JP5734010B2 - Power converter and control method thereof - Google Patents

Description

  The present invention relates to a power conversion device and a control method therefor, and more particularly to a power conversion device linked to a single-phase three-wire power system and a control method therefor.

  In recent years, distributed power supply devices such as solar cells, wind power generators, and fuel cells have begun to spread. At present, the DC power generated by the distributed power supply device is converted into AC power, and the AC power is converted into DC power and used in a device that consumes the power. Thus, since DC-AC conversion and AC-DC conversion are performed, a power loss occurs at each power conversion. Therefore, a DC power feeding system has been proposed that reduces conversion loss by transmitting DC power as it is and using it in equipment without converting the DC power generated by the distributed power supply into AC power.

  In such a DC power supply system, the power converter connected to the single-phase three-wire commercial power system converts the DC power generated by the distributed power supply device into AC power and supplies it to the commercial power system. A configuration is adopted in which AC power of a commercial power system is converted to DC power and supplied to equipment. This type of power converter is composed of an inverter bridge composed of a plurality of switching elements and an interconnected relay. The inverter bridge converts DC power to AC power by PWM (pulse width modulation) control, and connects Some are configured to be supplied to two wires (R-phase wire, T-phase wire) other than the neutral wire of the single-phase three-wire commercial power system via a system relay.

Japanese Patent Laid-Open No. 10-189287

  In the above-described conventional power converter, a smoothing circuit composed of two capacitors connected in series is connected to the DC side of the inverter bridge in order to suppress fluctuations in the input voltage to the inverter bridge. The midpoint of these two capacitors is connected to the neutral point of the single-phase three-wire commercial power system. By connecting the middle point of two capacitors to the neutral point of a single-phase three-wire commercial power system, the same voltage is applied between the R-phase line and the neutral line and between the neutral line and the T-phase line. It is for supplying.

  However, the two capacitors do not necessarily output the same voltage due to factors such as changes in the characteristics of the inverter bridge, control errors in PWM control, and leakage from the parasitic capacitance of the wiring. This causes a problem of unbalance. If voltage imbalance occurs between the two capacitors, the ground voltage at the midpoint of the two capacitors may become unstable.

  Therefore, the present invention has been made to solve such a problem, and an object of the present invention is to stabilize a ground voltage on the DC side in a power conversion apparatus linked to a single-phase three-wire power system. That is.

  According to an aspect of the present invention, there is provided a power conversion device linked to a single-phase three-wire AC power system, wherein the first and second power sources are connected in series between a DC positive bus and a DC negative bus. A smoothing circuit comprising a capacitor, a power conversion circuit connected to the DC terminal of the smoothing circuit, and performing power conversion between the DC power of the smoothing circuit and the AC power of the power system by a plurality of switching elements, and the DC terminal of the smoothing circuit Sensor for detecting a first DC voltage between the intermediate point of the inter-voltage, the DC positive bus and the smoothing circuit, and a second DC voltage between the midpoint of the DC negative bus and the smoothing circuit, and power conversion of the power conversion circuit And a control device for controlling the operation. The midpoint of the smoothing circuit is electrically connected to the neutral point of the power system. The control device includes a first switching control means for controlling the switching of the plurality of switching elements based on the deviation between the DC voltage target value and the voltage detection value between the DC terminals of the voltage sensor, and the first DC of the voltage sensor. Second switching control means for switching-controlling the plurality of switching elements based on a deviation between the voltage detection value and the second DC voltage detection value.

  Preferably, the second switching control unit discharges the first capacitor between the smoothing circuit and the power system when the first DC voltage detection value is larger than the second DC voltage detection value. The plurality of switching elements are subjected to switching control so that the current circulation path is formed.

  Preferably, the second switching control unit discharges the second capacitor between the smoothing circuit and the power system when the second DC voltage detection value is larger than the first DC voltage detection value. The plurality of switching elements are subjected to switching control so that the current circulation path is formed.

  Preferably, the first switching control means sets the duty ratio of the plurality of switching elements based on a deviation between the DC voltage target value and the DC terminal voltage detection value. The second switching control means calculates an ON period of the switching element for discharging the first or second capacitor based on a deviation between the first DC voltage detection value and the second DC voltage detection value. Then, the duty ratio is corrected according to the calculated ON period.

  Preferably, the power conversion device further includes a wiring that connects a midpoint of the smoothing circuit and a neutral point of the power system.

  According to another aspect of the present invention, there is provided a method for controlling a power converter connected to a single-phase three-wire power system, wherein the power converter is connected in series between a DC positive bus and a DC negative bus. A smoothing circuit comprising the first and second capacitors, and a power converter connected to a DC terminal of the smoothing circuit and performing power conversion between the DC power of the smoothing circuit and the AC power of the power system by a plurality of switching elements Voltage sensor for detecting a circuit, a DC terminal voltage of the smoothing circuit, a first DC voltage between the DC positive bus and the midpoint of the smoothing circuit, and a second DC voltage between the DC negative bus and the midpoint of the smoothing circuit Including. The midpoint of the smoothing circuit is electrically connected to the neutral point of the power system. The control method includes a step of controlling the power conversion operation of the power conversion circuit. The controlling step includes a step of performing switching control of the plurality of switching elements based on a deviation between the DC voltage target value and the voltage detection value between the DC terminals of the voltage sensor, the first DC voltage detection value of the voltage sensor, and the second And switching control of the plurality of switching elements based on a deviation from the detected DC voltage value.

  Preferably, the step of performing switching control of the plurality of switching elements based on a deviation between the first DC voltage detection value and the second DC voltage detection value includes the first DC voltage detection value being the second DC voltage detection value. When the value is larger than the value, the plurality of switching elements are subjected to switching control so that a current circulation path for discharging the first capacitor is formed between the smoothing circuit and the power system.

  Preferably, the step of performing switching control of the plurality of switching elements based on a deviation between the first DC voltage detection value and the second DC voltage detection value includes the second DC voltage detection value being the first DC voltage detection value. When the value is larger than the value, the plurality of switching elements are subjected to switching control so that a current circulation path for discharging the second capacitor is formed between the smoothing circuit and the power system.

  Preferably, the step of performing switching control of the plurality of switching elements based on the deviation between the DC voltage target value and the detected voltage value between the DC terminals of the voltage sensor is based on the deviation between the DC voltage target value and the detected voltage value between the DC terminals. Based on this, the duty ratios of the plurality of switching elements are set. The step of switching controlling the plurality of switching elements based on the deviation between the first DC voltage detection value and the second DC voltage detection value includes the first DC voltage detection value and the second DC voltage detection value. Based on the deviation, the ON period of the switching element for discharging the first or second capacitor is calculated, and the duty ratio is corrected according to the calculated ON period.

  According to the present invention, in the configuration in which the midpoint of the smoothing circuit composed of two capacitors connected in series is electrically connected to the neutral point of the single-phase three-wire power system, the voltage of the two capacitors By switching control of the power conversion circuit in accordance with the difference, a current circulation path for discharging the capacitor can be formed between the smoothing circuit and the power system. Thereby, the voltage imbalance of the two capacitors can be kept in balance, and the ground voltage at the midpoint of the smoothing circuit can be stabilized.

1 is a diagram schematically showing an overall configuration of a DC power supply system to which a power conversion device according to an embodiment of the present invention is applied. It is a circuit diagram which shows the detailed structure of the DC / AC converter in FIG. It is a figure explaining the power conversion operation | movement of a DC / AC converter. It is a figure explaining the switching control for compensating the voltage imbalance of capacitor Ch and Cl. It is a figure explaining the switching control shown in FIG. 4 in detail. FIG. 5 is a time waveform diagram for explaining the switching control shown in FIG. 4. It is a figure which shows the control structure of the control part in FIG. It is a figure which shows the modification 1 of the control structure of the control part in FIG. It is a figure which shows the modification 2 of the control structure of the control part in FIG. It is a circuit diagram which shows the detailed structure of the modification of the DC / AC converter which concerns on this Embodiment. It is a circuit diagram which shows the detailed structure of the AC / DC converter in FIG. It is a figure explaining the power conversion operation | movement of an AC / DC converter. It is a figure explaining the power conversion operation | movement of an AC / DC converter. It is a figure explaining the switching control for compensating the voltage imbalance of capacitor Ch and Cl. It is a figure which shows the control structure of the control part in FIG.

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

(Configuration of DC power supply system)
FIG. 1 is a diagram schematically showing an overall configuration of a DC power supply system to which a power converter according to an embodiment of the present invention is applied.

  Referring to FIG. 1, the DC power supply system according to the present embodiment includes a DC bus 1, a photovoltaic power generation system 2, a storage battery system 3, and a grid power system 4.

  The DC bus 1 supplies DC power to the DC load group 5. The DC load group 5 is, for example, an electric device such as an air conditioner, a refrigerator, a washing machine, a television, a lighting device, or a personal computer used at home. Alternatively, it may be an electric device such as a computer, a copier or a facsimile used in an office, or an electric device such as a showcase or a lighting device used in a store. A solar power generation system 2, a storage battery system 3, and a grid power system 4 are connected to the DC bus 1.

  In the DC power supply system according to the present embodiment, a case will be described in which each of DC bus 1, solar power generation system 2, storage battery system 3, grid power system 4, and DC load group 5 is provided. There is no limitation on the number, and it may be one or more.

DC bus 1 is composed of a positive bus PL and a negative bus NL, which are a pair of power lines.
The photovoltaic power generation system 2 includes a solar cell 20 and a DC / DC converter 30. The DC / DC converter 30 is disposed between the solar cell 20 and the DC bus 1, converts the DC power received from the solar cell 20 into a voltage, and supplies it to the DC bus 1. The voltage conversion operation in the DC / DC converter 30 is performed by a control unit (not shown) according to the output voltage of the solar battery 20 and the voltage of the DC bus 1 (the line voltage between the positive bus PL and the negative bus NL). It is controlled according to the switching command.

  The storage battery system 3 includes a storage battery 10 and a DC / DC converter 12. The storage battery 10 is a chargeable / dischargeable power storage element, and typically includes a secondary battery such as a lithium ion secondary battery or a nickel metal hydride battery. As an example, the storage battery 10 has a rated voltage of 380 V and 10 Ah. The DC / DC converter 12 is disposed between the storage battery 10 and the DC bus 1, converts the DC power received from the storage battery 10 into a voltage, and supplies it to the DC bus 1. Further, the DC / DC converter 12 converts the DC power received from the DC bus 1 into a voltage and supplies it to the storage battery 10. The voltage conversion operation in the DC / DC converter 12 is controlled according to a switching command from a control unit (not shown) according to the output voltage of the storage battery 10 and the voltage of the DC bus 1.

  The grid power system 4 exchanges DC power with the DC bus 1. The grid power system 4 includes a DC / AC converter 50, an AC / DC converter 70, and a grid power 40.

  System power 40 is power received from an electric power company or the like (for example, AC 200V). The system power 40 is supplied from a single-phase three-wire commercial AC power system. In the single-phase three-wire commercial AC power system, the neutral wire is grounded via the resistor Rg1, and AC 200V is supplied using two wires other than the neutral wire (R-phase wire RL and T-phase wire TL). Supply.

  The DC / AC converter 50 and the AC / DC converter 70 are connected in parallel between the DC bus 1 and the system power 40. The DC / AC converter 50 converts the DC power received from the DC bus 1 into AC power and supplies it to the system power 40. On the other hand, the AC / DC converter 70 converts AC power received from the system power 40 into DC power and supplies it to the DC bus 1. In the direct current power supply system according to the present embodiment, system power is purchased from an electric power company or the like (power purchase) via AC / DC converter 50, and surplus power is purchased via electric power company or the like via DC / AC converter 70. It is possible to sell to (power sale).

(Configuration of DC / AC converter)
Next, with reference to the drawings, a configuration of DC / AC converter 50 which is one form of the power conversion device according to the embodiment of the present invention will be described.

FIG. 2 is a circuit diagram showing a detailed configuration of the DC / AC converter 50 in FIG.
Referring to FIG. 2, DC / AC converter 50 includes capacitors Ch and Cl, DC / AC converter 52, interconnection reactors 54 and 56, DC voltage detectors 58, 62 and 64, and a controller. 60.

  Capacitors Ch and Cl are connected in series between positive bus PL and negative bus NL constituting DC bus 1, and constitute a smoothing circuit. Capacitors Ch and Cl have the same capacity. Thereby, an intermediate potential between the positive bus PL and the negative bus NL can be generated at the midpoint of the capacitors Ch and Cl (point B in the figure). Further, the midpoint B of the capacitor Ch and the capacitor Cl is grounded via the resistor Rg2. Therefore, for example, when the line voltage between positive bus PL and negative bus NL is 380 V (rated voltage of storage battery 3), the potentials at positive bus PL, negative bus NL, and midpoint B are +190 V, -190 V, and ground potential, respectively. (0V).

  The DC / AC conversion unit 52 converts the DC power received from the DC bus 1 into AC power according to the switching control signals S1 to S4 from the control unit 60, and outputs the AC power to the system power 40. The DC / AC conversion unit 52 includes transistors Q1 to Q4 that are switching elements and diodes D1 to D4. Transistors Q1 and Q3 are connected in series between positive bus PL and negative bus NL constituting DC bus 1. An intermediate point between transistors Q1 and Q3 is connected to R-phase line RL. Interconnection reactor 54 is connected to R-phase line RL.

  Transistors Q2 and Q4 are connected in series between positive bus PL and negative bus NL. An intermediate point between transistors Q2 and Q4 is connected to T-phase line TL. Interconnection reactor 56 is connected to T-phase line TL. Between the collectors and emitters of the transistors Q1 to Q4, diodes D1 to D4 that flow current from the emitter side to the collector side are respectively connected.

  For example, IGBTs (Insulated Gate Bipolar Transistors) can be used as the transistors Q1 to Q4. Alternatively, a power switching element such as a power MOSFET (Metal Oxide Semiconductor Field-Effect Transistor) may be used.

  The DC voltage detection unit 58 is connected between the positive bus PL and the negative bus NL, detects the DC power voltage Vdc supplied from the DC bus 1 to the DC / AC converter 50, and the detection result is a control unit. Output to 60.

  The DC voltage detection unit 62 is connected between the positive bus PL and the midpoint B, detects the voltage Vdc_h across the capacitor Ch (corresponding to the voltage between the positive bus PL and the midpoint B), and detects it. The result is output to the control unit 60.

  The DC voltage detector 64 is connected between the middle point B and the negative bus NL, detects the voltage Vdc_l (corresponding to the voltage between the middle point B and the negative bus NL) at both ends of the capacitor Cl, and detects this. The result is output to the control unit 60.

  Based on voltage Vdc received from DC voltage detection unit 58, voltage Vdc_h received from DC voltage detection unit 62, and voltage Vdc_l received from DC voltage detection unit 64, control unit 60 follows control structure described below. Switching control signals S1 to S4 for controlling on / off of the transistors Q1 to Q4 are generated, and the DC / AC converter 52 is controlled.

Hereinafter, the power conversion operation of the DC / AC converter 50 will be described with reference to FIG.
During the power conversion operation, the control unit 60 generates the switching control signals S1 to S4 so that the DC voltage Vdc becomes a predetermined voltage target value Vdc *. Voltage target value Vdc * can be determined in advance, such as 380 V, and stored in a storage unit (not shown). Or you may make it acquire a desired voltage value suitably by communicating between the control part 60 and the exterior of a direct-current power feeding system.

  In the transistors Q1 to Q4 constituting the full bridge circuit, the control unit 60 alternates two switch pairs with the transistors Q1 and Q4 as one set of switch pairs and the transistors Q2 and Q3 as another set of switch pairs. Turn on and off. In FIG. 3, only the switch pair in the on state is represented by a solid line.

  As shown in FIG. 3, in the half cycle of the system power (hereinafter referred to as “period 1”), the control unit 60 turns off the switch pair of the transistors Q2 and Q3, while switching the transistors Q1 and Q4. The pair is turned on / off at a predetermined duty ratio (the ratio of the on period to the switching period of the transistors Q1 to Q4). In this period 1, the current flows from the DC side to the AC side through the transistors Q1 and Q4 during the ON period of the transistors Q1 and Q4, thereby energizing from the DC side to the AC side. In the off period of the transistors Q1 to Q4, the electromagnetic energy in the current smoothing connected reactors 54 and 56 is returned to the DC side through the system power 40 and the diodes D2 and D3. When the electromagnetic energy in the current smoothing linked reactors 54 and 56 decreases during the off period of the transistors Q1 to Q4, the circuit is not energized.

  Subsequently, in the next half cycle of the system power (hereinafter referred to as “period 2”), the control unit 60 turns off the switch pair of the transistors Q1 and Q4, while switching the switch pair of the transistors Q2 and Q3. Turn on / off at a predetermined duty ratio. In this period 2, current flows from the DC side to the AC side through the transistors Q 2 and Q 3 during the ON period of the transistors Q 2 and Q 3, thereby energizing from the DC side to the AC side. The electromagnetic energy in the current smoothing connected reactors 54 and 56 is returned to the DC side through the system power 40 and the diodes D1 and D4 during the off period of the transistors Q1 to Q4. When the electromagnetic energy in the current smoothing linked reactors 54 and 56 decreases during the off period of the transistors Q1 to Q4, the circuit is not energized.

  In this way, by alternately turning on and off the two switch pairs of the full bridge circuit, AC power having a sine wave output waveform can be output between the R phase line RL and the T phase line TL. . In the above-described period 1 and period 2, when the DC voltage Vdc is larger than the voltage target value Vdc *, the control unit 60 increases the energy transmitted to the system power 40 side so that the energy of each switch pair is increased. Increase the duty ratio. On the other hand, when DC voltage Vdc is smaller than voltage target value Vdc *, control unit 60 decreases the duty ratio of each switch pair so as to reduce the energy transmitted to system power 40 side.

  However, in the above-described switching control of the transistors Q1 to Q4, a set of switches is caused by a change in characteristics of the transistors Q1 to Q4 and the capacitors Ch and Cl, a shift of a switching control signal due to a control error of the control unit 60, and the like There may be a difference in switching timing between the two transistors constituting the pair. When a deviation occurs in the switching timing, the amount of charge stored in the capacitor Ch and the amount of charge stored in the capacitor Cl differ in the smoothing circuit. This causes a problem that an imbalance occurs between the voltage Vdc_h of the capacitor Ch and the voltage Vdc_l of the capacitor Cl. If voltage imbalance occurs between the capacitors Ch and Cl in this way, the ground voltage at the midpoint B of the smoothing circuit may become unstable.

  In order to avoid such a problem, in the DC / AC converter according to the present embodiment, the voltage Vdc_h of the capacitor Ch with respect to the original switching control signals S1 to S4 generated based on the voltage target value Vdc *. And a correction process for compensating for an imbalance between the voltage Vdc_l of the capacitor Cl.

  Hereinafter, a switching control signal correction process according to an embodiment of the present invention will be described in detail with reference to the drawings.

  FIG. 4 is a diagram for explaining switching control for compensating for the voltage imbalance of the capacitors Ch and Cl.

  Referring to FIG. 4, in the switching control according to the embodiment of the present invention, in period 1 (half cycle of system power) shown in FIG. 2, transistors Q1 and Q4 are turned on and transistors Q1 to Q4 are turned off. A period for correcting the potential at the middle point B of the smoothing circuit (hereinafter referred to as “middle point potential correction period”) is provided between the periods.

  During the midpoint potential correction period, the control unit 60 fixes the transistors Q1 to Q4 on and off so that the voltage Vdc_h of the capacitor Ch and the voltage Vdc_l of the capacitor Cl are equal. For example, when the voltage Vdc_h of the capacitor Ch is larger than the voltage Vdc_l of the capacitor Cl, the transistor Q1 is fixed on and the transistors Q2 to Q4 are fixed off as shown in FIG.

  In this way, in the midpoint potential correction period, as shown in FIG. 5, between the smoothing circuit and the system power 40, the transistor Q1, the interconnection reactor 54, the R phase line RL, the neutral point A, the resistance Rg1, and the like. A current circulation path from the ground to the resistance Rg2 to the middle point B for discharging the capacitor Ch is formed, and a discharge current flows through the current circulation path. When the voltage Vdc_h of the capacitor Ch is equal to the voltage Vdc_l of the capacitor Cl and the midpoint potential correction period ends, the transistors Q1 to Q4 are all turned off.

  FIG. 6 is a time waveform diagram for explaining the switching control shown in FIG. FIG. 6 shows time waveforms of the switching control signals S1 and S4 in the period 1 shown in FIGS. FIG. 6 shows switching control signals S1, S4 for controlling on / off of the transistors Q1, Q4. On the other hand, the switching control signals S2 and S3 for turning on / off the transistors Q2 and Q3 are fixed to the L (logic low) level in the period 1, and therefore are not shown.

  Referring to FIG. 6, in period 1, switching control signals S <b> 1 and S <b> 4 having a predetermined duty ratio are output from control unit 60. During the period (period T in the figure) when the switching control signals S1 and S4 are at the H (logic high) level, the transistors Q1 and Q4 are both turned on. On the other hand, in the period when switching control signals S1 and S4 are at L level, transistors Q1 and Q4 are both turned off.

  Control unit 60 calculates a voltage deviation from the difference between voltage Vdc_h received from DC voltage detection unit 62 and voltage Vdc_l received from DC voltage detection unit 64, and calculates a midpoint potential correction period according to the voltage deviation. To do.

  When the voltage Vdc_h of the capacitor Ch is larger than the voltage Vdc_l of the capacitor Cl, when the midpoint potential correction period is ΔT, the switching control signals S1 and S4 have the ΔT when the switching control signal S1 is at the H level. It is corrected so as to become longer. In FIG. 6, as an example, the switching control signal S1 is corrected so that the period during which it is at the H level is T + ΔT / 2, and the switching control signal S4 is such that the period during which it is at the H level is T−ΔT / 2. It is corrected. Thus, in the midpoint potential correction period, the transistor Q1 can be fixed on, while the transistor Q4 can be fixed off, and a current circulation path for discharging the capacitor Ch shown in FIG. 5 is formed. Can do.

  Note that when the voltage Vdc_l of the capacitor Cl is larger than the voltage Vdc_h of the capacitor Ch, the switching control signals S1 and S4 have a period during which the switching control signal S4 is H level longer by the midpoint potential correction period ΔT. It is corrected as follows. Thereby, in the midpoint potential correction period, the transistor Q4 can be fixed on, while the transistor Q1 can be fixed off, and a current circulation path for discharging the capacitor Cl can be formed.

  As described above, according to the power conversion device of the present embodiment, the middle point B of the capacitors Ch and Cl is electrically connected to the neutral point A of the single-phase three-wire commercial AC power system. Thus, a current circulation path for discharging the capacitor Ch or Cl can be formed between the smoothing circuit and the system power 40 via the DC / AC converter 50. As a result, the voltage Vdc_h of the capacitor Ch and the voltage Vdc_l of the capacitor Cl can be kept in balance, and the ground voltage at the midpoint B of the smoothing circuit can be stabilized.

FIG. 7 is a diagram showing a control structure of the control unit 60 in FIG.
Referring to FIG. 7, control unit 60 includes subtraction units 600, 603 to 605, addition units 601, 602, PI control units 610, 615, carrier wave comparators 611-614, NOT circuits 621, 624, including.

  Subtraction unit 600 calculates a voltage deviation from the difference between DC voltage Vdc and voltage target value Vdc *, and outputs the result to PI control unit 610.

  The PI control unit 610 includes at least a proportional element (P) and an integral element (I), and receives a voltage deviation from the subtraction unit 600, in accordance with the input voltage deviation. A duty ratio d is calculated as a ratio of the ON period to the switching period (carrier wave signal period) of each of the transistors Q1 to Q4.

  The subtraction unit 605 calculates a voltage deviation from the difference between the voltage Vdc_h and the voltage Vdc_l and outputs the voltage deviation to the PI control unit 615.

  When receiving a voltage deviation from the subtracting unit 605, the PI control unit 615 calculates an ON period of the transistor as a midpoint potential correction period according to the input voltage deviation. Then, the PI control unit 615 calculates a correction amount Δd of the duty ratio necessary to ensure the calculated on period.

  The adding unit 601 adds the duty ratio correction amount Δd input from the PI control unit 615 to the duty ratio d input from the PI control unit 610 to generate a duty ratio command (d + Δd). Adder 601 outputs the generated duty ratio command to carrier wave comparator 611.

  The adding unit 602 adds the duty ratio correction amount Δd input from the PI control unit 615 to the duty ratio d input from the PI control unit 610 to generate a duty ratio command (d + Δd). Adder 602 outputs the generated duty ratio command to carrier wave comparator 612.

  The subtracting unit 603 subtracts the duty ratio correction amount Δd input from the PI control unit 615 from the duty ratio d input from the PI control unit 610 to generate a duty ratio command (d−Δd). Adder 601 outputs the generated duty ratio command to carrier wave comparator 613.

  The subtracting unit 604 subtracts the duty ratio correction amount Δd input from the PI control unit 615 from the duty ratio d input from the PI control unit 610 to generate a duty ratio command (d−Δd). Adder 602 outputs the calculated duty ratio command to carrier wave comparator 614.

  The carrier wave comparator 611 compares the duty ratio command (d + Δd) with a carrier wave signal (for example, a triangular wave signal), inverts a binary signal corresponding to the comparison result by the NOT circuit 621, and turns on / off the transistor Q1. A switching control signal S1 for controlling OFF is generated.

  The carrier wave comparator 612 compares the duty ratio command (d + Δd) and the carrier wave signal, and generates a switching control signal S2 for controlling on / off of the transistor Q2, which is a binary signal corresponding to the comparison result.

  The carrier wave comparator 613 compares the duty ratio command (d−Δd) and the carrier wave signal, and generates a switching control signal S3 for controlling on / off of the transistor Q3 composed of a binary signal according to the comparison result. To do.

  The carrier wave comparator 614 compares the duty ratio command (d−Δd) and the carrier wave signal, inverts a binary signal corresponding to the comparison result by the NOT circuit 624, and controls on / off of the transistor Q4. The switching control signal S4 is generated.

  In FIG. 7, the configuration has been described in which the switching control signals S1 to S4 are corrected by adding / subtracting the correction amount Δd to / from the duty ratio d of each switching control signal. As shown, only one of the two switching control signals respectively corresponding to the two transistors constituting one switch pair may be corrected.

  Further, in FIG. 6, a period during which only the transistor Q1 is turned on by controlling the switching control signal S1 to be continuously at the H level even after the switching control signal S4 has transitioned to the L level (midpoint potential correction period). However, the midpoint potential correction period may be provided at any timing. For example, the switching control signal S1 may be shifted to the H level before the switching control signal S4. Alternatively, a period in which only the switching control signal S1 is at the H level may be provided at a timing different from the period in which the switching control signals S1 and S4 are at the H level.

  In the present embodiment, the bipolar switching method has been described as the switching control of the power converter. However, the application of the present invention is not limited to the bipolar switching method. In other words, the present invention applies to all switching methods that are provided with an energy transmission period in which the transistors Q1 and Q4 or the transistors Q2 and Q3 are turned on and a midpoint potential correction period in which any one of the transistors Q1 to Q4 is turned on. It is possible to apply.

[Modification 1]
FIG. 8 is a diagram showing a first modification of the control structure of the control unit 60 in FIG.

  Referring to FIG. 8, control unit 60 </ b> A according to Modification 1 is different from control unit 60 shown in FIG. 7 only in that subtracting units 603 and 604 are not provided.

  In the configuration shown in FIG. 8, the adding unit 601 adds the duty ratio correction amount Δd input from the PI control unit 615 to the duty ratio d input from the PI control unit 610 to generate a duty ratio command (d + Δd). Generate. The carrier wave comparator 611 compares the duty ratio command (d + Δd) with the carrier wave signal, inverts a binary signal corresponding to the comparison result by the NOT circuit 621, and performs switching for controlling on / off of the transistor Q1. A control signal S1 is generated.

  The adding unit 602 adds the duty ratio correction amount Δd input from the PI control unit 615 to the duty ratio d input from the PI control unit 610 to generate a duty ratio command (d + Δd). The carrier wave comparator 612 compares the duty ratio command (d + Δd) and the carrier wave signal, and generates a switching control signal S2 for controlling on / off of the transistor Q2, which is a binary signal corresponding to the comparison result.

  The carrier wave comparator 613 compares the duty ratio d with the carrier wave signal, and generates a switching control signal S3 for controlling on / off of the transistor Q3 composed of a binary signal corresponding to the comparison result.

  The carrier wave comparator 614 compares the duty ratio d and the carrier wave signal, inverts a binary signal corresponding to the comparison result by the NOT circuit 624, and controls the switching control signal S4 for controlling on / off of the transistor Q4. Is generated.

  As described above, in the first modification, the midpoint is obtained by adding the correction amount Δd to the duty ratio d of one of the two switching control signals respectively corresponding to the two transistors constituting one switch pair. A potential correction period is secured.

[Modification 2]
FIG. 9 is a diagram illustrating a second modification of the control structure of the control unit 60 in FIG.

  Referring to FIG. 9, control unit 60 </ b> B according to modification 2 is different from control unit 60 shown in FIG. 7 only in that adders 601 and 602 are not provided.

  In the configuration shown in FIG. 9, the subtracting unit 603 subtracts the duty ratio correction amount Δd input from the PI control unit 615 from the duty ratio d input from the PI control unit 610 to obtain a duty ratio command (d−Δd ) Is generated. The carrier wave comparator 611 compares the duty ratio command (d−Δd) with the carrier wave signal, inverts a binary signal corresponding to the comparison result by the NOT circuit 621, and controls on / off of the transistor Q1. Switching control signal S1 is generated.

  The subtracting unit 604 subtracts the duty ratio correction amount Δd input from the PI control unit 615 from the duty ratio d input from the PI control unit 610 to generate a duty ratio command (d−Δd). The carrier wave comparator 612 compares the duty ratio command (d−Δd) and the carrier wave signal, and generates a switching control signal S2 for controlling on / off of the transistor Q2 composed of a binary signal according to the comparison result. To do.

  The carrier wave comparator 613 compares the duty ratio d with the carrier wave signal, and generates a switching control signal S3 for controlling on / off of the transistor Q3 composed of a binary signal corresponding to the comparison result.

  The carrier wave comparator 614 compares the duty ratio d and the carrier wave signal, inverts a binary signal corresponding to the comparison result by the NOT circuit 624, and controls the switching control signal S4 for controlling on / off of the transistor Q4. Is generated.

  As described above, in the second modification, the midpoint is obtained by subtracting the correction amount Δd from one duty ratio d of two switching control signals respectively corresponding to two transistors constituting one switch pair. A potential correction period is secured.

  In the above-described embodiment, as one form of the power converter, the middle point B and the single-phase three-wire are grounded by grounding the middle point B of the capacitors Ch and Cl constituting the smoothing circuit via the resistor Rg2. The DC / AC converter in which the neutral point A of the AC power system is electrically connected has been illustrated, but the present invention relates to the neutral point B of the smoothing circuit and the neutrality of the single-phase three-wire AC power system. The present invention can be applied to a DC / AC converter in which the point A is electrically connected. For example, as shown in FIG. 10, the present invention is applicable to a configuration in which the midpoint B of the smoothing circuit is connected to a neutral line of a single-phase three-wire AC power system. In the configuration shown in FIG. 10, a resistor Rg <b> 3 is inserted and connected to the neutral line connecting the neutral point B of the smoothing circuit and the neutral point A of the single-phase three-wire AC power system. The resistor Rg3 is connected to prevent an excessive current from flowing between the neutral point B and the neutral point A. However, the resistor Rg3 does not necessarily need to be connected to the neutral line. For example, the resistance Rg3 includes a wiring impedance of a neutral line connecting the neutral point B and the neutral point A.

(Configuration of AC / DC converter)
FIG. 11 is a circuit diagram showing a detailed configuration of the AC / DC converter 70 in FIG.

  Referring to FIG. 11, AC / DC converter 70 has basically the same circuit configuration as DC / AC converter 50 shown in FIG. 2, and includes capacitors Ch and Cl, AC / DC converter 72, and the like. Interconnection reactors 74, 76, DC voltage detection units 78, 82, 84, and control unit 80.

  Capacitors Ch and Cl are connected in series between positive bus PL and negative bus NL. A midpoint B of the capacitors Ch and Cl is grounded via a resistor Rg2.

  The AC / DC conversion unit 72 converts the AC power received from the system power 40 into DC power according to the switching control signals S11 to S14 from the control unit 80, and outputs the DC power to the DC bus 1. AC / DC conversion unit 72 includes transistors Q11 to Q14, which are switching elements, and diodes D11 to D14. Transistors Q11 and Q13 are connected in series between positive bus PL and negative bus SL. An intermediate point between transistors Q11 and Q13 is connected to R-phase line RL. Interconnection reactor 74 is connected to R-phase line RL.

  Transistors Q12 and Q14 are connected in series between positive bus PL and negative bus SL. An intermediate point between transistors Q12 and Q14 is connected to T-phase line TL. Interconnection reactor 76 is inserted and connected to T-phase line TL. Between the collectors and emitters of the transistors Q11 to Q14, diodes D11 to D14 that flow current from the emitter side to the collector side are respectively connected.

  For example, IGBTs can be used as the transistors Q11 to Q14. Alternatively, a power switching element such as a power MOSFET may be used.

  DC voltage detector 78 is connected between positive bus PL and negative bus NL, detects voltage value Vdc of DC power supplied from AC / DC converter 72 to DC bus 1, and controls the detection result. To the unit 80.

  DC voltage detection unit 82 is connected between positive bus PL and midpoint B, and detects voltage Vdc_h across capacitor Ch (corresponding to the voltage between positive bus PL and midpoint B) and detects it. The result is output to the control unit 80.

  The DC voltage detector 84 is connected between the middle point B and the negative bus NL, detects the voltage Vdc_l (corresponding to the voltage between the middle point B and the negative bus NL) at both ends of the capacitor Cl, and detects that. The result is output to the control unit 80.

  Based on voltage Vdc received from DC voltage detection unit 78, voltage Vdc_h received from DC voltage detection unit 82, and voltage Vdc_l received from DC voltage detection unit 84, control unit 80 follows control structure described below. Switching control signals S11 to S14 for controlling on / off of the transistors Q11 to Q14 are generated, and the AC / DC converter 72 is controlled.

Hereinafter, the power conversion operation of the AC / DC converter 70 will be described with reference to FIG.
During the power conversion operation, control unit 80 generates switching control signals S11 to S14 such that DC voltage Vdc becomes a predetermined voltage target value Vdc *. Voltage target value Vdc * can be determined in advance, such as 380 V, and stored in a storage unit (not shown). Or you may make it acquire a desired voltage value suitably by communicating between the control part 80 and the exterior of a direct current power supply system.

  As shown in FIG. 12, in the half cycle of the system power, control unit 80 turns off the switch pair of transistors Q11 and Q12 while turning on / off the switch pair of transistors Q13 and Q14 at a predetermined duty ratio. Let In the on period of transistors Q13 and Q14 (hereinafter referred to as “period 1-1”), electromagnetic current is accumulated in interconnection reactors 74 and 76 due to current flowing through transistors Q13 and Q14. Subsequently, in the off period of the transistors Q13 and Q14, that is, the off period of the transistors Q11 to Q14 (hereinafter referred to as “period 2”), the connection point between the diodes D11 and D13 and the connection point of the diodes D12 and D14 is between. The electromagnetic energy emitted from the interconnecting reactors 74 and 76 is received, and this electromagnetic energy is rectified to DC power.

  In FIG. 12, the transistors Q11 to Q14 are all turned off in the period 2 in which AC power is rectified to DC power by the diodes D11 and D14 constituting the bridge circuit. It is good also as a structure which carries out synchronous rectification as an ON state.

  When the DC voltage Vdc is greater than the voltage target value Vdc *, the control unit 80 reduces the duty ratio of the switch pair so as to reduce the energy transmitted to the DC side, that is, to shorten the period 1-1. Decrease. On the other hand, when the DC voltage Vdc is smaller than the voltage target value Vdc *, the control unit 80 increases the duty of the switch pair so as to increase the energy transmitted to the DC side, that is, to lengthen the period 1-1. Increase the ratio.

  Here, in the on-period (period 1-1) of the transistors Q13 and Q14, as shown in FIG. 12A, electromagnetic energy is accumulated in the interconnecting reactors 74 and 76, and at the same time, as shown in FIG. ) Between the smoothing circuit and the system power 40, the capacitor Cl, which is the interconnection reactor 76, the transistor Q14, the capacitor Cl, the middle point B, the resistance Rg2, the grounding, the resistance Rg1, and the neutral point A, is discharged. Current circulation path is formed, and a discharge current flows through this current circulation path. In other words, capacitor Cl is discharged during the on period (period 1-1) of transistors Q13 and Q14.

  On the other hand, FIG. 13 shows a state where the power conversion operation is performed by a control mode different from that in FIG. In FIG. 13, in the half cycle of the system power, control unit 80 turns off the switch pair of transistors Q13 and Q14, and turns on / off the switch pair of transistors Q11 and Q12 at a predetermined duty ratio. In the on period of transistors Q11 and Q12 (hereinafter referred to as “period 1-2”), current flows through transistors Q11 and Q12, so that electromagnetic energy is accumulated in interconnection reactors 74 and 76. Subsequently, in the off period of the transistors Q11 and Q12, that is, the off period (period 2) of the transistors Q11 to Q14, the interconnection reactor 74, between the connection point of the diodes D11 and D13 and the connection point of the diodes D12 and D14, The electromagnetic energy emitted from 76 is received, and this electromagnetic energy is rectified into DC power.

  When the DC voltage Vdc is larger than the voltage target value Vdc *, the control unit 80 reduces the duty ratio of the switch pair so as to reduce the energy transmitted to the DC side, that is, to shorten the period 1-2. Decrease. On the other hand, when the DC voltage Vdc is smaller than the voltage target value Vdc *, the control unit 80 increases the duty of the switch pair so as to increase the energy transmitted to the DC side, that is, to lengthen the period 1-2. Increase the ratio.

  In FIG. 13, in the ON period (period 1-2) of the transistors Q11 and Q12, as shown in FIG. 13A, electromagnetic energy is accumulated in the interconnecting reactors 74 and 76, and at the same time ( b), between the smoothing circuit and the system power 40, the neutral point A, the resistance Rg1, the ground, the resistance Rg2, the middle point B, the capacitor Ch, the transistor Q11, and the interconnection reactor 74 are discharged. Current circulation path is formed, and a discharge current flows through this current circulation path. That is, the capacitor Ch is discharged during the ON period (period 1-2) of the transistors Q11 and Q12.

  As described above, when the power conversion operation is performed according to the control mode of FIG. 12, capacitor Cl is discharged in the on period (period 1-1) of transistors Q13 and Q14. On the other hand, when the power conversion operation is performed according to the control mode of FIG. 13, capacitor Ch is discharged during the on period (period 1-2) of transistors Q11 and Q12.

  Therefore, in the AC / DC converter 80 according to the present embodiment, as a correction process for compensating for the voltage imbalance between the capacitors Ch and Cl, the voltage deviation between the voltage Vdc_h of the capacitor Ch and the voltage Vdc_l of the capacitor Cl is obtained. Accordingly, the control mode of FIG. 12 and the control mode of FIG. 13 are switched and executed.

  FIG. 14 is a diagram for explaining switching control for compensating for the voltage imbalance of the capacitors Ch and Cl.

  Referring to FIG. 14, in the switching control according to the embodiment of the present invention, control unit 80 compares voltage Vdc_h of capacitor Ch with voltage Vdc_l of capacitor Ch, and according to the comparison result, FIG. One of the switching control shown in A) and the switching control shown in FIG.

  Specifically, referring to FIG. 14A, when voltage Vdc_l is larger than voltage Vdc_h, transistors Q11 and Q12 are turned off in a half cycle of the system power as described in FIG. Transistors Q13 and Q14 are turned on / off at a predetermined duty ratio. Thereby, the capacitor Cl can be discharged in the ON period (period 1-1) of the transistors Q13 and Q14.

  On the other hand, referring to FIG. 14B, when voltage Vdc_h is larger than voltage Vdc_l, transistors Q13 and Q14 are turned off in the half cycle of the system power as described in FIG. Transistors Q11 and Q12 are turned on / off at a predetermined duty ratio. Thereby, the capacitor Ch can be discharged in the ON period (period 1-2) of the transistors Q11 and Q12.

  As described above, according to the power conversion device of the present embodiment, the middle point B of the capacitors Ch and Cl is electrically connected to the neutral point A of the single-phase three-wire commercial AC power system. Thus, a current circulation path for discharging the capacitor Ch or Cl can be formed between the smoothing circuit and the system power 40 via the AC / DC converter 70. As a result, the voltage Vdc_h of the capacitor Ch and the voltage Vdc_l of the capacitor Cl can be kept in balance, and the ground voltage at the midpoint B of the smoothing circuit can be stabilized.

FIG. 15 is a diagram showing a control structure of the control unit 80 in FIG.
Referring to FIG. 15, control unit 80 includes a subtraction unit 800, a PI control unit 802, a carrier wave comparator 804, a selection unit 806, and a comparison unit 808.

  Subtraction unit 800 calculates a voltage deviation from the difference between DC voltage Vdc and voltage target value Vdc *, and outputs the result to PI control unit 802.

  The PI control unit 802 includes at least a proportional element P and an integral element I. When receiving a voltage deviation from the subtracting unit 800, the PI control unit 802 includes switching periods (carrier waves) of the transistors Q11 to Q14 according to the input voltage deviation. The duty ratio d as a ratio of the ON period to the signal period) is calculated.

  A carrier wave comparator 804 compares the duty ratio d with a carrier wave signal (for example, a triangular wave signal), and performs switching control for controlling on / off of the transistors Q11 to Q14 composed of binary signals according to the comparison result. Signals S11 to S14 are generated. Specifically, carrier wave comparator 804 uses switching control signals S11 and S12 for controlling on / off of the switch pair of transistors Q11 and Q12 as one set of switching control signals, and sets the switch pair of transistors Q13 and Q14. Two sets of switching control signals (S11, S12) and (S13, S14) are generated using the switching control signals S13 and S14 as another set of switching control signals in order to control on / off.

  Comparison unit 808 compares voltage Vdc_h received from DC voltage detection unit 82 with voltage Vdc_l received from DC voltage detection unit 84. Then, the comparison unit 808 outputs a signal indicating the comparison result to the selection unit 806.

  The selection unit 806 selects one of the two sets of switching control signals (S11, S12) and (S13, S14) based on the comparison result signal received from the comparison unit 808. Specifically, when it is determined from the comparison result signal that the voltage Vdc_h is larger than the voltage Vdc_l, the selection unit 806 selects the switching control signal (S13, S14). In this case, the selection unit 806 fixes the non-selected switching control signals (S11, S12) to the L level.

  On the other hand, when it is determined from the comparison result signal that the voltage Vdc_l is larger than the voltage Vdc_h, the selection unit 806 selects the switching control signal (S11, S12). In this case, the selection unit 806 fixes the non-selected switching control signals (S13, S14) to the L level.

  Note that the AC / DC converter in this embodiment is provided with a midpoint potential correction period by the control method shown in FIG. 14, but as shown in FIG. 4B, any of the transistors Q11 to Q14 is provided. Alternatively, the potential at the midpoint B may be corrected by providing a period during which one of them is turned on.

  In the above-described embodiment, as one form of the power converter, the middle point B and the single-phase three-wire are connected by grounding the middle point B of the capacitors Ch and Cl constituting the smoothing circuit via the resistor Rg2. The AC / DC converter in which the neutral point A of the AC power system is electrically connected is illustrated. However, the present invention relates to the neutral point B of the smoothing circuit and the neutrality of the single-phase three-wire AC power system. The present invention can be applied to an AC / DC converter in which the point A is electrically connected. For example, as described with reference to FIG. 10, the present invention is applicable to a configuration in which the midpoint B of the smoothing circuit is connected to a neutral line of a single-phase three-wire AC power system.

  Moreover, in this Embodiment, although the structure which installs a DC / AC converter and an AC / DC converter in parallel between a direct current bus and system power was shown as a power converter device, both were integrated. The power conversion control according to the present invention can also be applied to the bidirectional power converter.

  The embodiment disclosed this time should be considered as illustrative in all points and not restrictive. The scope of the present invention is shown not by the above description of the embodiments but by the scope of claims for patent, and is intended to include meanings equivalent to the scope of claims for patent and all modifications within the scope.

  1 DC bus, 2 photovoltaic power generation system, 3 storage battery, 4 grid power system, 5 DC load group, 20 solar battery, 30 DC / DC converter, 40 grid power, 50 DC / AC converter, 52 DC / AC conversion Unit, 54, 56, 74, 76 interconnected reactor, 58, 62, 64, 78, 82, 84 DC voltage detection unit, 60, 60A, 60B, 80 control unit, 70 AC / DC converter, 72 AC / DC Converter unit, 600, 603 to 605, 800 subtraction unit, 601, 602 addition unit, 610, 615, 802 PI control unit, 611-614, 804 carrier wave comparator, 621, 624 NOT circuit, 806 selection unit, 808 comparison Part, Ch, Cl capacitor, D1-D4, D11-D14 diode, Q1-Q4, Q11-Q14 transistor.

Claims (11)

A power conversion device linked to a single-phase three-wire AC power system,
A smoothing circuit comprising first and second capacitors connected in series between a DC positive bus and a DC negative bus;
A first power conversion circuit connected to a DC terminal of the smoothing circuit, for converting DC power of the smoothing circuit into AC power of the power system ;
Detects a DC terminal voltage of the smoothing circuit, a first DC voltage between the DC positive bus and a midpoint of the smoothing circuit, and a second DC voltage between the DC negative bus and a midpoint of the smoothing circuit. A voltage sensor;
A first control device for controlling a power conversion operation of the first power conversion circuit,
A midpoint of the smoothing circuit is electrically connected to a neutral point of the power system,
The first power conversion circuit is connected in series between the first and second switching elements connected in series between the DC positive bus and the DC negative bus, and between the DC positive bus and the DC negative bus. And a full bridge circuit including third and fourth switching elements ,
The first control device includes:
The first switch pair composed of the first and fourth switching elements and the second and third switching elements with a duty ratio corresponding to the deviation between the DC voltage target value and the detected voltage value between the DC terminals of the voltage sensor. First switching control means for alternately turning on and off a second switch pair comprising elements ;
Depending on the deviation between the first DC voltage detection value and the second DC voltage detection value of the voltage sensor , in each of the first and second switch pairs, between an on period and an off period. And a second switching control means for providing a midpoint potential correction period for fixing one of the switching elements to ON while fixing the other switching element to OFF . The second switching control means includes:
When the first DC voltage detection value is larger than the second DC voltage detection value , the first switching element is fixed to ON during the midpoint potential correction period for the first switch pair. On the other hand, the fourth switching element is fixed to OFF, and the third switching element is fixed to ON during the midpoint potential correction period for the second switch pair, while the second switching element is The power conversion device according to claim 1, wherein a current circulation path for discharging the first capacitor is formed between the smoothing circuit and the power system by fixing a switching element to be off . The second switching control means includes:
When the second DC voltage detection value is larger than the first DC voltage detection value , the fourth switching element is fixed to ON during the midpoint potential correction period for the first switch pair. While the first switching element is fixed off and the second switching element is fixed on during the midpoint potential correction period for the second switch pair, while the third switching element The power conversion device according to claim 1 , wherein a current circulation path for discharging the second capacitor is formed between the smoothing circuit and the power system by fixing a switching element to be off . The first switching control means sets a duty ratio of each of the first and second switch pairs according to a deviation between the DC voltage target value and the DC terminal voltage detection value,
The second switching control means calculates the midpoint potential correction period based on a deviation between the first DC voltage detection value and the second DC voltage detection value, and calculates the calculated midpoint potential correction. The power converter according to claim 2 or 3 which corrects said duty ratio according to a period .   The power converter according to any one of claims 1 to 4, further comprising a wiring that connects a neutral point of the smoothing circuit and a neutral point of the power system. A second power conversion circuit connected to the DC terminal of the smoothing circuit and converting AC power of the power system into DC power of the smoothing circuit ;
A second control device for controlling a power conversion operation of the second power conversion circuit ,
The second power conversion circuit is connected in series between the DC positive bus and the DC negative bus, and the fifth and sixth switching elements connected in series between the DC positive bus and the DC negative bus. And a full bridge circuit including the seventh and eighth switching elements ,
The second control device includes :
Control to turn on / off the third switch pair composed of the fifth and seventh switching elements at a duty ratio corresponding to a deviation between the DC voltage target value and the DC terminal voltage detection value, and the duty ratio And third switching control means configured to be able to execute control to turn on and off the fourth switch pair composed of the sixth and eighth switching elements ,
The third switching control means includes:
When the first DC voltage detection value is larger than the second DC voltage detection value, control to turn on / off the third switch pair is executed, and the second DC voltage detection value is The power conversion device according to claim 1 , wherein control is performed to turn on and off the fourth switch pair when the detected value is greater than the first DC voltage detection value . A method for controlling a power converter connected to a single-phase three-wire power system,
The power converter is
A smoothing circuit comprising first and second capacitors connected in series between a DC positive bus and a DC negative bus;
A first power conversion circuit connected to a DC terminal of the smoothing circuit, for converting DC power of the smoothing circuit into AC power of the power system;
Detects a DC terminal voltage of the smoothing circuit, a first DC voltage between the DC positive bus and a midpoint of the smoothing circuit, and a second DC voltage between the DC negative bus and a midpoint of the smoothing circuit. Including a voltage sensor,
A midpoint of the smoothing circuit is electrically connected to a neutral point of the power system,
The first power conversion circuit is connected in series between the first and second switching elements connected in series between the DC positive bus and the DC negative bus, and between the DC positive bus and the DC negative bus. And a full bridge circuit including third and fourth switching elements,
The control method includes a step of controlling a power conversion operation of the first power conversion circuit,
The step of controlling the first power conversion circuit includes:
The first switch pair composed of the first and fourth switching elements and the second and third switching elements with a duty ratio corresponding to the deviation between the DC voltage target value and the detected voltage value between the DC terminals of the voltage sensor. Alternately turning on and off a second switch pair of elements;
Depending on the deviation between the first DC voltage detection value and the second DC voltage detection value of the voltage sensor, in each of the first and second switch pairs, between an on period and an off period. And a step of providing a midpoint potential correction period for fixing one switching element to be turned on while fixing the other switching element to be turned off . The step of providing the midpoint potential correction period includes the step of providing the midpoint potential correction period for the first switch pair when the first DC voltage detection value is greater than the second DC voltage detection value. The first switching element is fixed to ON while the fourth switching element is fixed to OFF, and the third switching element is turned on during the midpoint potential correction period for the second switch pair. The current circulation path for discharging the first capacitor is formed between the smoothing circuit and the power system by fixing the second switching element to OFF while fixing the second switching element to OFF. the method of the power conversion apparatus according to 7. The step of providing the midpoint potential correction period includes the step of providing the midpoint potential correction period for the first switch pair when the second DC voltage detection value is greater than the first DC voltage detection value. The fourth switching element is fixed to ON while the first switching element is fixed to OFF, and the second switching element is turned on during the midpoint potential correction period for the second switch pair. The current circulation path for discharging the second capacitor is formed between the smoothing circuit and the power system by fixing the third switching element to OFF while fixing the third switching element to OFF. The control method of the power converter device of Claim 7 . The step of alternately turning on and off sets a duty ratio of each of the first and second switch pairs according to a deviation between the DC voltage target value and the DC terminal voltage detection value,
The step of providing the midpoint potential correction period calculates the midpoint potential correction period based on a deviation between the first DC voltage detection value and the second DC voltage detection value, and the calculated midpoint The method of controlling a power converter according to claim 8 or 9, wherein the duty ratio is corrected according to a potential correction period. The power conversion device further includes a second power conversion circuit that is connected to a DC terminal of the smoothing circuit and converts AC power of the power system into DC power of the smoothing circuit,
The second power conversion circuit is connected in series between the DC positive bus and the DC negative bus, and the fifth and sixth switching elements connected in series between the DC positive bus and the DC negative bus. And a full bridge circuit including the seventh and eighth switching elements,
The control method further includes a step of controlling a power conversion operation of the second power conversion circuit,
The step of controlling the second power conversion circuit includes:
Control to turn on / off the third switch pair composed of the fifth and seventh switching elements at a duty ratio corresponding to a deviation between the DC voltage target value and the DC terminal voltage detection value, and the duty ratio The fourth switch pair including the sixth and eighth switching elements can be controlled to be turned on / off, and the first DC voltage detection value is larger than the second DC voltage detection value. In this case, control for turning on and off the third switch pair is executed, and when the second DC voltage detection value is larger than the first DC voltage detection value, the fourth switch pair is controlled. The control method of the power converter device according to claim 7, wherein control for turning on / off is executed.

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