Power Conversion Device
A power conversion unit includes a plurality of arms each having a plurality of converter cells connected to each other in cascade. A converter control unit that controls the power conversion unit includes a voltage command value computing unit to compute an output voltage command value for each arm, and a gate signal generating unit to generate an on/off control signal of each switching element of each converter cell in accordance with the output voltage command value. A voltage command value computing unit is configured to generate the output voltage command value with compensation for an AC vibration component, extracted for each arm, of stored energy of power storage elements of a plurality of converter cells included in the arm.
1 . A power conversion device connected to an AC power grid, comprising:
a power conversion unit including at least one arm having a plurality of converter cells connected to each other in cascade; and
a converter control unit to control the power conversion unit,
each of the converter cells including
a pair of input/output terminals,
a plurality of switching elements, and
a power storage element electrically connected to the input/output terminals through the switching elements,
the converter control unit including
a voltage command value computing unit to compute an output voltage command value for each of the at least one arm, and
a gate signal generating unit to generate an on/off control signal of each of the switching elements of each of the converter cells of each the arm in accordance with the output voltage command value,
wherein the voltage command value computing unit generates the output voltage command value with compensation for an AC vibration component, extracted for each arm, of a predetermined frequency of stored energy of the power storage elements of the converter cells included in the arm, wherein
the AC vibration component of stored energy of the power storage elements is extracted from temporal change for each arm in average value of voltage detection values of the power storage elements of the converter cells included in the arm, or it is extracted from temporal change in arm power estimate value obtained by multiplying an estimate value of the arm voltage and a command value of the arm current.
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5 . The power conversion device according to claim 1 , wherein
the power conversion device is connected in parallel with the AC power grid,
the power conversion unit has three phase arms in delta connection, and
connection ends of the three phase arms are respectively connected to three phases of the AC power grid through a rector component.
6 . The power conversion device according to claim 5 , wherein
the converter control unit further includes
an output current control unit to generate a first voltage command value for each phase for controlling an output current of the power conversion device in accordance with a command value,
a circulating current control unit to generate a second voltage command value for controlling circulating current that circulates in the power conversion device to pass through the arms of the three phases, such that voltages of the power storage elements of all the converter cells in the power conversion unit are balanced, and
a modulation index correcting unit to extract the AC vibration component for each arm and generate a modulation index correction signal having a frequency component in accordance with the AC vibration component, and
the voltage command value computing unit generates the output voltage command value to compensate for the AC vibration component of stored energy, based on the first voltage command value, the second voltage command value, and the modulation index correction signal.
7 . The power conversion device according to claim 6 , wherein the AC vibration component of stored energy of the power storage elements is extracted from temporal change for each arm in arm power estimate value in accordance with multiplication of an estimate value of arm voltage applied to the converter cells included in the arm and a command value of arm current passing through the arm.
8 . The power conversion device according to claim 6 , wherein
the circulating current control unit calculates a command value of the circulating current for balancing voltages of the storage elements of all the converter cells in the power conversion unit and computes the second voltage command value to control the circulating current in accordance with the command value, and
the command value of the circulating current is increased in a first state in which a command value of the output current of the power conversion device is smaller than a first preset value and an imbalance amount in voltage of the power storage elements of the converter cells of all the three phases is greater than a second preset value, compared to a second state that is not the first state.
9 . The power conversion device according to claim 8 , wherein
the AC vibration component of stored energy of the power storage elements is extracted from temporal change for each arm in arm power estimate value in accordance with multiplication of an estimate value of arm voltage applied to the arm and a command value of arm current passing through the arm, and
the command value of arm current is determined in accordance with a sum of a current command value for each phase determined from the command value of the output current and the command value of the circulating current.
10 . The power conversion device according to claim 1 , wherein
the power conversion device is connected between DC lines on a positive electrode side and a negative electrode side and the AC power grid, and
for each of three phases of the AC power grid, the power conversion unit includes the arms connected between an AC end connected to the AC power grid through a reactor component and the DC line on the positive electrode side and between the AC end and the DC line on the negative electrode side.
11 . The power conversion device according to claim 10 , wherein
the converter control unit further includes
an output current control unit to generate a first voltage command value for each phase for controlling an output current of the power conversion device in accordance with a command value,
a circulating current control unit to generate a second voltage command value for each phase for controlling circulating current of each phase that circulates in the power conversion device to pass through the arms on the positive electrode side and the negative electrode side in each of the phases, such that voltages of the power storage elements of the converter cells in the phase are balanced in each of the phases,
a voltage balance control unit to generate a third voltage command value for each phase for balancing voltages of the power storage elements between the converter cells in the arm on the positive electrode side and the converter cells in the arm on the negative electrode side in each of the phases, and
a modulation index correcting unit to extract the AC vibration component for each arm and generate a modulation index correction signal having a frequency component in accordance with the AC vibration component,
the voltage command value computing unit generates the output voltage command value for each arm to compensate for the AC vibration component of stored energy, based on the first voltage command value, the second voltage command value, the third voltage command value, a fourth voltage command value that is a command value of DC voltage applied to each of the arms, and the modulation index correction signal, and
the first voltage command value and the third voltage command value are reflected in the output voltage command value, with polarities opposite to each other between the arm on the positive electrode side and the arm on the negative electrode side in each of the phases.
12 . The power conversion device according to claim 11 , wherein the AC vibration component of stored energy of the power storage elements is extracted from temporal change for each arm in arm power estimate value in accordance with multiplication of an estimate of arm voltage applied to the converter cells included in the arm and a command value of arm current set for each phase corresponding to the arm.
13 . The power conversion device according to claim 11 , wherein
in each of the phases, in order to balance voltages of the power storage elements of the converter cells in the phase, the circulating current control unit calculates a command value of the circulating current for each phase and computes the second voltage command value for each phase to control the circulating current in accordance with the command value, and
the command value of the circulating current for each phase is increased in a first state in which a command value of the output current of the power conversion device is smaller than a first preset value and an imbalance amount in voltage of the power storage elements of the converter cells in the phase is greater than a second preset value, compared to a second state that is not the first state.
14 . The power conversion device according to claim 13 , wherein the AC vibration component of stored energy of the power storage elements is extracted from temporal change for each arm in arm power estimate value in accordance with multiplication of an estimate value of arm voltage applied to the converter cells included in the arm and a command value of arm current set for each phase corresponding to the arm, and
the command value of arm current is determined in accordance with a sum of a current command value for each phase determined from the command value of the output current and the command value of the circulating current for each phase.
15 . The power conversion device according to claim 1 , wherein the predetermined frequency includes a frequency of an integer multiple of a grid frequency of the AC power grid.