JP2018019492A - Power conversion device and control method for the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a power conversion device having high reliability, and a control method therefor.SOLUTION: According to one embodiment, there is provided a power conversion device comprising a neutral point clamp type converter, first and second voltage detectors, and a control section. The converter includes a positive potential terminal, a negative potential terminal, a neutral point terminal, a plurality of AC terminals, and a plurality of switching elements, and converts AC power supplied from each AC terminal by switching of each switching element into DC power having three levels. The first voltage detector detects a first DC voltage between the positive potential terminal and the neutral point terminal. The second voltage detector detects a second DC voltage between the negative potential terminal and the neutral point terminal. The control section controls each switching element such that the DC voltage is equal to a predetermined command value and the first and second DC voltages are equal to each other. Further, the control section varies the command value when a voltage difference between the first and second DC voltages is equal to a predetermined value or more.SELECTED DRAWING: Figure 1

Description

  Embodiments described herein relate generally to a power conversion device and a control method thereof.

  There is a power conversion device using a neutral point clamp type converter that converts AC power into three-level DC power of positive potential, negative potential, and neutral point potential. In the neutral point clamp type converter, when the voltage difference between the DC voltage between the positive potential and the neutral point potential and the DC voltage between the negative potential and the neutral point potential increases, This causes fluctuations in the DC voltage and generation of unnecessary harmonics. For this reason, in a power conversion device using a neutral point clamp type converter, the fluctuation of the neutral point potential is suppressed and the operation of the converter is controlled so that each DC voltage becomes equal (hereinafter referred to as balance control). ) Has been done.

  For example, it has been proposed to perform balance control of each DC voltage by flowing a reactive current to the AC side. In this method, balance control of each DC voltage can be performed without changing the voltage value of each DC voltage.

  However, in the method of supplying reactive current to the AC side, the AC voltage may be varied. For example, when the AC side is a power system, fluctuations in the system voltage are caused. For this reason, in the power converter, it is desired to suppress the voltage fluctuation on the AC side when performing balance control of each DC voltage and improve the reliability.

JP2013-1443836A

  Embodiments of the present invention provide a highly reliable power conversion device and a control method thereof.

  According to the embodiment of the present invention, there is provided a power conversion device including a neutral point clamp type converter, a first voltage detector, a second voltage detector, and a control unit. The converter has three DC terminals, a positive potential terminal, a negative potential terminal, and a neutral point terminal, a plurality of AC terminals, and a plurality of switching elements, and by switching the plurality of switching elements, AC power supplied from the plurality of AC terminals is converted into DC power having three levels of a positive potential, a negative potential, and a neutral point potential. The first voltage detector detects a first DC voltage between the positive potential terminal and the neutral point terminal. The second voltage detector detects a second DC voltage between the negative potential terminal and the neutral point terminal. The control unit, based on the detection result of the first voltage detector and the detection result of the second voltage detector, the DC voltage of the DC power becomes equal to a predetermined DC voltage command value, and the first voltage detector The switching of the plurality of switching elements is controlled so that the DC voltage and the second DC voltage are equal to each other. The control unit varies the DC voltage command value when an absolute value of a voltage difference between the first DC voltage and the second DC voltage is a predetermined value or more.

  Provided are a highly reliable power conversion device and a control method therefor, in which voltage fluctuation on the AC side when performing balance control of each DC voltage is suppressed.

It is a block diagram showing typically the power converter concerning an embodiment. It is a flowchart which represents typically an example of operation | movement of the power converter device which concerns on embodiment.

Each embodiment will be described below with reference to the drawings.
The drawings are schematic or conceptual, and the relationship between the thickness and width of each part, the size ratio between the parts, and the like are not necessarily the same as actual ones. Further, even when the same part is represented, the dimensions and ratios may be represented differently depending on the drawings.
Note that, in the present specification and each drawing, the same elements as those described above with reference to the previous drawings are denoted by the same reference numerals, and detailed description thereof is omitted as appropriate.

FIG. 1 is a block diagram schematically illustrating a power conversion device according to the embodiment.
As illustrated in FIG. 1, the power conversion device 10 includes a first converter 11, a second converter 12, a control unit 14, a pair of DC capacitors 16 and 18, and current detectors 20 u, 20 v, and 20 w. And voltage detectors 21 and 22.

  The first converter 11 has three AC terminals 11u, 11v, and 11w and three DC terminals 11p, 11n, and 11c. Each AC terminal 11u, 11v, 11w is connected to the AC power source 2 via, for example, the interconnecting reactors 4u, 4v, 4w and the circuit breakers 6u, 6v, 6w. Thereby, AC power from the AC power supply 2 is supplied to the first converter 11. Each of the AC terminals 11u, 11v, and 11w is connected to each phase of the three-phase AC power of the AC power supply 2.

  The AC power source 2 is, for example, various power generators or a commercial power system. The AC power supplied from the AC power source 2 is, for example, three-phase AC power. The AC power of the AC power supply 2 is not limited to three phases, but may be a single phase. The interconnecting reactors 4u, 4v, 4w and the circuit breakers 6u, 6v, 6w are provided as necessary and can be omitted. Interconnection reactors 4 u, 4 v, 4 w and circuit breakers 6 u, 6 v, 6 w may be provided in power converter 10 or may be provided separately from power converter 10.

  The first converter 11 converts the input three-phase AC power (first three-phase AC power) into DC power having three levels of a positive potential P, a negative potential N, and a neutral point potential O. Hereinafter, the DC terminals 11p, 11n, and 11c are referred to as a positive potential terminal 11p, a negative potential terminal 11n, and a neutral point terminal 11c, respectively.

The first converter 11 sets the positive potential terminal 11p to the positive potential P, sets the negative potential terminal 11n to the negative potential N, and sets the neutral point terminal 11c to the neutral point potential O. The neutral point potential O is an intermediate potential between the positive potential P and the negative potential N. Thus, the first converter 11 generates a DC voltage V _PO (first DC voltage) between the positive potential terminal 11p and the neutral point terminal 11c, and the neutral point terminal 11c and the negative potential terminal 11n. A DC voltage V _ON (second DC voltage) is generated between them. Therefore, the DC voltage V _DC between the positive potential terminal 11p and the negative potential terminal 11n is V _DC = V _PO + V _ON .

  The second converter 12 is connected to the DC terminals 11p, 11n, and 11c, and is connected to the AC load 8. The AC load 8 is, for example, a power system different from the AC power supply 2. The second converter 12 converts the DC power input from the first converter 11 into three-phase AC power (second three-phase AC power) having a desired frequency and voltage value corresponding to the AC load 8, and Three-phase AC power is supplied to the AC load 8.

  In this way, the power conversion device 10 once converts the three-phase AC power of the AC power supply 2 into DC power, converts the DC power into another three-phase AC power, and supplies it to the AC load 8. The power conversion device 10 is used for, for example, a BTB (Back To Back) system interconnection facility. The first converter 11 is a so-called three-level converter. The second converter 12 is a three-level inverter.

  In the power conversion device 10, the AC power on the AC load 8 side can be converted into AC power corresponding to the AC power source 2 and supplied to the AC power source 2. In this case, the first converter 11 functions as an inverter, and the second converter 12 functions as a converter.

  The AC load 8 is not limited to the power system, and may be, for example, an induction motor. The power conversion device 10 may be used for drive control of an induction motor.

The DC capacitor 16 is provided between the positive potential terminal 11p and the neutral point terminal 11c. The DC capacitor 16 is electrically connected between the positive potential terminal 11p and the neutral point terminal 11c. The DC capacitor 18 is provided between the neutral point terminal 11c and the negative potential terminal 11n. The DC capacitor 18 is electrically connected between the neutral point terminal 11c and the negative potential terminal 11n. The DC capacitors 16 and 18 smooth the DC voltages V _PO and V _ON . In other words, the DC capacitors 16 and 18 are smoothing capacitors.

  The current detectors 20u, 20v, and 20w detect the current values of the alternating currents iu, iv, and iw flowing through the AC terminals 11u, 11v, and 11w of the first converter 11, respectively. In other words, each of the current detectors 20u, 20v, 20w detects a line current of each phase of the AC power supply 2. The current detectors 20u, 20v, and 20w input the current values of the alternating currents iu, iv, and iw to the control unit 14, respectively.

The voltage detector 21 detects the voltage value of the DC voltage _VPO of the DC capacitor 16 and inputs the detected voltage value of the DC voltage _VPO to the control unit 14. The voltage detector 22 detects the voltage value of the DC voltage V _ON of the DC capacitor 18 and inputs the detected voltage value of the DC voltage V _ON to the control unit 14.

  The control unit 14 controls the operations of the first converter 11 and the second converter 12 based on the detection results of the current detectors 20u, 20v, 20w and the voltage detectors 21, 22. The control unit 14 controls the conversion from the three-phase AC power to the DC power by the first converter 11. The control unit 14 controls the conversion from DC power to three-phase AC power by the second converter 12. The operation of the second converter 12 may be controlled by a controller other than the controller 14.

  The first converter 11 includes a plurality of switching elements 41 and a plurality of rectifying elements 42 and 43. In this example, the first converter 11 has twelve switching elements 41, twelve rectifying elements 42, and six rectifying elements 43. Each switching element 41 is connected in a three-phase bridge. Each rectifying element 42 is connected to each switching element 41 in antiparallel.

  The switching element 41 has a pair of main terminals and a control terminal. The control terminal is used for switching between an on state in which a current flows between the main terminals and an off state in which no current substantially flows between the main terminals. For the switching element 41, for example, a self-extinguishing element such as a gate turn off thyristor (GTO) or an insulated gate bipolar transistor (IGBT) is used. The control terminal is, for example, a gate terminal.

  The first converter 11 is a three-phase bridge type and has six arms AU, AV, AW, AX, AY, and AZ. The configuration of each phase arm AU, AV, AW, AX, AY, AZ connected to each AC terminal 11u, 11v, 11w is substantially the same. Therefore, here, the two arms AU and AX connected to the AC terminal 11u (U phase) will be described as an example.

  The positive-side arm AU includes two switching elements SU1 and SU2 connected in series, rectifying elements DU1 and DU2 connected in antiparallel to these switching elements SU1 and SU2, and each switching element SU1 and SU2. And a rectifying element DU3 connected between the series connection point and the neutral point terminal 11c.

  Similarly, the negative arm AX includes two switching elements SX1 and SX2 connected in series, rectifying elements DX1 and DX2 connected in antiparallel to these switching elements SX1 and SX2, and each switching element. And a rectifying element DX3 connected between the series connection point of SX1 and SX2 and the neutral point terminal 11c.

  Both arms AU, AX are connected in series between a positive potential terminal 11p and a negative potential terminal 11n, and the series connection point of both arms AU, AX is connected to a U-phase AC terminal 11u. The potential at the series connection point of the switching elements SU1 and SU2 is clamped to the neutral point potential O via the rectifying element DU3. Similarly, the potential at the series connection point of the switching elements SX1, SX2 is clamped to the neutral point potential O via the rectifying element DX3. The rectifying elements DU1, DU2, DX1, DX2 (each rectifying element 42) are so-called free-wheeling diodes. The rectifying elements DU3 and DX3 (each rectifying element 43) are so-called clamp diodes.

  The configuration of the arms AV and AW is substantially the same as the configuration of the arm AU. The configuration of the arms AY and AZ is substantially the same as the configuration of the arm AX. Accordingly, the potential of the AC terminals 11u, 11v, and 11w is clamped to any one of the three levels of the positive potential terminal 11p, the negative potential terminal 11n, and the neutral point terminal 11c according to the switching of each switching element 41. Is done. The first converter 11 is a so-called neutral-point-clamped (NPC) type converter (converter).

  The second converter 12 includes a plurality of switching elements 51 and a plurality of rectifying elements 52 and 53. Similar to the first converter 11, the second converter 12 is a neutral point clamp type converter (inverter). Since the configuration of the second converter 12 is substantially the same as the configuration of the first converter 11, detailed description thereof is omitted. In the second converter 12, the side connected to the DC capacitors 16 and 18 is the DC side (corresponding to the positive potential terminal 11p, the negative potential terminal 11n, and the neutral point terminal 11c), and the side connected to the AC load 8 is On the AC side (corresponding to AC terminals 11u, 11v, 11w).

  The control unit 14 includes a DC voltage difference detector 60, an oscillator 61, a switch 62, a DC voltage controller 63, a current controller 64, a DC balance controller 65, a sign detector 66, and a multiplier. 67, an adder 68, and a PWM controller 69.

The DC voltage difference detector 60 receives the voltage values of the DC voltage V _PO and the DC voltage V _ON detected by the voltage detectors 21 and 22. The DC voltage difference detector 60 calculates the voltage difference Vd between the input voltage values. The DC voltage difference detector 60 calculates the voltage difference Vd by, for example, Vd = V _PO −V _ON . Then, the DC voltage difference detector 60 inputs the calculated voltage difference Vd to the oscillator 61 and the DC balance controller 65.

  The oscillator 61 is connected to the DC voltage difference detector 60 and the switch 62. The oscillator 61 determines whether or not the absolute value of the input voltage difference Vd is greater than or equal to a predetermined value. When the oscillator 61 determines that the absolute value of the voltage difference Vd is less than the predetermined value, the oscillator 61 inputs “0” to the switch 62. On the other hand, when the oscillator 61 determines that the absolute value of the voltage difference Vd is greater than or equal to a predetermined value, the oscillator 61 inputs a pulse signal that alternately repeats “0” and “1” with a predetermined period to the switch 62. “0” is, for example, “Low”, and “1” is, for example, “High”. Further, “0” is, for example, 0V, and “1” is, for example, + 5V.

  The period of the pulse signal is, for example, 1 second. Each duty ratio of “0” and “1” is, for example, 50%. That is, when the oscillator 61 determines that the absolute value of the voltage difference Vd is equal to or greater than a predetermined value, the oscillator 61 inputs a pulse signal that switches “0” and “1” every 0.5 seconds to the switch 62. The period and duty ratio of the pulse signal are not limited to the above and can be arbitrarily set.

  The switch 62 is connected to the oscillator 61 and the DC voltage controller 63. When “0” is input from the oscillator 61, the switch 62 inputs 1.0 pu (per unit) of a predetermined DC voltage command value Edc to the DC voltage controller 63. On the other hand, when “1” is input from the oscillator 61, the switch 62 inputs 0.9 pu of a predetermined DC voltage command value Edc to the DC voltage controller 63.

The DC voltage command value Edc is a command value of the DC voltage _VDC between the positive potential terminal 11p and the negative potential terminal 11n. DC voltage command value Edc is the command value of the DC voltage _{V PO} between the positive potential terminal 11p and the neutral point terminal 11c, or, the DC voltage _{V ON} between the neutral point and the terminal 11c and the negative potential terminal 11n It may be a command value. The DC voltage command value Edc is, for example, a predetermined constant value. The DC voltage command value Edc may be a variable input from an external controller or the like.

  As described above, when “0” is input from the oscillator 61, the switch 62 inputs the first DC voltage command value to the DC voltage controller 63, and when “1” is input from the oscillator 61, A second DC voltage command value different from the one DC voltage command value is input to the DC voltage controller 63. The switch 62 switches the DC voltage command value Edc input to the DC voltage controller 63 according to the determination result of the absolute value of the voltage difference Vd.

  In this example, the second DC voltage command value is lower than the first DC voltage command value. The second DC voltage command value may be higher than the first DC voltage command value. However, when the second DC voltage command value is set higher than the first DC voltage command value, for example, the occurrence of overvoltage is a concern. Therefore, the second DC voltage command value is preferably set lower than the first DC voltage command value. Thereby, the reliability of the power converter device 10 can be improved, for example. For example, the second DC voltage command value is preferably 0.9 times or more and less than 1.0 times the first DC voltage command value. More preferably, the second DC voltage command value is 0.90 times or more and less than 0.95 times. That is, the second DC voltage command value is preferably lowered by about 5% with respect to the first DC voltage command value.

  Thus, the control unit 14 uses the first DC voltage command value when the absolute value of the voltage difference Vd is less than a predetermined value. And the control part 14 switches a 1st DC voltage command value and a 2nd DC voltage command value alternately, when the absolute value of the voltage difference Vd is more than predetermined value. The controller 14 varies the DC voltage command value Edc when the absolute value of the voltage difference Vd is greater than or equal to a predetermined value.

Predetermined determining a voltage difference Vd value is, for example, is about 1% to 5% of the DC voltage _{V PO,} rated voltage of _{V ON.} For example, when the voltage difference Vd varies by 5% or more of the rated voltage, the control unit 14 varies the DC voltage command value Edc.

The DC voltage controller 63 receives the DC voltage command value Edc from the switch 62 and the voltage values of the DC voltage V _PO and the DC voltage V _ON detected by the voltage detectors 21 and 22. The The DC voltage controller 63 calculates the effective current command value Iq of the AC currents iu, iv, and iw flowing through the AC terminals 11u, 11v, and 11w based on the input DC voltage command value Edc and each voltage value. . The effective current command value Iq is, for example, a q-axis component current command value obtained by dq conversion (three-phase two-phase conversion) of the three-phase AC currents iu, iv, and iw. DC voltage controller 63 calculates the AC currents iu needed to bring the DC voltage _{V PO} and the DC voltage _{V ON} to the DC voltage command value Edc, iv, an active current command value Iq of iw.

  The DC voltage controller 63 calculates the effective current command value Iq from the DC voltage command value Edc and each voltage value by a control method such as PI control, PID control, or IP control, or modern control theory, for example. . Then, the DC voltage controller 63 inputs the calculated effective current command value Iq to the current controller 64.

  The current controller 64 receives the effective current command value Iq and the current values of the alternating currents iu, iv, iw detected by the current detectors 20u, 20v, 20w. The current controller 64 calculates voltage command values veu, vev, and for each of the three phases based on the input effective current command value Iq and each current value.

  The current controller 64 calculates the voltage command values veu, vev, and vew from the effective current command value Iq and each current value by performing, for example, two-phase three-phase conversion and PI control. Each voltage command value veu, vev, vew is, for example, a sinusoidal signal. Further, the current controller 64 inputs the calculated voltage command values veu, vev, and view to the code detector 66 and the adder 68.

The DC balance controller 65 is connected to the DC voltage difference detector 60 and the multiplier 67. The DC balance controller 65 calculates a correction command value Vb for each voltage command value veu, vev, and view based on the voltage difference Vd input from the DC voltage difference detector 60. The DC balance controller 65 calculates the correction command value Vb from the voltage difference Vd by, for example, PI control. The correction command value Vb is a correction value for each of the voltage command values veu, vev, and vew for making the voltage difference Vd substantially zero. In other words, the correction command value Vb is a correction value for performing DC voltage balance control so that the DC voltages V _PO and V _ON are equal to each other. The DC balance controller 65 inputs the calculated correction command value Vb to the multiplier 67.

The sign detector 66 detects the sign Vs of the correction command value Vb to be added to each voltage command value veu, vev, view based on each voltage command value veu, vev, view input from the current controller 64. Code detector 66, for example, in the case of increasing the DC voltage _{V ON} respect DC voltage _{V PO,} when setting a code Vs to "+1", lowering the DC voltage _{V ON} respect DC voltage _{V PO,} The code Vs is set to “−1”. For example, the sign detector 66 detects the sign Vs from the amplitude or phase of each voltage command value veu, vev, and view. The code detector 66 inputs the detected code Vs to the multiplier 67.

  The multiplier 67 is connected to the DC balance controller 65, the sign detector 66, and the adder 68. The multiplier 67 multiplies the correction command value Vb input from the DC balance controller 65 by the code Vs input from the code detector 66. The multiplier 67 inputs the corrected correction command value Vb ′ to the adder 68. The adder 68 corrects the voltage command values veu, vev, and vew by adding the correction command value Vb ′ to the voltage command values veu, vev, and vew.

The DC balance controller 65, the sign detector 66, the multiplier 67, and the adder 68, for example, add each DC voltage corresponding to the voltage difference Vd to each voltage command value veu, vev, and vew, thereby adding each DC voltage. V _PO, so _{V ON} are equal to each other, each of the voltage command value veu, vev, corrects the vew. For example, the voltage command values veu, vev, and view may be corrected by correcting the amplitudes of the voltage command values veu, vev, and view. The correction of the voltage command values veu, vev, and vew may be performed by any method capable of making the DC voltages V _PO and V _ON equal to each other. The adder 68 inputs the corrected voltage command values veu ′, vev ′, and view ′ to the PWM controller 69.

The PWM controller 69 generates control signals SG _{01 to} SG ₁₂ for the switching elements 41 of the first converter 11 based on the input voltage command values veu ′, vev ′, and view ′. The control signals SG _{01 to} SG ₁₂ are signals for switching each switching element 41 on and off. The control signals SG _{01 to} SG ₁₂ are, for example, gate signals.

The PWM controller 69 generates the control signals SG _{01 to} SG ₁₂ by, for example, comparing the sinusoidal voltage command values veu ′, vev ′, and view ′ with a triangular wave carrier wave. The PWM controller 69 controls the switching of each switching element 41 by PWM (Pulse Width Modulation) control.

The PWM controller 69 inputs the generated control signals SG _{01 to} SG ₁₂ to the respective control terminals of the switching elements 41. Thus, on-off of the switching elements 41 is switched in response to the control signal _SG 01 _{to SG 12.} Each DC voltage V _PO , V _ON is controlled to a voltage value corresponding to the DC voltage command value Edc. Further, balance control is performed so that the DC voltages V _PO and V _ON are substantially equal to each other according to the correction command value Vb ′.

FIG. 2 is a flowchart schematically illustrating an example of the operation of the power conversion device according to the embodiment.
As illustrated in FIG. 2, when the control unit 14 of the power conversion device 10 starts operation, the control unit 14 performs a first operation based on the detection results of the current detectors 20u, 20v, 20w and the voltage detectors 21, 22. The switching of each switching element 41 of the converter 11 is controlled, and the AC power supplied from each AC terminal 11u, 11v, 11w is converted into three levels of DC power (step S01 in FIG. 2). At this time, the control unit 14 controls switching of each switching element 51 of the second converter 12. Thereby, the control unit 14 converts DC power into AC power and supplies the AC power 8 with the AC power.

In the conversion from AC power to DC power, the control unit 14 sets each switching element so that the DC voltage V _DC between the positive potential terminal 11p and the negative potential terminal 11n becomes equal to a predetermined DC voltage command value Edc. 41 switching is controlled.

For example, the control unit 14 calculates the effective current command value Iq based on the DC voltage command value Edc and the voltage values of the DC voltages V _PO and V _ON , and the effective current command value Iq and the AC currents iu, Based on the current values of iv and iw, voltage command values veu, vev and vew are calculated, and switching of each switching element 41 is controlled based on the voltage command values veu, vev and vew. Thereby, switching of each switching element 41 can be controlled so that the DC voltage V _DC becomes equal to the DC voltage command value Edc.

The control unit 14, in the conversion from AC power to DC power, the DC voltage V _PO DC voltage V _ON is to be equal to each other, it controls the switching of the switching elements 41.

For example, the control unit 14 calculates a correction command value Vb ′ according to the voltage difference Vd, and corrects each voltage command value veu, vev, and view by adding the correction command value Vb ′. Accordingly, as the DC voltage V _PO DC voltage V _ON are equal to each other, it is possible to control the switching of the switching elements 41.

After starting the conversion from AC power to DC power, the control unit 14 acquires the voltage values of the DC voltages V _PO and V _ON from the voltage detectors 21 and 22 (step S02 in FIG. 2). Then, the control unit 14, the DC voltage _{V PO,} the absolute value of the voltage difference Vd in _{V ON} is determined whether more than a predetermined value (step S03 in FIG. 2).

  When it is determined that the absolute value of the voltage difference Vd is less than the predetermined value, the control unit 14 performs conversion from AC power to DC power using the first DC voltage command value (step S04 in FIG. 2). . On the other hand, when the control unit 14 determines that the absolute value of the voltage difference Vd is equal to or larger than the predetermined value, the control unit 14 alternately switches between the first DC voltage command value and the second DC voltage command value, and converts the AC power into the DC Conversion to electric power is performed (step S05 in FIG. 2). That is, when it is determined that the absolute value of the voltage difference Vd is equal to or greater than a predetermined value, the control unit 14 varies the DC voltage command value Edc.

  For example, when the absolute value of the voltage difference Vd is less than a predetermined value, the control unit 14 outputs “0” from the oscillator 61 to the switch 62, and outputs a predetermined DC from the switch 62 to the DC voltage controller 63. The voltage command value Edc of 1.0 pu is input as the first DC voltage command value. Thereby, conversion from AC power to DC power is performed using the DC voltage command value Edc.

  When the absolute value of the voltage difference Vd is equal to or greater than a predetermined value, the control unit 14 outputs, for example, a pulse signal that alternately repeats “0” and “1” with a predetermined period from the oscillator 61 to the switch 62. To do. When “0” is input from the oscillator 61, the switch 62 inputs 1.0 pu of the DC voltage command value Edc to the DC voltage controller 63 as the first DC voltage command value. "Is input to the DC voltage controller 63 as the second DC voltage command value, 0.9 pu of the DC voltage command value Edc. Thus, when the absolute value of the voltage difference Vd is equal to or greater than a predetermined value, the first DC voltage command value and the second DC voltage command value are alternately switched to perform conversion from AC power to DC power. it can.

  Thus, in the power converter device 10 according to the present embodiment, the DC voltage command value Edc is changed when the absolute value of the voltage difference Vd is equal to or greater than a predetermined value. Thereby, the alternating current of the effective current component can be passed through each of the alternating current terminals 11 u, 11 v, 11 w of the first converter 11.

Each DC voltage _{V PO,} in order to perform the balance control of the _{V ON,} for example, the AC terminals 11u, 11v, 11 w to alternating current iu, iv, it is necessary to flow for 10% of the rated current of iw. For this reason, for example, when the load becomes light, when the current flowing through each of the AC terminals 11u, 11v, and 11w becomes small, the effect of balance control tends to be low, and the voltage difference Vd tends to increase. .

  When the voltage difference Vd becomes large, it is conceivable to increase the balance control effect by flowing reactive currents to the AC terminals 11u, 11v, and 11w. However, if a reactive current flows through the AC power supply 2 side, the AC voltage of the AC power supply 2 may fluctuate. For example, when the AC power supply 2 is an electric power system, fluctuations in the system voltage are caused.

  On the other hand, in the power conversion device 10 according to the present embodiment, as described above, when the absolute value of the voltage difference Vd is equal to or larger than a predetermined value, the effective current component is changed by changing the DC voltage command value Edc. Is supplied to each AC terminal 11u, 11v, 11w. Thereby, the effect of balance control can be enhanced, the voltage difference Vd can be suppressed, and fluctuations in the voltage on the AC power supply 2 side can be suppressed as compared with the case where a reactive current is passed. Thus, in the power converter 10, the voltage fluctuation on the AC side when performing balance control of each DC voltage can be suppressed, and the reliability can be improved.

  When the absolute value of the voltage difference Vd becomes less than the predetermined value after the absolute value of the voltage difference Vd becomes equal to or greater than the predetermined value and the DC voltage command value Edc is changed, the control unit 14 changes the DC voltage command value Edc. Stop fluctuations. In this case, hysteresis may be given to the determination of the absolute value of the voltage difference Vd. For example, when the absolute value of the voltage difference Vd becomes less than the second predetermined value after the absolute value of the voltage difference Vd becomes equal to or greater than the first predetermined value and the DC voltage command value Edc is changed, the control unit 14 Then, the fluctuation of the DC voltage command value Edc is stopped. The second predetermined value is made smaller than the first predetermined value. Thereby, it is possible to suppress repetition of the operation for changing and the operation for stopping the change.

  In the above embodiment, when the absolute value of the voltage difference Vd is greater than or equal to a predetermined value, the first DC voltage command value and the second DC voltage command value are alternately switched to vary the DC voltage command value Edc. The change in the DC voltage command value Edc is not limited to this, and may be changed periodically to three or more values, for example. For example, the command value may be changed according to the magnitude of the voltage difference Vd.

  Although several embodiments of the present invention have been described, these embodiments are presented by way of example and are not intended to limit the scope of the invention. These novel embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the scope of the invention. These embodiments and modifications thereof are included in the scope and gist of the invention, and are included in the invention described in the claims and the equivalents thereof.

  2 ... AC power source, 4u, 4v, 4w ... Interconnected reactor, 6u, 6v, 6w ... Circuit breaker, 8 ... AC load, 10 ... Power converter, 11 ... First converter, 11c, 11p, 11n ... DC terminal 11u, 11v, 11w ... AC terminal, 12 ... second converter, 14 ... control unit, 16, 18 ... DC capacitor, 20u, 20v, 20w ... current detector, 21, 22 ... voltage detector, 41 ... switching Element, 42, 43 ... Rectifying element, 51 ... Switching element, 52, 53 ... Rectifying element, 60 ... DC voltage difference detector, 61 ... Oscillator, 62 ... Switch, 63 ... DC voltage controller, 64 ... Current controller 65 ... DC balance controller, 66 ... Sign detector, 67 ... Multiplier, 68 ... Adder, 69 ... PWM controller

Claims (4)

A plurality of alternating current terminals; a plurality of alternating current terminals; and a plurality of switching elements, wherein the plurality of alternating current terminals are switched by the switching of the plurality of switching elements. A neutral point clamp type converter that converts the AC power supplied from the DC power into three levels of positive potential, negative potential, and neutral point potential;
A first voltage detector for detecting a first DC voltage between the positive potential terminal and the neutral point terminal;
A second voltage detector for detecting a second DC voltage between the negative potential terminal and the neutral point terminal;
Based on the detection result of the first voltage detector and the detection result of the second voltage detector, the DC voltage of the DC power becomes equal to a predetermined DC voltage command value, and the first DC voltage and the first voltage detector A controller that controls switching of the plurality of switching elements so that two DC voltages are equal to each other;
With
The said control part is a power converter device which fluctuates the said DC voltage command value, when the absolute value of the voltage difference of the said 1st DC voltage and the said 2nd DC voltage is more than predetermined value. The controller is
A first DC voltage command value and a second DC voltage command value different from the first DC voltage command value,
When the absolute value of the voltage difference is less than the predetermined value, using the first DC voltage command value,
The power converter according to claim 1, wherein when the absolute value of the voltage difference is equal to or greater than the predetermined value, the first DC voltage command value and the second DC voltage command value are alternately switched.   The power converter according to claim 2, wherein the second DC voltage command value is smaller than the first DC voltage command value. A plurality of alternating current terminals; a plurality of alternating current terminals; and a plurality of switching elements, wherein the plurality of alternating current terminals are switched by the switching of the plurality of switching elements. A neutral point clamp type converter that converts the AC power supplied from the DC power into three levels of positive potential, negative potential, and neutral point potential;
A first voltage detector for detecting a first DC voltage between the positive potential terminal and the neutral point terminal;
A second voltage detector for detecting a second DC voltage between the negative potential terminal and the neutral point terminal;
A method for controlling a power conversion device comprising:
Based on the detection result of the first voltage detector and the detection result of the second voltage detector, the DC voltage of the DC power becomes equal to a predetermined DC voltage command value, and the first DC voltage and the first voltage detector Controlling the switching of the plurality of switching elements so that two DC voltages are equal to each other, and converting the AC power into the DC power;
Changing the DC voltage command value when the absolute value of the voltage difference between the first DC voltage and the second DC voltage is greater than or equal to a predetermined value;
A control method for a power conversion device having

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