JP7528856B2 - Power conversion device and control method thereof - Google Patents

Description

This disclosure relates to a power conversion device and a control method thereof.

A power conversion device capable of providing a single-phase three-wire independent output is proposed, for example, in Patent Document 1. The power conversion device of Patent Document 1 includes a two-phase half-bridge inverter and a converter that adjusts the midpoint potential of the DC bus. In this inverter, a series body of a first high-side switching element and a second low-side switching element, and a series body of a third high-side switching element and a fourth low-side switching element are connected between the two wires of the DC bus. The converter that controls the midpoint potential of the DC bus controls the potential of the interconnection point of the series body of a pair of capacitors connected between the two wires of the DC bus so that it is the midpoint of the potential between the two wires of the DC bus. This interconnection point is directly connected to the neutral wire of the single-phase three-wire.

The inverter control unit controls the opening and closing of four switching elements using unipolar pulse width modulation control (PWM (Pulse Width Modulation) control). The first switching element and the second switching element are alternately turned on to output a U-phase voltage between the first voltage line (U line) and the neutral line (O line) of the single-phase three-wire system. The third switching element and the fourth switching element are alternately turned on to output a W-phase voltage between the second voltage line (W line) and the neutral line (O line) of the single-phase three-wire system.

JP 2014-87160 A (FIG. 1)

In the above 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. As a result, 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 terminal. 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, a smoothing reactor must be placed on each of the U line and W line. This makes it difficult to miniaturize the power conversion device. In addition, large iron loss occurs in each reactor.

On the other hand, when supplying an independent 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, the U-phase voltage and the W-phase voltage cannot be matched by controlling the midpoint potential as described above.

The objective of this disclosure is to achieve miniaturization and suppression of phase voltage imbalance in a power conversion device that can provide independent output in a single-phase three-wire system.

This disclosure includes the following inventions, however, the inventions are defined by the claims.

(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 for supplying a DC voltage to the inverter;
a series body including a first capacitor and a second capacitor, the series body being provided between two wires of the DC bus and having a mutual connection point at a midpoint voltage;
a midpoint voltage control unit provided between two lines of the DC bus and 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 line and a second phase voltage between a second voltage line and the neutral line of the AC power line;
a neutral reactor that is provided on an electric path that connects the interconnection point to the neutral conductor and has a ground potential on the interconnection point side;
A control unit that performs unipolar pulse width modulation control on the inverter and controls the midpoint 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 is
controlling the inverter so that a line voltage between the first voltage line and the second voltage line becomes a target value;
controlling the midpoint voltage control unit to adjust the midpoint voltage so that the first phase voltage and the second phase voltage are equal to each other in absolute value;
It is a power conversion device.

(Method of controlling a power conversion device)
From the viewpoint of a control method, a control method for a power conversion device that provides an AC output to the AC circuit, the control method being performed by a control unit, 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 that is provided between the two wires of the DC bus and controls the midpoint voltage; a neutral line reactor that is provided on an electric circuit that connects the interconnection point to a neutral line of the AC electric circuit and has a ground potential on the interconnection point side; and a control unit that performs unipolar pulse width modulation control on the inverter and controls the midpoint voltage control unit, the control unit comprising:
controlling the inverter so that a line voltage between a first voltage line and a second voltage line of the AC current path becomes a target value;
controlling the midpoint voltage control unit to adjust the midpoint voltage so that the first phase voltage and the second phase voltage viewed from the neutral line of the AC current circuit become equal to each other in absolute value;
A method for controlling a power conversion device.

According to the present disclosure, it is possible to achieve miniaturization and suppression of phase voltage imbalance in a power conversion device that can provide independent output in a single-phase three-wire system.

FIG. 1 is a circuit diagram illustrating an example of a power conversion device. FIG. 2 is a control block diagram relating to the control of the inverter and the control of the midpoint voltage control unit executed in the control unit. FIG. 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 the AC circuit, and no load is applied between the W line and the O line. FIG. 4 is another control block diagram different from that of FIG. 2, which relates to the control of the inverter and the control of the midpoint voltage control unit executed in the control unit. FIG. 5 is another control block diagram different from FIG. 2 and FIG. 4, which relates to the control of the inverter and the control of the midpoint voltage control unit executed in the control unit. FIG. 6 is a graph showing an AC output waveform when a half-wave rectifying resistive load is connected only between the U line and the O line of the AC circuit, and no load is applied between the W line and the O line.

[Description of the embodiments of the present disclosure]
The gist of the present disclosure includes at least the following.

(1) Disclosed is a power conversion device that provides an AC output to a single-phase three-wire AC circuit, comprising: 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 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; 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 neutral line reactor that is provided on an electric circuit connecting the interconnection point to the neutral line and has a ground potential on the interconnection point side; and a control unit that performs unipolar pulse width modulation control of the inverter and controls the midpoint voltage control unit. The AC reactor has a core common to two lines extending from the output terminal of the inverter to the first voltage line and the second voltage line, and 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, and controls the midpoint voltage control unit to adjust the midpoint voltage so that the first phase voltage and the second phase voltage become equal to each other in absolute value.

In such a power conversion device, during an independent operation, the inverter does not control the phase voltages, but operates to adjust the line voltage between the first voltage line and the second voltage line to a target value. The control unit controls the midpoint voltage control unit 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 limited to the midpoint value, and may be biased to one side from the midpoint value.
By providing a neutral reactor, it is possible to perform unipolar pulse width modulation control of the inverter while using an AC reactor with a common core. With unipolar pulse width modulation control, the frequency of the pulse voltage applied to the AC reactor is twice that of the bipolar method. This reduces the inductance of the AC reactor, contributing to miniaturization. In this way, in addition to the miniaturization achieved by the common core structure, it is possible to further miniaturize the AC reactor.
In addition, since the interconnection point side from the neutral reactor is at ground potential, the voltage to ground of the DC bus is stabilized, and common mode noise can be reduced even if the inverter is controlled by pulse width modulation using a unipolar method.

(2) In the power conversion device of (1), the control unit can control the midpoint voltage control unit so that the midpoint voltage is shifted from an intermediate value of voltages 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 first phase voltage and the second phase voltage can be made equal to each other in absolute value by shifting the midpoint voltage from the intermediate value.

(3) In the power conversion device of (1) or (2), when there is a difference between the first phase voltage and the second phase voltage, the control unit can apply a positive or negative bias voltage to the midpoint voltage to reduce the difference.
By biasing the midpoint voltage by applying a bias voltage rather than necessarily setting it to an intermediate value, the first phase voltage and the second phase voltage can be adjusted to be equal to each other in absolute value even if the first phase AC load and the second phase AC load are unequal.

(4) In the power conversion device according to any one of (1) to (3), the AC reactors are connected in a summing manner.
In this case, the AC reactor can be made smaller.

(5) In the power conversion device according to any one of (1) to (4), the first phase voltage and the second phase voltage are instantaneous values or effective values.
Either instantaneous values or effective values can be used for the first phase voltage and the second phase voltage. 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.

(6) From the viewpoint of a control method, the control method includes 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, a neutral line reactor provided on an electric circuit that connects the interconnection point to a neutral line of the AC circuit and having a ground potential on the interconnection point side, and A control method for a power conversion device that provides AC output to the AC circuit, comprising: a control unit that performs unipolar pulse width modulation control on the inverter and controls the midpoint voltage control unit; 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; and the midpoint voltage control unit is controlled to adjust the midpoint voltage so that the first phase voltage and the second phase voltage as viewed from the neutral line of the AC circuit become equal in absolute value.

According to this control method for a power conversion device, during an independent operation, the inverter does not control the phase voltages, but operates to adjust the line voltage between the first voltage line and the second voltage line to a target value. 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 limited to the midpoint value, and may be biased to one side from the midpoint value.
By providing a neutral reactor, it is possible to perform unipolar pulse width modulation control of the inverter even when using an AC reactor having a common core. With unipolar pulse width modulation control, the frequency of the pulse voltage applied to the AC reactor is twice that of the bipolar method. This reduces the inductance of the AC reactor, contributing to miniaturization. In this way, in addition to the miniaturization achieved by the common core structure, it is possible to further miniaturize the AC reactor.
In addition, since the interconnection point side from the neutral reactor is at ground potential, the voltage to ground of the DC bus is stabilized, and common mode noise can be reduced even if the inverter is controlled by pulse width modulation using a unipolar method.

[Details of the embodiment of the present disclosure]
Hereinafter, specific examples of the power conversion device and the control method thereof according to the present disclosure will be described with reference to the drawings.

Configuration of power conversion device
1 is a circuit diagram showing an example of a power conversion device. The power conversion device 100 is provided between a DC power source 20 and an AC electric circuit 31. The power conversion device 100 is capable of grid-connected operation with a commercial power system or independent operation in a state where it is disconnected from the commercial power system, but in this disclosure, the independent operation will be described.

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

The power conversion device 100 includes a DC/DC converter 1 connected to a DC power source 20, a DC bus 2 on the high-voltage side of the DC/DC converter, a smoothing capacitor 3 connected between the two lines of the DC bus 2, a midpoint voltage control unit 4 connected between the two lines of the DC bus 2, a series body 5 of DC bus capacitors 5H and 5L connected between the 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 the two lines of the DC bus 2, an AC reactor 8, AC side capacitors 9u and 9w, voltage sensors 10u and 10w, a control unit 11, and a neutral line reactor 14. The DC side voltage sensor may be composed of a voltage sensor that detects the voltage between the two lines of the DC bus 2 and a voltage sensor that detects the voltage at either end of the DC bus capacitors 5H and 5L.

The voltage sensor 6H detects the voltage _VH across the DC bus capacitor 5H and sends a detection signal to the control unit 11. The voltage sensor 6L detects the voltage _VL across the DC bus capacitor 5L and sends a detection signal to the control unit 11. The capacitances of the DC bus capacitors 5H and 5L are the same.
The voltage sensor 10u detects a voltage V _uo across the AC side capacitor 9u and sends a detection signal to the control unit 11. The voltage sensor 10w detects a voltage V _wo across the AC side capacitor 9w and sends a detection signal to the control unit 11. The AC side capacitors 9u and 9w have the same capacitance.

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, and 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 a control unit 11.

If the voltage of the DC power supply 20 is sufficiently high, the DC/DC converter 1 can be omitted. Furthermore, if the voltage of the DC power supply 20 is too high, a step-down circuit can be used as the DC/DC converter. The voltage between the two lines of the DC bus 2 is smoothed by the smoothing capacitor 3.

The midpoint 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 midpoint voltage control unit 4 is controlled by the control unit 11.

The voltage that the midpoint voltage control unit 4 is trying to directly control is the voltage _VL . If the voltage between the two lines of the DC bus 2 is _VB , then the voltage _VH is ( _VB - _VL ). These voltages are expressed as follows:
V _L =V _H =(V _B /2);
V _L > (V _B /2) > V _H , or V _L < (V _B /2) < V _H
A midpoint M of the series body 5, which is the interconnection point of the DC bus capacitors 5H, 5L, is at ground potential and is connected to the O line on the AC side via a neutral line reactor 14.

The inverter 7 has 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 202 V between the U and W lines of the AC circuit 31 via a filter circuit made up of the AC reactor 8 and AC side capacitors 9u and 9w. The AC reactor 8, which smoothes the output of the inverter 7, has the U and W lines wound around a common core, and is connected in a summing manner to reinforce the magnetic flux against the normal current.

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. 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 _uw between the U line and the W line may be provided instead of either one of the voltages V _uo and V _wo . In short, it is sufficient if the voltages V _uw , V _uo , and V _wo can be obtained, including by detection or calculation.

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

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

Control of power conversion device
Next, the control (control method) of the inverter 7 and the midpoint voltage control unit 11 in the power conversion device 100 configured as above will be described.

(Common points for each example)
Generally, there are unipolar and bipolar PWM control of an inverter. In the bipolar method, the amplitude of the pulse voltage applied to the AC reactor is larger than in the unipolar method, and iron loss is larger. Therefore, in the present disclosure, the unipolar method is adopted. By providing the neutral line reactor 14, it is possible to perform PWM control in the unipolar method while using the AC reactor 8 of the summing connection with a common core. Since the AC reactor 8 of the summing connection with a common core is used, when the PWM control of the unipolar method is performed, the frequency of the pulse voltage applied to the AC reactor 8 is doubled. Therefore, the inductance can be reduced, and the AC reactor 8 can be made smaller.

In addition, since the electrical path is grounded between the midpoint M and the neutral reactor 14, the impedance between the midpoint M of the DC bus 2 and the ground is reduced, and the ground potential of the DC bus 2 is stabilized. Therefore, even if unipolar PWM control is performed on the inverter 7, the generation of common mode noise can be reduced.

(First Example)
FIG. 2 is a control block diagram of the control of the inverter 7 and the control of the midpoint voltage control unit 5 executed in the control unit 11. Such control can be realized entirely by software, but can also be partially configured by hardware. In FIG. 2, a target value of the AC voltage (instantaneous value) is indicated from a V _uw voltage reference indication unit B1. A deviation between this target value and the voltage V _uw between the U line and the W line that is actually detected is obtained by a comparator B2. The deviation is added to the target value via a proportional integral calculation unit B3 (adder B4). A voltage command signal generation unit B5 generates a voltage command signal based on the corrected voltage value obtained by the addition. The voltage command signal is compared with a carrier signal (triangular wave or sawtooth wave) output by a carrier signal generation unit B6 by a PWM control unit B7, and becomes a PWM control signal for the switching elements Q1 and Q2. Further, the voltage command signal whose sign has been inverted by an inverter B8 is compared with the carrier signal output by a carrier signal generating section B6 by a PWM control section B9 to become a PWM control signal for the switching elements Q3 and Q4.

In this way, the PWM control signal is generated from the voltage command signal, its inverted signal, and the carrier signal, and thereby the switching elements Q1, Q2, Q3, and Q4 are controlled. This control is a unipolar PWM type control in which the pair of switching elements Q1, Q2 is controlled based on the voltage command signal, and the pair of switching elements Q3, Q4 is controlled based on the inverted signal.
As a result, the inverter 7 operates so that the voltage V _uw between the U line and the W line coincides with the target value.

2, an adder B10 adds a U-phase voltage Vuo detected between the U line and the _O line and a W-phase voltage _Vwo detected between the W line and the O line. Since the U-phase voltage _Vuo and the W-phase voltage _Vwo are in opposite phase to each other and have opposite signs, the addition result is either 0 or a value obtained by adding the sign of the larger absolute value to the difference in absolute value. This value passes through a proportional-plus-integral calculation unit B11 and becomes a correction value that is added by an adder B13 to a midpoint voltage reference indicated by a midpoint voltage reference indication unit B12. For example, if the voltage between two lines of a DC bus is _VB , then the midpoint voltage reference is the intermediate value ( _VB /2).

The result of the addition in the adder B13 is compared with the voltage _VL in a comparator B14. The deviation of the comparison result, i.e., the deviation from _VL (bias voltage), is sent via a proportional-plus-integral calculation unit B15 to a voltage command signal generation unit B16. A voltage command signal is then generated in the voltage command signal generation unit B16. The voltage command signal is compared in a PWM control unit B18 with a carrier signal (triangular wave or sawtooth wave) output by a carrier signal generation unit B17, and becomes a PWM control signal for the switching elements Q5 and Q6.

In this way, if there is a difference between the U-phase voltage _Vuo and the W-phase voltage _Vwo , the midpoint voltage is corrected to deviate from the midpoint value in accordance with the difference. Basically, if the midpoint voltage control unit 4 controls the voltages _VL and _VH to be equal to each other, the U-phase voltage _Vuo and the W-phase voltage _Vwo will have voltage waveforms with roughly the same amplitude and inverted phase. However, if there is a large difference in the capacity of the loads connected between the U line and the O line and between the W line and the O line, simply making the voltages _VL and _VH equal will not result in the amplitudes of the U-phase voltage _Vuo and the W-phase voltage _Vwo matching each other.

Therefore, as described above, a correction value is added to the midpoint voltage reference so that the sum of the U-phase voltage _Vuo and the W-phase voltage _Vwo is always zero.
Alternatively, a necessary correction value may be applied so that the U-phase voltage _Vuo or the W-phase voltage _Vwo coincides with the control target value.
In short, it is only necessary to control the U-phase voltage _Vuo and the W-phase voltage _Vwo so that they are equal to each other in absolute value.

FIG. 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 the AC circuit and no load is applied between the W line and the O line. The horizontal axis indicates time [seconds], and the vertical axis indicates 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 in a phase-inverted relationship with each other 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 if 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 values are obtained. In addition, the midpoint voltage V _L fluctuates slightly from moment to moment, and the phase voltage on the AC side is adjusted by biasing the midpoint voltage.

(Second Example)
Fig. 4 is another control block diagram different from Fig. 2, relating to the control of the inverter 7 and the control of the midpoint voltage control unit 5 executed by the control unit 11. The difference from Fig. 2 is the midpoint voltage correction unit P1 at the upper left. In the midpoint voltage correction unit P1 at the upper left, an effective value calculation unit B20 calculates an effective value based on the U-phase voltage _Vuo . Similarly, an effective value calculation unit B21 calculates an effective value based on the W-phase voltage _Vwo .

The U-phase effective value and the W-phase effective value are compared with each other in a comparator B22. The difference obtained by the comparison is subjected to proportional integration by a proportional integral calculation unit B23, and then multiplied by a multiplier B25 with the sign indicated by an AC sign indication unit B24. This multiplication means that since the signs of the effective values of the U-phase and W-phase are the same, it is determined whether to add or subtract a correction value depending on the deviation of the U-phase and W-phase effective values. The correction value processed by the multiplier B25 is added to a midpoint voltage reference indicated by a midpoint voltage reference indication unit B26 (adder B27). For example, if the voltage between two lines of the DC bus is _VB , the midpoint voltage reference is the midpoint value ( _VB /2).

The result of the addition in the adder B27 is compared with the voltage _VL in a comparator B28. The deviation of the comparison result, i.e., the deviation from _VL (bias voltage), is sent to the voltage command signal generating unit B16 via a proportional-plus-integral calculation unit B29. The control block diagram relating to the subsequent processing and the control of the inverter 7 is the same as that in FIG.

When power is supplied to a load connected between the U line and the O line, in a period when the U phase voltage is positive, current flows from the DC bus capacitor 5H between the positive side electric circuit of the DC bus 2 and the midpoint M. In a period when the U phase voltage is negative, current flows from the DC bus capacitor 5L between the midpoint M and the negative side electric circuit of the DC bus 2. Therefore, if the effective value of the U phase voltage _Vuo is smaller than the effective value of the W phase voltage _Vwo , the effective values of the _U phase voltage _Vuo and the W phase voltage _Vwo will match each other by controlling the DC bus midpoint voltage _VL so _{that VH} > _VL in the positive period and VL > VH in the negative period.

For reference, the power conversion device described in the above-mentioned Patent Document 1 only performs midpoint voltage control so that the voltages _VL and _VH are equal, and does not perform control to bias the voltages (apply a bias voltage).

(Third Example)
Fig. 5 is another control block diagram different from Figs. 2 and 4, which relates to the control of the inverter 7 and the control of the midpoint voltage control unit 5 executed by the control unit 11. The difference from Fig. 2 is the midpoint voltage correction unit P2 at the upper left. In the midpoint voltage correction unit P2 at the upper left, an effective value calculation unit B20 calculates an effective value based on the U-phase voltage _Vuo . Similarly, an effective value calculation unit B21 calculates an effective value based on the W-phase voltage _Vwo .

The U-phase effective value and the W-phase effective value are compared with each other in a comparator B22. The difference obtained by the comparison is subjected to proportional integration by a proportional integral calculation unit B23, and then multiplied by a multiplier B25 with the sign indicated by an AC sign indication unit B24. This multiplication means that since the signs of the effective values of the U-phase and W-phase are the same, it is determined whether to add or subtract a correction value depending on the deviation of the U-phase and W-phase effective values. The correction value processed by the multiplier B25 is added to a midpoint voltage reference indicated by a midpoint voltage reference indication unit B26 (adder B27). For example, if the voltage between two lines of the DC bus is _VB , the midpoint voltage reference is the midpoint value ( _VB /2).

The sum of the adder B27 is sent to the voltage command signal generator B16. The subsequent processing and the control block diagram for the control of the inverter 7 are the same as those in Figures 2 and 4.

In the third example, as in the second example, when power is supplied to a load connected between the U line and the O line, in a period when the U-phase voltage is positive, a current flows from the DC bus capacitor 5H between the positive electric circuit of the DC bus 2 and the midpoint M. In a period when the U-phase voltage is negative, a current flows from the DC bus capacitor 5L between the midpoint M and the negative electric circuit of the DC bus 2. Therefore, when the effective value of the U-phase voltage _Vuo is smaller than the effective value of the W-phase voltage _Vwo , if the DC bus midpoint voltage _VL is controlled so that _VH > _VL in the positive period and _VL > _VH in the negative period, the effective values of the U-phase voltage _Vuo and the W-phase voltage _Vwo will match each other.

FIG. 6 is a graph showing an 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 no load is applied between the W line and the O line. The horizontal axis indicates time [seconds], and the vertical axis indicates voltage [V]. The AC frequency is 50 Hz. The midpoint voltage control is according to the 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 in a phase-inverted relationship with each other 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 if 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 values are obtained. In addition, the midpoint voltage V _L fluctuates slightly from moment to moment, and the phase voltage on the AC side is adjusted by biasing the midpoint voltage.

Summary of disclosure
The above disclosure can be generalized and expressed as follows.
The power conversion device 100, which provides an AC output to a single-phase three-wire AC circuit, includes a series body ₅ of a DC bus capacitor 5H and a DC bus capacitor 5L, which are provided between two wires of a DC bus 2 and have a midpoint voltage _VL at their interconnection point, and a midpoint voltage control unit 4, which is provided between the two wires of the DC bus 2 and controls the midpoint voltage VL. The control unit 11 of the power conversion device 100 performs unipolar pulse width modulation control of the inverter 7 and controls the midpoint voltage control unit 4. The AC reactor 8 has a core common to two wires extending from the output terminal of the inverter 7 to the U line and the W line. The neutral line reactor 14 is provided on the electric circuit connecting the interconnection point to the neutral line and is at ground potential on the interconnection point side. 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 controls the midpoint voltage control unit 4 to adjust the midpoint voltage _VL so that the U-phase voltage _Vuo and the W-phase voltage _Vwo become equal to each other in absolute value.

In such a power conversion device 100, during independent operation, the inverter 7 operates to adjust the line voltage between the U line and the W line to a target value without controlling the phase voltage. The control unit 11 controls the midpoint voltage control unit 4 to adjust the midpoint voltage _VL so that the U-phase voltage and the W-phase voltage become equal to each other in absolute value. The AC reactor 8, which has a core common to the two voltage lines (U line, W line), can be made compact.

By providing the neutral reactor 14, it is possible to perform unipolar pulse width modulation control on the inverter 7 while using the AC reactor 8 having a common core. With unipolar pulse width modulation control, the frequency of the pulse voltage applied to the AC reactor 8 is twice that of the bipolar method. This makes it possible to reduce the inductance of the AC reactor 8, which contributes to miniaturization. In this way, in addition to the miniaturization achieved by the common core structure, it is possible to further miniaturize the AC reactor 8.
In addition, since the interconnection point side viewed from the neutral line reactor 14 is at ground potential, the voltage to ground of the DC bus 2 is stabilized, and common mode noise can be reduced even if the inverter 7 is controlled by pulse width modulation using a unipolar method.

The control unit 11 can control the midpoint voltage control unit 4 so that the midpoint voltage _VL deviates from the median value of the voltages between the two lines of the DC bus 2. As a result, even if the AC loads of the U phase and the W phase are unequal, the midpoint voltage deviates from the median value, so that the U-phase voltage _Vuo and the W-phase voltage _Vwo can become equal to each other in absolute values.

In other words, when there is a difference between the U-phase voltage _Vuo and the W-phase voltage _Vwo , the control unit 11 can reduce the difference by applying a positive or negative bias voltage to the midpoint voltage _VL . In this way, by biasing the midpoint voltage _VL by applying a bias voltage rather than necessarily setting it to an intermediate value, it is possible to adjust the U-phase voltage _Vuo and the W-phase voltage _Vwo to be equal to each other in absolute values even if the AC loads of the U phase and the W phase are unequal.

The AC reactor 8 is connected in a additive manner, and therefore can be made compact.
In addition, either instantaneous values or effective values can be used for the U-phase voltage and the W2-phase voltage. 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.

《Addendum》
The embodiments disclosed herein should be considered to be illustrative and not restrictive in all respects. The scope of the present invention is defined by the claims, and it is intended to include all modifications within the scope and meaning equivalent to the claims.

REFERENCE SIGNS LIST 1 DC/DC converter 2 DC bus 3 Smoothing capacitor 4 Midpoint voltage control unit 5 Series (of capacitors) 5H, 5L DC bus capacitor 6H, 6L Voltage sensor 7 Inverter 8 AC reactor 9u, 9w AC side capacitor 10u, 10w Voltage sensor 11 Control unit 12, 13 DC reactor 14 Neutral line reactor 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 indication unit B2 Comparator B3 Proportional integral calculation unit B4 Adder B5 Voltage command signal generation unit B6 Carrier signal generation section B7 PWM control section B8 Inversion section B9 PWM control section B10 Adder B11 Proportional and integral calculation section B12 Midpoint voltage reference indication section B13 Adder B14 Comparator B15 Proportional and integral calculation section B16 Voltage command signal generation section B17 Carrier signal generation section B18 PWM control section B20, B21 Effective value calculation section B22 Comparator B23 Proportional and integral calculation section B24 AC sign indication section B25 Multiplier B26 Midpoint voltage reference indication section B27 Adder B28 Comparator B29 Proportional and integral calculation section M Midpoint P1, P2 Midpoint voltage correction section

Claims (6)

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 for supplying a DC voltage to the inverter;
a series body including a first capacitor and a second capacitor, the series body being provided between two wires of the DC bus and having a mutual connection point at a midpoint voltage;
a midpoint voltage control unit provided between two lines of the DC bus and 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 line and a second phase voltage between a second voltage line and the neutral line of the AC power line;
a neutral reactor that is provided on an electric path that connects the interconnection point to the neutral conductor and has a ground potential on the interconnection point side;
A control unit that performs unipolar pulse width modulation control on the inverter and controls the midpoint 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 is
controlling the inverter so that a line voltage between the first voltage line and the second voltage line becomes a target value;
controlling the midpoint voltage control unit to adjust the midpoint voltage so that the first phase voltage and the second phase voltage are equal to each other in absolute value;
Power conversion equipment. The power conversion device according to claim 1, wherein the control unit controls the midpoint voltage control unit so that the midpoint voltage is shifted from the midpoint value of the voltage between the two lines of the DC bus. The power conversion device according to claim 1 or 2, wherein the control unit, when there is a difference between the first phase voltage and the second phase voltage, applies a positive or negative bias voltage to the midpoint voltage to reduce the difference. The power conversion device according to any one of claims 1 to 3, wherein the AC reactor is connected in a complementary manner. The power conversion device according to any one of claims 1 to 4, wherein the first phase voltage and the second phase voltage are instantaneous values or effective values. a control method for a power conversion device that provides an AC output to the AC circuit, the control method 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 that is provided between the two wires of the DC bus and controls the midpoint voltage; a neutral line reactor that is provided on an electric circuit that connects the interconnection point to a neutral line of the AC electric circuit and has a ground potential on the interconnection point side; and a control unit that performs unipolar pulse width modulation control on the inverter and controls the midpoint voltage control unit, the control unit comprising:
controlling the inverter so that a line voltage between a first voltage line and a second voltage line of the AC current path becomes a target value;
controlling the midpoint voltage control unit to adjust the midpoint voltage so that the first phase voltage and the second phase voltage viewed from the neutral line of the AC current circuit become equal to each other in absolute value;
A method for controlling a power conversion device.

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