JP2008011665A - Power converting system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a power converting system which is preferable to attain the system linkage of a fuel cell to a system power supply. <P>SOLUTION: In a DC-DC converter of a full-bridge system using high-frequency insulating transformers, primary sides of the transformers are connected in a two-parallel manner. Secondary sides of the transformers are connected in a two-series manner. Secondary side outputs which are connected in series and formed into one system are connected to one set of rectifier circuits of a next stage. Furthermore, an input current of the DC-DC converter is controlled by repetitive control. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a power conversion device. The present invention particularly relates to a power conversion device that converts a direct current into a direct current having different characteristics.

  As an alternative to conventional mainstream power generation methods, power generation methods exemplified by fuel cells and solar cells have attracted attention. Fuel cells and solar cells are suitable for generating low-voltage and large-capacity power. Such power is desired to be linked to a higher voltage system. A technique for converting low-voltage and large-capacity power into high-voltage power that can be connected to the grid is desired.

  Japanese Patent Laid-Open No. 2004-26883 discloses a grid interconnection that aims to reduce power loss in a grid interconnection inverter that inputs a power source having a low voltage and a large current, such as a fuel cell, and to improve output current ripple of a DC power source. An inverter device is described.

Patent Document 2 discloses a DC-DC converter for reducing switching loss and enabling the use of a low breakdown voltage MOS-FET having a low on-resistance when a MOS-FET is used as a switching element. Is described.
JP 2004-274893 A Japanese Patent No. 3463807

An object of the present invention is to provide a power conversion device suitable for boosting the voltage of a low-voltage, large-current power supply.
Another object of the present invention is to provide a power conversion device suitable for boosting the voltage of a low-voltage, large-current power supply with high efficiency.
Still another object of the present invention is to provide a compact power converter that boosts the voltage of a low-voltage, large-current power supply.
Still another object of the present invention is to provide a power conversion device that suppresses input current ripple of a low-voltage, high-current power supply and boosts the voltage.

  In the following, means for solving the problem will be described using the numbers used in [Best Mode for Carrying Out the Invention] in parentheses. These numbers are added to clarify the correspondence between the description of [Claims] and [Best Mode for Carrying Out the Invention]. However, these numbers should not be used to interpret the technical scope of the invention described in [Claims].

  In a full bridge type DC-DC converter using a high-frequency insulating transformer, the primary side of the transformer is connected in parallel. Two secondary sides of the transformer are connected in series. The secondary outputs connected in series and combined into one system are connected to a set of rectifier circuits in the next stage. Furthermore, the input current of the DC-DC converter is controlled by repetitive control.

  A current converter according to the present invention includes a first converter (14) that converts a direct current supplied from a power supply (2) into a high-frequency current by a first full bridge circuit and supplies the high-frequency current to a first terminal pair; A second converter (16) that converts a direct current supplied in parallel with the first converter (14) from 2) into a high-frequency current by a second full-bridge circuit and supplies the high-frequency current to the second terminal pair; A first transformer part (HTr1) having a primary winding connected to the terminal pair and a second transformer part (HTr2) having a primary winding connected to the second terminal pair are provided. The secondary winding of the first transformer unit (HTr1) and the secondary winding of the second transformer unit (HTr2) are connected in series to output an output current to the lower stage.

  The power conversion device according to the present invention includes a rectification unit (20) that rectifies an output current.

  In the power converter according to the present invention, the direct current is supplied from the power source (2) to the first full bridge circuit and the second full bridge circuit without passing through the reactor.

  In the power conversion device according to the present invention, the first full bridge circuit is configured using a MOSFET.

  In the power converter according to the present invention, the current waveform generated by the first full bridge circuit and the current waveform generated by the second full bridge circuit have the same waveform and are out of phase.

  The power conversion device according to the present invention includes a first conversion unit (14) and a second conversion unit (16) so that the magnitude of the current supplied from the power source (2) to the power conversion device approaches the current command value. A repetitive control unit (34) that performs repetitive control on the switching operation is provided.

  The power conversion device according to the present invention includes interconnection units (22, 24, 26, and 28) that convert an output current so as to be linked to a current supplied from the system power supply (30).

  The power conversion device according to the present invention performs switching operations between the first conversion unit (14) and the second conversion unit (16) so that the current supplied from the power source (2) to the power conversion device approaches the current command value. A repetitive control unit (34) that performs repetitive control at a predetermined frequency is provided. The predetermined frequency is at least an integral multiple of the frequency of the system power supply (30), preferably at least twice.

  The grid interconnection system according to the present invention includes a power source (34) for generating a direct current, and a direct current supplied from the power source (34) is converted into a high-frequency current by a first full bridge circuit and supplied to the first terminal pair. The direct current supplied in parallel with the first converter (14) and the first converter (14) from the power source (34) is converted into a high-frequency current by the second full bridge circuit and supplied to the second terminal pair. A second transformer (16), a first transformer (HTr1) having a primary winding connected to the first terminal pair, and a first transformer having a primary winding connected to the second terminal pair. 2 transformer section (HTr2). The secondary winding of the first transformer unit (HTr1) and the secondary winding of the second transformer unit (HTr2) are connected in series to output an output current to the lower stage. The grid interconnection system further includes a linkage unit (22, 24, 26, 28) that converts the output current so as to be linked to the current supplied by the grid power supply (30).

  In the grid interconnection system according to the present invention, the power source (2) includes a fuel cell.

ADVANTAGE OF THE INVENTION According to this invention, the power converter device suitable for boosting the voltage of the power supply of a low voltage large current is provided.
Furthermore, according to the present invention, there is provided a power conversion device suitable for boosting the voltage of a low-voltage, large-current power supply with high efficiency.
Furthermore, according to the present invention, there is provided a compact power conversion device that boosts the voltage of a low-voltage, high-current power supply.
Furthermore, according to the present invention, there is provided a power conversion device that boosts a voltage by suppressing input current ripple of a power source of low voltage and large current.

  The best mode for carrying out the present invention will be described below with reference to the drawings. FIG. 1 shows a grid interconnection circuit using the power conversion device according to the present embodiment. The grid interconnection circuit is connected to the fuel cell 2. The fuel cell 2 may be any type such as a solid oxide type (SOFC), a molten carbonate type (MCFC), a solid polymer type (PEFC), a phosphoric acid type (PAFC), a direct methanol type (DMFC), and the like. Instead of the fuel cell 2, it may be connected to a device that generates a direct current that can easily obtain other types of low-voltage high-current, for example, a secondary battery or a solar cell. Moreover, although the system | strain linked in FIG. 1 is shown with the single phase power supply, a three phase power supply may be sufficient.

  An LC filter 4 is connected between the positive electrode and the negative electrode of the fuel cell 2. A DC-DC converter 12 is connected to the positive-side wiring L1 and the negative-side wiring L2 on the rear stage side of the LC filter 4. The DC-DC converter 12 includes a first conversion unit 14, a second conversion unit 16, a transformer unit 18 and a rectification unit 20. The DC-DC converter 12 converts the direct current generated by the fuel cell 2 and supplied through the LC filter 4 into a higher-voltage direct current and outputs it to the subsequent stage side. A smoothing capacitor 22 is connected to the rear stage side of the DC-DC converter 12. An inverter circuit 24 that converts direct current supplied from the DC-DC converter 12 via the smoothing capacitor 22 to alternating current is connected to the subsequent stage of the smoothing capacitor 22. An AC filter 26 that reduces AC harmonic components supplied from the inverter circuit 24 is connected to the rear stage side of the inverter circuit 24. The downstream side of the AC filter 26 is connected to, for example, a single-phase 200 V system power supply 30 via a connection relay 28.

  The wiring L1 is provided with an ammeter 6 that detects a current flowing through the wiring L1 and generates a current detection value DCI. The grid interconnection circuit includes a subtracter 8. The subtracter 8 outputs the difference between the current command value DCI * input from an external circuit and the current detection value DCI. The grid interconnection circuit further includes a current controller 10. The current controller 10 controls the switching elements of the first conversion unit 14 and the second conversion unit 16 based on the difference output from the subtracter 8.

  FIG. 2 shows a circuit of the DC-DC converter 12. The first conversion unit 14 includes a full bridge circuit including switching elements G1, G2, G3, and G4. The second conversion unit 16 includes a full bridge circuit including switching elements G5, G6, G7, and G8. As the switching elements G1 to G8, an IGBT, a bipolar transistor, a MOSFET, or the like can be used. In the present embodiment, it is preferable to use a MOSFET whose on-voltage is smaller than that of IGBT in that it is important to obtain a high step-up ratio.

  The drain side of the switching element G1 is connected to the wiring L1. The source side of the switching element G1 is connected to the drain side of the switching element G2. A node u1 is provided between the source side of the switching element G1 and the drain side of the switching element G2. The source side of the switching element G2 is connected to the wiring L2.

  The drain side of the switching element G3 is connected to the wiring L1. The source side of the switching element G3 is connected to the drain side of the switching element G4. A node v1 is provided between the switching element G3 and the switching element G4. The source side of the switching element G4 is connected to the wiring L2.

  The drain side of the switching element G5 is connected to the wiring L1. The source side of the switching element G5 is connected to the drain side of the switching element G6. A node u2 is provided between the switching element G5 and the switching element G6. The source side of the switching element G6 is connected to the wiring L2.

  The drain side of the switching element G7 is connected to the wiring L1. The source side of the switching element G7 is connected to the drain side of the switching element G8. A node v2 is provided between the switching element G7 and the switching element G8. The source side of the switching element G8 is connected to the wiring L2.

  The transformer unit 18 includes high-frequency insulating transformers HTr1 and HTr2. The iron core of the high frequency insulation transformer HTr1 and the iron core of the high frequency insulation transformer HTr2 are separated. The node u1 is connected to the first end of the primary winding of the high frequency insulation transformer HTr1. The node v1 is connected to the second end of the primary winding of the high frequency insulation transformer HTr1. The node u2 is connected to the first end of the primary winding of the high frequency insulation transformer HTr2. The node v2 is connected to the second end of the primary winding of the high frequency insulation transformer HTr2. That is, the first conversion unit 14 and the second conversion unit 16 are connected in parallel between the wirings L1 and L2 and the transformer unit 18.

  Thus, the booster circuit using the full bridge circuit and the high-frequency insulating transformer is suitable for obtaining a large boost ratio. Therefore, it is suitable for a power converter that boosts the voltage of the fuel cell 2 that is a power source of low voltage and large current for grid connection. If a large step-up ratio is obtained, the grid connection can be established using the fuel cell 2 with a small number of cells, so that the cost is reduced. Since the primary side of the transformer unit 18 is parallel, if the input current of the DC-DC converter 12 is the same, the current flowing through each transformer is smaller than when the voltage is boosted by a single harmonic isolation transformer. Therefore, copper loss can be reduced when windings having the same wire diameter are used.

  The size of the iron core of the transformer needs to be selected so that the maximum value of the magnetic flux density falls within a certain upper limit determined by the material. Since the transformer unit 18 includes two transformers, two small iron cores are used, and the overall size (the transformer unit inside the DC-DC converter 12 is compared with the case where a single iron core is used. 18 occupied volume) is reduced.

  The first end of the secondary winding of the high frequency isolation transformer HTr1 (the side corresponding to the first end of the primary winding) and the second end of the secondary winding of the high frequency isolation transformer HTr2 (the second end of the primary winding) Is connected to the rectifier 20 which is a full-wave rectifier circuit. The second end of the secondary winding of the high-frequency isolation transformer HTr1 (the side corresponding to the second end of the primary winding) and the first end of the secondary winding of the high-frequency isolation transformer HTr2 (the first end of the primary winding) Are connected to each other. That is, the high-frequency insulating transformer HTr1 and the high-frequency insulating transformer HTr2 are connected in series on the secondary winding side and output an output current to the lower stage side. A rectifying unit 20 that rectifies the output current is connected to the lower side.

  Since two transformers are connected in series on the secondary side of the transformer unit 18, the turn ratio of the secondary side to the primary side of each transformer is half that of a single transformer. For this reason, in the case of the same winding, the resistance is halved. Further, since the two transformers are connected in series on the secondary side of the transformer 18, the circulating current generated between the two transformers can be eliminated in principle. Thereby, it is not necessary to individually control the current for each of the two transformers, and the current can be controlled by one controller.

  The transformer unit 18 may include three or more high-frequency insulating transformers. In that case, the primary side is connected to three or more converters connected in parallel. Three or more secondary windings are connected in series.

  FIG. 3 is a diagram for explaining the current controller 10. A difference between the current command value DCI * generated by the subtractor 8 and the current detection value DCI is input to the current controller 10. The current controller 10 includes a proportional-integral compensator 32 and outputs a control signal so as to execute proportional-integral control on the current value. The current controller 10 further includes a repeat compensator 34. The repetitive compensator 34 outputs a control signal for performing repetitive control so as to reduce the ripple of the input current input to the DC-DC converter 12 at a predetermined frequency. The predetermined frequency is preferably at least an integral multiple of the frequency of the system power supply 30, and preferably at least 2 times, for example, 2.5 times. The control signal output from the proportional-integral compensator 32 and the control signal output from the repeat compensator 34 are added by an adder 36 and output as a combined control signal. The controller 10 controls the switching timing of the switching elements G1 to G8 using the composite control signal.

  By adopting repetitive control, a reduction in input current ripple is achieved. In particular, a large boost ratio and a small input current ripple can be achieved at the same time by combining with a boost circuit using a full bridge circuit and a high frequency isolation transformer in the present embodiment. If the input current ripple is reduced, the reactor of the LC filter 4 installed in the input stage of the DC-DC converter 12 can be omitted. That is, the direct current from the fuel cell 2 is supplied to the full bridge circuit of the first converter 14 and the full bridge circuit of the second converter 16 without going through the reactor. Therefore, the loss generated in the reactor can be eliminated, and the conversion efficiency of the DC-DC converter is improved.

  The power conversion device having the above configuration operates as follows. The fuel cell 2 generates a direct current. The voltage of the direct current is, for example, about 40V to 70V. The direct current is reduced in noise in the LC filter 26 and input to the DC-DC converter 12.

  The ammeter 6 outputs a current detection value DCI. A current command value DCI * is given from the outside of the power converter. The subtracter 8 calculates and outputs the difference between the current command value DCI * and the current detection value DCI. The current controller 10 outputs a composite control signal for performing proportional integral compensation and repetitive compensation using the difference.

  Switching elements G <b> 1 to G <b> 8 are operated by a composite control signal from the current control unit 10. When the switching elements G1 and G4 are turned on at the same time and the switching elements G2 and G3 are turned off at the same time, the first end of the primary winding of the high-frequency insulating transformer HTr1 becomes a positive voltage and the second end becomes a negative voltage. When the switching elements G1 and G4 are turned off at the same time and the switching elements G2 and G3 are turned on at the same time, the first end of the primary winding of the high-frequency insulating transformer HTr1 becomes a negative voltage and the second end becomes a positive voltage. An alternating magnetic field generated by this repetition generates a high-voltage alternating current with respect to the primary side in the secondary winding of the high-frequency insulating transformer HTr1.

  When the switching elements G5 and G8 are turned on at the same time and the switching elements G6 and G7 are turned off at the same time, the first end of the primary winding of the high-frequency insulating transformer HTr2 becomes a positive voltage and the second end becomes a negative voltage. When the switching elements G5 and G8 are turned on at the same time and the switching elements G6 and G7 are turned off at the same time, the first end of the primary winding of the high frequency insulating transformer HTr2 becomes a negative voltage and the second end becomes a positive voltage. An alternating magnetic field generated by this repetition generates a high-voltage alternating current with respect to the primary side in the secondary winding of the high-frequency insulating transformer HTr2.

  The alternating current between the secondary winding of the high-frequency isolation transformer HTr1 and the secondary winding of the high-frequency isolation transformer HTr2 connected in series is converted into a direct current by the rectifier 20 and supplied to the subsequent stage side of the DC-DC converter 12. . The DC voltage is 360V, for example. The direct current is smoothed by the smoothing capacitor 22 and converted into alternating current by the inverter circuit 24. Further, the harmonic component is reduced by the AC filter 26, and the interconnection with the system power supply 30 is made via the interconnection relay 28.

  FIG. 4 is a timing chart showing an example of the operation of the switching elements G1 to G8. The operations of the switching elements G1 to G8 are controlled using a sawtooth carrier wave. The current controller 10 generates a first carrier wave for controlling the switching elements G1 to G4 and a second carrier wave for controlling the switching elements G5 to G8.

  The sawtooth waveform shown in FIG. 4A shows the waveform of the first carrier wave. The first carrier wave has a periodic waveform, and one period is included at times t1 to t4. At time t1, the magnitude of the first carrier wave signal is zero. In the period from time t1 to t4, the magnitude of the first carrier wave signal increases linearly and reaches a predetermined maximum value at time t4. At time t4, the magnitude of the first carrier wave signal suddenly decreases in a pulse manner and reaches zero. That is, the first carrier wave has a sawtooth shape that rises to the right.

  The sawtooth waveform shown in FIG. 4B shows the waveform of the second carrier wave. The second carrier wave has a periodic waveform, and one period is included at times t1 to t4 that are equal to the first carrier wave. At time t1, the magnitude of the first carrier wave signal suddenly rises from zero and reaches a predetermined maximum value that is set to be the same as that of the first carrier wave. In the period from time t1 to time t4, the magnitude of the second carrier wave signal decreases linearly and reaches zero at time t4. At time t4, the magnitude of the signal of the second carrier wave suddenly increases in a pulse manner and reaches a predetermined maximum value. That is, the second carrier wave has a sawtooth shape with a downward slope. The fall time of the first carrier wave is equal to the rise time of the second carrier wave.

  The current controller 10 switches the switching elements G1 to G4 using the result of comparing the magnitude of the first carrier wave signal shown in FIG. 4A and the command value (the magnitude of the above-described combined control signal). Control. When the command value is larger (time t1 to t3), the switching elements G1 and G4 are controlled to be on, and the switching elements G2 and G3 are controlled to be off. When the magnitude of the carrier wave signal is larger (time t3 to t4), the switching elements G1 and G4 are controlled to be off, and the switching elements G2 and G3 are controlled to be on.

  The current controller 10 controls the switching elements G5 to G8 using the result of comparing the magnitude of the second carrier wave signal shown in FIG. 4 (b) with the same command value as in FIG. 4 (a). To do. When the magnitude of the carrier wave signal is larger (time t1 to t2), the switching elements G5 and G8 are controlled to be off, and the switching elements G6 and G7 are controlled to be on. When the command value is larger (time t2 to t4), the switching elements G5 and G8 are controlled to be on and the switching elements G6 and G7 are controlled to be off.

  According to such control, the ratio of the time that the switching elements G1 and G4 are on is equal to the ratio of the time that the switching elements G5 and G8 are on. The proportion of time that the switching elements G2 and G3 are on is equal to the proportion of time that the switching elements G6 and G7 are on. The current waveform generated by the first converter 14 and the current waveform generated by the second converter 16 have the same waveform and are out of phase. According to such control, there is a time (t1 to t2, t3 to t4) in which the on / off of the switching elements G1 and G4 and the on / off of the switching elements G5 and G8 are reversed. At the same time, on / off of switching elements G2 and G3 and on / off of switching elements G6 and G7 are reversed. Therefore, the effect that current ripple is reduced is achieved.

  FIG. 5 is a timing chart showing another example of the operation of the switching elements G1 to G8. The operations of the switching elements G1 to G8 are controlled using a sawtooth carrier wave. The current controller 10 generates a first carrier wave for controlling the switching elements G1 to G4 and a second carrier wave for controlling the switching elements G5 to G8.

  The sawtooth waveform shown in FIG. 5A shows the waveform of the third carrier wave. The third carrier wave has a periodic waveform, and one period is included at times t11 to t15. At time t11, the magnitude of the third carrier wave signal is zero. In the period from time t11 to t15, the magnitude of the third carrier wave signal increases linearly and reaches a predetermined maximum value at time t15. At time t15, the magnitude of the third carrier wave signal suddenly decreases in a pulse manner and reaches zero. That is, the third carrier wave has a sawtooth shape that rises to the right.

  The sawtooth waveform shown in FIG. 5B shows the waveform of the fourth carrier wave. The fourth carrier wave is homologous with the third carrier wave and has the same period. The third carrier wave and the fourth carrier wave are out of phase.

  The current controller 10 switches the switching elements G1 to G4 using the result of comparing the magnitude of the third carrier wave signal shown in FIG. 5A and the command value (the magnitude of the above-described combined control signal). Control. When the command value is larger (time t11 to t14), the switching elements G1 and G4 are controlled to be on, and the switching elements G2 and G3 are controlled to be off. When the magnitude of the carrier wave signal is larger (time t14 to t15), the switching elements G1 and G4 are controlled to be off, and the switching elements G2 and G3 are controlled to be on.

  The current controller 10 controls the switching elements G5 to G8 using the result of comparing the magnitude of the fourth carrier wave signal shown in FIG. 5 (b) with the same command value as in FIG. 5 (a). To do. When the command value is larger (time to t12), the switching elements G5 and G8 are controlled to be on and the switching elements G6 and G7 are controlled to be off. When the magnitude of the carrier wave signal is larger (time t12 to t13), the switching elements G5 and G8 are controlled to be off, and the switching elements G6 and G7 are controlled to be on.

  According to such control, the ratio of the time that the switching elements G1 and G4 are on is equal to the ratio of the time that the switching elements G5 and G8 are on. The proportion of time that the switching elements G2 and G3 are on is equal to the proportion of time that the switching elements G6 and G7 are on. The current waveform generated by the first converter 14 and the current waveform generated by the second converter 16 have the same waveform and are out of phase. According to such control, there is a time (˜t11, t12 to t13) in which the on / off of the switching elements G1 and G4 and the on / off of the switching elements G5 and G8 are reversed. At the same time, on / off of switching elements G2 and G3 and on / off of switching elements G6 and G7 are reversed. Therefore, the effect that the current ripple is reduced as in the example of FIG. 4 is achieved.

FIG. 1 shows a system interconnection circuit using a DC-DC converter. FIG. 2 shows a circuit of a DC-DC converter. FIG. 3 is a diagram for explaining the current controller. FIG. 4 is a timing chart showing the operation of the switching element. FIG. 5 is a timing chart showing the operation of the switching element.

Explanation of symbols

2 ... Fuel cell 4 ... Input filter 6 ... Ammeter 8 ... Subtractor 10 ... Current controller 12 ... DC-DC converter 14 ... First converter 16 ... Second converter 18 ... Transformer 20 ... Rectifier 22 ... Smoothing Capacitor 24 ... Inverter circuit 26 ... AC filter 28 ... Interconnection relay 30 ... System power supply G1-G8 ... Switching elements HTr1, HTr2 ... High frequency insulation transformer 32 ... Proportional integral compensator 34 ... Repetitive compensator 36 ... Adder

Claims (11)

A first converter that converts a direct current supplied from a power source into a high-frequency current by a first full bridge circuit and supplies the high-frequency current to the first terminal pair;
A second converter that converts a direct current supplied in parallel with the first converter from the power source into a high-frequency current by a second full-bridge circuit and supplies the high-frequency current to the second terminal pair;
A first transformer unit including a primary winding connected to the first terminal pair;
A second transformer part having a primary winding connected to the second terminal pair,
The secondary winding of the first transformer unit and the secondary winding of the second transformer unit are connected in series and output an output current to the lower stage. The power conversion device according to claim 1,
And a rectifier configured to rectify the output current. The power conversion device according to claim 1 or 2,
The DC current is supplied from the power source to the first full bridge circuit and the second full bridge circuit without going through a reactor. The power conversion device according to any one of claims 1 to 3,
The first full bridge circuit is a power conversion device configured using a MOSFET. The power conversion device according to any one of claims 1 to 4,
The current conversion waveform generated by the first full bridge circuit and the current waveform generated by the second full bridge circuit are out of phase. The power conversion device according to any one of claims 1 to 5,
Further, a repetitive control unit that performs repetitive control on the switching operation of the first conversion unit and the second conversion unit so that the magnitude of the current supplied from the power source to the power conversion device approaches the current command value. A power conversion device comprising: The power conversion device according to any one of claims 1 to 6,
Furthermore, the power converter device which comprises the connection part which converts the said output current so that it may connect with the electric current which a system power supply supplies. The power conversion device according to claim 7,
Further, iterative control for performing repetitive control at a predetermined frequency for the switching operation of the first conversion unit and the second conversion unit so that the current supplied from the power source to the power conversion device approaches the current command value. Comprising
The predetermined frequency is an integer multiple or more of the frequency of the system power supply. The power conversion device according to claim 8, wherein
Further, iterative control for performing repetitive control at a predetermined frequency for the switching operation of the first conversion unit and the second conversion unit so that the current supplied from the power source to the power conversion device approaches the current command value. Comprising
The predetermined frequency is at least twice the frequency of the system power supply. A power source that generates direct current,
A first converter that converts a direct current supplied from the power source into a high-frequency current by a first full bridge circuit and supplies the high-frequency current to the first terminal pair;
A second converter that converts a direct current supplied in parallel with the first converter from the power source into a high-frequency current by a second full-bridge circuit and supplies the high-frequency current to the second terminal pair;
A first transformer unit including a primary winding connected to the first terminal pair;
A second transformer part having a primary winding connected to the second terminal pair,
The secondary winding of the first transformer unit and the secondary winding of the second transformer unit are connected in series to output an output current to the lower stage,
Furthermore, the grid connection system which comprises the interconnection part which converts the said output current so that it may link with the electric current which a system power supply supplies. A grid interconnection system according to claim 10, wherein
The power source includes a fuel cell.

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