Active filter pre-charging for converter having active filter unit

Abstract

The electrical converter (10) comprises: a main stage (12) adapted to convert a DC voltage into an intermediate voltage, the intermediate voltage comprising at least two voltage levels; a filter cell stage (14) having filter cells (30) for each phase of the intermediate voltage, each filter cell (30) being adapted to add or subtract the cell voltage of the filter cell (30) to or from the intermediate voltage; and a pre-charge circuit (44) adapted to pre-charge the filter unit (30) and the DC link (20) of the main stage (12) and comprising an auxiliary power source (46) connected between one phase of the output (34) of the electrical converter (10) and a neutral point (NP) of said DC link (20) of the main stage (12) wherein all phases of the output (34) of the electrical converter (10) are connected via an electrical load (16). A method for pre-charging a filter unit (30) during start-up of an electrical converter (10) comprises: pre-charging the filter unit (30), the pre-charging of the filter unit (30) comprising the steps of: switching a main stage (12) such that all phases of an output (24) of the main stage (12) are directly connected to a neutral point (NP) of a DC link (20) of the main stage (12); switching at least one of the filter units (30) to a rectification state in which the rectified current flows through a unit capacitor (38) of the filter unit (30); and turning on the pre-charge circuit (44) and charging the cell capacitor (38) of the at least one filter cell (30) by the pre-charge circuit (44) until the nominal cell voltage is reached at the cell capacitor (38) of the filter cell (30).

Description

Technical Field

[0001] The present invention relates to the field of high power converter control. In particular, the present invention relates to a method, a computer program, a computer readable medium and a controller for precharging a filter unit during start-up of an electrical converter having the filter unit. The present invention also relates to an electrical converter. Background Art

[0002] Specifically, in medium voltage applications, the 3L(A)NPC+AF topology gives a cost-effective electrical converter topology with high power quality. Here, 3L means 3 levels, (A)NPC means (active) neutral point clamping and AF means active filter. 3L(A)NPC is also referred to as the main stage of the converter hereinafter. This topology has been proposed several years ago. However, optimized hardware design, reliable control and fail-safe operation remain challenges.

[0003] Due to the excellent voltage quality, the electric converter with active filter unit can be used to drive sensitive loads, such as direct-on-line (DOL) machines. Furthermore, if the electric converter with active filter unit is used as active rectifier unit, it enables compliance with strict grid harmonic limits without high switching frequencies of the main stage.

[0004] One of the main advantages of the 3L(A)NPC+AF topology is the small voltage step that can be generated. The small voltage step reduces the dv / dt and overvoltage on the machine side, as well as the current THD (Total Harmonic Distortion) and harmonic losses involved in the machine.

[0005] In order to fully benefit from these small voltage steps, the active filter unit of the electric converter needs to be charged to its nominal voltage from the beginning when the electric converter starts operating. This is especially important for sensitive loads such as DOL machines. If the AF unit is not charged, the electric converter may create voltage steps with large magnitudes, which may damage the insulation of the DOL machine and introduce torque ripple and extensive machine losses.

[0006] Therefore, before the electrical converter starts operating, the active filter unit should be pre-charged. Finding efficient ways to achieve this goal is an unsolved challenge so far.

[0007] Existing pre-charging methods for the AF cells of an electrical converter with a 3L(A)NPC+AF topology can be summarized as follows:

[0008] Externally powered AF units are one possibility. This approach is common, for example, in cascaded H-bridge converters, where each AF unit is powered via a diode bridge powered by an isolation transformer winding set. However, this is an expensive solution and is not suitable for 3L(A)NPC+AF topologies due to the lack of a multi-winding transformer.

[0009] Another possibility is to provide a dedicated pre-charging circuit for each AF unit powered by external auxiliary power. In most cases, the gate drive circuits of the AF units are powered using an isolated auxiliary power supply. An isolated / non-isolated dedicated pre-charging circuit fed by the auxiliary power can be designed for each active filter unit. However, this involves additional expensive hardware.

[0010] Another possibility is to start the electric converter operation with an uncharged AF unit and charge it slowly during closed-loop operation. This method has the above-mentioned disadvantage that the electric converter only works when an electric load, such as a motor or a machine, is directly connected to the electric converter, which therefore supplies large voltage steps to the electric load until the AF unit is charged. These voltage steps may damage, for example, the motor winding insulation. Summary of the invention

[0011] One object of the invention is to improve the control of an electrical multilevel electrical converter comprising a filter unit. Another object of the invention is to avoid any damage to an electrical load connected to the electrical converter with a filter unit when the electrical converter with a filter unit starts operating.

[0012] These objects are achieved by the subject-matter of the independent claims. Further exemplary embodiments are evident from the dependent claims and the following description.

[0013] One aspect of the invention relates to a method for precharging a filter unit during startup of an electrical converter. The electrical converter may be a medium voltage converter or a high voltage converter adapted to handle voltages up to 6.6 kV or higher. It may be configured to drive a motor or machine, or any other physical system acting as an electrical load for the electrical converter.

[0014] According to one embodiment of the invention, the electrical converter comprises a main stage, which is adapted to convert a DC voltage into a multiphase intermediate voltage, the multiphase intermediate voltage comprising at least two voltage levels. The main stage may comprise one or more (A) NPC half-bridges, which are connected in parallel to the DC link. The output of the half-bridge may be an intermediate voltage, which may be a multiphase voltage, in particular a three-phase voltage. Depending on the topology of the main stage, the output voltage may have two, three, five or more voltage levels. In the case of a 3L (A) NPC half-bridge, there may be three output voltage levels.

[0015] According to one embodiment of the present invention, the electrical converter further comprises a filter cell stage (hereinafter also referred to as "cell stage"), the filter cell stage having a filter cell for each phase of the intermediate voltage, wherein each filter cell is adapted to add a cell voltage of the filter cell to the intermediate voltage, or to subtract a cell voltage of the filter cell from the intermediate voltage. Each filter cell may comprise a cell capacitor providing a cell voltage, the cell capacitor being connected in parallel to two half bridges, the two half bridges providing an input and an output of the filter cell. The filter cell stage may be regarded as an active filter (AF) of the electrical converter, such that each filter cell may be regarded as an active filter cell (AF cell).

[0016] According to one embodiment of the invention, the electrical converter further comprises a pre-charging circuit adapted to pre-charge the filter unit and the DC link of the primary stage. To this end, the pre-charging circuit may comprise an auxiliary power source connected between a phase of the output of the electrical converter and the neutral point of the DC link of the primary stage. The neutral point may be a connection point between an upper DC link capacitor and a lower DC link capacitor of the DC link of the primary stage. For example, the auxiliary power source of the pre-charging circuit may be configured as a transformer winding that creates a single-phase AC voltage.

[0017] According to one embodiment of the invention, all phases of the output of the electrical converter are connected via an electrical load, such as a motor or electrical machine, or any other physical system. Thus, although the pre-charging circuit is connected to only one phase of the output of the electrical converter, the filter units in all other phases can be charged by the pre-charging circuit also via the electrical load (e.g., via the machine winding).

[0018] According to one embodiment of the present invention, the method comprises: pre-charging the filter unit, the pre-charging the filter unit comprising the following steps: switching the main stage so that all phases of the output of the main stage are directly connected to the neutral point of the DC link of the main stage; switching at least one of the filter units to a rectified state, in which the rectified current flows through the cell capacitor of the filter unit; and turning on the pre-charging circuit, and charging the cell capacitor of the at least one filter unit by the pre-charging circuit until a nominal (i.e., predefined) cell voltage is reached at the cell capacitor of the filter unit. Whether the nominal cell voltage is reached can be determined by comparing it with the output voltage of the filter unit, which can be directly measured in the filter unit.

[0019] Specifically, the pre-charging of the filter units may include the following steps: switching the main stage so that all phases of the output of the main stage are directly connected to the neutral point of the DC link of the main stage; switching all filter units to a rectified state, in which a rectified current flows through the cell capacitor of each filter unit; and turning on the pre-charging circuit, and charging the cell capacitors of all filter units by the pre-charging circuit until the nominal cell voltage is reached. The auxiliary power source of the pre-charging circuit will then charge the cell capacitors of the filter units in all phases simultaneously, because they are all connected in parallel to the pre-charging circuit through the electrical load and the neutral point of the DC link.

[0020] Using this method, the main stage stops operating when precharging the filter unit to the nominal cell voltage is performed. Thus, any damage to the electrical load by a full voltage step that may occur before the nominal cell voltage is reached can be avoided or excluded. In particular, precharging the filter unit can be performed before the main stage starts operating, i.e., during startup of the electrical converter.

[0021] According to one embodiment of the present invention, the current generated by the pre-charge circuit is an alternating current. As described above and below, the AC current generated by the pre-charge circuit can be rectified by switching the filter unit to its rectification state during pre-charging of the filter unit, in which the rectified current flows through the unit capacitor. For example, the auxiliary power source of the pre-charge circuit can be configured as a transformer winding that creates a single-phase AC voltage. Due to the high short-circuit impedance of the auxiliary transformer, the pre-charge current can be limited to a few amperes in any case.

[0022] According to one embodiment of the present invention, the method further comprises: precharging the DC link of the main stage. Precharging the DC link of the main stage can be performed before or after precharging the filter unit, and comprises the following steps: switching each filter unit so that the output of each filter unit is directly connected to the input of each filter unit (e.g., to the output of the main stage), thereby bypassing the unit capacitor; switching the main stage so that, depending on the sign of the voltage applied between the neutral point of the DC link and the output of the main stage in the corresponding phase, the upper DC link capacitor or the lower DC link capacitor is charged; turning on the precharging circuit and charging the DC link capacitor by the alternating current generated by the precharging circuit until the nominal (i.e., predefined) DC link voltage is reached. Whether the nominal DC link voltage is reached can be determined by comparing it with the upper DC link voltage and the lower DC link voltage, which are obtained, for example, from measurements at the output of the main stage or at the DC link of the main stage.

[0023] According to one embodiment of the invention, when the nominal cell voltage of the filter cell is reached and / or the nominal DC link voltage of the DC link is reached, the pre-charging circuit is switched off and / or its auxiliary power source is disconnected from the electrical converter.

[0024] The pre-charging of the filter cells may for example be repeated whenever a cell voltage, for example measured in at least one of the filter cells, drops below a predetermined cell voltage threshold. The predetermined cell voltage threshold may for example be equal to or less than the nominal cell voltage. Furthermore, the pre-charging of the DC link of the main stage may be repeated whenever a DC link voltage, for example measured at the DC link of the main stage, drops below a predetermined DC link voltage threshold. The predetermined DC link voltage threshold may for example be equal to or less than the nominal DC link voltage.

[0025] According to one embodiment of the present invention, each filter unit includes two half-bridges connected in parallel with the unit capacitor. Each half-bridge may have at least two semiconductor switches (such as IGBTs), each of the at least two semiconductor switches being bypassed by a freewheeling diode, so that the corresponding filter unit is switched to the rectification state of the corresponding filter unit by turning off all semiconductor switches of the corresponding filter unit, in which the rectified current passes through the unit capacitor. The corresponding filter unit may be switched to a direct connection state by turning on only all upper semiconductor switches or only all lower semiconductor switches connected between the input and output of the filter unit, in which the unit capacitor of the corresponding filter unit is bypassed. For example, two upper semiconductor switches and two lower semiconductor switches may be connected between the input and output of the filter unit, so that the filter unit has a total of four semiconductor switches.

[0026] According to one embodiment of the present invention, each phase of the main stage includes at least four freewheeling diodes and at least four semiconductor switches (e.g., IGBTs), each of the at least four semiconductor switches being bypassed by one of the freewheeling diodes, so that by turning off all the semiconductor switches of the main stage, the upper DC link capacitor or the lower DC link capacitor is precharged by the freewheeling diode bypassing the semiconductor switch, depending on the sign of the precharge current. In addition, by turning on the semiconductor switch connecting the output of the main stage to the neutral point, the output of the main stage is directly connected to the neutral point of the DC link, thereby bypassing the DC link capacitor. For example, each phase of the main stage may include an (A) NPC half-bridge, which is connected to the DC link in parallel and includes six freewheeling diodes. In an NPC configuration, four semiconductor switches may be provided. In an ANPC configuration, 6 semiconductor switches may be provided.

[0027] According to one embodiment of the invention, the electrical converter further comprises a passive output filter in each phase of the output of the electrical converter, the passive output filter being connected in series between the corresponding output of the filter unit stage and the output of the electrical converter. The pre-charge circuit can, for example, be connected to the phase of the output of the electrical converter, between the inductance and the RC group (i.e., the resistor and capacitor connected in series) of the passive output filter provided in the phase. For example, the passive output filter can be configured as an EMC filter (electromagnetic compatibility filter).

[0028] In this embodiment, the passive output filter star point, where the RC banks of all phases are connected together, may be connected, for example, directly to the neutral point of the DC link of the primary stage. Alternatively, the star point of the passive output filter may be a floating star point, ie without any other connection.

[0029] According to one embodiment of the invention, the step of precharging the filter unit is performed by a filter unit precharging controller, and the step of precharging the DC link of the primary stage is performed by a DC link precharging controller.

[0030] Other aspects of the invention relate to a computer program for precharging a filter unit during startup of an electrical converter, the computer program being adapted to perform the steps of the method described herein when executed by a processor, and a computer readable medium having such a computer program stored therein. Precharging the filter unit and precharging the DC link of the main stage may both be executed as software on one or more processors.

[0031] The filter unit pre-charge controller may have a processor, and pre-charging the filter unit may be performed on the processor. The method steps related to pre-charging the filter unit may be stored in a memory of the filter unit pre-charge controller.

[0032] The DC link pre-charge controller may also have a processor, and pre-charging the DC link of the primary stage may also be performed on the processor.Method steps related to pre-charging the DC link of the primary stage may be stored in a memory of the DC link pre-charge controller.

[0033] The computer readable medium may be a floppy disk, a hard disk, a USB (Universal Serial Bus) storage device, a RAM (Random Access Memory), a ROM (Read Only Memory), an EPROM (Erasable Programmable Read Only Memory) or a FLASH memory. The computer readable medium may also be a data communication network, such as the Internet, that allows downloading program code. In general, the computer readable medium may be a non-transitory or transient medium.

[0034] Another aspect of the invention relates to a controller for controlling an electric converter. The controller is adapted to perform the method described herein. The controller may include a filter unit pre-charge controller for performing the step of pre-charging the filter unit according to the method described herein. The controller may include a DC link pre-charge controller for performing the step of pre-charging the DC link of the main stage according to the method described herein. The controller may also include a main operation controller for controlling the normal operation of the electric converter, for example, the operation of the electric converter that begins after the above-mentioned startup is completed.

[0035] The controller may include one or more processors in which the method is executed as software. The method may also be implemented at least partially or completely in hardware such as a DSP or FPGA.

[0036] Another aspect of the invention relates to an electrical converter controlled as described herein.

[0037] According to one embodiment of the present invention, an electric converter comprises: a main stage, the main stage is adapted to convert a DC voltage into a multi-phase intermediate voltage, the multi-phase intermediate voltage comprising at least two voltage levels; and a filter unit stage, the filter unit stage having a filter unit for each phase of the intermediate voltage, wherein each filter unit is adapted to add the unit voltage of the filter unit to the intermediate voltage, or to subtract the unit voltage of the filter unit from the intermediate voltage. According to one embodiment of the present invention, the electric converter also includes a pre-charging circuit, the pre-charging circuit is adapted to pre-charge the filter unit and the DC link of the main stage, and also includes an auxiliary power source, the auxiliary power source is connected between one phase of the output of the electric converter and the neutral point of the DC link of the main stage. It may also include a controller such as described herein. According to one embodiment of the present invention, all phases of the output of the electric converter are connected via a motor or machine, or any other physical system acting as an electrical load of the electric converter.

[0038] In general, it has to be understood that features of the method described above and below may be features of the computer program, computer readable medium, controller and electrical converter described above and below, and vice versa.

[0039] These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] The subject matter of the invention will be explained in more detail hereinafter with reference to exemplary embodiments which are illustrated in the attached drawings.

[0041] Figure 1 A schematic circuit diagram of an electrical converter according to an embodiment of the present invention is shown.

[0042] Figure 2a and Figure 2b The schematic circuit diagram shows Figure 1 Two exemplary configurations of one phase of the primary stage of an electrical converter.

[0043] Figure 3 A schematic block diagram of a controller according to an embodiment of the present invention is shown.

[0044] Figure 4a A diagram illustrating pre-charging of a DC link of a primary stage to a nominal DC link voltage according to one embodiment of the present invention is shown.

[0045] Figure 4b A diagram illustrating pre-charging of a filter cell in each of three phases of an electrical converter to a nominal cell voltage according to one embodiment of the invention is shown.

[0046] Figure 4c The diagram shows Figure 4b The motor current in each of the three phases during pre-charging of the filter unit is shown.

[0047] In principle, identical parts are provided with the same reference symbols in the figures. DETAILED DESCRIPTION

[0048] Figure 1 An electrical converter 10 is shown having a main stage 12 and a filter unit stage 14. The electrical converter 10 is adapted to convert a first AC voltage provided by a power grid 15 into an output voltage to be supplied to an electrical load 16, which in this example may be an electrical machine.

[0049] The primary stage 12 comprises a rectifier 18, which may be a passive diode rectifier and is adapted to convert an AC voltage from the grid 15 into a DC voltage, which is supplied to a (main) DC link 20 of the primary stage 12. The DC link 20 comprises an upper DC link capacitor 19 and a lower DC link capacitor 21, wherein a neutral point NP is located between the upper DC link capacitor 19 and the lower DC link capacitor 21 as a connection point.

[0050] In this example, the main stage 12 includes three output converters 22 (for only one phase a, b or c, in Figure 1 Indicated schematically and in Figure 2a or Figure 2b1 ). Each output converter 22 is adapted to transform a DC voltage at the DC link 20 to a phase of an intermediate voltage, which is provided at a respective phase output 24 of the main stage 12. The output converters 22 are connected in parallel to the DC link 20, for example via clamping inductors and / or resistors 28 (not shown) or directly.

[0051] As per Figure 2a and Figure 2b As shown in more detail in the example of , each of the output converters 22 may include a 3-level neutral point clamped (3LNPC) half-bridge 23, which in this example may be based on IGBTs. However, other topologies and semiconductor types are also possible.

[0052] exist Figure 2a In the example shown, each phase of the main stage 12, ie each output converter 22, comprises an NPC half-bridge 23 which is connected in parallel to the DC link 20 and comprises six freewheeling diodes D1-D6. Figure 2a In the exemplary NPC configuration shown, four semiconductor switches T1-T4 (such as IGBTs) may be provided, each of the four semiconductor switches T1-T4 being bypassed by one of the freewheeling diodes D1-D4, so that by turning off all the semiconductor switches T1-T4 of the main stage 12, the upper DC link capacitor 19 or the lower DC link capacitor 21 of the DC link 20 may be pre-charged by the freewheeling diodes D1-D4 bypassing the semiconductor switches T1-T4, depending on the sign of the charging current or voltage applied between the neutral point NP of the DC link 20 and the corresponding output 24 of the output converter 22.

[0053] Alternatively, if Figure 2b As schematically depicted in , each half bridge 23 may be an ANPC half bridge, which is connected in parallel to the DC link 20 and includes six freewheeling diodes D1-D6. In this case, six semiconductor switches T1-T6 (e.g., IGBTs) may be provided, each of the six semiconductor switches T1-T6 being bypassed by one of the freewheeling diodes D1-D6, so that by turning off all the semiconductor switches T1-T6 of the main stage 12, the upper DC link capacitor 19 or the lower DC link capacitor 21 of the DC link 20 may be precharged by the freewheeling diodes D1-D4 bypassing the semiconductor switches T1-T4, depending on the sign of the charging current or voltage applied between the neutral point NP of the DC link 20 and the corresponding output 24 of the output converter 22.

[0054] Furthermore, other topologies are possible for the entire main stage 12. In general, the main stage 12 may be adapted to provide a two-level intermediate voltage or a multi-level intermediate voltage at the output 24, the two-level intermediate voltage or the multi-level intermediate voltage optionally having more than one phase. Figure 2a or Figure 2b The half-bridge 23 shown is adapted to generate three different voltage levels for respective phases (a, b and c) of the intermediate voltage.

[0055] For each phase of the main stage 12, the filter unit stage 14 comprises a filter unit 30, which can be regarded as an active filter (AF) and optionally a passive output filter 32, which are connected in series between the corresponding output 12 of the main stage 24 and the output 34 of the electrical converter 10. More than one filter unit 30 may be connected in series and / or in parallel between the output 24 and the output 34 of the phase.

[0056] Each filter unit 30 comprises two half-bridges 36 connected in parallel with a unit capacitor 38. A first midpoint of one of the half-bridges 36 is connected to the output 24 of the primary stage 12. A second midpoint of the other half-bridge 36 provides an output 40 of the filter unit 30 and is connected via an optional passive filter 32 to a corresponding output 34 of the electrical converter 10.

[0057] The filter unit 30 comprises different switching states, which can be achieved by correspondingly switching the semiconductor switches (two semiconductor switches per half bridge in this example) of the filter unit 30, which semiconductor switches can be, for example, IGBTs. In a first switching state (also referred to herein as a "direct connection state"), the filter unit 30 directly connects the output 24 of the main stage 12 to the output 40 of the filter unit 30. This can be achieved by switching on only the two upper semiconductor switches or only the two lower semiconductor switches connected between the input 24 and the output 40 of the filter unit 30. In a second switching state, the filter unit 30 connects the cell capacitor 38 between the output 24 and the output 40, so that the cell capacitor voltage of the cell capacitor 38 is added to the intermediate voltage provided at the output 24. In a third switching state, the filter unit 30 connects the cell capacitor 38 between the output 24 and the output 40, so that the cell capacitor voltage of the cell capacitor 38 is subtracted from the intermediate voltage provided at the output 24. In this way, the intermediate voltage from the main stage 12 (which is usually shaped like a step function due to the limited number of levels of the main stage 12) can be converted to a voltage (i.e., the output voltage) that is closer to a sinusoidal and / or continuous output voltage reference. In the fourth switching state (also denoted herein as "rectification state"), the rectified current flows through the cell capacitor 38, which is achieved in this example by turning off all four semiconductor switches of the filter unit 30.

[0058] Each of the passive filters 32 may include an inductor, a resistor and / or a capacitor for electrically filtering the output voltage to suppress even further higher harmonics. In this example, each passive filter 32 is configured as an EMC filter (electromagnetic compatibility filter) comprising an inductor and an RC group (i.e. a resistor and a capacitor connected in series) connected in series between the output 40 of the respective (active) filter unit 30 and a star point 42 to which all passive filters 32 are connected. Figure 1 In FIG. 4 , the star point 42 is a floating star point. However, it may alternatively be connected to the neutral point NP of the DC link 20 of the primary stage 12 .

[0059] The electrical conversion 10 further comprises a pre-charging circuit 44 adapted to pre-charge the filter unit 30 and the DC link 20 of the primary stage 12. To this end, the pre-charging circuit 44 comprises an auxiliary power source 46. The pre-charging circuit 44 is connected between one phase of the output 34 of the electrical converter 10 and the neutral point NP of the DC link 20 of the primary stage 12.

[0060] In this example, the auxiliary power source 46 of the pre-charging circuit 44 may be configured as a transformer winding that creates a single-phase AC voltage. Due to the high short-circuit impedance of the auxiliary transformer, the pre-charging current may be limited to a few amperes in any case.

[0061] All phases of the output 34 of the electrical converter 10 are connected via the electrical load 16, which is a motor as in this example, or any other electrical machine or physical system. Therefore, although the pre-charging circuit 46 is connected to only one phase (e.g., phase a) of the output 34 of the electrical converter 10, the filter units 30 in all other phases (i.e., phase b and phase c in this example) can be charged by the pre-charging circuit 46 also via the electrical load 16 (e.g., via the machine winding).

[0062] Figure 3 1 shows a block diagram of a controller 50 adapted to perform the method described herein. Using the controller 50, Figure 1 The electric converter 10 can be controlled. In this example, the controller 50 includes: a filter unit pre-charge controller 54, the filter unit pre-charge controller 54 is used to perform the step of pre-charging the filter unit 30 according to the method described herein; a DC link pre-charge controller 52, the DC link pre-charge controller 52 is used to perform the step of pre-charging the DC link 20 of the main stage 12 according to the method described herein; and a main operation controller 56, the main operation controller 56 is used to control the normal operation of the electric converter 10. The blocks of the controller 50 also indicate the method steps of the control method performed by the controller 50. Therefore, the block of the filter unit pre-charge controller 54 can also be regarded as the step of pre-charging the filter unit 30 according to the method. The block of the DC link pre-charge controller 52 can also be regarded as the step of pre-charging the DC link 20 of the main stage 12 according to the method.

[0063] In the following, exemplary embodiments of possible steps for precharging the (active) filter unit and the DC link of the primary stage of the electric converter during start-up of the electric converter according to the method of the invention will be described. Figures 1 to 4c The start-up may be followed by normal operation of the electric converter 10 performed by block 56 of the controller 50, which may also be interrupted later if the cell voltage of the filter unit 30 drops below a predetermined cell voltage threshold or the DC link voltage of the DC link 20 drops below a predetermined DC link voltage threshold, to repeat the pre-charging of the filter unit 30 and / or the DC link 20 by block 52 or block 54, respectively.

[0064] In this example, as described above, Figure 1The auxiliary power source 46 of the pre-charging circuit 44 in is configured as a transformer winding creating a single-phase AC voltage, the transformer winding being connected between the phase a of the output 34 of the electric converter 10 and the neutral point NP of the main DC link 20 of the primary stage 12. Due to the high short-circuit impedance of the auxiliary transformer, the charging current of the pre-charging circuit 44 is in any case limited to a few amperes.

[0065] In this example, during startup of the electrical converter 10, Figure 3 The controller 50 first pre-charges the upper DC capacitor 19 and the lower DC capacitor 21 of the DC link 20 of the main stage 12 to a predetermined nominal DC link voltage, which is determined by Figure 3 The DC link pre-charge controller 52 is then executed. Then, the cell capacitor 38 of the filter unit 30 is Figure 3 The filter cell pre-charge controller 54 is charged to a predetermined nominal cell voltage. The reverse sequence, ie charging the filter cell 30 first and then charging the DC link 20 of the main stage 12, is also possible.

[0066] First, all filter units 30 switch to zero vector (ie, directly connected state), for example, Figure 1 The two upper semiconductor switches or the two lower semiconductor switches (such as IGBTs) in the H bridge of the corresponding filter unit 30 are turned on. Figure 2a / Figure 2b All semiconductor switches T1-T4 or respectively T1-T6 (eg IGBTs) in the main stage 12 are shown to be turned off. As a result, the filter unit 30 is bypassed and the auxiliary winding AC voltage of the pre-charging circuit 44 begins to flow through the freewheeling diodes D1-D6 (see Figure 2a / Figure 2b ) charges the DC link 20 of the main stage 12.

[0067] When the DC link 12 of the main stage 20 has reached its nominal voltage, pre-charging of the filter unit 30 is performed by the filter unit pre-charging controller 54 by setting the switching state of the main stage to (000), in which the outputs 24 of the main stage 12 in all phases are connected to the neutral point NP of the DC link 20. All semiconductor switches of the filter unit 30 are turned off, so that the filter unit 30 is in its rectifying state. Now, the auxiliary winding AC voltage source 46 will charge the filter unit 30 because they are both connected in parallel to the single-phase AC voltage source 16 through the electrical load 46 and the neutral point NP of the DC link 20. If the predetermined nominal filter unit voltage is reached, the pre-charging circuit 44 can be turned off or disconnected.

[0068] This is Figures 4a to 4c The following figure is additionally illustrated:

[0069] Figure 4a Two graphs are shown illustrating precharging of the upper DC link capacitor 19 (curve 58) and the lower DC link capacitor 21 (curve 60) of the DC link 20 to the nominal DC link voltage as described in the above example. Both graphs show curves 58 and 60 which are ideally identical. Figure 4b Three diagrams are shown illustrating precharging of the filter unit 30 in each of the three phases (phase a: curve 62; phase b: curve 64; phase c: curve 66) of the electrical converter 10 to the nominal cell voltage, which is performed immediately after precharging the DC link 20 in the above example. All three diagrams show ideally identical curves 62, 64 and 66. Figure 4c A diagram illustrating the motor current in each of the three phases during pre-charging of the filter unit 30 or during pre-charging of the DC link 20 as described above is shown. Here, the magnitude of the motor current in phase a (curve 68) is twice the magnitude of the motor current in phases b and c (curves 70 and 72), as can be seen from Figure 1 As can be seen, the motor current in phase a is split into motor currents in phases b and c.

[0070] Although the pre-charge circuit 44 is connected to only phase a, all other active filter units 30 (i.e., in phases b and c) will also be charged via the machine winding of the electrical load 16. The machine winding is connected in parallel with respect to the auxiliary power source 46 to three AF ​​units 30. The current vector generated in the machine (electrical load 16) does not rotate, it only oscillates on the alpha axis of the two-dimensional vector representation of the complex current. Therefore, the rotor of the machine cannot start to rotate.

[0071] As mentioned above, the star point 42 of the passive EMC filter 32 is at Figure 1 In the example shown it is floating. The method is also applicable to an EMC filter star point 32 connected to the neutral point NP of the DC link 20 of the primary stage 12 .

[0072] If the DC link 20 or the cell capacitor 38 of the active filter unit 30 is discharged below a predetermined threshold, the pre-charge circuit 44 can be connected again and the previously described switching sequence may be repeated.

[0073] Although the present invention has been illustrated and described in detail in the drawings and the above description, such illustrations and descriptions should be regarded as illustrative or exemplary rather than restrictive; the present invention is not limited to the disclosed embodiments. Those skilled in the art and those who practice the claimed invention can understand and implement other changes to the disclosed embodiments from a study of the drawings, the disclosure and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single processor or controller or other unit may implement the functions of several items recited in the claims. The fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any figure marks in the claims should not be interpreted as limiting the scope.

Claims

1. A method for precharging a filter unit (30) during startup of an electrical converter (10), The electrical converter (10) comprises: a main stage (12) adapted to convert a DC voltage into a multi-phase intermediate voltage comprising at least two voltage levels; a filter cell stage (14), the filter cell stage (14) having a filter cell (30) for each phase of the intermediate voltage, each filter cell (30) being adapted to add a cell voltage of the filter cell (30) to the intermediate voltage or to subtract a cell voltage of the filter cell (30) from the intermediate voltage; and a precharging circuit (44), the precharging circuit (44) being adapted to precharge the filter cell (30) and the DC link (20) of the main stage (12), and comprising an auxiliary power source (46), the auxiliary power source (46) being connected between one phase of the output (34) of the electrical converter (10) and a neutral point (NP) of the DC link (20) of the main stage (12), all phases of the output (34) of the electrical converter (10) being connected via an electrical load (16); The method comprises: pre-charging the filter unit (30), and pre-charging the filter unit (30) comprises the following steps: Switching the primary stage (12) such that all phases of the output (24) of the primary stage (12) are directly connected to the neutral point (NP) of the DC link (20) of the primary stage (12); switching at least one of the filter units (30) to a rectifying state in which a rectified current flows through a cell capacitor (38) of the filter unit (30); and The pre-charging circuit (44) is turned on and the cell capacitor (38) of the at least one filter cell (30) is charged by the pre-charging circuit (44) until a nominal cell voltage is reached at the cell capacitor (38) of the filter cell (30).

2. The method according to claim 1, The precharging of the filter units (30) is accomplished by switching all the filter units (30) to the rectifying state simultaneously, so that the unit capacitors (38) of all the filter units (30) are charged simultaneously by the precharging circuit (44).

3. The method according to claim 1 or 2, The current generated by the pre-charging circuit (44) is an alternating current.

4. The method according to any one of the preceding claims, Also includes: Precharging the DC link (20) of the primary stage (12), the precharging of the DC link (20) of the primary stage (12) being performed before or after precharging the filter unit (30), and comprising the following steps: switching each filter unit (30) such that the output (40) of each filter unit (30) is directly connected to the input (24) of each filter unit (30), thereby bypassing the unit capacitor (38); switching the primary stage (12) so that, depending on the sign of the voltage applied between the neutral point (NP) of the DC link (20) and the output (24) of the primary stage (12), either an upper DC link capacitor (19) or a lower DC link capacitor (21) is charged; The pre-charging circuit (44) is turned on and the DC link capacitors (19, 21) are charged by the alternating current generated by the pre-charging circuit (44) until a nominal DC link voltage of the DC link (20) is reached.

5. The method according to any one of the preceding claims, The pre-charging circuit (44) is switched off when the nominal cell voltage of the filter unit (30) and / or the nominal DC link voltage of the DC link (20) is reached.

6. The method according to any one of the preceding claims, wherein the precharging of the filter cells (30) is repeated each time the cell voltage of at least one of the filter cells (30) drops below a predetermined cell voltage threshold; and / or The pre-charging of the DC link (20) of the primary stage (12) is repeated each time the DC link voltage of the DC link (20) drops below a predetermined DC link voltage threshold.

7. The method according to any one of the preceding claims, wherein the electrical converter (10) further comprises a passive output filter (32) in each phase of the output (34) of the electrical converter (10), the passive output filter (32) being connected in series between the respective output (40) of the filter unit stage (14) and the output (34) of the electrical converter (10); And the pre-charging circuit (44) is preferably connected to the phase of the output (34) of the electrical converter (10), between the inductance (L) and the RC group of the passive output filter (32) provided in that phase.

8. The method according to any one of the preceding claims, The auxiliary power source (46) of the pre-charging circuit (44) is configured as a transformer winding that creates a single-phase AC voltage.

9. The method according to any one of the preceding claims, wherein each filter unit (30) comprises two half bridges (36), the two half bridges (36) being connected in parallel with the unit capacitor (38); and Each half bridge (36) has at least two semiconductor switches, each of the at least two semiconductor switches being bypassed by a freewheeling diode; causing the corresponding filter unit (30) to be switched to a rectifying state by turning off all semiconductor switches of the corresponding filter unit (30), in which a rectified current flows through the unit capacitor (38); and The corresponding filter unit (30) is switched to a direct connection state by turning on only all upper semiconductor switches or only all lower semiconductor switches connected between the input (24) and the output (40) of the filter unit (30), in which the cell capacitor (38) of the corresponding filter unit (30) is bypassed.

10. The method according to any one of the preceding claims, Each phase of the main stage (12) comprises at least four freewheeling diodes (D1-D6) and at least four semiconductor switches (T1-T4; T1-T6), and each of the at least four semiconductor switches (T1-T4; T1-T6) is connected by the freewheeling diode (D1-D4; D1-D6) is bypassed by a freewheeling diode; By turning off all semiconductor switches (T1-T4; T1-T6) of the primary stage (12), the upper DC link capacitor (19) or the lower DC link capacitor (21) is charged via the freewheeling diodes (D1-D4) bypassing the semiconductor switches (T1-T4) depending on the sign of the current; and By turning on at least two semiconductor switches (T2-T3; T2-T3 / T5-T6) of the primary stage (12), the output (24) of the primary stage (12) is directly connected to the neutral point (NP) of the DC link (20), thereby bypassing the DC link capacitors (19, 21).

11. A computer program for pre-charging a filter unit (30) during start-up of an electrical converter (10), the computer program being adapted to perform the steps of the method according to any of the preceding claims when executed by a processor.

12. A computer-readable medium having stored therein the computer program according to claim 11.

13. A controller (50) for controlling an electrical converter (10), the controller (50) being adapted to perform the method according to any one of claims 1 to 10 and comprising: A filter unit pre-charge controller (54), the filter unit pre-charge controller (54) being used to execute the step of pre-charging the filter unit (30); A DC link pre-charge controller (52) is provided for executing the step of pre-charging the DC link (20) of the primary stage (12).

14. An electrical converter (10), comprising: a main stage (12) adapted to convert a DC voltage into a multi-phase intermediate voltage comprising at least two voltage levels; a filter cell stage (14) having a filter cell (30) for each phase of the intermediate voltage, wherein each filter cell (30) is adapted to add a cell voltage of the filter cell (30) to the intermediate voltage or to subtract a cell voltage of the filter cell (30) from the intermediate voltage; a pre-charging circuit (44) adapted to pre-charge the filter unit (30) and the DC link (20) of the primary stage (12), and comprising an auxiliary power source (46) connected between a phase of the output (34) of the electrical converter (10) and a neutral point (NP) of the DC link (20) of the primary stage (12), The controller (50) according to claim 13, All phases of the output (34) of the electrical converter (10) are connected via an electrical load (16).

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