JP2024047909A - Power Conversion Equipment - Google Patents

Abstract

A power conversion device capable of achieving a desired power conversion operation even in a state in which a DC voltage is smaller than the peak value of an AC voltage is provided.
[Solution] A power conversion device 1 has a first DC capacitor C1, a second DC capacitor C2, a first switching leg 5, and a second switching leg 6. A positive side arm 9 and a negative side arm 10 include a plurality of bipolar unit converters H. A control device 20 performs controls (1) and (2) described below when the total voltage of the first DC capacitor C1 and the second DC capacitor C2 is smaller than the peak value of the AC power source AC. (1) The positive side arm 9 outputs a bipolar voltage when the first switching leg 5 is outputting a positive voltage, and outputs a positive voltage when the first switching leg 5 is outputting a negative voltage. (2) The negative side arm 10 outputs a positive voltage when the second switching leg 6 is outputting a positive voltage, and outputs a bipolar voltage when the second switching leg 6 is outputting a negative voltage.
[Selected Figure] Figure 1

Description

An embodiment of the present invention relates to a power conversion device that converts AC power and DC power.

Research and development is underway on modular multilevel converters (MMCs), which are power conversion devices that convert AC power to DC power and vice versa, and which connect multiple unit converters in multiple stages. MMCs can reduce harmonic voltages that accompany the switching of semiconductor switches by providing multiple levels of output voltage. This makes it possible to reduce the size of large-capacity filters, which increase the cost and weight of power conversion devices.

To further miniaturize the device, the neutral-point clamped modular multilevel converter (NPC-MMC) has been proposed as a circuit configuration that reduces the number of unit converters in an MMC (Patent Document 1). The NPC-MMC has a switching leg consisting of two capacitors that divide the DC voltage and semiconductor switches connected in parallel to each capacitor, and has a circuit configuration in which unit converters are connected in multiple stages to the AC output of the switching leg. This makes it possible to achieve the same power conversion operation with roughly half the number of unit converters as conventional MMCs.

However, when the NPC-MMC is connected to a device (power source or load) whose DC voltage fluctuates relatively greatly, such as a storage battery or solar panel, the voltage at the DC terminal drops depending on the operating conditions, making it impossible to continue normal operation. For this reason, it is important that the power conversion device continues to operate even if the DC power supply voltage fluctuates.

Patent Publication 2015-146692 Patent Publication No. 2021-87262

Hirofumi Akagi and Makoto Hagiwara: "Classification and Name of Modular Multilevel Cascade Converters (MMCc)", The Institute of Electrical Engineers of Japan National Convention, 4-043 (2010)

Conventionally, a boost chopper was connected between the converter and a storage battery, etc., to boost the converter's DC voltage to a certain level. However, the loss and size of the boost chopper were problems, and it was not possible to fully utilize the small size and high efficiency of the NPC-MMC.

Patent Document 2 and Non-Patent Document 1 disclose a configuration that uses an MMC (or called MMCc Double Star Bridge Cell) that employs a bipolar bridge circuit capable of outputting a negative voltage as the unit converter, allowing operation to continue even if the voltage at the DC terminal drops. However, the NPC-MMC has two switching legs and a DC capacitor on the DC side to reduce the number of unit converters. Because the operation of the switching legs and the DC voltage affect the control of the unit converter, simply changing the unit converter to bipolarity does not allow the desired power conversion operation to be achieved when the DC voltage drops.

The embodiment of the present invention has been made to solve the above problem, and aims to provide a power conversion device that can achieve the desired power conversion operation even when the DC voltage is smaller than the peak value of the AC voltage.

The power conversion device of the embodiment has the following configuration.
(1) First DC capacitor.
(2) a second DC capacitor connected in series with the first DC capacitor;
(3) a first switching leg connected in parallel with the first DC capacitor;
(4) a second switching leg connected in parallel with the second DC capacitor;
(5) A positive arm including a plurality of bipolar unit converters connected in series between an AC terminal connected to an AC power source and an output terminal of the first switching leg.
(6) a negative arm including a plurality of bipolar unit converters connected in series between the AC terminal and the output terminal of the second switching leg;
(7) A control circuit connected to the first DC capacitor, the second DC capacitor, the first switching leg, the second switching leg, the positive arm, and the negative arm, for detecting or controlling input/output of these devices.
(8) When the total voltage of the first DC capacitor and the second DC capacitor is smaller than a peak value of the AC power supply, the control circuit
the positive arm outputs a bipolar voltage when the first switching leg is outputting a positive voltage, and outputs a positive voltage when the first switching leg is outputting a negative voltage,
The negative arm performs control so as to output a voltage of positive polarity when the second switching leg is outputting a positive output, and a voltage of both polarities when the second switching leg is outputting a negative output.

The power conversion device according to the embodiment may employ the following configuration.
(1) The unit converter includes a cell capacitor, and the voltage of the cell capacitor multiplied by the number of bipolar unit converters is greater than the peak value of the AC power supply.

(2) When generating a gate signal to control the unit converter, the control circuit determines the phase of a positive carrier group that generates the gate signal for the unit converter in the positive arm and a negative carrier group that generates the gate signal for the unit converter in the negative arm based on the total voltage of the first DC capacitor and the second DC capacitor.

(3) When the total voltage of the first DC capacitor and the second DC capacitor is smaller than the peak value of the AC power supply, the phase of the positive carrier group that generates the gate signal of the unit converter in the positive arm and the phase of the negative carrier group that generates the gate signal of the unit converter in the negative arm are shifted by 90 degrees.

(4) Change the duty ratio of the gate signal provided to the first switching leg and the duty ratio of the gate signal provided to the second switching leg.

(5) The control circuit is a circuit that generates gate signals to control the unit converter, the first switching leg, and the second switching leg,
Detecting a voltage across the first DC capacitor and the second DC capacitor;
When the first switching leg has a positive output, a voltage output from the positive side arm includes a voltage of the first DC capacitor, and when the second switching leg has a negative output, a voltage output from the negative side arm includes a voltage of the second DC capacitor.
A gate signal for the unit converter is generated.

(6) The control circuit is a circuit that generates gate signals for the first switching leg, the second switching leg, and the plurality of unit converters,
A cell capacitor voltage average value control section;
a circulating current control unit that outputs a voltage command value based on the feedback manipulated variable current value output from the average value control unit,
When a DC voltage source is connected to the positive terminal of the first DC capacitor and the negative terminal of the second DC capacitor,
Calculating a feedback control amount current value that sets an average value of the cell capacitor voltages included in each of the plurality of unit converters to a predetermined value by the average value control;
Based on the voltage command value from the circulating current control unit, a circulating current passing through the positive arm and the negative arm is controlled to follow a circulating current command value including the feedback manipulated variable current value.

(7) The control circuit further includes a DC capacitor voltage control unit,
When a DC current source is connected to the positive terminal of the first DC capacitor and the negative terminal of the second DC capacitor,
a first feedback manipulated variable current value that sets an average value of the cell capacitor voltages included in each of the plurality of unit converters as a predetermined value;
a second feedback manipulated variable current value that sets the sum of the voltages of the first DC capacitor and the second DC capacitor to a predetermined value;
The circulating current passing through the positive arm and the negative arm is controlled so as to follow a circulating current command value including the sum.

1 is a circuit diagram showing an overall configuration of a first embodiment. FIG. 2 is a circuit diagram showing a bridge cell according to the first embodiment. 3 is a circuit diagram showing each mode in the first embodiment. FIG. 4 is a graph showing operational waveforms according to the first embodiment; 13 is a graph showing harmonics contained in an AC voltage in a state where the DC voltage is smaller than the AC voltage peak value in the second embodiment. FIG. 11 is a graph showing harmonics contained in an AC voltage in a state where a DC voltage is greater than a peak value of an AC voltage in the second embodiment. 13 is a graph showing operational waveforms according to the third embodiment. FIG. 13 is a circuit diagram showing a configuration of a control circuit in a fourth embodiment. FIG. 13 is a circuit diagram showing a configuration of a control circuit in a fifth embodiment.

[1. First embodiment]
[1-1. Configuration]
A first embodiment of the present invention will be described with reference to Fig. 1. A power conversion device 1 of this embodiment is connected between an AC power source AC and a DC voltage source DC. The power conversion device 1 has a first DC capacitor C1 and a second DC capacitor C2 connected in series to divide a positive side DC terminal 2 and a negative side DC terminal 3 to create a neutral point 4 potential. A first switching leg 5 and a second switching leg 6 are connected in parallel to the first DC capacitor C1 and the second DC capacitor C2. The first switching leg 5 is configured by connecting two switching elements S1 and S2 in series, and the second switching leg 6 is configured by connecting two switching elements S3 and S4 in series.

In this embodiment, the first DC capacitor C1 and the second DC capacitor C2 are described as being connected together for the three phases, but they may also be provided separately for the three phases and connected in parallel.

A leg having a positive arm 9 and a negative arm 10 formed by connecting unit converters H in series is connected between the first output terminal 7 of the first switching leg 5 connected in parallel with the first DC capacitor C1 and the second output terminal 8 of the second switching leg 6 connected in parallel with the second DC capacitor C2. The number of unit converters H connected in series can be half that of a conventional MMC. The connection point between the positive arm 9 and the negative arm 10 is the AC terminal 11. In FIG. 1, the positive arm 9 and the negative arm 10 are connected via a buffer reactor 12 to suppress ripple current associated with switching, but they may be connected via a coupling reactor or a transformer.

The first DC capacitor C1, the second DC capacitor C2, the first switching leg 5, the second switching leg 6, the positive arm 9, and the negative arm 10 are connected to a control circuit 20. The control circuit 20 detects or controls the input and output of these devices, and performs the following control when the total voltage of the first DC capacitor C1 and the second DC capacitor C2 is smaller than the peak value of the AC power source AC.

(1) The positive arm 9 outputs a bipolar voltage when the first switching leg 5 outputs a positive voltage, and outputs a positive voltage when the first switching leg 5 outputs a negative voltage.
(2) The negative arm 10 outputs a voltage of positive polarity when the second switching leg 6 outputs a positive voltage, and outputs a voltage of both polarities when the second switching leg 6 outputs a negative voltage.

Figure 2 shows a bridge cell 13, which is an example of a bipolar unit converter H. The bridge cell 13 is composed of four switching elements Sb1 to Sb4 and a cell capacitor Cc. If the voltage of the cell capacitor Cc is Vc, the output of the unit converter H in Figure 2 can output voltages of Vc, -Vc, or 0 depending on the switching state of the switching elements Sb1 to Sb4. Since it can output positive polarity Vc and negative polarity -Vc, the unit converter H of this configuration is called a bipolar unit converter. To keep the cell capacitor voltage Vc constant, the unit converter H has a capacitor voltage detection circuit 14 and sends a voltage detection signal Vs to the control circuit 20.

Each switching element Sb1 to Sb4 is driven based on a gate signal Gs sent from the control circuit 20. That is, the control circuit 20 determines the phase of the positive carrier group that generates the gate signal Gs of the unit converter H in the positive arm 9 and the negative carrier group that generates the gate signal Gs of the unit converter H in the negative arm 10 based on the total voltage of the first DC capacitor C1 and the second DC capacitor C2. A specific example of phase determination will be described in the second embodiment.

[1-2. Action]
The operation of the power conversion device 1 of this embodiment will be described below. In the circuit shown in Fig. 1, the voltage to be output by the bipolar unit converter H changes depending on the operation of the first switching leg 5 and the second switching leg 6.

Figures 3 (1) to (4) show modes 1 to 4 classified according to the operating state of the switching legs. Mode 1 is when the first switching leg 5 and the second switching leg 6 are positive outputs. At this time, the positive arm 9 needs to output a voltage of Vdc/2-Vac, and the negative arm 10 needs to output a voltage of Vac. Mode 2 is when the first switching leg 5 and the second switching leg 6 are negative outputs. At this time, the positive arm 9 needs to output a voltage of -Vac, and the negative arm 10 needs to output a voltage of Vdc/2+Vac. Mode 3 is when the first switching leg 5 is a positive output and the second switching leg 6 is a negative output. At this time, the positive arm 9 needs to output a voltage of Vdc/2-Vac, and the negative arm 10 needs to output a voltage of Vdc/2+Vac. Mode 4 is when the first switching leg 5 is a negative output and the second switching leg 6 is a positive output. At this time, the positive arm 9 needs to output a voltage of -Vac, and the negative arm 10 needs to output a voltage of Vac.

Figure 4 shows waveforms using modes 1 and 2 when the DC voltage is smaller than the peak value of the AC voltage. In a positive half cycle of the AC voltage, the positive arm 9 outputs a bipolar voltage and the negative arm 10 outputs a positive voltage, thereby realizing the state of mode 1. In a negative half cycle of the AC voltage, the positive arm 9 outputs a positive voltage and the negative arm 10 outputs a bipolar voltage, thereby realizing the state of mode 2. This makes it possible to select an appropriate mode even when the DC voltage is smaller than the peak value of the AC voltage, and to achieve the desired power conversion operation. Since the voltage of each arm 9, 10 is at most the peak value of the AC voltage, it is desirable for the total value of the cell capacitor voltages Vc of each arm 9, 10 to be equal to or greater than the peak value of the AC voltage.

In addition, the cell capacitor voltage Vc during operation must be kept constant at or above the above-mentioned value. However, when the DC voltage drops and each arm 9, 10 outputs voltages of both polarities, the average voltage output by each arm 9, 10 drops. Therefore, it is necessary to control the circulating current Iz passing through the positive and negative arms 9, 10 to increase in accordance with the drop in the average voltage output by each arm 9, 10. The circulating current Iz can be calculated, for example, according to Iz = (Ip + In) / 2.

[1-3. Effects]
According to this embodiment, when the total voltage of the first DC capacitor C1 and the second DC capacitor C2 is smaller than the peak value of the AC power supply AC, the control circuit 20 monitors the total voltage of the first DC capacitor C1 and the second DC capacitor C2 and the positive and negative of the outputs of the first switching leg 5 and the second switching leg 6, and controls the polarities of the output voltages of the positive side arm 9 and the negative side arm 10, so as to execute the four modes. As a result, even if the DC voltage is smaller than the peak value of the AC voltage, an appropriate mode can be selected, and a desired power conversion operation can be realized.

[2. Second embodiment]
This embodiment shows an example of a multilevel converter having a plurality of unit converters H. A multilevel converter is a converter that basically has few harmonics, but the reduction rate of harmonics varies depending on the operating state and modulation method. Fig. 5 is a diagram showing harmonic voltages in a state where the voltage of the direct current voltage source DC is smaller than the peak value of the voltage of the alternating current power source AC in the circuit of Fig. 1.

In this embodiment, each arm 9, 10 is composed of three unit converters H, and uses the so-called phase-shift PWM modulation method, in which a gate signal is generated by comparing a group of triangular wave carriers shifted by 120 degrees with a voltage command value Va.

Figure 5 (1) shows the case where the phases of the carrier groups used in the positive and negative arms 9, 10 are aligned, and Figure 5 (2) shows the case where the phases of the carrier groups used in the positive and negative arms 9, 10 are shifted by 90 degrees. It can be seen that harmonics are reduced more when the phases are shifted than when the phases are aligned. Note that the phase angle is the angle in the carrier period.

Figure 6 shows the harmonic voltages in the circuit of Figure 1 when the DC voltage is greater than the peak value of the AC voltage. Figure 6 (1) shows the case where the phases of the carrier groups used in the positive and negative arms are aligned, while Figure 6 (2) shows the case where the phases of the carrier groups used in the positive and negative arms are shifted. In this case, unlike the case of Figure 5, the harmonics are larger when the phases are shifted than when the phases are aligned. In other words, this shows that in order to reduce harmonics, the way the phase is shifted needs to be changed depending on the magnitude of the DC voltage.

For example, when the DC voltage is smaller than the peak value of the AC voltage, it is desirable to shift the phase of the positive carrier group that generates the gate signal Gs of the unit converter H in the positive arm 9 and the negative carrier group that generates the gate signal Gs of the unit converter H in the negative arm 10 by 90 degrees.

Based on the above findings, in the second embodiment, when the control circuit 20 generates the gate signal Gs that controls the unit converter H, the phases of the positive carrier group that generates the gate signal Gs of the unit converter H in the positive arm 9 and the negative carrier group that generates the gate signal Gs of the unit converter H in the negative arm 10 are determined to be shifted by 90 degrees based on the total voltage of the first DC capacitor C1 and the second DC capacitor C2.

According to the second embodiment having such a configuration, the DC voltage is compared with the peak value of the AC voltage, and depending on the magnitude of the two, the phase shift of the gate signal Gs of the unit converter H of the positive arm 9 and the negative frame 10 is changed, thereby reducing harmonics in the AC voltage and enabling the AC voltage to be stabilized.

[3. Third embodiment]
As the DC voltage decreases, the loss increases when the circulating current Iz increases in response to a decrease in the average voltage value output from each arm 9, 10. In the third embodiment, the on/off duties of the first switching leg 5 and the second switching leg 6 are changed to enable use of mode 3.

That is, in the third embodiment, the control circuit 20 performs control to change the duty ratio of both the gate signal Gs provided to the first switching leg 5 by increasing the duty ratio of the gate signal Gs provided to the second switching leg 6 and decreasing the duty ratio of the gate signal Gs provided to the first switching leg 5. For example, in the first embodiment, as shown in FIG. 4, both switching elements S1 and S3 have a duty ratio of 0.5, but in the third embodiment, as shown in FIG. 7, the duty ratio of switching element S1 is made larger than 0.5 and the duty ratio of switching element S3 is made smaller than 0.5 to suppress an increase in the circulating current.

Figure 7 shows waveforms when the on-off duties of the first switching leg 5 and the second switching leg 6 in the third embodiment are changed. In Figure 7, the control circuit 20 makes the on-period of switching element S1 longer than the positive half cycle of the AC voltage, and makes the on-period of switching element S3 shorter than the positive half cycle of the AC voltage. This makes it possible to suppress a decrease in the average voltage value output by each arm 9, 10, and to suppress an increase in the circulating current Iz.

[4. Fourth embodiment]
8 shows an example of the control circuit 20 when a voltage source such as a storage battery is connected to the DC terminals. As described in the above embodiment, each of the arms 9 and 10 has a unit converter H, a first switching leg 5, and a second switching leg 6, so that the control circuit 20 of this embodiment has a circuit that creates a gate signal Gs to be provided to the unit converter H provided in each of the arms 9 and 10.

In FIG. 8, the AC current command value is input to the AC current control unit 21, and is output as the gate signal Gs of the switching elements S1 to S4 via the comparator 22. The AC current control unit 21 outputs control signals for modes 1 and 2 of the positive arm 9 and the negative arm 10. Of these, the control signals for modes 1 and 2 of the positive arm 9 are output as the unit converter gate signal Gs of the positive arm 9 via the inverter 23, the mode changeover switch 24, and the phase shift PWM 25. The control signals for modes 1 and 2 of the negative arm 10 are output as the unit converter gate signal Gs of the positive arm 9 via the mode changeover switch 24 and the phase shift PWM 25. A preset phase difference between the positive and negative carriers is input to the phase shift PWM 25 of the negative arm 10.

On the other hand, in order to control the circulating current Iz, an average value control unit 26 of the cell capacitor voltage Vc is provided, and the feedback manipulated variable current value Fi output from there is input to the circulating current control unit 27. The voltage command value Va output from the circulating current control unit 27 is added to the control signals of modes 1 and 2 of the positive arm 9 and the negative arm 10 output from the AC current control unit 21 via an adder 29 via a 1/2 calculator 28. In this case, the first DC capacitor voltage is added to the control signal of mode 1 of the positive arm 9, and the second DC capacitor voltage is added to the control signal of mode 2 of the negative arm 10. That is, the voltages of the first DC capacitor C1 and the second DC capacitor C2 are detected, and a gate signal Gs of the unit converter H is generated so that when the first switching leg 5 is a positive output, the voltage output by the positive arm 9 includes the voltage of the first DC capacitor C1, and when the second switching leg 6 is a negative output, the voltage output by the negative arm 10 includes the voltage of the second DC capacitor C2.

In this embodiment, the average value control unit 26 of the cell capacitor voltage Vc calculates the feedback control amount current value Fi that controls the average value of the cell capacitor voltages Vc of all the unit converters H to a predetermined value. The circulating current control unit 27 generates a voltage command value Va so as to follow the circulating current command value including the feedback control amount current value Fi.

In mode 1, as shown in FIG. 3(1), the positive arm 9 adds the voltage of the first DC capacitor C1 to the voltage command value Va and outputs the result. In mode 2, as shown in FIG. 3(2), the negative arm 10 adds the voltage of the second DC capacitor C2 to the voltage command value Va and outputs the result. This makes it possible to control the cell capacitor voltage Vc to an appropriate value even if the DC voltage drops.

That is, when a DC voltage source DC is connected to the positive terminal of the first DC capacitor C1 and the negative terminal of the second DC capacitor C2, the average value control unit 26 of the cell capacitor voltage Vc calculates a feedback control amount current value Fi that sets the average value of the voltages of the cell capacitors Cc included in each of the multiple unit converters H to a predetermined value. Then, a gate signal Gs is generated for the first switching leg 5, the second switching leg 6, and the multiple unit converters H so that the circulating current Iz passing through the positive arm 9 and the negative arm 10 follows the circulating current command value including the feedback control amount current value Fi.

In this embodiment having such a configuration, when a voltage source is connected to the DC terminal, the voltages of the first DC capacitor C1 and the second DC capacitor C2 are maintained, so the AC current command value can be freely given. When an AC current flows, the cell capacitor voltage Vc decreases, but as described above, the AC current command value is controlled to an appropriate value by the average value control unit 26 of the cell capacitor voltage Vc and the circulating current control unit 27.

[5. Fifth embodiment]
Fig. 9 shows a control block when a current source such as a solar power generation is connected to the DC terminal. When a current source is connected, it is necessary to actively control the voltages of the first DC capacitor C1 and the second DC capacitor C2. Therefore, in this embodiment, the control circuit 20 includes a DC capacitor voltage control unit 30 in the front stage of the AC current control unit 21 in addition to the average value control unit 26 of the cell capacitor voltage Vc shown in Fig. 8.

The cell capacitor voltage Vc average value control unit 26 calculates a first feedback control amount current value Fi1 that controls the average value of the cell capacitor voltages Vc of all unit converters H to a predetermined value. The DC capacitor voltage control unit 30 calculates a second feedback control amount current value Fi2 that sets the average value of the voltages of the first DC capacitor C1 and the second DC capacitor C2 to a predetermined value. The circulating current control unit 27 calculates the sum of the first feedback control amount current value Fi1 and the second feedback control amount current value Fi2 using an adder 29, and generates a voltage command value Va to follow the circulating current command value including the value of this sum.

In this embodiment, a multiplier 31 is provided on the output side of the average value control unit 26 and the DC capacitor voltage control unit 30, and the AC current and circulating current input to the AC current control unit 21 and the circulating current control unit 27 are multiplied by gains K1 to K4, which are predetermined constants. In other words, since the AC current and circulating current required to control the DC capacitor voltage and the cell capacitor voltage are different, the DC capacitor voltage and the cell capacitor voltage can be controlled with high precision by multiplying them by predetermined gains represented by K1 to K4 using the multiplier 31.

That is, when a DC current source DC is connected to the positive terminal of the first DC capacitor C1 and the negative terminal of the second DC capacitor C2, the control circuit 20 uses the average value control unit 26 of the cell capacitor voltage Vc and the DC capacitor voltage control unit 30 to calculate the sum of the first feedback control amount current value Fi1, which sets the average value of the voltages of the cell capacitors Cc included in each of the multiple unit converters H to a predetermined value, and the second feedback control amount current value Fi2, which sets the average value of the voltages of the first DC capacitor C1 and the second DC capacitor C2 to a predetermined value, by the adder 29. Then, the control circuit 20 generates gate signals Gs for the first switching leg 5, the second switching leg 6, and the multiple unit converters H so that the circulating current Iz passing through the positive arm 9 and the negative arm 10 follows the circulating current command value including the sum.

In this embodiment having such a configuration, in mode 1, as shown in FIG. 3 (1), the positive arm 9 adds the voltage of the first DC capacitor C1 to the voltage command value Va and outputs it. In mode 2, as shown in FIG. 3 (2), the negative arm 10 adds the voltage of the second DC capacitor C2 to the voltage command value Va and outputs it. This makes it possible to control the cell capacitor voltage Vc to an appropriate value even if the DC voltage drops.

In addition, when a current source is connected to the DC terminal, it is also necessary to control the voltages of the first DC capacitor C1 and the second DC capacitor C2. For this reason, the average value of the cell capacitor voltage Vc is controlled, and the AC current is controlled to follow the AC current command value obtained by multiplying and adding an appropriate gain to the operation amount of the DC capacitor voltage control unit 30. This makes it possible to control the cell capacitor voltage Vc and the DC capacitor voltage to appropriate values.

6. Other embodiments
The present invention is not limited to the above-described embodiment, and in the implementation stage, the components can be modified and embodied without departing from the gist of the invention. In addition, various inventions can be formed by appropriately combining the multiple components disclosed in the above-described embodiment. For example, some components may be deleted from all the components shown in the embodiment. Furthermore, components from different embodiments may be appropriately combined.

S1 to S4...Switching element C1...First DC capacitor C2...Second DC capacitor Cc...Cell capacitor H...Unit converter Vs...Voltage detection signal Va...Voltage command value Gs...Gate signal Fi...Feedback operation amount current value 1...Power conversion device 2...Positive side DC terminal 3...Negative side DC terminal 4...Neutral point 5...First switching leg 6...Second switching leg 7...First output terminal 8...Second output terminal 9...Positive side arm 10...Negative side arm 11...AC terminal 12...Buffer reactor 13...Bridge cell 14...Capacitor voltage detection circuit 20...Control circuit 21...AC current control unit 22...Comparator 23...Inverter 24...Mode changeover switch 25...Phase shift PWM
26: Average value control unit 27: Circulating current control unit 28: 1/2 calculator 29: Adder 30: DC capacitor voltage control unit 31: Multiplier

Claims (8)

A first DC capacitor;
a second DC capacitor connected in series with the first DC capacitor;
a first switching leg connected in parallel with the first DC capacitor;
a second switching leg connected in parallel with the second DC capacitor;
a positive arm including a plurality of bipolar unit converters connected in series between an AC terminal connected to an AC power source and an output terminal of the first switching leg;
a negative arm including a plurality of bipolar unit converters connected in series between the AC terminal and an output terminal of the second switching leg;
a control circuit connected to the first DC capacitor, the second DC capacitor, the first switching leg, the second switching leg, the positive side arm, and the negative side arm, and configured to detect or control input/output of these devices;
Equipped with
The control circuit includes:
When the total voltage of the first DC capacitor and the second DC capacitor is smaller than the peak value of the AC power supply,
the positive arm outputs a bipolar voltage when the first switching leg is outputting a positive voltage, and outputs a positive voltage when the first switching leg is outputting a negative voltage,
The power conversion device according to claim 1, wherein the negative arm is controlled to output a voltage of positive polarity when the second switching leg is outputting a positive output, and a voltage of both polarities when the second switching leg is outputting a negative output. The power conversion device according to claim 1, characterized in that the unit converter includes a cell capacitor, and the voltage of the cell capacitor x the number of bipolar unit converters is greater than the peak value of the AC power supply. The power conversion device according to claim 1 or 2, characterized in that, when generating a gate signal to control the unit converter, the control circuit determines the phase of a positive carrier group that generates the gate signal of the unit converter in the positive arm and a negative carrier group that generates the gate signal of the unit converter in the negative arm based on the total voltage of the first DC capacitor and the second DC capacitor. The power conversion device according to claim 1 or 2, characterized in that when the total voltage of the first DC capacitor and the second DC capacitor is smaller than the peak value of the AC power supply, the phases of the positive carrier group that generates the gate signal of the unit converter in the positive arm and the negative carrier group that generates the gate signal of the unit converter in the negative arm are shifted by 90 degrees. The power conversion device according to claim 1 or 2, characterized in that the duty ratio of the gate signal provided to the first switching leg and the duty ratio of the gate signal provided to the second switching leg are changed. The control circuit is a circuit that generates gate signals to control the unit converter, the first switching leg, and the second switching leg,
Detecting a voltage across the first DC capacitor and the second DC capacitor;
When the first switching leg has a positive output, a voltage output from the positive side arm includes a voltage of the first DC capacitor, and when the second switching leg has a negative output, a voltage output from the negative side arm includes a voltage of the second DC capacitor.
3. The power conversion device according to claim 1, further comprising: a gate signal for the unit converter. The control circuit is a circuit that generates gate signals for the first switching leg, the second switching leg, and the plurality of unit converters,
A cell capacitor voltage average value control section;
a circulating current control unit that outputs a voltage command value based on the feedback manipulated variable current value output from the average value control unit,
When a DC voltage source is connected to the positive terminal of the first DC capacitor and the negative terminal of the second DC capacitor,
Calculating a feedback control amount current value that sets an average value of the cell capacitor voltages included in each of the plurality of unit converters to a predetermined value by the average value control;
7. The power conversion device according to claim 6, wherein a circulating current passing through the positive side arm and the negative side arm is controlled so as to follow a circulating current command value including the feedback control amount current value, based on the voltage command value from the circulating current control unit. The control circuit further includes a DC capacitor voltage control unit,
When a DC current source is connected to the positive terminal of the first DC capacitor and the negative terminal of the second DC capacitor,
a first feedback manipulated variable current value that sets an average value of cell capacitor voltages included in each of the plurality of unit converters as a predetermined value;
a second feedback manipulated variable current value that sets the sum of the voltages of the first DC capacitor and the second DC capacitor to a predetermined value;
8. The power conversion device according to claim 7, wherein a circulating current passing through the positive arm and the negative arm is controlled to follow a circulating current command value including the sum.