Power conversion device and three-phase alternating current power supply device

1. A power conversion device that converts power between a DC unit and a three-phase AC, the power conversion device comprising:

a first-phase conversion device configured to covert power between the DC unit and a first phase of the three-phase AC, via a first reactor;

a second-phase conversion device configured to covert power between the DC unit and a second phase of the three-phase AC, via a second reactor;

a third-phase conversion device configured to covert power between the DC unit and a third phase of the three-phase AC, via a third reactor; and

a control unit configured to control the first-phase conversion device, the second-phase conversion device, and the third-phase conversion device, wherein

the first-phase conversion device, the second-phase conversion device, and the third-phase conversion device each include a DC/DC conversion circuit and a single-phase power conversion circuit, and

for each of the first-phase conversion device, the second-phase conversion device, and the third-phase conversion device,

when an absolute value of a voltage target value for the AC exceeds DC voltage of each DC unit, the control unit causes the DC/DC conversion circuit to operate to achieve the absolute value of the voltage target value and causes the single-phase power conversion circuit to only perform necessary polarity inversion, and

when the absolute value of the voltage target value is smaller than the DC voltage, the control unit stops operation of the DC/DC conversion circuit and causes the single-phase power conversion circuit to operate to achieve the voltage target value.

2. The power conversion device according to claim 1 , wherein

an output smoothing capacitor is provided at each of stages subsequent to the first reactor, the second reactor, and the third reactor,

a smoothing capacitor is provided between the DC/DC conversion circuit and the single-phase power conversion circuit, and

for each of the first-phase conversion device, the second-phase conversion device, and the third-phase conversion device, the control unit controls output of the AC power based on: an output current target value obtained based on a value of input power from the corresponding DC unit and a value of phase voltage of the three-phase AC; and a current target value for the corresponding DC/DC conversion circuit calculated from the following expression representing the current target value Iin* for the DC/DC conversion circuit:

Iin *={( Iinv*×Vinv *)+( sCVo *)× Vo*}/V DC ,

the current target value Iinv* for the single-phase power conversion circuit being represented as follows:

Iinv*=Ia*+sCaVa,

where

Ia* is the output current target value,

Vinv* is a voltage target value for the single-phase power conversion circuit,

Ca is an electrostatic capacitance of the output smoothing capacitor,

Va is a voltage value of phase voltage of the three-phase AC,

C is an electrostatic capacitance of the smoothing capacitor provided between the DC/DC conversion circuit and the single-phase power conversion circuit,

Vo* is a voltage target value for the DC/DC conversion circuit,

V DC is a DC input voltage value,

s is a Laplace operator.

3. The power conversion device according to claim 2 , wherein

the control unit calculates the voltage target value Vinv* for the single-phase power conversion circuit as the voltage target value for the DC/DC conversion circuit by the following expression:

Vinv*=Va+sLaIinv*,

where La is an inductance that is common among the first reactor, the second reactor, and the third reactor.

4. The power conversion device according to claim 1 , wherein

the DC unit is composed of a first DC unit, a second DC unit, and a third DC unit provided for the first phase, the second phase, and the third phase, respectively, the DC units being independent of each other without sharing either positive terminals or negative terminals.

5. The power conversion device according to claim 1 , wherein

the DC unit is one unit that is common among the first phase, the second phase, and the third phase, and isolation transformers are provided between: the first-phase conversion device, the second-phase conversion device, and the third-phase conversion device; and the respective phases of the three-phase AC.

6. The power conversion device according to claim 1 , wherein

the power conversion device is connected to the three-phase AC in either connection manner of three-phase four-line connection or three-phase three-line connection.

7. A three-phase AC power supply device comprising:

a DC power supply;

a first-phase conversion device configured to supply AC power via a first reactor to a first phase of the three-phase AC, based on DC power inputted from the DC power supply;

a second-phase conversion device configured to supply AC power via a second reactor to a second phase of the three-phase AC, based on DC power inputted from the DC power supply;

a third-phase conversion device configured to supply AC power via a third reactor to a third phase of the three-phase AC, based on DC power inputted from the DC power supply; and

a control unit configured to control the first-phase conversion device, the second-phase conversion device, and the third-phase conversion device, wherein

the first-phase conversion device, the second-phase conversion device, and the third-phase conversion device each include a DC/DC conversion circuit and a single-phase power conversion circuit, and

for each of the first-phase conversion device, the second-phase conversion device, and the third-phase conversion device,

when an absolute value of a voltage target value for the AC to be outputted exceeds inputted DC voltage, the control unit causes the DC/DC conversion circuit to operate to generate the absolute value of the voltage target value and causes the single-phase power conversion circuit to only perform necessary polarity inversion, and

when the absolute value of the voltage target value is smaller than the inputted DC voltage, the control unit stops operation of the DC/DC conversion circuit and causes the single-phase power conversion circuit to operate to generate the voltage target value.

8. The three-phase AC power supply device according to claim 7 , wherein

the DC power supply is composed of a first DC power supply, a second DC power supply, and a third DC power supply provided for the first phase, the second phase, and the third phase, respectively, the DC power supplies being independent of each other without sharing either positive terminals or negative terminals.

9. The three-phase AC power supply device according to claim 7 , wherein

the DC power supply is one power supply that is common among the first phase, the second phase, and the third phase, and isolation transformers are provided between: the first-phase conversion device, the second-phase conversion device, and the third-phase conversion device; and the respective phases of the three-phase AC.

10. The three-phase AC power supply device according to claim 7 , wherein

the first-phase conversion device, the second-phase conversion device, and the third-phase conversion device are connected to the three-phase AC in either connection manner of three-phase four-line connection or three-phase three-line connection.

11. The three-phase AC power supply device according to claim 7 , wherein

each of the first DC power supply, the second DC power supply, and the third DC power supply is a concentrator photovoltaic panel configured to operate so as to track the sun.

12. The three-phase AC power supply device according to claim 7 , wherein

the three-phase AC power supply device is system-interconnected with the three-phase AC system.

13. The three-phase AC power supply device according to claim 7 , wherein

the three-phase AC power supply device is configured to output the three-phase AC by autonomous operation.

14. The power conversion device according to claim 2 , wherein

the DC unit is composed of a first DC unit, a second DC unit, and a third DC unit provided for the first phase, the second phase, and the third phase, respectively, the DC units being independent of each other without sharing either positive terminals or negative terminals.

15. The power conversion device according to claim 3 , wherein

the DC unit is composed of a first DC unit, a second DC unit, and a third DC unit provided for the first phase, the second phase, and the third phase, respectively, the DC units being independent of each other without sharing either positive terminals or negative terminals.

16. The power conversion device according to claim 2 , wherein

the DC unit is one unit that is common among the first phase, the second phase, and the third phase, and isolation transformers are provided between: the first-phase conversion device, the second-phase conversion device, and the third-phase conversion device; and the respective phases of the three-phase AC.

17. The power conversion device according to claim 3 , wherein

the DC unit is one unit that is common among the first phase, the second phase, and the third phase, and isolation transformers are provided between: the first-phase conversion device, the second-phase conversion device, and the third-phase conversion device; and the respective phases of the three-phase AC.

18. The power conversion device according to claim 2 , wherein

the power conversion device is connected to the three-phase AC in either connection manner of three-phase four-line connection or three-phase three-line connection.

19. The power conversion device according to claim 3 , wherein

the power conversion device is connected to the three-phase AC in either connection manner of three-phase four-line connection or three-phase three-line connection.