Energy storage system

Abstract

To provide a power storage system which is capable of regulating a voltage of a neutral conductor and improved in efficiency further than the prior arts.SOLUTION: A power storage system 10 comprises an inverter 40 connected to a single-phase three-wire type power system, a bidirectional DC / DC converter 20, and a control section 50. The control section 50 alternately performs discharge of a first capacitor C1 and charge of a second capacitor C2 by turning on / turning off switch elements SW1, SW3 and SW5 in a case (1) where a first voltage V1 is higher than a second voltage V2, and alternately performs charge of the first capacitor C1 and discharge of the second capacitor C2 by turning on / turning off switch elements SW2, SW4 and SW5 in a case (2) where the first voltage V1 is lower than the second voltage V2.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a power storage system that is connected to a storage battery and a single-phase three-wire commercial power system for use.

Background Art

[0002] As a power conversion device used by being connected to a single-phase three-wire commercial power system, a power conversion device 100 for solar power generation shown in FIG. 6 is known (see Patent Document 1). This power conversion device 100 includes a boost chopper circuit (DC / DC converter) 102 including capacitors Ca and Cb that boosts the voltage output from the solar cell 101 to the voltage required by the inverter 103. According to this power conversion device 100, by using capacitors Ca and Cb of the same capacity, the voltage after boosting can be divided equally.

[0003] However, when the U-phase - O-phase (neutral phase) load and the W-phase - O-phase load connected to the AC output terminal side of the inverter 103 of this power conversion device 100 become unbalanced, the voltage after boosting cannot be divided equally. For example, when the U-phase - O-phase load is larger than the W-phase - O-phase load, the power conversion device 100 outputs a lower voltage than usual to the U-phase - O-phase load and a higher voltage than usual to the W-phase - O-phase load.

[0004] Patent Document 2 describes a power conversion device (power storage system) intended to suppress the above unbalance. As shown in FIG. 7, this power storage system 200 includes a DC / DC converter 202 that boosts the output voltage of the power generation unit 201 (such as a storage battery), an inverter 204 that converts the boosted voltage into an AC voltage and outputs this through a filter 205, and a neutral line voltage adjustment circuit 203 connected between these.

[0005] The neutral wire voltage adjustment circuit 203 includes adjustment switch elements SWa and SWb connected in series, and capacitors Ca and Cb connected in series. This neutral wire voltage adjustment circuit 203 allows the voltage across capacitor Ca and the voltage across capacitor Cb to be matched by switching the adjustment switch elements SWa and SWb on and off to charge or discharge capacitors Ca and Cb. In other words, this neutral wire voltage adjustment circuit 203 allows the voltage between the U-phase and O-phase to be matched with the voltage between the W-phase and O-phase.

[0006] However, a problem with this energy storage system 200 was that when the output voltage of the power generation unit 201 (storage battery) was relatively low, the boost ratio in the DC / DC converter 202 increased, reducing the efficiency of the DC / DC converter 202. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 9-65657 [Patent Document 2] Japanese Patent Publication No. 2021-93861 [Overview of the project] [Problems that the invention aims to solve]

[0008] The present invention has been made in view of the above circumstances, and aims to provide an energy storage system that can adjust the voltage of the neutral wire and is more efficient than conventional systems. [Means for solving the problem]

[0009] To solve the above problems, a power storage system according to one aspect of the present invention comprises an inverter whose AC side input / output terminals are connected to a single-phase three-wire power system, a bidirectional DC / DC converter whose first input / output terminals are connected to a storage battery and whose second input / output terminals are connected to the DC side input / output terminals of the inverter, and a control unit that controls the bidirectional DC / DC converter, wherein the bidirectional DC / DC converter comprises a first power line connected to the first input / output terminals and the second input / output terminals, a second power line having a lower potential than the first power line, a third power line connected to the neutral terminal constituting the AC side input / output terminals of the inverter, and a third power line connected to the first power line and the third power line The control unit includes a capacitor, a second capacitor connected to a second power line and a third power line, a first voltmeter for measuring the first voltage, which is the voltage across the first capacitor, a second voltmeter for measuring the second voltage, which is the voltage across the second capacitor, and a plurality of switch elements. The control unit has the following configuration: (1) When the first voltage is greater than the second voltage, it alternately discharges the first capacitor and charges the second capacitor by switching the switch elements on and off; and (2) When the first voltage is less than the second voltage, it alternately charges the first capacitor and discharges the second capacitor by switching the switch elements on and off.

[0010] With this configuration, if a discrepancy occurs between the first voltage, which is the voltage across the first capacitor connected between the first and third power lines, and the second voltage, which is the voltage across the second capacitor connected between the second and third power lines, the discrepancy can be resolved by discharging one of the capacitors while charging the other. In other words, with this configuration, the potential of the third power line can be set to a value exactly midway between the potentials of the first and second power lines.

[0011] Furthermore, in order to solve the above problems, a power storage system according to another aspect of the present invention comprises an inverter whose AC side input / output terminals are connected to a single-phase three-wire power system, a bidirectional DC / DC converter whose first input / output terminals are connected to a storage battery and whose second input / output terminals are connected to the DC side input / output terminals of the inverter, and a control unit that controls the bidirectional DC / DC converter, wherein the bidirectional DC / DC converter comprises a first power line connected to the first input / output terminals and the second input / output terminals, a second power line having a lower potential than the first power line, a third power line connected to the neutral terminal constituting the AC side input / output terminals of the inverter, a first switch element interposed in the first power line, a second switch element interposed in the second power line, and a third power line connected to the first power line between the first switch element and the second input / output terminal The configuration includes a first capacitor connected to a power line, a second capacitor connected to a second power line and a third power line between a second switch element and a second input / output terminal, a third switch element connected to a first power line and a third power line between a first switch element and a first input / output terminal, a fourth switch element connected to a second power line and a third power line between a second switch element and a first input / output terminal, a fifth switch element and a third capacitor connected to a first power line between a first switch element and a first input / output terminal and a second power line between a second switch element and a first input / output terminal, and a freewheeling diode connected in parallel to each of the first, second, third, fourth, and fifth switch elements.

[0012] With this configuration, if a discrepancy occurs between the first voltage, which is the voltage across the first capacitor connected between the first and third power lines, and the second voltage, which is the voltage across the second capacitor connected between the second and third power lines, the discrepancy can be resolved by discharging one of the capacitors while charging the other. In other words, with this configuration, the potential of the third power line can be set to a value exactly midway between the potentials of the first and second power lines.

[0013] Furthermore, this configuration allows for a lower step-down ratio in the bidirectional DC / DC converter when charging the battery compared to stepping down the voltage between the first and second power lines to the voltage required by the battery. Therefore, this configuration enables highly efficient charging of the battery.

[0014] Furthermore, this configuration allows for a lower boost ratio in the bidirectional DC / DC converter when discharging the battery compared to boosting the battery output voltage to the voltage required by the inverter. Therefore, this configuration enables highly efficient discharge of the battery.

[0015] The energy storage system relating to one aspect and the energy storage system relating to the other aspect may further include a DC / DC converter whose input terminal is connected to a solar cell and whose output terminal is connected to the DC input / output terminal of an inverter.

[0016] The bidirectional DC / DC converter of the energy storage system relating to the other aspect described above may further include a first voltmeter for measuring a first voltage, which is the voltage across the first capacitor; a second voltmeter for measuring a second voltage, which is the voltage across the second capacitor; and a third voltmeter for measuring a third voltage, which is the voltage across the third capacitor.

[0017] The control unit of the energy storage system relating to the other aspect described above may, for example, (1) control the system to alternately discharge the first capacitor and the second capacitor by turning the first and second switch elements on and off while keeping the third, fourth and fifth switch elements off so that the third voltage becomes the target voltage, and (2) alternate charging the first capacitor and the second capacitor by turning the third, fourth and fifth switch elements on and off while keeping the first and second switch elements off so that the first and second voltages match. (3) Control that alternates between the two, by switching the first, third, and fifth switch elements on and off while keeping the second and fourth switch elements off so that the first and second voltages match, thereby alternately discharging the first capacitor and charging the second capacitor; and (4) Control that alternates between charging the first capacitor and discharging the second capacitor by switching the second, fourth, and fifth switch elements on and off while keeping the first and third switch elements off so that the first and second voltages match. [Effects of the Invention]

[0018] According to the present invention, it is possible to adjust the voltage of the neutral wire and to provide a more efficient energy storage system than conventional ones. [Brief explanation of the drawing]

[0019] [Figure 1] This is a circuit diagram showing the energy storage system and its surrounding components according to the present invention. [Figure 2] This figure shows the battery charging control performed by the control unit of the energy storage system according to the present invention. [Figure 3] This figure shows the battery discharge control performed by the control unit of the energy storage system according to the present invention. [Figure 4] This figure shows the first balance adjustment control performed by the control unit of the energy storage system according to the present invention. [Figure 5] This figure shows the second balance adjustment control performed by the control unit of the energy storage system according to the present invention. [Figure 6]It is a circuit diagram showing a conventional power conversion device for solar power generation. [Figure 7] It is a circuit diagram showing a conventional power storage system provided with a neutral voltage adjustment circuit.

Embodiment for Carrying Out the Invention

[0020] Hereinafter, embodiments of the power storage system according to the present invention will be described with reference to the accompanying drawings.

[0021] FIG. 1 shows a power storage system 10 according to an embodiment of the present invention. As shown in the figure, the power storage system 10 includes an inverter 40 whose AC side input / output terminals 41a, 41b, 41c are connected to a single-phase three-wire system terminal 63 and a self-supporting output terminal 64 via a filter 62 and a relay group (system relay 65, bypass relay 66, and self-supporting relay 67), a bidirectional DC / DC converter 20 whose first input / output terminals 24a, 24b are connected to a storage battery 60 and whose second input / output terminals 25a, 25b are connected to the DC side input / output terminals 42a, 42b of the inverter 40, a DC / DC converter 30 whose input terminals 31a, 31b are connected to a solar cell 61 and whose output terminals 32a, 32b are connected to the DC side input / output terminals 42a, 42b of the inverter 40, a capacitor C4 connected to the DC side input / output terminals 42a, 42b of the inverter 40, and a control unit 50 that controls at least the bidirectional DC / DC converter 20.

[0022] When there is no abnormality such as a power failure in the commercial power system connected to the system terminal 63, the system relay 65 and the bypass relay 66 are in a closed state (on state), and the self-supporting relay 67 is in an open state (off state). Thereby, at least one of the system power, the power generated by the solar cell 61, and the discharge power of the storage battery 60 can be supplied to an important load connected to the self-supporting output terminal 64. Also, thereby, the storage battery 60 can be charged with at least one of the system power and the power generated by the solar cell 61.

[0023] When an abnormality occurs in the commercial power grid connected to the grid terminal 63, the grid relay 65 and the bypass relay 66 are set to the open state (off), and the independent relay 67 is set to the closed state (on). This allows at least one of the power generated by the solar cell 61 and the power discharged by the storage battery 60 to be supplied to the critical load connected to the independent output terminal 64.

[0024] The DC / DC converter 30 is configured to boost the voltage input to the input terminals 31a and 31b to the voltage required by the inverter 40, and to output the boosted voltage from the output terminals 32a and 32b.

[0025] The inverter 40 is configured to convert the voltage (DC voltage) input to the DC input / output terminals 42a and 42b into AC voltage, and output the converted AC voltage from the AC input / output terminals 41a and 41c. The inverter 40 is also configured to convert the voltage (AC voltage) input to the AC input / output terminals 41a and 41c into DC voltage, and output the converted DC voltage from the DC input / output terminals 42a and 42b. The AC input / output terminal (neutral terminal) 41b of the inverter 40 is connected to the third power line 28 of the bidirectional DC / DC converter 20, which will be described later.

[0026] The bidirectional DC / DC converter 20 is configured to boost the voltage input to the first input / output terminals 24a and 24b and output the boosted voltage from the second input / output terminals 25a and 25b. The bidirectional DC / DC converter 20 is also configured to step down the voltage input to the second input / output terminals 25a and 25b and output the step-down voltage from the first input / output terminals 24a and 24b.

[0027] Specifically, the bidirectional DC / DC converter 20 includes a first power line 26 connected to the first input / output terminal 24a and the second input / output terminal 25a, a second power line 27 connected to the first input / output terminal 24b and the second input / output terminal 25b and having a lower potential than the first power line 26, a third power line 28 connected to the AC side input / output terminal 41b of the inverter 40, a first switch element SW1 interposed in the first power line 26, a second switch element SW2 interposed in the second power line 27, a first capacitor C1 connected to the first power line 26 and the third power line 28 between the first switch element SW1 and the second input / output terminal 25a, a second capacitor C2 connected to the second power line 27 and the third power line 28 between the second switch element SW2 and the second input / output terminal 25b, and the first switch element SW1 and the first input / output The device comprises a third switch element SW3 connected to the first power line 26 and the third power line 28 between the power terminals 24a, a fourth switch element SW4 connected to the second power line 27 and the third power line 28 between the second switch element SW2 and the first input / output terminal 24b, a fifth switch element SW5 and a third capacitor C3 connected to the first power line 26 between the first switch element SW1 and the first input / output terminal 24a and the second power line 27 between the second switch element SW2 and the first input / output terminal 24b, a coil L interposed in the first power line 26 between the fifth switch element SW5 and the third capacitor C3, and freewheeling diodes D1, D2, D3, D4, D5 connected in parallel to each of the switch elements SW1, SW2, SW3, SW4, and SW5. The freewheeling diodes D1, D2, D3, D4, and D5 may be parasitic diodes (internal diodes) of the switching elements SW1, SW2, SW3, SW4, and SW5, or they may be external diodes configured separately from the switching elements SW1, SW2, SW3, SW4, and SW5.

[0028] Furthermore, the bidirectional DC / DC converter 20 includes a first voltmeter 21 for measuring the first voltage V1, which is the voltage across the first capacitor C1; a second voltmeter 22 for measuring the second voltage V2, which is the voltage across the second capacitor C2; and a third voltmeter 23 for measuring the third voltage V3, which is the voltage across the third capacitor C3.

[0029] The first capacitor C1 and the second capacitor C2 have the same capacitance. Therefore, ideally, the potential of the third power line 28 is exactly halfway between the potential of the first power line 26 and the potential of the second power line 27.

[0030] The first voltmeter 21 outputs a signal corresponding to the measured first voltage V1 to the control unit 50. Similarly, the second voltmeter 22 outputs a signal corresponding to the measured second voltage V2 to the control unit 50, and the third voltmeter 23 outputs a signal corresponding to the measured third voltage V3 to the control unit 50.

[0031] The switch elements SW1, SW2, SW3, SW4, and SW5 are made up of IGBTs (Insulated Gate Bipolar Transistors) and are switched from the ON state to the OFF state or from the OFF state to the ON state by the drive signals S1, S2, S3, S4, and S5 output by the control unit 50. In other words, the switch elements SW1, SW2, SW3, SW4, and SW5 are controlled on / off by the control unit 50.

[0032] The control unit 50 consists of a microprocessor (MPU, Micro Processor Unit) and is configured to generate and output drive signals S1, S2, S3, S4, and S5 based on voltages V1, V2, and V3.

[0033] The control unit 50 may refer to the status of the commercial power grid (whether or not an abnormality such as a power outage has occurred) and the power generation status of the solar cell 61 when generating the drive signals S1, S2, S3, S4, and S5. The control unit 50 may also be configured to control the on / off status of the switch elements constituting the DC / DC converter 30 and inverter 40, and the on / off status of the relays 65, 66, and 67.

[0034] The on / off control of the switch elements SW1, SW2, SW3, SW4, and SW5 by the control unit 50 will be explained in more detail below.

[0035] (Battery charging control) The control unit 50 can perform battery charging control to charge the battery 60 with grid power or power generated by the solar cell 61 when there is no abnormality in the commercial power grid. In this control, the control unit 50 turns the first and second switch elements SW1 and SW2 on / off while keeping the third, fourth and fifth switch elements SW3, SW4 and SW5 off, so that the third voltage V3 measured by the third voltmeter 23 becomes the target voltage (the voltage required by the battery 60).

[0036] More specifically, the control unit 50 repeatedly creates the following states in this order: the first switch element SW1 shown in Figure 2(A) is ON and the other switch elements SW2, SW3, SW4, SW5 are OFF; all switch elements SW1, SW2, SW3, SW4, SW5 shown in Figure 2(B) are OFF; the second switch element SW2 shown in Figure 2(C) is ON and the other switch elements SW1, SW3, SW4, SW5 are OFF; and all switch elements SW1, SW2, SW3, SW4, SW5 shown in Figure 2(D) are OFF. As a result, the first capacitor C1 and the second capacitor C2 discharge alternately, and the storage battery 60 is charged with the first voltage V1 and the second voltage V2 reduced voltage.

[0037] The timing of the switch from the state shown in Figure 2(A) to the state shown in Figure 2(B), i.e., the on-duty cycle of the first switch element SW1, can be determined, for example, by PWM control. Similarly, the timing of the switch from the state shown in Figure 2(C) to the state shown in Figure 2(D), i.e., the on-duty cycle of the second switch element SW2, can also be determined, for example, by PWM control.

[0038] In this battery charging control, the step-down ratio of the bidirectional DC / DC converter 20, that is, the step-down ratio when stepping down the first voltage V1 and the second voltage V2 to the target voltage, is half the step-down ratio when stepping down the voltage across the fourth capacitor C4 to the target voltage. Therefore, this control allows for efficient charging of the battery 60.

[0039] Note that the arrows in Figure 2 indicate the flow of current. The arrows in Figures 3, 4, and 5 also indicate the flow of current.

[0040] (Battery discharge control) The control unit 50 can perform battery discharge control to discharge the battery 60 in order to supply the power required for critical loads, etc. The control unit 50 can perform this control regardless of whether or not there is an abnormality in the commercial power grid. In this control, the control unit 50 turns on / off the third, fourth, and fifth switch elements SW3, SW4, SW5 while keeping the first and second switch elements SW1 and SW2 off, so that the first voltage V1 measured by the first voltmeter 21 and the second voltage V2 measured by the second voltmeter 22 each become a target voltage (half the voltage required by the inverter 40).

[0041] More specifically, the control unit 50 repeatedly creates the following states in this order: the fifth switch element SW5 shown in Figure 3(A) is ON and the other switch elements SW1, SW2, SW3, SW4 are OFF; the fourth switch element SW4 shown in Figure 3(B) is ON and the other switch elements SW1, SW2, SW3, SW5 are OFF; the fifth switch element SW5 shown in Figure 3(C) is ON and the other switch elements SW1, SW2, SW3, SW4 are OFF; and the third switch element SW3 shown in Figure 3(D) is ON and the other switch elements SW1, SW2, SW4, SW5 are OFF. As a result, the output voltage (third voltage V3) of the storage battery 60 is boosted, and the first capacitor C1 and the second capacitor C2 are charged alternately.

[0042] The timing of the switch from the state shown in Figure 3(A) to the state shown in Figure 3(B), i.e., the on-duty cycle of the fifth switch element SW5 (the off-duty cycle of the fourth switch element SW4), can be determined, for example, by PWM control. Similarly, the timing of the switch from the state shown in Figure 3(C) to the state shown in Figure 3(D), i.e., the on-duty cycle of the fifth switch element SW5 (the off-duty cycle of the third switch element SW3), can also be determined, for example, by PWM control.

[0043] In this battery discharge control, the boost ratio of the bidirectional DC / DC converter 20, that is, the boost ratio when boosting the third voltage to the target voltage, is half the boost ratio when boosting the third voltage to the voltage across the fourth capacitor C4. Therefore, this control allows the battery 60 to be discharged efficiently.

[0044] (First balance adjustment control) The control unit 50 can perform a first balance adjustment control when an abnormality occurs in the commercial power grid, causing critical loads to be driven by the power generated by the solar cells 61, and when the balance between the U-phase-O phase and W-phase-O phase connections of the critical load is disrupted, causing the first voltage V1 to be greater than the second voltage V2. In this control, the control unit 50 turns on / off the first, third, and fifth switch elements SW1, SW3, SW5 while keeping the second and fourth switch elements SW2, SW4 off, so that the first voltage V1 measured by the first voltmeter 21 and the second voltage V2 measured by the second voltmeter 22 become the target voltage (half the voltage required by the inverter 40).

[0045] More specifically, the control unit 50 repeatedly creates the following states in this order: the first switch element SW1 shown in Figure 4(A) is ON and the other switch elements SW2, SW3, SW4, SW5 are OFF; all switch elements SW1, SW2, SW3, SW4, SW5 shown in Figure 4(B) are OFF; the fifth switch element SW5 shown in Figure 4(C) is ON and the other switch elements SW1, SW2, SW3, SW4 are OFF; and the third switch element SW3 shown in Figure 4(D) is ON and the other switch elements SW1, SW2, SW4, SW5 are OFF. As a result, the discharge of the first capacitor C1 and the charging of the second capacitor C2 occur alternately, and the first voltage V1 and the second voltage V2 match the target voltage.

[0046] When the first switch element SW1 is turned on, power is transferred from the first capacitor C1 to the battery 60 or the third capacitor C3, as shown in Figure 4(A). Then, when the third switch element SW3 is turned on, power is transferred from the battery 60 or the third capacitor C3 to the second capacitor C2, as shown in Figure 4(D). In other words, in this control, power is transferred from the first capacitor C1 to the second capacitor C2 via the battery 60 or the third capacitor C3.

[0047] The timing of the switch from the state shown in Figure 4(A) to the state shown in Figure 4(B), i.e., the on-duty cycle of the first switch element SW1, can be determined, for example, by PWM control. Similarly, the timing of the switch from the state shown in Figure 4(C) to the state shown in Figure 4(D), i.e., the on-duty cycle of the fifth switch element SW5 (the off-duty cycle of the third switch element SW3), can also be determined, for example, by PWM control.

[0048] This first balance adjustment control allows the first voltage V1 and the second voltage V2 to be quickly matched by the transfer of power from the first capacitor C1 to the second capacitor C2. In other words, this control allows the potential of the third power line 28, which acts as the neutral line, to be quickly brought to the ideal potential.

[0049] (Second balance adjustment control) The control unit 50 can perform a second balance adjustment control when an abnormality occurs in the commercial power grid, causing critical loads to be driven by the power generated by the solar cells 61, and when the balance between the U-phase-O phase and W-phase-O phase connections of the critical load is disrupted, causing the second voltage V2 to be greater than the first voltage V1. In this control, the control unit 50 turns on / off the second, fourth, and fifth switch elements SW2, SW4, SW5 while keeping the first and third switch elements SW1, SW3 off, so that the first voltage V1 measured by the first voltmeter 21 and the second voltage V2 measured by the second voltmeter 22 become the target voltage (half the voltage required by the inverter 40).

[0050] More specifically, the control unit 50 repeatedly creates the following states in this order: the second switch element SW2 shown in Figure 5(A) is ON and the other switch elements SW1, SW3, SW4, SW5 are OFF; all switch elements SW1, SW2, SW3, SW4, SW5 shown in Figure 5(B) are OFF; the fifth switch element SW5 shown in Figure 5(C) is ON and the other switch elements SW1, SW2, SW3, SW4 are OFF; and the fourth switch element SW4 shown in Figure 5(D) is ON and the other switch elements SW1, SW2, SW3, SW5 are OFF. As a result, the discharge of the second capacitor C2 and the charging of the first capacitor C1 occur alternately, and the first voltage V1 and the second voltage V2 match the target voltage.

[0051] When the second switch element SW2 is turned on, power is transferred from the second capacitor C2 to the battery 60 or the third capacitor C3, as shown in Figure 5(A). Then, when the fourth switch element SW4 is turned on, power is transferred from the battery 60 or the third capacitor C3 to the first capacitor C1, as shown in Figure 5(D). In other words, in this control, power is transferred from the second capacitor C2 to the first capacitor C1 via the battery 60 or the third capacitor C3.

[0052] The timing of the switch from the state shown in Figure 5(A) to the state shown in Figure 5(B), i.e., the on-duty cycle of the second switch element SW2, can be determined, for example, by PWM control. Similarly, the timing of the switch from the state shown in Figure 5(C) to the state shown in Figure 5(D), i.e., the on-duty cycle of the fifth switch element SW5 (the off-duty cycle of the fourth switch element SW4), can also be determined, for example, by PWM control.

[0053] This second balance adjustment control allows the first voltage V1 and the second voltage V2 to be quickly matched by the transfer of power from the second capacitor C2 to the first capacitor C1. In other words, this control allows the potential of the third power line 28, which acts as the neutral line, to be quickly brought to the ideal potential.

[0054] Thus, in the energy storage system 10 according to the embodiment of the present invention, the potential of the third power line 28, which acts as the neutral line, is always maintained at an ideal potential (a value exactly midway between the potential of the first power line 26 and the potential of the second power line 27). Furthermore, because the energy storage system 10 has small boost and buck ratios in the bidirectional DC / DC converter 20, the battery 60 can be charged and discharged with higher efficiency than conventional systems.

[0055] Furthermore, the energy storage system 10 according to an embodiment of the present invention includes a fifth switch element SW5 connected in parallel to a series connection of third and fourth switch elements SW3 and SW4, and by simply turning on this fifth switch element SW5, a current path as shown in Figure 3(A) can be formed. In other words, with the energy storage system 10, switching losses can be reduced compared to the case where an equivalent current path cannot be formed without turning on two switch elements (third and fourth switch elements SW3 and SW4).

[0056] Although embodiments of the energy storage system according to the present invention have been described above, the configuration of the present invention is not limited thereto. For example, the DC / DC converter 30 and inverter 40 are not limited to those shown in Figure 1. [Explanation of Symbols]

[0057] 10 Energy storage systems 20 Bidirectional DC / DC Converter 21. First Voltmeter 22. Second Voltmeter 23. Third Voltmeter 24a, 24b First input / output terminals 25a, 25b Second input / output terminals 26. First power line 27. Second power line 28. Third Power Line 30 DC / DC Converters 31a, 31b Input terminals 32a,32b output end 40 Inverters 41a, 41b, 41c AC side input / output terminal 42a,42b DC side input / output terminal 50 Control Unit 60 Battery 61 Solar Cells 62 filters 63 System terminal 64 Standalone output terminals 65 System Relay 66 Bypass Relay 67 Self-supporting relay C1, C2, C3, C4 Capacitors D1, D2, D3, D4, D5 Freewheeling diodes L Coil SW1, SW2, SW3, SW4, SW5 Switch elements

Claims

1. A power storage system comprising: an inverter whose AC input / output terminals are connected to a single-phase three-wire power grid; a bidirectional DC / DC converter whose first input / output terminals are connected to a storage battery and whose second input / output terminals are connected to the DC input / output terminals of the inverter; and a control unit that controls the bidirectional DC / DC converter, The aforementioned bidirectional DC / DC converter is A first power line connected to the first input / output terminal and the second input / output terminal, and a second power line having a lower potential than the first power line, A third power line connected to the neutral terminal that constitutes the AC input / output terminal of the inverter, A first capacitor connected to the first power line and the third power line, A second capacitor connected to the second power line and the third power line, A first voltmeter for measuring the first voltage, which is the voltage across the first capacitor, A second voltmeter for measuring the second voltage, which is the voltage across the second capacitor, Multiple switching elements, Includes, The control unit (1) when the first voltage is greater than the second voltage, alternately discharges the first capacitor and charges the second capacitor by turning the switch element on and off, and (2) when the first voltage is less than the second voltage, alternately charges the first capacitor and discharges the second capacitor by turning the switch element on and off. A power storage system characterized by the following features.

2. A power storage system comprising: an inverter whose AC input / output terminals are connected to a single-phase three-wire power grid; a bidirectional DC / DC converter whose first input / output terminals are connected to a storage battery and whose second input / output terminals are connected to the DC input / output terminals of the inverter; and a control unit that controls the bidirectional DC / DC converter, The aforementioned bidirectional DC / DC converter is A first power line connected to the first input / output terminal and the second input / output terminal, and a second power line having a lower potential than the first power line, A third power line connected to the neutral terminal that constitutes the AC input / output terminal of the inverter, A first switch element interposed in the first power line, A second switching element interposed in the second power line, A first capacitor is connected to the first power line and the third power line between the first switch element and the second input / output terminal, A second capacitor is connected to the second power line and the third power line between the second switch element and the second input / output terminal, A third switch element is connected to the first power line and the third power line between the first switch element and the first input / output terminal, A fourth switch element is connected to the second power line and the third power line between the second switch element and the first input / output terminal, A fifth switch element and a third capacitor are connected to the first power line between the first switch element and the first input / output terminal, and to the second power line between the second switch element and the first input / output terminal. A freewheeling diode connected in parallel to each of the first, second, third, fourth, and fifth switching elements and A power storage system characterized by including the following.

3. The DC / DC converter further comprises an input terminal connected to a solar cell and an output terminal connected to the DC-side input / output terminal of the inverter. The energy storage system according to claim 1 or 2, characterized in that it is the same as described in claim 1 or 2.

4. The aforementioned bidirectional DC / DC converter is A first voltmeter for measuring the first voltage, which is the voltage across the first capacitor, A second voltmeter for measuring the second voltage, which is the voltage across the second capacitor, A third voltmeter for measuring the third voltage, which is the voltage across the third capacitor, Includes The energy storage system according to claim 2, characterized in that it is as described above.

5. The control unit is capable of controlling the alternating discharge of the first capacitor and the second capacitor by switching the first and second switch elements on and off while keeping the third, fourth, and fifth switch elements off, so that the third voltage becomes the target voltage. The energy storage system according to feature 4.

6. The control unit can control the alternating charging of the first capacitor and the second capacitor by switching the third, fourth, and fifth switch elements on and off while keeping the first and second switch elements off, so that the first and second voltages match. The energy storage system according to feature 4.

7. The control unit can alternately discharge the first capacitor and charge the second capacitor by switching the first, third, and fifth switch elements on and off while keeping the second and fourth switch elements off, so that the first and second voltages match. The energy storage system according to feature 4.

8. The control unit can control the alternating charging of the first capacitor and discharging of the second capacitor by switching the second, fourth, and fifth switch elements on and off while keeping the first and third switch elements off, so that the first and second voltages match. The energy storage system according to feature 4.

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