Inverter device

Abstract

To provide an inverter device capable of suppressing influence of a current ripple.SOLUTION: An inverter device includes a plurality of cell inverters connected in series. Each of the plurality of cell inverters includes: a modular multilevel converter including a plurality of normal cell inverters and a spare cell inverter; and a control unit for controlling the modular multilevel converter. The control unit allows the plurality of normal cell inverters to perform multilevel operation at normal time when bearing corrosion of a motor driven by the modular multilevel converter is not detected, and allows the normal cell inverters and the spare cell inverter to perform multilevel operation when bearing corrosion is detected. DC voltage carried by each cell inverter of the plurality of cell inverters when bearing corrosion is detected is lower than DC voltage carried by each normal cell inverter of the plurality of normal cell inverters at normal time during multilevel operation.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present disclosure relates to an inverter device.

Background Art

[0002] Patent Document 1 discloses a rolling bearing capable of detecting the grease iron powder content by a sensor. Non-Patent Document 1 discloses a mechanism in which bearing electrolytic corrosion occurs due to the common-mode voltage when driving an inverter and countermeasures by reducing the carrier frequency.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Non-Patent Documents

[0004]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] As described in Patent Document 1, bearing electrolytic corrosion can be detected by detecting iron powder generated by the electrolytic corrosion of the bearing. Further, according to Non-Patent Document 1, the progress of bearing electrolytic corrosion can be suppressed by suppressing the common-mode current by reducing the carrier frequency. However, when the carrier frequency is reduced, there is a risk that the influence of the current ripple of harmonics generated at the carrier frequency and odd multiples of the carrier frequency increases.

[0006] Generally, when electrolytic corrosion occurs, it may progress rapidly. If the electrolytic corrosion progresses, it may lead to bearing damage. In that case, it will result in a serious failure of the motor and require a long-term shutdown for repair. In the case of motors used in production equipment such as factories, an unexpected shutdown will have a significant impact on the production process. Therefore, even for repair, it is desirable to perform a planned shutdown with a predetermined preparation period in advance. Accordingly, when the occurrence of electrolytic corrosion is detected, it is desirable to continue operating the motor while suppressing the progress of electrolytic corrosion until a period when a planned shutdown is possible.

[0007] An object of the present disclosure is to obtain an inverter device that can suppress the progress of bearing electrolytic corrosion of a motor while suppressing an increase in current ripple.

Means for Solving the Problems

[0008] The inverter device according to the first disclosure has a plurality of cell inverters connected in series. The plurality of cell inverters include a modular multilevel converter including a plurality of normal cell inverters and a spare cell inverter, and a control unit that controls the modular multilevel converter. When the control unit does not detect bearing electrolytic corrosion of a motor driven by the modular multilevel converter during normal times, it causes the plurality of normal cell inverters to perform a multilevel operation. When the bearing electrolytic corrosion is detected, it causes the plurality of normal cell inverters and the spare cell inverter to perform the multilevel operation. In the multilevel operation, the DC voltage borne by each cell inverter of the plurality of cell inverters at the time of detection of the bearing electrolytic corrosion is lower than the DC voltage borne by each normal cell inverter of the plurality of normal cell inverters during normal times.

[0009] The inverter device according to the second disclosure has, for each of a plurality of phases, a plurality of cell inverters connected in series, the plurality of cell inverters including a modular multilevel converter including a plurality of normal cell inverters and a spare cell inverter, and a control unit that causes the plurality of normal cell inverters to execute a multilevel operation. When the control unit detects bearing electrolytic corrosion of a motor driven by the modular multilevel converter, the control unit controls the normal cell inverters and the spare cell inverter so that a detected current of a zero-phase current transformer connected to outputs of the plurality of phases of the modular multilevel converter approaches zero.

Advantages of the Invention

[0010] According to the inverter device according to the first disclosure, the DC voltage borne by each cell inverter at the time of detecting bearing electrolytic corrosion is lower than the DC voltage borne by each normal cell inverter in normal times. Therefore, while suppressing an increase in current ripple, it is possible to suppress the progress of bearing electrolytic corrosion of the motor. According to the inverter device according to the second disclosure, the spare cell inverter is controlled so that the detected current of the zero-phase current transformer approaches zero at the time of detecting bearing electrolytic corrosion. Thereby, the common mode current can be suppressed. In this way, since the common mode current can be suppressed by a method other than reducing the carrier frequency, it is possible to suppress the progress of bearing electrolytic corrosion of the motor while suppressing an increase in current ripple.

Brief Description of the Drawings

[0011]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Embodiments for Carrying Out the Invention

[0012] The inverter devices according to the respective embodiments will be described with reference to the drawings. The same or corresponding components may be denoted by the same reference numerals, and repeated descriptions may be omitted.

[0013] Embodiment 1. FIG. 1 is a diagram for explaining the configuration of an inverter device 11 according to Embodiment 1. The inverter device 11 includes an MMC (Modular Multilevel Converter) 12 and an inverter control unit 15 that controls the MMC 12. The MMC 12 is configured to output an AC voltage from each of, for example, three phases of R phase, S phase, and T phase. An electric motor 21 is connected to the output of the MMC 12. The electric motor 21 is driven by the three-phase AC voltage output by the MMC 12.

[0014] The MMC 12 has an input transformer 6. A three-phase AC voltage from an AC power supply 10 is input to the input transformer 6. In the input transformer 6, the primary winding and the secondary winding are insulated from each other, and the secondary windings are also insulated from each other. Nine sets of three-phase secondary windings are respectively connected to the input sides of the cell inverters of the R phase, S phase, and T phase, three sets at a time.

[0015] In the MMC 12, each of the three phases consisting of the R phase, S phase, and T phase has a plurality of cell inverters connected in series. The plurality of cell inverters constituting each phase include a plurality of normal cell inverters 13 and at least one spare cell inverter 14.

[0016] In the example of FIG. 1, the R-phase includes normal cell inverters 13R1 and 13R2, and a standby cell inverter 14R. One end of the output of the normal cell inverter 13R1 is connected to the neutral point, the other end of the output of the normal cell inverter 13R1 is connected to one end of the output of the normal cell inverter 13R2, and the other end of the output of the normal cell inverter 13R2 is connected to one end of the output of the standby cell inverter 14R. The other end of the output of the standby cell inverter 14R becomes the R-phase output of the MMC 12 and is connected to the R-phase AC input of the motor 21 via the CT 16 described later.

[0017] The S-phase includes normal cell inverters 13S1 and 13S2, and a standby cell inverter 14S. One end of the output of the normal cell inverter 13S1 is connected to the neutral point, the other end of the output of the normal cell inverter 13S1 is connected to one end of the output of the normal cell inverter 13S2, and the other end of the output of the normal cell inverter 13S2 is connected to one end of the output of the standby cell inverter 14S. The other end of the output of the standby cell inverter 14S becomes the S-phase output of the MMC 12 and is connected to the S-phase AC input of the motor 21 via the CT 16.

[0018] The T-phase includes normal cell inverters 13T1 and 13T2, and a standby cell inverter 14T. One end of the output of the normal cell inverter 13T1 is connected to the neutral point, the other end of the output of the normal cell inverter 13T1 is connected to one end of the output of the normal cell inverter 13T2, and the other end of the output of the normal cell inverter 13T2 is connected to one end of the output of the standby cell inverter 14T. The other end of the output of the standby cell inverter 14T becomes the T-phase output of the MMC 12 and is connected to the T-phase AC input of the motor 21 via the CT 16.

[0019] In this way, the MMC 12 has a multi-level configuration in which cell inverters that output single-phase voltages are connected in multiple stages. Note that the neutral point of the MMC 12 is grounded.

[0020] Each cell inverter and the inverter control unit 15 transmit and receive signals such as a gate signal S1c, a gate signal S1i, a DC voltage S2, a cell inverter failure signal S3, and an output short-circuit circuit operation signal S4.

[0021] The electric motor 21 is provided with a bearing electrolytic corrosion detection unit 22. The bearing electrolytic corrosion detection unit 22 detects the bearing electrolytic corrosion of the electric motor 21 based on, for example, the amount of iron powder contained in the grease of the bearing of the electric motor 21. When the bearing electrolytic corrosion detection unit 22 detects iron powder contained in the bearing grease, it transmits an electrolytic corrosion detection signal S5 to the inverter control unit 15 to notify the occurrence of bearing electrolytic corrosion. The bearing electrolytic corrosion detection unit 22 may transmit the electrolytic corrosion detection signal S5 when the amount of iron powder exceeds a predetermined value. Further, the bearing electrolytic corrosion detection unit 22 may transmit the amount of iron powder as the electrolytic corrosion detection signal S5. The method for detecting bearing electrolytic corrosion is not limited to the method based on the amount of iron powder contained in the bearing grease, and any method can be adopted.

[0022] CTs (Current Transformers) 16 are provided at the outputs of each phase of the MMC 12. The CT 16 detects the output current of each phase and transmits it to the inverter control unit 15 as an output current signal S6. Note that the CT 16 may be provided only for two of the three phases. When the three phases are balanced, if the currents of two phases are detected, the current of the other phase can be estimated. Further, the CT 16 may be provided outside the MMC 12.

[0023] FIG. 2 is a diagram for explaining the configuration of the cell inverter according to Embodiment 1. The configurations of the normal cell inverter 13 and the standby cell inverter 14 may be the same. The input terminal P1 of the cell inverter is connected to the three-phase secondary windings of the input transformer 6. The AC input of the converter section 32 having a three-phase bridge configuration is connected to the input terminal P1 via a fuse 31. A capacitor 34 is connected between the DC circuit sides of the converter section 32, that is, between the positive electrode side and the negative electrode side.

[0024] The converter section 32 has three sets of upper and lower arms. The connection points, which are the AC inputs of the three sets of upper and lower arms, are each connected to the three-phase input terminals P1. Each arm includes a switching element 32a and an antiparallel diode 32b. A gate drive circuit 32c is connected to the gate of each switching element 32a. The converter section 32 is, for example, a PWM converter capable of PWM (Pulse Width Modulation) control.

[0025] Based on the gate signal S1c input from the inverter control section 15, the control circuit 38 controls the on / off of the switching element 32a via the gate drive circuit 32c. Thereby, the control circuit 38 converts the three-phase AC power input from the input terminals P1 into DC power and outputs it to the capacitor 34 on the DC circuit side.

[0026] The voltage detection circuit 33 is provided in parallel with the capacitor 34. The voltage detection circuit 33 detects the voltage of the capacitor 34. The detected voltage is transmitted to the inverter control section 15 as a DC voltage S2 via the control circuit 38. Based on the DC voltage S2, the inverter control section 15 outputs the gate signal S1c so that the output DC voltage of the converter section 32, that is, the voltage of the capacitor 34, follows the DC target voltage.

[0027] A capacitor 34 is connected to the DC side circuit of an inverter section 36 via a fuse 35. The inverter section 36 is a single-phase bridge circuit and has two sets of upper and lower arms. The connection points of the two sets of upper and lower arms are AC outputs, which are respectively connected to output terminals P2 and P3. Each arm includes a switching element 36a and an antiparallel diode 36b. A gate drive circuit 36c is connected to the gate of each switching element 36a. Here, the switching element 36a connected between the positive electrode side of the DC circuit and the output terminal P3 is referred to as a switching element Q1. The switching element 36a connected between the output terminal P3 and the negative electrode side of the DC circuit is referred to as a switching element Q2. The switching element 36a connected between the positive electrode side of the DC circuit and the output terminal P2 is referred to as a switching element Q3. The switching element 36a connected between the output terminal P2 and the negative electrode side of the DC circuit is referred to as a switching element Q4.

[0028] Based on the gate signal S1i input from the inverter control unit 15, the control circuit 38 controls the on / off of the switching element 36a via the gate drive circuit 36c. The inverter control unit 15 outputs the gate signal S1i so that the output current signal S6 of the inverter section 36 detected by the CT16 follows the target current based on the output current signal S6. That is, the MMC12 is controlled so that the output current of the MMC12 follows the target current. The target current is generated in the inverter control unit 15 so that the motor 21 rotates at the target rotational speed.

[0029] The cell inverter failure signal S3 is output when the cell inverter fails. For example, when any one of the fuses 31 and 35 blows, a fuse blow detection signal is output from the blown fuse. The cell inverter failure signal S3 is output by the control circuit 38 in response to, for example, the fuse blow detection signal. Also, the control circuit 38 may output the cell inverter failure signal S3 when the DC voltage of the capacitor 34 becomes equal to or lower than a predetermined voltage, or when a failure occurs in the control circuit 38 or the gate drive circuits 32c and 36c.

[0030] When the inverter control unit 15 receives the cell inverter failure signal S3, it sets the output voltage of the normal cell inverter 13 to 0V. For example, an output short-circuit circuit 37 is connected between the output terminals P2 and P3, which are the output of the inverter unit 36. The output short-circuit circuit 37 is a circuit for maintaining the AC output (between the output terminals P2 and P3) of the cell inverter in a short-circuited state when a failure occurs in the cell inverter. For example, the inverter control unit 15 outputs an output short-circuit circuit operation signal S4 according to the cell inverter failure signal S3. When the control circuit 38 receives the output short-circuit circuit operation signal S4 from the inverter control unit 15, it transmits the output short-circuit circuit operation signal to the output short-circuit circuit 37. The output short-circuit circuit 37 operates the output short-circuit circuit 37 according to the output short-circuit circuit operation signal.

[0031] The output short-circuit circuit 37 may be a mechanical switch or a bidirectional semiconductor switch. The output short-circuit circuit 37 may have any configuration as long as it can maintain the short-circuited state between the output terminals P2 and P3. For example, the output short-circuit circuit 37 may be provided at a location different from that in FIG. 2. Also, the output short-circuit circuit 37 may be omitted, and the control circuit 38 or the like may maintain the switching element Q1 and the switching element Q3, or the switching element Q2 and the switching element Q4 in the on state by some means to also serve as the function of the output short-circuit circuit 37.

[0032] FIG. 3 is a flowchart for explaining the operation of the inverter control unit 15 according to Embodiment 1. In step S101, when the inverter control unit 15 does not detect bearing electrolytic corrosion and a failure of the cell inverter during normal operation, it operates only with the normal cell inverter 13. The inverter control unit 15 controls the converter unit 32 of the normal cell inverter 13 so that the DC voltage of the normal cell inverter 13 follows the first voltage set value, and PWM-controls the inverter unit 36 of the normal cell inverter 13 at the first carrier frequency so that the output current of the MMC 12 follows the current command value.

[0033] Normally, the standby cell inverter 14 is not involved in multi-level operation, and when viewed from the outside, the output of the inverter section 36 of the standby cell inverter 14 is equivalently in a state where the output voltage is zero. Specifically, there are the following three methods, and any means may be adopted.

[0034] In the first means, without operating the output short-circuit circuit 37, the switching element Q1 and the switching element Q3 are turned on, and the switching element Q2 and the switching element Q4 are turned off, so that the output voltage becomes zero.

[0035] In the second means, without operating the output short-circuit circuit 37, the switching element Q1 and the switching element Q3 are turned off, and the switching element Q2 and the switching element Q4 are turned on, so that the output voltage becomes zero.

[0036] In the third means, the output short-circuit circuit 37 is operated, and the switching element Q1, the switching element Q2, the switching element Q3, and the switching element Q4 are turned off, so that the output voltage becomes zero.

[0037] At this time, all the switching elements 32a constituting the converter section 32 of the standby cell inverter 14 are in the off state. The capacitor 34 of the standby cell inverter 14 is charged by the antiparallel diode 32b constituting the converter section 32. However, since the output of the inverter section 36 is zero, the ripple current becomes very small. Also, since the current flowing through other electrical components is small, the temperature rise can be suppressed. In this way, the component life can be maintained and the performance as a spare can be ensured.

[0038] Next, in step S102, the inverter control unit 15 determines whether there is a failure in the normal cell inverter 13. This determination is made based on, for example, the reception or non-reception of the cell inverter failure signal S3. If it is determined that there is a failure in the normal cell inverter 13, the process proceeds to step S104. If it is determined that there is no failure in the normal cell inverter 13, the process proceeds to step S114.

[0039] In step S104, when the normal cell inverter 13 fails, the inverter control unit 15 sets the output of the failed normal cell inverter 13 to 0V as described above. The inverter control unit 15 further operates the spare cell inverter 14 of the same phase as the failed normal cell inverter 13 to continue the operation. At this time, the DC voltage of the operating spare cell inverter 14 is set to follow the first voltage set value and is operated. Then, instead of setting the output voltage of the spare cell inverter 14 to 0V, the output voltage of the spare cell inverter 14 is controlled so that its output follows the current command value as the MMC12. At this time, the inverter section 36 of each cell inverter used for operation is PWM-controlled at the first carrier frequency.

[0040] In step S114, the inverter control unit 15 determines whether or not the electric erosion detection signal S5 is received. The inverter control unit 15 may also determine whether or not the amount of iron powder notified by the electric erosion detection signal S5 is equal to or greater than a predetermined value. In step S114, it is sufficient that the inverter control unit 15 can determine the presence or absence of bearing electric erosion based on the electric erosion detection signal S5. When the electric erosion detection signal S5 is not received, or when the amount of iron powder notified by the electric erosion detection signal S5 is less than the predetermined value, the inverter control unit 15 maintains the normal state. When the electric erosion detection signal S5 is received, or when the amount of iron powder notified by the electric erosion detection signal S5 is equal to or greater than the predetermined value, the process proceeds to step S116.

[0041] In step S116, the inverter control unit 15 operates the spare cell inverter 14 for all three phases and performs the operation using the normal cell inverter 13 and the spare cell inverter 14. At this time, the inverter control unit 15 sets the output amplitude of the MMC12, that is, the total value of the output voltages of the normal cell inverter 13 and the spare cell inverter 14, to the same three-phase AC voltage amplitude as in the normal state. This can be achieved by controlling the converter section 32 of each cell inverter so that the DC voltages of the normal cell inverter 13 and the spare cell inverter 14 follow the second voltage set value. The adjustment of the DC voltage can be performed using the converter section 32 of each cell inverter. The second voltage set value is smaller than the first voltage set value.

[0042] This control will be described with reference to FIGS. 4 and 5. FIG. 4 is a diagram showing an example of the output voltage waveforms of each phase during normal operation according to Embodiment 1. FIG. 5 is a diagram showing an example of the output voltage waveforms of each phase at the time of detecting bearing electrolytic corrosion according to Embodiment 1. In the examples of FIGS. 4 and 5, the amplitude of the three-phase alternating voltage output by the inverter device 11 is V amp , the number of series stages of the normal cell inverter 13 per phase during normal operation when bearing electrolytic corrosion is not detected is n g , the DC voltage of each cell inverter is V DCg . Also, the number of stages per phase of the spare cell inverter 14 operated at the time of detecting bearing electrolytic corrosion is n a , and the DC voltage of each cell inverter at this time is V DCa . Also, let the modulation ratio of PWM be m. Note that FIGS. 4 and 5 are examples with the modulation ratio m = 1.

[0043] The inverter control unit 15 uses the converter unit 32 to adjust V DCg , V DCa so that the following equations (1) and (2) hold in each cell inverter. The inverter control unit 15 receives the DC voltage S2 from each cell inverter and controls the converter unit 32 in the cell inverter so that the DC voltage of the cell inverter in operation follows a predetermined voltage set value. That is, the first voltage set value, which is the voltage set value during normal operation when bearing electrolytic corrosion is not detected, corresponds to V DCg , and the second voltage set value, which is the voltage set value in step S116 when bearing electrolytic corrosion is detected, corresponds to V DCa .

[0044]

Equation

[0045] During normal operation, the inverter control unit 15 causes the plurality of normal cell inverters 13 to perform a multi-level operation. Further, when detecting bearing electrolytic corrosion, the inverter control unit 15 causes the plurality of normal cell inverters 13 and the spare cell inverter 14 to perform a multi-level operation. During normal operation and when detecting bearing electrolytic corrosion, the amplitude V of the multi-level waveform output by the multi-level operation amp is the same. Therefore, when detecting bearing electrolytic corrosion, by further operating the n a -stage spare cell inverter 14 per phase, the DC voltage V DCg during normal operation can be reduced to V DCa . Here, combining the PWM voltage waveforms from the cell inverters to output a multi-level waveform as shown in FIGS. 4 and 5 is called a multi-level operation.

[0046] In the examples of FIGS. 4 and 5, n g = 2 and n a = 1. During normal operation, since it operates with two normal cell inverters 13 connected in series, the output voltages of each phase for positive and negative are in two levels. On the other hand, when detecting bearing electrolytic corrosion, since it operates with two normal cell inverters 13 and one spare cell inverter 14 connected in series, the output voltages of each phase for positive and negative are in three levels. The number of stages of the normal cell inverter 13 and the spare cell inverter 14 is not limited to that shown in FIG. 1.

[0047] In step S118, further, when detecting bearing electrolytic corrosion, the inverter control unit 15 operates by reducing the carrier frequency of the inverter unit 36 of each cell inverter (normal cell inverter 13 and spare cell inverter 14) constituting the MMC12 to a second carrier frequency lower than the first carrier frequency compared to normal operation.

[0048] Next, the effects of the present embodiment will be described. According to Non-Patent Document 1, the electrolytic corrosion of the bearings of an electric motor is generally caused by the common-mode current resulting from the switching of the inverter. The common-mode current increases as the frequency of the common-mode voltage, which depends on the PWM carrier frequency of the inverter, increases, and as the amplitude of the common-mode voltage, which depends on the DC voltage of the inverter, increases. Simply reducing only the PWM carrier frequency increases the current ripple flowing into the electric motor, which is the load, as will be described later, and this is not preferable. In contrast, in the present embodiment, in the multi-level operation, by operating the MMC 12 using the preliminary cell inverter 14 as shown in step S116, the DC voltage of each cell inverter can be reduced. Therefore, when detecting electrolytic corrosion of the bearings, the DC voltage V DCa borne by each normal cell inverter 13 during normal times is the DC voltage V DCg is lower. By reducing the DC voltage in this way, the common-mode voltage can be reduced, so the common-mode current can be suppressed. Therefore, in the present embodiment, the progress of electrolytic corrosion of the bearings can be suppressed.

[0049] Furthermore, the inverter unit 36 generates voltage ripples of the carrier frequency and odd multiples of the carrier frequency. The voltage ripple becomes a current ripple and flows into the electric motor 21. The harmonic current ripple depends on the DC voltage of the inverter unit 36 of the cell inverter. The harmonic current ripple is not preferable because it gives unnecessary torque pulsations and the like to the electric motor 21. Here, in the case of an inductive load such as an electric motor in general, the flowing harmonic current ripple is proportional to the DC voltage and inversely proportional to the harmonic frequency. Therefore, by increasing the number of series-connected cell inverters to be operated and reducing the DC voltage, even if the carrier frequency of the inverter unit 36 of the cell inverter is reduced, an increase in the harmonic current ripple can be suppressed. The second carrier frequency can be reduced until the ratio of the second voltage setting value to the first voltage setting value is the same as the ratio of the second carrier frequency to the first carrier frequency.

[0050] As described above, in this embodiment, since the DC voltage of the cell inverter that constitutes the MMC12 is reduced in step S116, the frequency of the common mode voltage can be further reduced by reducing the carrier frequency of the inverter unit 36 of the cell inverter that constitutes the MMC12 in step S118. Therefore, the common mode current can be suppressed, and an increase in the harmonic current ripple due to the reduction of the carrier frequency of the inverter unit 36 of the cell inverter can be suppressed. Therefore, in this embodiment, the progress of bearing electrolytic corrosion can be suppressed. In particular, in this embodiment, even when the carrier frequency of the inverter unit 36 of the cell inverter is reduced to suppress the progress of bearing electrolytic corrosion, the influence of the ripple due to the reduction of the carrier frequency of the inverter unit 36 of the cell inverter can be suppressed while suppressing the progress of bearing electrolytic corrosion.

[0051] Note that the DC voltage borne by the cell inverter may be reduced without reducing the carrier frequency of the inverter unit 36 of the cell inverter. Also in this case, the influence of the current ripple can be suppressed by the effect of reducing the DC voltage.

[0052] FIG. 6 is a diagram showing a hardware configuration example of a processing circuit included in the inverter control unit 15 according to Embodiment 1. Each function of the inverter control unit 15 can be realized by a processing circuit. As one aspect, the processing circuit includes at least one processor 91 and at least one memory 92. As another aspect, the processing circuit includes at least one dedicated hardware 93.

[0053] When the processing circuit includes the processor 91 and the memory 92, each function is realized by software, firmware, or a combination of software and firmware. At least one of the software and the firmware is described as a program. At least one of the software and the firmware is stored in the memory 92. The processor 91 realizes each function by reading and executing the program stored in the memory 92. That is, the inverter control unit 15 can also be realized by a computer and a program. Note that the program can be stored in a storage medium or provided through a network.

[0054] When the processing circuit includes dedicated hardware 93, the processing circuit is, for example, a single circuit, a composite circuit, a programmed processor, or a combination thereof. Each function of the inverter control unit 15 may be partially or entirely configured by the hardware 93, or may be partially or entirely configured as a program executed by the processor 91.

[0055] The program executed by the processor 91 can be said to be configured to cause the processor 91 to execute at least the following first to third processes. The first process is a process of causing a plurality of normal cell inverters 13 to perform a multi-level operation during normal times when bearing electrolytic corrosion is not detected. The second process is a process of causing a plurality of normal cell inverters 13 and the spare cell inverter 14 to perform a multi-level operation when bearing electrolytic corrosion is detected. The third process is a process of making the DC voltage borne by each cell inverter at the time of detection of bearing electrolytic corrosion lower than the DC voltage borne by the normal cell inverter 13 during normal times in the multi-level operation.

[0056] The processor 91 may be, for example, a CPU (Central Processing Unit), RISC (Reduced Instruction Set Computer), DSP (Digital Signal Processor), FPGA (Field-Programmable Gate Array), ASIC (Application Specific Integrated Circuit), PLD (Programmable Logic Device), or another processing unit. Further, the processor 91 may be a combination of two or more of them.

[0057] The memory 92 is a volatile or non-volatile storage medium such as an HDD (Hard Disk Drive), SSD (Solid State Drive), DRAM (Dynamic Random Access Memory), or semiconductor memory. Further, the memory 92 may be a combination of two or more of them. The memory 92 is connected to each part of the inverter control unit 15 via, for example, a bus (not shown) so that various information can be input and output. The memory 92 stores, for example, programs necessary for the operation of each part of the inverter control unit 15, and various information is written and read by each part of the inverter control unit 15.

[0058] The memory 92 may be provided outside the inverter control unit 15 and connected to the inverter control unit 15 by wire or wirelessly. The memory 92 may be an external storage medium such as a memory card, DVD (Digital Versatile Disc), or the like, or may be online storage or the like.

[0059] Further, the present embodiment can also be realized as a control method for the inverter device 11 in which the above first to third processes are performed.

[0060] The above-described modifications can be appropriately applied to the inverter device according to the following embodiments. Note that since the inverter device according to the following embodiments has many common points with those of Embodiment 1, the description will focus on the differences from Embodiment 1.

[0061] Embodiment 2. FIG. 7 is a diagram for explaining the configuration of the inverter device 211 according to Embodiment 2. In this embodiment, a ZCT (Zero-phase Current Transformer) 217 is connected to the outputs of a plurality of phases of the MMC 12, and the inverter control unit 15 is replaced by an inverter control unit 15A, which is different from Embodiment 1. Other configurations are the same as those of Embodiment 1 and the description thereof is omitted. The ZCT 217 is commonly connected to the outputs of the R phase, S phase, and T phase. The ZCT 217 detects a zero-phase current and transmits a zero-phase current detection signal S7 to the inverter control unit 15A.

[0062] FIG. 8 is a flowchart for explaining the operation of the inverter control unit 15A according to Embodiment 2. The operations in steps S101, S102, S104, and the No in step S114 are the same as those in Embodiment 1. Normally, the inverter control unit 15A causes a plurality of normal cell inverters 13 to perform a multi-level operation, and the spare cell inverter 14 is not involved in the multi-level operation. In step S114, when the corrosion detection signal S5 is not received, or when the amount of iron powder notified by the corrosion detection signal S5 is smaller than a predetermined value, the operation is the same as that in Embodiment 1, and the inverter control unit 15A maintains the normal state.

[0063] Next, if the electric erosion detection signal S5 is received in step S114, or if the amount of iron powder notified by the electric erosion detection signal S5 is equal to or greater than a predetermined value, the process proceeds to step S126. In step S126, even when bearing electric erosion is detected, the inverter control unit 15A continues the multi-level operation with the DC voltage set to the first voltage set value for the plurality of normal cell inverters 13, and operates so that the MMC12 output current follows a predetermined current. Further, the inverter control unit 15A controls each phase's spare cell inverter 14 based on the zero-phase current detection signal S7 so that the zero-phase detected current approaches zero.

[0064] By setting the output of ZCT217 to zero, the common mode current can be canceled. In the operation of reducing the zero-phase detected current in step S126, for example, the three-phase spare cell inverters 14 are switched at the same timing to maintain the three-phase balanced state. Thereby, the influence on the operation of the motor 21 can be suppressed.

[0065] At this time, the converter section 32 of the spare cell inverter 14 may be controlled so that the DC voltage of the spare cell inverter 14 follows a third voltage set value different from the first voltage set value. The third voltage set value may be set so that the output voltage of the MMC12 does not exceed the allowable input voltage of the motor 21. Alternatively, when a normal cell inverter 13 of a certain phase is outputting the peak voltage of the phase voltage in the multi-level operation, the spare cell inverter 14 may be controlled to output a zero voltage so that the peak voltage of each phase of the MMC12 does not exceed the allowable voltage of the motor 21. In this case, a period during which the zero-phase current suppression operation by the spare cell inverter 14 is temporarily stopped every 60 degrees of the electrical angle of the motor 21 will occur, but overall in one cycle, the common mode current will be suppressed. Note that the carrier frequency of the inverter section 36 of the spare cell inverter 14 in step S126 may be a frequency capable of suppressing the common mode current.

[0066] According to this embodiment, the spare cell inverter 14 is operated so as to reduce the zero-phase detection current. Thereby, the common-mode current can be suppressed, and the progress of bearing electrolytic corrosion can be suppressed. In this embodiment, since the common-mode current can be suppressed by a method other than reducing the carrier frequency of the inverter unit 36, the influence of the harmonic current ripple on the load side described above can be suppressed.

[0067] Also in this embodiment, each function of the inverter control unit 15A can be realized by the processing circuit shown in FIG. 6. In this case, it can be said that the program executed by the processor 91 is configured to cause the processor 91 to execute at least the following first and second processes. The first process is a process of causing a plurality of normal cell inverters 13 to perform a multi-level operation. The second process is a process of controlling the spare cell inverters of each phase of the MMC 12 so that the detection current of the ZCT 217 approaches zero when bearing electrolytic corrosion is detected.

[0068] Further, this embodiment can also be realized as a control method of an inverter device in which the above-described first and second processes are performed.

[0069] The technical features described in each embodiment may be used in appropriate combination.

Explanation of Reference Numerals

[0070] 6 Input transformer, 10 AC power supply, 11 Inverter device, 12 MMC, 13 Normal cell inverter, 14 Spare cell inverter, 15, 15A Inverter control unit, 21 Electric motor, 22 Bearing electrolytic corrosion detection unit, 31 Fuse, 32 Converter unit, 32a Switching element, 32b Anti-parallel diode, 32c Gate drive circuit, 33 Voltage detection circuit, 34 Capacitor, 35 Fuse, 36 Inverter unit, 36a Switching element, 36b Anti-parallel diode, 36c Gate drive circuit, 37 Output short-circuit circuit, 38 Control circuit, 91 Processor, 92 Memory, 93 Hardware, 211 Inverter device, P1 Input terminal, P2 Output terminal, Q1~Q4 Switching elements

Claims

1. It has a plurality of cell inverters connected in series, and the plurality of cell inverters include a modular multilevel converter including a plurality of normal cell inverters and a spare cell inverter, a control unit for controlling the modular multilevel converter, and is provided with, The control unit, when the bearing electrolytic corrosion of the motor driven by the modular multilevel converter is not detected, causes the plurality of normal cell inverters to perform a multilevel operation, when detecting the bearing electrolytic corrosion, causes the plurality of normal cell inverters and the spare cell inverter to perform the multilevel operation, In the multilevel operation, the DC voltage borne by each cell inverter of the plurality of cell inverters at the time of detecting the bearing electrolytic corrosion is lower than the DC voltage borne by each normal cell inverter of the plurality of normal cell inverters during normal times. An inverter device characterized by this.

2. The control unit reduces the carrier frequency of the inverter section of the modular multilevel converter when detecting the bearing electrolytic corrosion, as compared with normal times. The inverter device according to claim 1.

3. The bearing electrolytic corrosion is detected based on the amount of iron powder contained in the grease of the bearing of the motor. The inverter device according to claim 1 or 2.

4. The amplitude of the multilevel waveform output by the multilevel operation is the same during normal times and at the time of detecting the bearing electrolytic corrosion, During normal times, the spare cell inverter does not participate in the multilevel operation. The inverter device according to claim 1 or 2.

5. Each of a plurality of phases has a plurality of cell inverters connected in series, and the plurality of cell inverters include a modular multilevel converter including a plurality of normal cell inverters and a spare cell inverter, A control unit that causes the plurality of normal cell inverters to perform a multi-level operation, is provided, When the control unit detects bearing electrolytic corrosion of an electric motor driven by the modular multi-level converter, the control unit controls the normal cell inverter and the spare cell inverter so that the detected current of a zero-phase current transformer connected to outputs of the plurality of phases of the modular multi-level converter approaches zero. An inverter device characterized by this.

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