Power conversion device and control method thereof

Abstract

To achieve miniaturization and suppression of phase voltage imbalance in a power conversion device capable of providing a self-sustaining output in a single-phase three-wire system.SOLUTION: A power conversion device that provides AC output to a single-phase three-wire AC circuit includes a series body of a first capacitor and a second capacitor, provided between two lines of a DC bus and having a midpoint voltage at an interconnection point, a midpoint voltage control unit provided between two lines of the DC bus and controlling the midpoint voltage, and a control unit that performs bipolar pulse width modulation control on the inverter and controls the midpoint voltage control unit, and an AC reactor has a core common to two lines from the output end of the inverter to a first voltage line and a second voltage line, and the control unit controls the inverter such that the line voltage between the first voltage line and the second voltage line reaches a target value, and adjusts the midpoint voltage by controlling the midpoint voltage control unit such that the absolute values of the first phase voltage and the second phase voltage are equal to each other.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present disclosure relates to a power conversion device and a control method thereof.

Background Art

[0002] A power conversion device capable of providing a single-phase three-wire independent output has been proposed, for example, in Patent Document 1. The power conversion device of Patent Document 1 includes an inverter of a two-phase half-bridge and a converter that adjusts the midpoint potential of the DC bus. This inverter has a series body of a high-side first switching element and a low-side second switching element, and a series body of a high-side third switching element and a low-side fourth switching element connected between two lines of the DC bus. The converter that controls the midpoint potential of the DC bus controls the potential of the interconnection point in the series body of a pair of capacitors connected between two lines of the DC bus to be intermediate between the potentials of the two lines of the DC bus. This interconnection point is directly connected to the neutral line of the single-phase three-wire.

[0003] The control unit of the inverter controls the opening and closing of the four switching elements by unipolar pulse width modulation control (PWM (Pulse Width Modulation) control). By alternately turning on the first switching element and the second switching element, a U-phase voltage between the first voltage line (U line) of the single-phase three-wire and the neutral line (O line) is output. By alternately turning on the third switching element and the fourth switching element, a W-phase voltage between the second voltage line (W line) of the single-phase three-wire and the neutral line (O line) is output.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] In the above-described power conversion device, the opening and closing of the first switching element and the fourth switching element, and the opening and closing of the second switching element and the third switching element are not synchronized with each other. For this reason, the ground potential of the DC bus fluctuates greatly due to switching, and a large amount of common-mode noise is generated at the AC output terminals. In order to suppress this common-mode noise, an expensive and large filter circuit including a common-mode choke coil and a Y capacitor is required. In addition, reactors for smoothing must be arranged on the U-line and the W-line, respectively. Therefore, it is difficult to miniaturize the power conversion device. Also, large iron losses occur in each reactor.

[0006] On the other hand, when supplying a self-powered output to a single-phase three-wire AC load, if there is an imbalance between the U-phase load and the W-phase load, the U-phase voltage and the W-phase voltage will not match. In this case, with the control of the neutral point potential as described above, the U-phase voltage and the W-phase voltage cannot be made to match.

[0007] The present disclosure aims to achieve miniaturization and suppression of phase voltage imbalance in a power conversion device that can provide a self-powered output in a single-phase three-wire system.

Means for Solving the Problems

[0008] The present disclosure includes the following inventions. However, the invention is defined by the scope of the claims.

[0009] (Power Conversion Device) Disclosed is a power conversion device that provides an AC output to a single-phase three-wire AC circuit, an inverter, an AC reactor provided between the inverter and the AC circuit, a DC bus that supplies a DC voltage to the inverter, a series body of a first capacitor and a second capacitor provided between two lines of the DC bus, the interconnected point of which becomes the neutral point voltage, a neutral point voltage control unit provided between two lines of the DC bus for controlling the neutral point voltage, An AC-side voltage sensor that acquires a first-phase voltage between a first voltage line and a neutral line of the AC circuit and a second-phase voltage between a second voltage line and the neutral line of the AC circuit, a control unit that performs bipolar pulse-width modulation control on the inverter and controls the neutral-point voltage control unit. The AC reactor has a core common to two lines extending from an output terminal of the inverter to the first voltage line and the second voltage line, The control unit, controls the inverter so that the line voltage between the first voltage line and the second voltage line becomes a target value, controls the neutral-point voltage control unit to adjust the neutral-point voltage so that the first-phase voltage and the second-phase voltage are equal to each other in absolute value.

[0010] (Control method of power conversion device) For a power conversion device including an inverter, an AC reactor provided between the inverter and a single-phase three-wire AC circuit, a DC bus that supplies a DC voltage to the inverter, a series body of a first capacitor and a second capacitor provided between two lines of the DC bus and having an interconnection point as a neutral-point voltage, a neutral-point voltage control unit provided between two lines of the DC bus and controlling the neutral-point voltage, and a control unit that performs bipolar pulse-width modulation control on the inverter and controls the neutral-point voltage control unit, and provides an AC output to the AC circuit, a control method by the control unit, controls the inverter so that the line voltage between the first voltage line and the second voltage line of the AC circuit becomes a target value, controls the neutral-point voltage control unit to adjust the neutral-point voltage so that the first-phase voltage and the second-phase voltage seen from the neutral line of the AC circuit are equal to each other in absolute value.

Advantages of the Invention

[0011] According to the present disclosure, in a power conversion device that can provide a self-supporting output in a single-phase three-wire system, miniaturization and suppression of unbalance in the phase voltage can be achieved.

Brief Description of the Drawings

[0012]

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Embodiments for Carrying Out the Invention

[0013] [Description of Embodiments of the Present Disclosure] The embodiments of the present disclosure mainly include at least the following.

[0014] (1) The power conversion device of the present disclosure is a power conversion device that provides an AC output to a single-phase three-wire AC circuit, and includes an inverter, an AC reactor provided between the inverter and the AC circuit, a DC bus that supplies a DC voltage to the inverter, a series body of a first capacitor and a second capacitor provided between two lines of the DC bus and having an interconnected point as a neutral point voltage, a neutral point voltage control unit provided between two lines of the DC bus for controlling the neutral point voltage, an AC side voltage sensor for acquiring a first phase voltage between a first voltage line and a neutral line of the AC circuit and a second phase voltage between a second voltage line and the neutral line of the AC circuit, and a control unit that performs bipolar pulse width modulation control on the inverter and controls the neutral point voltage control unit. The AC reactor has a core common to two lines extending from an output end of the inverter to the first voltage line and the second voltage line, and the control unit controls the inverter so that a line voltage between the first voltage line and the second voltage line becomes a target value, and controls the neutral point voltage control unit to adjust the neutral point voltage so that the absolute values of the first phase voltage and the second phase voltage are equal to each other.

[0015] In such a power conversion device, during independent operation, the inverter operates to adjust the line voltage between the first voltage line and the second voltage line to a target value without controlling the phase voltage. The control unit controls the neutral point voltage control unit to adjust the neutral point voltage so that the absolute values of the first phase voltage and the second phase voltage are equal to each other. The neutral point voltage is not necessarily an intermediate value and may deviate to either one from the intermediate value. Also, by bipolar pulse width modulation control, the ground voltage of the DC bus is stabilized, common mode noise can be reduced, and an AC reactor having a core common to two lines of the voltage line can be used. As a result, the noise filter and the AC reactor can be miniaturized, and thus the power conversion device can be miniaturized.

[0016] (2) In the power conversion device of (1) above, the control unit controls the neutral point voltage control unit so that the neutral point voltage deviates from an intermediate value of the voltage between two lines of the DC bus. In this case, even if the AC load of the first phase and the AC load of the second phase are unequal, the neutral point voltage deviates from the intermediate value, so that the first phase voltage and the second phase voltage can be made equal to each other in absolute value.

[0017] (3) In the power conversion device of (1) or (2) above, the control unit controls the neutral point voltage control unit to adjust the neutral point voltage so that the phase of the first phase voltage coincides with the phase of the inverted voltage obtained by inverting the second phase voltage. Thereby, while performing phase voltage adjustment on the AC side, the neutral point control current can be reduced as compared with the case where the phase difference between the first phase voltage and the inverted voltage is not adjusted. As a result, a DC reactor with a small current rating can be used, so that the increase in size, cost, and loss of the DC reactor can be suppressed.

[0018] (4) In the power conversion device of (1) or (2) above, the control unit controls the neutral point voltage control unit to adjust the neutral point voltage so that the phase of the first phase voltage or the phase of the inverted voltage obtained by inverting the second phase voltage coincides with the phase of the line voltage. Thereby, while performing phase voltage adjustment on the AC side, the neutral point control current can be reduced as compared with the case where the phase difference between the first phase voltage or the inverted voltage and the line voltage is not adjusted. As a result, a DC reactor with a small current rating can be used, so that the increase in size, cost, and loss of the DC reactor can be suppressed. Further, since it is only necessary to calculate the phase of one of the first phase voltage and the second phase voltage, the circuit configuration can be further simplified.

[0019] (5) In the power conversion device of (3) above, when there is a phase difference between the first phase voltage and the inverted voltage, the control unit applies a positive or negative bias voltage to the neutral point voltage to reduce the phase difference. By applying a positive or negative bias voltage to the neutral point voltage based on the phases of the first phase voltage and the inverted voltage, it is possible to adjust the phase difference between the first phase voltage and the inverted voltage to approach zero (or become zero).

[0020] (6) In the power conversion device of (4) above, when there is a phase difference between the first-phase voltage or the inversion voltage and the line voltage, the control unit applies a positive or negative bias voltage to the neutral point voltage to reduce the phase difference. By applying a positive or negative bias voltage to the neutral point voltage based on the phase of the first-phase voltage or the inversion voltage and the phase of the line voltage, it is possible to adjust the phase difference between the first-phase voltage or the inversion voltage and the line voltage to approach zero (or become zero).

[0021] (7) In the power conversion device of (3) or (4) above, the control unit applies a positive or negative bias voltage to the neutral point voltage based on at least one of the first-phase voltage at the zero-crossing detection of the line voltage and the second-phase voltage at the zero-crossing detection of the line voltage to reduce the phase difference between the first-phase voltage and the inversion voltage. Accordingly, since it is only necessary to apply a bias voltage based on each voltage at the zero-crossing detection of the line voltage instead of the phases of the first-phase voltage and the second-phase voltage, there is no need to calculate the phase, and the circuit configuration can be further simplified.

[0022] (8) In the power conversion device of (1) or (2) above, when there is a difference between the first-phase voltage and the second-phase voltage, the control unit applies a positive or negative bias voltage to the neutral point voltage to reduce the difference. Rather than necessarily making the neutral point voltage the intermediate value, by biasing it by applying a bias voltage, even if the first-phase AC load and the second-phase AC load are unequal, the first-phase voltage and the second-phase voltage can be adjusted to be equal to each other in absolute value.

[0023] (9) In the power conversion device of any one of (5) to (8) above, the bias voltage is a rectangular wave, a sine wave, or a triangular wave.

[0024] (10) In the power conversion device of (1) or (2) above, the first-phase voltage and the second-phase voltage are instantaneous values or effective values. Either instantaneous or effective values ​​can be used for the first and second phase voltages. When instantaneous values ​​are used, the control response is fast but the control is somewhat oversensitive. When effective values ​​are used, the control response is slower than that of instantaneous values, but the control is stable and not oversensitive.

[0025] (11) In the power conversion device according to any one of (1) to (10), the AC reactors are connected in a summing manner. In this case, the AC reactor can be made smaller.

[0026] (12) From the viewpoint of a control method, the control method is for a power conversion device that provides AC output to the AC circuit, the power conversion device comprising: an inverter; an AC reactor provided between the inverter and a single-phase three-wire AC circuit; a DC bus that supplies a DC voltage to the inverter; a series body of a first capacitor and a second capacitor provided between two wires of the DC bus and having an interconnection point at a midpoint voltage; a midpoint voltage control unit provided between the two wires of the DC bus and controlling the midpoint voltage; and a control unit that performs bipolar pulse width modulation control on the inverter and controls the midpoint voltage control unit, the control method comprising the steps of: controlling the inverter so that a line voltage between a first voltage line and a second voltage line of the AC circuit becomes a target value; and controlling the midpoint voltage control unit to adjust the midpoint voltage so that a first phase voltage and a second phase voltage as viewed from a neutral wire of the AC circuit become equal to each other in absolute values.

[0027] According to such a control method for a power conversion device, during an independent operation, the inverter operates to adjust the line voltage between the first voltage line and the second voltage line to a target value without controlling the phase voltage. As a control method, the midpoint voltage control unit is controlled to adjust the midpoint voltage so that the first phase voltage and the second phase voltage are equal to each other in absolute value. The midpoint voltage is not necessarily the midpoint value, and may be biased to one side from the midpoint value. In addition, the bipolar pulse width modulation control stabilizes the DC bus voltage to ground and reduces common mode noise.

[0028] [Details of Embodiments of the Present Disclosure] Hereinafter, specific examples of the power conversion device and its control method of the present disclosure will be described with reference to the drawings.

[0029] 《Configuration of Power Conversion Device》 FIG. 1 is a circuit diagram showing an example of a power conversion device. The power conversion device 100 is provided between a DC power supply 20 and an AC circuit 31. The power conversion device 100 can perform grid-connected operation with a commercial power system or autonomous operation in a state of being disconnected from the commercial power system. In the present disclosure, autonomous operation will be described.

[0030] The AC circuit 31 is single-phase three-wire and has a U wire and a W wire of the voltage line and an O wire of the grounded neutral line. A U-phase load 32u is connected to the U phase (between the U wire and the O wire) of the AC circuit 31. A W-phase load 32w is connected to the W phase (between the W wire and the O wire) of the AC circuit 31. 101 V is applied between the U wire and the O wire, 101 V with a phase opposite to that of the U phase is applied between the W wire and the O wire, and 202 V is applied between the U wire and the W wire.

[0031] The power conversion device 100 includes a DC / DC converter 1 connected to the DC power supply 20, a DC bus 2 on the high-voltage side of this DC / DC converter, a smoothing capacitor 3 connected between two lines of the DC bus 2, a neutral point voltage control unit 4 connected between two lines of the DC bus 2, a series body 5 of DC bus capacitors 5H and 5L connected between two lines of the DC bus 2, a voltage sensor 6H connected to both ends of the DC bus capacitor 5H, a voltage sensor 6L connected to both ends of the DC bus capacitor 5L, an inverter 7 connected between two lines of the DC bus 2, an AC reactor 8, AC side capacitors 9u and 9w, voltage sensors 10u and 10w, and a control unit 11. Note that the DC side voltage sensor may be composed of a voltage sensor that detects the voltage between two lines of the DC bus 2 and a voltage sensor that detects the voltage between both ends of either one of the DC bus capacitors 5H and 5L.

[0032] The voltage sensor 6H is the voltage V across both ends of the DC bus capacitor 5H HDetect it and send the detection signal to the control unit 11. The voltage sensor 6L detects the voltage V across both ends of the DC bus capacitor 5L L and sends the detection signal to the control unit 11. The capacitances of the DC bus capacitors 5H and 5L are the same as each other. The voltage sensor 10u detects the voltage V across both ends of the AC side capacitor 9u uo and sends the detection signal to the control unit 11. The voltage sensor 10w detects the voltage V across both ends of the AC side capacitor 9w wo and sends the detection signal to the control unit 11. The capacitances of the AC side capacitors 9u and 9w are the same as each other.

[0033] The DC / DC converter 1 is configured by connecting a DC reactor 12, a high-side switching element Q7, and a low-side switching element Q8 as shown in the figure. A diode d7 is connected in anti-parallel to the switching element Q7, and a diode d8 is connected in anti-parallel to the switching element Q8. This DC / DC converter 1 is a boost circuit that boosts the voltage of the DC power supply 20 to the voltage required for the DC bus 2. The DC / DC converter 1 is controlled by the control unit 11.

[0034] Note that when the voltage of the DC power supply 20 is sufficiently high, the DC / DC converter 1 can be omitted. Furthermore, when the voltage of the DC power supply 20 is too high, a buck circuit may be adopted as the DC / DC converter. The voltage between the two lines of the DC bus 2 is smoothed by the smoothing capacitor 3.

[0035] The neutral point voltage control unit 4 is a DC / DC converter and is configured by connecting a high-side switching element Q5, a low-side switching element Q6, and a DC reactor 13 as shown in the figure. The DC reactor 13 is connected to the midpoint M of the series body 5, which is the interconnection point of the DC bus capacitors 5H and 5L. A diode d5 is connected in anti-parallel to the switching element Q5, and a diode d6 is connected in anti-parallel to the switching element Q6. The neutral point voltage control unit 4 is controlled by the control unit 11.

[0036] What the midpoint voltage control unit 4 is directly trying to control is the voltage V L . Assuming the voltage between the two lines of the DC bus 2 is V B , then the voltage V H is (V B - V L ). These voltages can be V L = V H = (V B / 2), or V L > (V B / 2) > V H , or V L < (V B / 2) < V H . The midpoint M of the series body 5, which is the connection point of the DC bus capacitors 5H and 5L, is directly connected to the O line on the AC side, so it has a ground potential as the "potential".

[0037] The inverter 7 is equipped with four switching elements Q1, Q2, Q3, and Q4. Diodes d1, d2, d3, and d4 are connected in anti-parallel to the switching elements Q1, Q2, Q3, and Q4 respectively. The inverter 7 outputs 202V between the U line and the W line of the AC circuit 31 through a filter circuit composed of an AC reactor 8 and AC side capacitors 9u and 9w. The AC reactor 8 that smoothes the output of the inverter 7 has windings of the U line and the W line wound around a common core and is in a sum connection that strengthens the magnetic flux with respect to the normal current.

[0038] The voltage V uw between the U line and the W line of the AC circuit 31 is the difference between the voltage V uo detected by the voltage sensor 10u and the voltage V wo detected by the voltage sensor 10w. Note that a voltage sensor that directly detects the voltage V uw between the U line and the W line may be provided. Also, a voltage sensor that detects the voltage V uo , V wo may be provided to detect the voltage V uw between the U line and the W line instead of either one of them. In short, the voltages V uw , V uo, V wo It only needs to be in a state where it can be acquired, including detection or calculation.

[0039] The control unit 11 includes, for example, a computer, and the computer executes software (computer program) to realize necessary control functions. The software is stored in a storage device (not shown) of the control unit 11.

[0040] Note that although all of the switching elements Q1 to Q8 illustrated in FIG. 1 are IGBTs (Insulated Gate Bipolar Transistors), MOSFETs (Metal - Oxide - Semiconductor Field Effect Transistors) can be used instead.

[0041] 《Control of Power Conversion Device》 Next, the control (control method) for the inverter 7 and the neutral - point voltage control unit 4 in the power conversion device 100 configured as described above will be described.

[0042] (First Example) FIG. 2 is a control block diagram regarding the control of the inverter 7 and the control of the neutral - point voltage control unit 4 executed in the control unit 11. Such control can be entirely realized by software, but can also be partially configured by hardware. In FIG. 2, V uw A target value of the AC voltage (instantaneous value) is instructed from the voltage reference instruction unit B1. The deviation between this target value and the actually detected voltage V between the U - line and the W - line uw is obtained by the comparator B2. The deviation is added to the target value through the proportional - integral operation unit B3 (adder B4). Based on the corrected voltage value obtained by the addition, the voltage command signal generation unit B5 generates a voltage command signal. The voltage command signal is compared with the carrier signal (triangle wave or sawtooth wave) output by the carrier signal generation unit B6 in the PWM control unit B7 to become a PWM control signal.

[0043] This PWM control signal is generated from a common voltage command signal and a carrier signal, whereby the switching elements Q1, Q2, Q3, and Q4 are controlled. This control is a bipolar PWM control method in which the opening and closing of the pair of switching elements Q1 and Q4 are synchronized and complementary, and the opening and closing of the pair of switching elements Q2 and Q3 are synchronized.

[0044] As a result, the inverter 7 operates so that the voltage V between the U line and the W line uw matches the target value. By bipolar PWM control, the ground potential of the DC bus is stabilized. Therefore, common-mode noise can be reduced. Also, since the cores of the reactors of the U line and the W line can be shared and connected in series, the AC reactor 8 (Fig. 1) can be integrated into one and miniaturized.

[0045] For reference, in the power conversion device of Patent Document 1 described above, when different voltage command signals are applied to the pairs of switching elements Q1, Q2 and Q3, Q4 so that the phase voltages V uo , V wo match their respective target values, the opening and closing of the pair of switching elements Q1, Q4 in the diagonal direction of the bridge and the opening and closing of the pair of switching elements Q2, Q3 are not synchronized. For this reason, the ground potential of the DC bus fluctuates due to switching, and a large current flows through the AC reactor. Also, if the cores of the AC reactor of the U line and the AC reactor of the W line are shared and connected in series, the control becomes impossible. Therefore, the AC reactors cannot be integrated into one and miniaturized.

[0046] Next, in Fig. 2, the U-phase voltage V detected between the U line and the O line uo and the W-phase voltage V detected between the W line and the O line wo are added by the adder B8. The U-phase voltage V uo and the W-phase voltage V woThey are in opposite phases to each other and their signs are reversed. Therefore, the addition result will be either 0 or a value obtained by attaching the sign of the larger absolute value to the difference between the absolute values. This value becomes a correction value that is added by the adder B11 to the midpoint voltage reference indicated by the midpoint voltage reference instruction unit B10 via the proportional-integral operation unit B9. The midpoint voltage reference is, for example, the voltage between two lines of the DC bus, denoted as V B If we set it as B , the intermediate value is (V

[0047] The addition result in the adder B11 is compared with the voltage V L in the comparator B12. The deviation of the comparison result, that is, the deviation (bias voltage) from V L , is sent to the voltage command signal generation unit B14 via the proportional-integral operation unit B13. Then, a voltage command signal is generated in the voltage command signal generation unit B14. The voltage command signal is compared with the carrier signal (triangle wave or sawtooth wave) output by the carrier signal generation unit B15 in the PWM control unit B16, and becomes a PWM control signal for the switching elements Q5, Q6.

[0048] In this way, when there is a difference between the U-phase voltage V uo and the W-phase voltage V wo , the midpoint voltage is corrected so as to deviate from the intermediate value according to the difference. The midpoint voltage control unit 4 basically controls the voltages V L , V H to be equal to each other. In this case, the U-phase voltage V uo and the W-phase voltage V wo will generally have voltage waveforms with the same amplitude and opposite phases. However, when the difference in the capacitance of the loads connected between the U-line - O-line and the W-line - O-line is large, simply equalizing the voltages V L , V H will not make the amplitudes of the U-phase voltage V uo and the W-phase voltage V wo match each other.

[0049] Therefore, as described above, a correction value is added to the midpoint voltage reference so that the sum of the U-phase voltage V uo and the W-phase voltage V wo is always zero. In addition, correction values necessary for the U-phase voltage V uo or the W-phase voltage V wo to respectively match their control target values may be given. In short, the U-phase voltage V uo and the W-phase voltage V wo may be controlled so that their absolute values are equal to each other.

[0050] Figure 3 is a graph showing an AC output waveform when a resistive load is connected only between the U-line and the O-line of an AC circuit and the W-line and the O-line are unloaded. The numerical values on the horizontal axis are time [seconds], and the numerical values on the vertical axis are voltage [V]. The AC frequency is 50 Hz. The AC waveform with the larger amplitude is the voltage V uw between the U-line and the W-line. The two AC waveforms with the smaller amplitude and opposite phase inversion relationship are the U-phase voltage V uo , and the W-phase voltage V wo . The DC waveform is the midpoint voltage V L of the DC bus. From this result, it can be seen that even when the load is uneven, the line voltage V uw , the U-phase voltage V uo , and the W-phase voltage V wo matching the target value are obtained. Also, the midpoint voltage V L fluctuates slightly moment by moment, and AC side phase voltage adjustment is performed by biasing the midpoint voltage.

[0051] (Second Example) Figure 4 is another control block diagram different from Figure 2 regarding the control of the inverter 7 executed in the control unit 11 and the control of the midpoint voltage control unit 4. The difference from Figure 2 is the upper left midpoint voltage correction unit P1. In the upper left midpoint voltage correction unit P1, the effective value calculation unit B20 calculates the effective value based on the U-phase voltage V uo . Similarly, the effective value calculation unit B21 calculates the effective value based on the W-phase voltage V wo .

[0052] The effective value of the U phase and the effective value of the W phase are compared with each other in the comparator B22. The difference obtained by the comparison is multiplied by the sign indicated from the AC sign indicator B24 by the multiplier B25 after passing through the proportional-integral operation by the proportional-integral operation unit B23. Since the signs of both the U phase and the W phase are the same as the effective values, this multiplication means determining whether to add or subtract a correction value according to the deviation of the effective values of the U phase and the W phase. The correction value processed by the multiplier B25 is added to the midpoint voltage reference indicated by the midpoint voltage reference indicator B26 (adder B27). The midpoint voltage reference is, for example, the voltage between two lines of the DC bus as V B If it is set as, it is the intermediate value of (V B / 2).

[0053] The addition result in the adder B27 is compared with the voltage V L in the comparator B28. The deviation of the comparison result, that is, the deviation (bias voltage) from V L is sent to the voltage command signal generation unit B14 through the proportional-integral operation unit B29. The subsequent processing and the control block diagram regarding the control of the inverter 7 are the same as those in FIG. 2.

[0054] When supplying power to the load connected between the U line and the O line, current flows from the DC bus capacitor 5H between the plus-side circuit of the DC bus 2 and the midpoint M during the period when the U-phase voltage is positive. During the period when the U-phase voltage is negative, current flows from the DC bus capacitor 5L between the midpoint M and the minus-side circuit of the DC bus 2. Therefore, when the effective value of the U-phase voltage V uo is smaller than the effective value of the W-phase voltage V wo , by controlling the DC bus midpoint voltage V H >V L during the positive period and V L >V H during the negative period, the effective values of the U-phase voltage V L and the W-phase voltage V uo , V wo will match each other.

[0055] For reference, in the power conversion device described in the aforementioned Patent Document 1, the voltages V L , V HIt only performs neutral point voltage control so that it becomes equal, and does not perform control to bias the voltage (apply a bias voltage).

[0056] (Third example) FIG. 5 is another control block diagram different from FIGS. 2 and 4 regarding the control of the inverter 7 executed in the control unit 11 and the control of the neutral point voltage control unit 4. The difference from FIG. 2 is the upper left neutral point voltage correction unit P2. In the upper left neutral point voltage correction unit P2, the effective value calculation unit B20 calculates the effective value based on the U-phase voltage V uo and similarly, the effective value calculation unit B21 calculates the effective value based on the W-phase voltage V wo .

[0057] The effective value of the U-phase and the effective value of the W-phase are compared with each other in the comparator B22. The obtained difference is multiplied by the sign indicated by the AC sign indicator B24 by the proportional-integral operation unit B23 through proportional-integration. This multiplication means that since the signs of both the U-phase and the W-phase are the same as effective values, it determines whether to add or subtract a correction value according to the deviation of the effective values of the U-phase and the W-phase. The correction value passed through the process of the multiplier B25 is added to the neutral point voltage reference indicated by the neutral point voltage reference indicator B26 (adder B27). The neutral point voltage reference is, for example, the voltage between two lines of the DC bus is V B , and the intermediate value is (V B / 2).

[0058] The addition result in the adder B27 is sent to the voltage command signal generation unit B14. The subsequent processing and the control block diagram regarding the control of the inverter 7 are the same as those in FIGS. 2 and 4.

[0059] Also in the case of the third example, similar to the second example, when supplying power to the load connected between the U line and the O line, during the period when the U-phase voltage is positive, current flows from the DC bus capacitor 5H between the plus side circuit of the DC bus 2 and the neutral point M. During the period when the U-phase voltage is negative, current flows from the DC bus capacitor 5L between the neutral point M and the minus side circuit of the DC bus 2. Therefore, the U-phase voltage V uoIf the effective value of wo is smaller than the effective value of the W-phase voltage V H >V L in the positive period and V L >V H in the negative period so that the DC bus midpoint voltage V L is controlled, the effective values of the U-phase voltage V uo , and the W-phase voltage V wo will match each other.

[0060] Figure 6 is a graph showing the AC output waveform when a half-wave rectified resistive load is connected only between the U-line and the O-line of the AC circuit and the W-line and the O-line are unloaded. The numerical values on the horizontal axis are time [seconds], and the numerical values on the vertical axis are voltage [V]. The AC frequency is 50 Hz. The midpoint voltage control is based on the above third example. The AC waveform with the larger amplitude is the voltage V uw between the U-line and the W-line. The two AC waveforms with the smaller amplitude and opposite phase inversion relationship are the U-phase voltage V uo , and the W-phase voltage V wo . The DC waveform is the midpoint voltage V L of the DC bus. From this result, it can be seen that even when the load is uneven, the line voltage V uw , the U-phase voltage V uo , and the W-phase voltage V wo that match the target value are obtained. Also, the midpoint voltage V L varies slightly moment by moment, and the AC side phase voltage adjustment is performed by biasing the midpoint voltage.

[0061] (Example 4) In the above second example, in order to make the effective value of the U-phase voltage V uo match the effective value of the W-phase voltage V wo , a current (hereinafter referred to as "midpoint control current I L ") is passed from the midpoint voltage control unit 4 to the DC reactor 13. This current flows through the DC bus capacitor 5H or the DC bus capacitor 5L, and the DC bus midpoint voltage V L is controlled.

[0062] Here, in an AC circuit, the greater the degree of imbalance between the load connected between the U line and the O line and the load connected between the W line and the O line, the more the neutral point control current I uo required to make the effective value of the U-phase voltage V wo match the effective value of the W-phase voltage V L tends to increase, as revealed by the inventors' research.

[0063] FIG. 7 and FIG. 8 are both graphs showing the AC output waveform (a) and the neutral point control current (b) in the power conversion device 100 of the second example. FIG. 7 is a graph when the U line - O line of the AC circuit is unloaded and a 1.5 kW resistive load is connected between the W line and the O line. FIG. 8 is a graph when the U line - O line of the AC circuit is unloaded and a 3 kW resistive load is connected between the W line and the O line. That is, FIG. 8 shows a state with a greater degree of load imbalance than FIG. 7.

[0064] In the graphs of FIG. 7 and FIG. 8, the numerical values on the horizontal axis are time [seconds], the numerical values on the vertical axis of (a) are voltage [V], and the numerical values on the vertical axis of (b) are current [A]. The AC frequency is 50 Hz. The AC waveform with the larger amplitude is the voltage V uw between the U line and the W line. The two AC waveforms with the smaller amplitude are the inverted voltages V uo and V wo obtained by inverting the U-phase voltage V ow (=-V wo ). In FIGS. 3 and 6, the U-phase voltage V uo and the W-phase voltage V wo were shown, but from FIG. 7 onwards, in order to compare the phases of the U-phase voltage V uo and the W-phase voltage V wo , the inverted voltage V ow is shown.

[0065] The effective value of the neutral point control current I L is 31.1 A in FIG. 7(b) (load difference is 1.5 kW), while it is 62.0 A in FIG. 8(b) (load difference is 3 kW), indicating that the greater the degree of load imbalance, the more the neutral point control current I L increases.

[0066] A larger neutral point control current I L To allow a larger current to flow, a DC reactor 13 with a higher current rating is required, which causes problems such as an increase in size and cost of the DC reactor 13. Also, as the current rating of the DC reactor 13 increases, the losses in the DC reactor 13 increase, resulting in the problem of increased losses in the DC reactor 13.

[0067] Based on the above, the inventors focused on the phase difference between the U-phase voltage V uo and the W-phase voltage V wo while compensating to make the absolute values of the U-phase voltage V L and the W-phase voltage V uo equal, and invented a method to suppress the increase in the neutral point control current I ow .

[0068] Here, in Fig. 7(a), the phase difference between the U-phase voltage V uo and the inverted voltage V ow is 4.34°, while in Fig. 8(a), the phase difference between the U-phase voltage V uo and the inverted voltage V ow is 8.55°. Thus, as the degree of load imbalance increases, the phase difference between the U-phase voltage V uo and the inverted voltage V ow also increases.

[0069] In the second example above, while performing neutral point control so that the effective values of the U-phase voltage V uo and the W-phase voltage V wo match each other, control is not performed such that the phase of the U-phase voltage V uo and the phase of the inverted voltage V ow match.

[0070] Fig. 9 is a diagram for explaining the signal output from the AC symbol indicating section B24 of the second example. Fig. 9(a) is an enlarged graph showing a part of Fig. 8(a), and Fig. 9(b) is a graph showing the signal output from the AC symbol indicating section B24 of the second example. In Fig. 9(a), for the sake of explanation, the U-phase voltage V uoand the W-phase voltage V wo of the reverse voltage V ow and the phase difference therebetween are exaggeratedly shown.

[0071] In the case of the second example, the AC sign indicating unit B24 outputs +1 or -1 according to the sign of the line voltage V uw . That is, the output phase of the AC sign indicating unit B24 coincides with the phase of the line voltage V uw . In such a case, although the effective values of the U-phase voltage V uo and the W-phase voltage V wo can be made to match each other, a phase difference may occur between the U-phase voltage V uo and the reverse voltage V ow .

[0072] Therefore, in this example, the neutral point voltage control unit 4 is controlled so that the absolute values of the U-phase voltage V uo and the W-phase voltage V wo become equal to each other, and the phase of the U-phase voltage V uo coincides with the phase of the reverse voltage V ow to adjust the neutral point voltage V L . That is, the neutral point voltage control unit 4 is controlled so that the phases of the U-phase voltage V uo and the W-phase voltage V wo become 180° to adjust the neutral point voltage V L . Thereby, while performing the phase voltage adjustment on the AC side, compared with the case where the phase difference between the U-phase voltage V uo and the reverse voltage V ow is not adjusted, the neutral point control current I L is reduced.

[0073] FIG. 10 is a control block diagram showing the internal configuration of the AC sign indicating unit B241 in this example. The power conversion device 10 in this example is different in configuration from the second example in that it includes an AC sign indicating unit B241 instead of the AC sign indicating unit B24 in the second example (FIG. 4), and the other points are common.

[0074] In the AC sign indicating unit B241, the phase calculation unit B30 calculates the phase of the U-phase voltage V uo based on the U-phase voltage V uo . The phase calculation unit B31 calculates the phase of the W-phase voltage Vwo Based on this, the inversion voltage V ow phase (i.e., the phase obtained by inverting the phase of the W-phase voltage V wo by 180°) is calculated. The phase calculation unit B32 calculates the phase of the line voltage V uw based on the line voltage V uw . The phase calculation units B30 to B32 may be, for example, a zero-crossing detection circuit or a phase-locked loop (PLL) circuit.

[0075] The phase of the U-phase and the phase of the W-phase are compared with each other in the comparator B33. The obtained deviation is added to the phase of the line voltage V uw through proportional-integral by the proportional-integral operation unit B34 (adder B35). The AC sign indicating unit B241 outputs the phase corrected by addition as the phase output φ to the multiplier B25. In the multiplier B25, the difference in the effective value of each phase that has passed through the comparator B22 and the proportional-integral operation unit B23 is multiplied by the value indicated by the AC sign indicating unit B241.

[0076] That is, the control unit 11, based on the phase of the U-phase voltage V uo and the phase of the W-phase voltage V wo , applies a positive or negative bias voltage to the neutral point voltage V L to reduce the phase difference between the U-phase voltage V uo and the W-phase voltage V wo . In this way, the neutral point voltage V L is not necessarily set to the intermediate value, but is biased by applying a bias voltage, so that even if the AC load of the U-phase and the AC load of the W-phase are unequal, the U-phase voltage V uo and the W-phase voltage V wo can be adjusted to be equal to each other in absolute value. Further, based on the phase of the U-phase voltage V uo and the phase of the W-phase voltage V wo , by applying a positive or negative bias voltage to the neutral point voltage V L , the phase difference between the U-phase voltage V uo and the inversion voltage V ow can be adjusted to approach zero (or become zero).

[0077] FIG. 11 is a diagram for explaining the signal output from the AC sign indicating section B241. FIG. 11(a) is a graph showing an enlarged AC output waveform in the power conversion device 100 of this example, and FIGS. 11(b) to (d) are graphs respectively showing an example of the signal output from the AC sign indicating section B241.

[0078] The AC sign indicating section B241 performs phase output φ by feedback control by the proportional integral operation section B34 so that the difference between the phase of the U phase and the phase of the W phase becomes 0. Since the phase output φ is obtained according to the difference between the phase of the U phase and the phase of the W phase, it does not always match the phase of the line voltage V uw . As shown in FIG. 11(b), the phase output φ is, for example, a rectangular wave. When the phase output φ is a rectangular wave, the phase output φ has the following values.

[0079]

Equation

[0080] Note that the phase output φ may be a sine wave (sin φ) as shown in FIG. 11(c). Also, the phase output φ may be a triangular wave as shown in FIG. 11(d). When the phase output φ is a triangular wave, the phase output φ has the following values.

[0081]

Equation

[0082] As shown in FIG. 11(a), by making the phases of the U-phase voltage V uo and the inverted voltage V ow coincide with each other by the AC sign indicating section B241, the neutral point control current I L can be reduced.

[0083] FIG. 12 is a graph showing the AC output waveform (a) and the neutral point control current (b) in the power conversion device 100 of this example. FIG. 12 is a graph when the U line - O line of the AC circuit is under no load and a 3 kW resistive load is connected between the W line - O line. The numerical value on the horizontal axis is time [seconds], the numerical value on the vertical axis of (a) is voltage [V], and the numerical value on the vertical axis of (b) is current [A]. The AC frequency is 50 Hz. The AC waveform with the larger amplitude is the voltage V between the U line - W line uw is. The two AC waveforms with the smaller amplitude are the U - phase voltage V uo and the inverted voltage V of the W - phase ow .

[0084] As shown in FIG. 12(a), the phase difference between the U - phase voltage V uo and the inverted voltage V ow (that is, the difference between the phase of the U - phase voltage V uo and the phase obtained by inverting the phase of the W - phase voltage V wo by 180°) is almost 0. And the effective value of the neutral point control current I L in FIG. 12(b) is 39.7 A, which is lower than the value (62.0 A) in FIG. 8(b) where the load difference is 3 kW as in the example of FIG. 12.

[0085] As described above, by adjusting the neutral point voltage V uo so that the phases of the U - phase voltage V ow and the inverted voltage V L substantially coincide, the neutral point control current I L can be reduced. As a result, a reactor with a small current rating can be used as the DC reactor 13, so that the increase in size, cost, and loss of the DC reactor 13 can be suppressed.

[0086] (Example 5) FIG. 13 is a control block diagram showing the internal configuration of the AC symbol indicating unit B242 in this example. The power conversion device 100 of this example is different in configuration from the fourth example in that it includes an AC symbol indicating unit B242 instead of the AC symbol indicating unit B241 in the fourth example (FIG. 10), and the other points are common.

[0087] In the AC sign indicating section B241, the three phase operation sections B30 to B32 calculate the phases of the U-phase voltage V uo , the W-phase voltage V wo and the line voltage V uw respectively. On the other hand, in the AC sign indicating section B242, using the fact that the phase of the line voltage V uw is the intermediate value between the phase of the U-phase voltage V uo and the phase of the W-phase voltage V wo , it controls to make the difference between the phase of the U-phase voltage V uo and the phase of the line voltage V uw zero. Since the phase of the W-phase voltage V wo is linked to the phase of the U-phase voltage V uo , by this control, the phase of the inversion voltage V ow is also made to coincide with the phase of the line voltage V uw .

[0088] In the AC sign indicating section B242, the comparator B33 compares the phase of the U-phase voltage V uo calculated by the phase operation section B30 with the phase of the line voltage V uw calculated by the phase operation section B32 to calculate the deviation. The deviation is added to the phase of the line voltage V uw through proportional integral by the proportional integral operation section B34 (adder B35). The AC sign indicating section B242 outputs the phase corrected by addition as the phase output φ to the multiplier B25.

[0089] That is, the control section 11 applies a positive or negative bias voltage to the neutral point voltage V uo based on the phase of the U-phase voltage V uw and the phase of the line voltage V L to reduce the phase difference between the U-phase voltage V uo and the line voltage V uw . In this way, instead of necessarily making the neutral point voltage V L the intermediate value, by biasing it by applying a bias voltage, even if the AC loads of the U-phase and the W-phase are unequal, the U-phase voltage V uo and the W-phase voltage V wo can be adjusted to be equal to each other in absolute value. Furthermore, the U-phase voltage Vuo The phase of the and the W-phase voltage V wo Based on the phase of the and the midpoint voltage V L By applying a positive or negative bias voltage to U-phase voltage V uo and the inverted voltage V ow The phase difference between and can be adjusted to approach zero (or become zero).

[0090] According to the AC sign indicating section B242, one phase calculation section B31 can be omitted compared to the AC sign indicating section B241, so the circuit configuration can be further simplified.

[0091] Note that the AC sign indicating section B242 may be controlled to make the difference between the phase of the inverted voltage V ow and the phase of the line voltage V uw zero. In this case, the comparator B33 calculates the deviation by comparing the phase of the inverted voltage V ow calculated by the phase calculation section B31 with the phase of the line voltage V uw calculated by the phase calculation section B32.

[0092] That is, in this case, the control section 11 applies a positive or negative bias voltage to the midpoint voltage V ow based on the phase of the inverted voltage V uw and the phase of the line voltage V L to reduce the phase difference between the inverted voltage V ow and the line voltage V uw . Since the phase of the U-phase voltage V uo is linked to the phase of the inverted voltage V ow , by this control, the phase of the U-phase voltage V uo is also made to coincide with the phase of the line voltage V uw .

[0093] (Example 6) FIG. 14 is a control block diagram showing the internal configuration of the AC sign indicating section B243 in this example. The power conversion device 100 in this example is different in configuration from the fourth example (FIG. 10) in that it includes the AC sign indicating section B243 instead of the AC sign indicating section B241 in the fourth example, and the other points are common.

[0094] FIG. 15 is a graph showing an enlarged view of each voltage at the zero-crossing detection of the line voltage V uw in FIG. 9(a). The instantaneous value of the line voltage V uw at the time when the instantaneous value becomes zero (i.e., at the zero-crossing detection) of the U-phase voltage V uo is referred to as "instantaneous value X1". Also, the instantaneous value of the inverted voltage V uw of the W-phase voltage V wo at the zero-crossing detection of the line voltage V ow is referred to as "instantaneous value X2". Here, the instantaneous value X1 is the positive and negative absolute value of the instantaneous value X2 (X1 = -X2). That is, the instantaneous value X1 of the U-phase voltage V uw at the zero-crossing detection of the line voltage V uo is equal to the instantaneous value (-X2) of the W-phase voltage V wo .

[0095] In this example, by controlling so that the instantaneous value X1 of the U-phase voltage V uw at the zero-crossing detection of the line voltage V uo becomes zero, the phase of the U-phase voltage V uo is made to coincide with the phase of the line voltage V uw . Since the phase of the W-phase voltage V wo is linked to the phase of the U-phase voltage V uo , by this control, the phase of the inverted voltage V ow is also made to coincide with the phase of the line voltage V uw .

[0096] In the AC sign indicating section B243, the instantaneous value calculation section B36 calculates the instantaneous value X1 of the U-phase voltage V uo at the zero-crossing detection of the line voltage V uw based on the U-phase voltage V uw and the line voltage V uo . The comparator B33 compares the instantaneous value X1 calculated by the instantaneous value calculation section B36 with zero and calculates a deviation. The deviation is added to the phase of the line voltage V uw through proportional integral by the proportional integral calculation section B34 (adder B35). The AC sign indicating section B243 outputs the phase corrected by the addition as the phase output φ to the multiplier B25.

[0097] The instantaneous value X1 is controlled to be zero by the phase output φ. According to the AC sign indicating unit B243, since two phase calculation units B30 and B31 can be omitted compared to the AC sign indicating unit B241, the circuit configuration can be further simplified.

[0098] Note that the AC sign indicating unit B243 may control such that the instantaneous value of the W-phase voltage V uw at the zero crossing detection of the line voltage V wo becomes zero (the instantaneous value is equal to the instantaneous value X1 as described above). In this case, the instantaneous value calculation unit B36 calculates the instantaneous value of the W-phase voltage V wo and the line voltage V uw based on the line voltage V uw at the zero crossing detection of the line voltage V wo of the W-phase voltage V

[0099] That is, the control unit 11 applies a positive or negative bias voltage to the neutral point voltage V uw based on the instantaneous value X1 (the voltage of the U-phase voltage V uo at the zero crossing detection of the line voltage V uw or the instantaneous value -X2 (the voltage of the W-phase voltage V wo at the zero crossing detection of the line voltage V L ) to reduce the phase difference between the U-phase voltage V uo and the inverted voltage V ow .

[0100] (Example 7) FIG. 16 is a control block diagram showing the internal configuration of the AC sign indicating unit B244 in this example. The power conversion device 100 in this example is different in configuration from the sixth example (FIG. 14) in that it includes the AC sign indicating unit B244 instead of the AC sign indicating unit B243 in the sixth example, and the other points are common.

[0101] In the sixth example, control is performed so that the instantaneous value X1 becomes zero. In contrast, in this example, the instantaneous value X1 of the U-phase voltage V uw at the zero crossing detection of the line voltage V uo and the inverted voltage V uw at the zero crossing detection of the line voltage V owControl is performed so that the difference from the instantaneous value X2 becomes zero. That is, control is performed so that the instantaneous value X1 and the instantaneous value X2 become equal to each other.

[0102] In the AC sign indicating unit B244, the instantaneous value calculation unit B37 calculates the instantaneous value X2 of the reverse voltage V ow and the line voltage V uw based on the line voltage V uw at the zero crossing detection of the line voltage V ow The comparator B33 compares the instantaneous value X1 calculated by the instantaneous value calculation unit B36 with the instantaneous value X2 calculated by the instantaneous value calculation unit B37 to calculate a deviation. The deviation is added to the phase of the line voltage V uw through proportional integral by the proportional integral calculation unit B34 (adder B35). The AC sign indicating unit B244 outputs the phase corrected by addition as the phase output φ to the multiplier B25.

[0103] The phase output φ controls the instantaneous value X1 and the instantaneous value X2 to be equal to each other. According to the AC sign indicating unit B244, since two phase calculation units B30 and B31 can be omitted compared to the AC sign indicating unit B241, the circuit configuration can be further simplified.

[0104] That is, the control unit 11 applies a positive or negative bias voltage to the neutral point voltage V L based on the instantaneous value X1 and the instantaneous value X2, thereby reducing the phase difference between the U-phase voltage V uo and the reverse voltage V ow .

[0105] "Summary of the Disclosure" The above disclosure can be generally expressed as follows. The power conversion device 100 that provides an AC output to a single-phase three-wire AC circuit is provided between two lines of the DC bus 2, and the series body 5 of the DC bus capacitor 5H and the DC bus capacitor 5L whose interconnected point becomes the neutral point voltage V L and is provided between two lines of the DC bus 2, and the neutral point voltage V LA midpoint voltage control unit 4 for controlling is provided. The control unit 11 of the power conversion device 100 performs bipolar pulse width modulation control on the inverter 7 and controls the midpoint voltage control unit 4. The AC reactor 8 has a core common to two lines extending from the output terminal of the inverter 7 to the U line and the W line. The control unit 11 controls the inverter 7 so that the line voltage between the U line and the W line becomes a target value, and the U-phase voltage V uo and the W-phase voltage V wo The midpoint voltage control unit 4 is controlled so that the absolute values of and are equal to each other, and the midpoint voltage V L is adjusted.

[0106] In such a power conversion device 100, during self-sustained operation, the inverter 7 operates to adjust the line voltage between the U line and the W line to the target value without controlling the phase voltage. The control unit 11 controls the midpoint voltage control unit 4 to adjust the midpoint voltage V L so that the U-phase voltage and the W-phase voltage are equal to each other in absolute value. The midpoint voltage V L is not necessarily the intermediate value and may deviate (shift) to either one from the intermediate value. Also, by bipolar pulse width modulation control, the voltage to ground of the DC bus 2 is stabilized, common-mode noise can be reduced, and an AC reactor 8 having a core common to two lines (U line, W line) of the voltage line can be used. As a result, the AC reactor 8 can be miniaturized, and the power conversion device 100 can be miniaturized.

[0107] The control unit 11 can control the midpoint voltage control unit 4 so that the midpoint voltage V L deviates from the intermediate value of the voltage between the two lines of the DC bus 2. Thereby, even if the AC load of the U phase and the AC load of the W phase are unequal, the midpoint voltage deviates from the intermediate value, so that the U-phase voltage V uo and the W-phase voltage V wo can be equal to each other in absolute value.

[0108] In other words, when there is a difference between the U-phase voltage V uo and the W-phase voltage V wo , the control unit 11 controls the midpoint voltage V LA positive or negative bias voltage can be applied to reduce the difference. Thus, the midpoint voltage V L is not necessarily set to the intermediate value. By applying a bias voltage to cause a deviation, even if the AC load of the U phase and the AC load of the W phase are unequal, the U-phase voltage V uo and the W-phase voltage V wo can be adjusted so that their absolute values are equal to each other.

[0109] Since the AC reactor 8 is connected in series, it can be miniaturized. In addition, as the U-phase voltage and the W2-phase voltage, either the instantaneous value or the effective value can be used. Using the instantaneous value results in fast control response but slightly overly sensitive control. Using the effective value results in a control response that is slower than the instantaneous value but stable control that does not become overly sensitive.

[0110] <Supplementary Note> It should be considered that the embodiments disclosed this time are illustrative in all respects and not restrictive. The scope of the present invention is indicated by the claims, and all modifications within the meaning and scope equivalent to the claims are intended to be included.

Explanation of Reference Numerals

[0111] 1 DC / DC converter 2 DC bus 3 Smoothing capacitor 4 Midpoint voltage control unit 5 (Series body of capacitors) 5H, 5L DC bus capacitors 6H, 6L Voltage sensors 7 Inverter 8 AC reactor 9u, 9w AC side capacitors 10u, 10w Voltage sensors 11 Control unit 12, 13 DC reactors 20 DC power supply 31 AC circuit 32u U-phase load 32w W-phase load 100 Power conversion device d1, d2, d3, d4, d5, d6, d7, d8 Diodes Q1, Q2, Q3, Q4, Q5, Q6, Q7, Q8 Switching elements B1 V uw Voltage reference indicating section B2 Comparator B3 Proportional-integral computing section B4 Adder B5 Voltage command signal generation section B6 Carrier signal generation section B7 PWM control section B8 Adder B9 Proportional-integral computing section B10 Midpoint voltage reference indicating section B11 Adder B12 Comparator B13 Proportional-integral computing section B14 Voltage command signal generation section B15 Carrier signal generation section B16 PWM control section B20, B21 RMS value computing section B22 Comparator B23 Proportional-integral computing section B24 AC sign indicating section B25 Multiplier B26 Midpoint voltage reference indicating section B27 Adder B28 Comparator B29 Proportional-integral computing section B30, B31, B32 Phase computing section B33 Comparator B34 Proportional-integral computing section B35 Adder B36, B37 Instantaneous value computing section B241, B242, B243, B244 AC sign indicating section M Midpoint P1, P2 Midpoint voltage correction section X1, X2 Instantaneous values

Claims

1. A power conversion device that provides an AC output to a single-phase three-wire AC power circuit, an inverter, an AC reactor provided between the inverter and the AC power circuit, a DC bus that supplies a DC voltage to the inverter, a series combination of a first capacitor and a second capacitor provided between two lines of the DC bus, where the interconnection point is the midpoint voltage, a midpoint voltage control unit provided between two lines of the DC bus for controlling the midpoint voltage, an AC side voltage sensor that acquires a first phase voltage between a first voltage line and a neutral line of the AC power circuit, and a second phase voltage between a second voltage line and the neutral line of the AC power circuit, a control unit that performs bipolar pulse width modulation control on the inverter and controls the midpoint voltage control unit, the AC reactor has a common core for two lines extending from the output terminal of the inverter to the first voltage line and the second voltage line, the control unit, controls the inverter so that the line voltage between the first voltage line and the second voltage line becomes a target value, controls the midpoint voltage control unit to adjust the midpoint voltage so that the absolute values of the first phase voltage and the second phase voltage are equal to each other, power conversion device.

2. the control unit controls the midpoint voltage control unit so that the midpoint voltage deviates from the intermediate value of the voltage between two lines of the DC bus, the power conversion device according to claim 1.

3. the control unit controls the midpoint voltage control unit to adjust the midpoint voltage so that the phase of the first phase voltage coincides with the phase of the inverted voltage obtained by inverting the second phase voltage, the power conversion device according to claim 1 or claim 2.

4. The control unit controls the neutral point voltage control unit to adjust the neutral point voltage so that the phase of the first-phase voltage or the phase of the inverted voltage obtained by inverting the second-phase voltage coincides with the phase of the line voltage. The power conversion device according to claim 1 or claim 2.

5. When there is a phase difference between the first-phase voltage and the inverted voltage, the control unit applies a positive or negative bias voltage to the neutral point voltage to reduce the phase difference. The power conversion device according to claim 3.

6. When there is a phase difference between the first-phase voltage or the inverted voltage and the line voltage, the control unit applies a positive or negative bias voltage to the neutral point voltage to reduce the phase difference. The power conversion device according to claim 4.

7. Based on at least one of the first-phase voltage at the zero-crossing detection of the line voltage and the second-phase voltage at the zero-crossing detection of the line voltage, the control unit applies a positive or negative bias voltage to the neutral point voltage to reduce the phase difference between the first-phase voltage and the inverted voltage. The power conversion device according to claim 3 or claim 4.

8. When there is a difference between the first-phase voltage and the second-phase voltage, the control unit applies a positive or negative bias voltage to the neutral point voltage to reduce the difference. The power conversion device according to claim 1 or claim 2.

9. The bias voltage is a rectangular wave, a sine wave, or a triangular wave. The power conversion device according to any one of claims 5 to 8.

10. The first-phase voltage and the second-phase voltage are instantaneous values or effective values. The power conversion device according to claim 1 or claim 2.

11. The AC reactor is connected in series. The power conversion device according to any one of claims 1 to 10.

12. An inverter, an AC reactor provided between the inverter and a single-phase three-wire AC power line, a DC bus that supplies a DC voltage to the inverter, a series body of a first capacitor and a second capacitor provided between two lines of the DC bus and having an interconnection point as a neutral point voltage, a neutral point voltage control unit provided between two lines of the DC bus and controlling the neutral point voltage, and a control unit that performs bipolar pulse width modulation control on the inverter and controls the neutral point voltage control unit. A control method by the control unit for a power conversion device that provides an AC output to the AC power line, comprising: Controlling the inverter so that the line voltage between a first voltage line and a second voltage line of the AC power line becomes a target value; Adjusting the neutral point voltage by controlling the neutral point voltage control unit so that the absolute values of a first phase voltage and a second phase voltage seen from the neutral line of the AC power line are equal to each other; A control method for a power conversion device.

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