Power converter

Abstract

To provide a power conversion device that equalizes a first holding voltage Vc1 and a second holding voltage Vc2 from alow load to a high load without using a balancer.SOLUTION: A power conversion device includes: a DC / AC conversion unit 10 that includes a first capacitor C1 and a second capacitor C2 connected in series between a positive pole P and a negative pole N of a voltage E of a DC power supply and outputs AC power by switching either a zero pole Z which is a connection point between the positive pole P, the first capacitor C1, and the second capacitor C2, or the negative pole N; and a processing unit 20 that controls switching of the DC / AC conversion unit 10 according to a current command and a voltage command. If it is determined that the active power is equal to or higher than a threshold value, the processing unit 20 corrects the voltage command with a DC signal, and if it is determined that the active power is less than the threshold value, the current changes and the current command is corrected with an AC signal whose average value per unit time is DC.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a technique for equalizing the holding voltages of capacitors in a series capacitor circuit provided in an input DC power supply.

Background Art

[0002] As a power conversion device that converts a DC power supply input from a solar power generation system, a storage battery, etc. into an AC voltage, a three-level type that can output a waveform closer to a sine wave is often used compared to a two-level type. In a three-level type power conversion device, a neutral point is provided that divides the DC voltage between the positive and negative electrodes. Further, in a three-level type power conversion device, a first capacitor is provided between the positive electrode and the neutral point, and a second capacitor is provided between the neutral point and the negative electrode. In such a power conversion device, the holding voltage of the first capacitor is output as the positive side of the AC waveform, and the holding voltage of the second capacitor is output as the negative side of the AC waveform.

[0003] Therefore, in the power conversion device, it is necessary to equalize (balance) the holding voltage of the first capacitor and the holding voltage of the second capacitor so that they are equal to each other. As a technique for equalizing the holding voltages of both capacitors, for example, the techniques described in Patent Document 1 and Patent Document 2 are known.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, the technology described in Patent Document 1 requires a separate balancer, such as a reactor or switching element, to equalize the holding voltage of both capacitors, which leads to problems such as increased size and cost due to the increase in capacitance. The technology described in Patent Document 2 has a problem in that the equalization effect is weakened at low loads when the current flowing is small.

[0006] In view of these circumstances, the object of the present invention is to provide a technology that can maintain the equalization effect even at low loads without using a balancer. [Means for solving the problem]

[0007] A power conversion device according to one aspect of the present invention includes a first capacitor and a second capacitor connected in series between the positive and negative electrodes of a DC power supply, and comprises a DC / AC conversion unit that outputs AC power by switching to either the positive electrode, the zero electrode which is the connection point of the first and second capacitors, or the negative electrode, and a processing unit that controls the switching of the DC / AC conversion unit according to a current command and a voltage command, wherein the processing unit determines whether the active power of the AC power is above a threshold, corrects the voltage command with a DC signal if it determines that the active power is above a threshold, and corrects the current command with an AC signal if it determines that the active power is below a threshold. According to this embodiment of the power conversion device, it becomes possible to maintain the equalization effect even at low loads without using a balancer. [Brief explanation of the drawing]

[0008] [Figure 1] This figure shows the circuit configuration of the power conversion device according to the embodiment. [Figure 2] This is a block diagram showing the configuration of the processing unit in a power converter. [Figure 3] This figure shows the circuit configuration of a power conversion device related to an application example. [Figure 4]This diagram shows the circuit configuration of a conventional example (Patent Document 1). [Figure 5] This figure shows the circuit configuration of a conventional example (Patent Document 2). [Modes for carrying out the invention]

[0009] Hereinafter, a power conversion device according to an embodiment of the present invention will be described with reference to the drawings. Figure 1 is a diagram showing the circuit configuration of a power conversion device 1 that takes a DC power supply voltage E as input and converts it into, for example, a single-phase AC voltage for output.

[0010] As shown in the figure, the power converter 1 has a DC / AC conversion unit 10 and a processing unit 20. The DC / AC conversion unit 10 is a three-level inverter that outputs a DC voltage E at three levels: positive pole P, zero pole Z, and negative pole N, and includes a first capacitor C1, a second capacitor C2, and switch circuits Q1 to Q6.

[0011] The first capacitor C1 and the second capacitor C2 are connected in series between the positive electrode P and the negative electrode N, and divide the DC voltage E that makes up the input. The connection point of the first capacitor C1 and the second capacitor C2 is the zero pole Z. In switch circuits Q1 to Q6, the switching element and the diode element are connected in parallel in opposite directions. Of these, switch circuits Q1 to Q4 are connected in series in four stages between the positive electrode P and the negative electrode N. The connection point of switch circuits Q2 and Q3 is the output point Out.

[0012] Switch circuit Q5 is provided between the connection point of switch circuits Q1 and Q2 and the zero pole Z, and blocks the current flowing from the connection point towards the zero pole Z. Therefore, the connection point of switch circuits Q1 and Q2 is clamped to the potential of the zero pole Z by switch circuit Q5. Switch circuit Q6 is provided between the zero pole Z and the connection point of switch circuits Q3 and Q4, and blocks the current flowing from the zero pole Z to the connection point. Therefore, the connection point of switch circuits Q3 and Q4 is clamped to the potential of the zero pole Z by switch circuit Q6. Note that the switch circuits Q5 and Q6 may be replaced with diode elements having a reverse withstand voltage.

[0013] The first holding voltage Vc1 held by the first capacitor C1, the second holding voltage Vc2 held by the second capacitor C2, the current i_d flowing from the output point Out to the load, and the current i_d flowing from the output point Out to the load are respectively detected by sensors, and these detection results are respectively supplied to the processing unit 20. Based on these detection results, voltage commands, and current commands, the processing unit 20 controls the on and off of the switching elements in the switch circuits Q1 to Q4.

[0014] Next, the processing unit 20 will be described. The operations in the processing unit 20 are executed by digital processing using, for example, a DSP (Digital Signal Processor), but here, for the sake of convenience, it will be described as being executed by analog processing.

[0015] FIG. 2 is a block diagram showing the configuration of the processing unit 20. As shown in the figure, the processing unit 20 includes a voltage difference calculation unit 210, an active power determination unit 220, a voltage correction value calculation unit 230, a current correction value calculation unit 240, and a control unit 250.

[0016] The voltage difference calculation unit 210 includes an addition unit 211 and an LPF (Low Pass Filter) 212. The addition unit 211 subtracts the second holding voltage Vc2 input to the subtraction input terminal (-) from the first holding voltage Vc1 input to the addition input terminal (+), and outputs the subtraction result. In this description, the addition unit refers to a unit that performs not only two-input addition but also general addition and subtraction such as subtraction of one input from the other input and addition of a positive value and a negative value.

[0017] The LPF 212 passes the low-frequency component of the output of the addition unit 211, blocks the high-frequency component, and outputs it as a DC voltage difference Dc. If the voltage difference Dc is zero, it indicates that the first holding voltage Vc1 and the second holding voltage Vc2 are equal and balanced. If the voltage difference Dc is a positive value, it indicates that Vc1 > Vc2. If the voltage difference Dc is a negative value, it indicates that Vc1 < Vc2.

[0018] The active power determination unit 220 includes an active power calculation unit 221, a comparator 222, a switch 223, a NOT circuit 224, and a switch 225. The active power calculation unit 221 inputs the voltage e_d and the current i_d output from the DC / AC conversion unit 10, and calculates the active power Pw. For example, the active power calculation unit 221 multiplies the instantaneous values of the voltage e_d and the current i_d and averages the multiplication result to calculate the active power Pw.

[0019] If the active power Pw supplied to the negative input terminal (-) of the comparator 222 is greater than or equal to the threshold value Pth supplied to the positive input terminal (+), the comparator 222 outputs a signal Det of L level. If the active power Pw is less than the threshold value Pth, the comparator 222 outputs a signal Det of H level.

[0020] The switches 223 and 225 are each single-pole double-throw types. If the signal Det is at L level, the switch 223 selects the coefficient "1" as shown by the solid line in the figure. If the signal Det is at H level, the switch 223 selects the coefficient "0" as shown by the dashed line in the figure, and outputs the selected coefficient to the current correction value calculation unit 240. The switch 225 selects the coefficient "0" or "1" according to the signal obtained by inverting the logic level of the signal Det by the NOT circuit 224. Specifically, if the signal Det is at L level, the switch 225 selects the coefficient "0" as shown by the solid line in the figure. If the signal Det is at H level, the switch 225 selects the coefficient "1" as shown by the dashed line in the figure, and outputs the selected coefficient to the voltage correction value calculation unit 230. Therefore, the active power determination unit 220 exclusively supplies the coefficient "0" or "1" to the voltage correction value calculation unit 230 and the current correction value calculation unit 240.

[0021] The voltage correction value calculation unit 230 includes an adjustment unit 231 and a multiplication unit 232. The adjustment unit 231 is, for example, an ACR (Automatic Current Regulator) that automatically adjusts the current of the voltage difference Dc. The multiplication unit 232 multiplies the voltage difference Dc adjusted by the adjustment unit 231 by a coefficient selected by the switch 225, and supplies the product of this multiplication as a voltage correction value e_am to the control unit 250.

[0022] The current correction value calculation unit 240 includes a positive / negative determination unit 241, a switch 242, a multiplication unit 243, an adjustment unit 244, an absolute value calculation unit 245, an LPF 246, an adjustment unit 247, and a multiplication unit 248. The positive / negative determination unit 241 determines the positive or negative sign of the voltage difference Dc and switches the selection of the switch 242 accordingly. The switch 242 is a single-pole double-throw type, with a positive half-wave of the cosine wave supplied to one of its two input terminals and a negative half-wave of the cosine wave supplied to the other input terminal.

[0023] In this configuration, if the voltage difference Dc is determined to be positive by the positive / negative determination unit 241, a positive half-wave is selected by the switch 242. If the voltage difference Dc is determined to be negative, a negative half-wave is selected, and the selected half-wave is supplied to one of the two input terminals of the multiplication unit 243.

[0024] The adjustment unit 244 automatically adjusts the current of the voltage difference Dc. The absolute value calculation unit 245 outputs the absolute value of the voltage difference Dc adjusted by the adjustment unit 244. The LPF 246 blocks the high-frequency component of the absolute value of the voltage difference Dc, and the adjustment unit 247 automatically adjusts the current of the voltage difference Dc from which the high-frequency component has been blocked and supplies it to one of the two input terminals of the multiplication unit 248. The other input terminal of the multiplication unit 248 is supplied with a coefficient output from the active power determination unit 220. The multiplication unit 248 multiplies the coefficient output from the active power determination unit 220 by the absolute value of the voltage difference Dc, which has its high-frequency components blocked and has been adjusted by the adjustment unit 247, and supplies the result to the other input terminal of the multiplication unit 243. The multiplication unit 243 multiplies the positive or negative half-wave of the cosine wave selected by the switch 242 by the multiplication result of the multiplication unit 248, and supplies the product of the multiplication as the current correction value i_am to the control unit 250.

[0025] The control unit 250 includes adders 251 and 252, an adjustment unit 253, adders 254 and 255, and a λ conversion unit 206. The addition unit 251 adds the current correction value i_am to the current command i_p supplied from an external source. The current command i_p is the target value of the AC current output from the DC / AC conversion unit 10. The adder 252 subtracts the current i_d output from the DC / AC converter 10 from the summation result of the adder 251. Therefore, the subtraction result by the adder 252 becomes the current deviation i_er, which indicates how much the current actually output in the feedback control deviates from the target current.

[0026] The adjustment unit 253 automatically adjusts the current of the current deviation i_er. The addition unit 254 adds the adjusted current deviation i_er to the voltage command e_p supplied from an external source. The voltage command e_p is the target value of the AC voltage output from the DC / AC conversion unit 10. The addition unit 255 adds the voltage correction value e_am to the summation result of the addition unit 254. The λ conversion unit 256 converts the summation result of the summation unit 255 into a modulation index λ. The modulation index λ is compared with a carrier (not shown in the diagram). This comparison generates a PWM signal, which is supplied as the gate signal for switch circuits Q1 to Q4.

[0027] Next, the operation of the power converter 1 will be explained. For convenience, in order to explain the basic processing of the processing unit 20, we will assume a configuration in which the voltage difference calculation unit 210, the active power determination unit 220, the voltage correction value calculation unit 230, and the current correction value calculation unit do not exist, and only the control unit 250 exists. In this configuration, the control unit 250 controls the switching of switch circuits Q1 to Q4 so that the current i_d output from the DC / AC converter 10 matches the target current command i_p. Specifically, the control unit 250 controls the current deviation i_er to zero, that is, so that the current i_d output from the DC / AC converter 10 matches the current command i_p, using feedback, and also controls the voltage e_d output by the voltage command e_p using feedforward.

[0028] Next, the operation of the configuration of an embodiment in which the processing unit 20 includes a voltage difference calculation unit 210, an active power determination unit 220, a voltage correction value calculation unit 230, and a current correction value calculation unit 240 will be described.

[0029] If the active power determination unit 220 determines that the active power Pw is equal to or greater than the threshold Pth, it supplies a coefficient "1" to the voltage correction value calculation unit 230 and a coefficient "0" to the current correction value calculation unit 240. Therefore, the voltage correction value e_am calculated by the voltage correction value calculation unit 230 is obtained by adjusting the voltage difference Dc between the first holding voltage Vc1 and the second holding voltage Vc2 by the adjustment unit 231. On the other hand, the current correction value i_am calculated by the current correction value calculation unit 240 is zero regardless of the output by the adjustment unit 247 and the selection by the switch 242.

[0030] If the active power Pw is greater than or equal to the threshold Pth, the voltage command e_p is corrected by adding a voltage correction value e_am, and the voltage e_d output from the DC / AC converter 10 is controlled according to the corrected voltage command. More specifically, if the active power Pw is greater than or equal to the threshold Pth, the AC voltage command e_p is offset by a voltage correction value e_am corresponding to the voltage difference Dc, and the unbalanced state of the first holding voltage Vc1 and the second holding voltage Vc2 is controlled to be balanced.

[0031] If the active power determination unit 220 determines that the active power Pw is less than the threshold Pth, it supplies a coefficient of "0" to the voltage correction value calculation unit 230 and a coefficient of "1" to the current correction value calculation unit 240. Therefore, the voltage correction value e_am calculated by the voltage correction value calculation unit 230 is zero regardless of the voltage difference Dc between the first holding voltage Vc1 and the second holding voltage Vc2. On the other hand, the current correction value i_am calculated by the current correction value calculation unit 240 is a half-wave corresponding to the polarity of the voltage difference Dc, and the amplitude of the half-wave is a magnitude corresponding to the absolute value of the voltage difference Dc. Note that the current correction value i_am becomes DC when viewed as a time average value.

[0032] If the active power Pw is less than the threshold Pth, the current command i_p is corrected by adding a current correction value i_am, and the current i_d output from the DC / AC converter 10 is feedback controlled so that the current deviation i_er between the corrected current command (i_p + i_am) and the current i_d becomes zero. In detail, if the active power Pw is less than the threshold Pth, the feedback control of the output current is combined with balance control of the first holding voltage Vc1 and the second holding voltage Vc2. Therefore, since balance control is performed by distorting the current command i_p by the current correction value i_am, even if the current output from the DC / AC conversion unit 10 is small and the active power is below the threshold, the equalization of the first holding voltage Vc1 and the second holding voltage Vc2 can be maintained.

[0033] Next, we will explain the specific operation of the first holding voltage Vc1 and the second holding voltage Vc2. When switch circuits Q1 and Q2 are turned on, the power discharged from the first capacitor C1 is stored in an inductor (not shown) connected to the load, for example. Next, when switch circuits Q1 and Q2 are turned off, the power stored in the inductor recirculates through the zero pole Z, the second capacitor C2, the diode of switch circuit Q4, and the diode of switch circuit Q3 in sequence, thereby charging the second capacitor C2. Therefore, turning switch circuits Q1 and Q2 on and off lowers the first holding voltage Vc1 held by the first capacitor C1 and raises the second holding voltage Vc2 held by the second capacitor C2.

[0034] On the other hand, when switch circuits Q3 and Q4 are turned on, the second capacitor C2 discharges, and the discharged power is stored, for example, in an inductor connected to the load. Next, when switch circuits Q3 and Q4 are turned off, the power stored in the inductor recirculates through the diode of switch circuit Q2 and the diode of switch circuit Q1, the positive electrode P, the first capacitor C1, and the zero electrode Z in sequence, so that the first capacitor C1 is charged. Therefore, turning switch circuits Q3 and Q4 on and off increases the first holding voltage Vc1 and decreases the second holding voltage Vc2.

[0035] In this embodiment, if Vc1 > Vc2 and the voltage difference becomes positive, the voltage command e_p is corrected by the voltage correction value e_am, or the current command i_p is corrected by the current correction value i_am. As a result, the on / off frequency (on / off time) of switch circuits Q1 and Q2 becomes higher (longer) than the on / off frequency (longer time) of switch circuits Q3 and Q4. Therefore, the first holding voltage Vc1 decreases and the second holding voltage Vc2 increases, correcting the Vc1 > Vc2 condition.

[0036] On the one hand, if Vc1 < Vc2 and the voltage difference becomes negative, the voltage command e_p is corrected by the voltage correction value e_am, or the current command i_p is corrected by the current correction value i_am. As a result, the on / off frequency (on / off time) of the switch circuits Q3 and Q4 becomes higher (longer time) than that of the switch circuits Q1 and Q2. For this reason, the first holding voltage Vc1 increases, the second holding voltage Vc2 decreases, and the state of Vc1 < Vc2 is corrected.

[0037] Therefore, in this embodiment, if a difference occurs between the first holding voltage Vc1 and the second holding voltage Vc2, balance control is executed to make the difference zero. Also, when the active power Pw is greater than or equal to the threshold value Pth in this embodiment, since the voltage correction value e_am is a value corresponding to the voltage difference Dc, it functions as an operation amount in the equalization control of the first holding voltage Vc1 and the second holding voltage Vc2. Also, when the active power Pw is less than the threshold value Pth, since the current correction value i_am is the product of a half-wave corresponding to the polarity of the voltage difference Dc and the absolute value of the voltage difference Dc, it functions as an operation amount in the equalization control of the first holding voltage Vc1 and the second holding voltage Vc2. Therefore, in this embodiment, whether the active power Pw is greater than or equal to the threshold value Pth or less than the threshold value Pth, the first holding voltage Vc1 and the second holding voltage Vc2 are quickly equalized.

[0038] [ In explaining the effects of this embodiment, the configurations and problems in Patent Documents 1 and 2 are examined in detail.

[0039] First, as shown in FIG. 4, in the power conversion device 1a described in Patent Document 1, a balancer BL is provided to equalize the first holding voltage Vc1 held by the first capacitor C1 and the second holding voltage Vc2 held by the second capacitor C2. Specifically, in the power conversion device 1a, a series circuit of switch circuits Q11 and Q12 is provided between the positive electrode P and the negative electrode N, and a reactor L1 is provided between the zero electrode Z and the connection point of the switch circuits Q11 and Q12.

[0040] In this configuration, for example, if Vc1 > Vc2, the switch circuit Q11 is turned on, and the first capacitor C1 is discharged. The power discharged at this time is stored in the reactor L1. Next, the switch circuit Q11 is turned off, and the power stored in the reactor L1 is returned to the second capacitor C2 via the diode of the switch circuit Q12. This return increases the second holding voltage Vc2, and the first holding voltage Vc1 and the second holding voltage Vc2 are equalized.

[0041] However, as mentioned above, the technology described in Patent Document 1 requires a balancer BL consisting of a reactor L1 and switch circuits Q11 and Q12 for equalization, which leads to problems such as increased size and cost due to increased capacity.

[0042] Next, as shown in Figure 5, the power converter 1b described in Patent Document 2 detects a first holding voltage Vc1 and a second holding voltage Vc2 for equalization and controls the AC voltage of each phase according to the voltage difference between them. In the power converter 1b, for example, if Vc1 > Vc2, the switch circuit between the positive pole P and the zero pole Z is turned on to discharge the first capacitor C1. The power discharged at this time is stored in an inductor (omitted in Figure 5) connected to the AC load. Next, the switch circuit between the positive pole P and the zero pole Z is turned off, and the power stored in the inductor is returned to the second capacitor C2 via the diode in the switch circuit between the zero pole Z and the negative pole N. This return increases the second holding voltage Vc2, equalizing the first holding voltage Vc1 and the second holding voltage Vc2.

[0043] However, the technology described in Patent Document 2 exhibits an equalization effect under high load conditions where a large amount of current flows, but under low load conditions, the equalization effect is weak because the current flowing is small even if the AC voltage is controlled. Therefore, as mentioned above, there is a problem in that it is difficult to equalize the voltage when the first holding voltage Vc1 and the second holding voltage Vc2 become rapidly unbalanced under low load conditions.

[0044] In contrast, in this embodiment, if the active power is above a threshold, the voltage command e_p is corrected with a DC voltage correction value e_am, thereby equalizing the first holding voltage Vc1 and the second holding voltage Vc2. On the other hand, if the active power is below a threshold, the current correction value i_am is added to the feedback control current command i_p, distorting the current command, thereby equalizing the first holding voltage Vc1 and the second holding voltage Vc2. Therefore, in the power converter 1 according to this embodiment, it is possible to equalize the first holding voltage Vc1 and the second holding voltage Vc2 from low load to high load without using a balancer.

[0045] In the embodiment described above, the processing in the processing unit 20 was explained in analog terms, but as mentioned above, digital processing is also possible. In the case of digital processing, the first holding voltage Vc1, the second holding voltage Vc2, and the currents i_d and i_d are each converted from analog to digital and supplied to the processing unit 20. In addition, the current command i_p and the voltage command e_p are supplied to the processing unit 20 digitally. Furthermore, the system should be configured so that operations such as addition, subtraction, multiplication, comparison, and filtering are processed digitally.

[0046] Furthermore, although the embodiment shows a configuration in which the DC / AC conversion unit 10 outputs single-phase AC, it may also be configured to output three-phase AC of U, V, and W, as shown in Figure 3. When outputting three-phase AC, the processing content shown in Figure 2 should be applied to each of the U, V, and W phases.

[0047] From the various forms exemplified above, the following types of characteristics can be understood, for example.

[0048] A power converter according to one aspect 1 of the present disclosure includes a first capacitor and a second capacitor connected in series between the positive and negative electrodes of a DC power supply, and comprises a DC / AC converter that outputs AC power by switching to either the positive electrode, the zero electrode which is the connection point of the first and second capacitors, or the negative electrode, and a processing unit that controls the switching of the DC / AC converter according to a current command and a voltage command, wherein the processing unit determines whether the active power of the AC power is above a threshold, and if it determines that it is above the threshold, it corrects the voltage command with a DC signal, and if it determines that it is below the threshold, it corrects the current command with an AC signal in which the current changes and the average value per unit time becomes DC.

[0049] According to Embodiment 1, if the active power is above a threshold, the voltage command is corrected by a DC signal to equalize the voltage difference between the first holding voltage and the second holding voltage, thereby equalizing the first and second holding voltages. On the other hand, if the active power is below a threshold, the current command is corrected by an AC signal that changes the current and whose average value per unit time is DC, so the current command is distorted, and even if the output current of the DC / AC converter is small and the active power is below a threshold, the first and second holding voltages are equalized. Therefore, in the power conversion device according to Embodiment 1, it becomes possible to equalize the first holding voltage and the second holding voltage from low load to high load without using a balancer.

[0050] A DC signal is a signal whose voltage can be considered constant over a unit of time. The voltage command is the target value of the AC voltage output from the DC / AC converter, used to control the switching of the DC / AC converter. The current command is the target value of the AC current output from the DC / AC converter, used to control the switching of the DC / AC converter. Correcting a voltage command with a DC signal means adding the DC signal to the voltage command and then controlling it with the resulting voltage command. Similarly, correcting a current command with an AC signal means adding the AC signal to the current command and then controlling it with the resulting current command.

[0051] In the specific embodiment 2 of embodiment 1, the AC signal is a half-wave AC. Since the current of a half-wave AC changes over time, it can be used as an AC signal to correct current commands.

[0052] In the specific embodiment 3 of embodiment 1, the DC signal is the voltage difference between the first holding voltage held by the first capacitor and the second holding voltage held by the second capacitor. According to embodiment 3, if the active power is above a threshold, the voltage command is corrected by the voltage difference between the first holding voltage and the second holding voltage, so that the equalization of the first holding voltage and the second holding voltage is performed quickly.

[0053] In a specific embodiment 4 of embodiment 3, the AC signal is obtained by multiplying a positive or negative AC wave, selected according to the polarity of the voltage difference, by the absolute value of the voltage difference. According to embodiment 4, if the active power is below a threshold, the current command is corrected by the product obtained by multiplying the positive or negative AC wave, selected according to the polarity of the voltage difference between the first holding voltage and the second holding voltage, by the absolute value of the voltage difference, thereby quickly equalizing the first holding voltage and the second holding voltage.

[0054] In a specific embodiment 5 of embodiment 1, the processing unit includes a control unit, an active power determination unit that determines whether the active power is equal to or greater than the threshold, a voltage difference calculation unit that calculates the voltage difference between the voltage held by the first capacitor and the voltage held by the second capacitor, a voltage correction value calculation unit that calculates a voltage correction value according to the voltage difference, and a current correction value calculation unit that calculates a current correction value according to the voltage difference. If the determination unit determines that the active power is equal to or greater than the threshold, the control unit corrects the voltage command with the voltage correction value, and if the determination unit determines that the active power is less than the threshold, the control unit corrects the current command with the current correction value. According to embodiment 5, the processing content in the processing unit can consist of an active power determination unit, a voltage difference calculation unit, a voltage correction value calculation unit, and a current correction value calculation unit. In addition, in embodiment 5, if the active power is above a threshold, the current correction value may be reduced to zero or negligible, and if the active power is below a threshold, the voltage correction value may be reduced to zero or negligible. [Explanation of Symbols]

[0055] 10...DC / AC conversion unit, 20...Processing unit, 200...Control unit, 210...Voltage difference calculation unit, 220...Active power determination unit, 230...Voltage correction value calculation unit, 240...Current correction value calculation unit, C1...First capacitor, C2...Second capacitor.

Claims

1. A DC / AC converter includes a first capacitor and a second capacitor connected in series between the positive and negative terminals of a DC power supply, and switches between the positive terminal, the connection point of the first and second capacitors, or the negative terminal, and the output point, so that AC power is output from the output point in three levels. A processing unit controls the switching of the DC / AC conversion unit according to current commands and voltage commands, It has, The aforementioned processing unit, Control unit and An active power determination unit that determines whether the active power of the AC power is equal to or greater than the threshold, A voltage difference calculation unit calculates the voltage difference between the voltage held by the first capacitor and the voltage held by the second capacitor, A voltage correction value calculation unit that calculates a voltage correction value corresponding to the voltage difference, A current correction value calculation unit that calculates a current correction value corresponding to the voltage difference, It has, The control unit, If the active power determination unit determines that the value is above the threshold, the voltage command is corrected based on the voltage correction value. If the active power determination unit determines that the value is below the threshold, the current command is corrected based on the current correction value. A power conversion device characterized by the following features.

2. The current correction value calculation unit is: A selection unit that selects a positive half-wave of the cosine wave if the voltage difference is positive, and selects a negative half-wave of the cosine wave if the voltage difference is negative. A multiplier unit multiplies the half-wave selected by the selection unit by the absolute value of the voltage difference and outputs it as the current correction value. The power conversion device according to claim 1, characterized by including the following:

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