i1_in, i2_in, 13_in) of the output filter (5) are measured and the total current (i0_in) is formed by adding the measured filter input currents (i1_in, i2_in, i3_in).

4. Method according to one of the preceding claims, wherein the target voltage space vector (uα,β_s) of the space vector modulation is formed as a function of filter output currents (il_out, i2_out, i3_out) of the output filter (5).

5. Self-commutated inverter (1), which is supplied from a direct voltage circuit (3), the inverter (1) comprising - electronic switches (S1, S2, S3) for generating a three-phase output voltage, - a three-phase output filter (5) arranged downstream of the electronic switches (S1, S2, S3) and coupled to the direct voltage circuit (3), so that common mode portions of filter input currents (i1_in, 12_in, i3_in) of the output filter (5) can flow into the direct voltage circuit (3), - a control unit (7) which is designed to activate the electronic switches (S1, S2, S3) with a space vector modulation, with which filter output currents (i1_out, i2_out, i3_out) of the output filter (5) are controlled in a closed-loop manner and a zero-phase-sequence system voltage of filter input voltages of the output filter (5) is controlled in a closed-loop manner as a function of a target voltage space vector (uα,β_s) of the space vector modulation and the filter input currents (i1_in, 12_in, i3_in) of the output filter (5) so that oscillations in a zero-phase-sequence system of the filter input currents (i1_in, i2_in, i3_in) are suppressed, wherein the control unit (7) is designed to control in a closed-loop manner the zero-phase-sequence system voltage of the filter input voltages by means of a zero-phase-sequence system shift (µ), which defines a duration ratio of durations, in which one of the zero-phase-sequence voltage space vectors is applied in each case during a clock period of the space vector modulation, characterised in that the control unit (7) has a space vector modulator (29), which is designed to determine a first shift portion (µ1) of the zero-phase-sequence system shift (µ) as a function of the target voltage space vector (uα,β_s) of the space vector modulation, wherein the control unit (7) has a P controller (25), which is designed to form a second shift portion (µ2) of the zero-phase-sequence system shift (µ) as a function of a total current (i0_in), which is formed from the filter input currents (i1_in, i2_in, 13_in) of the output filter (5), wherein the zero-phase-sequence system shift (µ) can be formed by adding the two shift portions (µ1, µ2), wherein by means of the first shift portion (µ1) a time duration ratio can be defined, with which the target voltage space vector (uα,β_s) can be realised free of a zero-phase-sequence system and wherein a zero-phase-sequence system voltage of the filter input voltages can be effected by the second shift portion (µ2) and dampens oscillations in the zero-phase-sequence system of the filter input currents (i1_in, 12_in, i3_in).

6. Inverter (1) according to claim 5, wherein the control unit (7) is embodied to form the target voltage space vector (uα,β_s) of the space vector modulation as a function of the filter output currents (i1_out, i2_out, i3_out) of the output filter (5).

7. Inverter (1) according to one of claims 5 or 6 with a first measuring apparatus (17) for measuring the filter input currents (i1_in, i2_in, i3_in) and / or a second measuring apparatus (19) for measuring the filter output currents (il_out, i2_out, i3_out) of the output filter (5).