JP2019193377A - Series multiple power converter - Google Patents

Abstract

To realize high operating efficiency and high voltage utilization while reducing a harmonic component contained in an AC output voltage in a series multiple power converter.SOLUTION: A series multiple power converter 1 includes: first to M-th stage converters 28 to 25 connected in series by M transformers 21 to 24; and a control device 11 for performing PWM control of an AC output voltage of the converter at each stage so that a voltage obtained by serially combining the AC output voltages of the converters at each stage matches an AC voltage command value. The PWM control includes: a multi-pulse mode for generating a control command for the converter so as to output multiple pulse voltages in a half cycle of the AC voltage command value; and a single pulse mode for generating a control command for the converter so as to output a single pulse voltage in the above section. The control device 11 controls the converters 28 to 26 from the first stage to the (M-1)-th stage in the single pulse mode and controls the M-th stage converter 25 in the multi-pulse mode.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a serial multiple power converter.

  Japanese Patent Laying-Open No. 2013-258841 (Patent Document 1) discloses a serial multiple power conversion device in which a plurality of converters are multiplexed in series. In patent document 1, the method of multiplexing the alternating current side with a transformer is employ | adopted. In this method, the AC voltage terminal of each converter is connected to the secondary winding of each transformer, and the primary winding of each transformer is connected in series and connected to the power system. A voltage obtained by serially combining the AC output voltages of the converters is output to the power system.

JP2013-258841A

  The serial multiple power converter can reduce the harmonic components contained in the AC output voltage waveform of the entire device by combining the output voltages of a plurality of converters, resulting in harmonics flowing into the power system. The current can be reduced. However, there is a concern that the size of the apparatus increases as the number of converters to be multiplexed increases. Moreover, since the power loss of the entire device increases due to the power loss generated by each converter, there is a concern that the operation efficiency of the serial multiple power conversion device may be reduced.

  The present invention has been made in consideration of the above-described problems, and an object of the present invention is to reduce high-frequency components contained in an AC output voltage and achieve high operating efficiency and high voltage in a series multiple power converter. It is to realize the utilization rate.

  A serial multiple power conversion device according to an aspect of the present invention is a serial multiple power conversion device linked to an AC power system, and M units (M is 2) in which primary windings are multiplexed and connected in series to the AC power system. And the first to M-th stage converters connected to the secondary windings of the M transformers and performing bidirectional power conversion between AC power and DC power, respectively. And a control device that performs PWM control of the AC output voltage of the converter at each stage so that the voltage obtained by serially combining the AC output voltages of the converters at each stage matches the AC voltage command value. The converter at each stage is configured by a single-phase full bridge circuit. The PWM control includes a multi-pulse mode for generating a control command for the converter so as to output a plurality of pulse voltages in a half cycle of the AC voltage command value, and a converter for outputting a single pulse voltage in the above interval. And a single pulse mode for generating a control command. The control device controls the first to (M−1) th stage converters in the single pulse mode, and controls the Mth stage converter in the multi-pulse mode.

  According to the present invention, in the serial multiple power conversion device, high operating efficiency and high voltage utilization can be realized while reducing harmonic components contained in the AC output voltage.

It is a schematic block diagram which shows the main circuit structure of the serial multiple power converter device 1 which concerns on Embodiment 1 of this invention. It is a figure which shows the specific circuit structure of the converter shown in FIG. It is a figure which shows the circuit structure of the x phase single phase full bridge | bridging 3 level circuit contained in a converter. It is a wave form diagram for demonstrating control of the converter at the time of multipulse mode application. It is a wave form diagram for demonstrating control of the converter at the time of single pulse mode application. It is a wave form diagram for demonstrating the PWM control of a 4-stage serial multiple converter. It is a wave form diagram for demonstrating the PWM control of a 4 step | paragraph serial multiple converter at the time of the fall of a system voltage. It is a wave form diagram for demonstrating the PWM control of a 4 step | paragraph serial multiple converter at the time of the fall of a system voltage. It is a wave form diagram for demonstrating the PWM control of a 4 step | paragraph serial multiple converter at the time of the fall of a system voltage. FIG. 5 is a schematic diagram for conceptually explaining switching between a single long pulse mode and a single short pulse mode. It is a circuit block diagram which shows the part regarding control of a 4-stage serial multiple converter of the control apparatus shown in FIG. It is a block diagram which shows the structure of the PWM control part shown in FIG. It is a block diagram which shows the structural example of the 4th stage control part shown in FIG. It is a block diagram which shows the structural example of the 1st step control part shown in FIG. 12 to a 3rd step control part. It is a block diagram which shows the structural example of the single long pulse mode control part shown in FIG. It is a block diagram which shows the structural example of the single short pulse mode control part shown in FIG. It is an operation | movement waveform diagram of the 4-stage serial multiple converter at the time of a rated output. It is an operation | movement waveform diagram of the 4-stage serial multiple converter at the time of two-phase modulation application.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the drawings, the same or corresponding parts are denoted by the same reference numerals, and the description thereof will not be repeated.

[Embodiment 1]
FIG. 1 is a schematic block diagram showing a main circuit configuration of serial multiple power conversion device 1 according to Embodiment 1 of the present invention. The serial multiple power conversion apparatus 1 is typically applied to a BTB (Back to Back) system used for power transfer in an asynchronous AC system or a different frequency AC system. Specific examples include a 50-60 Hz frequency converter.

  Referring to FIG. 1, serial multiple power conversion device 1 is connected between power system 2 and power system 3 having different AC frequencies. Power systems 2 and 3 have a plurality of phases (for example, three phases). The serial multiple power conversion device 1 includes AC voltage detectors 12 and 52, current detectors 13 and 53, a DC voltage detector 14, a power conversion unit 10, and a control device 11.

  The AC voltage detector 12 detects an AC voltage of the power system 2 and outputs a signal indicating the detected value to the control device 11. The current detector 13 detects a current flowing through the power system 2 and outputs a signal indicating the detected value to the control device 11. The AC voltage detector 52 detects the AC voltage of the power system 3 and outputs a signal indicating the detected value to the control device 11. The current detector 53 detects a current flowing through the power system 3 and outputs a signal indicating the detected value to the control device 11.

  The power conversion unit 10 is connected between the power system 2 and the power system 3. Specifically, the power conversion unit 10 is connected to one power system 2 via transformers 21 to 24 and connected to the other power system 3 via transformers 61 to 64. Power conversion unit 10 includes transformers 21 to 24, converters 25 to 28, DC buses 15 to 17, smoothing capacitors C1 and C2, converters 65 to 68, and transformers 61 to 64. DC buses 15 to 17 and smoothing capacitors C1 and C2 constitute a DC circuit of each converter.

  The primary windings of the transformers 21 to 24 are connected to the power system 2. The primary windings of the transformers 21 to 24 are connected in series with each other and are star-connected. As a result, a voltage obtained by combining the AC output voltages of converters 25 to 28 in series with each other is output to power system 2. The AC terminal of each phase power conversion circuit of converters 25-28 is connected to one end 33 of each phase secondary winding of transformers 21-24, respectively. AC terminals of other phase power conversion circuits connected to the smoothing capacitors C1 and C2 are connected to the other ends 34 of the phase secondary windings of the transformers 21 to 24, respectively. Converters 25-28 constitute a polyphase full bridge circuit.

  For example, a power conversion circuit of a converter 25 that converts a DC voltage between the DC positive bus 15 and the DC negative bus 17 into each phase AC voltage is connected to one end 33 of each phase secondary winding of the transformer 21. The Connected to the other end 34 of each phase secondary winding of the transformer 21 is a power conversion circuit that converts a DC voltage between the DC positive bus 15 and the DC negative bus 17 into an AC voltage of each phase. The DC circuit of the two power conversion circuits connected to each phase secondary winding is common.

  Each of converters 25 to 28 is composed of a power converter using a self-extinguishing element such as GCT (Gate Commutated Turn-Off thyristor) or IGBT (Insulated Gate Bipolar Transistor). In the present embodiment, converters 25 to 28 are each configured as a three-level circuit. Three DC voltage terminals of each of converters 25 to 28 are connected to DC positive bus 15, DC neutral point bus 16 and DC negative bus 17, respectively.

  Smoothing capacitors C1 and C2 are connected in series between DC positive bus 15 and DC negative bus 17, and smooth the voltage between DC positive bus 15 and DC negative bus 17. A DC neutral point bus 16 is connected to a connection point between the smoothing capacitors C1 and C2.

  The DC voltage detector 14 detects the voltage between the terminals of the smoothing capacitor C1 and the voltage between the terminals of the smoothing capacitor C2, and outputs a signal indicating the detected value to the control device 11.

  Each of converters 65 to 68 is configured by a power converter using a self-extinguishing element such as GCT or IGBT, similarly to converters 25 to 28. Converters 65-68 are all configured as a three-level circuit. Three DC voltage terminals of each of converters 65 to 68 are connected to DC positive bus 15, DC neutral point bus 16 and DC negative bus 17, respectively.

  The AC terminals of converters 65-68 are connected to the secondary windings of transformers 61-64, respectively. Specifically, the AC terminal of each phase power conversion circuit of converters 65-68 is connected to one end 69 of each phase of the secondary windings of transformers 61-64, respectively. AC terminals of other phase power conversion circuits connected to the smoothing capacitors C1 and C2 are connected to the other ends 70 of the phases of the secondary windings of the transformers 61 to 64, respectively. Each of converters 65-68 constitutes a polyphase full bridge circuit.

  The primary windings of the transformers 61 to 64 are connected to the power system 3. The primary windings of the transformers 61 to 64 are connected in series with each other and are star-connected. As a result, a voltage obtained by combining each AC output voltage of converters 65 to 68 in series with each other is output to power system 3.

  Thus, the power conversion unit 10 has a configuration in which the DC terminals of the converters 25 to 28 and the DC terminals of the converters 65 to 68 are connected via the DC buses 15 to 17. In the example of FIG. 1, the DC terminals of the converters 25 to 28 are connected to a common DC circuit, but the DC terminals of the converters 25 to 28 may be connected to independent DC circuits. . Similarly, the DC terminals of the converters 65 to 68 may be connected to independent DC circuits.

  The power conversion unit 10 accommodates power from one power system 2 (or power system 3) to the other power system 3 (or power system 2). Specifically, in the case where power is interchanged from the power system 2 to the power system 3, the converters 25 to 28 convert the AC voltages supplied from the transformers 21 to 24 to desired DC voltages, respectively, and convert the converted DC voltages. Voltage is supplied to converters 65-68 via DC buses 15-17. Converters 65 to 68 convert the DC voltages supplied from converters 25 to 28 via DC buses 15 to 17 into desired AC voltages, respectively. The transformers 61 to 64 output a voltage obtained by serially combining the AC output voltages of the converters 65 to 68 to the power system 3.

  In addition, when the electric power is transferred from the electric power system 3 to the electric power system 2, the converters 65 to 68 convert the AC voltages respectively supplied from the transformers 61 to 64 into desired DC voltages, and the converted DC voltages. Is supplied to converters 25-28 via DC buses 15-17. Converters 25 to 28 convert the DC voltage supplied from converters 65 to 68 via DC buses 15 to 17 into desired AC voltages, respectively. The transformers 21 to 24 output a voltage obtained by serially combining the AC output voltages of the converters 25 to 28 to the power system 2.

  Here, in this specification, in order to distinguish the four converters 25-28 in which the alternating current side is multiplexed in series by the transformers 21-24, the converter 28 is referred to as "first stage converter 28". The converter 27 may be referred to as a “second stage converter 27”, the converter 26 may be referred to as a “third stage converter 26”, and the converter 25 may be referred to as a “fourth stage converter 25”. . In addition, the entire converters 25 to 28 may be referred to as “four-stage serial multiple converter 20”.

  Further, in order to distinguish the four converters 65 to 68 multiplexed on the AC side in series by the transformers 61 to 64 from each other, the converter 68 is referred to as a “first stage converter 68”, and the converter 67 Are referred to as “second stage converter 67”, converter 66 may be referred to as “third stage converter 66”, and converter 65 may be referred to as “fourth stage converter 65”. Further, the entire converters 65 to 68 may be referred to as “four-stage serial multiple converter 60”.

  In each of the four-stage serial multiple converters 20 and 60, the first-stage converter to the fourth-stage converter need only be serially multiplexed, and the order of arrangement is not particularly limited.

  FIG. 2 is a diagram showing a specific circuit configuration of converters 25 to 28 and converters 65 to 68 shown in FIG. Since the converters 25 to 28 and the converters 65 to 68 have the same circuit configuration, FIG. 2 representatively illustrates the circuit configuration of the converter 25.

  Referring to FIG. 2, converter 25 is configured as a three-level circuit, and includes switching elements S1R to S8R, S1S to S8S, S1T to S8T, and diodes D1R to D12R, D1S to D12S, and D1T to D12T. The switching element is, for example, a GCT, but is not limited to this as long as it is a self-extinguishing type switching element.

  Here, in order to comprehensively describe the configuration of each phase of the converter 25, the symbols R, S, and T are collectively denoted by a symbol “x”. The anode of switching element S1x is connected to DC positive bus 15 and the cathode is connected to the anode of switching element S2x. The cathode of the switching element S2x is connected to one end 33 of the x-phase secondary winding. The anode of the switching element S3x is connected to one end 33 of the x-phase secondary winding, and the cathode is connected to the anode of the switching element S4x. The cathode of the switching element S4x is connected to the DC negative bus 17. The diodes D1x to D4x are connected in antiparallel to the switching elements S1x to S4x, respectively. The cathode of diode D9x is connected to the connection point of switching elements S1x and S2x, and its anode is connected to DC neutral point bus 16. The cathode of diode D10x is connected to DC neutral point bus 16, and its anode is connected to the connection point of switching elements S3x and S4x.

  The anode of switching element S5x is connected to DC positive bus 15, and the cathode is connected to the anode of switching element S6x. The cathode of the switching element S6x is connected to the other end 34 of the x-phase secondary winding. The anode of the switching element S7x is connected to the other end 34 of the x-phase secondary winding, and the cathode is connected to the anode of the switching element S8x. The cathode of the switching element S8x is connected to the DC negative bus 17. The diodes D5x to D8x are connected in antiparallel to the switching elements S5x to S8x, respectively. The cathode of diode D11x is connected to the connection point of switching elements S5x and S6x, and its anode is connected to DC neutral point bus 16. The cathode of diode D12x is connected to DC neutral point bus 16, and the anode thereof is connected to the connection point of switching elements S7x and S8x. The diodes D1x to D4x and D5x to D8x function as freewheeling diodes, and the diodes D9x to 12x function as clamp diodes.

  That is, the switching elements S1x to S4x and the diodes D1x to D4x, D9x, D10x constitute one power conversion circuit, and the connection point of the switching elements S2x and S3x serves as an AC terminal, and the secondary winding of the x-phase of the transformer Connected to one end 33 of the line. Further, another power conversion circuit is configured by the switching elements S5x to S8x and the diodes D5x to D8x, D11x, and D12x, and the connection point of the switching elements S6x and S7x serves as an AC terminal, and the secondary winding of the x-phase of the transformer Connected to the other end 34 of the line. Therefore, the AC terminals of the two power conversion circuits are respectively connected to both ends of the x-phase secondary winding, and a single-phase full-bridge three-level circuit is configured by the two power conversion circuits.

  When power systems 2 and 3 are three-phase, a total of six power conversion circuits are connected in parallel between DC bus 15 and DC bus 17. In other words, three single-phase full-bridge three-level circuits are connected in parallel between the DC bus 15 and the DC bus 17.

  Returning to FIG. 1, the control device 11 controls the operations of the converters 25 to 28 and the converters 65 to 68. In the present embodiment, the control device 11 applies PWM (Pulse Width Modulation) control as a control method for the switching elements constituting each converter. The control device 11 receives signals from the AC voltage detectors 12 and 52, the current detectors 13 and 53, and the DC voltage detector 14 and executes PWM control. The control device 11 generates gate pulse signals GC1 to GC4 which are control commands for controlling the converters 25 to 28 by PWM control, and the generated gate pulse signals GC1 to GC4 to the converters 25 to 28, respectively. Output. Further, the control device 11 generates gate pulse signals GC5 to GC8 which are control commands for controlling the converters 65 to 68 by PWM control, and the generated gate pulse signals GC5 to GC8 are converted into the converters 65 to 68. Respectively.

  Next, PWM control of each converter in the control device 11 will be described. In the following description, PWM control of the converter will be described using the x-phase single-phase full-bridge three-level circuit shown in FIG.

  Referring to FIG. 3, the single-phase full-bridge three-level circuit is an a-phase power conversion circuit (hereinafter also referred to as an a-phase arm) connected in parallel between DC positive bus 15 and DC negative bus 17. And b-phase power conversion circuit (hereinafter also referred to as b-phase arm). The AC terminal of the a-phase arm (connection point of the switching elements S2x and S3x) is connected to one end 33 of the secondary winding of the x-phase, and the AC terminal of the b-phase arm (connection point of the switching elements S6x and S7x) is x Connected to the other end 34 of the phase secondary winding.

  Here, in the present embodiment, the PWM control of the converter includes a plurality of pulse voltages in a half cycle of the AC voltage command value (section of electrical angle 0 ° to 180 ° or section of electrical angle 180 ° to 360 °). A control mode for generating a control command (gate pulse signal GC) of the converter so as to output a signal, and a control for generating a control command for the converter so as to output a single pulse voltage in a half cycle of the AC voltage command value There is a mode. In the present specification, the former control mode is referred to as “multi-pulse mode”, and the latter control mode is referred to as “single pulse mode”. Hereinafter, each of the multi-pulse mode and the single pulse mode will be described in detail.

  FIG. 4 is a waveform diagram for explaining control of the converter when the multi-pulse mode is applied. FIG. 4 shows an on / off relationship between the x-phase AC voltage Vx and the switching elements S1x to S8x. Va in the figure is the output voltage at the AC terminal of the a-phase arm, Vb is the output voltage at the AC terminal of the b-phase arm, and the line output voltage Vab (difference between the voltage Va and the voltage Vb) is the x-phase output voltage. Corresponds to AC output voltage.

  In the multi-pulse mode, the level of the AC voltage command value Vx * (line voltage command value) and the carrier waves CW1 and CW2 are compared, and on / off combinations of the switching elements S1x to S4x are determined based on the comparison result. . Further, the AC voltage command value -Vx * and the levels of the carrier waves CW1 and CW2 are compared, and on / off combinations of the switching elements S5x to S8x are determined based on the comparison result.

  Carrier waves CW1 and CW2 are triangular wave signals, for example. The carrier wave CW1 has a maximum value of + Vd and a minimum value of 0. The carrier wave CW2 has a maximum value of 0 and a minimum value of -Vd. Vd corresponds to the DC voltage between the DC positive bus 15 and the DC negative bus 17. The frequencies and phases of the carriers CW1 and CW2 are the same. The carrier waves CW1 and CW2 have a frequency 2N times the AC voltage command value Vx * (for example, N = 8), and are signals synchronized with the AC voltage command value Vx *.

  The switching elements S1x to S4x of the a-phase arm are turned on / off at a timing at which the amplitudes of the AC voltage command value Vx * and the carrier waves CW1 and CW2 coincide. The b-phase arm switching elements S5x to S8x are turned on and off at the timing when the amplitudes of the AC voltage command value -Vx * and the carrier waves CW1 and CW2 coincide. In the a-phase arm, switching elements S1x and S3x are switched when the polarity of AC voltage command value Vx * is positive, and switching elements S2x and S4x are switched when the polarity of AC voltage command value Vx * is negative. Switching is performed. In the b-phase arm, the switching elements S5x and S7x are switched when the polarity of the AC voltage command value -Vx * is positive, and the switching elements S6x and S8x are switched when the polarity of the AC voltage command value -Vx * is negative. Switching is performed.

  Accordingly, as shown in FIG. 4, each of the output voltages Va and Vb takes three values of ± Vd / 2, 0, and the line-to-line output voltage Vab takes five values of ± Vd, ± Vd / 2, 0. . Although illustration is omitted, when the amplitude of the AC voltage command values Vx * and -Vx * is ½ or less of the amplitude of the carrier waves CW1 and CW2, the line output voltage Vab is ± Vd / 2,0. Takes 3 values.

  FIG. 5 is a waveform diagram for explaining the control of the converter when the single pulse mode is applied. FIG. 5 shows an on / off relationship between the x-phase AC voltage Vx and the switching elements S1x to S8x. Va in the figure is the output voltage at the AC terminal of the a-phase arm, Vb is the output voltage at the AC terminal of the b-phase arm, and the line output voltage Vab (difference between the voltage Va and the voltage Vb) is the x-phase output voltage. Corresponds to AC output voltage.

  In the single pulse mode, the level of the AC voltage command value Vx * (line voltage command value) and the threshold voltage Vth are compared, and on / off combinations of the switching elements S1x to S4x are determined based on the comparison result. . Further, the AC voltage command value -Vx * and the threshold voltage / Vth are compared, and based on the comparison result, the on / off combination of the switching elements S5x to S8x is determined. The threshold voltage Vth is 0 <Vth ≦ Vd / 2. The threshold voltage / Vth is a voltage obtained by offsetting the threshold voltage Vth by −Vd.

  The switching elements S1x to S4x of the a-phase arm are turned on / off at the timing when the amplitude of the AC voltage command value Vx * matches the threshold voltage Vth. The switching elements S5x to S8x of the b-phase arm are turned on / off at the timing when the amplitude of the AC voltage command value −Vx * matches the threshold voltage / Vth. In the a-phase arm, switching elements S1x and S3x are switched when the polarity of AC voltage command value Vx * is positive, and switching elements S2x and S4x are switched when the polarity of AC voltage command value Vx * is negative. Switching is performed. In the b-phase arm, the switching elements S5x and S7x are switched when the polarity of the AC voltage command value -Vx * is positive, and the switching elements S6x and S8x are switched when the polarity of the AC voltage command value -Vx * is negative. Switching is performed.

  As shown in FIG. 5, even in the single pulse mode, the output voltages Va and Vb take the three values ± Vd / 2 and 0, and the line output voltage Vab is the same as in the multi-pulse mode shown in FIG. Five values of ± Vd, ± Vd / 2, 0 are taken. However, in the multi-pulse mode, a plurality of pulse voltages are output from each of the a-phase arm and the b-phase arm during a half cycle of the AC voltage command value Vx, whereas in the single pulse mode, the AC voltage command value Vx is output. A single pulse voltage is output from each of the a-phase arm and the b-phase arm during the half cycle.

  As is clear from FIGS. 4 and 5, in the multi-pulse mode, two of the four switching elements are turned on and off once during one carrier period (period of one pulse of the carrier wave), and the remaining two are not turned on and off. The average switching frequency per switching element is 1/2 of the frequency of the carrier waves CW1 and CW2. On the other hand, in the single pulse mode, two of the four switching elements are turned on / off once during a half cycle of the AC voltage command value Vx *, and the remaining two are not turned on / off. The switching frequency is ½ of the frequency of the AC voltage command value Vx *. Thus, since the single pulse mode has a lower average switching frequency per switching element than the multi-pulse mode, it is possible to reduce power loss (switching loss) generated in the switching element. Thus, the single pulse mode can increase the operating efficiency of the converter than the multi-pulse mode.

  In the multi-pulse mode, in the case of the sine wave comparison PWM method as shown in FIG. 4, the maximum value of the fundamental wave component of the line-to-line output voltage is Vdc, so that the fundamental wave component of the AC output voltage of the converter can be increased. There is a limit to the voltage utilization rate. The voltage utilization rate is a scale indicating how much an AC voltage can be generated with respect to the same DC component. On the other hand, in the single pulse mode, if the amplitude of the AC voltage command value Vx * is made higher than the amplitude of the carrier wave, the fundamental wave component of the AC output voltage of the converter can be increased, thus increasing the voltage utilization rate. be able to.

  On the other hand, the single pulse mode has a problem that the harmonic component of the AC output voltage increases. On the other hand, in the multi-pulse mode, the waveform of the AC output voltage changes more finely than in the single-pulse mode, so that the waveform can be made closer to a sine wave, resulting in harmonics generated by the operation of the converter. Wave components can be reduced. Therefore, the multipulse mode can reduce the content rate of the harmonic component contained in the AC output voltage as compared with the single pulse mode.

  In the serial multiple power conversion device 1 according to the present embodiment, the multi-pulse mode and the single pulse mode are used together in the PWM control of a plurality of converters (four in FIG. 1) connected in series. The entire apparatus realizes high operating efficiency and high voltage utilization rate while reducing harmonic components contained in the AC output voltage.

  Hereinafter, PWM control of the four-stage serial multiple converter 20 including the converters 25 to 28 will be described. Note that the PWM control described below can also be applied to the four-stage serial multiple converter 60 including the converters 65 to 68.

  FIG. 6 is a waveform diagram for explaining the PWM control of the four-stage serial multiple converter 20. FIG. 6 shows that the x-phase (x is R, S, T) AC voltage command value Vx * and the half cycle (electrical angle) of the AC output voltage Vx from the first stage converter 28 to the fourth stage converter 25. A waveform of 0 ° to 180 °) is shown. In addition, the dotted line in a figure has shown the waveform of the system voltage of the electric power grid | system 2. FIG.

  Referring to FIG. 6, in the present embodiment, AC voltage command value Vx * is generated by superimposing a third harmonic component of a sine wave on a sine wave having the frequency of power system 2 as a fundamental frequency. The By superimposing the third-order harmonic component on the sine wave, the amplitude of the AC voltage command value Vx * becomes equal to or less than the amplitude of the carrier waves CW1 and CW2, and PWM control is possible over the entire period. Furthermore, since the third harmonic component does not affect the line voltage, by superimposing this component on the fundamental wave component, the maximum value of the fundamental wave amplitude of the line voltage becomes 2 / √3 times Vd, Compared with the sine wave comparison method, a voltage utilization factor about 15% higher can be obtained.

  The control device 11 performs PWM control of the AC output voltage of each stage converter so that the voltage obtained by serially combining the AC output voltages of the first stage converter 28 to the fourth stage converter 25 matches the AC voltage command value Vx. To do. Specifically, the control device 11 controls each of the first-stage converter 28, the second-stage converter 27, and the third-stage converter 26 in a single pulse mode, and includes the fourth-stage converter 25. Control by pulse mode.

  In the control of the first stage converter 28, the level of the AC voltage command value Vx * and the first threshold voltage Vth1 are compared, and on / off combinations of the switching elements S1x to S4x are determined based on the comparison result. The first threshold voltage Vth1 is 0 <Vth1 ≦ Vd / 2. Further, the levels of the AC voltage command value Vx * and the first threshold voltage / Vth1 are compared, and an on / off combination of the switching elements S5x to S8x is determined based on the comparison result. The first threshold voltage / Vth1 = Vd−Vth1. Note that the on / off combination of the switching elements S5x to S8x based on the comparison result of the AC voltage command value Vx * and the first threshold voltage / Vth1 in FIG. 6 is the AC voltage command value −Vx * shown in FIG. This is substantially the same as the combination of the switching elements S5x to S8x based on the comparison result with the threshold voltage / Vth. By applying the single pulse mode, the AC output voltage of the first stage converter 28 becomes a trapezoidal single pulse voltage having a height of Vd in the section of the electrical angle of 0 ° to 180 °. The single pulse voltage takes three values of 0, Vd / 2, and Vd.

  In the control of the second stage converter 27, the level of the AC voltage command value Vx * and the second threshold voltage Vth2 are compared, and an on / off combination of the switching elements S1x to S4x is determined based on the comparison result. The second threshold voltage Vth2 is Vd larger than the first threshold voltage Vth1 (Vth2 = Vth1 + Vd). Further, the levels of the AC voltage command value Vx * and the second threshold voltage / Vth2 are compared, and on / off combinations of the switching elements S5x to S8x are determined based on the comparison result. The second threshold voltage / Vth2 = Vd × 2-Vth1. Note that the on / off combination of the switching elements S5x to S8x based on the comparison result between the AC voltage command value Vx * and the second threshold voltage / Vth2 in FIG. 6 is the AC voltage command value −Vx * shown in FIG. This is substantially the same as the combination of the switching elements S5x to S8x based on the comparison result with the threshold voltage / Vth1. By applying the single pulse mode, the AC output voltage of the second stage converter 27 becomes a trapezoidal single pulse voltage having an amplitude Vd in the section of the electrical angle of 0 ° to 180 °. The single pulse voltage takes three values of 0, Vd / 2, and Vd.

  In the control of the third stage converter 26, the level of the AC voltage command value Vx * and the third threshold voltage Vth3 are compared, and an on / off combination of the switching elements S1x to S4x is determined based on the comparison result. The third threshold voltage Vth3 is Vd larger than the second threshold voltage Vth2, and Vd × 2 larger than the first threshold voltage Vth1 (Vth3 = Vth1 + Vd × 2). Further, the levels of the AC voltage command value Vx * and the third threshold voltage / Vth3 are compared, and on / off combinations of the switching elements S5x to S8x are determined based on the comparison result. The third threshold voltage / Vth3 = Vd × 3-Vth1. The combination of ON / OFF of the switching elements S5x to S8x based on the comparison result between the AC voltage command value Vx * and the third threshold voltage / Vth3 in FIG. 6 is the AC voltage command value −Vx * shown in FIG. This is substantially the same as the combination of the switching elements S5x to S8x based on the comparison result with the threshold voltage / Vth1. By applying the single pulse mode, the AC output voltage of the third stage converter 26 becomes a trapezoidal single pulse voltage having an amplitude Vd in the section of the electrical angle of 0 ° to 180 °. The single pulse voltage takes three values of 0, Vd / 2, and Vd.

  In the control of the fourth stage converter 25, the level of the AC voltage command value Vx and the carrier waves CW1 and CW2 are compared, and the combination of ON / OFF of the switching elements S1x to S4x is determined based on the comparison result. Further, the AC voltage command value -Vx * and the levels of the carrier waves CW1 and CW2 are compared, and on / off combinations of the switching elements S5x to S8x are determined based on the comparison result. However, for this comparison, a value obtained by subtracting Vd × 3 from the AC voltage command value Vx is used, and a value obtained by adding Vd × 3 to the AC voltage command value −Vx is used. This Vd × 3 is equivalent to the combined voltage of the AC output voltages of the converters 28 to 26 in the first to third stages.

  Note that the carrier waves CW1 and CW2 in FIG. 6 have the same amplitude and frequency as the carrier waves CW1 and CW2 in FIG. In FIG. 6, the carrier wave CW2 is inverted between positive and negative.

  In the fourth stage converter 25, the switching elements S1x to S4x are turned on and off at the timing when the amplitudes of the AC voltage command values (Vx * −Vd × 3) and the carrier waves CW1 and CW2 match, and the switching elements S5x to S8x are The AC voltage command value (−Vx * + Vd × 3) and the carrier waves CW1 and CW2 are turned on and off at the same timing. Note that the on / off combination of the switching elements S5x to S8x based on the comparison result between the AC voltage command value Vx * and the carrier wave CW2 in FIG. 6 is the combination of the AC voltage command value −Vx * and the carrier wave CW2 shown in FIG. This is substantially the same as the combination of the switching elements S5x to S8x based on the comparison result. By applying the multi-pulse mode, the AC output voltage of the fourth stage converter 25 takes three values of + Vd, + Vd / 2, 0 in the section of the electrical angle of 0 ° to 180 °.

  The AC output voltage of the four-stage serial multiple converter 20 is obtained by combining the AC output voltages of the first-stage converter 28 to the fourth-stage converter 25 in series. Therefore, as shown in FIG. 6, the AC output voltage is obtained by accumulating trapezoidal single pulse voltages from the first stage converter 28 to the third stage converter 26 in the section of the electrical angle 0 ° to 180 °. Further, the AC output voltage of the fourth stage converter 25 is further added.

  In other words, by accumulating single pulse voltages from the first stage converter 28 to the third stage converter 26, a trapezoidal base having a height of Vd × 3 is formed. This base has seven values of 0, Vd / 2, Vd, Vd × 3/2, Vd × 2, Vd × 5/2, and Vd × 3. Then, by placing a plurality of pulse voltages of the fourth stage converter 25 on this base, the value of the AC output voltage further takes three values of Vd × 3, Vd × 7/2, and Vd × 4. Become.

  As shown in FIG. 6, the AC voltage command value Vx * is divided into an interval I in which the voltage value monotonously increases from 0 to Vd × 3 in the interval of electrical angle 0 ° to 180 °, and the voltage value is Vd × 3 or more Vd. It can be roughly divided into a section II that fluctuates within a range of × 4 or less and a section III in which the voltage value monotonously decreases from Vd × 3 to 0. Among the AC output voltages of the four-stage serial multiple converter 20, the section I and the section III are formed by a single pulse voltage from the first stage converter 28 to the third stage converter 26, and the voltage value in the section II The undulation is formed by a plurality of pulse voltages of the fourth stage converter 25.

  As described above, in the PWM control of the AC output voltage of the four-stage serial multiple converter 20, the single-pulse mode is applied to the first-stage converter 28 to the third-stage converter 26, and the fourth-stage conversion is performed. By applying the multi-pulse mode to the converter 25, the power loss generated in the switching elements of the first-stage converter 28 to the third-stage converter 26, compared to the case where the multi-pulse mode is applied to each stage converter. (Switching loss) can be reduced.

  Further, by applying the single pulse mode from the first stage converter 28 to the third stage converter 26, a trapezoidal base having a height of Vd × 3 is formed by the output voltages of the three converters. Can do. When forming the same height base by controlling each stage converter in the multi-pulse mode, it is necessary to multiplex four or more converters. On the other hand, in the present embodiment, since a smaller number of converters are sufficient, it is possible to reduce the number of converter stages required to output an AC voltage that matches the AC voltage command value Vx *. This improves the voltage utilization factor of the serial multiple converter 20, and can contribute to a reduction in size, weight, and cost of the device structure of the serial multiple converter 20.

  Further, by applying the multi-pulse mode to the fourth stage converter 25, the waveform of the AC output voltage placed on the base can be brought close to the AC voltage command value Vx *. Therefore, the content rate of the harmonic component contained in the AC output voltage of the four-stage serial multiple converter 20 can be brought close to the same level as the content rate of the harmonic component of the AC voltage command value Vx *.

  As a result, according to the PWM control according to the present embodiment, the series multiple power conversion device 1 as a whole can achieve high operating efficiency and voltage utilization rate while reducing harmonic components contained in the AC output voltage. it can.

  Although the PWM control of the four-stage serial multiple converter 20 has been described in the first embodiment, the PWM control according to the present embodiment is performed from the first stage to the Mth stage (M is an integer of 2 or more). It can be generally applied to an M-stage serial multiple converter consisting of In this case, the control device 11 controls the first to (M−1) th stage converters in the single pulse mode and controls the Mth stage converter in the multi-pulse mode. In the section of the AC voltage command value Vx * with an electrical angle of 0 ° to 180 °, the AC output voltage of the M-stage serial multiple converter is a single trapezoidal shape of the converter from the first stage to the (M−1) -th stage. A base having a height of Vd × (M−1) is formed by accumulating pulse voltages. The AC output voltage is 3 (Vd × (M−1), Vd × (M−1) + Vd / 2, Vd × M) by placing a plurality of pulse voltages of the M-th stage converter on this base. Will take the value.

[Embodiment 2]
In the second embodiment, a method for generating the AC voltage command value Vx * used for the PWM control shown in the first embodiment will be described. In the following description, it is assumed that carrier waves CW1 and CW2 have a frequency that is 16 times the AC voltage command value Vx *, following the example of FIGS.

  When the carrier waves CW1 and CW2 have a frequency 16 times as high as the AC voltage command value Vx *, as shown in FIG. 4, the carrier wave of a total of 8 pulses in the section of the AC voltage command value Vx * with an electrical angle of 0 ° to 180 °. Will be included. That is, the 8-carrier cycle is included in the section of the electrical angle of 0 ° to 180 °.

  In the present embodiment, the first 1.5 carrier period (1.5 pulses) of these 8 carrier periods is assigned to section I, and the last 1.5 carrier period (1.5 pulses) is assigned to section III. These five intermediate carrier periods (5 pulses) are assigned to section II. That is, section I corresponds to a section of electrical angle 0 ° to 33.75 °, section II corresponds to a section of 33.75 ° to 146.25 °, and section III corresponds to a section of 146.25 ° to 180 °. It corresponds to.

  The third harmonic is superimposed so that the AC voltage command value Vx * is Vd × 3 at electrical angles of 33.75 ° and 146.25 °. For example, when the AC voltage of the power system 2 is the nominal rated voltage (system voltage 1 p.u., no load on the converter), the third harmonic superposition ratio (ratio of the third harmonic amplitude to the fundamental component) Is about 19%.

  The superposition ratio of the third harmonic can be changed according to the amplitude value of the AC voltage of the power system 2. For example, when the power system 2 is in steady operation at a frequency of 60 Hz and is at the maximum voltage, the electrical angle is 33.75 ° and 146.25 ° by setting the superposition ratio of the third harmonic to about 12%. The AC voltage command value Vx * at can be set to Vd × 3. Further, when the AC voltage of the power system 2 is the minimum voltage (system voltage 0.9 p.u., no load on the converter), the electrical angle 33. The AC voltage command value Vx * at 75 ° and 146.25 ° can be set to Vd × 3.

  Thus, the superposition ratio of the third harmonic becomes smaller as the AC voltage of the power system 2 is larger. The superimposition ratio of the third harmonic is calculated in advance according to the amplitude value of the AC voltage of the power system 2, and the relationship between the obtained AC voltage amplitude value and the third harmonic superposition ratio is tabulated. it can.

  According to the present embodiment, one carrier period (one pulse) is allocated from the first stage converter 28 to the third stage converter 26 for each of three carrier periods including the sections I and III. Two of the four switching elements of any converter are turned on and off once during the period.

  In section II, two of the four switching elements of the fourth stage converter 25 are turned on and off five times every five carrier periods. Accordingly, the fourth-stage converter 25 outputs the five pulse voltages for five carrier periods (for five pulses) in the section of the electrical angle 0 ° to 180 ° of the AC voltage command value Vx *. It will be controlled by.

  As a result, when viewed as a whole of the four-stage serial multiple converter 20, two of the four switching elements of the four converters 25 to 28 are once in every eight carrier periods for every one carrier period. It will be turned on and off one by one. This can be regarded as substantially equivalent to the operation in the case where the multi-pulse mode (8-pulse mode in FIG. 6) is applied to the entire four-stage serial multiple converter 20. Therefore, it is possible to obtain the effect of reducing the content rate of the harmonic component included in the AC output voltage, which is an advantage of the control by the multi-pulse mode.

  When the above-described method for generating the AC voltage command value Vx * is applied to the PWM control of the M-stage serial multiple converter, the carrier wave is 2N times the AC voltage command value Vx (N is an integer of 2 or more). When having a frequency, the section I is an section of electrical angle 0 ° to 180 × (M−1) / 2N °, and the section II is an electrical angle 180 × (M−1) / 2N ° to 180 × {1- (M -1) / 2N} [deg.], And section III is a section of 180 * {1- (M-1) / 2N} [deg.] To 180 [deg.]. The M-th stage converter is controlled by the multi-pulse mode in section II. The M-th stage converter is controlled in a {N- (M-1} pulse mode that outputs {N- (M-1)} pulse voltages in the section II.

[Embodiment 3]
In the first and second embodiments described above, the PWM control of the four-stage serial multiple converter 20 when the AC voltage of the power system 2 is the nominal rated voltage (1 p.u.) has been described. In the third embodiment, PWM control of the four-stage serial multiple converter 20 in a state where the system voltage is lower than the nominal rated voltage due to the occurrence of an accident in the power system 2 will be described.

  As shown in FIGS. 7 to 9, the control device 11 changes the PWM control of the four-stage serial multiple converter 20 according to the voltage drop degree of the power system 2. FIG. 7 is a waveform diagram for explaining the PWM control of the four-stage serial multiple converter 20 when the system voltage drops to about 5/8 of the nominal rated voltage. FIG. 8 is a waveform diagram for explaining the PWM control of the four-stage serial multiple converter 20 when the system voltage drops to about 3/8 of the nominal rated voltage. FIG. 9 is a waveform diagram for explaining the PWM control of the four-stage serial multiple converter 20 when the system voltage drops to about 1/10 of the nominal rated voltage. 7 to 9, the x-phase AC voltage command value Vx * and the half cycle of the AC output voltage from the first stage converter 28 to the fourth stage converter 25 (electrical angle 0 ° to 180 °). The waveform of (section) is shown. The broken line in the figure shows the waveform of the system voltage.

  First, referring to FIG. 7, when the system voltage is lowered to about 5/8 of the nominal rated voltage, the AC voltage command value Vx * is obtained by superimposing the third harmonic component on the system voltage. The waveform is as shown in FIG. In the example of FIG. 7, the superposition ratio of the third harmonic component is about 38%.

  The control device 11 PWMs each AC output voltage from the first stage converter 28 to the fourth stage converter 25 so that the AC output voltage of the four-stage serial multiple converter 20 matches the AC voltage command value Vx *. Control. Specifically, the control device 11 controls each of the first-stage converter 28, the second-stage converter 27, and the third-stage converter 26 in a single pulse mode, and includes the fourth-stage converter 25. Control is performed in a pulse mode (5 pulse mode in FIG. 7).

  However, the control device 11 controls the first stage converter 28 and the second stage converter 27 so as to output a single pulse voltage in the section of the electrical angle of 0 ° to 180 °, while The stage converter 26 is controlled to output a pulse voltage only during one carrier period.

  The control of the third stage converter 26 includes a section (0.5 carrier period) of electrical angle 22.5 ° to 33.75 ° in section I and an electrical angle 146.25 ° to 157.5 ° in section III. In the interval (for 0.5 carrier period). In these sections, the levels of the AC voltage command value Vx * and the carrier waves CW1 and CW2 are compared, and on / off combinations of the switching elements S1x to S4x are determined based on the comparison result. Further, the AC voltage command value -Vx * and the levels of the carrier waves CW1 and CW2 are compared, and on / off combinations of the switching elements S5x to S8x are determined based on the comparison result. However, for this comparison, a value obtained by subtracting Vd × 2 from the AC voltage command value Vx is used, and a value obtained by adding Vd × 2 to the AC voltage command value −Vx is used. This Vd × 2 is worth the combined voltage of the AC output voltages of the first stage converter 28 and the second stage converter 27. According to this, the third stage converter 26 outputs one pulse voltage in each of the section of electrical angle 22.5 ° to 33.75 ° and the section of electrical angle 146.25 ° to 157.5 °.

  In the present embodiment, in the single pulse mode, a single pulse voltage is applied in the half cycle of the AC voltage command value Vx * (section of electrical angle 0 ° to 180 ° or section of electrical angle 180 ° to 360 °). A control mode for generating a control command for the converter so as to output is referred to as a “single-length pulse mode”. On the other hand, a control mode for generating a converter control command so as to output a pulse voltage only during one carrier cycle of the half cycle of the AC voltage command value Vx * is referred to as a “single short pulse mode”. In the single short pulse mode, as indicated by hatching in FIG. 7, one pulse voltage is output per 0.5 carrier period in each of the sections I and III, so that the half period of the AC voltage command value Vx * 2 pulse voltage is output.

  In the example of FIG. 7, the first stage converter 28 and the second stage converter 27 are controlled by a single long pulse mode, the third stage converter 26 is controlled by a single short pulse mode, and the fourth stage converter 25 is controlled by the multi-pulse mode, the AC output voltage of the four-stage serial multiple converter 20 is obtained by adding the single pulse voltage of the first-stage converter 28 and the second-stage converter 27 to the third A plurality of pulse voltages of the stage converter 26 and the fourth stage converter 25 are added.

  Next, referring to FIG. 8, when the system voltage is reduced to about 3/8 of the nominal rated voltage, the AC voltage command value Vx * is obtained by superimposing the third harmonic component on the system voltage. Thus, the waveform is as shown in FIG. In the example of FIG. 8, the superposition ratio of the third harmonic component is about 38%.

  The control device 11 PWMs each AC output voltage from the first stage converter 28 to the fourth stage converter 25 so that the AC output voltage of the four-stage serial multiple converter 20 matches the AC voltage command value Vx *. Control. Specifically, the control device 11 controls the first stage converter 28 in the single long pulse mode, and controls each of the second stage converter 27 and the third stage converter 26 in the single short pulse mode. The fourth stage converter 25 is controlled by the multi-pulse mode (5-pulse mode in FIG. 8).

  FIG. 8 is different from FIG. 7 in that the control of the second stage converter 27 is switched from the single long pulse mode to the single short pulse mode. The control of the second-stage converter 27 is performed in the section of electrical angle 11.25 ° to 22.5 ° in section I (for 0.5 carrier period) and the electrical angle in section III of 157.5 ° to 168.75 °. In the interval (for 0.5 carrier period). In these sections, the levels of the AC voltage command value Vx * and the carrier waves CW1 and CW2 are compared, and on / off combinations of the switching elements S1x to S4x are determined based on the comparison result. Further, the AC voltage command value -Vx * and the levels of the carrier waves CW1 and CW2 are compared, and on / off combinations of the switching elements S5x to S8x are determined based on the comparison result. However, for this comparison, a value obtained by subtracting Vd from the AC voltage command value Vx is used, and a value obtained by adding Vd to the AC voltage command value -Vx is used. This Vd is equivalent to the AC output voltage of the first stage converter 28. According to this, as indicated by hatching in FIG. 8, the second stage converter in each of the section of electrical angle 11.25 ° to 22.5 ° and the section of electrical angle 157.5 ° to 168.75 °. 27 outputs one pulse voltage.

  The control of the third stage converter 26 is performed in the same manner as in FIG. 7. The electrical angle in the section I is 22.5 ° to 33.75 ° (0.5 carrier period) and the electrical angle 146.25 in the section III. It is performed in a section (for 0.5 carrier period) of ° to 157.5 °. As indicated by diagonal lines in FIG. 8, the third stage converter 26 has one pulse in each of the section of electrical angle 22.5 ° to 33.75 ° and the section of electrical angle 146.25 ° to 157.5 °. Output voltage.

  The first stage converter 28 is controlled by the single long pulse mode, the second stage converter 27 and the third stage converter 26 are controlled by the single short pulse mode, and the fourth stage converter 25 is controlled by the multi-pulse mode. By controlling, the AC output voltage of the four-stage serial multiple converter 20 is added to the single pulse voltage of the first-stage converter 28 by adding the multiple pulse voltages of the second-stage converter 27 to the fourth-stage converter 25. Will be.

  Next, referring to FIG. 9, when the system voltage is reduced to about 1/10 of the nominal rated voltage, the third harmonic component is not superimposed on the system voltage, so the AC voltage command value Vx * is The waveform is as shown in FIG.

  The control device 11 PWMs each AC output voltage from the first stage converter 28 to the fourth stage converter 25 so that the AC output voltage of the four-stage serial multiple converter 20 matches the AC voltage command value Vx *. Control. Specifically, the control device 11 controls each of the first-stage converter 28, the second-stage converter 27, and the third-stage converter 26 in a single short pulse mode, and controls the fourth-stage converter 25. Control is performed in a pulse mode (5 pulse mode in FIG. 9).

  9 differs from FIG. 8 in that the control of the first stage converter 28 is switched from the single long pulse mode to the single short pulse mode. The control of the first stage converter 28 is performed in the section of the electrical angle 0 ° to 11.25 ° in the section I (for 0.5 carrier period) and the section of the electrical angle 168.75 ° to 180 ° in the section III (0 .5 carrier period). In these sections, the levels of the AC voltage command value Vx * and the carrier waves CW1 and CW2 are compared, and on / off combinations of the switching elements S1x to S4x are determined based on the comparison result. Further, the AC voltage command value -Vx * and the levels of the carrier waves CW1 and CW2 are compared, and on / off combinations of the switching elements S5x to S8x are determined based on the comparison result. According to this, as indicated by hatching in FIG. 9, the first stage converter 28 has one pulse in each of the section of electrical angle 0 ° to 11.25 ° and the section of electrical angle 168.75 ° to 180 °. Output voltage.

  The control of the second stage converter 27 is performed in the same manner as in FIG. 8, the section of electrical angle 11.25 ° to 22.5 ° in section I (for 0.5 carrier period) and the electrical angle 157.5 in section III. It is performed in a section (degree 0.5 carrier period) of ° to 168.75 °. As indicated by the oblique lines in FIG. 9, the second stage converter 27 has one pulse in each of the section of electrical angle 11.25 ° to 22.5 ° and the section of electrical angle 157.5 ° to 168.75 °. Output voltage.

  The control of the third stage converter 26 is performed in the same manner as in FIG. 7 and FIG. 8, the section of the electrical angle 22.5 ° to 33.75 ° (for 0.5 carrier period) in the section I and the electrical angle in the section III. It is performed in the interval of 146.25 ° to 157.5 ° (for 0.5 carrier period). As indicated by diagonal lines in FIG. 8, the third stage converter 26 has one pulse in each of the section of electrical angle 22.5 ° to 33.75 ° and the section of electrical angle 146.25 ° to 157.5 °. Output voltage.

  By controlling the first-stage converter 28 to the third-stage converter 26 in the single short pulse mode and the fourth-stage converter 25 in the multi-pulse mode, the AC output voltage of the four-stage serial multiple converter 20 is controlled. Is formed with a total of 16 pulse voltages by one pulse voltage output every 0.5 carrier period.

  As is apparent from FIG. 9, the multi-pulse mode (8-pulse mode in FIG. 9) is applied to the entire 4-stage serial multiple converter 20 as the waveform of the AC output voltage of the entire 4-stage serial multiple converter 20. This is substantially the same as the operation of the case. Therefore, the effect of reducing the content rate of the harmonic component, which is an advantage of the control by the multi-pulse mode, can be obtained.

  In each of the first stage converter 28 to the third stage converter 26, the single long pulse mode is executed with priority over the single short pulse mode. Specifically, as shown in FIG. 10, the timing at which the AC voltage command value Vx * increases from 0 and matches the threshold voltage Vth is earlier than the timing at which the AC voltage command value Vx * and the carrier wave CW1 match. Sometimes a single long pulse mode is performed.

  On the other hand, when the system voltage decreases and the timing at which the AC voltage command value Vx * matches the carrier wave CW1 is earlier than the timing at which the AC voltage command value Vx * matches the threshold voltage Vth, the control of the converter is a single length. Switching from pulse mode to single short pulse mode. As a result, as shown in FIG. 7 to FIG. 9, as the degree of voltage drop increases, the single-stage pulse mode to the single-short pulse mode are sequentially performed from the third stage converter 26 to the first stage converter 28. It will be switched to.

  According to this, even when the system voltage is lowered, the entire four-stage serial multiple converter 20 can be controlled by the multi-pulse mode, so that the harmonic content in the AC output voltage can be reduced.

[Embodiment 4]
In the fourth embodiment, a control structure of control device 11 in serial multiple power conversion device 1 according to the present embodiment will be described.

  FIG. 11 is a circuit block diagram showing a part related to control of the four-stage serial multiple converter 20 in the control device 11 shown in FIG.

  In FIG. 11, the control device 11 includes subtractors 110 and 112, a current control unit 114, a two-phase / three-phase conversion unit 116, a 3f superimposition calculation unit 118, adders 120, 122, 124 and 132, a PWM control unit 126, Multipliers 128 and 130 and a square root 134 are included.

  The d-axis current feedback value Id and the q-axis current feedback value Iq are the d-axis component and the q-axis component of the alternating current flowing through the power system 2 detected by the current detector 13 (FIG. 1), and the three-phase alternating current IR , IS, IT detected values are obtained by three-phase / two-phase conversion. The d-axis current command Id * and the q-axis current command Iq * are the d-axis component and the q-axis component of the AC current command, and the three-phase current command values IR *, IS *, and IT * are converted into three-phase / two-phase. It was obtained.

  The subtractor 110 calculates a deviation Δd (= Id−Id *) between the d-axis current feedback value Id and the d-axis current command Id *. The subtractor 112 calculates a deviation Δq (= Iq−Iq *) between the q-axis current feedback value Iq and the q-axis current command Iq *.

  The current control unit 114 generates the d-axis voltage command Vd * and the q-axis voltage command Vq * so that each of the deviations Δd and Δq becomes zero. The current control unit 114 generates the d-axis voltage command Vd * and the q-axis voltage command Vq * by, for example, amplifying the deviations Δd and Δq according to proportional control or proportional-integral control.

  The two-phase / 3-phase converter 116 converts the d-axis voltage command Vd * and the q-axis voltage command Vq * into two-phase / 3-phase to thereby convert a three-phase AC voltage command value (R-phase voltage command value VR #, S-phase voltage command value VS #, T-phase voltage command value VT #) are generated.

  The 3f superposition calculation unit 118 calculates the superposition ratio of the third harmonic component that is superposed on each of the AC voltage command values VR #, VS #, and VT #. Based on the d-axis voltage command Vd * and the q-axis voltage command Vq * generated by the current control unit 114, the 3f superposition calculation unit 118 calculates a superposition ratio of the third harmonic component, and based on the calculated superposition ratio. Thus, the third harmonic component of each phase is generated. Specifically, as described in the second embodiment, the 3f superposition calculation unit 118 determines that the AC voltage command value Vx * is Vd × at the electrical angles of 33.75 ° and 146.25 ° of the AC voltage command value Vx *. The superposition ratio of the third harmonic is calculated so as to be 3. The 3f superimposing operation unit 118 calculates the third harmonic superposition ratio in advance according to the amplitude value of the AC voltage, and the relationship between the obtained AC voltage amplitude value and the third harmonic superposition ratio. Can be tabulated.

  Adder 120 adds R-phase voltage command value VR # and the R-phase third harmonic component to generate R-phase voltage command value VR *. Adder 122 adds S-phase voltage command value VS # and the S-phase third harmonic component to generate S-phase voltage command value VS *. Adder 124 adds T-phase voltage command value VT # and the T-phase third harmonic component to generate T-phase voltage command value VT *.

The multiplier 128 calculates a square value Vd * ² of the d-axis voltage command Vd *. The multiplier 130 calculates the square value Vq ^{* 2} q-axis voltage command Vq *. The adder 132 adds the square value Vd * ² of the d-axis voltage command and the square value Vq * ² of the q-axis voltage command. Square root 134 calculates the amplitude of the fundamental wave of AC voltage command value V # by calculating the square root (= (Vd * ² + Vq * ² ) ^1/2 ) of the addition result of adder 132.

  The PWM control unit 126 generates gate pulse signals GC1 to GC4 (FIG. 1) for controlling the four-stage serial multiple converter 20 based on the AC voltage command values VR *, VS *, and VT *. The PWM control unit 126 generates the gate pulse signals GC1 to GC4 based on the amplitude of the line voltage.

  FIG. 12 is a block diagram showing a configuration of PWM control unit 126 shown in FIG. Referring to FIG. 12, PWM control unit 126 includes a first stage control unit 141, a second stage control unit 142, a third stage control unit 143, a fourth stage control unit 144, and a carrier wave generator 135.

  Carrier wave generator 135 generates triangular wave signals as carrier waves CW1 and CW2. The carrier wave CW1 has a maximum value of + Vd and a minimum value of 0, as shown in FIG. The carrier wave CW2 has a maximum value of 0 and a minimum value of −Vd. The frequencies and phases of the carriers CW1 and CW2 are the same. The carrier waves CW1 and CW2 have a frequency 2N times the AC voltage command value Vx * (for example, N = 8), and are signals synchronized with the AC voltage command value Vx *.

  The first stage control unit 141 is a control command for controlling on / off of the switching elements S1x to S8x in the first stage converter 28 based on the AC voltage command value Vx * (VR *, VS *, VT *). A certain gate pulse signal GC1 is generated.

  The second stage control unit 142 is a control command for controlling on / off of the switching elements S1x to S8x in the second stage converter 27 based on the AC voltage command value Vx * (VR *, VS *, VT *). A certain gate pulse signal GC2 is generated.

  The third stage control unit 143 is a control command for controlling on / off of the switching elements S1x to S8x in the third stage converter 26 based on the AC voltage command value Vx * (VR *, VS *, VT *). A certain gate pulse signal GC3 is generated.

  The fourth stage control unit 144 is a control command for controlling on / off of the switching elements S1x to S8x in the fourth stage converter 25 based on the AC voltage command value Vx * (VR *, VS *, VT *). A certain gate pulse signal GC4 is generated.

  FIG. 13 is a block diagram illustrating a configuration example of the fourth stage control unit 144 illustrated in FIG. 12. Referring to FIG. 13, fourth stage control unit 144 includes a subtractor 150, a multiplier 152, comparators COM1 to COM4, and inverters I1 to I4.

  The subtracter 150 subtracts Vd × 3 from the AC voltage command value Vx *. Vd × 3 is equivalent to the combined voltage of the AC output voltages of the first-stage converter 28 to the third-stage converter 26.

  Multiplier 152 multiplies AC voltage command value Vx * by subtracting Vd × 3 (= Vx * −Vd × 3) by −1. The multiplication value of the multiplier 152 is worth the person who added Vd × 3 to the AC voltage command value −Vx *.

  The comparator COM1 receives the AC voltage command value Vx * −Vd × 3 at the non-inverting input terminal (+ terminal) and the carrier wave CW1 at the inverting input terminal (−terminal). The comparator COM1 compares the AC voltage command value Vx * −Vd × 3 with the level of the carrier wave CW1, and outputs a signal indicating the comparison result. The output signal of the comparator COM1 becomes a gate pulse signal of the switching element S1x of the fourth stage converter 25.

  The inverter I1 inverts the output signal of the comparator COM1. The output signal of the inverter I1 becomes a gate pulse signal of the switching element S3x of the fourth stage converter 25.

  The comparator COM2 receives the AC voltage command value Vx * −Vd × 3 at the non-inverting input terminal (+ terminal) and the carrier wave CW2 at the inverting input terminal (−terminal). The comparator COM2 compares the AC voltage command value Vx * −Vd × 3 and the level of the carrier wave CW2, and outputs a signal indicating the comparison result. The output signal of the comparator COM2 becomes a gate pulse signal of the switching element S2x of the fourth stage converter 25.

  The inverter I2 inverts the output signal of the comparator COM2. The output signal of the inverter I2 becomes a gate pulse signal of the switching element S4x of the fourth stage converter 25.

  The comparator COM3 receives the AC voltage command value −Vx * + Vd × 3 at the non-inverting input terminal (+ terminal) and the carrier wave CW1 at the inverting input terminal (−terminal). The comparator COM1 compares the AC voltage command value −Vx * + Vd × 3 with the level of the carrier wave CW1, and outputs a signal indicating the comparison result. The output signal of the comparator COM3 becomes a gate pulse signal of the switching element S5x of the fourth stage converter 25.

  The inverter I3 inverts the output signal of the comparator COM3. The output signal of the inverter I3 becomes a gate pulse signal of the switching element S7x of the fourth stage converter 25.

  The comparator COM4 receives the AC voltage command value −Vx * + Vd × 3 at the non-inverting input terminal (+ terminal) and the carrier wave CW2 at the inverting input terminal (−terminal). The comparator COM4 compares the AC voltage command value −Vx * + Vd × 3 with the level of the carrier wave CW2, and outputs a signal indicating the comparison result. The output signal of the comparator COM4 becomes a gate pulse signal of the switching element S6x of the fourth stage converter 25.

  The inverter I4 inverts the output signal of the comparator COM4. The output signal of the inverter I4 becomes a gate pulse signal of the switching element S8x of the fourth stage converter 25.

  FIG. 14 is a block diagram illustrating a configuration example of the first-stage control unit 141 to the third-stage control unit 143 illustrated in FIG. In FIG. 14, reference numerals 1, 2, and 3 are collectively denoted as “i” in order to comprehensively describe the configuration of the first stage control unit 141 to the third stage control unit 143.

  Referring to FIG. 14, i-th stage control unit 14 i includes a single long pulse mode control unit 160, a single short pulse mode control unit 162, a switch 164, and a mode switching unit 166.

  The single long pulse mode control unit 160 generates the gate pulse signal GCi based on the AC voltage command value Vx * by executing the single long pulse mode. The single short pulse mode control unit 162 generates the gate pulse signal GCi based on the AC voltage command value Vx * by executing the single short pulse mode.

  FIG. 15 is a block diagram illustrating a configuration example of the single long pulse mode control unit 160 illustrated in FIG. 14. Referring to FIG. 15, single-length pulse mode control unit 160 includes a multiplier 170, comparators COM11 to COM14, and inverters I11 to I14.

  Multiplier 170 calculates AC voltage command value -Vx * by multiplying AC voltage command value Vx * by -1.

  The comparator COM11 receives the AC voltage command value Vx * at the non-inverting input terminal (+ terminal) and the i-th threshold voltage Vthi at the inverting input terminal (−terminal). The comparator COM11 compares the AC voltage command value Vx * and the i-th threshold voltage Vthi and outputs a signal indicating the comparison result. The output signal of the comparator COM11 becomes a gate pulse signal of the switching element S1x of the i-th stage converter.

  The inverter I11 inverts the output signal of the comparator COM11. The output signal of the inverter I11 becomes a gate pulse signal of the switching element S3x of the i-th stage converter.

  The comparator COM12 receives the AC voltage command value Vx * at the non-inverting input terminal (+ terminal) and the i-th threshold voltage / Vthi at the inverting input terminal (−terminal). / Vthi is a voltage obtained by offsetting Vthi by −Vd. The comparator COM12 compares the AC voltage command value Vx * and the i-th threshold voltage / Vthi and outputs a signal indicating the comparison result. The output signal of the comparator COM12 becomes a gate pulse signal of the switching element S2x of the i-th stage converter.

  The inverter I12 inverts the output signal of the comparator COM12. The output signal of the inverter I12 becomes a gate pulse signal of the switching element S4x of the i-th converter.

  The comparator COM13 receives the AC voltage command value −Vx * at the non-inverting input terminal (+ terminal) and the i-th threshold voltage Vthi at the inverting input terminal (−terminal). The comparator COM13 compares the AC voltage command value -Vx * and the i-th threshold voltage Vthi and outputs a signal indicating the comparison result. The output signal of the comparator COM13 becomes a gate pulse signal of the switching element S5x of the i-th stage converter.

  The inverter I13 inverts the output signal of the comparator COM13. The output signal of the inverter I13 becomes a gate pulse signal of the switching element S7x of the i-th stage converter.

  The comparator COM14 receives the AC voltage command value −Vx * at the non-inverting input terminal (+ terminal) and the i-th threshold voltage / Vthi at the inverting input terminal (−terminal). The comparator COM14 compares the AC voltage command value -Vx * and the i-th threshold voltage / Vthi with each other, and outputs a signal indicating the comparison result. The output signal of the comparator COM14 becomes a gate pulse signal of the switching element S6x of the i-th stage converter.

  The inverter I14 inverts the output signal of the comparator COM14. The output signal of the inverter I14 becomes a gate pulse signal of the switching element S8x of the i-th stage converter.

  FIG. 16 is a block diagram showing a configuration example of the single short pulse mode control unit 162 shown in FIG. Referring to FIG. 16, single short pulse mode control unit 162 has the same configuration as fourth stage control unit 144 shown in FIG. However, the single short pulse mode control unit 162 is different from the fourth stage control unit 144 in that the subtractor 150 subtracts Vd × (i−1) from the AC voltage command value Vx *. This Vd × (i−1) is worth the combined voltage of the AC output voltages of the first to (i−1) th stage converters.

  Returning to FIG. 14, the switch 164 selects one of the gate pulse signal GCi generated by the single long pulse mode control unit 160 and the gate pulse signal GCi generated by the single short pulse mode control unit 162. Then, it is configured to output to the i-th stage converter.

The mode switching unit 166 uses the switch 164 to switch between the single long pulse mode and the single short pulse mode. The mode switching unit 166 selects the single long pulse mode and the mode according to the fundamental wave amplitude (= (Vd * ² + Vq * ² ) ^1/2 ) of the AC voltage command value V # calculated by the square root 134 (FIG. 11). Switch single short pulse mode.

  Here, the determination values for switching between the single long pulse mode and the single short pulse mode are set differently among the first stage converter 28, the second stage converter 27, and the third stage converter 26. Is done.

  Specifically, the third stage control unit 143 selects the single-long pulse mode when the fundamental wave amplitude of the AC voltage command value V # is 5/8 or more of the nominal rated voltage, while the fundamental wave amplitude is nominal. When the voltage is less than 5/8, the single short pulse mode is selected.

  The second stage control unit 142 selects the single long pulse mode when the fundamental wave amplitude is 3/8 or more of the nominal rated voltage, while the single short pulse mode is selected when the fundamental wave amplitude is less than 3/8 of the nominal rated voltage. Select pulse mode.

  The first stage control unit 141 selects the single long pulse mode when the fundamental wave amplitude is 1/8 or more of the nominal rated voltage, while the single stage control unit 141 selects the single short pulse mode when the fundamental wave amplitude is less than 1/8 of the nominal rated voltage. Select pulse mode.

  When the single short pulse mode is selected, the third stage control unit 143 has a section of electrical angle 22.5 ° to 33.75 ° and a section of electrical angle 146.25 ° to 157.5 °. In each, the gate pulse signal GC3 of the third stage converter 26 is generated so as to output one pulse voltage. In the case where the single short pulse mode is selected, the second stage control unit 142 is in each of the section of electrical angle 11.25 ° to 22.5 ° and the section of electrical angle 157.5 ° to 168.75 °. The gate pulse signal GC2 of the second stage converter 27 is generated so as to output one pulse voltage. When the single short pulse mode is selected, the first stage control unit 141 performs one pulse in each of the section of electrical angle 0 ° to 11.25 °° and the section of electrical angle 168.75 ° to 180 °. The gate pulse signal GC1 of the first stage converter 28 is generated so as to output a voltage.

  According to this, when the system voltage is lowered, the control of the first stage converter 28 to the third stage converter 26 is switched from the single long pulse mode to the single short pulse mode according to the degree of voltage drop. Accordingly, as shown in FIG. 7 to FIG. 9, the single long pulse mode to the single short pulse mode are sequentially arranged from the third stage converter 26 to the first stage converter 28 as the degree of voltage drop increases. It will be switched to. Thereby, even when the system voltage is lowered, the entire four-stage serial multiple converter 20 can be controlled by the multi-pulse mode, so that the harmonic content in the AC output voltage can be reduced.

[Embodiment 5]
According to the first to fourth embodiments described above, the R phase, S phase, and T phase of the fourth-stage converter 25 in the section II of the electrical angle of 33.75 ° to 146.25 ° of the AC voltage command value Vx *. These switching elements are always turned on / off, but only two-phase switching elements of the three phases can be turned on / off.

  FIG. 17 is an operation waveform diagram of the four-stage serial multiple converter 20 at the rated output. In FIG. 17, the four-stage serial multiple converter 20 outputs the maximum voltage. In this case, by setting the superposition ratio of the third harmonic to about 12%, the AC voltage command value Vx * can be set to Vd × 3 at the electrical angles of 33.75 ° and 146.25 °.

  In the fifth embodiment, among the AC voltage command values Vx * of each phase, the voltage command value of the phase having the maximum amplitude is made to coincide with Vd × 4, and the remaining two phase voltage command values are converted to the output line of the converter. Correct the voltage so that the voltage is maintained. FIG. 18 is an operation waveform diagram of the four-stage serial multiple converter 20 when the voltage command value for each phase is generated in this manner. In FIG. 18, the one-dot chain line is the original AC voltage command value Vx *.

  As shown in FIG. 18, the voltage command value of the phase having the maximum amplitude among the three phases is fixed to Vd × 4 in the section of the electrical angle of 60 ° to 120 °. In this section, the voltage command values of the other two phases are corrected so that the line voltage with the other two phases is not affected. In FIG. 18, the voltage command value is corrected because the voltage command value of the other phase is fixed to Vd × 4 in each of the electrical angle interval of 0 ° to 60 ° and the electrical angle range of 120 ° to 180 °. Has been. Such a modulation method is called two-phase modulation.

  According to the present embodiment, the single-phase full-bridge three-level circuit of the fourth-stage converter 25 does not switch for a period of 120 ° per cycle, so that the switching frequency of the switching element is smaller than that of the original 5-pulse mode. Decrease. Thereby, the loss of the converter by switching can be reduced. Therefore, it is possible to further increase the operation efficiency of the entire four-stage serial multiple converter 20.

[Example of change]
In the above-described embodiment, the BTB system is exemplified as the serial multiple power conversion device of the present invention, but the present invention is not limited to this configuration. For example, the serial multiple power converter of the present invention can also be applied to a self-excited SVC (static reactive power compensator).

  In the above-described embodiment, an example in which a plurality of converters constituting a serial multiple converter are configured by a three-level circuit has been described. However, a converter of each stage may be a two-level circuit or a multilevel having four or more levels. It is also possible to configure with a circuit.

  The embodiment disclosed this time should be considered as illustrative in all points and not restrictive. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

  DESCRIPTION OF SYMBOLS 1 Serial multiple power converter, 2, 3 Power system, 10 Power converter, 11 Control apparatus, 12, 52 AC voltage detector, 13, 53 Current detector, 14 DC voltage detector, 15 DC positive bus, 16 DC Neutral point bus, 17 DC negative bus, 20, 60 4 stage serial multiple converter, 25, 65 4th stage converter, 26, 66 3rd stage converter, 27, 67 2nd stage converter, 28, 68 First stage converter, 110, 112, 150 Subtractor, 114 Current control unit, 116 Two-phase / three-phase conversion unit, 118 3f superimposition operation unit, 120, 122, 124, 132 Adder, 126 PWM control unit, 128 , 130, 152, 170 multiplier, 134 square root, 135 carrier generator, 141 first stage control unit, 142 second stage control unit, 143 third stage control unit, 144 fourth stage control unit, 160 unit One long pulse mode control unit, 162 Single short pulse mode control unit, 164 switch, 166 mode switching unit, C1, C2 smoothing capacitor, COM1-COM4, COM11-COM14 comparator, CW1, CW2 carrier wave, GC1-GC8 gate pulse Signal, I1-I4, I11-I14 Inverter, S1R-S8R, S1S-S8S, S1T-S8T Switching element, D1R-D12R, D1S-D12S, D1T-D12T Diode.

Claims (7)

A serial multiple power conversion device linked to an AC power system,
M transformers (M is an integer of 2 or more) in which primary windings are connected in series to the AC power system;
A first to M-th converter connected to the secondary windings of the M transformers for performing bidirectional power conversion between AC power and DC power;
A control device that performs PWM control of the AC output voltage of the converter at each stage so that the voltage obtained by serially combining the AC output voltages of the converters at each stage matches the AC voltage command value;
Each stage of the converter is constituted by a single-phase full bridge circuit,
The PWM control includes a multi-pulse mode for generating a control command for the converter so as to output a plurality of pulse voltages in a half cycle of the AC voltage command value, and a single pulse voltage for outputting the single pulse voltage in the interval. A single pulse mode for generating a control command for the converter,
The control device controls the first to (M-1) th stage converters by the single pulse mode, and controls the Mth stage converters by the multipulse mode. Conversion device. The multi-pulse mode is configured to generate the control command by comparing the AC voltage command value with a carrier wave having a frequency 2N times (N is an integer equal to or greater than 2) the AC voltage command value,
The control device is configured to convert the M-th stage converter in a section of the AC voltage command value having an electrical angle of 180 × (M−1) / 2N ° or more and 180 × {1- (M−1) / 2N} ° or less. The serial multiple power conversion device according to claim 1, which is controlled by the multi-pulse mode. The single pulse mode is configured to generate the control command by comparing the AC voltage command value with a threshold voltage,
The control device has first to (M−1) th threshold voltages respectively set corresponding to the first to (M−1) th converters,
When the DC voltage of the converter at each stage is Vd, the Ith threshold voltage (I is an integer not smaller than 2 and not more than M-1) is larger than the first threshold voltage by Vd × (I−1). The serial multiple power conversion device according to claim 1 or 2, which is set to a voltage value. The control device is configured to generate the AC voltage command value by superimposing a third harmonic component of the sine wave on a sine wave having the frequency of the AC power system as a fundamental frequency. Including
When the voltage of the DC circuit of the converter at each stage is set to Vd, the voltage command generator generates the following (a) electrical angle of 0 ° in a section of the AC voltage command value from 0 ° to 180 °. In the section of 180 × (M−1) / 2N ° or less, the voltage value increases from 0 to (M−1) × Vd,
(B) In a section where the electrical angle is 180 × (M−1) / 2N ° or more and 180 × {1- (M−1) / 2N} ° or less, the voltage value is (M−1) × Vd or more and M × Vd or less. In the range of
(C) The voltage value decreases from (M−1) × Vd to 0 in the section of electrical angle 180 × {1− (M−1) / 2N} ° to 180 °,
The serial multiple power converter according to any one of claims 1 to 3, wherein the AC voltage command value is generated so as to satisfy The single pulse mode includes a single-length pulse mode in which the control command is generated by comparing the AC voltage command value and the threshold voltage, and 1 in a section of the AC voltage command value from 0 ° to 180 °. A single short pulse mode for generating the control command by comparing the AC voltage command value and the carrier wave only during a carrier cycle;
The serial multiple power conversion device according to claim 3, wherein the control device preferentially executes the single long pulse mode over the single short pulse mode.   When the amplitude of the AC voltage command value is reduced, the control device sequentially starts from the single long pulse mode to the single pulse from the (M-1) th stage converter to the first stage converter. The serial multiple power converter according to claim 5 which switches to a mode. The AC power system is a three-phase AC system,
The voltage command generation unit fixes the voltage value in the section of the electrical angle of 60 ° to 120 ° with respect to the AC voltage command value of each phase voltage to M × Vd, and the section of the electrical angle of 0 ° to 60 °. The serial multiple power conversion device according to claim 4, wherein the voltage value in the section of the electrical angle of 120 ° to 180 ° is corrected so as to maintain the line voltage with the other two-phase voltages.

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