Power conversion device, control device, and control method

Abstract

A technology is provided that can suppress the switching frequency of power conversion cells driven by the PD method in a multilevel power conversion device in which the output sections of multiple power conversion cells are connected in series for each of multiple phases. [Solution] A power conversion device 30 according to one embodiment of the present disclosure comprises a three-phase single-phase AC output unit 31X, each phase of which includes N power conversion cells 100, and a central control unit 33. The central control unit 33 drives all or some of the N power conversion cells 100 using the PD method, thereby gradually superimposing the outputs of all or some of the N power conversion cells 100, and when the number of usable power conversion cells 100 in each of the three-phase single-phase AC output units 31X is not the same, selectively uses a cell balance mode in which only the same number of power conversion cells 100 are used in each of the three-phase single-phase AC output units 31X, or a cell unbalance mode in which all usable power conversion cells 100 are used.

Description

[Technical Field]

[0001] The present disclosure relates to a power conversion device and the like. [Background technology]

[0002] For example, a multilevel power conversion device is known in which output sections of a predetermined number of power conversion cells, two or more, are connected in series for each of a plurality of phases (see Patent Document 1).

[0003] Patent Document 1 discloses a method for maintaining a balanced state of the line voltages and continuing to output AC voltages even when a power conversion cell in a certain phase becomes unusable due to a failure or the like, even though the number of power conversion cells used in the phase in question and the number of power conversion cells used in other phases remain in a state of mismatch. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 4553167 Summary of the Invention [Problem to be solved by the invention]

[0005] Incidentally, in the multilevel power conversion device as described above, a PD (Phase Disposition) method (also called a "level shift method") may be adopted in which the power conversion cells are driven based on a comparison between a carrier wave (triangular wave) having the same frequency and phase but with a shifted voltage level and a voltage command value for each predetermined number of power conversion cells.

[0006] However, when the PD method is adopted, if a power conversion cell in a certain phase becomes unusable and the number of power conversion cells used in the target phase does not match the number of power conversion cells used in other phases, it becomes necessary to increase the switching frequency of the target phase.

[0007] In view of the above problems, an object of the present invention is to provide a technology capable of suppressing the switching frequency of power conversion cells driven by the PD method in a multilevel power conversion device in which the output sections of multiple power conversion cells are connected in series for each of multiple phases. [Means for solving the problem]

[0008] In order to achieve the above object, in one embodiment of the present disclosure, a plurality of single-phase AC output units each outputting a single-phase AC; a control unit, each of the multiple-phase single-phase AC output units includes a predetermined number (two or more) of power conversion cells that converts power supplied from an external power supply into single-phase AC of a predetermined voltage and frequency and outputs the converted power; the output units of the predetermined number of power conversion cells are connected in series, thereby enabling single-phase AC to be output by superimposing outputs of the predetermined number of power conversion cells; and the multiple-phase single-phase AC output unit includes a short-circuit unit that is capable of short-circuiting the output unit for each of the predetermined number of power conversion cells; and by short-circuiting the short-circuit units corresponding to some of the power conversion cells among the predetermined number of power conversion cells, it is possible to output single-phase AC by superimposing outputs of only the remaining power conversion cells; The control unit controls the plurality of phases. phase for each AC output unit, based on a comparison between a voltage command value and triangular waves having the same frequency and phase as each other and shifted in voltage level, all or some of the predetermined number of power conversion cells are driven, thereby superimposing the outputs of all or some of the predetermined number of power conversion cells so as to be changed in a stepwise manner; the control unit may drive the power conversion cell by unipolar modulation based on a comparison between the triangular wave and the voltage command value, or may drive the power conversion cell by dipolar modulation based on a comparison between the triangular wave and the voltage command value, When the number of usable power conversion cells for each of the plurality of single-phase AC output units is not the same, When the power conversion cell is driven by the dipolar modulation, Each of the plurality of single-phase AC output units is connected to the plurality of single-phase AC output units. phase a first control state in which single-phase AC is output by superimposing only the outputs of the power conversion cells in a number corresponding to the minimum value of the number of power conversion cells usable for each AC output unit; Use , When the power conversion cell is driven by the unipolar modulation,The phase differences between the line voltages of adjacent phases among the plurality of phases are all the same. phase a second control state in which each of the AC output units is caused to output a single-phase AC by superimposing the outputs of all the power conversion cells that can be used; Use your attitude Use, A power converter is provided.

[0009] In another embodiment of the present disclosure, a plurality of single-phase AC output units each outputting a single-phase AC; Each of the multiple-phase single-phase AC output units includes a predetermined number (two or more) of power conversion cells that converts power supplied from an external power source into single-phase AC of a predetermined voltage and frequency and outputs the converted power, and the output units of the predetermined number of power conversion cells are connected in series, thereby enabling single-phase AC to be output by superimposing outputs of the predetermined number of power conversion cells, and includes a short-circuit unit that can short-circuit the output unit for each of the predetermined number of power conversion cells, and by short-circuiting the short-circuit unit corresponding to some of the power conversion cells among the predetermined number of power conversion cells, it is possible to output single-phase AC by superimposing outputs of only the remaining power conversion cells. A control device for controlling a power conversion device, The multiple phase single phase for each AC output unit, based on a comparison between a voltage command value and triangular waves having the same frequency and phase as each other and shifted in voltage level, all or some of the predetermined number of power conversion cells are driven, thereby superimposing the outputs of all or some of the predetermined number of power conversion cells so as to be changed in a stepwise manner; a case where the power conversion cell is driven by unipolar modulation based on a comparison between the triangular wave and the voltage command value, and a case where the power conversion cell is driven by dipolar modulation based on a comparison between the triangular wave and the voltage command value, When the number of usable power conversion cells for each of the single-phase AC output units of the plurality of phases is not all the same, When the power conversion cell is driven by the dipolar modulation, Each of the plurality of single-phase AC output units is connected to the plurality of single-phase AC output units. phase a first control state in which single-phase AC is output by superimposing only the outputs of the power conversion cells in a number corresponding to the minimum value of the number of power conversion cells usable for each AC output unit; Use , When the power conversion cell is driven by the unipolar modulation,The phase differences between the line voltages of adjacent phases among the plurality of phases are all the same. phase a second control state in which each of the AC output units is caused to output a single-phase AC by superimposing the outputs of all the power conversion cells that can be used; Use your attitude Use, A control device is provided.

[0010] In still another embodiment of the present disclosure, a plurality of single-phase AC output units each outputting a single-phase AC; Each of the multiple-phase single-phase AC output units includes a predetermined number (two or more) of power conversion cells that converts power supplied from an external power source into single-phase AC of a predetermined voltage and frequency and outputs the converted power, and the output units of the predetermined number of power conversion cells are connected in series, thereby enabling single-phase AC to be output by superimposing outputs of the predetermined number of power conversion cells, and includes a short-circuit unit that can short-circuit the output unit for each of the predetermined number of power conversion cells, and by short-circuiting the short-circuit unit corresponding to some of the power conversion cells among the predetermined number of power conversion cells, it is possible to output single-phase AC by superimposing outputs of only the remaining power conversion cells. A control method for controlling a power conversion device, comprising: The multiple phase single phase for each AC output unit, based on a comparison between a voltage command value and triangular waves having the same frequency and phase as each other and shifted in voltage level, all or some of the predetermined number of power conversion cells are driven, thereby superimposing the outputs of all or some of the predetermined number of power conversion cells so as to be changed in a stepwise manner; a case where the power conversion cell is driven by unipolar modulation based on a comparison between the triangular wave and the voltage command value, and a case where the power conversion cell is driven by dipolar modulation based on a comparison between the triangular wave and the voltage command value, When the number of usable power conversion cells for each of the single-phase AC output units of the plurality of phases is not all the same, When the power conversion cell is driven by the dipolar modulation, Each of the plurality of single-phase AC output units is connected to the plurality of single-phase AC output units. phase a first control state in which single-phase AC is output by superimposing only the outputs of the power conversion cells in a number corresponding to the minimum value of the number of power conversion cells usable for each AC output unit; Use , When the power conversion cell is driven by the unipolar modulation,The phase differences between the line voltages of adjacent phases among the plurality of phases are all the same. phase a second control state in which each of the AC output units is caused to output a single-phase AC by superimposing the outputs of all the power conversion cells that can be used; Use your attitude Use, A control method is provided. [Effects of the Invention]

[0011] According to the above-described embodiment, it is possible to suppress the switching frequency of the power conversion cells driven by the PD method in a multilevel power conversion device in which the output sections of multiple power conversion cells are connected in series for each of multiple phases. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a diagram illustrating a configuration of an example of a drive system. [Figure 2] FIG. 2 is a diagram illustrating a configuration of an example of a power conversion cell. [Figure 3] FIG. 2 is a diagram illustrating a configuration of an example of a control system of a power conversion device. [Figure 4] FIG. 10 is a diagram showing a specific example of a carrier waveform. [Figure 5] FIG. 10 is a diagram illustrating a specific example of a modulation method. [Figure 6] FIG. 2 is a diagram illustrating an example of an operating state of a power conversion device. [Figure 7] 10 is a diagram illustrating an example (cell balancing mode) of a control method for a power conversion device when there is an unusable power conversion cell. FIG. [Figure 8] FIG. 10 is a diagram illustrating an example of a method for controlling usable and unused power conversion cells (surplus cells) in a cell balancing mode. [Figure 9] 10 is a diagram illustrating another example (cell imbalance mode) of a control method for a power conversion device when an unusable power conversion cell is present. FIG. [Figure 10] FIG. 10 is a flowchart illustrating an example of a modulation scheme switching method. [Figure 11]FIG. 10 is a flowchart illustrating an example of a control process of the power conversion device when there is an unusable power conversion cell. [Figure 12] FIG. 10 is a time chart illustrating an example of the operation of the power conversion device when there is an unusable power conversion cell. DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, an embodiment will be described with reference to the drawings.

[0014] [Drive system configuration] The configuration of a drive system 1 including a power conversion device 30 according to this embodiment will be described with reference to FIG.

[0015] FIG. 1 is a diagram showing an example of the configuration of a drive system 1. As shown in FIG.

[0016] As shown in FIG. 1, the drive system 1 includes a three-phase AC power supply 10, a converter 20, a power conversion device 30, and a load 40.

[0017] A three-phase AC power supply 10 supplies three-phase AC to a converter 20 .

[0018] The converter 20 converts the three-phase AC supplied from the three-phase AC power supply 10 into a plurality of sets of three-phase AC and outputs the converted three-phase AC. The plurality of sets of three-phase AC are supplied to the power conversion device 30. For example, the converter 20 is a polyphase transformer.

[0019] The power conversion device 30 converts the multiple sets of three-phase AC supplied from the converter 20 into three-phase AC of a predetermined frequency and voltage, specifically, U-phase, V-phase, and W-phase AC, and outputs the converted three-phase AC. The three-phase AC supplied from the power conversion device 30 is supplied to the load 40. In this way, the power conversion device 30 can drive the load 40.

[0020] The load 40 is operated by the three-phase AC supplied from the power conversion device 30. The load 40 is, for example, an induction motor or a synchronous motor.

[0021] [Configuration of power conversion device] The configuration of the power conversion device 30 according to this embodiment will be described with reference to FIGS. 2 to 5 in addition to FIG.

[0022] FIG. 2 is a diagram showing an example of the configuration of the power conversion cell 100. FIG. 3 is a diagram showing an example of the configuration of a control system of the power conversion device 30. FIG. 4 is a diagram showing a specific example of a carrier waveform. FIG. 4 includes FIG. 4A showing an example of a carrier waveform according to the embodiment and FIG. 4B showing a carrier waveform according to a comparative example. FIG. 5 is a diagram showing a specific example of a modulation method. FIG. 5 includes FIG. 5A showing an example of a modulation method and FIG. 5B showing another example of the modulation method.

[0023] As shown in Figures 1 to 3, the power conversion device 30 includes a plurality of input sections 30I, an output section 30O, a U-phase AC output section 31U, a V-phase AC output section 31V, a W-phase AC output section 31W, a cell control section 32, and an overall control section 33.

[0024] The input units 30I receive a plurality of sets of three-phase AC power supplied from the converter 20. That is, the input units 30I receive a plurality of sets of three-phase AC power supplied from the converter 20.

[0025] The output unit 30O outputs the three-phase AC generated by the power conversion device 30 to the outside. The output unit 30O is electrically connected to a load 40 outside the power conversion device 30. The output unit 30O includes a U-phase output unit 30OU that outputs a U-phase AC, a V-phase output unit 30OV that outputs a V-phase AC, and a W-phase output unit 30OW that outputs a W-phase AC.

[0026] The U-phase AC output unit 31U outputs a U-phase AC based on the three-phase AC input to the input unit 301. Inside the power conversion device 30, the U-phase AC output unit 31U is electrically connected to the U-phase output unit 30OU.

[0027] The V-phase AC output unit 31V outputs a V-phase AC based on the three-phase AC input to the input unit 30I. Inside the power conversion device 30, the V-phase AC output unit 31V is electrically connected to the V-phase output unit 30OV.

[0028] The W-phase AC output unit 31W outputs a W-phase AC based on the three-phase AC input to the input unit 301. Inside the power conversion device 30, the W-phase AC output unit 31W is electrically connected to the W-phase output unit 30OW.

[0029] Hereinafter, when the U-phase AC output unit 31U, the V-phase AC output unit 31V, and the W-phase AC output unit 31W are described without distinguishing between them, they may be referred to as "single-phase AC output unit 31X" for convenience.

[0030] The U-phase AC output unit 31U includes N power conversion cells 100 (N is an integer of 2 or more).

[0031] In the U-phase AC output unit 31U, three-phase AC is input from each of the N input units 30I to the N power conversion cells 100. Then, in the U-phase AC output unit 31U, each of the N power conversion cells 100 converts the three-phase AC input from the corresponding input unit 30I into a single-phase AC of a predetermined voltage and frequency.

[0032] The V-phase AC output section 31V includes N power conversion cells 100, the same number as the U-phase AC output section 31U.

[0033] In the V-phase AC output unit 31V, three-phase AC is input from each of the N input units 30I to the N power conversion cells 100, similar to the U-phase AC output unit 31U. Then, in the V-phase AC output unit 31V, the N power conversion cells 100 each convert the three-phase AC input from the corresponding input unit 30I into single-phase AC of a predetermined voltage and frequency.

[0034] The W-phase AC output section 31W includes N power conversion cells 100, the same number as each of the U-phase AC output section 31U and the V-phase AC output section 31V.

[0035] In the W-phase AC output unit 31W, three-phase AC is input from each of the N input units 30I to the N power conversion cells 100, similar to the U-phase AC output unit 31U and the V-phase AC output unit 31V. Then, in the W-phase AC output unit 31W, the N power conversion cells 100 each output the three-phase AC input from the corresponding input unit 30I as a single-phase AC of a predetermined voltage and frequency.

[0036] Since the (3×N) power conversion cells 100 included in the power conversion device 30 all have the same configuration and the same specifications, the configuration of one power conversion cell 100 will be representatively described.

[0037] For example, as shown in FIG. 2, the power conversion cell 100 includes an input unit 100I, an output unit 100O, a rectifier unit 110, a DC link unit 115, a smoothing unit 120, an inverter unit 130, and a short-circuit unit 140.

[0038] The input unit 100I is electrically connected to the input unit 30I, and the three-phase AC supplied from the converter 20 is input to the input unit 100I via the input unit 30I. In other words, the input unit 100I receives the three-phase AC from the converter 20 via the input unit 100I.

[0039] The output unit 100O outputs the single-phase AC power generated by the power conversion cell 100.

[0040] The rectifier 110 converts the three-phase AC input through the input unit 100I into DC and outputs it to the DC link unit 115.

[0041] The DC link unit 115 electrically connects the rectifier unit 110 and the inverter unit 130. The DC link unit 115 includes a positive line PL and a negative line NL.

[0042] The smoothing unit 120 smoothes the DC of the DC link unit 115 .

[0043] The inverter unit 130 converts the DC from the DC link unit 115, which has been smoothed by the smoothing unit 120, into a single-phase AC of a predetermined voltage and frequency, and outputs it to the outside via the output unit 100O.

[0044] 2, the rectifier 110 includes, for example, a diode bridge circuit. The diode bridge circuit is a bridge-type full-wave rectifier circuit, and includes input lines ILR, ILS, and ILT and six rectifier diodes RD.

[0045] Three-phase AC currents of R phase, S phase, and T phase are input to the input lines ILR, ILS, and ILT, respectively, via the input section 100I.

[0046] The diode bridge circuit includes three sets of series-connected bodies (legs) of two rectifier diodes RD, and the three sets of legs are connected in parallel between the positive line PL and the negative line NL so that the forward direction of the rectifier diodes RD is from the negative line NL to the positive line PL. The tips of the input lines ILR, ILS, and ILT are connected to the midpoints of the two rectifier diodes RD in each of the three sets of legs.

[0047] For example, as shown in FIG. 2, the smoothing unit 120 includes a smoothing capacitor SC.

[0048] One of the two electrodes of the smoothing capacitor SC is electrically connected to the positive line PL, and the other is electrically connected to the negative line PL. The smoothing capacitor SC suppresses and smoothes the pulsation of the DC of the DC link unit 115 by repeatedly charging and discharging as appropriate.

[0049] For example, as shown in Fig. 2, one smoothing capacitor SC is provided. Alternatively, a plurality of smoothing capacitors SC may be provided. In the latter case, the plurality of smoothing capacitors SC may be connected in parallel or in series between the positive line PL and the negative line NL. Alternatively, the plurality of smoothing capacitors SC may be provided in such a manner that two or more series-connected bodies of two or more smoothing capacitors SC are connected in parallel between the positive line PL and the negative line NL.

[0050] Furthermore, in addition to the smoothing capacitor SC, a reactor may be provided in the smoothing unit 120. The reactor is provided, for example, on the positive line PL, and smoothes the DC of the DC link unit 115 while generating a voltage that appropriately prevents changes in the current.

[0051] For example, as shown in Fig. 2, the inverter unit 130 includes a full-bridge inverter circuit. The full-bridge inverter circuit includes four semiconductor switches SW, four freewheeling diodes FD, and output lines OL1 and OL2.

[0052] The full-bridge inverter circuit includes two sets of series-connected bodies (switch legs) of two semiconductor switches SW, and the two sets of switch legs are connected in parallel between a positive line PL and a negative line NL.

[0053] The semiconductor switch SW is primarily made of silicon (Si), for example. Alternatively, the semiconductor switch SW may be primarily made of a wide bandgap semiconductor material, such as silicon carbide (SiC), gallium nitride (GaN), gallium oxide (Ga2O3), or carbon (diamond (C)).

[0054] The freewheeling diode FD is connected in parallel to the semiconductor switch SW so that the forward direction is from the negative line NL to the positive line PL.

[0055] The output lines OL1 and OL2 output single-phase AC of a predetermined voltage and frequency generated by the full-bridge inverter circuit to the outside through the output unit 100O. The output line OL1 is drawn from the midpoint between the two semiconductor switches SW (i.e., upper and lower arms) in one of the two switch legs and electrically connected to the output unit 100O. The output line OL2 is drawn from the midpoint between the two semiconductor switches SW (i.e., upper and lower arms) in the other switch leg and electrically connected to the output unit 100O.

[0056] The short-circuiting unit 140 switches between a cut-off state in which the output lines OL1 and OL2 are electrically disconnected and a short-circuited state in which they are electrically connected. In this way, the short-circuiting unit 140 can short-circuit the output lines OL1 and OL2. Therefore, by short-circuiting the output lines OL1 and OL2, the short-circuiting unit 140 can prevent voltage from being output from the power conversion cell 100. Unless otherwise specified, the following description will be given assuming that the short-circuiting unit 140 is in the cut-off state.

[0057] In the single-phase AC output unit 31X, the output units 100O of the N power conversion cells 100 are connected in series. Hereinafter, the N power conversion cells 100 whose output units 100O are connected in series may be referred to as the "first-stage power conversion cell 100," the "second-stage power conversion cell 100," ..., and the "Nth-stage power conversion cell 100," starting from the power conversion cell 100 at one end opposite the load 40.

[0058] The output line OL1 of the first-stage power conversion cell 100 is connected to the neutral point 30NP through the output section 100O, and the output line OL2 is connected to the output line OL1 of the second-stage power conversion cell 100 through the output section 100O.

[0059] For the power conversion cells 100 in the second to (N-1)th stages, the output line OL1 is connected to the output line OL2 of the power conversion cell 100 in the preceding stage, and the output line OL2 is connected to the output line OL1 of the power conversion cell 100 in the subsequent stage.

[0060] For the Nth-stage power conversion cell 100, the output line OL1 is connected to the output line OL2 of the (N-1)th-stage power conversion cell 100 through the output unit 100O, and the output line OL2 is connected to the output unit 30O through the output unit 100O. Specifically, the output line OL2 of the Nth-stage power conversion cell 100 in the U-phase AC output unit 31U is connected to the U-phase output unit 30OU, the output line OL2 of the Nth-stage power conversion cell 100 in the V-phase AC output unit 31V is connected to the V-phase output unit 30OV, and the output line OL2 of the Nth-stage power conversion cell 100 in the W-phase AC output unit 31W is connected to the W-phase output unit 30OW.

[0061] The single-phase AC output unit 31X is configured such that the output units 100O of N power conversion cells 100 are connected in series, thereby superimposing the outputs of the power conversion cells 100 in stages and outputting a relatively high voltage single-phase AC (specifically, U-phase, V-phase, or W-phase AC).

[0062] Furthermore, when some of the N power conversion cells 100 are unusable, the single-phase AC output unit 31X can output single-phase AC (specifically, U-phase, V-phase, or W-phase AC) by gradually superimposing the outputs of the remaining power conversion cells 100 by putting the short-circuit sections 140 of the unusable power conversion cells 100 into a short-circuit state.

[0063] The use of the power conversion cell 100 means a state in which the power conversion cell 100 outputs a voltage corresponding to a voltage command value. Cases in which the power conversion cell 100 is unusable include, for example, a case in which the power conversion cell 100 experiences an abnormality (i.e., a failure) that prevents it from continuing to operate normally, or a case in which an abnormality (e.g., an overheating state) occurs in which it is preferable to temporarily stop operation. In addition, cases in which the power conversion cell 100 is unusable may also include, for example, a case in which maintenance work is being performed on the power conversion cell 100, which is specified in advance or which suddenly becomes necessary for some reason.

[0064] In the following, a specific example where five power conversion cells 100 are connected in series in the single-phase AC output section 31X (that is, where N=5) may be described (see FIGS. 4 and 6 to 8).

[0065] The cell control unit 32 is provided for each of the (3×N) power conversion cells 100, and drives and controls the target power conversion cell 100 under the control of the general control unit 33.

[0066] 3, a group of N cell control units 32 out of the (3×N) cell control units 32 that control the power conversion cells 100 included in the U-phase AC output unit 31U may be referred to as a "U-phase control unit 32U." Similarly, a group of N cell control units 32 out of the (3×N) cell control units 32 that control the power conversion cells 100 included in the V-phase AC output unit 31V may be referred to as a "V-phase control unit 32V." Similarly, a group of N cell control units 32 out of the (3×N) cell control units 32 that control the power conversion cells 100 included in the W-phase AC output unit 31W may be referred to as a "W-phase control unit 32W."

[0067] The functions of the cell control unit 32 are realized by any hardware or any combination of hardware and software, etc. For example, the cell control unit 32 is mainly configured with a computer including a processor, a memory device, an auxiliary storage device, and an interface device.

[0068] The central control unit 33 controls the (3×N) cell control units 32 and controls the operation of each power conversion cell 100 via the cell control units 32, thereby controlling the overall operation of the power conversion device 30.

[0069] The functions of the overall control unit 33 are realized by any hardware or any combination of hardware and software, etc. For example, the overall control unit 33 is mainly configured with a computer including a processor, a memory device, an auxiliary storage device, and an interface device.

[0070] 4A, this embodiment employs a PD method in which N series-connected power conversion cells 100 are driven based on a comparison between a voltage command value and N (five in this example) triangular waves serving as carrier waves having the same frequency and phase but shifted in voltage level, and the power conversion cells 100 are connected in series. This allows the single-phase AC output unit 31X to output a single-phase AC signal by superimposing the output voltages of the N power conversion cells 100 in stages.

[0071] For example, as shown in FIG. 4B , the comparative example employs a phase-shift (PS) system in which the power conversion cell 100 is driven based on a comparison between N (five in this example) triangular carrier waves with the same voltage amplitude and frequency but shifted in phase and a voltage command value. In the PS system, the slopes of the carrier waves may be different at the same time. As a result, different switching operations may occur simultaneously between the phases (specifically, the semiconductor switch SW of one phase is turned on and the semiconductor switch SW of the other phase is turned off). This may result in a two-level voltage change (so-called “two-stage voltage jump”) in the difference between the voltages output from the single-phase AC output units 31X of two adjacent phases, i.e., the line voltage. This may result in, for example, accelerated insulation degradation of the load 40. Furthermore, the harmonic components of the line voltage may become relatively large.

[0072] In contrast, in the PD system, one power conversion cell 100 performs a switching operation during one cycle of the carrier wave out of N power conversion cells 100. Therefore, in this embodiment, by adopting the PD system, it is possible to suppress two-level voltage changes in the line voltage and harmonic components.

[0073] 3, for N power conversion cells 100 included in a U-phase AC output unit 31U, the central control unit 33 compares N triangular waves with shifted voltage levels with a U-phase voltage command value Vu*, and generates PWM (Pulse Width Modulation) signals for driving the semiconductor switches SW of each of the N power conversion cells 100. Then, the central control unit 33 outputs the PWM signals for each of the N power conversion cells 100 to a cell control unit 32 included in a U-phase control unit 32U that controls the corresponding power conversion cell 100.

[0074] Similarly, the central control unit 33 compares the N triangular waves with shifted voltage levels with the V-phase voltage command value Vv* for the N power conversion cells 100 included in the V-phase AC output unit 31V, and generates PWM signals for driving the semiconductor switches SW of each of the N power conversion cells 100. The central control unit 33 then outputs the PWM signals for each of the N power conversion cells 100 to the cell control unit 32 included in the V-phase control unit 32V and which controls the target power conversion cells 100.

[0075] Similarly, the central control unit 33 compares the N triangular waves with shifted voltage levels with the W-phase voltage command value Vw* for the N power conversion cells 100 included in the W-phase AC output unit 31W, and generates PWM signals for driving the semiconductor switches SW of each of the N power conversion cells 100. The central control unit 33 then outputs the PWM signals for each of the N power conversion cells 100 to the cell control unit 32 included in the W-phase control unit 32W that controls the target power conversion cells 100.

[0076] The general control unit 33 converts the voltage command value into a pulse width to generate a PWM signal. In this case, in this embodiment, the general control unit 33 selectively uses two modulation methods, unipolar modulation and dipolar modulation.

[0077] 5A, in unipolar modulation, the PWM signal includes one ON signal F51 with a polarity corresponding to the voltage command value in one cycle. In other words, when using unipolar modulation, the integrated control unit 33 generates a PWM signal including an ON signal F51 with a polarity and pulse width corresponding to the voltage command value.

[0078] As shown in FIG. 5B , in dipolar modulation, the PWM signal includes, in one cycle, an ON signal F52 with a polarity corresponding to the voltage command value and a bias signal F53 with an opposite polarity. In other words, when using dipolar modulation, the integrated control unit 33 generates a PWM signal including an ON signal F52 with a polarity corresponding to the voltage command value and a pulse width obtained by adding the pulse width of the bias signal F53 to the pulse width corresponding to the voltage command value, and a bias signal F53 with an opposite polarity. This allows the power conversion cell 100 to operate appropriately in accordance with the voltage command value, even when the voltage command value is relatively small and the pulse width corresponding to the voltage command value is shorter than the minimum on / off time of the semiconductor switch SW. Furthermore, because the pulse width corresponding to the voltage command value is realized by superimposing the ON signal F52 and the bias signal F53, the power conversion cell 100 can output a voltage corresponding to the voltage command value with higher accuracy, which is particularly suitable when the voltage command value is relatively small. On the other hand, in dipolar modulation, the switching frequency of the power conversion cell 100 is twice as high as in unipolar modulation.

[0079] Each of the N cell controllers 32 included in the U-phase controller 32U generates a control signal for controlling the switching of the semiconductor switch SW of the power conversion cell 100 to be controlled in accordance with the PWM signal input from the integrated controller 33, and outputs the control signal to the power conversion cell 100 to be controlled. A voltage is applied from the drive circuit to the gate or base of the semiconductor switch SW included in the power conversion cell 100 based on the control signal, and as a result, the semiconductor switch SW of the power conversion cell 100 can perform a switching operation in accordance with the PWM signal. Therefore, the U-phase AC output unit 31U can output a U-phase AC by gradually superimposing the outputs of the N power conversion cells 100. Furthermore, when the short-circuit units 140 included in some of the N power conversion cells 100 are short-circuited, the U-phase AC output unit 31U can output a U-phase AC by gradually superimposing the outputs of the remaining two or more power conversion cells 100.

[0080] Similarly, the N cell controllers 32 included in the V-phase controller 32V each generate a control signal for controlling the switching of the semiconductor switch SW of the power conversion cell 100 to be controlled in accordance with the PWM signal input from the integrated controller 33, and output the control signal to the power conversion cell 100 to be controlled. A voltage is applied from the drive circuit to the gate or base of the semiconductor switch SW included in the power conversion cell 100 based on the control signal, and as a result, the semiconductor switch SW of the power conversion cell 100 can perform a switching operation in accordance with the PWM signal. Therefore, the V-phase AC output unit 31V can output a V-phase AC current by gradually superimposing the outputs of the N power conversion cells 100. Furthermore, when the short-circuit units 140 included in some of the N power conversion cells 100 are short-circuited, the V-phase AC output unit 31V can output a V-phase AC current by gradually superimposing the outputs of the remaining two or more power conversion cells 100.

[0081] Similarly, the N cell controllers 32 included in the W-phase controller 32W each generate a control signal for controlling the switching of the semiconductor switch SW of the power conversion cell 100 to be controlled in accordance with the PWM signal input from the integrated controller 33, and output the control signal to the power conversion cell 100 to be controlled. A voltage is applied from the drive circuit to the gate or base of the semiconductor switch SW included in the power conversion cell 100 based on the control signal, and as a result, the semiconductor switch SW of the power conversion cell 100 can perform a switching operation in accordance with the PWM signal. Therefore, the W-phase AC output unit 31W can output a W-phase AC by gradually superimposing the outputs of the N power conversion cells 100. Furthermore, when the short-circuit units 140 included in some of the N power conversion cells 100 are short-circuited, the W-phase AC output unit 31W can output a W-phase AC by gradually superimposing the outputs of the remaining two or more power conversion cells 100.

[0082] [Basic operating conditions of power conversion equipment] The basic operating state of the power conversion device 30 will be described with reference to Fig. 6. Specifically, the operating state of the power conversion device 30 when all the power conversion cells 100 included in the power conversion device 30 are available and in use will be described.

[0083] Fig. 6 is a diagram illustrating an example of an operating state of the power conversion device 30. Fig. 6 includes Figs. 6A and 6B. Fig. 6A is a diagram illustrating an example of the switching frequency of the power conversion cell 100. Fig. 6B is a diagram illustrating an example of the magnitudes and phases of the U-phase voltage Vu, the V-phase voltage Vv, the W-phase voltage Vw, the U-phase line voltage Vuv, the V-phase line voltage Vvw, and the W-phase line voltage Vwu.

[0084] The U-phase voltage Vu is a phase voltage output from the U-phase AC output unit 31U. The V-phase voltage Vv is a phase voltage output from the V-phase AC output unit 31V. The W-phase voltage Vw is a phase voltage output from the W-phase AC output unit 31W. The U-phase line voltage Vuv is a voltage equivalent to the difference between the U-phase voltage Vu and the V-phase voltage Vv. The V-phase line voltage Vvw is a voltage equivalent to the difference between the V-phase voltage Vv and the W-phase voltage Vw. The W-phase line voltage Vwu is a voltage equivalent to the difference between the W-phase voltage Vw and the U-phase voltage Vu.

[0085] In this example, each of the U-phase AC output unit 31U, the V-phase AC output unit 31V, and the W-phase AC output unit 31W includes five power conversion cells 100 (i.e., N=5). The same applies to Figs. 7 and 8 described below. In this example, all of the power conversion cells 100 included in each of the U-phase AC output unit 31U, the V-phase AC output unit 31V, and the W-phase AC output unit 31W are available and in use.

[0086] When the PD method is adopted, the sum of the switching frequencies of the power conversion cells 100 in use included in each of the U-phase AC output unit 31U, the V-phase AC output unit 31V, and the W-phase AC output unit 31W (hereinafter referred to as the "total switching frequency" for convenience) must be the same.

[0087] When all the power conversion cells included in each of the U-phase AC output unit 31U, the V-phase AC output unit 31V, and the W-phase AC output unit 31W are available and in use, the U-phase AC output unit 31U, the V-phase AC output unit 31V, and the W-phase AC output unit 31W each use the same number of power conversion cells 100 with the same specifications (five in this example). In this case, as shown in FIG. 6A , if the switching frequencies of the power conversion cells 100 included in each of the U-phase AC output unit 31U, the V-phase AC output unit 31V, and the W-phase AC output unit 31W are the same, the total switching frequency will automatically be the same. In this example, the switching frequencies of the power conversion cells 100 included in each of the U-phase AC output unit 31U, the V-phase AC output unit 31V, and the W-phase AC output unit 31W are the same, 500 Hz, and as a result, the total switching frequency will be the same, 2500 (= 500 × 5) Hz.

[0088] Furthermore, the U-phase AC output unit 31U, the V-phase AC output unit 31V, and the W-phase AC output unit 31W use the same number (five in this example) of power conversion cells 100 with the same specifications, and therefore can output voltages of the same magnitude. Therefore, as shown in FIG. 6B , the U-phase AC output unit 31U, the V-phase AC output unit 31V, and the W-phase AC output unit 31W are controlled by the integrated control unit 33 so that the U-phase voltage Vu, the V-phase voltage Vv, and the W-phase voltage Vw are the same magnitude and the phase difference between adjacent phases is 120°. As a result, the integrated control unit 33 controls the U-phase voltage Vuv, the V-phase voltage Vvw, and the W-phase voltage Vwu so that they are the same magnitude and the phase difference between adjacent line voltages is 120°, thereby achieving a balanced state among the U-phase voltage Vuv, the V-phase voltage Vvw, and the W-phase voltage Vwu. Therefore, the power conversion device 30 can appropriately drive the load 40 under the control of the general control unit 33.

[0089] [Cell balancing mode] An example of a control method (cell balancing mode) for the power conversion device 30 when there is an unusable power conversion cell 100 will be described with reference to FIGS.

[0090] FIG. 7 is a diagram illustrating an example of a control method (cell balancing mode) of the power conversion device 30 when an unusable power conversion cell 100 is present. FIG. 7 includes FIGS. 7A and 7B. FIG. 7A is a diagram illustrating an example of the switching frequency of the power conversion cell 100 in the cell balancing mode. FIG. 7B is a diagram illustrating an example of the magnitudes and phases of the U-phase voltage Vu, V-phase voltage Vv, W-phase voltage Vw, UV line voltage Vuv, VW line voltage Vvw, and WU line voltage Vwu in the cell balancing mode. FIG. 8 is a diagram illustrating an example of a control method of usable but unused power conversion cells (surplus cells) in the cell balancing mode.

[0091] In the following, in this example, the description will be given on the assumption that the unusable power conversion cells 100 are unable to output voltage because the short-circuiting section 140 is short-circuited. Furthermore, in this example, unless otherwise specified, the description will be given of a case where there are unusable power conversion cells 100 and the numbers of usable power conversion cells 100 in the U-phase AC output section 31U, the V-phase AC output section 31V, and the W-phase AC output section 31W are not all the same.

[0092] The cell balancing mode is an example of a control mode in which the central control unit 33 controls the power conversion cells 100. In the cell balancing mode, the central control unit 33 equalizes the number of power conversion cells 100 used by each of the U-phase AC output unit 31U, the V-phase AC output unit 31V, and the W-phase AC output unit 31W. Specifically, in the cell balancing mode, the central control unit 33 equalizes the number of power conversion cells used by each of the U-phase AC output unit 31U, the V-phase AC output unit 31V, and the W-phase AC output unit 31W to the minimum number of power conversion cells 100 that can be used by each of them. In this case, for the single-phase AC output unit 31X among the U-phase AC output unit 31U, the V-phase AC output unit 31V, and the W-phase AC output unit 31W in which the number of usable power conversion cells 100 is not the minimum, the central control unit 33 intentionally disables use of the number of usable power conversion cells 100 that is the difference from the minimum value. Hereinafter, in the cell balancing mode, the power conversion cells 100 that are unused despite being usable may be referred to as "surplus cells" for convenience.

[0093] 7, all five power conversion cells 100 included in each of the V-phase AC output unit 31V and the W-phase AC output unit 31W are usable. On the other hand, of the five power conversion cells 100 included in the U-phase AC output unit 31U, the fifth-stage power conversion cell 100 is unusable, and only the remaining four are usable. Therefore, in the cell balancing mode, for each of the V-phase AC output unit 31V and the W-phase AC output unit 31W in which the number of usable power conversion cells 100 is not the minimum among the U-phase AC output unit 31U, the V-phase AC output unit 31V, and the W-phase AC output unit 31W, the central control unit 33 intentionally disables the number of power conversion cells 100 that is the difference from the minimum value (in this example, one cell). This allows the general control unit 33 to adjust the number of power conversion cells 100 used in each of the U-phase AC output unit 31U, the V-phase AC output unit 31V, and the W-phase AC output unit 31W to the number of power conversion cells 100 available in the U-phase AC output unit 31U (four).

[0094] For example, the surplus cell is fixed to a specific power conversion cell 100. Specifically, for example, as shown in Fig. 7, the fifth-stage power conversion cell 100 of each of the V-phase AC output unit 31V and the W-phase AC output unit 31W is set as the surplus cell. The surplus cell may also be changed as appropriate. For example, for the single-phase AC output unit 31X including the surplus cell, the central control unit 33 sequentially changes the surplus cell among the available power conversion cells 100 so that the number of switching operations among the available power conversion cells 100 is uniform.

[0095] 8, in the surplus cell, the semiconductor switches SW of both upper arms of the two switch legs are kept on and the semiconductor switches SW of both lower arms are kept off under the control of the integrated control unit 33 and the cell control unit 32. This makes it possible to fix the line voltage between the output lines OL1 and OL2 to 0 V, thereby making it possible to disable the surplus cell.

[0096] As described above, when the PD method is adopted, the total switching frequencies of the power conversion cells 100 in use included in each of the U-phase AC output unit 31U, the V-phase AC output unit 31V, and the W-phase AC output unit 31W must be the same.

[0097] In the cell balancing mode, the U-phase AC output unit 31U, the V-phase AC output unit 31V, and the W-phase AC output unit 31W each use the same number of power conversion cells 100 with the same specifications (four in this example). In this case, as shown in Fig. 7A, if the switching frequencies of the power conversion cells 100 included in each of the U-phase AC output unit 31U, the V-phase AC output unit 31V, and the W-phase AC output unit 31W are the same, the total switching frequency will automatically be the same. In this example, the switching frequencies of the power conversion cells 100 included in each of the U-phase AC output unit 31U, the V-phase AC output unit 31V, and the W-phase AC output unit 31W are the same, 500 Hz, and as a result, the total switching frequency is the same, 2000 (= 500 × 4) Hz.

[0098] In the cell balancing mode, the U-phase AC output unit 31U, the V-phase AC output unit 31V, and the W-phase AC output unit 31W use the same number (four in this example) of power conversion cells 100 with the same specifications, and therefore can output voltages of the same magnitude. Therefore, as shown in FIG. 7B , the U-phase AC output unit 31U, the V-phase AC output unit 31V, and the W-phase AC output unit 31W are controlled by the integrated control unit 33 so that the U-phase voltage Vu, the V-phase voltage Vv, and the W-phase voltage Vw are the same magnitude and the phase difference between adjacent phases is 120°. As a result, the integrated control unit 33 controls the U-phase voltage Vuv, the V-phase voltage Vvw, and the W-phase voltage Vwu so that they are the same magnitude and the phase difference between adjacent line voltages is 120°, thereby achieving a balanced state among the U-phase voltage Vuv, the V-phase voltage Vvw, and the W-phase voltage Vwu. Therefore, the power conversion device 30 can appropriately drive the load 40 under the control of the integrated control unit 33 in the cell balancing mode.

[0099] In this way, by making the number of power conversion cells 100 used by the U-phase AC output unit 31U, the V-phase AC output unit 31V, and the W-phase AC output unit 31W the same, the integrated control unit 33 can appropriately drive the load 40, although the output voltage decreases from the maximum value.

[0100] It is also possible that the U-phase AC output unit 31U, the V-phase AC output unit 31V, and the W-phase AC output unit 31W all have unusable power conversion cells 100, and the number of unusable power conversion cells 100 is the same. In this case, the U-phase AC output unit 31U, the V-phase AC output unit 31V, and the W-phase AC output unit 31W use the same number of power conversion cells 100 with the same specifications. Therefore, the integrated control unit 33 can control the power conversion cells 100 of the U-phase AC output unit 31U, the V-phase AC output unit 31V, and the W-phase AC output unit 31W by a control method similar to that in the cell balancing mode, except for control of surplus cells. As a result, the power conversion device 30 can appropriately drive the load 40.

[0101] [Cell imbalance mode] With reference to FIG. 9, another example (cell imbalance mode) of the control method for the power conversion device 30 when there is an unusable power conversion cell 100 will be described.

[0102] Fig. 9 is a diagram illustrating another example (cell imbalance mode) of a control method for the power conversion device 30 when an unusable power conversion cell 100 is present. Fig. 9 includes Figs. 9A and 9B. Fig. 9A is a diagram illustrating an example of the switching frequency of the power conversion cell 100 in the cell imbalance mode. Fig. 9B is a diagram illustrating an example of the magnitudes and phases of the U-phase voltage Vu, V-phase voltage Vv, W-phase voltage Vw, U-V line voltage Vuv, V-W line voltage Vvw, and W-U line voltage Vwu in the cell imbalance mode.

[0103] In the following, in this example, the description will be given on the assumption that the unusable power conversion cells 100 are unable to output voltage because the short-circuiting section 140 is short-circuited. Furthermore, in this example, unless otherwise specified, the description will be given of a case where there are unusable power conversion cells 100 and the numbers of usable power conversion cells 100 in the U-phase AC output section 31U, the V-phase AC output section 31V, and the W-phase AC output section 31W are not all the same.

[0104] In the cell imbalance mode, the central control unit 33 uses all of the available power conversion cells 100 included in the power conversion device 30. Therefore, the number of power conversion cells 100 used by each of the U-phase AC output unit 31U, the V-phase AC output unit 31V, and the W-phase AC output unit 31W is not all the same.

[0105] 9, all five power conversion cells 100 included in each of the V-phase AC output unit 31V and the W-phase AC output unit 31W are usable. On the other hand, of the five power conversion cells 100 included in the U-phase AC output unit 31U, the fifth-stage power conversion cell 100 is unusable, and only the remaining four are usable. Therefore, in the cell imbalance mode, the U-phase AC output unit 31U outputs U-phase AC using four power conversion cells 100, while the V-phase AC output unit 31V and the W-phase AC output unit 31W each output V-phase and W-phase AC using five power conversion cells 100.

[0106] As described above, when the PD method is adopted, the total switching frequencies of the power conversion cells 100 in use included in each of the U-phase AC output unit 31U, the V-phase AC output unit 31V, and the W-phase AC output unit 31W must be the same.

[0107] In the cell imbalance mode, the U-phase AC output unit 31U, the V-phase AC output unit 31V, and the W-phase AC output unit 31W do not all use the same number of power conversion cells 100. Therefore, the fewer the number of usable power conversion cells 100 for each of the U-phase AC output unit 31U, the V-phase AC output unit 31V, and the W-phase AC output unit 31W, the higher the switching frequency of the power conversion cells 100.

[0108] For example, as shown in Fig. 9A, the V-phase AC output unit 31V and the W-phase AC output unit 31W can each use all five power conversion cells 100, and therefore, as in the case of Fig. 7A, the switching frequency of the power conversion cells 100 included in each is 500 Hz. On the other hand, the U-phase AC output unit 31U can only use four of the five power conversion cells 100, and therefore, the switching frequency of the power conversion cells 100 included in the U-phase AC output unit 31U is 625 (= 500 × 5 / 4) Hz, which is higher than the V-phase AC output unit 31V and the W-phase AC output unit 31W.

[0109] In the cell imbalance mode, the U-phase AC output unit 31U, the V-phase AC output unit 31V, and the W-phase AC output unit 31W do not all use the same number of power conversion cells 100, and therefore the magnitudes of the voltages of each phase are not all the same. Therefore, the integrated control unit 33 adjusts the phases of the U-phase voltage Vu, the V-phase voltage Vv, and the W-phase voltage Vw so that the U-phase voltage Vuv, the V-phase voltage Vvw, and the W-phase voltage Vwu are in a balanced state, that is, so that the voltages have the same magnitude and the phase differences between adjacent line voltages are all the same, 120°.

[0110] 9B , the central control unit 33 controls the U-phase AC output unit 31U, the V-phase AC output unit 31V, and the W-phase AC output unit 31W so that the phase difference between the U-phase voltage Vu and each of the V-phase voltage Vv and the W-phase voltage Vw is 126.5° and the phase difference between the V-phase voltage Vv and the W-phase voltage Vw is 117°. This allows the central control unit 33 to achieve a balanced state among the U-phase line voltage Vuv, the V-phase line voltage Vvw, and the W-phase line voltage Vwu. Therefore, the power conversion device 30 can appropriately drive the load 40 under the control of the central control unit 33 in the cell imbalance mode.

[0111] In this way, the central control unit 33 can appropriately drive the load 40 using all available power conversion cells 100 by appropriately adjusting the phase of the U-phase voltage Vu and the phases of the V-phase voltage Vv and W-phase voltage Vw.

[0112] [How to switch modulation methods] An example of a modulation scheme switching method will be described with reference to FIG.

[0113] FIG. 10 is a flow chart illustrating an example of a modulation scheme switching method.

[0114] The flowchart of FIG. 10 is repeatedly executed at predetermined processing intervals when the power conversion device 30 is in operation.

[0115] 10 , in step S102, the central control unit 33 acquires the output voltage Vo of the power conversion device 30. The output voltage Vo of the power conversion device 30 is, for example, an effective value of a sine wave corresponding to the voltage command value V* of the power conversion device 30, and is set based on the operating conditions of the load 40.

[0116] When the process of step S102 is completed, the central control unit 33 proceeds to step S104.

[0117] In step S104, the overall control unit 33 determines whether the output voltage Vo acquired in step S102 is equal to or greater than the threshold value Vth. If the output voltage Vo is equal to or greater than the threshold value Vth, the overall control unit 33 proceeds to step S106, and if the output voltage Vo is less than the threshold value Vth, the overall control unit 33 proceeds to step S108.

[0118] In step S104, the central control unit 33 may determine whether the output voltage Vo is greater than a threshold value Vth.

[0119] In step S106, the central control unit 33 generates a PWM signal for the power conversion cell 100 by dipolar modulation.

[0120] On the other hand, in step S108, the central control unit 33 generates a PWM signal for the power conversion cell 100 by unipolar modulation.

[0121] When the process of step S106 or step S108 is completed, the central control unit 33 ends the process of this flowchart.

[0122] In this way, the general control unit 33 uses unipolar modulation when the output voltage Vo of the power conversion device 30 is relatively large, and uses dipolar modulation when the output voltage Vo of the power conversion device 30 is relatively small. This allows the general control unit 33 to improve the accuracy of the actual output voltage with respect to the voltage command value when the output voltage of the power conversion device 30 is relatively small, and to suppress the switching frequency when the output voltage of the power conversion device 30 is relatively large.

[0123] [Control process of power conversion device when there is an unusable power conversion cell] 11, a description will be given of the control processing of the power conversion device 30 when there is an unusable power conversion cell 100. Specifically, a description will be given of the control processing of the power conversion device 30 when there is an unusable power conversion cell 100 and the numbers of usable power conversion cells 100 in the U-phase AC output unit 31U, the V-phase AC output unit 31V, and the W-phase AC output unit 31W are not all the same.

[0124] In the following, in this example, the explanation will be given on the assumption that the cell unbalanced mode is used in the case of unipolar modulation, and the cell balanced mode is used in the case of dipolar modulation.

[0125] FIG. 11 is a flowchart showing an example of a control process for the power conversion cells 100 when an unusable power conversion cell 100 is present.

[0126] The flowchart of Figure 11 is repeatedly executed, for example, when there is an unusable power conversion cell 100 and the numbers of usable power conversion cells 100 in the U-phase AC output unit 31U, the V-phase AC output unit 31V, and the W-phase AC output unit 31W are not all the same.

[0127] As shown in Fig. 11, in step S202, the overall control unit 33 determines whether or not the modulation method has been changed in the most recent processing of the flowchart in Fig. 10. If the modulation method has been changed, the overall control unit 33 proceeds to step S204, and if the modulation method has not been changed, the overall control unit 33 proceeds to step S210.

[0128] In step S204, the central control unit 33 changes the carrier frequency of the power conversion device 30 to the total switching frequency of a predetermined phase in accordance with the changed control mode. The predetermined phase is, for example, one of the U phase, V phase, and W phase in which the total switching frequency is the maximum in the cell imbalance mode (i.e., the phase in which the number of usable power conversion cells 100 included in the single-phase AC output unit 31X is the maximum).

[0129] For example, when switching from the cell balance mode of Fig. 7 to the cell unbalance mode of Fig. 9, the central control unit 33 increases the total switching frequency of the V-phase and W-phase from the state (2000 Hz) in Fig. 7A to the state (2500 Hz) in Fig. 9A, thereby increasing the carrier frequency of the power conversion device 30 from 2000 Hz to 2500 Hz. Also, when switching from the cell unbalance mode of Fig. 9 to the cell balance mode of Fig. 7, the central control unit 33 decreases the total switching frequency of the V-phase and W-phase from the state (2500 Hz) in Fig. 9A to the state (2000 Hz) in Fig. 7A, thereby decreasing the carrier frequency of the power conversion device 30 from 2500 Hz to 2000 Hz.

[0130] When the process of step S204 is completed, the overall control unit 33 proceeds to step S206.

[0131] In step S206, the overall control unit 33 determines whether the change of the carrier frequency has been completed, i.e., whether the carrier frequency has increased or decreased to the changed set value. If the change of the carrier frequency has been completed, the overall control unit 33 proceeds to step S208, and if the change of the carrier frequency has not been completed, the overall control unit 33 repeats the processing of this step until the change of the carrier frequency is completed.

[0132] In step S208, the central control unit 33 changes the control mode.

[0133] As a result, the central control unit 33 starts controlling the power conversion cells 100 of the power conversion device 30 in the changed control mode.

[0134] On the other hand, in step S210, the central control unit 33 maintains the current control mode.

[0135] As a result, the central control unit 33 continues to control the power conversion cells 100 of the power conversion device 30 in the current control mode.

[0136] When the process of step S208 or step S210 is completed, the central control unit 33 ends the process of this flowchart.

[0137] In this way, the overall control unit 33 can switch the control mode in accordance with the change in modulation method, such as selecting the cell balanced mode in the case of dipolar modulation and selecting the cell unbalanced mode in the case of unipolar modulation. Therefore, the overall control unit 33 can suppress a situation in which, for example, dipolar modulation and cell unbalanced mode are selected in combination, causing a further increase in the switching frequency of the semiconductor switch SW, and can suppress the switching frequency of the semiconductor switch SW.

[0138] [Operation of the power conversion device when there is an unavailable power conversion cell] 12, the operation of the power conversion device 30 when there is an unusable power conversion cell 100 will be described. Specifically, the operation of the power conversion device 30 when there is an unusable power conversion cell 100 and the numbers of usable power conversion cells 100 in the U-phase AC output unit 31U, the V-phase AC output unit 31V, and the W-phase AC output unit 31W are not all the same will be described.

[0139] FIG. 12 is a time chart illustrating an example of the operation of the power conversion device 30 when an unusable power conversion cell 100 is present. FIG. 12 includes FIGS. 12A, 12B, 12C, and 12D. FIG. 12A is a time chart illustrating an example of a change over time in the output voltage Vo of the power conversion device 30. FIG. 12B is a time chart illustrating an example of a change over time in the carrier frequency of the power conversion cells 100 included in the single-phase AC output unit 31X that does not have the largest number of usable power conversion cells 100 among the U-phase AC output unit 31U, the V-phase AC output unit 31V, and the W-phase AC output unit 31W. FIG. 12C is a time chart illustrating an example of a change over time in the output voltage of a power conversion cell 100 (always-used cell) that is used in both the cell balance mode and the cell unbalance mode among the usable power conversion cells 100 included in the power conversion device 30. FIG. 12D is a time chart illustrating an example of a change over time in the output voltage of a power conversion cell 100 (excess cell) that is unused in the cell balance mode among the usable power conversion cells 100 included in the power conversion device 30.

[0140] As shown in FIG. 12A, in this example, the output voltage Vo of the power conversion device 30 increases linearly from 0 (zero), and in the time range in which the output voltage Vo is less than the threshold value Vth, a PWM signal is generated by dipolar modulation.

[0141] Furthermore, in the time range in which the output voltage Vo is less than the threshold value Vth, the power conversion device 30 is controlled in the cell balancing mode, which corresponds to dipolar modulation. Therefore, as shown in Fig. 12B, the carrier frequency of the power conversion cells 100 included in the single-phase AC output unit 31X that does not have the largest number of usable power conversion cells 100 among the U-phase AC output unit 31U, the V-phase AC output unit 31V, and the W-phase AC output unit 31W is maintained at a relatively low set value.

[0142] At time t1, when the output voltage Vo reaches the threshold value Vth, the modulation method switches from dipolar modulation to unipolar modulation (see FIG. 10), and from time t1 onwards, a PWM signal is generated by unipolar modulation.

[0143] Furthermore, when the modulation method is switched from dipolar modulation to unipolar modulation at time t1, the carrier frequency of the power conversion cells 100 included in the single-phase AC output unit 31X that does not have the largest number of usable power conversion cells 100 among the U-phase AC output unit 31U, the V-phase AC output unit 31V, and the W-phase AC output unit 31W is changed in an increasing direction in accordance with the cell imbalance mode corresponding to the unipolar modulation (see step S204 in FIG. 11 ). Then, at time t2 after time t1, when the carrier frequency of the power conversion cells 100 included in the single-phase AC output unit 31X that does not have the largest number of usable power conversion cells 100 among the U-phase AC output unit 31U, the V-phase AC output unit 31V, and the W-phase AC output unit 31W reaches a set value corresponding to the cell imbalance mode, the control mode is switched from the cell balance mode to the cell imbalance mode (see steps S206 and S208 in FIG. 11 ).

[0144] 12C, in the time range before time t1, the PWM signal is output by dipolar modulation, so the normally-used cell outputs an ON voltage of the polarity corresponding to the voltage command value and a bias voltage of the opposite polarity. On the other hand, in the time range after time t1, the PWM signal is output by unipolar modulation, so the normally-used cell outputs only an ON voltage of the polarity corresponding to the voltage command value.

[0145] 12D, in the time range before time t2, the cell balance mode is used and the surplus cell is intentionally not used, so the output voltage of the surplus cell is maintained at 0 (zero). On the other hand, in the time range after time t2, the cell unbalance mode is used and the surplus cell is used, so the surplus cell outputs an on-voltage with a polarity corresponding to the voltage command value based on the PWM signal generated by unipolar modulation.

[0146] [Other embodiments] Another embodiment will now be described.

[0147] The above-described embodiment may be modified or changed as appropriate. Hereinafter, examples in which the above-described embodiment is modified or changed will be referred to as "modified examples" for convenience.

[0148] For example, in the above-described embodiment, the modulation method may be fixed to either dipolar modulation or unipolar modulation. In this case, the central control unit 33 may select the cell balance mode when the output voltage Vo is equal to or less than a predetermined threshold, and may select the cell unbalance mode when the output voltage Vo is greater than or equal to the threshold. In this case, the predetermined threshold may be the same as or different from the threshold Vth described above.

[0149] In the above-described embodiment and its modifications, in the cell balancing mode, the surplus cells may be made unused by bringing the short-circuiting unit 140 into a short-circuited state.

[0150] In the above-described embodiment and its modifications, the cell control unit 32 and the general control unit 33 may be provided outside the power conversion device 30.

[0151] In the above-described embodiment and its modifications, the short-circuiting portion 140 may be provided outside the power conversion cell 100 .

[0152] In the above-described embodiment and modified examples, the power conversion device 30 may output two-phase AC or four or more-phase AC instead of three-phase AC. That is, the power conversion device 30 includes M (M is an integer of 2 or more) single-phase AC output units 31X, thereby being able to output M-phase (M is an integer of 2 or more) AC.

[0153] [Effect] The operation of the power conversion device, the control device, and the control method according to this embodiment will be described.

[0154] In a first aspect of the present embodiment, there is provided a power conversion device including a multi-phase single-phase AC output unit and a control unit. The power conversion device is, for example, the above-described power conversion device 30. The multi-phase single-phase AC output unit is, for example, the above-described three-phase single-phase AC output unit 31X, i.e., U-phase AC output unit 31U, V-phase AC output unit 31V, and W-phase AC output unit 31W. The control unit is, for example, the above-described general control unit 33. Specifically, each of the multi-phase single-phase AC output units outputs a single-phase AC. Further, each of the multiple-phase single-phase AC output units includes a predetermined number (two or more) of power conversion cells that convert power supplied from an external power source into single-phase AC of a predetermined voltage and frequency and output the converted power. The output units of the predetermined number of power conversion cells are connected in series, thereby enabling the outputs of the predetermined number of power conversion cells to be superimposed to output single-phase AC. The output units include short-circuiting units that can short-circuit the output units for each of the predetermined number of power conversion cells. By short-circuiting the short-circuiting units corresponding to some of the power conversion cells among the predetermined number of power conversion cells, the single-phase AC can be output by superimposing only the outputs of the remaining power conversion cells. The external power source is, for example, a converter 20. The power conversion cells are, for example, the power conversion cells 100 described above. The predetermined number is, for example, N, an integer equal to or greater than two. The output units of the power conversion cells are, for example, the output unit 100O described above. The short-circuiting units are, for example, the short-circuiting units 140 described above. Furthermore, the control unit controls the multiple-phase single-phase AC. phase For each AC output unit, based on a comparison between a triangular wave having the same frequency and phase and whose voltage level is shifted and a voltage command value, all or some of the predetermined number of power conversion cells are driven, thereby superimposing the outputs of all or some of the predetermined number of power conversion cells so as to be changed in a stepwise manner. phasea first control state in which a single-phase AC is output by superimposing only the outputs of the power conversion cells in a number corresponding to the minimum value of the number of power conversion cells available for use in each AC output unit; and a second control state in which a single-phase AC is output by superimposing only the outputs of the power conversion cells in a number corresponding to the minimum value of the number of power conversion cells available for use in each AC output unit. phase and a second control state in which each AC output unit outputs a single-phase AC power by superimposing the outputs of all the available power conversion cells. The first control state is, for example, the cell balance mode described above. The second control state is, for example, the cell unbalance mode described above.

[0155] As a result, the power conversion device can operate in a single mode including an unusable power conversion cell under the assumption that the power conversion cells 100 are driven in the PD system. phase The power conversion device can use both the second control state in which the switching frequency of the available power conversion cells included in the AC output section increases and the first control state in which the switching frequency does not increase, thereby suppressing the switching frequency of the power conversion cells driven by the PD method.

[0156] In a second aspect of the present embodiment, based on the first aspect described above, the control unit may drive the power conversion cells by unipolar modulation based on a comparison between the triangular wave and the voltage command value, or may drive the power conversion cells by dipolar modulation based on a comparison between the triangular wave and the voltage command value. When the number of usable power conversion cells for each of the multiple-phase single-phase AC output units is not the same, the control unit may use the second control state when driving the power conversion cells by unipolar modulation, and may use the first control state when driving the power conversion cells by dipolar modulation.

[0157] This allows the power conversion device to avoid selecting a superimposed state between, for example, dipolar modulation, in which the switching frequency of the single-phase AC output unit is twice that of unipolar modulation, and a second control state, in which the switching frequency of the power conversion cells is increased, for a single-phase AC output unit among multiple single-phase AC output units that does not have the largest number of usable power conversion cells. Therefore, the power conversion device can further reduce the switching frequency of the power conversion cells driven by the PD method, thereby suppressing cost increases from the perspective of durability and reliability against switching frequency. Furthermore, the power conversion device can suppress an increase in heat generation from the semiconductor switches due to the suppression of the switching frequency, thereby enabling the miniaturization of coolers such as heat sinks and the reduction of the costs of the coolers.

[0158] In addition, in a third aspect of the present embodiment, based on the first or second aspect described above, when the number of usable power conversion cells for each of the single-phase AC output units of the plurality of phases is not the same, the control unit Device the first control state is used when the output voltage of the power converter is less than or equal to a threshold value; Device The second control state may be used when the output voltage of the inverter is equal to or greater than the threshold.

[0159] As a result, in the first control state, some of the usable power conversion cells are not used and the output is limited, but the power conversion device can suppress the influence of the output limit by using the first control state when the output voltage is relatively small. Therefore, the power conversion device can suppress the switching frequency of the power conversion cells while suppressing the influence of the output limit in the first control state.

[0160] In a fourth aspect of the present embodiment, based on any one of the first to third aspects described above, the control unit may, in the first control state, for a single-phase AC output unit among the multiple-phase single-phase AC output units in which the number of usable power conversion cells is greater than the minimum value, control the output voltages of the usable power conversion cells in excess of the minimum value to zero, thereby outputting single-phase AC power by superimposing only the outputs of the power conversion cells in a number corresponding to the minimum value. The usable power conversion cells in excess of the minimum value are, for example, the surplus cells described above.

[0161] This allows the power conversion device to quickly switch between the first control state and the second control state.

[0162] In a fifth aspect of the present embodiment, based on any one of the first to fourth aspects described above, the control unit may, in the first control state, for a single-phase AC output unit in which the number of usable power conversion cells among the multiple-phase single-phase AC output units is greater than the minimum value, switch the power conversion cells that do not contribute to the output voltage of the target single-phase AC output unit among the usable power conversion cells. The power conversion cells that do not contribute to the output voltage of the target single-phase AC output unit among the usable power conversion cells are, for example, the surplus cells described above.

[0163] This allows the power conversion device to suppress imbalances in the number of switching times among the available power conversion cells for single-phase AC output units having a number of available power conversion cells greater than the minimum value among multiple-phase single-phase AC output units.

[0164] In addition, in a sixth aspect of the present embodiment, there is provided a control device for controlling a power conversion device, the control device including: multiple-phase single-phase AC output units each outputting a single-phase AC; each of the multiple-phase single-phase AC output units including a predetermined number (two or more) of power conversion cells each converting power supplied from an external power source into single-phase AC of a predetermined voltage and frequency and outputting the converted power; the output units of the predetermined number of power conversion cells being connected in series, thereby enabling the output of the predetermined number of power conversion cells to be superimposed on each other to output single-phase AC; and a short-circuit unit capable of short-circuiting the output unit for each of the predetermined number of power conversion cells; and by short-circuiting the short-circuit unit corresponding to some of the power conversion cells among the predetermined number of power conversion cells, the control device is capable of outputting single-phase AC by superimposing only the outputs of the remaining power conversion cells. The control device is, for example, the above-mentioned general control unit 33. Specifically, the control device is phase For each AC output unit, all or some of the predetermined number of power conversion cells are driven based on a comparison between a triangular wave having the same frequency and phase and whose voltage level is shifted and a voltage command value, thereby superimposing the outputs of all or some of the predetermined number of power conversion cells so as to be changed in a stepwise manner. phase a first control state in which a single-phase AC is output by superimposing only the outputs of the power conversion cells in a number corresponding to the minimum value of the number of power conversion cells available for use in each AC output unit; and a second control state in which a single-phase AC is output by superimposing only the outputs of the power conversion cells in a number corresponding to the minimum value of the number of power conversion cells available for use in each AC output unit. phase and a second control state in which each of the AC output sections outputs a single-phase AC power by superimposing the outputs of all the power conversion cells that can be used.

[0165] As a result, the control device achieves the same functions and effects as the first aspect described above.

[0166] Furthermore, with respect to the control device, on the premise of the sixth aspect described above, aspects similar to the second to fifth aspects of the power conversion device can be realized.

[0167] As a result, the control device achieves the same functions and effects as the second to fifth aspects described above.

[0168] Furthermore, in a seventh aspect of the present embodiment, there is provided a control method for controlling a power conversion device including multiple single-phase AC output units each outputting a single-phase AC, wherein the multiple single-phase AC output units each include a predetermined number (two or more) of power conversion cells that convert power supplied from an external power supply into single-phase AC of a predetermined voltage and frequency and output the converted power, the output units of the predetermined number of power conversion cells are connected in series, thereby enabling single-phase AC to be output by superimposing outputs of the predetermined number of power conversion cells, and including short-circuit units that are capable of short-circuiting the output units for each of the predetermined number of power conversion cells, and by short-circuiting the short-circuit units corresponding to some of the power conversion cells among the predetermined number of power conversion cells, it is possible to output single-phase AC by superimposing outputs of only the remaining power conversion cells. Specifically, in the control method, a control device controls the multiple single-phase AC output units phase For each AC output unit, based on a comparison between a triangular wave having the same frequency and phase and a voltage level shifted and a voltage command value, all or some of the predetermined number of power conversion cells are driven, thereby superimposing the outputs of all or some of the predetermined number of power conversion cells so as to be changed in a stepwise manner. phase a first control state in which a single-phase AC is output by superimposing only the outputs of the power conversion cells in a number corresponding to the minimum value of the number of power conversion cells available for use in each AC output unit; and a second control state in which a single-phase AC is output by superimposing only the outputs of the power conversion cells in a number corresponding to the minimum value of the number of power conversion cells available for use in each AC output unit. phase and a second control state in which each of the AC output sections outputs a single-phase AC power by superimposing the outputs of all the power conversion cells that can be used.

[0169] As a result, the control method has the same functions and effects as the first aspect described above.

[0170] Furthermore, with regard to the control method as well, on the premise of the above-mentioned seventh aspect, aspects similar to the second to fifth aspects of the power conversion device can be realized.

[0171] As a result, the control method has the same functions and effects as the second to fifth aspects described above.

[0172] Although the embodiments have been described in detail above, the present disclosure is not limited to such specific embodiments, and various modifications and variations are possible within the scope of the gist described in the claims. [Explanation of symbols]

[0173] 1. Drive system 10 3-phase AC power supply 20 Converter 30 Power conversion device 30I Input section 30O Output section 30OU U phase output section 30OV V phase output section 30OW W phase output section 30NP neutral point 31U U-phase AC output section 31V V-phase AC output section 31W W-phase AC output section 31X Single-phase AC output unit 32 Cell control unit 32U U phase control section 32V V phase control section 32W W-phase control unit 33 General Control Unit 40 Load 100 Power Conversion Cells 100I Input section 100O Output section 110 Rectifier 115 DC link section 120 Smooth section 130 Inverter section 140 Short circuit FD freewheeling diode ILR input line ILS input line ILT input line NL Negative Line OL1 output line OL2 output line PL positive line RD rectifier diode SC smoothing capacitor SW Semiconductor switch

Claims

1. a plurality of single-phase AC output units each outputting a single-phase AC; a control unit, each of the multiple-phase single-phase AC output units includes a predetermined number (two or more) of power conversion cells that converts power supplied from an external power supply into single-phase AC of a predetermined voltage and frequency and outputs the converted power; the output units of the predetermined number of power conversion cells are connected in series, thereby enabling single-phase AC to be output by superimposing outputs of the predetermined number of power conversion cells; and the multiple-phase single-phase AC output unit includes a short-circuit unit that is capable of short-circuiting the output unit for each of the predetermined number of power conversion cells; and by short-circuiting the short-circuit units corresponding to some of the power conversion cells among the predetermined number of power conversion cells, it is possible to output single-phase AC by superimposing outputs of only the remaining power conversion cells; the control unit drives all or a part of the predetermined number of power conversion cells based on a comparison between a voltage command value and triangular waves having the same frequency and phase as each other and shifted in voltage level, for each of the single-phase AC output units of the plurality of phases, thereby superimposing the outputs of all or a part of the predetermined number of power conversion cells so as to be changed in a stepwise manner; the control unit may drive the power conversion cell by unipolar modulation based on a comparison between the triangular wave and the voltage command value, or may drive the power conversion cell by dipolar modulation based on a comparison between the triangular wave and the voltage command value, When the number of usable power conversion cells for each of the multiple-phase single-phase AC output units is not all the same, the control unit uses a first control state in which, when driving the power conversion cells by the dipolar modulation, each of the multiple-phase single-phase AC output units outputs a single-phase AC signal by superimposing only outputs of the power conversion cells in a number corresponding to a minimum value of the number of usable power conversion cells for each of the multiple-phase single-phase AC output units, and uses a second control state in which, when driving the power conversion cells by the unipolar modulation, each of the multiple-phase single-phase AC output units outputs a single-phase AC signal by superimposing outputs of all usable power conversion cells so that phase differences between line voltages of adjacent phases among the multiple phases are all the same. Power conversion device.

2. When the number of usable power conversion cells for each of the plurality of single-phase AC output units is not the same, the control unit uses the first control state when an output voltage of the power conversion device is smaller than a threshold value or equal to or less than the threshold value, and uses the second control state when the output voltage of the power conversion device is equal to or greater than the threshold value. The power conversion device according to claim 1 .

3. In the first control state, the control unit controls, for a single-phase AC output unit having a number of usable power conversion cells greater than the minimum value among the plurality of single-phase AC output units, output voltages of the usable power conversion cells in excess of the minimum value to zero, thereby outputting single-phase AC power by superimposing only outputs of the power conversion cells in a number corresponding to the minimum value. The power conversion device according to claim 1 or 2.

4. and in the first control state, for a single-phase AC output unit among the plurality of single-phase AC output units in which the number of usable power conversion cells is greater than the minimum value, the control unit switches the power conversion cells that do not contribute to an output voltage of the target single-phase AC output unit among the usable power conversion cells. The power conversion device according to claim 1 or 2.

5. a plurality of single-phase AC output units each outputting a single-phase AC; Each of the multiple-phase single-phase AC output units includes a predetermined number (two or more) of power conversion cells that converts power supplied from an external power source into single-phase AC of a predetermined voltage and frequency and outputs the converted power, and the output units of the predetermined number of power conversion cells are connected in series, thereby enabling single-phase AC to be output by superimposing outputs of the predetermined number of power conversion cells, and includes a short-circuit unit that can short-circuit the output unit for each of the predetermined number of power conversion cells, and by short-circuiting the short-circuit unit corresponding to some of the power conversion cells among the predetermined number of power conversion cells, it is possible to output single-phase AC by superimposing outputs of only the remaining power conversion cells. A control device for controlling a power conversion device, for each of the plurality of single-phase AC output units, all or a part of the predetermined number of power conversion cells are driven based on a comparison between a voltage command value and triangular waves having the same frequency and phase as each other and shifted in voltage level, thereby superimposing the outputs of all or a part of the predetermined number of power conversion cells so as to be changed in a stepwise manner; a case where the power conversion cell is driven by unipolar modulation based on a comparison between the triangular wave and the voltage command value, and a case where the power conversion cell is driven by dipolar modulation based on a comparison between the triangular wave and the voltage command value, When the number of usable power conversion cells for each of the single-phase AC output units for the multiple phases is not all the same, a first control state is used when driving the power conversion cells by the dipolar modulation, in which single-phase AC is output by superimposing only outputs of the power conversion cells in a number corresponding to the minimum value of the number of usable power conversion cells for each of the single-phase AC output units for the multiple phases, and a second control state is used when driving the power conversion cells by the unipolar modulation, in which single-phase AC is output by superimposing outputs of all usable power conversion cells for each of the single-phase AC output units for the multiple phases, in such a way that phase differences between line voltages of adjacent phases among the multiple phases are all the same. Control device.

6. a plurality of single-phase AC output units each outputting a single-phase AC; Each of the multiple-phase single-phase AC output units includes a predetermined number (two or more) of power conversion cells that converts power supplied from an external power source into single-phase AC of a predetermined voltage and frequency and outputs the converted power, and the output units of the predetermined number of power conversion cells are connected in series, thereby enabling single-phase AC to be output by superimposing outputs of the predetermined number of power conversion cells, and includes a short-circuit unit that can short-circuit the output unit for each of the predetermined number of power conversion cells, and by short-circuiting the short-circuit unit corresponding to some of the power conversion cells among the predetermined number of power conversion cells, it is possible to output single-phase AC by superimposing outputs of only the remaining power conversion cells. A control method for controlling a power conversion device, comprising: for each of the plurality of single-phase AC output units, all or a part of the predetermined number of power conversion cells are driven based on a comparison between a voltage command value and triangular waves having the same frequency and phase as each other and shifted in voltage level, thereby superimposing the outputs of all or a part of the predetermined number of power conversion cells so as to be changed in a stepwise manner; a case where the power conversion cell is driven by unipolar modulation based on a comparison between the triangular wave and the voltage command value, and a case where the power conversion cell is driven by dipolar modulation based on a comparison between the triangular wave and the voltage command value, When the number of usable power conversion cells for each of the single-phase AC output units for the multiple phases is not all the same, a first control state is used when driving the power conversion cells by the dipolar modulation, in which single-phase AC is output by superimposing only outputs of the power conversion cells in a number corresponding to the minimum value of the number of usable power conversion cells for each of the single-phase AC output units for the multiple phases, and a second control state is used when driving the power conversion cells by the unipolar modulation, in which single-phase AC is output by superimposing outputs of all usable power conversion cells for each of the single-phase AC output units for the multiple phases, in such a way that phase differences between line voltages of adjacent phases among the multiple phases are all the same. Control method.

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