Power conversion device

Abstract

To provide a power conversion apparatus having a series multi-transformer configuration in which, even when AC power of less than a prescribed voltage is output, switching unbalance among a plurality of switching elements and fluctuations of a neutral point potential can be suppressed.SOLUTION: In loss dispersion control in a power conversion apparatus having a series multi-transformer configuration, a control device for controlling operations of multiple three-level converters divides the multiple three-level converters into a first group and a second group, sets a low period for the second group during a high period of the first group, sets a high period for the second group during a low period of the first group, causes the three-level converters of the first group to output a voltage of a prescribed magnitude, and causes the three-level converters of the second group to output a voltage that has the prescribed magnitude but has a polarity opposite to that from the three-level converters of the first group. Accordingly, a voltage to be output to an AC circuit becomes equal to an intended output voltage.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] Embodiments of the present invention relate to a power conversion device.

Background Art

[0002] A power conversion device having a transformer series multi-configuration using a plurality of three-level converters and a transformer is known. The plurality of three-level converters are connected in parallel to a DC positive bus, a DC negative bus, and a DC neutral point bus. The plurality of three-level converters have a plurality of switching elements, and perform conversion from DC power to AC power by switching the plurality of switching elements.

[0003] The transformer has a plurality of primary windings connected to the AC side of each of the plurality of three-level converters, and a plurality of secondary windings magnetically coupled to the plurality of primary windings. The plurality of secondary windings are connected in series. The transformer is connected to an AC circuit via both ends of the plurality of secondary windings connected in series. Thereby, AC power of a voltage obtained by summing the voltages of the plurality of secondary windings can be supplied to the AC circuit.

[0004] In such a power conversion device having a transformer series multi-configuration, the voltage obtained by summing the primary winding voltages of the transformer can be shared by the plurality of three-level converters, and the withstand voltage of the components used in the plurality of three-level converters can be lowered.

[0005] On the other hand, in a power conversion device having a transformer series multi-configuration, when outputting AC power less than a predetermined voltage, there is a bias in the switching of the plurality of switching elements of the plurality of three-level converters, and the switching loss of a predetermined switching element may increase. Further, due to variations in the characteristics of the elements, etc., the neutral point potential of the DC neutral point bus may fluctuate, and the operation of the plurality of three-level converters may become unstable. However, there has been no case where both the operation of preventing an increase in the switching loss of a predetermined switching element and the suppression of the fluctuation of the neutral point potential are achieved simultaneously.

[0006] Therefore, in a power conversion device using a three-level converter with a transformer series multi-configuration, even when outputting AC power below a predetermined voltage, it is desirable to suppress the bias in the switching of a plurality of switching elements and the fluctuation of the neutral point potential.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0008] Embodiments of the present invention provide a power conversion device with a transformer series multi-configuration that can suppress the bias in the switching of a plurality of switching elements and the fluctuation of the neutral point potential even when outputting AC power below a predetermined voltage.

Means for Solving the Problems

[0009] According to an embodiment of the present invention, there are provided a DC positive bus connected to the high potential side of a DC circuit, a DC negative bus connected to the low potential side of the DC circuit, a pair of charge storage elements connected in series between the DC positive bus and the DC negative bus, a DC neutral point bus connected to the connection point of the pair of charge storage elements, three DC connection points, and a pair of AC connection points. A plurality of three-level converters are connected in parallel to the DC positive bus, the DC negative bus, and the DC neutral point bus via the three DC connection points. A plurality of primary windings are connected to the pair of AC connection points of each of the plurality of three-level converters. A plurality of secondary windings are magnetically coupled to the plurality of primary windings and connected in series. A transformer is connected to an AC circuit via both ends of the plurality of secondary windings connected in series. A control device controls the operation of each of the plurality of three-level converters. The plurality of three-level converters are full-bridge circuits having two legs, namely a first leg and a second leg, and have a plurality of switching elements. By switching the plurality of switching elements, conversion from DC power to AC power is performed. The control device controls the switching of the plurality of switching elements based on four signals, namely an upper carrier, a lower carrier, a first voltage command value, and a second voltage command value, to control the power conversion of each of the plurality of three-level converters. The upper carrier is a triangular wave signal having a minimum value set to be 0 or more and a frequency higher than the frequency of the AC voltage output from the plurality of three-level converters. The lower carrier is a triangular wave signal having a maximum value set to be 0 or less and a frequency higher than the frequency of the AC voltage output from the plurality of three-level converters. The first voltage command value is a voltage command value for the first leg, and the second voltage command value is a voltage command value for the second leg. When outputting AC power less than a predetermined voltage to the AC circuit, the control device performs loss dispersion control to make the upper period during which the second voltage command value becomes greater than 0 and the lower period during which the second voltage command value becomes less than 0 coincide with the upper period during which the first voltage command value becomes greater than 0 and the lower period during which the first voltage command value becomes less than 0. In the loss dispersion control, the plurality of three-level converters are divided into two groups, namely a first group and a second group.When the first voltage command value and the second voltage command value of the three-level converter in the first group are in the previous period, set the first voltage command value and the second voltage command value of the three-level converter in the second group in the next period. When the first voltage command value and the second voltage command value of the three-level converter in the first group are in the next period, set the first voltage command value and the second voltage command value of the three-level converter in the second group in the previous period. By adjusting the magnitudes of the first voltage command value and the second voltage command value of the three-level converter in the first group respectively, a voltage of a predetermined magnitude is output from the three-level converter in the first group. By adjusting the magnitudes of the first voltage command value and the second voltage command value of the three-level converter in the second group respectively, a voltage of a predetermined magnitude with a polarity opposite to that of the three-level converter in the first group is output from the three-level converter in the second group, so that a power conversion device is provided in which the voltage output to the AC circuit becomes the intended output voltage.

Advantages of the Invention

[0010] According to an embodiment of the present invention, even when outputting AC power less than a predetermined voltage, a power conversion device having a transformer series multi-configuration capable of suppressing the bias of switching of a plurality of switching elements and the fluctuation of the neutral point potential is provided.

Brief Description of the Drawings

[0011]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

[0012] Hereinafter, each embodiment will be described with reference to the drawings. Note that the drawings are schematic or conceptual, and the relationship between the thickness and width of each part, the ratio of the sizes between parts, etc. are not necessarily the same as the actual ones. Also, even when representing the same part, there are cases where their dimensions and ratios are represented differently in the drawings. In the present specification and each drawing, the same reference numerals are given to the same elements as those described above with respect to the already shown drawings, and detailed descriptions are appropriately omitted.

[0013] FIG. 1 is a circuit diagram schematically showing a power conversion device according to an embodiment. As shown in FIG. 1, the power conversion device 10 includes a DC positive bus 12a, a DC negative bus 12b, a DC neutral point bus 12c, a pair of charge storage elements 14 and 16, a plurality of three-level converters 18, a transformer 20, and a control device 22.

[0014] The DC positive bus 12a and the DC negative bus 12b are connected to a DC circuit (not shown). More specifically, the DC positive bus 12a is connected to the high-potential side terminal of the DC circuit, and the DC negative bus 12b is connected to the low-potential side terminal of the DC circuit.

[0015] The pair of charge storage elements 14 and 16 are connected in series between the DC positive bus 12a and the DC negative bus 12b. The pair of charge storage elements 14 and 16 smooth the voltage between the DC positive bus 12a and the DC negative bus 12b. In other words, the pair of charge storage elements 14 and 16 are smoothing capacitors.

[0016] The DC neutral point bus 12c is connected to the connection point of the pair of charge storage elements 14 and 16. In other words, the charge storage element 14 is provided between the DC positive bus 12a and the DC neutral point bus 12c, and the charge storage element 16 is provided between the DC neutral point bus 12c and the DC negative bus 12b.

[0017] The capacitance of the charge storage element 16 is substantially the same as the capacitance of the charge storage element 14. As a result, the potential of the DC neutral point bus 12c is set to half the potential of the DC positive bus 12a and the DC negative bus 12b. In other words, the connection point of the pair of charge storage elements 14 and 16 is the neutral point.

[0018] The plurality of three-level converters 18 have three DC connection points d1 to d3 and a pair of AC connection points a1 and a2. Each of the plurality of three-level converters 18 is connected to the DC positive bus 12a, the DC negative bus 12b, and the DC neutral point bus 12c via the three DC connection points d1 to d3. The DC connection point d1 is connected to the DC positive bus 12a, the DC connection point d2 is connected to the DC negative bus 12b, and the DC connection point d3 is connected to the DC neutral point bus 12c. In other words, the DC connection point d1 is the positive potential terminal, the DC connection point d2 is the negative potential terminal, and the DC connection point d3 is the neutral point terminal.

[0019] In other words, the plurality of three-level converters 18 are connected in parallel to the DC positive bus 12a, the DC negative bus 12b, and the DC neutral point bus 12c. As a result, in the power conversion device 10, the power flowing through the DC circuit can be shared by the plurality of three-level converters 18. For example, the rated current of the supplies used in the plurality of three-level converters 18 can be reduced.

[0020] The transformer 20 has a plurality of primary windings 20a and a plurality of secondary windings 20b. Each of the plurality of primary windings 20a is connected to the pair of AC connection points a1 and a2 of each of the plurality of three-level converters 18. Therefore, the number of the plurality of primary windings 20a is the same as the number of the plurality of three-level converters 18. One end of the primary winding 20a is connected to one AC connection point a1 of the three-level converter 18, and the other end of the primary winding 20a is connected to the other AC connection point a2 of the three-level converter 18.

[0021] The plurality of secondary windings 20b are magnetically coupled to the plurality of primary windings 20a. The number of the plurality of secondary windings 20b is the same as the number of the plurality of primary windings 20a. However, the number of the plurality of secondary windings 20b does not necessarily have to be the same as the number of the plurality of primary windings 20a. For example, a configuration in which one secondary winding 20b is magnetically coupled to the plurality of primary windings 20a may be used.

[0022] The plurality of secondary windings 20b are each connected in series. One end of the plurality of secondary windings 20b connected in series is connected to one terminal of the AC circuit. The other end of the plurality of secondary windings 20b connected in series is connected to the other terminal of the AC circuit. In this way, the power conversion device 10 is connected to an AC circuit (not shown) via the plurality of secondary windings 20b of the transformer 20.

[0023] The DC circuit is, for example, a DC power source. The AC circuit is, for example, an AC load. The plurality of three-level converters 18 convert the DC power supplied from the DC circuit into AC power and output the converted AC power to the primary winding 20a of the transformer 20. The transformer 20 serially synthesizes the AC power (voltage) output from the plurality of three-level converters 18 by the plurality of secondary windings 20b. The voltage across both ends of the plurality of secondary windings 20b connected in series (the voltage input to the AC circuit) is the sum of the voltages output from each of the plurality of secondary windings 20b. The transformer 20 supplies the synthesized AC power to the AC circuit.

[0024] In this way, the power conversion device 10 converts, for example, the DC power supplied from the DC circuit into AC power corresponding to the AC circuit and supplies the converted AC power to the AC circuit. The power conversion device 10 is, for example, a power conversion device having a transformer series multi-configuration using the plurality of three-level converters 18 and the transformer 20.

[0025] A plurality of three-level converters 18 convert the DC power supplied from the DC circuit into single-phase AC power. The power conversion device 10 supplies single-phase AC power to the AC circuit, for example, by serially combining the AC power output from the plurality of three-level converters 18 by means of the plurality of secondary windings 20b. The AC power of the AC circuit is, for example, single-phase AC power. However, the AC power of the AC circuit may also be three-phase AC power or the like. The power conversion device 10 may be configured, for example, to provide three series-connected bodies of the plurality of three-level converters 18 and supply three-phase AC power to the AC circuit by means of the three series-connected bodies.

[0026] The power conversion device 10 is applied, for example, to a BTB (Back to Back) system used for power transfer in an asynchronous AC system or a different-frequency AC system. The AC circuit may be, for example, a power grid or the like. The DC circuit may be, for example, another power conversion device that converts the AC power supplied from another power grid into DC power. The power conversion device 10 may further have a function of converting the AC power supplied from the AC circuit into DC power and supplying it to the DC circuit.

[0027] The control device 22 controls the operation of each of the plurality of three-level converters 18. In other words, the control device 22 controls the power conversion by each of the plurality of three-level converters 18.

[0028] FIG. 2 is a circuit diagram schematically showing a three-level converter according to the embodiment. As shown in FIG. 2, each three-level converter 18 has a plurality of switching elements 41, and a plurality of rectifying elements 42, 43. In this example, the three-level converter 18 has eight switching elements 41, eight rectifying elements 42, and four rectifying elements 43. Each switching element 41 is connected in a full-bridge configuration. Each rectifying element 42 is connected in anti-parallel to each switching element 41.

[0029] The switching element 41 has a pair of main terminals and a control terminal. The control terminal is used to switch between an on state in which current flows between the main terminals and an off state in which substantially no current flows between the main terminals. As the switching element 41, for example, a self-extinguishing element such as a GTO (Gate Turn Off thyristor) or an IGBT (Insulated Gate Bipolar Transistor) is used. The control terminal is, for example, a gate terminal.

[0030] The three-level converter 18 is a full-bridge circuit having two legs, i.e., a first leg LG1 and a second leg LG2. The first leg LG1 has a first arm AM1 and a second arm AM2. The second leg LG2 has a third arm AM3 and a fourth arm AM4. In other words, the three-level converter 18 has four arms. The first arm AM1 and the third arm AM3 are positive-side arms, and the second arm AM2 and the fourth arm AM4 are negative-side arms.

[0031] The positive-side first arm AM1 has two switching elements 41 connected in series, two rectifying elements 42 connected in anti-parallel to each of the two switching elements 41, and a rectifying element 43 connected between the series connection point of the two switching elements 41 and the DC connection point d3 (neutral point terminal).

[0032] The negative-side second arm AM2 has two switching elements 41 connected in series, two rectifying elements 42 connected in anti-parallel to each of the two switching elements 41, and a rectifying element 43 connected between the series connection point of the two switching elements 41 and the DC connection point d3 (neutral point terminal).

[0033] Both arms AM1 and AM2 are connected in series between a DC connection point d1 (positive potential terminal) and a DC connection point d2 (negative potential terminal), and the series connection point of both arms AM1 and AM2 becomes an AC connection point a1. The potential of the series connection point of the two switching elements 41 of the first arm AM1 is clamped to the neutral point potential via a rectifying element 43. Similarly, the potential of the series connection point of the two switching elements 41 of the second arm AM2 is clamped to the neutral point potential via a rectifying element 43. The rectifying element 42 is a so-called freewheeling diode. The rectifying element 43 is a so-called clamping diode.

[0034] The configuration of the third arm AM3 is substantially the same as that of the first arm AM1. The configuration of the fourth arm AM4 is substantially the same as that of the second arm AM2. The series connection point of the third arm AM3 and the fourth arm AM4 becomes an AC connection point a2.

[0035] In the three-level converter 18, according to the switching of each switching element 41, the potentials of the AC connection points a1 and a2 are connected to any of the three levels of the DC connection point d1 (positive potential terminal), the DC connection point d2 (negative potential terminal), and the DC connection point d3 (neutral point terminal). When the voltage between the DC positive bus 12a and the DC negative bus 12b is Vd, the voltage between the AC connection points a1 and a2 becomes five levels of +Vd, +Vd / 2, 0, -Vd / 2, and -Vd.

[0036] The plurality of three-level converters 18 perform conversion from DC power to AC power by switching the plurality of switching elements 41. The plurality of three-level converters 18 can also perform conversion from AC power to DC power by switching the plurality of switching elements 41. The three-level converter 18 is a so-called neutral-point-clamped (NPC) type converter (converter).

[0037] Hereinafter, when individually identifying the four switching elements 41 included in the first leg LG1, they are referred to as switching elements S11, S12, S13, and S14 in order from the DC connection point d1 side. Also, when individually identifying the four switching elements 41 included in the second leg LG2, they are referred to as switching elements S21, S22, S23, and S24 in order from the DC connection point d1 side.

[0038] FIG. 3 is a waveform diagram schematically showing an example of the operation of the power conversion device according to the embodiment. As shown in FIG. 3, the control device 22 controls the switching of the plurality of switching elements 41 of each three-level converter 18 based on the four signals of the upper carrier CW1, the lower carrier CW2, the first voltage command value VR1, and the second voltage command value VR2, thereby controlling the power conversion of each three-level converter 18.

[0039] The control device 22 generates a control signal GS11 for controlling the switching of the switching element S11, a control signal GS12 for controlling the switching of the switching element S12, a control signal GS13 for controlling the switching of the switching element S13, a control signal GS14 for controlling the switching of the switching element S14, a control signal GS21 for controlling the switching of the switching element S21, a control signal GS22 for controlling the switching of the switching element S22, a control signal GS23 for controlling the switching of the switching element S23, and a control signal GS24 for controlling the switching of the switching element S24 based on the four signals of the upper carrier CW1, the lower carrier CW2, the first voltage command value VR1, and the second voltage command value VR2. The control device 22 generates a plurality of control signals corresponding to each three-level converter 18, and inputs each generated control signal to the corresponding three-level converter 18, thereby controlling the operation of each three-level converter 18.

[0040] The upper carrier CW1 and the lower carrier CW2 are, for example, triangular wave signals. When the DC voltage between the DC positive bus 12a and the DC negative bus 12b is Vd, the maximum value of the upper carrier CW1 is +Vd, and the minimum value is 0. And the maximum value of the lower carrier CW2 is 0, and the minimum value is -Vd. The frequency and phase of the lower carrier CW2 are the same as those of the upper carrier CW1. The frequencies of the upper carrier CW1 and the lower carrier CW2 are higher than the frequency of the output AC voltage. The frequencies of the upper carrier CW1 and the lower carrier CW2 are set to, for example, an integer multiple of the frequency of the output AC voltage. The upper carrier CW1 and the lower carrier CW2 are, for example, signals synchronized with the output AC voltage.

[0041] In other words, the upper carrier CW1 and the lower carrier CW2 are carrier waves. The upper carrier CW1 and the lower carrier CW2 may be, for example, sawtooth wave signals or the like. The upper carrier CW1 may be an arbitrary triangular wave signal with a minimum value set to 0 or more and having a frequency higher than the frequency of the AC voltage output from each three-level converter 18. The lower carrier CW2 may be an arbitrary triangular wave signal with a maximum value set to 0 or less and having a frequency higher than the frequency of the AC voltage output from each three-level converter 18.

[0042] The first voltage command value VR1 is the voltage command value for the first leg LG1. When the first voltage command value VR1 is greater than the upper carrier CW1, the control device 22 generates a control signal GS11 that turns on the switching element S11, and when the first voltage command value VR1 is less than the upper carrier CW1, the control signal GS11 that turns off the switching element S11. In FIG. 3, an example of each control signal is shown in which the on state of the switching element 41 is Hi (high voltage state) and the off state of the switching element 41 is Lo (low voltage state).

[0043] When the first voltage command value VR1 is greater than the lower carrier CW2, the control device 22 generates a control signal GS12 to turn on the switching element S12. When the first voltage command value VR1 is less than the lower carrier CW2, the control device 22 generates a control signal GS12 to turn off the switching element S12.

[0044] Also, the control device 22 generates a control signal GS13 by inverting the Hi and Lo of the control signal GS11, and generates a control signal GS14 by inverting the Hi and Lo of the control signal GS12.

[0045] The second voltage command value VR2 is a voltage command value for the second leg LG2. Similar to the case of the first leg LG1, the control device 22 generates control signals GS21, GS22, GS23, and GS24 for each switching element 41 of the second leg LG2 by comparing the second voltage command value VR2 with the upper carrier CW1 and comparing the second voltage command value VR2 with the lower carrier CW2.

[0046] Figures 4(a) and 4(b) are waveform diagrams schematically showing an example of the reference operation of the power conversion device. Figure 4(a) is a waveform diagram schematically showing an example of the operation when the power conversion device 10 outputs AC power equal to or higher than a predetermined voltage to the AC circuit. As shown in Figure 4(a), when outputting AC power equal to or higher than a predetermined voltage to the AC circuit, the control device 22 uses the sine-wave-shaped first voltage command value VR1 and second voltage command value VR2 corresponding to the output AC power. The maximum value of the first voltage command value VR1 and the second voltage command value VR2 is +Vd, and the minimum value is -Vd. Also, the phase of the second voltage command value VR2 is set to be shifted by 180° with respect to the phase of the first voltage command value VR1.

[0047] At this time, the same value is added to the first voltage command value VR1 and the second voltage command value VR2, and the duty ratio at which the neutral point terminal is connected to the AC connection point is controlled. As a result, it is possible to suppress the occurrence of a bias between the voltage of the charge storage element 14 (the voltage between the DC positive bus 12a and the DC neutral point bus 12c) and the voltage of the charge storage element 16 (the voltage between the DC neutral point bus 12c and the DC negative bus 12b). In other words, it is possible to suppress the fluctuation of the neutral point potential.

[0048] The output voltage between the AC connection points a1 and a2 of the three-level converter 18 becomes a voltage corresponding to the difference between the first voltage command value VR1 and the second voltage command value VR2. Therefore, when a low voltage is output from the three-level converter 18, the amplitudes of the first voltage command value VR1 and the second voltage command value VR2 are each reduced and made closer to 0.

[0049] However, if the amplitudes of the first voltage command value VR1 and the second voltage command value VR2 are set to approach 0, there will be a bias in the switching of the plurality of switching elements 41, and the switching loss of a predetermined switching element 41 may increase. In other words, the load on a predetermined switching element 41 may increase, and the deterioration of the predetermined switching element 41 may be accelerated.

[0050] For this reason, when the power conversion device 10 outputs AC power less than a predetermined voltage to the AC circuit, it has been proposed to perform the loss dispersion control shown in FIG. 4(b). As shown in FIG. 4(b), in the loss dispersion control, while the difference between the first voltage command value VR1 and the second voltage command value VR2 is set to be small according to the output voltage, the period in which the second voltage command value VR2 is greater than 0 and the period in which it is less than 0 are made to coincide with the period in which the first voltage command value VR1 is greater than 0 and the period in which it is less than 0. In FIG. 4(b), the magnitude of the second voltage command value VR2 is substantially equal to the magnitude of the first voltage command value VR1, and the state where the second voltage command value VR2 substantially overlaps the first voltage command value VR1 is illustrated.

[0051] Hereinafter, a period during which the first voltage command value VR1 and the second voltage command value VR2 are greater than 0 is referred to as the upper period, and a period during which the first voltage command value VR1 and the second voltage command value VR2 are less than 0 is referred to as the lower period. In the loss dispersion control, the upper period and the lower period are repeated at a predetermined cycle. The switching cycle between the upper period and the lower period is calculated from, for example, the heat capacity of the element. Thereby, even when the power conversion device 10 outputs AC power less than a predetermined voltage to the AC circuit, it is possible to suppress the occurrence of bias in the switching of the plurality of switching elements 41.

[0052] On the other hand, if the first voltage command value VR1 and the second voltage command value VR2 are set so that each of the plurality of three-level converters 18 substantially simultaneously enters the upper period and the lower period, even if the same value is added to the first voltage command value VR1 and the second voltage command value VR2, the increase or decrease in the neutral point conduction current of the first leg LG1 is canceled out by the increase or decrease in the neutral point conduction current of the second leg LG2, and the neutral point potential cannot be controlled, and the neutral point potential may fluctuate. In other words, the voltages of the charge storage elements 14 and 16 may fluctuate.

[0053] Therefore, when performing the loss dispersion control, the control device 22 of the power conversion device 10 according to the present embodiment divides the plurality of three-level converters 18 into two groups, a first group and a second group.

[0054] Hereinafter, when performing the loss dispersion control, the first voltage command value VR1 for the three-level converter 18 in the first group is referred to as the first voltage command value VR1a, the second voltage command value VR2 for the three-level converter 18 in the first group is referred to as the second voltage command value VR2a, the first voltage command value VR1 for the three-level converter 18 in the second group is referred to as the first voltage command value VR1b, and the second voltage command value VR2 for the three-level converter 18 in the second group is referred to as the second voltage command value VR2b, respectively.

[0055] Note that in FIG. 3, DC first voltage command values VR1a, second voltage command values VR2a, first voltage command values VR1b, and second voltage command values VR2b are shown. The first voltage command values VR1a, second voltage command values VR2a, first voltage command values VR1b, and second voltage command values VR2b are not limited thereto and may be set to be sinusoidal.

[0056] As shown in FIG. 3, when the first voltage command values VR1a and second voltage command values VR2a of the first group of three-level converters 18 are in the upper period, the control device 22 sets the first voltage command values VR1b and second voltage command values VR2b of the second group of three-level converters 18 to the lower period, and when the first voltage command values VR1a and second voltage command values VR2a of the first group of three-level converters 18 are in the lower period, the control device 22 sets the first voltage command values VR1b and second voltage command values VR2b of the second group of three-level converters 18 to the upper period.

[0057] Further, the control device 22 adjusts the magnitudes (amplitudes) of the first voltage command values VR1a and second voltage command values VR2a respectively, so as to output a voltage of a predetermined magnitude from the first group of three-level converters 18. Then, the control device 22 adjusts the magnitudes (amplitudes) of the first voltage command values VR1b and second voltage command values VR2b respectively, so as to output a voltage of a predetermined magnitude with a polarity opposite to that of the first group of three-level converters 18 from the second group of three-level converters 18.

[0058] That is, when the control device 22 outputs a positive-side voltage from the first group of three-level converters 18, it outputs a negative-side voltage from the second group of three-level converters 18, and when the control device 22 outputs a negative-side voltage from the first group of three-level converters 18, it outputs a positive-side voltage from the second group of three-level converters 18.

[0059] As described above, the voltage output from the power conversion device 10 to the AC circuit is the sum of the voltages output from each of the plurality of secondary windings 20b. The control device 22 causes the first group of three-level converters 18 and the second group of three-level converters 18 to output voltages of opposite polarities, so that the voltage output from the power conversion device 10 to the AC circuit becomes the intended output voltage.

[0060] As a result, AC power with a relatively small voltage can be output from the power conversion device 10. Also, by providing an upper period and a lower period for each of the first voltage command value VR1a, the second voltage command value VR2a, the first voltage command value VR1b, and the second voltage command value VR2b, it is possible to suppress the occurrence of bias in the switching of the plurality of switching elements 41 even when the output voltage is set to be small.

[0061] Furthermore, the upper period and the lower period are made opposite between the first group of three-level converters 18 and the second group of three-level converters 18, and voltages of opposite polarities are output to the first group of three-level converters 18 and the second group of three-level converters 18. As a result, the neutral point potential can be controlled by adjusting the magnitudes of the voltages output from the first group of three-level converters 18 and the voltages output from the second group of three-level converters 18.

[0062] The control device 22 detects, for example, the neutral point potential, and adjusts the magnitudes of the first voltage command value VR1a, the second voltage command value VR2a, the first voltage command value VR1b, and the second voltage command value VR2b so that the neutral point potential becomes zero. As a result, fluctuations in the neutral point potential can be suppressed.

[0063] The control device 22 detects the neutral point potential, for example, by acquiring the voltage values of the charge storage elements 14 and 16 from a voltmeter or the like and calculating the difference between the voltage values of the charge storage elements 14 and 16. Note that the control device 22 is not limited to the above, and may detect the neutral point potential based on, for example, a command value of the neutral point potential input from a higher-level controller or the like.

[0064] The number of the three-level converters 18 in the second group is, for example, the same as the number of the three-level converters 18 in the first group. In other words, the number of the plurality of three-level converters 18 is an even number. In this case, the magnitude of the voltage output from the three-level converters 18 in the second group is set to be approximately the same as the magnitude of the voltage output from the three-level converters 18 in the first group. Thereby, AC power with a relatively small voltage can be output from the power conversion device 10.

[0065] However, the number of the three-level converters 18 in the second group does not necessarily have to be the same as the number of the three-level converters 18 in the first group. As long as the output of the plurality of three-level converters 18 is controlled so that the total magnitude of the voltage output from the three-level converters 18 in the second group is approximately the same as the total magnitude of the voltage output from the three-level converters 18 in the first group, the number of the three-level converters 18 in the second group may be different from the number of the three-level converters 18 in the first group.

[0066] As shown in FIG. 3, for example, the control device 22 generates a first voltage command value VR1a, a second voltage command value VR2a, a first voltage command value VR1b, and a second voltage command value VR2b for loss dispersion control based on a loss dispersion offset value OFV and neutral point potential control amounts CA1a, CA2a, CA1b, and CA2b.

[0067] The loss dispersion offset value OFV is set to an arbitrary value that can suppress the switching bias of the plurality of switching elements 41, for example. The loss dispersion offset value OFV is set to be, for example, a value intermediate between the maximum value and the minimum value of each of the upper carrier CW1 and the lower carrier CW2 (a value half of the peak-to-peak value of the triangular wave). Thereby, the switching bias of the plurality of switching elements 41 can be appropriately suppressed.

[0068] The neutral point potential control quantities CA1a, CA2a, CA1b, and CA2b are set according to the detection result of the neutral point potential. The magnitude of the neutral point potential control quantity CA2a for the second voltage command value VR2a is substantially the same as the magnitude of the neutral point potential control quantity CA1a for the first voltage command value VR1a with the polarity reversed. The magnitude of the neutral point potential control quantity CA2b for the second voltage command value VR2b is substantially the same as the magnitude of the neutral point potential control quantity CA1b for the first voltage command value VR1b with the polarity reversed.

[0069] For example, when the number of three-level converters 18 in the second group is the same as the number of three-level converters 18 in the first group, the magnitude of the neutral point potential control quantity CA1b for the three-level converter 18 in the second group is set to be approximately the same as the magnitude of the neutral point potential control quantity CA1a for the three-level converter 18 in the first group. However, the magnitude of the neutral point potential control quantity CA1a for the three-level converter 18 in the first group and the magnitude of the neutral point potential control quantity CA1b for the three-level converter 18 in the second group may be arbitrarily set according to the detection result of the neutral point potential and the like.

[0070] When current is flowing from the AC output terminal a1 of the first leg LG1 of the three-level converter 18 in the first group towards the primary winding 20a of the transformer 20, when the three-level converter 18 in the first group is in the upper period, the control device 22 generates the first voltage command value VR1a for the first leg LG1 by adding the neutral point potential control quantity CA1a to the positive-side loss dispersion offset value OFV, and generates the second voltage command value VR2a for the second leg LG2 by subtracting the neutral point potential control quantity CA2a from the positive-side loss dispersion offset value OFV. When the direction of the current is opposite, the addition and subtraction are reversed.

[0071] When current is flowing from the AC output terminal a1 of the first leg LG1 of the three-level converter 18 in the first group towards the primary winding 20a of the transformer 20, the control device 22, when the three-level converter 18 in the first group is in the next period, generates the first voltage command value VR1a for the first leg LG1 by subtracting the neutral point potential control amount CA1a from the negative-side loss dispersion offset value OFV, and generates the second voltage command value VR2a for the second leg LG2 by adding the neutral point potential control amount CA2a to the negative-side loss dispersion offset value OFV. When the direction of the current is opposite, the addition and subtraction are reversed.

[0072] That is, when current is flowing from the AC output terminal a1 of the first leg LG1 of the three-level converter 18 in the first group towards the primary winding 20a of the transformer 20, the control device 22 generates the first voltage command value VR1a by increasing the magnitude of the loss dispersion offset value OFV by the amount of the neutral point potential control amount CA1a, and generates the second voltage command value VR2a by decreasing the magnitude of the loss dispersion offset value OFV by the amount of the neutral point potential control amount CA2a.

[0073] Similar to the case of the first group, when current is flowing from the AC output terminal a1 of the first leg LG1 of the three-level converter 18 in the second group towards the primary winding 20a of the transformer 20, the control device 22 generates the first voltage command value VR1b for the second group by decreasing the magnitude of the loss dispersion offset value OFV by the amount of the neutral point potential control amount CA1b, and generates the second voltage command value VR2b for the second group by increasing the magnitude of the loss dispersion offset value OFV by the amount of the neutral point potential control amount CA2b. Note that the secondary windings 20b of the transformer 20 are connected in series, the current values of each primary winding 20a have almost no variation, and the directions of the output currents of each three-level converter 18 may be treated as the same in practical applications.

[0074] As described above, the control device 22 controls the operation of the three-level converters 18 in the first group by comparing the upper carrier CW1 and the lower carrier CW2 with the first voltage command value VR1a generated, and by comparing the upper carrier CW1 and the lower carrier CW2 with the second voltage command value VR2a generated. Then, the control device 22 controls the operation of the three-level converters 18 in the second group by comparing the upper carrier CW1 and the lower carrier CW2 with the first voltage command value VR1b generated, and by comparing the upper carrier CW1 and the lower carrier CW2 with the second voltage command value VR2b generated.

[0075] In the power conversion device 10 having a transformer series multi-configuration using a plurality of three-level converters 18 and transformers 20, as shown in FIG. 3, in order to suppress the generation of harmonics, the phases of the upper carrier CW1 and the lower carrier CW2 are shifted in each of the plurality of three-level converters 18.

[0076] For example, the phases of the upper carrier CW1 and the lower carrier CW2 of the three-level converter 18 in the second stage are set to be shifted by a quarter cycle of the upper carrier CW1 and the lower carrier CW2 with respect to the phases of the upper carrier CW1 and the lower carrier CW2 of the three-level converter 18 in the first stage.

[0077] For example, when the control device 22 sets two three-level converters 18 as one set, with one being the three-level converter 18 in the first group and the other being the three-level converter 18 in the second group, the control device 22 sets the phases of the upper carrier CW1 and the lower carrier CW2 of the three-level converter 18 in the second group to be shifted by a quarter cycle of the upper carrier CW1 and the lower carrier CW2 with respect to the phases of the upper carrier CW1 and the lower carrier CW2 of the three-level converter 18 in the first group. Thereby, even when the plurality of three-level converters 18 are divided into the first group and the second group, the generation of harmonics can be suppressed.

[0078] As described above, when the phases of the upper carrier CW1 and the lower carrier CW2 are shifted, the upper period and the lower period do not completely oppose each other between the three-level converters 18 of the first group and the three-level converters 18 of the second group, and there are periods during which each of the three-level converters 18 of the first group and the three-level converters 18 of the second group becomes the upper period or the lower period.

[0079] That is, in loss dispersion control, the plurality of three-level converters 18 have four states: a state in which the three-level converters 18 of the first group are in the upper period and the three-level converters 18 of the second group are in the lower period, a state in which the three-level converters 18 of the first group are in the lower period and the three-level converters 18 of the second group are in the upper period, a state in which each of the three-level converters 18 of the first group and the three-level converters 18 of the second group are in the upper period, and a state in which each of the three-level converters 18 of the first group and the three-level converters 18 of the second group are in the lower period.

[0080] As described above, the period in which the upper period and the lower period are aligned between the three-level converters 18 of the first group and the three-level converters 18 of the second group is set, for example, to a quarter cycle of the upper carrier CW1 and the lower carrier CW2. The period in which the upper period and the lower period are aligned between the three-level converters 18 of the first group and the three-level converters 18 of the second group is shorter than the period in which the upper period and the lower period are different.

[0081] Note that the length of the period in which the upper period and the lower period are aligned is not limited to a quarter cycle of the upper carrier CW1 and the lower carrier CW2, and may be any length shorter than the period in which the upper period and the lower period are aligned. It is preferable to set the period in which the upper period and the lower period are aligned to be as short as possible. The period in which the upper period and the lower period are aligned may not necessarily be provided.

[0082] FIG. 5 is a block diagram schematically showing an example of a control device according to an embodiment. FIG. 5 schematically shows an example of the control device 22 when performing the operation shown in FIG. 3. In FIG. 5, an AC motor 2 is shown as an example of an AC circuit. Also, in FIG. 5, two three-level converters 18 are taken as a set, with one being the three-level converter 18 of the first group and the other being the three-level converter 18 of the second group, and only a part of the control device 22 corresponding to the two three-level converters 18 of one set is extracted and illustrated. The power conversion device 10 has, for example, a plurality of sets of three-level converters 18. In this case, the configuration of the part corresponding to other sets of the control device 22 is substantially the same as the configuration of the part corresponding to the two three-level converters 18 of one set shown in FIG. 5. Note that the power conversion device 10 may be configured to have two three-level converters 18 of one set.

[0083] As shown in FIG. 5, the control device 22 includes a rotational coordinate conversion circuit 50, a current command value calculation circuit 51, a current controller 52, an inverse coordinate conversion circuit 53, a carrier generation circuit 54, an offset output circuit 55, a polarity calculation circuit 56, a neutral point potential control circuit 57, a multiplier 58, a control signal generation circuit 59, a circuit 61 for the first group, and a circuit 62 for the second group.

[0084] Also, as shown in FIG. 5, the power conversion device 10 further includes voltage detectors 80 and 82, and current detectors 84, 86, and 88.

[0085] The voltage detector 80 detects the voltage of the charge storage element 14. The voltage detector 82 detects the voltage of the charge storage element 16. The current detector 84 detects the AC current output from the three-level converter 18 of the first group. The current detector 84 detects, for example, the AC current of the first-stage three-level converter 18 of a plurality of three-level converters 18. The current detector 86 detects the AC current output from the three-level converter 18 of the second group. The current detector 86 detects, for example, the AC current of the second-stage three-level converter 18 of a plurality of three-level converters 18.

[0086] The current detector 88 detects the alternating current in a plurality of secondary windings 20b connected in series with the transformer 20. The current detectors 84 and 86, in other words, detect the alternating current in a plurality of primary windings 20a of the transformer 20. The current detector 88, in other words, detects the alternating current supplied from the transformer 20 to the AC motor 2 (AC circuit). Further in other words, the current detector 88 detects the output current on the AC side of the power conversion device 10.

[0087] The rotation coordinate conversion circuit 50 receives the detection results of the current detectors 84, 86, and 88 respectively, and the electrical angle θ of the alternating current supplied to the AC motor 2. The electrical angle θ is input to the rotation coordinate conversion circuit 50 from, for example, the rotation angle detection circuit 4 provided on the AC circuit side. The rotation angle detection circuit 4 detects, for example, the rotation angle (mechanical angle) of the AC motor 2, calculates the electrical angle θ corresponding to the detected rotation angle, and inputs the calculated electrical angle θ to the rotation coordinate conversion circuit 50. The electrical angle θ, in other words, is the phase θ of the alternating current supplied to the AC circuit.

[0088] Based on the detection results of the current detectors 84, 86, and 88 respectively and the electrical angle θ (phase θ) input thereto, the rotation coordinate conversion circuit 50 performs so-called dq conversion (Park conversion) to rotationally convert the current signals detected by the current detectors 84, 86, and 88 into current signals in a coordinate system (dq coordinate system) synchronized with the electrical angle θ. The rotation coordinate conversion circuit 50 inputs the converted current signals to the current controller 52.

[0089] The current command value calculation circuit 51 calculates, for example, the current command values of the alternating current output from the plurality of three-level converters 18 based on command values input from a higher-level controller or the like, and inputs the calculated current command values to the current controller 52. The current command values are, for example, the current command values of the d-axis component and the q-axis component of the alternating current output from the plurality of three-level converters 18.

[0090] The current controller 52 calculates the voltage command values of the AC voltages of the plurality of three-level converters 18 for outputting an AC current corresponding to the current command value from the plurality of three-level converters 18 based on the current signal input from the rotation coordinate conversion circuit 50 and the current command value input from the current command value calculation circuit 51. The current controller 52 calculates the voltage command value, for example, by performing PI control based on the difference between the current signal and the current command value. The voltage command value is, for example, the voltage command value of the d-axis component and the voltage command value of the q-axis component of the AC voltage output from the plurality of three-level converters 18. However, the method of calculating the voltage command value by the current controller 52 is not limited to PI control, and any calculation method that can appropriately output an AC current corresponding to the current command value from the plurality of three-level converters 18 may be used. The current controller 52 inputs the calculated voltage command value to the inverse coordinate conversion circuit 53.

[0091] When the voltage command value is input from the current controller 52 to the inverse coordinate conversion circuit 53, the same electrical angle θ as that of the rotation coordinate conversion circuit 50 is also input. The inverse coordinate conversion circuit 53 converts the voltage command value in the rotation coordinate system into the voltage command value in the three-phase coordinate system by performing inverse dq conversion (inverse Park conversion) on the input voltage command value and electrical angle θ. The inverse coordinate conversion circuit 53 inputs the converted voltage command value to the circuit 61 for the first group and the circuit 62 for the second group as the reference voltage command value.

[0092] The carrier generation circuit 54 generates the upper carrier CW1 and the lower carrier CW2 of each of the plurality of three-level converters 18 based on preset amplitude information, frequency information, phase information, and the like. The carrier generation circuit 54 inputs the generated upper carrier CW1 and lower carrier CW2 to the offset output circuit 55 and the control signal generation circuit 59.

[0093] The offset output circuit 55 operates when performing loss dispersion control for outputting AC power below a predetermined voltage. When performing loss dispersion control, the offset output circuit 55 generates a loss dispersion offset value OFV for each of the plurality of three-level converters 18 based on the upper carrier CW1 and the lower carrier CW2 of each of the plurality of three-level converters 18 input from the carrier generation circuit 54. The offset output circuit 55 generates the loss dispersion offset value OFV so as to be a value intermediate between the maximum value and the minimum value of each of the upper carrier CW1 and the lower carrier CW2, for example, as described above.

[0094] The offset output circuit 55 inputs the generated loss dispersion offset value OFV to the polarity operation circuit 56. Also, the offset output circuit 55 inputs the loss dispersion offset value OFV corresponding to the three-level converters 18 of the first group to the circuit 61 for the first group and inputs the loss dispersion offset value OFV corresponding to the three-level converters 18 of the second group to the circuit 62 for the second group.

[0095] The loss dispersion offset value OFV is input to the polarity operation circuit 56 from the offset output circuit 55, and the detection results of the current detectors 84 and 86 are also input. Based on the input loss dispersion offset value OFV and the detection results of the current detectors 84 and 86, the polarity operation circuit 56 performs an operation on the polarities of the neutral point potential control amounts CA1a, CA2a, CA1b, and CA2b used for loss dispersion control.

[0096] FIG. 6 is an explanatory diagram schematically showing an example of a table used for polarity operation. The polarity operation circuit 56 performs a polarity operation based on the table shown in FIG. 6. The polarity operation circuit 56 determines the polarities of the output currents of the plurality of three-level converters 18 based on the detection results of the current detectors 84 and 86. In this example, the direction in which current flows from the first leg LG1 through the primary winding 20a to the second leg LG2 is defined as positive, and the opposite direction is defined as negative. In other words, in this example, the direction in which current flows from the AC connection point a1 to the AC connection point a2 is defined as positive, and the opposite direction is defined as negative.

[0097] Further, the polarity operation circuit 56 determines the respective upper periods and lower periods of the first group of three-level converters 18 and the second group of three-level converters 18 based on the loss dispersion offset values OFV of the respective plurality of three-level converters 18.

[0098] Then, the polarity operation circuit 56 determines the polarities of the neutral point potential control amounts CA1a, CA2a, CA1b, and CA2b based on the table shown in FIG. 6, based on the determination result of the polarity of the output current and the determination results of the upper period and the lower period. The polarity operation circuit 56 inputs the determined polarity to the multiplier 58.

[0099] As shown in FIG. 6, when the first group of three-level converters 18 is in the upper period and the second group of three-level converters 18 is in the lower period, or when the first group of three-level converters 18 is in the lower period and the second group of three-level converters 18 is in the upper period, the neutral point potential can be controlled in the direction shown in FIG. 6. In FIGS. 5 and 6, the positive potential of the DC connection point d1 is represented as P, the negative potential of the DC connection point d2 is represented as N, and the neutral point potential of the DC connection point d3 is represented as O.

[0100] For example, when the polarities of the output currents of the plurality of three-level converters 18 are positive, the first group of three-level converters 18 is in the upper period, and the second group of three-level converters 18 is in the lower period, when the output of the multiplier 58 is a positive value, the energy between the positive potential P and the neutral point potential O can be released, and the energy between the neutral point potential O and the negative potential N can be accumulated. In other words, when it is necessary to discharge the charge storage element 14 and charge the charge storage element 16, when the neutral point potential control circuit 57 outputs a positive value, it is not necessary to invert the polarity with the multiplier 58.

[0101] When the polarities of the output currents of the plurality of three-level converters 18 are positive, and the first group of three-level converters 18 is in the next period and the second group of three-level converters 18 is in the previous period, when the output of the multiplier 58 is a positive value, the energy between the positive potential P and the neutral point potential O can be accumulated, and the energy between the neutral point potential O and the negative potential N can be released. In other words, when it is necessary to discharge the charge storage element 14 and charge the charge storage element 16, when the neutral point potential control circuit 57 outputs a positive value, it is necessary to invert the polarity with the multiplier 58.

[0102] On the other hand, when both the first group of three-level converters 18 and the second group of three-level converters 18 are in the previous period or the next period, the energy accumulation or release between the positive potential P and the neutral point potential O by the first group of three-level converters 18, and the energy accumulation or release between the neutral point potential O and the negative potential N are cancelled out by the second group of three-level converters 18, so the neutral point potential cannot be controlled. Therefore, it is preferable to make the state where both the first group of three-level converters 18 and the second group of three-level converters 18 are in the previous period or the next period as short as possible. Thereby, the controllability of the neutral point potential can be enhanced. For example, the state where both the first group of three-level converters 18 and the second group of three-level converters 18 are in the previous period or the next period is set to a quarter cycle of the upper carrier CW1 and the lower carrier CW2 as described above. Thereby, in the plurality of three-level converters 18, while suppressing the generation of harmonics, the controllability of the neutral point potential can be enhanced.

[0103] The detection results of voltage detectors 80 and 82 are input to the neutral point potential control circuit 57. Based on the detection results of voltage detectors 80 and 82, the neutral point potential control circuit 57 calculates the magnitudes of the neutral point potential control amounts CA1a, CA2a, CA1b, and CA2b. For example, the neutral point potential control circuit 57 calculates the magnitudes of the neutral point potential control amounts CA1a, CA2a, CA1b, and CA2b according to the magnitude of the neutral point potential (the voltage difference between the charge storage elements 14 and 16). For example, the neutral point potential control circuit 57 calculates the magnitudes of the neutral point potential control amounts CA1a, CA2a, CA1b, and CA2b such that the magnitudes of the neutral point potential control amounts CA1a, CA2a, CA1b, and CA2b increase as the neutral point potential increases. The neutral point potential control circuit 57 inputs the calculated magnitudes of the neutral point potential control amounts CA1a, CA2a, CA1b, and CA2b to the multiplier 58.

[0104] The multiplier 58 multiplies the magnitudes of the neutral point potential control amounts CA1a, CA2a, CA1b, and CA2b input from the neutral point potential control circuit 57 by the polarities input from the polarity operation circuit 56. The multiplier 58 uses the multiplication result as the neutral point potential control amounts CA1a, CA2a, CA1b, and CA2b and inputs them to the first group circuit 61 and the second group circuit 62.

[0105] The first group circuit 61 includes adders 61a, 61b, 61c and a subtractor 61d. When performing loss dispersion control, the first group circuit 61 adds the loss dispersion offset value OFV input from the offset output circuit 55 to the reference voltage command value input from the inverse coordinate conversion circuit 53 by the adder 61a, and adds the neutral point potential control amount CA1a input from the multiplier 58 to the addition result of the adder 61a by the adder 61b, thereby calculating the first voltage command value VR1a for the first leg LG1 of the three-level converter 18 of the first group.

[0106] When the loss dispersion control is performed, the circuit 61 for the first group adds the loss dispersion offset value OFV input from the offset output circuit 55 to the reference voltage command value input from the inverse coordinate conversion circuit 53 by the adder 61c, and subtracts the neutral point potential control amount CA2a input from the multiplier 58 from the addition result of the adder 61c by the subtractor 61d, thereby calculating the second voltage command value VR2a for the second leg LG2 of the three-level converter 18 in the first group.

[0107] The circuit 61 for the first group inputs the calculated first voltage command value VR1a and second voltage command value VR2a to the control signal generation circuit 59.

[0108] The circuit 62 for the second group includes an adder 62a, a subtractor 62b, and adders 62c and 62d. When the loss dispersion control is performed, the circuit 62 for the second group adds the loss dispersion offset value OFV input from the offset output circuit 55 to the reference voltage command value input from the inverse coordinate conversion circuit 53 by the adder 62a, and subtracts the neutral point potential control amount CA1b input from the multiplier 58 from the addition result of the adder 62a by the subtractor 62b, thereby calculating the first voltage command value VR1b for the first leg LG1 of the three-level converter 18 in the second group.

[0109] When the loss dispersion control is performed, the circuit 62 for the second group adds the loss dispersion offset value OFV input from the offset output circuit 55 to the reference voltage command value input from the inverse coordinate conversion circuit 53 by the adder 62c, and adds the neutral point potential control amount CA2b input from the multiplier 58 to the addition result of the adder 62c by the adder 62d, thereby calculating the second voltage command value VR2b for the second leg LG2 of the three-level converter 18 in the second group.

[0110] The circuit 62 for the second group inputs the calculated first voltage command value VR1b and second voltage command value VR2b to the control signal generation circuit 59.

[0111] In this way, the control device 22 calculates the first voltage command value VR1a for the first leg LG1 of the three-level converter 18 in the first group by adding the neutral point potential control amount CA1a to the voltage command value to which the loss dispersion offset value OFV is added, calculates the second voltage command value VR2a for the second leg LG2 of the three-level converter 18 in the first group by subtracting the neutral point potential control amount CA2a from the voltage command value to which the loss dispersion offset value OFV is added, calculates the first voltage command value VR1b for the first leg LG1 of the three-level converter 18 in the second group by subtracting the neutral point potential control amount CA1b from the voltage command value to which the loss dispersion offset value OFV is added, and calculates the second voltage command value VR2b for the second leg LG2 of the three-level converter 18 in the second group by adding the neutral point potential control amount CA2b to the voltage command value to which the loss dispersion offset value OFV is added. Thereby, the first voltage command value VR1a, the second voltage command value VR2a, the first voltage command value VR1b, and the second voltage command value VR2b shown in FIG. 3 can be calculated.

[0112] Based on the upper carrier CW1 and the lower carrier CW2 input from the carrier generation circuit 54, the first voltage command value VR1a and the second voltage command value VR2a input from the circuit 61 for the first group, and the first voltage command value VR1b and the second voltage command value VR2b input from the circuit 62 for the second group, the control signal generation circuit 59 generates each control signal as described with reference to FIG. 3, and inputs the generated control signals to each three-level converter 18.

[0113] Thereby, as described with reference to FIG. 3, the upper period and the lower period are made opposite between the three-level converter 18 in the first group and the three-level converter 18 in the second group, and voltages of opposite polarities can be output to the three-level converter 18 in the first group and the three-level converter 18 in the second group.

[0114] As described above, according to the power conversion device 10 according to the present embodiment, when the control device 22 outputs AC power less than a predetermined voltage to the AC circuit, the plurality of three-level converters 18 are divided into two groups, a first group and a second group. When the upper period of the three-level converter 18 in the first group, the three-level converter 18 in the second group is set to the lower period. When the lower period of the three-level converter 18 in the first group, the three-level converter 18 in the second group is set to the upper period. By causing a voltage of a predetermined magnitude to be output from the three-level converter 18 in the first group and causing a voltage of a predetermined magnitude with a polarity opposite to that of the three-level converter 18 in the first group to be output from the three-level converter 18 in the second group, the voltage output to the AC circuit becomes the intended output voltage.

[0115] By providing an upper period and a lower period for each of the first voltage command value VR1a, the second voltage command value VR2a, the first voltage command value VR1b, and the second voltage command value VR2b, it is possible to suppress the occurrence of bias in the switching of the plurality of switching elements 41 of the plurality of three-level converters 18 even when the output voltage is set to be small.

[0116] Then, by making the upper period and the lower period opposite between the three-level converter 18 in the first group and the three-level converter 18 in the second group, and causing the three-level converter 18 in the first group and the three-level converter 18 in the second group to output voltages with opposite polarities, the neutral point potential can be controlled.

[0117] Therefore, it is possible to provide a power conversion device 10 having a transformer series multi-configuration that can suppress the bias in the switching of the plurality of switching elements 41 and the fluctuation of the neutral point potential even when outputting AC power less than a predetermined voltage.

[0118] Although several embodiments of the present invention have been described, these embodiments are presented by way of example and are not intended to limit the scope of the invention. These novel embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the gist of the invention. These embodiments and their modifications are included in the scope and gist of the invention, and are also included in the invention described in the claims and the equivalent scope thereof.

Description of Reference Numerals

[0119] 2... AC motor, 4... Rotation angle detection circuit, 10... Power conversion device, 12a... DC positive bus, 12b... DC negative bus, 12c... DC neutral point bus, 14, 16... Charge storage element, 18... Three-level converter, 20... Transformer, 20a... Primary winding, 20b... Secondary winding, 22... Control device, 41... Switching element, 42, 43... Rectifying element, 50... Rotational coordinate conversion circuit, 51... Current command value calculation circuit, 52... Current controller, 53... Inverse coordinate conversion circuit, 54... Carrier generation circuit, 55... Offset output circuit, 56... Polarity calculation circuit, 57... Neutral point potential control circuit, 58... Multiplier, 59... Control signal generation circuit, 61... Circuit for the first group, 62... Circuit for the second group, 80, 82... Voltage detector, 84, 86, 88... Current detector

Claims

1. A DC positive bus connected to the high potential side of the DC circuit, A DC negative bus connected to the low potential side of the DC circuit, A pair of charge storage elements connected in series between the DC positive bus and the DC negative bus, A DC neutral point bus connected to the connection point of the pair of charge storage elements, Having three DC connection points and a pair of AC connection points, and a plurality of three-level converters connected in parallel to the DC positive bus, the DC negative bus, and the DC neutral point bus via the three DC connection points, A plurality of primary windings connected to the respective pair of AC connection points of the plurality of three-level converters, and a plurality of secondary windings magnetically coupled to the plurality of primary windings and connected in series, and a transformer connected to the AC circuit via both ends of the plurality of secondary windings connected in series, A control device for controlling the operation of each of the plurality of three-level converters, Comprising, The plurality of three-level converters are full-bridge circuits having two legs, a first leg and a second leg, and have a plurality of switching elements, and convert DC power into AC power by switching the plurality of switching elements, The control device controls the switching of the plurality of switching elements based on four signals, an upper carrier, a lower carrier, a first voltage command value, and a second voltage command value, thereby controlling the power conversion of each of the plurality of three-level converters, The upper carrier is a triangular wave signal having a minimum value set to 0 or more and having a frequency higher than the frequency of the AC voltage output from the plurality of three-level converters, The lower carrier is a triangular wave signal having a maximum value set to 0 or less and having a frequency higher than the frequency of the AC voltage output from the plurality of three-level converters, The first voltage command value is a voltage command value for the first leg, The second voltage command value is a voltage command value for the second leg, When the control device outputs AC power less than a predetermined voltage to the AC circuit, Loss dispersion control is performed to make the upper period in which the second voltage command value becomes greater than 0 and the lower period in which it becomes less than 0 coincide with the upper period in which the first voltage command value becomes greater than 0 and the lower period in which it becomes less than 0, In the loss dispersion control, the plurality of three-level converters are divided into two groups, a first group and a second group, When the first voltage command value and the second voltage command value of the three-level converter in the first group are in the previous period, set the first voltage command value and the second voltage command value of the three-level converter in the second group in the next period. When the first voltage command value and the second voltage command value of the three-level converter in the first group are in the next period, set the first voltage command value and the second voltage command value of the three-level converter in the second group in the previous period. By adjusting the magnitudes of the first voltage command value and the second voltage command value of the three-level converter in the first group, a voltage of a predetermined magnitude is output from the three-level converter in the first group. By adjusting the magnitudes of the first voltage command value and the second voltage command value of the three-level converter in the second group, a voltage of a predetermined magnitude with a polarity opposite to that of the three-level converter in the first group is output from the three-level converter in the second group, A power conversion device that enables the voltage output to the AC circuit to be the intended output voltage.

2. In the loss dispersion control, the plurality of three-level converters have four states: a state where the three-level converter in the first group is in the previous period and the three-level converter in the second group is in the next period, a state where the three-level converter in the first group is in the next period and the three-level converter in the second group is in the previous period, a state where each of the three-level converter in the first group and the three-level converter in the second group is in the previous period, and a state where each of the three-level converter in the first group and the three-level converter in the second group is in the next period. The power conversion device according to claim 1, wherein the period in which the previous period and the next period are the same for the three-level converter in the first group and the three-level converter in the second group is shorter than the period in which the previous period and the next period are different.

3. The power conversion device according to claim 2, wherein the period in which the previous period and the next period are the same for the three-level converter in the first group and the three-level converter in the second group is set to a quarter cycle of the upper carrier and the lower carrier.

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