EP-4120542-B1 - CONTROL DEVICE FOR FREQUENCY CONVERTER

Inventors

• WATANABE, Kenta
• KAWAZOE, HIRONARI
• KIKUCHI, AKIRA
• NAKAIDE, Yosuke
• BANDO, AKIRA

Dates

Publication Date

20260506

Application Date

20210309

Claims (3)

1. A control device (109) of a frequency converter (103) in which a rotating machine (101) including a synchronous machine is interconnected to a power system (102), the frequency converter (103) including a plurality of sets of arms (105) in which a plurality of unit converters (200) are connected in series, capacitors (204) are connected in parallel in the plurality of unit converters (200), one ends of the series connected unit converters (200) are connected to respective phases of an alternating current, and the other ends of the series connected unit converters (200) are connected to terminals of a direct current, the control device (109) comprising: an AC voltage removal calculation unit (401) configured to detect at least voltages of the capacitors (204) in the plurality of unit converters (200) that form the arms (105), extract a DC component from the capacitor voltages, and input the DC component; an arm voltage command generation unit configured to generate an arm voltage command value; a carrier signal generation unit (113) configured to generate a carrier signal; and a gate pulse generation unit (111) that is provided with a modulated wave generator (301) configured to correct the arm voltage command value to be supplied by the arm voltage command generation unit and that uses the corrected arm voltage command value and the carrier signal to determine a gate command for each of the plurality of unit converters (200) in respective arms, the control device (109) being further characterized in that the modulated wave generator (301) uses the DC component calculated in the AC voltage removal calculation unit to correct the arm voltage command value so that magnitude of the voltages of the capacitors (204) in the unit converters (200) of the arms (105) does not fluctuate, wherein, to correct the arm voltage command value, V UP *, the modulates wave generator (301) causes an average value calculation unit (303) to calculate an average value, V CUPave , of the capacitors' voltages, V CUP_N' , for the unit converters (200) in an arm (105) and outputs a modulates wave, V UP_N *, according to the Formula V UP _ N ∗ = V UP ∗ + Gain V CUPave − V CUP _ N ′ ⋅ cosθ i , wherein V UP * defines the arm voltage command value, θ i is a current phase angle and Gain defines a control gain, such as a proportional gain, the AC voltage removal calculation unit (401) changes a filter time constant according to a phase of the rotating machine (101) in order to remove an AC voltage determined by a rotation frequency of the rotating machine (101), and the AC voltage removal calculation unit (401) calculates an integrated voltage obtained by integrating capacitor voltages until a desired phase threshold is reached, divides the integrated voltage by the number of times of integration to calculate an average voltage of the unit converters (200) in a desired period, and maintains the average voltage until the phase threshold is reached for a next time, thereby removing the AC voltage.
2. The control device (109) of a frequency converter (103) according to claim 1, wherein the carrier signal generation unit (113) determines a carrier frequency to be applied to a power conversion device (104) based on the rotation frequency of the rotating machine (101), and the carrier signal generation unit (113) has a function of preventing a voltage imbalance of the capacitor voltages by increasing a pulse number for PWM driving the frequency converter (103) in an operation region in which the rotation frequency of the rotating machine (101) is equal to or less than a specified value.
3. The control device (109) of a frequency converter (103) according to claim 1, wherein the carrier signal generation unit (113) has a function of increasing the pulse number for PWM driving the frequency converter (103) in an operation region in which a rotation angular frequency of the rotating machine (101) is less than 80% of a rated rotation angular frequency.

Description

Technical Field The present invention relates to a control device of a frequency converter as defined by the attached claims. Background Art An MMC converter includes a voltage source such as a capacitor and a storage battery that are energy storage elements, and a unit converter including a half-bridge circuit. The unit converter generates a desired voltage by controlling a PWM modulation factor of the half-bridge circuit. Voltages of the capacitor and the storage battery that are energy storage elements fluctuate by repeating charging and discharging at a cycle determined by an output AC frequency of the MMC converter. A plurality of sets of the unit converters are connected in series to form a two-terminal arm converter, first terminals of the arm converter are connected to respective phase terminals of an AC power source, and a star-connected second terminal is connected to a terminal of a DC power source. The arm converter connected to the respective phases as described above generates a desired AC voltage to perform an AC current control, and at the same time, superimposes a DC current to implement a power conversion with the DC power source. A control of the MMC converter includes a function (hereinafter, referred to as an inter-stage balance control) of balancing an average voltage of capacitors between unit converters by a current control of adjusting an arm current to an AC current command and a DC current command from the outside and by mutually adjusting PWM modulation factors of half-bridge circuits provided in the unit converters in an arm, and a function (hereinafter, referred to as an inter-phase balance control) of balancing total accumulated energy of capacitors in arm converters between the arm converters. In order to implement the inter-phase balance control, a circuit element for preventing a circulating current between the arm converters is required. Patent Literature 1 proposes to provide a circulating current prevention reactor between a first terminal of an arm converter and a terminal of an AC power source for the purpose of preventing a circulating current. A device disclosed in Patent Literature 1 forms a double star type MMC converter (hereinafter referred to as a DSMMC converter) which is one form of an MMC converter. In addition, Patent Literature PTL 2 discloses a power conversion device capable of suppressing a DC component at a transformer side. Specifically, a power conversion device is provided between a DC circuit including a positive-side DC line and a negative-side DC line as well as an AC circuit, and a positive-side arm and a negative-side arm are connected in series between the positive-side DC line and the negative-side DC line so that an AC current path, such as an AC terminal, is extended from a connection point of the positive-side arm and the negative side arm. Moreover, in Patent Literature PTL 3, likewise a power conversion device including a main circuit unit having a plurality of arms to which outputs of a plurality of converters are connected in series are described. Herein, a driving unit is particularly stipulated to generate a driving signal for driving a plurality of semiconductor switching elements in a main circuit unit according to a voltage command and output the signal to the plurality of semiconductor switching elements, an AC control unit which generates AC voltage commands for a number of phases on a AC side for controlling AC power of a target to be controlled, and a DC control unit which generates a DC voltage command for setting an average value of DC voltages of the plurality of capacitors in the main circuit unit to a predetermined value. Furthermore, Patent Literature PTL 4 also discloses an electric power conversion device including a power conversion unit and a converter control unit. Herein, the power conversion unit comprises three arms connected to an AC circuit while the converter control unit further includes a phase DC voltage control unit, a negative sequence current command value calculation unit, an output current control unit, a circulating current control unit, a voltage command value calculation unit, and a gate signal generation unit, and imbalance of cell DC capacitor voltages among phases due to grid imbalance is controlled by circulating current and negative sequence current. Lastly, in Patent Literature PTL 5, a method and a system for reducing a waveform distortion in a power conversion device is defined. For this, a respective power conversion device is provided with a plurality of cells connected in series and each having a plurality of switching elements and at least one capacitor. A phase information generation unit for generating phase information and on/off control units for turning on/off the switching elements of each of the cells at a timing based on the phase information and cell-specific information of each of the cells are further provided while the on/off control units preferably control the on/off states