JP7690251B2 - Power Conversion Equipment - Google Patents

Description

An embodiment of the present invention relates to a power conversion device.

A power conversion device with a transformer series multiplexing configuration using multiple three-level converters and transformers is known. The multiple three-level converters are connected in parallel to a DC positive bus, a DC negative bus, and a DC neutral bus. The multiple three-level converters have multiple switching elements, and convert DC power to AC power by switching the multiple switching elements.

The transformer has multiple primary windings connected to the AC sides of the multiple three-level converters, and multiple secondary windings magnetically coupled to the multiple primary windings. The multiple secondary windings are connected in series. The transformer is connected to the AC circuit via both ends of the multiple secondary windings connected in series. This makes it possible to supply AC power with a voltage equal to the sum of the voltages of the multiple secondary windings to the AC circuit.

In such a power conversion device with a multiple series transformer configuration, the total voltage of the primary windings of the transformers can be shared by multiple three-level converters, making it possible to lower the withstand voltage of the equipment used in the multiple three-level converters.

On the other hand, in a power conversion device with a multiple series transformer configuration, when outputting AC power below a certain voltage, bias may occur in the switching of multiple switching elements of multiple three-level converters, which may increase the switching loss of a certain switching element. In addition, due to variations in element characteristics, the neutral point potential of the DC neutral point bus may fluctuate, which may cause the operation of multiple three-level converters to become unstable. However, there have been no cases in which both an operation to prevent an increase in switching loss of a certain switching element and a suppression of fluctuations in the neutral point potential were simultaneously achieved.

For this reason, in a power conversion device using a three-level converter with a multiplexed series transformer configuration, it is desirable to be able to suppress bias in the switching of multiple switching elements and fluctuations in the neutral point potential even when outputting AC power below a certain voltage.

JP 2019-193377 A

An embodiment of the present invention provides a power conversion device with a multiplexed series transformer configuration that can suppress bias in the switching of multiple switching elements and fluctuations in the neutral point potential even when outputting AC power below a specified voltage.

According to an embodiment of the present invention, a three-level converter having 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 a DC circuit, a pair of charge storage elements connected in series between the DC positive bus and the DC negative bus, a DC neutral bus connected to the connection point of the pair of charge storage elements, three DC connection points, and a pair of AC connection points, and connected in parallel to the DC positive bus, the DC negative bus, and the DC neutral bus via the three DC connection points, a plurality of primary windings connected to the pair of AC connection points of each 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 an AC circuit via both ends of the plurality of secondary windings connected in series, and the plurality of three-level converters. and a control device that controls the operation of each of the three-level converters, wherein the multiple three-level converters are full bridge circuits having two legs, a first leg and a second leg, and have multiple switching elements, and perform conversion from DC power to AC power by switching the multiple switching elements, and the control device controls the power conversion of each of the multiple three-level converters by controlling the switching of the multiple switching elements based on four signals, including an upper carrier, a lower carrier, a first voltage command value, and a second voltage command value, and the upper carrier is a triangular wave signal having a minimum value set to 0 or more and a frequency higher than a frequency of the AC voltage output from the multiple three-level converters, and the lower carrier is a triangular wave signal having a maximum value set to 0 or less and a frequency higher than a frequency of the AC voltage output from the multiple three-level converters. a triangular wave signal having a frequency higher than a predetermined frequency, the first voltage command value being a voltage command value for the first leg, and the second voltage command value being a voltage command value for the second leg, and when outputting AC power of less than a predetermined voltage to the AC circuit, the control device performs loss dispersion control to match an upper period in which the second voltage command value is greater than 0 and a lower period in which the second voltage command value is less than 0 with the upper period in which the first voltage command value is greater than 0 and the lower period in which the first voltage command value is less than 0, and in the loss dispersion control, divides the plurality of three-level converters into two groups, a first group and a second group, and, during the upper period of the first voltage command value and the second voltage command value of the three-level converter of the first group, A power conversion device is provided that sets the first voltage command value and the second voltage command value of the three-level converter of the first group to the upper period during the lower period of the first voltage command value and the second voltage command value of the three-level converter of the first group, adjusts the magnitudes of the first voltage command value and the second voltage command value of the three-level converter of the first group, thereby outputting a voltage of a predetermined magnitude from the three-level converter of the first group, and adjusts the magnitudes of the first voltage command value and the second voltage command value of the three-level converter of the second group to output a voltage of a predetermined magnitude of the opposite polarity to that of the three-level converter of the first group from the three-level converter of the second group, thereby making the voltage output to the AC circuit the intended output voltage.

According to an embodiment of the present invention, a power conversion device with a multiplexed series transformer configuration is provided that can suppress bias in switching of multiple switching elements and fluctuations in neutral point potential even when outputting AC power below a predetermined voltage.

1 is a circuit diagram illustrating a power conversion device according to an embodiment of the present invention; 1 is a circuit diagram illustrating a three-level converter according to an embodiment. 4 is a waveform diagram illustrating an example of an operation of the power conversion device according to the embodiment. FIG. 4A and 4B are waveform diagrams that diagrammatically show an example of a reference operation of the power conversion device. FIG. 2 is a block diagram illustrating an example of a control device according to the embodiment. FIG. 11 is an explanatory diagram illustrating an example of a table used for polarity calculation.

Each embodiment will be described below with reference to the drawings.
The drawings are schematic or conceptual, and the relationship between the thickness and width of each part, the size ratio between parts, etc. are not necessarily the same as in reality. Even when the same part is shown, the dimensions and ratios of each part may be different depending on the drawing.
In this specification and each drawing, elements similar to those described above with reference to the previous drawings are given the same reference numerals and detailed descriptions thereof will be omitted as appropriate.

FIG. 1 is a circuit diagram illustrating 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 bus 12c, a pair of charge storage elements 14, 16, a plurality of three-level converters 18, a transformer 20, and a control device 22.

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 a terminal on the high potential side of the DC circuit, and the DC negative bus 12b is connected to a terminal on the low potential side of the DC circuit.

The pair of charge storage elements 14, 16 are connected in series between the DC positive bus 12a and the DC negative bus 12b. The pair of charge storage elements 14, 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, 16 are smoothing capacitors.

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

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 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.

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

In other words, the multiple three-level converters 18 are connected in parallel to the DC positive bus 12a, the DC negative bus 12b, and the DC neutral bus 12c. This allows the power conversion device 10 to share the power flowing through the DC circuit among the multiple three-level converters 18. For example, the rated current of the equipment used in the multiple three-level converters 18 can be lowered.

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 a pair of AC connection points a1, 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.

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

The multiple secondary windings 20b are connected in series. One end of the multiple secondary windings 20b connected in series is connected to one terminal of the AC circuit. The other end of the multiple 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 multiple secondary windings 20b of the transformer 20.

The DC circuit is, for example, a DC power source. The AC circuit is, for example, an AC load. The multiple 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 combines the AC power (voltage) output from the multiple three-level converters 18 using the multiple secondary windings 20b. The voltage across both ends of the multiple secondary windings 20b connected in series (the voltage input to the AC circuit) is the sum of the voltages output from each of the multiple secondary windings 20b. The transformer 20 supplies the combined AC power to the AC circuit.

In this way, the power conversion device 10 converts, for example, DC power supplied from a 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 with a multiple transformer series configuration using multiple three-level converters 18 and transformers 20.

The multiple 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 multiple three-level converters 18 using multiple 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 be three-phase AC power or the like. The power conversion device 10 may be configured to provide, for example, three series connections of multiple three-level converters 18, and supply three-phase AC power to the AC circuit through the three series connections.

The power conversion device 10 is applied to, for example, a BTB (Back to Back) system used for power transfer between asynchronous AC systems and different frequency AC systems. The AC circuit may be, for example, a power system. The DC circuit may be, for example, another power conversion device that converts AC power supplied from another power system into DC power. The power conversion device 10 may further have a function of converting AC power supplied from the AC circuit into DC power and supplying it to the DC circuit.

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

FIG. 2 is a circuit diagram illustrating a three-level converter according to an embodiment.
2, each three-level converter 18 has a plurality of switching elements 41 and a plurality of rectifying elements 42 and 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.

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

The three-level converter 18 is a full-bridge circuit having two legs, 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.

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

The second arm AM2 on the negative side 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 terminal).

Both arms AM1 and AM2 are connected in series between DC connection point d1 (positive potential terminal) and DC connection point d2 (negative potential terminal), and the series connection point of both arms AM1 and AM2 becomes 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 the rectifier 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 the rectifier element 43. The rectifier element 42 is a so-called freewheel diode. The rectifier element 43 is a so-called clamp diode.

The configuration of the third arm AM3 is substantially the same as the configuration of the first arm AM1. The configuration of the fourth arm AM4 is substantially the same as the configuration of the second arm AM2. The series connection point between the third arm AM3 and the fourth arm AM4 is the AC connection point a2.

In the three-level converter 18, the potential of each of the AC connection points a1 and a2 is connected to one of three levels: DC connection point d1 (positive potential terminal), DC connection point d2 (negative potential terminal), and DC connection point d3 (neutral terminal) depending on the switching of each switching element 41. 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 has five levels: +Vd, +Vd/2, 0, -Vd/2, and -Vd.

The multiple three-level converters 18 convert DC power to AC power by switching the multiple switching elements 41. The multiple three-level converters 18 can also convert AC power to DC power by switching the multiple switching elements 41. The three-level converters 18 are so-called neutral-point-clamped (NPC) type converters.

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

FIG. 3 is a waveform diagram illustrating 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 power conversion of each of the three-level converters 18 by controlling the switching of a plurality of switching elements 41 of each of the three-level converters 18 based on four signals, namely, 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 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 four signals, 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 of the three-level converters 18, and controls the operation of each of the three-level converters 18 by inputting each of the generated control signals to the corresponding three-level converter 18.

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. 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 frequency of the upper carrier CW1 and the lower carrier CW2 is higher than the frequency of the output AC voltage. The frequency of the upper carrier CW1 and the lower carrier CW2 is set, for example, to 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.

The upper carrier CW1 and the lower carrier CW2 are, in other words, carrier waves. The upper carrier CW1 and the lower carrier CW2 may be, for example, sawtooth signals. The upper carrier CW1 may be any triangular wave signal whose minimum value is set to 0 or more and whose frequency is higher than the frequency of the AC voltage output from each three-level converter 18. The lower carrier CW2 may be any triangular wave signal whose maximum value is set to 0 or less and whose frequency is higher than the frequency of the AC voltage output from each three-level converter 18.

The first voltage command value VR1 is a voltage command value for the first leg LG1. The control device 22 generates a control signal GS11 that turns on the switching element S11 when the first voltage command value VR1 is greater than the upper carrier CW1, and turns off the switching element S11 when the first voltage command value VR1 is smaller than the upper carrier CW1. Note that FIG. 3 shows an example of each control signal that turns the on state of the switching element 41 to Hi (high voltage state) and the off state of the switching element 41 to Lo (low voltage state).

The control device 22 generates a control signal GS12 that turns on the switching element S12 when the first voltage command value VR1 is greater than the lower carrier CW2, and turns off the switching element S12 when the first voltage command value VR1 is less than the lower carrier CW2.

The control device 22 also inverts the Hi and Lo of the control signal GS11 to generate the control signal GS13, and inverts the Hi and Lo of the control signal GS12 to generate the control signal GS14.

The second voltage command value VR2 is a voltage command value for the second leg LG2. As in 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.

4A and 4B are waveform diagrams that diagrammatically show an example of a reference operation of the power conversion device.
Fig. 4(a) is a waveform diagram that shows an example of an operation when AC power of a predetermined voltage or more is output from the power conversion device 10 to the AC circuit. As shown in Fig. 4(a), when AC power of a predetermined voltage or more is output to the AC circuit, the control device 22 uses a first voltage command value VR1 and a second voltage command value VR2 that are sinusoidal according to the AC power to be output. 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. The phase of the second voltage command value VR2 is set to be shifted by 180° from the phase of the first voltage command value VR1.

At this time, the same value is added to the first voltage command value VR1 and the second voltage command value VR2 to control the time ratio during which the neutral terminal is connected to the AC connection point. This makes it possible to prevent bias from occurring between the voltage of the charge storage element 14 (the voltage between the DC positive bus 12a and the DC neutral bus 12c) and the voltage of the charge storage element 16 (the voltage between the DC neutral bus 12c and the DC negative bus 12b). In other words, it is possible to prevent the neutral potential from fluctuating.

The output voltage between the AC connection points a1 and a2 of the three-level converter 18 is a voltage that corresponds 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 reduced and brought closer to zero.

However, if the amplitudes of the first voltage command value VR1 and the second voltage command value VR2 are set to approach 0, there is a possibility that the switching of the multiple switching elements 41 will become unbalanced, and the switching loss of a certain switching element 41 will increase. In other words, the load on the certain switching element 41 will increase, and this may hasten the deterioration of the certain switching element 41.

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

In the following, the period in 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 the period in 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 loss dispersion control, the upper period and the lower period are repeated at a predetermined cycle. The cycle for switching between the upper period and the lower period is calculated, for example, from the thermal capacity of the elements. This makes it possible to suppress bias in the switching of the multiple switching elements 41 even when AC power of less than a predetermined voltage is output from the power conversion device 10 to the AC circuit.

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 multiple three-level converters 18 is substantially simultaneously in 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 will be cancelled out by the increase or decrease in the neutral point conduction current of the second leg LG2, making it impossible to control the neutral point potential, and there is a possibility that the neutral point potential will fluctuate. In other words, there is a possibility that the voltages of the charge storage elements 14 and 16 will fluctuate.

For this reason, the control device 22 of the power conversion device 10 according to this embodiment divides the multiple three-level converters 18 into two groups, a first group and a second group, when performing loss dispersion control to output AC power below a predetermined voltage from the AC connection points a1 and a2.

In the following, when loss dispersion control is performed, the first voltage command value VR1 for the three-level converter 18 of the first group will be referred to as the first voltage command value VR1a, the second voltage command value VR2 for the three-level converter 18 of the first group will be referred to as the second voltage command value VR2a, the first voltage command value VR1 for the three-level converter 18 of the second group will be referred to as the first voltage command value VR1b, and the second voltage command value VR2 for the three-level converter 18 of the second group will be referred to as the second voltage command value VR2b.

Note that FIG. 3 shows 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 in a DC form. 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 are not limited to this, and may be set to a sinusoidal wave form.

As shown in FIG. 3, the control device 22 sets the first voltage command value VR1b and the second voltage command value VR2b of the three-level converter 18 of the second group to the lower period during the upper period of the first voltage command value VR1a and the second voltage command value VR2a of the three-level converter 18 of the first group, and sets the first voltage command value VR1b and the second voltage command value VR2b of the three-level converter 18 of the second group to the upper period during the lower period of the first voltage command value VR1a and the second voltage command value VR2a of the three-level converter 18 of the first group.

The control device 22 also adjusts the magnitude (amplitude) of each of the first voltage command value VR1a and the second voltage command value VR2a to cause the three-level converter 18 of the first group to output a voltage of a predetermined magnitude. The control device 22 also adjusts the magnitude (amplitude) of each of the first voltage command value VR1b and the second voltage command value VR2b to cause the three-level converter 18 of the second group to output a voltage of a predetermined magnitude with the opposite polarity to that of the three-level converter 18 of the first group.

In other words, when the control device 22 is causing the three-level converter 18 of the first group to output a positive voltage, the control device 22 is causing the three-level converter 18 of the second group to output a negative voltage, and when the control device 22 is causing the three-level converter 18 of the first group to output a negative voltage, the control device 22 is causing the three-level converter 18 of the second group to output a positive voltage.

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 multiple 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 polarity, so that the voltage output from the power conversion device 10 to the AC circuit becomes the intended output voltage.

This allows AC power of a relatively small voltage to be output from the power conversion device 10. In addition, by providing upper and lower periods 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 bias in the switching of the multiple switching elements 41 even when the output voltage is set to a small value.

Furthermore, the upper and lower periods are reversed between the three-level converters 18 of the first group and the three-level converters 18 of the second group, and the three-level converters 18 of the first group and the three-level converters 18 of the second group output voltages of opposite polarity. This allows the neutral point potential to be controlled by adjusting the magnitude of the voltage output from the three-level converters 18 of the first group and the voltage output from the three-level converters 18 of the second group.

The control device 22, for example, detects 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. This makes it possible to suppress fluctuations in the neutral point potential.

The control device 22 detects the neutral point potential by, for example, acquiring the voltage values of each of the charge storage elements 14, 16 from a voltmeter or the like and calculating the difference between the voltage values of each of the charge storage elements 14, 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 for the neutral point potential input from a higher-level controller or the like.

The number of three-level converters 18 in the second group is, for example, the same as the number of three-level converters 18 in the first group. In other words, the number 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. This allows AC power of a relatively small voltage to be output from the power conversion device 10.

However, the number of three-level converters 18 in the second group does not necessarily have to be the same as the number of three-level converters 18 in the first group. The output of the multiple three-level converters 18 only needs to be controlled so that the total magnitude of the voltages output from the three-level converters 18 in the second group is approximately the same as the total magnitude of the voltages output from the three-level converters 18 in the first group, and the number of three-level converters 18 in the second group may be different from the number of three-level converters 18 in the first group.

As shown in FIG. 3, 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, for example, the loss dispersion offset value OFV and the neutral point potential control amounts CA1a, CA2a, CA1b, and CA2b.

The loss variance offset value OFV is set to, for example, any value that can suppress bias in the switching of the multiple switching elements 41. The loss variance offset value OFV is set to, for example, an intermediate value between the maximum and minimum values of the upper carrier CW1 and the lower carrier CW2 (half the peak-to-peak value of the triangular wave). This makes it possible to appropriately suppress bias in the switching of the multiple switching elements 41.

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

For example, if 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 amount CA1b for the three-level converters 18 in the second group is set to be approximately the same as the magnitude of the neutral point potential control amount CA1a for the three-level converters 18 in the first group. However, the magnitude of the neutral point potential control amount CA1a for the three-level converters 18 in the first group and the magnitude of the neutral point potential control amount CA1b for the three-level converters 18 in the second group may be set arbitrarily based on the detection result of the neutral point potential, etc.

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

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

That is, when a current flows from the AC output terminal a1 of the first leg LG1 of the three-level converter 18 of the first group to 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 variance offset value OFV by the neutral point potential control amount CA1a, and generates the second voltage command value VR2a by decreasing the magnitude of the loss variance offset value OFV by the neutral point potential control amount CA2a.

In the case of the second group, as in the case of the first group, under the condition that a current flows from the AC output terminal a1 of the first leg LG1 of the three-level converter 18 to the primary winding 20a of the transformer 20, the control device 22 generates the first voltage command value VR1b of the second group by decreasing the magnitude of the loss variance offset value OFV by the neutral point potential control amount CA1b, and generates the second voltage command value VR2b of the second group by increasing the magnitude of the loss variance offset value OFV by the neutral point potential control amount CA2b. Note that the secondary windings 20b of the transformer 20 are connected in series, and there is almost no variation in the current values of the primary windings 20a, so that the directions of the output currents of the three-level converters 18 may be treated as being the same in practical terms.

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

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

For example, the phase of the upper carrier CW1 and the lower carrier CW2 of the second stage three-level converter 18 is set to be shifted by a quarter period of the upper carrier CW1 and the lower carrier CW2 with respect to the phase of the upper carrier CW1 and the lower carrier CW2 of the first stage three-level converter 18.

For example, when two three-level converters 18 are grouped into one set, one of which is a first group of three-level converters 18 and the other is a second group of three-level converters 18, the control device 22 sets the phase of the upper carrier CW1 and the lower carrier CW2 of the second group of three-level converters 18 to be shifted by a quarter period of the upper carrier CW1 and the lower carrier CW2 relative to the phase of the upper carrier CW1 and the lower carrier CW2 of the first group of three-level converters 18. This makes it possible to suppress the generation of harmonics even when multiple three-level converters 18 are divided into a first group and a second group.

As described above, when the phases of the upper carrier CW1 and the lower carrier CW2 are shifted, the upper and lower periods are not completely reversed between the three-level converters 18 of the first group and the three-level converters 18 of the second group, and there are periods in which the three-level converters 18 of the first group and the three-level converters 18 of the second group are in the upper period or the lower period.

That is, in the loss dispersion control, the multiple three-level converters 18 have four states: 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; 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; 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 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.

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

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

FIG. 5 is a block diagram illustrating an example of a control device according to the embodiment.
FIG. 5 is a schematic diagram of 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. In addition, in FIG. 5, two three-level converters 18 are set as one set, one of which is a first group of three-level converters 18 and the other is a second group of three-level converters 18. In this case, only a part of the control device 22 corresponding to one set of two three-level converters 18 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 of the control device 22 corresponding to the other set is substantially the same as the configuration of the part corresponding to one set of two three-level converters 18 shown in FIG. 5. Note that the power conversion device 10 may have one set of two three-level converters 18.

As shown in FIG. 5, the control device 22 has a rotational coordinate transformation circuit 50, a current command value calculation circuit 51, a current controller 52, an inverse coordinate transformation 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 first group circuit 61, and a second group circuit 62.

As shown in FIG. 5, the power conversion device 10 further includes voltage detectors 80 and 82 and current detectors 84, 86, and 88.

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 first group of three-level converters 18. The current detector 84 detects, for example, the AC current of the first stage three-level converter 18 of the multiple three-level converters 18. The current detector 86 detects the AC current output from the second group of three-level converters 18. The current detector 86 detects, for example, the AC current of the second stage three-level converter 18 of the multiple three-level converters 18.

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

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

The rotating coordinate transformation circuit 50 performs a so-called dq transformation (Park transformation) based on the input detection results of the current detectors 84, 86, and 88 and the electrical angle θ (phase θ) to perform a rotating coordinate transformation of the current signals detected by the current detectors 84, 86, and 88 into current signals in a coordinate system (dq coordinates) synchronized with the electrical angle θ. The rotating coordinate transformation circuit 50 inputs the converted current signals to the current controller 52.

The current command value calculation circuit 51 calculates the current command value of the AC current output from the multiple three-level converters 18 based on, for example, a command value input from a higher-level controller, and inputs the calculated current command value to the current controller 52. The current command value is, for example, the current command value of the d-axis component and the current command value of the q-axis component of the AC current output from the multiple three-level converters 18.

Based on the current signal input from the rotation coordinate transformation 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 of the AC voltage of the multiple three-level converters 18 for outputting the AC current corresponding to the current command value from the multiple three-level converters 18. The current controller 52 calculates the voltage command value by performing PI control, for example, 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 multiple three-level converters 18. However, the calculation method of the voltage command value by the current controller 52 is not limited to PI control, and any calculation method that can appropriately output the AC current corresponding to the current command value from the multiple three-level converters 18 may be used. The current controller 52 inputs the calculated voltage command value to the inverse coordinate transformation circuit 53.

The inverse coordinate transformation circuit 53 receives the voltage command value from the current controller 52 and the electrical angle θ, which is the same as that of the rotating coordinate transformation circuit 50. Based on the input voltage command value and electrical angle θ, the inverse coordinate transformation circuit 53 performs an inverse dq transformation (inverse Park transformation) on the voltage command value to convert the voltage command value in the rotating coordinate system into a voltage command value in three-phase coordinates. The inverse coordinate transformation circuit 53 inputs the converted voltage command value to the first group circuit 61 and the second group circuit 62 as a reference voltage command value.

The carrier generation circuit 54 generates the upper carrier CW1 and the lower carrier CW2 for each of the multiple three-level converters 18 based on preset amplitude information, frequency information, phase information, etc. 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.

The offset output circuit 55 operates when performing loss dispersion control to output 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 multiple three-level converters 18 based on the upper carrier CW1 and lower carrier CW2 of each of the multiple three-level converters 18 input from the carrier generation circuit 54. The offset output circuit 55 generates the loss dispersion offset value OFV so that it is an intermediate value between the maximum and minimum values of the upper carrier CW1 and lower carrier CW2, for example, as described above.

The offset output circuit 55 inputs the generated loss dispersion offset value OFV to the polarity calculation circuit 56. The offset output circuit 55 also inputs the loss dispersion offset value OFV corresponding to the three-level converter 18 of the first group to the first group circuit 61, and inputs the loss dispersion offset value OFV corresponding to the three-level converter 18 of the second group to the second group circuit 62.

The polarity calculation circuit 56 receives the loss dispersion offset value OFV from the offset output circuit 55 and the detection results of the current detectors 84 and 86. Based on the input loss dispersion offset value OFV and the detection results of the current detectors 84 and 86, the polarity calculation circuit 56 calculates the polarity of the neutral point potential control amounts CA1a, CA2a, CA1b, and CA2b used for loss dispersion control.

FIG. 6 is an explanatory diagram that illustrates an example of a table used in polarity calculation.
The polarity calculation circuit 56 performs polarity calculation based on the table shown in Fig. 6. The polarity calculation circuit 56 determines the polarity of the output currents of the multiple three-level converters 18 based on the detection results of the current detectors 84, 86. In this example, the direction in which the current flows from the first leg LG1 to the second leg LG2 via the primary winding 20a is considered to be positive, and the opposite direction is considered to be negative. In other words, in this example, the direction in which the current flows from the AC connection point a1 to the AC connection point a2 is considered to be positive, and the opposite direction is considered to be negative.

The polarity calculation circuit 56 also determines the upper and lower periods of each of the three-level converters 18 in the first group and the three-level converters 18 in the second group based on the loss dispersion offset values OFV of each of the multiple three-level converters 18.

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

As shown in FIG. 6, when the three-level converter 18 of the first group is in the upper period and the three-level converter 18 of the second group is in the lower period, or when the three-level converter 18 of the first group is in the lower period and the three-level converter 18 of the second group is in the upper period, the neutral point potential can be controlled in the direction shown in FIG. 6. Note that in FIG. 5 and FIG. 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.

For example, if the polarity of the output current of the multiple three-level converters 18 is 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, energy can be released between the positive potential P and the neutral point potential O, and energy can be stored between the neutral point potential O and the negative potential N. In other words, when an operation is required to discharge the charge storage element 14 and charge the charge storage element 16, if the neutral point potential control circuit 57 outputs a positive value, there is no need to invert the polarity in the multiplier 58.

When the polarity of the output current of the multiple three-level converters 18 is positive, 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, when the output of the multiplier 58 is a positive value, energy can be stored between the positive potential P and the neutral point potential O, and energy can be released between the neutral point potential O and the negative potential N. In other words, when it is necessary to discharge the charge storage element 14 and charge the charge storage element 16, if the neutral point potential control circuit 57 outputs a positive value, it is necessary to invert the polarity in the multiplier 58.

On the other hand, when the three-level converters 18 of the first group and the three-level converters 18 of the second group are both in the upper period or the lower period, the three-level converters 18 of the second group cancel out the energy storage or release between the positive potential P and the neutral point potential O by the three-level converters 18 of the first group, and the energy storage or release between the neutral point potential O and the negative potential N, so that the neutral point potential cannot be controlled. Therefore, it is preferable to make the state in which the three-level converters 18 of the first group and the three-level converters 18 of the second group are both in the upper period or the lower period as short as possible. This can improve the controllability of the neutral point potential. For example, the state in which the three-level converters 18 of the first group and the three-level converters 18 of the second group are both in the upper period or the lower period is set to a quarter period of the upper carrier CW1 and the lower carrier CW2 as described above. This can improve the controllability of the neutral point potential while suppressing the generation of harmonics in multiple three-level converters 18.

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

The multiplier 58 multiplies the magnitude of the neutral point potential control amounts CA1a, CA2a, CA1b, and CA2b input from the neutral point potential control circuit 57 by the polarity input from the polarity calculation circuit 56. The multiplier 58 sets 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.

The first group circuit 61 has adders 61a, 61b, and 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 transformation circuit 53 in the adder 61a, and adds the neutral point potential control amount CA1a input from the multiplier 58 to the sum of the adder 61a in 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.

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 transformation circuit 53 in the adder 61c, and subtracts the neutral point potential control amount CA2a input from the multiplier 58 from the sum of the adder 61c in the subtractor 61d, thereby calculating the second voltage command value VR2a for the second leg LG2 of the three-level converter 18 of the first group.

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

The second group circuit 62 has an adder 62a, a subtractor 62b, and adders 62c and 62d. When performing loss dispersion control, the second group circuit 62 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 transformation circuit 53 in the adder 62a, and subtracts the neutral point potential control amount CA1b input from the multiplier 58 from the sum of the adder 62a in the subtractor 62b to calculate the first voltage command value VR1b for the first leg LG1 of the three-level converter 18 of the second group.

When performing loss dispersion control, the second group circuit 62 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 transformation circuit 53 in an adder 62c, and also adds the neutral point potential control amount CA2b input from the multiplier 58 to the sum of the adder 62c in an adder 62d, thereby calculating the second voltage command value VR2b for the second leg LG2 of the three-level converter 18 of the second group.

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

In this way, the control device 22 calculates a first voltage command value VR1a for the first leg LG1 of the three-level converter 18 of the first group by adding the neutral point potential control amount CA1a to the voltage command value to which the loss variance offset value OFV has been added, calculates a second voltage command value VR2a for the second leg LG2 of the three-level converter 18 of the first group by subtracting the neutral point potential control amount CA2a from the voltage command value to which the loss variance offset value OFV has been added, calculates a first voltage command value VR1b for the first leg LG1 of the three-level converter 18 of the second group by subtracting the neutral point potential control amount CA1b from the voltage command value to which the loss variance offset value OFV has been added, and calculates a second voltage command value VR2b for the second leg LG2 of the three-level converter 18 of the second group by adding the neutral point potential control amount CA2b to the voltage command value to which the loss variance offset value OFV has been added. This makes it possible to calculate 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.

The control signal generation circuit 59 generates each control signal as described with reference to FIG. 3 based on the upper carrier CW1 and 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 first group circuit 61, and the first voltage command value VR1b and the second voltage command value VR2b input from the second group circuit 62, and inputs each generated control signal to each three-level converter 18.

As a result, as described with respect to FIG. 3, the upper and lower periods are reversed between the three-level converters 18 of the first group and the three-level converters 18 of the second group, and the three-level converters 18 of the first group and the three-level converters 18 of the second group can output voltages of opposite polarity.

As described above, according to the power conversion device 10 of this embodiment, when the control device 22 outputs AC power of less than a predetermined voltage to the AC circuit, the control device 22 divides the multiple three-level converters 18 into two groups, a first group and a second group, sets the three-level converters 18 of the second group to the lower period during the upper period of the three-level converters 18 of the first group, sets the three-level converters 18 of the second group to the upper period during the lower period of the three-level converters 18 of the first group, and outputs a voltage of a predetermined magnitude from the three-level converters 18 of the first group, and outputs a voltage of a predetermined magnitude of the opposite polarity to the three-level converters 18 of the first group from the three-level converters 18 of the second group, so that the voltage output to the AC circuit becomes the intended output voltage.

By providing upper and lower periods 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 prevent bias in the switching of the multiple switching elements 41 of the multiple three-level converters 18 even when the output voltage is set to a small value.

Then, the upper and lower periods are reversed between the three-level converters 18 of the first group and the three-level converters 18 of the second group, and the three-level converters 18 of the first group and the three-level converters 18 of the second group are caused to output voltages of opposite polarity, thereby controlling the neutral point potential.

Therefore, it is possible to provide a power conversion device 10 with a multiplexed series transformer configuration that can suppress bias in the switching of multiple switching elements 41 and fluctuations in the neutral point potential even when outputting AC power below a certain voltage.

Although several embodiments of the present invention have been described, these embodiments are presented as examples and are not intended to limit the scope of the invention. These novel embodiments can be embodied in various other forms, and various omissions, substitutions, and modifications 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 included in the scope of the invention and its equivalents described in the claims.

2...AC motor, 4...rotation angle detection circuit, 10...power conversion device, 12a...DC positive bus, 12b...DC negative bus, 12c...DC neutral 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...rectifier element, 50...rotation 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...first group circuit, 62...second group circuit, 80, 82...voltage detector, 84, 86, 88...Current detector

Claims (3)

A DC positive bus connected to a high potential side of the DC circuit;
A DC negative bus connected to a 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 bus connected to a connection point of the pair of charge storage elements;
a plurality of three-level converters each having three DC connection points and a pair of AC connection points, the converters being connected in parallel to the DC positive bus, the DC negative bus, and the DC neutral bus via the three DC connection points;
a transformer including a plurality of primary windings connected to the pair of AC connection points of each 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, the transformer being connected to an 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;
Equipped with
the plurality of three-level converters are full bridge circuits having two legs, i.e., a first leg and a second leg, each of which has a plurality of switching elements, and which perform conversion from DC power to 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 including 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 a frequency higher than a frequency of the AC voltages output from the plurality of three-level converters;
the lower carrier is a triangular wave signal whose maximum value is set to 0 or less and whose frequency is higher than the frequency of the AC voltages 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 having a voltage less than a predetermined voltage to the AC circuit,
performing a loss dispersion control such that an upper period during which the second voltage command value is greater than 0 and a lower period during which the second voltage command value is less than 0 coincides with the upper period during which the first voltage command value is greater than 0 and the lower period during which the first voltage command value is 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;
During the upper period of the first voltage command value and the second voltage command value of the three-level converter of the first group, the first voltage command value and the second voltage command value of the three-level converter of the second group are set to the lower period;
During the lower period of the first voltage command value and the second voltage command value of the three-level converter of the first group, the first voltage command value and the second voltage command value of the three-level converter of the second group are set to the upper period;
outputting a voltage of a predetermined magnitude from the three-level converter of the first group by adjusting the magnitudes of the first voltage command value and the second voltage command value of the three-level converter of the first group;
By adjusting the magnitudes of the first voltage command value and the second voltage command value of the three-level converter of the second group, a voltage of a predetermined magnitude and of a polarity opposite to that of the three-level converter of the first group is output from the three-level converter of the second group,
A power conversion device that ensures that the voltage output to the AC circuit is an intended output voltage. the plurality of three-level converters have four states in the loss dispersion control, the three-level converters of the first group being in the upper period and the three-level converters of the second group being in the lower period, the three-level converters of the first group being in the lower period and the three-level converters of the second group being in the upper period, each of the three-level converters of the first group and the three-level converters of the second group being in the upper period, and each of the three-level converters of the first group and the three-level converters of the second group being in the lower period,
The power conversion device according to claim 1 , wherein a period in which the upper period and the lower period are aligned in the first group of three-level converters and the second group of three-level converters is shorter than a period in which the upper period and the lower period are different from each other. 3. The power conversion device according to claim 2, wherein the period during which the upper period and the lower period are aligned in the first group of three-level converters and the second group of three-level converters is set to a quarter cycle of the upper carrier and the lower carrier.

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