Power conversion system
In a power conversion system in which one or more converter cells are connected in series to form an arm for each phase, a control device includes a voltage command generating unit for generating a positive arm voltage command and a negative arm voltage command for each phase. The voltage command generating unit includes an AC current control unit, a circulating current control unit, and a command distributing unit. On the basis of inputted voltage commands, the command distributing unit subtracts voltage drop portions due to inductance values in the arms from respective voltages assigned as outputs of the positive arm and the negative arm, to distribute voltage components, thereby determining the positive arm voltage command and the negative arm voltage command.
1. A power conversion system comprising:
a power converter for performing power conversion between plural-phase AC and DC, the power converter including a plurality of legs each having a positive arm and a negative arm connected in series, with a connection point therebetween connected to each phase AC line, the plurality of legs being connected in parallel between positive and negative DC buses; and
a control device for controlling the power converter, wherein
each of the positive arm and the negative arm of each leg is composed of at least one converter cells connected in series and including: a series unit of a plurality of semiconductor switching elements connected in series to each other; and a DC capacitor connected in parallel to the series unit, a terminal of the semiconductor switching element being used as an output terminal,
the control device includes a voltage command generating unit for generating a first voltage command for the positive arm and a second voltage command for the negative arm, to perform output control for each converter cell in the positive arm and the negative arm, and
the voltage command generating unit includes:
a current control unit for calculating control commands for controlling an AC current component flowing through each phase AC line, and a circulating current component for each phase which circulates between the legs; and
a command distributing unit for, on the basis of the control commands and a DC voltage command value for voltage between the DC buses, subtracting voltage drop portions due to inductance values in the positive arm and the negative arm from voltages assigned as outputs of the positive arm and the negative arm, to determine the first voltage command and the second voltage command.
2. The power conversion system according to claim 1 , wherein
each phase AC line is connected to an AC circuit, and
the voltage command generating unit generates the first voltage command for the positive arm and the second voltage command for the negative arm, using each phase voltage of the AC circuit as a feedforward component.
3. The power conversion system according to claim 1 , wherein
the control commands calculated by the current control unit includes: a first control command calculated so that AC current flowing through each phase AC line becomes close to an AC current command; and a second control command calculated so that circulating current for each phase which circulates between the legs becomes close to a circulating current command.
4. The power conversion system according to claim 1 , wherein
the control commands calculated by the current control unit includes a third control command and a fourth control command calculated so that arm currents respectively flowing through the positive arm and the negative arm of each leg become close to respective set current commands, and
the current control unit controls the arm currents, thereby controlling the AC current component and the circulating current component.
5. The power conversion system according to claim 1 , wherein
a reactor is provided in series in at least one of the positive arm and the negative arm of each leg.
6. The power conversion system according to claim 5 , wherein
the command distributing unit uses an inductance of the provided reactor as the inductance values for calculation of the first voltage command and the second voltage command.
7. The power conversion system according to claim 5 , wherein
the reactor is provided in series only in the negative arm of each leg.
8. The power conversion system according to claim 7 , wherein
the reactor is provided on a negative side of the converter cell in the negative arm.
9. The power conversion system according to claim 5 , wherein
the reactors having inductances equal to each other are provided in series in both the positive arm and the negative arm of each leg.
10. The power conversion system according to claim 1 , wherein
the voltage command generating unit generates a modulation index correcting signal on the basis of voltage of the DC capacitor in each converter cell, and corrects the first voltage command and the second voltage command from the command distributing unit by dividing the first voltage command and the second voltage command by the modulation index correcting signal.
11. The power conversion system according to claim 1 , wherein
the voltage drop portions are due to inductance values from which a parallel inductance value of the positive arm and the negative arm has been removed.