DE102022120268A1 - Method for reducing a voltage ripple in the intermediate circuit of a 3-level inverter - Google Patents

Abstract

The invention relates to a method for operating a 3-level inverter (1), which has an intermediate circuit (2) with three connections (DC+, DC-, N) and at least three switching sections (3a, 3b, 3c), each with a phase tap ( u, v, w) and three switching devices (S1, S2, S3), for which a switching pattern (SMa, SMb) is determined to modulate a working point-specific output voltage (Va), through which the switching devices (3a, 3b, 3c). Connect the phase tap (u, v, w) of the respective switching section (3a, 3b, 3c) to one of the connections (DC+, DC-, N) of the intermediate circuit (2) to generate a voltage vector (v2p, v2n, v4, v5). ,wherein in order to reduce an input-side voltage ripple of an input voltage (VDC) present on the intermediate circuit, it is determined for each voltage vector (v2p, v2n, v4, v5) of the switching pattern (SMa, SMb) whether it causes an input voltage increase or an input voltage reduction, - the Voltage vectors (v2p, v2n, v4, v5) are assigned to switching pattern sections (A+, A-), and - the switching pattern sections (A+, A-) are combined to form the switching pattern (SMa, SMb) in such a way that an input voltage-increasing switching pattern section (A+) and an input voltage reducing switching pattern portion (A-) for increasing a frequency and decreasing an amplitude of the voltage ripple alternately.

Description

The invention relates to a method for operating a 3-level inverter, which has an intermediate circuit with three connections and at least three switching sections connected to the intermediate circuit, each with a phase tap for electrical connection to one phase of an electric drive machine, each switching section having three Has switching devices. The switching devices are each connected to one of the connections of the intermediate circuit. A switching pattern is determined for the switching devices in order to modulate an operating point-specific output voltage, through which the switching devices are converted into different switching states and thereby connect the phase tap of the respective switching section to one of the connections of the intermediate circuit, generating a voltage vector. The invention also relates to a control device, a power module and a motor vehicle.

In this case, interest is focused on 3-level inverters or three-stage inverters for electrically powered motor vehicles, i.e. electric or hybrid vehicles. Such 3-level inverters are part of a high-voltage on-board electrical system of the motor vehicle and are connected between an electrical energy storage device, for example a traction battery, and an electric drive motor of the high-voltage on-board electrical system. They serve to convert a direct voltage provided by the electrical energy storage into a multi-phase alternating voltage for phases of the electric drive machine. Also, the 3-level inverter can be used as a power factor correction filter for a 3-phase charger.

Inverters usually have three half-bridges connected in parallel and an intermediate circuit connected to the half-bridges. Each half bridge has two switching devices, a first switching device being designed as a high-side switching device and being connected to a high-side connection of the intermediate circuit, and a second switching device being designed as a low-side switching device and being connected to a Low-side connection of the intermediate circuit is connected. In addition to the half bridges, 3-level inverters have three third switching devices, which are designed as midpoint switching devices and are each electrically connected to a tap between the first and second switching devices of a half bridge and to a midpoint connection of the intermediate circuit. Such a 3-level inverter can provide three voltage levels for the phases of the electrical machine per switching section, which consists of a half bridge and a midpoint switching device, and thus has, in addition to the eight switching states of a conventional 2-level inverter, 19 further switching states for modulating a operating point-specific output voltage. Using these 27 switching states, a desired sinusoidal shape of the output voltage with a particularly low harmonic component can be generated, thereby reducing losses in the electrical machine. The 3-level inverter also has particularly low switching losses.

To protect other components of the high-voltage electrical system, there are usually specifications as to what voltage ripple each component can cause on the input side. With a 3-level inverter, this voltage ripple is created, which is a voltage signal of any frequency and waveform superimposed on the input DC voltage, for example due to the switching processes of the switching devices. This voltage ripple can be fed back into the intermediate circuit on the input side and cause charging and discharging processes of the capacitors there, which cause the capacitors to heat up in an undesirable manner. Therefore, the voltage ripple can be influenced by the design of the capacitors of the intermediate circuit by sizing them in such a way that they reduce the voltage ripple. However, this dimensioning results in an increased space requirement for the capacitors, increased weight and increased costs of the 3-level inverter.

It is the object of the present invention to provide a simple and cost-effective solution for reducing voltage ripples of a 3-level inverter for a motor vehicle.

This object is achieved according to the invention by a method, a control device, a power electronics module and a motor vehicle with the features according to the respective independent patent claims. Advantageous embodiments of the invention are the subject of the dependent claims, the description and the figures.

A method according to the invention is used to operate a 3-level inverter. The 3-level inverter has an intermediate circuit with three connections and at least three switching sections connected to the intermediate circuit, each with a phase tap for electrical connection to one phase of an electric drive machine. Each switching section has three switching devices, each of which is connected to one of the connections of the intermediate circuit and for which an operating point is modulated specific output voltage a switching pattern is determined. The switching pattern converts the switching devices into different switching states and thereby connects the phase tap of the respective switching section to one of the connections of the intermediate circuit, generating a voltage vector.

To reduce an input-side voltage ripple of an input voltage applied to the intermediate circuit, it is first determined for each voltage vector of the switching pattern whether it causes an input voltage change in the form of an input voltage increase or an input voltage reduction. The voltage vectors are assigned to switching pattern sections, with each switching pattern section having at least one voltage vector and the switching pattern sections being assembled into the switching pattern in such a way that a switching pattern section with at least one input voltage-increasing voltage vector and a switching pattern section with at least one input voltage-reducing voltage vector for increasing a frequency and for reducing a Alternate the amplitude of the tension ripple.

The invention also includes a control device which is designed to carry out a method according to the invention, as well as a power electronics module with a 3-level inverter and a control device according to the invention. A motor vehicle according to the invention comprises an electrical energy storage device, an electric machine and a power electronics module according to the invention, wherein the 3-level inverter is connected on the input side to the electrical energy storage device and on the output side to phases of a stator of the electric machine. The 3-level inverter is designed, for example, to convert the DC voltage provided by the electrical energy storage, which is present as an input voltage at the intermediate circuit of the 3-level inverter, into a multi-phase alternating voltage and to provide it as an output voltage for the phases of the electrical machine.

The 3-level inverter has the intermediate circuit on the input side, which in particular has at least one series connection of at least two capacitors. The switching sections or switching bridges each have a first switching device in the form of a high-side switching device, a second switching device in the form of a low-side switching device and a third switching device in the form of a midpoint switching device. Each switching device can have at least one switch in the form of a semiconductor switch, for example an IGBT or power MOSFET. The high-side switching device and the low-side switching device of a switching section are connected to form a half bridge. Each switching section has a phase tap, which is connected to the associated phase of the electrical machine. The connections of the series connection of capacitors, which form a high-side connection and a low-side connection of the intermediate circuit, are connected to poles of the electrical energy storage and to the half-bridges of the switching sections. A midpoint connection, or midpoint for short, is provided between the capacitors of the at least one series circuit, which is connected to the midpoint switching devices of the switching sections. The half bridges each form a vertical bridge branch of the respective switching section and the midpoint switching devices form a horizontal bridge branch of the respective switching section. To provide a bidirectional blocking capability, the midpoint switching devices can each have two midpoint switches connected in anti-series or each have one bidirectional midpoint switch.

Due to the three switching devices per switching bridge, the 3-level inverter has 27 switching states with three switching bridges, with each switching state generating a voltage vector that can be represented as a space vector in a vector diagram. With a closed high-side switching device of a switching section, the phase tap of this switching section is electrically connected to the high-side connection and a high-side potential is thus applied to the associated phase. With a closed low-side switching device of a switching section, the phase tap of this switching section is electrically connected to the low-side connection and thus a low-side potential is applied to the associated phase. With a closed midpoint switching device of a switching section, the phase tap of this switching section is electrically connected to the midpoint connection and thus a midpoint potential is applied to the associated phase.

For example, three switching states are created in which either all high-side switches are closed and thus all phase taps and phases are connected to the high-side connection of the intermediate circuit, or all low-side switches are closed and thus all phase taps and phases are connected are connected to the low-side connection of the intermediate circuit, or all midpoint switches are closed and thus all phase taps and phases are connected to the midpoint connection of the intermediate circuit, zero voltage vectors, which are represented as zero voltage space vectors in the space vector diagram be sent. The zero voltage space vectors can thus be generated redundantly.

Six further switching states, which do not involve the midpoint switching devices, generate six voltage vectors, which are represented as long, outer voltage space vectors in the space vector diagram and form corner points of an outer hexagon. These six long, outer voltage space vectors correspond to the basic voltage space vectors of a 2-level inverter. Six further switching states involve the midpoint switching devices and generate six further voltage vectors, which are depicted as middle, outer voltage space vectors in the space vector diagram and lie on the side edges of the outer hexagon between two long, outer voltage space vectors.

Twelve further switching states, which also involve the midpoint switching devices, generate twelve short, internal voltage space vectors, with two switching states each providing two identical voltage space vectors, i.e. voltage space vectors with the same length and the same phase angle. This means that each of the short, inner voltage space vectors can be generated redundantly. The short, inner stress space vectors span an inner hexagon, with two redundant stress space vectors each forming a corner point of the inner hexagon. The mutually redundant voltage space vectors have an opposite, complementary effect on a current in the center connection of the intermediate circuit.

The outer hexagon is divided into six triangular sectors, with corner points of each sector formed by the zero voltage space vectors and two adjacent long, outer voltage space vectors. Each sector is in turn divided into four triangular segments, with two redundant voltage space vectors located at at least one corner point of each segment. In order to modulate any operating point-specific output voltage using the voltage vectors, it is determined in which sector and in which segment of the sector the operating point-specific output voltage space vector representing the output voltage lies. This operating point-specific output voltage space vector can then be generated by pulse width modulation of the voltage space vector spanning this segment. For this purpose, a switching pattern is determined in which a sequence and a duration of the segment-specific voltage space vectors used for modulation are defined.

The 3-level inverter generates voltage ripples, which occur on the input side of the capacitors of the intermediate circuit and which are applied to the input voltage. This voltage ripple can damage other components in the high-voltage electrical system, which is why its amplitude must not exceed a predetermined threshold value. According to the invention, the voltage ripple is reduced using a special, voltage ripple-reducing switching pattern. The invention is based on the knowledge that each voltage vector of the switching pattern has a vector-specific influence on the input voltage of the 3-level inverter depending on its phase position. In other words, the voltage vectors cause a vector-specific input voltage change. Some voltage vectors have the effect of increasing the input voltage, i.e. causing a voltage swing, and some voltage vectors have the effect of decreasing the input voltage, i.e. causing a voltage drop. Therefore, the voltage vectors in the switching pattern are arranged in such a way that switching pattern sections with at least one input voltage increasing voltage vector and switching pattern sections with at least one input voltage reducing voltage vector alternate. This increases the frequency of the voltage ripple and reduces the amplitude of the voltage ripple. By optimizing the switching pattern with regard to the voltage ripple, it is advantageously possible to avoid oversizing the capacitors of the intermediate circuit. The 3-level inverter can therefore be designed to be particularly cost-effective, space-saving and weight-saving.

In particular, the voltage vectors are divided into switching pattern sections in such a way that the input voltage changes of the switching pattern sections provided by the voltage vectors are the same in magnitude or differ from one another by at most a predetermined threshold value. For example, a switching pattern section can have at least two voltage vectors, the respective influences on the input voltage change, viewed in absolute terms, being smaller than the respective influences on the input voltage change of the other voltage vectors. Therefore, these two voltage vectors are assigned to a switching pattern section so that their influences on the input voltage change can add up. At least two voltage vectors whose absolute contribution to the input voltage change would not be large enough on their own can be bundled in a switching pattern section, so that the input voltage change caused by this switching pattern section is approximately the same size as the opposite input voltage Change in voltage of the adjacent switching pattern section.

The embodiments presented with reference to the method according to the invention and their advantages apply accordingly to the control device according to the invention, to the power electronics module according to the invention and to the motor vehicle according to the invention.

Further features of the invention emerge from the claims, the figures and the description of the figures. The features and combinations of features mentioned above in the description as well as the features and combinations of features mentioned below in the description of the figures and/or shown in the figures alone can be used not only in the combination specified in each case, but also in other combinations or on their own.

• 1 ein Schaltplan einer Ausgestaltung eines 3-Level-Inverters;
• 2 eine Darstellung eines Zeigerdiagramms des 3-Level-Inverters;
• 3 eine Darstellung von Zeitanteilen unterschiedlicher Spannungsvektoren innerhalb des Schaltmusters; und
• 4a, 4b eine Darstellung von zwei spannungsrippelreduzierenden Schaltmustervarianten zum Modulieren einer Ausgangsspannung des 3-Level-Inverters.

The invention will now be explained in more detail using a preferred exemplary embodiment and with reference to the drawings. Show it:

• 1 a circuit diagram of an embodiment of a 3-level inverter;
• 2 a representation of a phasor diagram of the 3-level inverter;
• 3 a representation of time components of different voltage vectors within the switching pattern; and
• 4a , 4b a representation of two voltage ripple-reducing switching pattern variants for modulating an output voltage of the 3-level inverter.

In the figures, identical and functionally identical elements are provided with the same reference numerals.

1 shows a 3-level inverter 1 for an electrified motor vehicle. The 3-level inverter 1 has an intermediate circuit 2 on the input side, which here has three intermediate circuit branches 2a, 2b, 2c, each with a series connection of two capacitors C1, C2. Connections in the form of a high-side connection DC+ and a low-side connection DC- of the intermediate circuit 2 are connected to poles of an electrical energy storage device of the motor vehicle, which provides the intermediate circuit 2 with a direct voltage V _DC . Half the direct voltage V _DC /2 is present at each capacitor C1, C2. The 3-level inverter 1 also has three switching sections 3a, 3b, 3c, which are each electrically connected to one of the intermediate circuit branches 2a, 2b, 2c. However, it can also be provided that the intermediate circuit 2 has only one intermediate circuit branch with two series-connected capacitors C1, C2, to which the switching sections 3a, 3b, 3c are connected. Each switching section 3a, 3b, 3c has a phase tap u, v, w, which is connected to a phase Phu, Phv, Phw of an electric drive machine of the motor vehicle.

Each switching section 3a, 3b, 3c has three switching devices S1, S2, S3, which can each have at least one switch designed as a semiconductor switch, for example IGBTs. The switching devices S1 and S2 are connected to a half bridge HB of the respective switching section 3a, 3b, 3c. The first switching devices S1 are high-side switching devices and are connected to the high-side connection DC+ of the intermediate circuit 2, which is connected to a positive pole of the electrical energy storage, and to a tap A of the respective half bridge HB. The second switching devices S2 are low-side switching devices and are connected to the tap of the respective half bridge HB and to the low-side connection DC of the intermediate circuit 2, which is connected to a negative pole of the electrical energy storage. The third switching devices S3 are midpoint switching devices and are connected to the tap and to a further connection of the intermediate circuit 2 in the form of a midpoint connection N of the respective intermediate circuit branch 2a, 2b, 2c between the two capacitors C1, C2. The midpoint switching devices S3 each have two midpoint switches S3a, S3b connected in anti-series.

The 3-level inverter 1 can assume 27 switching states, by means of which a desired, operating point-specific output voltage can be modulated for application to the phases Phu, Phv, Phw. Each switching state generates a voltage vector, which in an in 2 shown space vector diagram 4 can be represented with the axes α, β. In the space vector diagram 4, the different switching states (u, v, w) are shown at discrete points with u, v, w = 0, 1, 2, where u indicates which of the switching devices S1, S2, S3 is closed in the first switching section 3a , v indicates which of the switching devices S1, S2, S3 is closed in the second switching section 3b, and w indicates which of the switching devices S1, S2, S3 is closed in the third switching section 3c. “0” means that the respective low-side switching device S2 is closed and thus connects the respective phase tap u, v, w to the low-side connection DC-. “1” means that the respective midpoint switching device S3 is closed and thus connects the respective phase tap u, v, w to the midpoint connection N. “2” means that the respective high-side switching device S1 is closed and thus connects the respective phase tap u, v, w with the high-side connection DC+. The discrete points mark an arrowhead of a voltage space vector generated by the respective switching state, which has a specific length and a specific phase angle. It is already clear that switching states exist that generate identical voltage vectors. For example, the switching states “221” and “110” produce identical voltage vectors. The switching states “000”, “111”, “222” also generate identical voltage vectors, namely a zero voltage vector v0 ₁ , v0 ₂ , v0 ₃ .

The space vector diagram is divided into six sectors K1, K2, K3, K4, K5, K6, with each sector K1, ..., K6 divided into four segments Q1, Q2, Q3, Q4 (shown here only for the first sector K1). is. If a certain, operating point-specific output voltage is to be achieved, for example the in 2 output voltage vector Va shown are modulated, it is first determined in which sector K1, ..., K6 and in which segment Q1, ..., Q4 of the respective sector K1, ..., K6 the respective output voltage vector V _a lies. For example, the output voltage vector V _a lies in the first sector K1 in the fourth segment Q4 and can therefore be represented by those voltage vectors v2p, generated by switching state "221", v2n, generated by switching state "110", v4, generated by switching state "210", and v5, generated by switching state “220” are modulated or approximated, which form corner points of the fourth segment Q4 and thus surround the output voltage vector V _a to be generated.

As a rule, the voltage vectors v2p, v2n, v4, v5 are set in sequence so that the switching actions during the transition between two switching states are minimal. Here, however, the voltage vectors v2p, v2n, v4 and v5 are arranged such that an amplitude of the voltage ripple applied to the input voltage V _DC is minimized and a frequency of the voltage ripple is maximized. Depending on the phase position, voltage vectors v2p, v2n, v4 and v5 have a different influence on the input voltage V _DC of the 3-level inverter 1. In order to minimize the amplitude of the voltage ripple at the input, it is determined how the influence of the voltage vectors v2p, v2n , v4, v5, which are set within a period as part of the space vector modulation, to the input voltage V _DC . In 3 The total time duration d0 of the voltage vector v4, d1 of the voltage vector v5 and d2 of the voltage vectors v2p and v2n within a modulation period are shown as an example.

The voltage vector v4 has the effect of reducing the input voltage, while the voltage vectors v5, v2p, v2n have the effect of increasing the input voltage. Then a voltage ripple-reducing switching pattern SMa (see 4a) or SMb (see 4b) with switching pattern sections A+, A-. At least one voltage vector v2p, v2n, v4, v5 is assigned to each switching pattern section A+, A-. The switching pattern SMa has seven switching pattern sections A+, A-, one of the input voltage increasing voltage vectors v2a, v2n, v5 being arranged in the switching pattern sections A+ and the input voltage reducing voltage vector v4 being arranged in the switching pattern sections A-. The voltage vectors v2p, v2n, v4, v5 and thus the switching pattern sections A+, A- are arranged alternately in such a way that voltage vectors v2p, v2n, v5, which increase the input voltage V _DC , and voltage vectors v4, which lower the input voltage V _DC , alternate. This increases the frequency of the voltage ripple and reduces the amplitude.

4b shows a switching pattern SMb for the case that voltage vectors, for example v2n, exist which have a smaller influence on the input voltage V _DC than the other voltage vectors. Therefore, voltage vectors v2n, v5, which have the same magnitude but absolutely a smaller influence on the input voltage V _DC , are bundled in a switching pattern section A+, so that the voltage swing, i.e. the increase in the input voltage V _DC , and the voltage reduction, i.e. the reduction the input voltage V _DC , must be balanced as well as possible. Here too, switching pattern sections A-, which have the effect of lowering the input voltage, and switching pattern sections A+, which have the effect of increasing the input voltage, are arranged alternately with one another.

Claims (6)

Method for operating a 3-level inverter (1), which has an intermediate circuit (2) with three connections (DC+, DC-, N) and at least three switching sections (3a, 3b, 3c) connected to the intermediate circuit (2). each has a phase tap (u, v, w) for electrically connecting to one phase (Phu, Phw, Phw) of an electric drive machine, each switching section (3a, 3b, 3c) having three switching devices (S1, S2, S3). , which are each connected to one of the connections (DC+, DC-, N) of the intermediate circuit (2) and for which a switching pattern (SMa, SMb) is determined to modulate an operating point-specific output voltage (V _a ), through which the switching devices (3a , 3b, 3c) into different switching states the and thereby the phase tap (u, v, w) of the respective switching section (3a, 3b, 3c) to generate a voltage vector (v2p, v2n, v4, v5) with one of the connections (DC+, DC-, N) of the intermediate circuit (2), characterized in that in order to reduce an input-side voltage ripple of an input voltage (V _DC ) present on the intermediate circuit - it is determined for each voltage vector (v2p, v2n, v4, v5) of the switching pattern (SMa, SMb) whether it has a Input voltage change in the form of an input voltage increase or an input voltage reduction - the voltage vectors (v2p, v2n, v4, v5) are assigned to switching pattern sections (A+, A-), each switching pattern section (A+, A-) having at least one voltage vector (v2p, v2n , v4, v5), and - the switching pattern sections (A+, A-) are assembled into the switching pattern (SMa, SMb) in such a way that there is a switching pattern section (A+) with at least one input voltage increasing voltage vector (v2p, v2n, v5) and a Switching pattern section (A-) alternates with at least one input voltage-reducing voltage vector (v4) for increasing a frequency and for reducing an amplitude of the voltage ripple. Procedure according to Claim 1 , characterized in that the voltage vectors (v2p, v2n, v4, v5) are assigned to the switching pattern sections (A+, A-) in such a way that the input voltage changes of the switching pattern sections (A+, A.) provided by the voltage vectors (v2p, v2n, v4, v5). -) are the same in amount or differ from each other by a maximum of a predetermined threshold value. Procedure oh Claim 2 , characterized in that at least one switching pattern section (A+) has at least two voltage vectors (v2n, v5), whose individual input voltage changes add up to a total input voltage change of the switching pattern section (A+). Control device for a power electronics module of a motor vehicle, which is designed to carry out a method according to one of the preceding claims. Power electronics module for a motor vehicle with: - a 3-level inverter (1), which has an intermediate circuit (2) with three connections (DC+, DC-, N) and at least three switching sections (3a, 3b) connected to the intermediate circuit (2). , 3c) each with a phase tap (u, v, w) for electrically connecting to one phase (Phu, Phv, Phw) of an electric drive machine, each switching section (3a, 3b, 3c) having three switching devices (S1, S2 , S3), which are each connected to one of the connections (DC+, DC-, N), and - a control device Claim 4 . Motor vehicle with an electrical energy storage, an electric drive machine and a power electronics module Claim 5 , wherein the 3-level inverter (1) is connected on the input side to the electrical energy storage and on the output side to phases (Phu, Phv, Phw) of the electric drive machine.

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