Power conversion device
A power conversion device includes a transformer, a plurality of converter cells, and a control circuit that controls semiconductor switching elements in each of the converter cells to be turned on and off. The transformer includes: a primary winding group being connected in multiple phases to an AC power supply including multiple phases; and a plurality of secondary winding groups. Each secondary winding group includes, in each of the multiple phases, secondary windings each formed of a single-phase open winding. Each converter cell converts a single-phase AC voltage between AC nodes connected to the respective secondary windings into a DC voltage by control of the semiconductor switching elements to be turned on and off, and outputs the converted DC voltage between a pair of DC nodes. The DC nodes of the plurality of converter cells are connected in series between a first DC terminal and a second DC terminal.
1. A power conversion device that performs power conversion between a plurality of AC terminals connected to an AC power supply having multiple phases and a first DC terminal and a second DC terminal, the power conversion device comprising:
a transformer including
a plurality of primary windings connected in multiple phases to respective AC terminals, and
a plurality of secondary windings each formed of a single-phase open winding; and
a plurality of converter cells disposed to correspond to the plurality of secondary windings, wherein
the secondary windings are disposed to form a plurality of secondary winding groups each having secondary windings corresponding to respective multiple phases,
each of the converter cells converts a single-phase AC voltage between a pair of AC nodes each connected to a corresponding one of the secondary windings into a DC voltage by control of a plurality of semiconductor switching elements to be turned on and off, and outputs the DC voltage between a pair of DC nodes,
DC nodes of the converter cells are connected in series between the first DC terminal and the second DC terminal, and
the power conversion device further comprises
a control circuit that controls each of the semiconductor switching elements to be turned on and off so as to control an AC current on each of the AC nodes and the DC voltage between DC nodes in each of the converter cells, wherein
the control circuit includes
a voltage control unit that generates an AC voltage command value between the AC nodes for controlling the AC current according to an AC current command value that has an amplitude for controlling the DC voltage to be set at a voltage command value in each of the converter cells, and
a modulation unit that generates signals for controlling respective semiconductor switching elements to be turned on and off according to comparison between each of carrier waves and the AC voltage command value generated by the voltage control unit, and
the AC current command value is generated to have a phase in synchronization with a phase of an AC voltage in one phase of the multiple phases of the AC power supply, wherein the one phase corresponds to a phase of one secondary winding of the secondary windings to which a corresponding one of the converter cells is connected.
2. The power conversion device according to claim 1 , wherein
the voltage control unit generates the AC current command value according to a sum of a positive-phase current command value and a negative-phase current command value,
the positive-phase current command value shows a sinusoidal current that has:
an amplitude for controlling an average value of the DC voltages of the converter cells to be set at the voltage command value; and
a phase in synchronization with the AC voltage, and
the negative-phase current command value shows a sinusoidal current that has an amplitude and a phase for suppressing a difference between phases of the DC voltages of the converter cells.
3. The power conversion device according to claim 1 , wherein
each of the converter cells includes a plurality of capacitors that are connected in series between DC nodes and that are equal in capacitance, and
the AC voltage command value is generated with addition of a DC bias component for equalizing voltages of the capacitors.
4. The power conversion device according to claim 1 , wherein
the AC current command value is generated as an AC current obtained by superimposing a third harmonic current on a sinusoidal current.
5. The power conversion device according to claim 2 , wherein
the AC current command value is generated by superimposing a third harmonic current on a sinusoidal current that is based on the sum of the positive-phase current command value and the negative-phase current command value.
6. The power conversion device according to claim 1 , wherein
in the modulation unit, the carrier waves are shifted in phase from each other between the converter cells connected to the respective secondary windings in a same phase of the multiple phases.
7. The power conversion device according to claim 1 , wherein
the first DC terminal and the second DC terminal are connected to a modular multilevel converter for motor driving.
8. The power conversion device according to claim 1 , wherein
each of the converter cells is formed of a three-level rectifier.
9. The power conversion device according to claim 1 , wherein
each of the converter cells is formed of a Vienna rectifier.
10. The power conversion device according to claim 1 , wherein
each of the converter cells is formed of a neutral-point clamped three-level converter.
11. The power conversion device according to claim 2 , wherein
each of the converter cells includes a plurality of capacitors that are connected in series between DC nodes and that are equal in capacitance, and
the AC voltage command value is generated with addition of a DC bias component for equalizing voltages of the capacitors.