Method for operating an inverter, inverter and electric drive system

Abstract

The invention relates to a change from an active short circuit to the free wheel of an inverter in an electric drive system. To this end, a half bridge of the inverter is temporarily set to a switching state that corresponds to the switching state of the free wheel. Then, the current curve in this half bridge is evaluated to check whether a change to the free wheel is permissible or not.

Description

[0001] Description

[0002] Title:

[0003] Method for operating an inverter, inverter and electric drive system

[0004] Technical area

[0005] The present invention relates to a method for operating an inverter, in particular an inverter of an electric drive system. The present invention further relates to an inverter in which such a method is implemented, as well as to an electric drive system with such an inverter.

[0006] background

[0007] Electric drive systems are used in numerous applications. For example, such electric drive systems are also used in fully or at least partially electrically powered motor vehicles. A power converter can be used to convert electrical energy from an energy source, such as the traction battery of an electric vehicle, into an electrical voltage suitable for controlling an electrical machine. In addition, so-called safe operating states can also be set in the power converter, for example an active short circuit, in which the switching elements are controlled such that the terminals of the electrical machine are short-circuited. Furthermore, a so-called freewheel can be provided, in which all switching elements in the half-bridges of the power converter are open.

[0008] For example, the publication DE 10 2014222 256 A1 describes a method for switching the operating state of an electrical machine from freewheeling to an active short circuit. In a multi-phase electrical machine, it is proposed that the individual phases be successively switched to the corresponding switching states.

[0009] Disclosure of the invention

[0010] The present invention provides a method for operating an inverter, an inverter, and an electric drive system having the features of the independent patent claims. Further advantageous embodiments are the subject of the dependent patent claims.

[0011] Accordingly, it is provided:

[0012] A method for operating an inverter, in particular for operating an inverter for an electric drive system. The inverter comprises a plurality of half-bridges, each with two switching elements. The method comprises a step of setting an active short circuit in the inverter. In such an active short circuit, either the upper switching elements of all half-bridges or the lower switching elements of all half-bridges are closed. In this way, all phase connections of an electrical machine connected to the inverter are electrically connected to one another, i.e., short-circuited. The method further comprises a step of opening the two switching elements in a first half-bridge of the inverter. In other words, a switching state corresponding to the switching state for a freewheeling mode is set in a first of the half-bridges of the inverter.The method further comprises a step for determining an electrical parameter that characterizes a phase current profile in the direction of the first half-bridge in which both switching elements are open. Furthermore, the method comprises a step for setting a freewheeling mode in the inverter, in which all switching elements of all half-bridges are open if a phase current profile in the direction of the first half-bridge satisfies a predetermined switching condition.

[0013] The determination of an electrical parameter that characterizes a course of a phase current in the direction of the first half-bridge can, for example, be carried out using at least one current measuring device. Of course, the phase current can assume positive and negative values, so that the formulation phase current in the direction of the first half-bridge should in no way be interpreted in a restrictive manner that only the current flowing in the direction of the first half-bridge is determined, but naturally also includes the phase current in the opposite direction, which flows away from the first half-bridge. Depending on the arrangement within the inverter, one or more current measuring devices are preferably used to determine the electrical parameter, which enable the determination of a parameter that characterizes the course of a phase current in the direction of the first half-bridge.Such a current measuring device can preferably be arranged within a switching element of the half-bridge and, in particular, can detect currents through an intrinsic diode connected in parallel or a body diode of a switching element. Such a current measuring device is preferably arranged in a supply or discharge line of the current through a switching element of a half-bridge, preferably at the phase current terminal of a half-bridge.

[0014] Furthermore, it is planned:

[0015] An inverter with multiple half-bridges and a control device. The multiple half-bridges each comprise two switching elements. In particular, two semiconductor switching elements can be provided in each half-bridge, with a diode being provided in parallel with each semiconductor switching element. The control device is designed to provide control signals for the switching elements in the multiple half-bridges. Furthermore, the control device is designed to carry out a method according to the invention for operating the inverter.

[0016] Finally, it is planned:

[0017] An electric drive system comprising an electric machine and an inverter according to the invention. The inverter is connected to the electric machine via an AC voltage connection. Furthermore, the inverter is designed to be connected to a DC voltage source via a DC voltage connection. Advantages of the invention

[0018] An electric drive system typically includes an electrical power converter, such as an inverter, which can convert a DC voltage supplied at the input into an AC voltage to control an electrical machine. Under certain conditions, a so-called safe state can also be set in this power converter. Such a safe state can, for example, be an active short circuit, in which the switching elements of the power converter are controlled in such a way that the phase connections of an electrical machine connected to the power converter are short-circuited. For this purpose, all upper switching elements or all lower switching elements in the half-bridges of the power converter can be closed, for example.For example, the upper switching elements can be those switching elements that are connected to a positive connection point of a DC voltage connection of the power converter, while the lower switching elements are connected to a negative connection point of the DC voltage connection.

[0019] In addition, the so-called freewheeling mode is also known as a safe operating state. In this mode, all switching elements in the power converter are open. If the electrical machine connected to the power converter is moving, an electrical voltage can be induced in the phase windings of the electrical machine. If the electrical voltage from the electrical machine exceeds the input voltage on the DC side, an electrical current can flow through the diodes in the power converter provided parallel to the switching elements from the electrical machine to a battery or similar connected on the DC side of the power converter. Such a current flow is undesirable under certain circumstances and should therefore be avoided or at least limited where possible.Diodes which are provided in parallel with the switching elements of the power converter can be understood in the sense of the present invention as: separate electrical elements, integrated electrical elements and / or also an element which, through the semiconductor structure of the switching element or a parasitic semiconductor structure of the switching element, in particular in the case of MOSFET, performs a function corresponding to a diode behavior.

[0020] It is therefore an idea of ​​the present invention to switch from an active short circuit to a freewheeling mode only when no or at least no significant current flow from the electrical machine to the DC side is to be expected. For this purpose, the invention provides that in an inverter, a switching state corresponding to a freewheeling mode is initially set only in a first half-bridge. The course of the electrical current in this half-bridge can then be evaluated. If, according to this current course, no or at least no significant current flow from the electrical machine to the DC side is to be expected, the freewheeling mode can then be set in all half-bridges.

[0021] In this way, by means of a brief change from the switching state of the active short circuit to the switching state of a freewheeling mode in only one half-bridge of an inverter, it can be easily checked whether, in a freewheeling mode, a current flow from the electrical machine to the DC side of the inverter would occur due to the electrical voltage provided by a connected electrical machine.

[0022] In contrast to conventional methods, in which the criteria for switching between active short circuit and freewheeling are based solely on estimates, such as a relationship between the speed of the electrical machine and the expected induced electrical voltage, the method according to the invention allows the resulting electrical current flow to be verified under actual, real-world conditions. This allows for a very precise and reliable assessment of the operating conditions and the potentially occurring electrical current flow from the electrical machine to the DC voltage connection. Any necessary safety margins, such as those typically required for theoretical estimates, can be eliminated.According to one embodiment, the method for operating the inverter switches from the active short circuit to freewheeling mode if the magnitude of the phase current in the direction of the first half-bridge does not exceed a predetermined threshold value after a current zero crossing. If, for example, a switching state corresponding to freewheeling is set in one of the half-bridges shortly before a current zero crossing, it can be checked as a condition for enabling freewheeling mode whether the electrical current in this half-bridge remains at zero after the zero crossing or at least does not exceed a predetermined threshold value. If the switching state for freewheeling is only set in this half-bridge shortly after a current zero crossing, it can be checked whether the electrical current in this half-bridge tends to increase or decrease.

[0023] According to one embodiment, the switching element in a half-bridge of the inverter opens when the phase current profile in the direction of the first half-bridge is at least approximately in the region of a zero crossing. In this case, no load, or at least only a very small load, needs to be switched when a switching element is opened. Preferably, the switching state, which corresponds to freewheeling, can be set in the half-bridge at times when the electrical current is exactly zero, or, if necessary, as shortly as possible before this time.

[0024] According to one embodiment, to set the freewheeling mode in the inverter, the switching elements in the first half-bridge, in which both switching elements were previously opened, remain open. Subsequently, the switching elements in the other half-bridges of the inverter are opened to set the freewheeling mode. In particular, the switching elements in the other half-bridges can be opened at times at which an electrical current in the respective half-bridges is as close as possible to a zero current crossing. Thus, no or only a small load needs to be switched when the switching elements are opened. According to one embodiment, after determining the electrical parameter that characterizes a course of the phase current in the direction of the first half-bridge, the switching state for an active short circuit is first set again.To do this, the switching element in this half-bridge, which was previously opened to check the current flow, is closed. The phase current curve toward the first half-bridge is then evaluated. Following this evaluation, the inverter is set to freewheeling mode if the phase current curve toward the first half-bridge fulfills the predetermined switching condition. Since the inverter has since returned to the active short-circuit state, this evaluation can be performed more quickly using a processing device with relatively low computing power.

[0025] According to one embodiment, the method comprises a step for determining a speed of an electrical machine connected to the inverter. In this case, the steps of opening the two switching elements, subsequently determining the electrical parameter, and switching to freewheeling mode can only be carried out if the determined speed of the electrical machine falls below a predetermined limit. Above this limit, an electrical voltage from the electrical machine can be expected which would almost certainly lead to a current flow through the inverter if freewheeling mode were set. Thus, at such high speeds of the electrical machine, a check for a possible switch to freewheeling can be dispensed with.

[0026] The above embodiments and developments can be combined with each other as desired, where appropriate. Further embodiments, developments, and implementations of the invention also include combinations of features of the invention not explicitly mentioned above or described below with respect to the exemplary embodiments. In particular, those skilled in the art will also add individual aspects as improvements or additions to the respective basic forms of the invention. Brief Description of the Drawings

[0027] Further features and advantages of the invention are explained below with reference to the figures. These show:

[0028] Fig. 1: a schematic representation of an electric drive system according to an embodiment;

[0029] Fig. 2: a current-time diagram for a current curve in an inverter according to an embodiment;

[0030] Fig. 3: a current-time diagram for a current curve in an inverter according to another embodiment;

[0031] Fig. 4: a current-time diagram for a current curve in an inverter according to yet another embodiment; and

[0032] Fig. 5: a flowchart underlying a method for controlling an inverter according to an embodiment.

[0033] Description of the embodiments

[0034] Figure 1 shows a schematic representation of a basic circuit diagram for an electric drive system with an inverter 1 and an electric machine 2 according to one embodiment. A DC voltage source 3, such as a battery, in particular a traction battery of an electric vehicle, can be connected to a DC voltage terminal of the inverter 1. An electric machine 2 can be connected to an AC voltage terminal of the inverter 1. The inverter 1 can convert the DC voltage provided at the DC voltage terminal into an AC voltage suitable for controlling the electric machine 2.If necessary, the inverter 1 can also be designed to convert an alternating voltage provided by the electric machine 2 in generator mode into a direct voltage suitable for charging a battery connected to the direct voltage connection.

[0035] The inverter 1 can include several half-bridges, each with two switching elements S1 to S6. Each half-bridge comprises two series-connected switching elements S1 to S6, each of which is electrically connected to one another at a node. The inverter 1 can include one such half-bridge for each phase of an alternating voltage to be provided at an alternating voltage terminal of the power converter 1. The nodes of the respective half-bridges, at which the two switching elements S1 to S6 are electrically connected to one another, are each connected to a connection point of the alternating voltage terminal of the inverter 1.

[0036] The other connections of the switching elements S1 to S6, i.e. those connections of switching elements S1 to S6 which are not connected to the AC voltage connection of inverter 1, are connected to a DC voltage connection of inverter 1. In this case, all switching elements S1, S3, S5 which are connected to a connection point for a first (e.g. positive) polarity of the DC voltage provided at the DC voltage connection are electrically connected to one another. Analogously, all switching elements S2, S4, S6 which are connected to a connection point for a second (e.g. negative) polarity of the DC voltage provided at the DC voltage connection are also electrically connected to one another. The switching elements S1, S3, S5 which are connected to the connection point of the DC voltage connection for the first, positive polarity are also referred to as upper switching elements.Analogously, the switching elements S2, S4, S6, which are connected to the connection point of the DC voltage connection for the second, negative polarity, can be referred to as lower switching elements.

[0037] The switching elements S1 to S6 can be controlled by a control device 10. For this purpose, the control device 10 can provide a control signal for each of the switching elements S1 to S6, which is suitable for opening or closing the respective switching element S1 to S6. In this way, a desired alternating voltage for controlling the electrical machine 2 can be generated from the direct voltage provided on the input side, for example by means of a pulse-width modulated control of the switching elements S1 to S6.

[0038] Furthermore, a so-called active short circuit can be set in inverter 1. In such an active short circuit, the upper switching elements S1, S3, and S5 can be closed, while the lower switching elements S2, S4, and S6 are open. Alternatively, the upper switching elements S1, S3, and S5 can be opened, while the lower switching elements S2, S4, and S6 are closed. In this way, all phase connections of electrical machine 2 are electrically connected to one another, i.e., short-circuited.

[0039] In another operating mode, a so-called freewheeling mode can be set. In this mode, all switching elements S1 to S6 in the half-bridges of inverter 1 are open. If the electrical machine 2 is moving, an electrical voltage can be induced in the phase windings of the electrical machine 2. If the electrical voltage applied by the electrical machine 2 to the AC voltage connection of inverter 1 is sufficiently high, an electrical current can flow through diodes arranged in parallel with the switching elements S1 to S6 to the DC voltage connection of inverter 1. Such an electrical current during freewheeling mode should be limited or completely prevented if necessary.

[0040] If a change from an active short circuit to a freewheeling mode is to be made, it must therefore be checked before such a change whether a current flow from the electrical machine 2 at the AC voltage connection of the inverter 1 to the DC voltage connection of the inverter 1 is to be expected. For this purpose, the control device 10 can set a switching state in a first of the half-bridges of the inverter 1 in which both switching elements of a first half-bridge, S1 + S2, S3 + S4, or S5 + S6, are open. The switching elements of the remaining half-bridges initially remain in the switching state that corresponds to the currently set active short circuit.

[0041] Preferably, this switching state, in which both switching elements in the first half-bridge are open, is set at a time when the electrical current through the previously closed switching element is at least approximately in the region of a zero crossing. For example, the corresponding switch can be opened shortly before the electrical current through this switch reaches the zero crossing. Alternatively, the switch can also be opened precisely at the zero crossing or shortly after the zero crossing.

[0042] After this switching element has been opened and a switching state corresponding to active freewheeling has thus been set in a half-bridge of the inverter 1, an electrical parameter that characterizes the course of a phase current in the direction of the first half-bridge or characterizes the electrical current in this half-bridge can be detected, determined, or monitored. In particular, the electrical current that flows in the corresponding phase connection or through the diodes provided parallel to the open switching elements during this switching state can be monitored. For example, the current course can be detected by current sensors 11. The current sensors 11 can provide a sensor signal to the control device 10 that corresponds to an electrical current at the phase connections or the half-bridges.

[0043] If a significant current flow is detected, which may continue to increase during the detection or monitoring period, this can be considered an indication that, when a full freewheeling mode is set in inverter 1, an electrical current will also flow from electrical machine 2 through the diodes arranged in parallel with switching elements S1 to S6. If such a current flow is not desired, the active short circuit in inverter 1 can be maintained. For this purpose, for example, the previously opened switching element can be closed again.If, on the other hand, no current flow is detected during the period described above in which both switching elements are open in a half-bridge, or if a current flow is detected that fulfills a previously specified condition, this can be interpreted as an indication that in the case of freewheeling, the electrical voltage from the electrical machine 2 applied to the AC voltage terminal of the inverter 1 is sufficiently low so that no current flow will occur from the AC voltage terminal to the DC voltage terminal in the inverter 1.

[0044] If, based on the determination, evaluation, or monitoring of the phase current profile in the direction of the first half-bridge, it is determined that a switch to freewheeling mode is possible, the switching state with the two open switching elements in the half-bridge can be maintained, for example. Subsequently, the previously closed switching element in the remaining half-bridges can also be opened. In particular, the previously closed switching elements can be opened at times when the electrical current through the switching elements is at least approximately in the range of a zero crossing.

[0045] Alternatively, after setting the freewheeling mode in the first half-bridge and subsequently determining the phase current profile in the direction of the first half-bridge, it is also possible to first close the previously open switching element again and thus return to the fully active short circuit. The previously recorded electrical current profile during the period with the two open switching elements in a half-bridge can then be evaluated in the control device 10 to check whether a switch to freewheeling mode is possible. Such processing of the previously recorded current profile is particularly useful when the control device 10, for example, has only limited computing power, so that a rapid evaluation of the current profile in real time is not possible.If this subsequent evaluation of the current waveform after returning to the active short circuit determines that freewheeling mode is possible, freewheeling mode can then be activated by opening all switching elements S1 to S6. Here, too, the previously closed switching elements S1 to S6 can preferably be opened at times when the electrical current through the respective switching elements is at least approximately in the range of a zero crossing.

[0046] Figure 2 shows a schematic representation of a current-time diagram for the phase currents l_l, l_2 and l_3 in an electric drive system during the testing of a possible change from an active short circuit to a freewheeling mode. At time t1 in the half-bridge for the current I_1, the switching element closed for the active short circuit is opened exactly at the zero crossing. At time t2, this switching element is then closed again to return to the active short circuit. In the time period between t1 and t2, no electrical current flows in the corresponding phase. It can therefore be assumed that when changing to freewheeling, the electrical voltage provided by the electrical machine 2 is sufficiently low that no electrical current will flow from the AC voltage connection to the DC voltage connection during freewheeling. A change from an active short circuit to freewheeling is therefore possible.

[0047] Figure 3 shows a further schematic representation of a current-time diagram for the phase currents l_l, l_2 and l_3 in an electric drive system during the testing of a possible change from an active short circuit to a freewheeling mode. Here, too, at time t1 in the half-bridge for the current I_1, the switching element closed for the active short circuit is opened exactly at the zero crossing, and at time t2 this switching element is closed again to return to the active short circuit. In contrast to Figure 2, during the time interval between t1 and t2 an electric current flows in the phase with the half-bridge in which both switching elements are open. From this it can be concluded that the electrical voltage from the electric machine 2 is so high that in the event of a freewheeling mode an electric current would flow through the diodes arranged in parallel with the switching elements S1 to S6.

[0048] Figure 4 shows another schematic representation of a current-time diagram for the phase currents l_l, l_2 and l_3 in an electrical drive system during the testing of a possible change from an active short circuit to a freewheeling mode. In this case, the switching element in one of the half-bridges opens at a time t3 which is shortly after the zero crossing of the current I_1 in the corresponding phase. At time t4 this switching element is closed again. Thus, even when this switching element opens, a small electrical current flows in the corresponding phase. If this electrical current I_1 tends to decrease over time, this can be interpreted as an indication that a freewheeling mode is possible. If, on the other hand, the electrical current tends to increase during this phase, the active short circuit should be maintained.

[0049] Analogous evaluation is also possible if the switching element opens shortly before the zero crossing. However, in this case, the current waveform should be monitored over a period that includes the (theoretical) zero crossing in the current waveform.

[0050] Figure 5 shows a flowchart underlying a method for operating an inverter 1 for an electric drive system according to one embodiment. The method can, in principle, comprise any steps as previously described in connection with the control of the electric drive system. Similarly, the previously described drive system, in particular the inverter 1 and the control device 10 provided in the inverter 1, can also comprise any components required to implement the method described below.

[0051] In step 100, an active short circuit is set in the inverter. In such an active short circuit, as already explained above, either the upper switching elements S1, S3, and S5 are closed and the lower switching elements S2, S4, and S6 are open, or alternatively, the lower switching elements S2, S4, and S6 are closed while the upper switching elements S1, S3, and S5 are open. In step 200, the previously closed switching element in a first half-bridge of inverter 1 is opened. In the remaining half-bridges of inverter 1, the switching elements closed for the active short-circuit remain closed.

[0052] In step 300, an electrical parameter is then determined which characterizes a course of a phase current in the direction of the first half-bridge in which both switching elements are open.

[0053] In step 400, the phase current curve toward the first half-bridge is evaluated. If a predetermined condition for switching to freewheeling is met, a freewheeling mode is then set in inverter 1. In this case, all switching elements S1 to S6 in the inverter's half-bridges are opened. If, however, the predetermined switching condition is not met, inverter 1 continues to operate in active short-circuit mode.

[0054] The method for checking whether a change from an active short circuit to freewheeling mode is possible can in principle be carried out at any time during operation of the inverter 1 in the active short circuit. For example, during operation in the active short circuit, a check can be carried out regularly, in particular periodically at predetermined time intervals, to determine whether a change from the active short circuit to freewheeling mode is possible. Furthermore, the check for a possible change to freewheeling mode can also be carried out only if predetermined operating conditions are met. For example, a speed of the electrical machine 2 connected to the inverter 1 can be determined. In this case, for example, a change from the active short circuit to freewheeling mode can only be considered if the speed of the electrical machine 2 falls below a predetermined value.At higher speeds, for example, it can be assumed that an electrical voltage is induced in the phase windings of the electric machine 2, which would lead to a current flow in the inverter 1 from the AC voltage connection to the DC voltage connection. Furthermore, the transition from the active short circuit to freewheeling mode can, of course, also be linked to any other conditions. For example, a transition to freewheeling mode can only occur if additional operating conditions in the electrical system are met.

[0055] Drive system, in particular the electric machine and / or the DC voltage source 3 connected to the DC voltage, in particular a traction battery, are met. In summary, the present invention relates to the transition from an active short circuit to freewheeling in an inverter of an electric drive system. For this purpose, a switching state corresponding to the switching state of the freewheeling is briefly set in a half-bridge of the inverter. The current curve in this half-bridge is then evaluated to check whether a transition to freewheeling is permissible or not.

Claims

Claims 1. A method for operating an inverter (1) for an electric drive system, wherein the inverter (1) comprises a plurality of half-bridges each having two switching elements (S1-S6), and wherein the method comprises the following steps: Setting (100) an active short circuit in the inverter (1); Opening (200) the switching elements in a first half-bridge of the inverter (1); Determining (300) an electrical parameter that characterizes a course of a phase current in the direction of the first half-bridge in which both switching elements are open; Setting (400) a freewheeling mode in the inverter in which all switching elements (S1-S6) of all half-bridges are open if the course of the phase current in the direction of the first half-bridge fulfills a predetermined change condition.

2. The method according to claim 1, wherein the active short circuit is switched to the freewheeling mode if the magnitude of the phase current in the direction of the first half-bridge does not exceed a predetermined threshold value from a current zero crossing.

3. Method according to claim 1 or 2, wherein the two switching elements in a half-bridge of the inverter are opened when a course of the phase current in the direction of the first half-bridge is at least approximately in the region of a zero crossing.

4. Method according to one of claims 1 to 3, wherein for setting (400) the freewheeling mode in the inverter (1) the switching elements in the first half-bridge, in which previously both switching elements were opened remain open, and then the switching elements in the other half-bridges of the inverter (1) are opened. Method according to one of claims 1 to 3, wherein after determining (300) the electrical parameter, firstly a switching element in the first half-bridge is closed again in order to set an active short circuit; and then the course of the phase current in the direction of the first half-bridge is evaluated, and after the evaluation, a freewheeling mode is set in the inverter (1) if the course of the phase current in the direction of the first half-bridge satisfies the predetermined changeover condition.Method according to one of claims 1 to 5, comprising a step for determining a rotational speed of an electrical machine (2) connected to the inverter (1), wherein the steps of opening (200) the two switching elements, subsequently determining (300) the electrical parameter, and setting (400) to freewheeling mode are only carried out if the determined rotational speed of the electrical machine (2) falls below a predetermined limit value. Inverter (1), comprising: a plurality of half-bridges, each comprising two switching elements (S1-S6), a diode being provided in parallel with each switching element (S1-S6); and a control device (10) designed to provide control signals for the switching elements (S1-S6) in the plurality of half-bridges. wherein the control device (10) is designed to carry out a method according to one of claims 1 to 6. An electric drive system, comprising: an electric machine (2); and an inverter (1) according to claim 7; wherein the inverter (1) is connected to the electric machine (2) at an AC voltage terminal, and wherein the inverter (1) is designed to be connected to a DC voltage source at a DC voltage terminal.

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