Power conversion device and power storage system

Abstract

To achieve appropriate power storage while suppressing increase in size and cost of a power storage system.SOLUTION: A controller 210 controls a bi-directional DC / DC converter 300 to execute a discharge operation of converting a DC power from a power storage device 600 into a voltage and supplying the voltage to a DC bus 220, a charge operation of converting a DC power from the DC bus 220 into a voltage and supplying the voltage to the power storage device 600, and a voltage maintenance operation of maintaining a voltage of the DC bus 220 to a predetermined voltage. A DC / DC converter 500 performs control so as to reduce an output voltage of the DC / DC converter 500 with increase in output current of the DC / DC converter 500 in a case where the output current becomes larger than a threshold corresponding to a power that can be acquired from a solar battery 10.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a power conversion device and a power storage system. [Background technology]

[0002] Technologies that utilize renewable energy sources such as sunlight, solar heat, hydropower, wind power, and geothermal heat are attracting attention. Electric power generated by a power generation device that uses renewable energy to generate electricity is consumed by a load, stored in a power storage device, or fed back to an AC grid. The load, power storage device, and AC grid handle various forms of electricity. For this reason, the electricity generated by the power generation device is converted into various forms by a power conversion device, such as a DC (Direct Current) / DC converter or an AC (Alternate Current) / DC converter.

[0003] For example, Patent Document 1 describes a power conversion device that includes a first DC / DC converter that performs voltage conversion on DC power from a power generation device, an inverter that converts the DC power from the first DC / DC converter into AC power, and a second DC / DC converter that performs voltage conversion on a DC voltage supplied from a power storage device. In the power conversion device described in Patent Document 1, output terminals of the two DC / DC converters are connected to a DC bus, but Patent Document 1 does not describe in detail how the two DC / DC converters are controlled. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2002-354677 Summary of the Invention [Problem to be solved by the invention]

[0005] However, when the output terminals of two DC / DC converters are connected to a DC bus, the controls of the two DC / DC converters may interfere with each other, resulting in unstable DC bus voltage. In this case, the utilization rate of generated power may decrease, the operation of the power conversion device may stop, or the power conversion device may fail, potentially preventing proper power storage. One possible method for stabilizing the DC bus voltage is to connect a high-voltage, large-capacity capacitor to the DC bus. However, adopting this method inevitably increases the size and cost of the power storage system. Therefore, a technology that achieves proper power storage while suppressing the increase in size and cost of the power storage system is desired.

[0006] The present disclosure has been made in view of the above-mentioned problems, and aims to achieve appropriate power storage while suppressing increases in the size and cost of a power storage system. [Means for solving the problem]

[0007] In order to solve the above problem, a power conversion device according to an embodiment of the present disclosure includes: a DC / DC converter that converts the voltage of DC power from the power generation device and supplies it to the DC bus; a bidirectional AC / DC converter having a first input / output terminal connected to the DC bus and a second input / output terminal connected to at least one of an AC system and an AC load; a bidirectional DC / DC converter having a third input / output terminal connected to the DC bus and a fourth input / output terminal connected to a power storage device; a discharging operation in which DC power from the power storage device is voltage-converted and supplied to the DC bus; a charging operation in which DC power from the DC bus is voltage-converted and supplied to the power storage device; and a charging operation in which the voltage of the DC bus is converted to a predetermined value. No. 1 a first control unit that controls the bidirectional DC / DC converter to perform a voltage maintaining operation that maintains a voltage at a predetermined level; The DC / DC converter is configured to output a current of the DC / DC converter at a threshold value corresponding to the power obtainable from the power generation device. If the voltage does not exceed the threshold, the DC / DC converter is controlled so that the output voltage of the DC / DC converter maintains a second voltage higher than the first voltage, and the output current does not exceed the threshold.If it exceeds the limit, the output current increases. Note The power supply includes a second control unit that controls the DC / DC converter so that the output voltage decreases. [Effects of the Invention]

[0008] According to the above configuration, it is possible to achieve appropriate power storage while suppressing an increase in size and cost of the power storage system. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a diagram illustrating a configuration of a power storage system according to a first embodiment. [Figure 2] Configuration diagram of a case according to the first embodiment [Figure 3] Circuit diagram of a bidirectional inverter according to the first embodiment [Figure 4] Circuit diagram of a DC / DC converter according to a first embodiment [Figure 5] FIG. 1 is an explanatory diagram of control of a DC / DC converter according to a first embodiment; [Figure 6] Graph showing output characteristics of the DC / DC converter according to the first embodiment. [Figure 7] FIG. 10 is an explanatory diagram of the operation of the power conversion device when the generated power decreases during charging. [Figure 8] Graph showing output changes when generated power decreases during charging [Figure 9] Graph showing the change in output when the generated power increases during charging [Figure 10] Graph showing output changes when power consumption decreases during charging [Figure 11] Graph showing the change in output when power consumption increases during charging [Figure 12] Graph showing output changes when generated power decreases during discharge [Figure 13] Graph showing the change in output when the generated power increases during discharge [Figure 14] Graph showing the change in output when power consumption decreases during discharge [Figure 15]Graph showing the change in output when power consumption increases during discharge [Figure 16] An explanatory diagram of the operation of a power conversion device when power consumption increases and transitions from charging to discharging. [Figure 17] A graph showing the change in output when power consumption increases and the device transitions from charging to discharging. [Figure 18] A graph showing the change in output when the generated power decreases and the system transitions from charging to discharging. [Figure 19] Graph showing the change in output when the power generation increases and the system transitions from discharging to charging [Figure 20] A graph showing the change in output when power consumption decreases and the transition from discharging to charging occurs [Figure 21] Graph showing output changes when generated power increases during charging and AC input [Figure 22] Graph showing output changes when generated power decreases during charging and AC input [Figure 23] Configuration diagram of a power storage system according to a second embodiment DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, a power storage system according to an embodiment of the technology disclosed herein will be described with reference to the drawings. Note that in the following embodiments, the same components are denoted by the same reference numerals. Also, the size ratios and shapes of the components shown in each drawing are not necessarily the same as those in the actual implementation.

[0011] (Embodiment 1) FIG. 1 is a configuration diagram of a power storage system 1000 according to this embodiment. The power storage system 1000 is a system that stores power supplied from a solar cell 10 or an AC system 20, and supplies the stored power to the AC system 20 or an AC load 30. The solar cell 10 is an electric power device that converts light energy into electrical energy. The solar cell 10 outputs a DC voltage of, for example, about several tens of volts. The solar cell 10 is an example of a power generation device.

[0012] The AC system 20 is a power system that supplies and receives AC power. The AC system 20 is, for example, a commercial power system provided by an electric power company. The voltage input and output by the AC system 20 is, for example, an AC voltage of about 100V or 200V. The AC system 20 has a terminal 21 that can be connected to an AC input / output terminal 202. The AC load 30 is a load that consumes AC power. The AC load 30 is, for example, an electrical device that operates on AC voltage. The voltage handled by the AC load 30 is, for example, an AC voltage of about 100V or 200V. The AC load 30 has a terminal 31 that can be connected to the AC input / output terminal 202.

[0013] As shown in Fig. 1, the power storage system 1000 includes a power conversion device 100 and a power storage device 600. The power conversion device 100 includes a bidirectional inverter 200 and a DC / DC converter 500. The bidirectional inverter 200 is a device that bidirectionally converts direct current and alternating current. In this embodiment, the bidirectional inverter 200 has a function of converting direct current into voltage. As shown in Fig. 1, the bidirectional inverter 200 includes a DC input / output terminal 201, an AC input / output terminal 202, a DC input terminal 203, a control unit 210, a DC bus 220, a bidirectional DC / DC converter 300, and a bidirectional AC / DC converter 400.

[0014] The DC input / output terminal 201 is a terminal for inputting and outputting direct current. The AC input / output terminal 202 is a terminal for inputting and outputting alternating current. The DC input terminal 203 is a terminal for inputting direct current. In this embodiment, the terminals may be in any form. For example, the terminals may be conductors that are connected to a terminal block, that are incorporated into an outlet, or that are incorporated into a plug. The terminals may be integrated or divided as appropriate. For example, the DC input / output terminal 201 and the terminal 302 that is connected to the DC input / output terminal 201 may be integrated into the DC input / output terminal 201. The AC input / output terminal 202 that is connected to the terminals 21 and 31 may be divided into two terminals.

[0015] The control unit 210 controls the bidirectional DC / DC converter 300 and the bidirectional AC / DC converter 400. The control unit 210 includes, for example, a microcomputer including a CPU (Central Processing Unit), ROM (Read Only Memory), RAM (Random Access Memory), RTC (Real Time Clock), digital input / output, an A / D (Analog / Digital) converter, a D / A (Digital / Analog) converter, etc., and a semiconductor memory such as a flash memory. The microcomputer operates according to an operating program stored in the flash memory, for example.

[0016] The control unit 210 controls the bidirectional DC / DC converter 300 to perform a discharging operation, a charging operation, and a voltage maintaining operation. The discharging operation is an operation of converting the voltage of DC power from the power storage device 600 and supplying the converted power to the DC bus 220. The charging operation is an operation of converting the voltage of DC power from the DC bus 220 and supplying the converted power to the power storage device 600. The voltage maintaining operation is an operation of maintaining the voltage of the DC bus 220 at a predetermined voltage. The voltage maintaining operation will be described in detail later. The control unit 210 is an example of a first control unit.

[0017] The DC bus 220 is a bus to which a DC voltage is applied. The DC bus voltage applied to the DC bus 220 is, for example, a DC voltage of about 400 V. A bidirectional DC / DC converter 300, a bidirectional AC / DC converter 400, and a DC / DC converter 500 are connected to the DC bus 220.

[0018] The bidirectional DC / DC converter 300 is a device that bidirectionally converts DC voltage. The bidirectional DC / DC converter 300 converts the voltage of DC power from the power storage device 600 and supplies the converted voltage to the DC bus 220. For example, the bidirectional DC / DC converter 300 boosts a DC voltage of approximately several tens of volts supplied from the power storage device 600 to a DC voltage of approximately 400 V and supplies the DC voltage to the DC bus. The bidirectional DC / DC converter 300 also converts the voltage of DC power from the DC bus 220 and supplies the DC voltage to the power storage device 600. For example, the bidirectional DC / DC converter 300 reduces a DC voltage of approximately 400 V supplied from the DC bus 220 to a DC voltage of approximately several tens of volts and supplies the DC voltage to the power storage device 600. The bidirectional DC / DC converter 300 includes a terminal 301 connected to the DC bus 220 and a terminal 302 connected to the power storage device 600 via the DC input / output terminal 201. The terminal 301 is an example of a third input / output terminal. Terminal 302 is an example of a fourth input / output terminal.

[0019] The bidirectional AC / DC converter 400 is a device that bidirectionally converts AC and DC. The bidirectional AC / DC converter 400 converts AC power from the AC system 20 into DC power and supplies it to the DC bus 220. For example, the bidirectional AC / DC converter 400 converts an AC voltage of about 100 V or 200 V supplied from the AC system 20 into a DC voltage of about 400 V and supplies it to the DC bus 220. The bidirectional AC / DC converter 400 also converts DC power from the DC bus 220 into AC power and supplies it to the AC system 20 or the AC load 30. For example, the bidirectional AC / DC converter 400 converts a DC voltage of about 400 V supplied from the DC bus 220 into an AC voltage of about 100 V or 200 V and supplies it to the AC system 20 or the AC load 30. The bidirectional AC / DC converter 400 includes a terminal 401 connected to the DC bus 220 and a terminal 402 connected to at least one of the AC system 20 and the AC load 30 via the AC input / output terminal 202. The terminal 401 is an example of a first input / output terminal. The terminal 402 is an example of a second input / output terminal.

[0020] The DC / DC converter 500 is a device that converts DC voltage. The DC / DC converter 500 converts the DC voltage from the solar cell 10 and supplies the converted voltage to the DC bus 220. The DC / DC converter 500, for example, boosts a DC voltage of several tens of volts supplied from the solar cell 10 to a DC voltage of about 400 volts and supplies the boosted voltage to the DC bus 220. The DC / DC converter 500 is a device that efficiently acquires DC power from the solar cell 10 and supplies the power to the DC bus 220. The DC / DC converter 500 includes a DC output terminal 501 and a control unit 510. The DC output terminal 501 is an output terminal of the DC / DC converter 500 that can be connected to the DC input terminal 203. The DC output terminal 501 is connected to the DC bus 220 via the DC input terminal 203.

[0021] The control unit 510 controls the overall operation of the DC / DC converter 500. The control unit 510 includes, for example, a microcomputer including a CPU, ROM, RAM, RTC, digital input / output, an A / D converter, a D / A converter, etc., and a semiconductor memory such as a flash memory. The microcomputer operates according to an operating program stored in the flash memory, for example.

[0022] When the output current of the DC / DC converter 500 exceeds a threshold value corresponding to the power obtainable from the solar cell 10, the control unit 510 controls the DC / DC converter 500 so that the output voltage of the DC / DC converter 500 decreases as the output current increases. In other words, the control unit 210 controls the current and voltage output characteristics of the DC / DC converter 500. Details of the operation of the control unit 210 will be described later. The control unit 510 is an example of a second control unit.

[0023] The power storage device 600 stores power supplied from the solar cell 10 or the AC system 20 via the power conversion device 100. The power storage device 600 also supplies the stored power to the AC system 20 or the AC load 30 via the power conversion device 100. The power storage device 600 includes a terminal 601. The charging voltage and discharging voltage of the power storage device 600 are, for example, DC voltages of about several tens of volts. The power storage device 600 stores, for example, about 3 kWh of power.

[0024] In this embodiment, the DC / DC converter 500 can be electrically separated from the bidirectional DC / DC converter 300 and the bidirectional AC / DC converter 400. That is, the DC input terminal 203 of the bidirectional inverter 200 and the DC output terminal 501 of the DC / DC converter 500 are detachable. Furthermore, the AC input / output terminal 202 of the bidirectional inverter 200 and the terminal 21 of the AC system 20 are detachable, and the AC input / output terminal 202 of the bidirectional inverter 200 and the terminal 31 of the AC load 30 are detachable.

[0025] Furthermore, in this embodiment, in order to facilitate transportation of bidirectional inverter 200 and power storage device 600, power storage system 1000 includes case 700 that stores bidirectional inverter 200 and power storage device 600. As shown in FIG. 2 , case 700 includes plug 701, plug 702, a plurality of outlets 703, a plurality of wheels 704, and housing 710.

[0026] The plug 701 is a plug for connecting the bidirectional inverter 200 to the AC system 20. The conductors included in the plug 701 correspond to the AC input / output terminals 202. Furthermore, the conductors included in a power outlet (not shown) into which the plug 701 is inserted correspond to the terminals 21. The plug 702 is a plug for connecting the bidirectional inverter 200 to the DC / DC converter 500. The conductors included in the plug 702 correspond to the DC input terminals 203. Furthermore, the conductors included in the power outlet (not shown) into which the plug 702 is inserted correspond to the DC output terminals 501.

[0027] Each of the multiple outlets 703 is a plug for connecting the bidirectional inverter 200 to the AC load 30. The conductors included in the outlets 703 correspond to the AC input / output terminals 202. The conductors included in a plug (not shown) inserted into the outlets 703 correspond to the terminals 31. The multiple wheels 704 are wheels for facilitating movement of the case 700. The housing 710 is a member that houses the bidirectional inverter 200 and the power storage device 600.

[0028] The case 700 is connected to the solar cell 10 or the AC system 20 when charging the power storage device 600. The case 700 is connected to the AC load 30 when discharging the power storage device 600. The case 700 is used, for example, as an AC power source that charges the power storage device 600 under normal circumstances and can be moved to the location where the AC load 30 is used in the event of a disaster.

[0029] Next, the configuration of the bidirectional inverter 200 will be described with reference to Fig. 3. The bidirectional inverter 200 includes a bidirectional DC / DC converter 300 connected to the power storage device 600 via a DC input / output terminal 201 and a terminal 204, and a bidirectional AC / DC converter 400 connected to the AC system 20 or an AC load 30 via an AC input / output terminal 202 and an AC input / output terminal 205. The bidirectional DC / DC converter 300 and the bidirectional AC / DC converter 400 are connected via a DC input terminal 203 and a terminal 206. The AC load 30 and various terminals are not shown in Fig. 3.

[0030] The bidirectional DC / DC converter 300 includes an IGBT (Insulated Gate Bipolar Transistor) 311, an IGBT 312, a diode 321, a diode 322, an inductor 330, and a capacitor 340. The IGBT 311 and the IGBT 312 are connected in series between a DC input terminal 203 and a terminal 206. The collector of the IGBT 311 is connected to the DC input terminal 203, the emitter of the IGBT 311 is connected to the collector of the IGBT 312, and the emitter of the IGBT 312 is connected to the terminal 206.

[0031] The anode of diode 321 is connected to the emitter of IGBT 311, and the cathode of diode 321 is connected to the collector of IGBT 311. The anode of diode 322 is connected to the emitter of IGBT 312, and the cathode of diode 322 is connected to the collector of IGBT 312. One end of inductor 330 is connected to DC input / output terminal 201, and the other end of inductor 330 is connected to the emitter of IGBT 311. One end of capacitor 340 is connected to DC input / output terminal 201, and the other end of capacitor 340 is connected to the emitter of IGBT 312.

[0032] The control unit 210 applies voltages of opposite logical levels to the gate of IGBT 311 and the gate of IGBT 312. Therefore, the on-duty of the voltage signal applied to the gate of IGBT 311 is the same as the off-duty of the voltage signal applied to the gate of IGBT 312. The control unit 210 adjusts the duty ratio of the voltage signals applied to the gates of IGBT 311 and IGBT 312, thereby adjusting the direction and magnitude of the current flowing between the DC bus 220 and the power storage device 600. The IGBT 311 and IGBT 312 are examples of switch elements.

[0033] The bidirectional AC / DC converter 400 includes an IGBT 411, an IGBT 412, an IGBT 413, an IGBT 414, a diode 421, a diode 422, a diode 423, a diode 424, an inductor 430, and a capacitor 440. The IGBT 411 and the IGBT 412 are connected in series between a DC input terminal 203 and a terminal 206. The collector of the IGBT 411 is connected to the DC input terminal 203, the emitter of the IGBT 411 is connected to the collector of the IGBT 412, and the emitter of the IGBT 412 is connected to the terminal 206. The IGBT 413 and the IGBT 414 are connected in series between the DC input terminal 203 and the terminal 206. The collector of the IGBT 413 is connected to the DC input terminal 203 , the emitter of the IGBT 413 is connected to the collector of the IGBT 414 , and the emitter of the IGBT 414 is connected to the terminal 206 .

[0034] The anode of diode 421 is connected to the emitter of IGBT 411, and the cathode of diode 421 is connected to the collector of IGBT 411. The anode of diode 422 is connected to the emitter of IGBT 412, and the cathode of diode 422 is connected to the collector of IGBT 412. The anode of diode 423 is connected to the emitter of IGBT 413, and the cathode of diode 423 is connected to the collector of IGBT 413. The anode of diode 424 is connected to the emitter of IGBT 414, and the cathode of diode 424 is connected to the collector of IGBT 414. One end of inductor 430 is connected to AC input / output terminal 202, and the other end of inductor 430 is connected to the emitter of IGBT 411. The emitter of IGBT 413 is connected to terminal 206. One end of the capacitor 440 is connected to the DC input terminal 203 , and the other end of the capacitor 440 is connected to the emitter of the IGBT 414 .

[0035] The control unit 210 applies a voltage of the same logical level to the gate of the IGBT 411 and the gate of the IGBT 414. The control unit 210 also applies a voltage of the same logical level to the gate of the IGBT 412 and the gate of the IGBT 413. The control unit 210 adjusts the voltages applied to the gates of the IGBT 411, IGBT 412, IGBT 413, and IGBT 414, thereby adjusting the direction and magnitude of the current flowing between the DC bus 220 and the AC system 20 or the AC load 30.

[0036] Next, the configuration of DC / DC converter 500 will be described with reference to Fig. 4. DC / DC converter 500 converts the voltage of DC power supplied from solar cell 10 via terminals 502 and 504, and supplies the converted voltage to DC bus 220 via DC output terminals 501 and 503. DC / DC converter 500 includes a two-stage converter: a boost converter 511 as a front-stage converter, and an LLC converter 512 as a rear-stage converter. Although not shown in Fig. 4, DC / DC converter 500 includes control unit 510, as described above.

[0037] The boost converter 511 is a circuit that adjusts the power from the solar cell 10 by performing MPPT (Maximum Power Point Tracking) control. The MPPT control is a control that automatically determines the optimum voltage x current value that can maximize the output when the solar cell 10 generates power. The boost converter 511 includes an N-channel MOSFET 521, an N-channel MOSFET 522, a diode 531, a diode 532, a diode 541, a diode 542, an inductor 551, an inductor 552, a capacitor 561, a capacitor 562, a capacitor 563, and a capacitor 564.

[0038] Diodes 531 and 532 are parasitic diodes of N-channel MOSFET 521 and N-channel MOSFET 522, respectively. Capacitor 561 and capacitor 562 are electrolytic capacitors and have polarity. The drain of N-channel MOSFET 521, the anode of diode 541, and one end of inductor 551 are connected together. The source of N-channel MOSFET 521, the source of N-channel MOSFET 522, the negative terminal of capacitor 561, the negative terminal of capacitor 562, one end of capacitor 564, and terminal 504 are connected together. The drain of N-channel MOSFET 522, the anode of diode 542, and one end of inductor 552 are connected together.

[0039] The cathode of diode 541, the positive terminal of capacitor 562, and one end of capacitor 563 are connected together. The cathode of diode 542, the other end of capacitor 563, and the other end of capacitor 564 are connected together. The other end of inductor 551, the other end of inductor 552, the positive terminal of capacitor 561, and terminal 502 are connected together. Control unit 510 performs MPPT control by controlling the gate voltage of N-channel MOSFET 521 and the gate voltage of N-channel MOSFET 521.

[0040] The LLC converter 512 is a circuit that adjusts the output current and output voltage. The LLC converter 512 includes an N-channel MOSFET 523, an N-channel MOSFET 524, a diode 533, a diode 534, a diode 543, a diode 544, a diode 545, a diode 546, an inductor 553, an inductor 554, a transformer 555, a capacitor 565, and a capacitor 566. The diode 533 and the diode 534 are parasitic diodes of the N-channel MOSFET 523 and the N-channel MOSFET 524, respectively. The capacitor 566 is an electrolytic capacitor and has polarity. The LLC converter 512 includes a resonant circuit including the inductor 553, the inductor 554, and the capacitor 565.

[0041] The drain of N-channel MOSFET 523 is connected to the cathode of diode 541. The source of N-channel MOSFET 523, the drain of N-channel MOSFET 524, and one end of inductor 553 are connected. The source of N-channel MOSFET 524 is connected to terminal 504. The other end of inductor 553 is connected to one end of capacitor 565. One end of inductor 554, one end of the primary inductor of transformer 555, and the other end of capacitor 565 are connected. The other end of inductor 554, the other end of the primary inductor of transformer 555, and terminal 504 are connected.

[0042] The cathode of diode 543, the cathode of diode 545, the positive terminal of capacitor 566, and DC output terminal 501 are connected together. The anode of diode 543, the cathode of diode 544, and one end of the secondary inductor of transformer 555 are connected together. The anode of diode 544, the anode of diode 546, the negative terminal of capacitor 566, and terminal 503 are connected together. The other end of the secondary inductor of transformer 555, the anode of diode 545, and the cathode of diode 546 are connected together. Control unit 510 adjusts the output voltage and output current by controlling the gate voltage of N-channel MOSFET 523 and the gate voltage of N-channel MOSFET 524.

[0043] In the DC / DC converter 500, the boost converter 511 extracts maximum power from the solar cell 10 at an optimal voltage and current, and the LLC converter 512 adjusts the output voltage and output current. Therefore, employing the DC / DC converter 500 provides a high degree of design flexibility. For example, even if the input voltage from the solar cell 10 is approximately 400 V, the output voltage to the DC bus 220 can be designed to be approximately 40 V. Also, even if the input voltage from the solar cell 10 is approximately 40 V, the output voltage to the DC bus 220 can be designed to be approximately 400 V. It is preferable to adjust the winding ratio of the transformer 555 to match the median value of the relationship between the input voltage and the output voltage. The input voltage and output current can be adjusted by adjusting the on / off timing of the N-channel MOSFET 523 and the N-channel MOSFET 524.

[0044] Next, the voltage maintaining operation executed by the control unit 210 will be described with reference to Fig. 5. The control unit 210 detects the current flowing through each part of the bidirectional DC / DC converter 300 and the voltage of each part of the bidirectional DC / DC converter 300, and controls the above-mentioned duty ratio according to the detection result so that the voltage of the DC bus 220 (hereinafter referred to as "DC bus voltage" as appropriate) is maintained at a predetermined voltage (hereinafter referred to as "target DC bus voltage" as appropriate).

[0045] The control unit 210 functions as an adder 211, a constant voltage control unit 212, an adder 213, and a constant current control unit 214. The adder 211 calculates a differential voltage Vs by subtracting a DC bus voltage Vb from a target DC bus voltage Vp. The constant voltage control unit 212 calculates an adjustment current Ia from the differential voltage Vs. The constant voltage control unit 212 calculates Ia from Vs using, for example, proportional integral (PI) control.

[0046] The adder 213 calculates the difference current Is by subtracting the DC bus current Ib from the adjustment current Ia. The DC bus current is the current that flows from the power storage device 600 to the DC bus 220. The constant current control unit 214 obtains the command value M from the difference current Is. The constant current control unit 214 calculates M from Is by, for example, PI control. The command value is a command value for the bidirectional DC / DC converter 300, and is, for example, the duty ratio of a voltage signal supplied to the gate of the IGBT 311 or the IGBT 312. The frequency of this voltage signal is, for example, 16 kHz.

[0047] In this way, when the DC bus voltage is less than the target DC bus voltage, the control unit 210 increases the current flowing from the power storage device 600 to the DC bus 220 or decreases the current discharged from the DC bus 220 to the power storage device 600, thereby bringing the DC bus voltage closer to the target DC bus voltage. Furthermore, when the DC bus voltage exceeds the target DC bus voltage, the control unit 210 decreases the current flowing from the power storage device 600 to the DC bus 220 or increases the current discharged from the DC bus 220 to the power storage device 600, thereby bringing the DC bus voltage closer to the target DC bus voltage.

[0048] Next, the output characteristics of the DC / DC converter 500 will be described with reference to Fig. 6. The output characteristics of the DC / DC converter 500 are characteristics that indicate the correspondence relationship between the output current of the DC / DC converter 500 and the output voltage of the DC / DC converter 500. Hereinafter, unless otherwise specified, the output current of the DC / DC converter 500 will be simply referred to as the output current, and the output voltage of the DC / DC converter 500 will be simply referred to as the output voltage. Pmax, which is the maximum amount of power that can be extracted from the solar cell 10, is basically determined by the intensity of light irradiating the solar cell 10. Therefore, the stronger the light irradiating the solar cell 10, the larger Pmax.

[0049] Incidentally, because output power is the product of output current and output voltage, there is room to adjust the ratio of output current to output voltage when extracting Pmax. For example, one method is to fix the output voltage Vout to the target DC bus voltage Vp and set the output current Iout to Pmax / Vp. In this case, the output characteristics of a typical DC / DC converter 500 are such that Vout is Vp when Iout is in the range equal to or less than Pmax / Vp, and Vout is 0 V when Iout exceeds Pmax / Vp.

[0050] However, when such output characteristics are adopted, for example, if the amount of solar radiation decreases and Pmax decreases, Vout may be controlled to suddenly decrease from Vp to 0 V before Iout decreases. Furthermore, if the amount of solar radiation subsequently increases and Pmax increases, Vout may be controlled to suddenly increase from 0 V to Vp. In other words, when such output characteristics are adopted, Vout fluctuates significantly with respect to changes in Iout. In this case, Vb, which is the DC bus voltage, may fluctuate significantly from Vp. In this case, the outputs of the two DC / DC converters connected in parallel to the DC bus 220 may become unstable. In other words, in this case, the control of the DC / DC converter 500 by the control unit 510 and the control of the bidirectional DC / DC converter 300 by the control unit 210 may interfere with each other, and the fluctuations in Vb may not converge.

[0051] Therefore, in this embodiment, instead of the output characteristics described above, the output characteristics shown in Fig. 6 are adopted. That is, in this embodiment, when Iout is equal to or less than Ith, which is a threshold value according to Pmax, Vout is maintained at Vh, which is higher than Vp, and when Iout exceeds Ith, an output characteristic is adopted in which Vout decreases as Iout increases. P1 is the point where Iout is 0 A and Vout is Vh. P2 is the point where Iout is Ith and Vout is Vh. P3 is the point where Iout is I3 and Vout is Vp. P4 is the point where Iout is Ih and Vout is 0 V.

[0052] As shown in Figure 6, the output characteristics are represented by a line segment connecting P1 and P2 and a line segment connecting P2 and P4. P3 is the intersection of the line segment connecting P2 and P4 and the line connecting the points where Vout = Vp. When the DC bus voltage Vb is stable, Iout and Vout are the values ​​indicated by P3. The product of Iout and Vout is less than or equal to Pmax. Therefore, Vh, Ith, and Ih are determined so that P1, P2, P3, and P4 are all located to the lower left of the dotted curve representing Iout × Vout = Pmax.

[0053] First, Vh is determined to be a value several tens of percent higher than Vp, for example. Furthermore, Ith and Ih are determined to be values ​​such that the slope of the line segment connecting P2 and P4 matches the slope of the dotted curve representing Iout × Vout = Pmax near P3, and the distance between this dotted curve and P3 is extremely close to zero. That is, when Iout exceeds Ith, the control unit 510 controls the DC / DC converter 500 so that the product of Iout and Vout remains substantially constant in a range close to Pmax. In this case, Vout can be reduced as Iout increases while maintaining power extraction close to Pmax. Note that when the amount of solar radiation decreases and Pmax decreases, the line segment connecting P2 and P4 shifts in the direction of decreasing Iout, i.e., to the left.

[0054] When such output characteristics are employed, for example, if the amount of solar radiation decreases and Pmax decreases, Vout is controlled to decrease gradually. Furthermore, if the amount of solar radiation subsequently increases and Pmax increases, Vout is controlled to increase gradually. In other words, when such output characteristics are employed, Vout gradually changes in response to changes in Iout. In this case, Vb, which is the DC bus voltage, does not change abruptly. In this case, the outputs of the two DC / DC converters connected in parallel to the DC bus 220 are stabilized. In other words, in this case, the change in Vb in response to the control of the DC / DC converter 500 by the control unit 510 is appropriately absorbed by the control of the bidirectional DC / DC converter 300 by the control unit 210, and Vb converges to a value close to Vp.

[0055] The reason why control units 210 and 510 maintain Vb at a stable value will be described below.

[0056] First, with reference to Fig. 7, the operation of the power conversion device when the generated power decreases during charging will be described. When the amount of solar radiation decreases, the power generated by the solar cell 10 (hereinafter referred to as "generated power" as appropriate) decreases. When the generated power decreases, the control unit 510, for example, reduces the output voltage output by the DC / DC converter 500 while maintaining the output current output by the DC / DC converter 500. When the output voltage decreases, the DC bus voltage decreases. On the other hand, when the control unit 210 detects a decrease in the DC bus voltage, it reduces the charging current, which is the current supplied from the DC bus 220 to the power storage device 600 via the DC input / output terminal 201, and increases the DC bus voltage. As the output voltage increases due to the increase in the DC bus voltage, the control unit 510 reduces the output current.

[0057] According to this control, even when the power generated by the solar cell 10 decreases, the power supplied from the solar cell 10 to the power storage device 600 is reduced, thereby making it possible to maintain the power supplied from the solar cell 10 to the AC grid 20 or the AC load 30. Furthermore, according to this control, it is possible to reduce fluctuations in the DC bus voltage.

[0058] Below, with reference to Fig. 8, a description will be given of the change in output of the DC / DC converter 500 when the generated power decreases during charging. In Fig. 8, the solid line graph shows the output characteristics before the generated power decreases, and the dashed line graph shows the output characteristics after the generated power decreases. Ia1 is the load current flowing from the AC input / output terminal 202 to the AC system 20 or the AC load 30. Ia1 is smaller than I12. I10 is the output current before the generated power decreases. I12 is the output current after the generated power decreases. P10 is the point corresponding to the output voltage and output current before the generated power decreases. When the generated power decreases, the output characteristics shift in the direction of decreasing the output current.

[0059] Here, for example, when maintaining the output current at I10, the control unit 510 reduces the output voltage from Vp toward V11. That is, the output voltage and output current change in the direction from P10 toward P11. Here, the DC bus voltage decreases as the output voltage decreases. Note that the control unit 510 adjusts the output current and output voltage by, for example, adjusting the timing of turning on / off the gates of the N-channel MOSFET 523 and the N-channel MOSFET 524.

[0060] When the control unit 210 detects a drop in the DC bus voltage, it reduces the charging current and increases the DC bus voltage toward the target DC bus voltage. The control unit 210 adjusts the charging current, for example, by adjusting the duty ratio of the voltage signal supplied to the gates of the IGBTs 311 and 312. The control unit 510 reduces the output current from I10 toward I12 as the output voltage increases with the increase in the DC bus voltage. Therefore, the output voltage and output current change from P11 toward P12. When the output voltage and output current reach Vp and I12, respectively, the changes in the output voltage and output voltage cease. As a result, when the charging current decreases by the amount corresponding to the decrease in the output current, the DC bus voltage increases to the target DC bus voltage and stabilizes.

[0061] Next, referring to FIG. 9, a description will be given of the change in output of the DC / DC converter 500 when the generated power increases during charging. In FIG. 9, the solid line graph shows the output characteristics before the generated power increases, and the dashed line graph shows the output characteristics after the generated power increases. When the generated power increases, the output characteristics shift in the direction in which the output current increases. Here, for example, when maintaining the output current at I10, the control unit 510 increases the output voltage from Vp toward V13. In other words, the output voltage and output current change in the direction from P10 toward P13. In this case, the increase in output voltage causes the DC bus voltage to increase.

[0062] When the control unit 210 detects an increase in the DC bus voltage, it increases the charging current, lowering the DC bus voltage toward the target DC bus voltage. The control unit 510 increases the output current from I10 toward I14 in response to a decrease in output voltage caused by a decrease in the DC bus voltage. As a result, the output voltage and output current change from P13 toward P14. When the output voltage and output current reach Vp and I14, respectively, the changes in the output voltage and output voltage cease. In this way, when the charging current increases by the amount corresponding to the increase in the output current, the DC bus voltage decreases to the target DC bus voltage and stabilizes.

[0063] Next, referring to FIG. 10, a description will be given of the change in output of the DC / DC converter 500 when power consumption decreases during charging. In FIG. 10, the solid line graph shows the output characteristics before the power consumption decreases, and the dashed line graph shows the output characteristics after the power consumption decreases. When the charging current is constant, if the load current decreases from Ia2 to Ia3 due to a decrease in power consumption, the DC bus voltage increases from Vp toward V15. As the output voltage increases due to the increase in the DC bus voltage, the control unit 510 decreases the output current from I10 toward I15. Note that I15 = I10 - Ia2 + Ia3. As a result, the output voltage and output current change from P10 toward P15.

[0064] On the other hand, when the control unit 210 detects an increase in the DC bus voltage, it increases the charging current, thereby decreasing the DC bus voltage toward the target DC bus voltage. The control unit 510 increases the output current from I15 toward I10 in response to a decrease in output voltage caused by a decrease in the DC bus voltage. Therefore, the output voltage and output current change from P15 toward P10. When the output voltage and output current reach Vp and I10, respectively, the changes in the output voltage and output voltage cease. In this way, when the charging current increases by the amount corresponding to the decrease in the load current, the DC bus voltage decreases to the target DC bus voltage and stabilizes.

[0065] Next, referring to FIG. 11, a description will be given of the change in output of the DC / DC converter 500 when power consumption increases during charging. In FIG. 11, the solid line graph shows the output characteristics before the power consumption increases, and the dashed line graph shows the output characteristics after the power consumption increases. When the charging current is constant, if the load current increases from Ia2 to Ia4 due to an increase in power consumption, the DC bus voltage decreases from Vp toward V16. As the output voltage decreases due to the decrease in the DC bus voltage, the control unit 510 increases the output current from I10 toward I16. Note that I16 = I10 - Ia2 + Ia4. As a result, the output voltage and output current change from P10 toward P16.

[0066] On the other hand, when the control unit 210 detects a drop in the DC bus voltage, it reduces the charging current and increases the DC bus voltage toward the target DC bus voltage. As the output voltage increases with the increase in the DC bus voltage, the control unit 510 reduces the output current from I16 toward I10. Therefore, the output voltage and output current change from P16 toward P10. When the output voltage and output current reach Vp and I10, respectively, the changes in the output voltage and output voltage cease. In this way, when the charging current decreases by the amount corresponding to the increase in the load current, the DC bus voltage drops to the target DC bus voltage and stabilizes.

[0067] Below, with reference to Fig. 12, a description will be given of the change in output of DC / DC converter 500 when the generated power decreases during discharge. In Fig. 12, the solid line graph shows the output characteristics before the generated power decreases, and the dashed line graph shows the output characteristics after the generated power decreases. Ia5 is a load current greater than I10. When the generated power decreases, the output characteristics shift in the direction of decreasing output current.

[0068] Here, for example, when maintaining the output current at I10, the control unit 510 reduces the output voltage from Vp toward V11. On the other hand, when the control unit 210 detects a decrease in the DC bus voltage due to a decrease in the output voltage, it increases the discharge current and raises the DC bus voltage toward the target DC bus voltage. As the output voltage increases due to an increase in the DC bus voltage, the control unit 510 reduces the output current from I10 toward I12. Then, when the output voltage reaches Vp and the output current reaches I12, the output voltage and the change in the output voltage subside. In this way, when the discharge current increases by the amount corresponding to the decrease in the output current, the DC bus voltage rises to the target DC bus voltage and stabilizes.

[0069] Next, referring to Fig. 13, a description will be given of the change in output of the DC / DC converter 500 when the generated power increases during discharge. In Fig. 13, the solid line graph shows the output characteristics before the generated power increases, and the dashed line graph shows the output characteristics after the generated power increases. Ia6 is a load current greater than I14. As the generated power increases, the output characteristics shift in the direction in which the output current increases. Here, for example, when maintaining the output current at I10, the control unit 510 increases the output voltage from Vp toward V13.

[0070] When the control unit 210 detects an increase in the DC bus voltage due to an increase in the output voltage, it reduces the discharge current and decreases the DC bus voltage toward the target DC bus voltage. The control unit 510 increases the output current from I10 toward I14 in response to a decrease in the output voltage due to a decrease in the DC bus voltage. Therefore, the output voltage and output current change in the direction from P13 toward P14. When the output voltage and output current reach Vp and I14, respectively, the changes in the output voltage and output voltage cease. In this way, when the discharge current decreases by the amount corresponding to the increase in the output current, the DC bus voltage decreases to the target DC bus voltage and stabilizes.

[0071] Next, referring to FIG. 14, a description will be given of the change in output of the DC / DC converter 500 when power consumption decreases during discharge. In FIG. 14, the solid line graph shows the output characteristics before the power consumption decreases, and the dashed line graph shows the output characteristics after the power consumption decreases. When the discharge current is constant, if the load current decreases from Ia7 to Ia8 due to a decrease in power consumption, the DC bus voltage increases from Vp toward V15. As the output voltage increases due to the increase in the DC bus voltage, the control unit 510 decreases the output current from I10 toward I15. Note that I15 = I10 - Ia7 + Ia8. As a result, the output voltage and output current change in the direction from P10 toward P15.

[0072] On the other hand, when the control unit 210 detects an increase in the DC bus voltage, it reduces the discharge current, lowering the DC bus voltage toward the target DC bus voltage. The control unit 510 increases the output current from I15 toward I10 in response to a decrease in output voltage caused by a decrease in the DC bus voltage. Therefore, the output voltage and output current change in the direction from P15 toward P10. When the output voltage and output current reach Vp and I10, respectively, the changes in the output voltage and output voltage cease. In this way, when the discharge current decreases by the amount corresponding to the decrease in the load current, the DC bus voltage decreases to the target DC bus voltage and stabilizes.

[0073] Next, referring to FIG. 15, a description will be given of the change in output of the DC / DC converter 500 when power consumption increases during discharge. In FIG. 15, the solid line graph shows the output characteristics before the power consumption increases, and the dashed line graph shows the output characteristics after the power consumption increases. When the charging current is constant, if the load current increases from Ia9 to Ia10 due to an increase in power consumption, the DC bus voltage decreases from Vp toward V16. As the output voltage decreases due to the decrease in the DC bus voltage, the control unit 510 increases the output current from I10 toward I16. Note that I16 = I10 - Ia9 + Ia10. As a result, the output voltage and output current change from P10 toward P16.

[0074] On the other hand, when the control unit 210 detects a drop in the DC bus voltage, it increases the discharge current, raising the DC bus voltage toward the target DC bus voltage. As the output voltage increases with the rise in the DC bus voltage, the control unit 510 reduces the output current from I16 toward I10. Therefore, the output voltage and output current change in the direction from P16 toward P10. When the output voltage and output current reach Vp and I10, respectively, the changes in the output voltage and output voltage cease. In this way, when the discharge current increases by the amount corresponding to the increase in the load current, the DC bus voltage drops to the target DC bus voltage and stabilizes.

[0075] Next, with reference to FIG. 16 , the operation of the power conversion device when the power consumption increases and the device transitions from charging to discharging will be described. As the power consumption of the AC load increases, the load current output from the AC input / output terminal 202 increases. As the load current increases, the DC bus voltage decreases. On the other hand, when the control unit 210 detects a decrease in the DC bus voltage, it reduces the charging current and increases the DC bus voltage. Here, if the DC bus voltage does not reach the target DC bus voltage even when the charging current is set to 0 A, the control unit 210 seamlessly switches the power storage device 600 from a charging state to a discharging state. That is, instead of supplying a charging current to the power storage device 600, the control unit 210 seamlessly switches to acquiring a discharging current from the power storage device 600. The control unit 210 increases the discharging current until the DC bus voltage reaches the target DC bus voltage.

[0076] According to such control, even when the power consumption of the AC load 30 increases, the power storage device 600 can be seamlessly switched from a charging state to a discharging state, thereby making it possible to supply power to the AC load 30 appropriately.

[0077] Next, referring to FIG. 17, a description will be given of the output change of the DC / DC converter 500 when power consumption increases and the converter transitions from charging to discharging. In FIG. 17, the solid line graph shows the output characteristics before the power consumption increases, and the dashed line graph shows the output characteristics after the power consumption increases. When the charging current is constant, if the load current increases from Ia11 to Ia12 due to an increase in power consumption, the DC bus voltage decreases from Vp toward V17. As the output voltage decreases due to the decrease in the DC bus voltage, the control unit 510 increases the output current from I10 toward I17. Note that I17 = I10 - Ia11 + Ia12. As a result, the output voltage and output current change from P10 toward P17.

[0078] On the other hand, when the control unit 210 detects a drop in the DC bus voltage, it reduces the charging current and increases the DC bus voltage toward the target DC bus voltage. If the DC bus voltage is still below the target DC bus voltage even when the charging current is set to 0 A, the control unit 210 switches the power storage device 600 from the charging state to the discharging state. Thereafter, the control unit 210 increases the discharging current until the DC bus voltage reaches the target DC bus voltage. Meanwhile, the control unit 510 decreases the output current from I17 toward I10 as the output voltage increases with the rise in the DC bus voltage. Therefore, the output voltage and output current change from P17 toward P10. When the output voltage and output current reach Vp and I10, the changes in the output voltage and output voltage cease. In this way, as the charging current decreases and the discharging current increases by the amount corresponding to the increase in the load current, the DC bus voltage decreases to the target DC bus voltage and stabilizes.

[0079] In this embodiment, the bidirectional DC / DC converter 300 includes IGBTs 311 and 312 that adjust the direction and amount of current flowing between the power storage device 600 and the DC bus 220. The control unit 210 controls the IGBTs 311 and 312 so that, when switching from a discharging operation to a charging operation, the current from the power storage device 600 to the DC bus 220 decreases and then continuously increases the current from the DC bus 220 to the power storage device 600, and so that, when switching from a charging operation to a discharging operation, the current from the DC bus 220 to the power storage device 600 decreases and then continuously increases the current from the power storage device 600 to the DC bus 220. Therefore, according to this embodiment, there is no time lag when switching between charging and discharging, and the DC bus voltage can be smoothly brought close to the target DC bus voltage.

[0080] Next, referring to FIG. 18, a description will be given of the output change of the DC / DC converter 500 when power consumption increases and the converter transitions from charging to discharging. In FIG. 17, the solid line graph shows the output characteristics before the power consumption increases, and the dashed line graph shows the output characteristics after the power consumption increases. When the charging current is constant, if the load current increases from Ia11 to Ia12 due to an increase in power consumption, the DC bus voltage decreases from Vp toward V17. As the output voltage decreases due to the decrease in the DC bus voltage, the control unit 510 increases the output current from I10 toward I17. Note that I17 = I10 - Ia11 + Ia12. As a result, the output voltage and output current change from P10 toward P17.

[0081] On the other hand, when the control unit 210 detects a drop in the DC bus voltage, it reduces the charging current and increases the DC bus voltage toward the target DC bus voltage. If the DC bus voltage is still below the target DC bus voltage even when the charging current is set to 0 A, the control unit 210 switches the power storage device 600 from the charging state to the discharging state. Thereafter, the control unit 210 increases the discharging current until the DC bus voltage reaches the target DC bus voltage. Meanwhile, the control unit 510 decreases the output current from I17 toward I10 as the output voltage increases with the rise in the DC bus voltage. Therefore, the output voltage and output current change from P17 toward P10. When the output voltage and output current reach Vp and I10, the changes in the output voltage and output voltage cease. In this way, as the charging current decreases and the discharging current increases by the amount corresponding to the increase in the load current, the DC bus voltage decreases to the target DC bus voltage and stabilizes.

[0082] Below, with reference to Fig. 18, we will explain the change in output of DC / DC converter 500 when the generated power decreases and the system transitions from charging to discharging. In Fig. 18, the solid line graph shows the output characteristics before the generated power decreases, and the dashed line graph shows the output characteristics after the generated power decreases. Ia13 is a load current that is smaller than I10 and larger than I12. When the generated power decreases, the output characteristics shift in the direction of decreasing output current.

[0083] Here, for example, when maintaining the output current at I10, the control unit 510 reduces the output voltage from Vp toward V11. On the other hand, when the control unit 210 detects a decrease in the DC bus voltage due to a decrease in the output voltage, it increases the discharge current and raises the DC bus voltage toward the target DC bus voltage. If the DC bus voltage is still lower than the target DC bus voltage even when the charging current is set to 0 A, the control unit 210 switches the power storage device 600 from a charging state to a discharging state. Thereafter, the control unit 210 increases the discharge current until the DC bus voltage reaches the target DC bus voltage. As the output voltage increases with the increase in the DC bus voltage, the control unit 510 reduces the output current from I10 toward I12. When the output voltage reaches Vp and the output current reaches I12, the output voltage and the change in the output voltage cease. In this way, the charging current decreases and the discharging current increases by the amount corresponding to the decrease in the output current, and the DC bus voltage rises to the target DC bus voltage and stabilizes.

[0084] Next, referring to Fig. 19, a description will be given of the output change of DC / DC converter 500 when the generated power increases and the converter transitions from discharging to charging. In Fig. 19, the solid line graph shows the output characteristics before the generated power increases, and the dashed line graph shows the output characteristics after the generated power increases. Ia14 is a load current that is smaller than I10 and larger than I14. As the generated power increases, the output characteristics shift in the direction in which the output current increases. Here, for example, when maintaining the output current at I10, control unit 510 increases the output voltage from Vp toward V13.

[0085] When the control unit 210 detects an increase in the DC bus voltage due to an increase in the output voltage, it reduces the discharge current and decreases the DC bus voltage toward the target DC bus voltage. If the DC bus voltage exceeds the target DC bus voltage even when the discharge current is set to 0 A, the control unit 210 switches the power storage device 600 from a discharging state to a charging state. Thereafter, the control unit 210 increases the charging current until the DC bus voltage reaches the target DC bus voltage. As the output voltage decreases due to the decrease in the DC bus voltage, the control unit 510 increases the output current from I10 toward I14. As a result, the output voltage and output current change from P13 toward P14. When the output voltage reaches Vp and the output current reaches I14, the changes in the output voltage and output voltage cease. In this way, as the discharge current decreases and the charging current increases by the amount corresponding to the increase in the output current, the DC bus voltage decreases to the target DC bus voltage and stabilizes.

[0086] Next, referring to FIG. 20, the output change of the DC / DC converter 500 when the power consumption decreases and the converter transitions from discharging to charging will be described. In FIG. 20, the solid line graph shows the output characteristics before the power consumption decreases, and the dashed line graph shows the output characteristics after the power consumption decreases. Ia15 is a load current greater than I10. Ia16 is a load current smaller than I10. When the discharge current is constant, if the load current decreases from Ia15 to Ia16 due to a decrease in power consumption, the DC bus voltage increases from Vp toward V15. As the output voltage increases due to the increase in the DC bus voltage, the control unit 510 decreases the output current from I10 toward I15. Note that I15 = I10 - Ia15 + Ia16. As a result, the output voltage and output current change from P10 toward P15.

[0087] On the other hand, when the control unit 210 detects an increase in the DC bus voltage, it reduces the discharge current and decreases the DC bus voltage toward the target DC bus voltage. If the DC bus voltage exceeds the target DC bus voltage even when the discharge current is set to 0 A, the control unit 210 switches the power storage device 600 from a discharging state to a charging state. Thereafter, the control unit 210 increases the charging current until the DC bus voltage reaches the target DC bus voltage. As the output voltage decreases due to the decrease in the DC bus voltage, the control unit 510 increases the output current from I15 toward I10. As a result, the output voltage and output current change from P15 toward P10. When the output voltage reaches Vp and the output current reaches I10, the changes in the output voltage and output voltage cease. In this way, as the discharge current decreases and the charging current increases by the amount corresponding to the decrease in the load current, the DC bus voltage decreases to the target DC bus voltage and stabilizes.

[0088] Next, referring to FIG. 21, a description will be given of the output change of the DC / DC converter 500 when the generated power increases during charging and AC input. During AC input, current is supplied from the AC system 20 to the DC bus 220 via the AC input / output terminal 202. In FIG. 21, the solid line graph shows the output characteristics before the generated power increases, and the dashed line graph shows the output characteristics after the generated power increases. Ia17 is a load current smaller than I10. Ic1 is a charging current larger than I14. Ic2 is a charging current larger than Ic1. As the generated power increases, the output characteristics shift in the direction of increasing the output current. Here, for example, when maintaining the output current at I10, the control unit 510 increases the output voltage from Vp toward V13.

[0089] When the control unit 210 detects an increase in the DC bus voltage due to an increase in the output voltage, it increases the charging current, thereby decreasing the DC bus voltage toward the target DC bus voltage. The control unit 510 increases the output current from I10 toward I14 in response to a decrease in the output voltage due to a decrease in the DC bus voltage. As a result, the output voltage and output current change in the direction from P13 toward P14. When the output voltage and output current reach Vp and I14, respectively, the changes in the output voltage and output voltage cease. In this way, when the charging current increases by the amount corresponding to the increase in the output current, the DC bus voltage decreases to the target DC bus voltage and stabilizes.

[0090] Below, with reference to Fig. 22, a description will be given of the change in output of DC / DC converter 500 when the generated power decreases during charging and AC input. In Fig. 22, the solid line graph shows the output characteristics before the generated power decreases, and the dashed line graph shows the output characteristics after the generated power decreases. Ia18 is a load current smaller than I12. Ic3 is a charging current larger than Ic4. Ic4 is a charging current larger than I10. When the generated power decreases, the output characteristics shift in the direction of decreasing output current.

[0091] Here, for example, when maintaining the output current at I10, the control unit 510 reduces the output voltage from Vp toward V11. On the other hand, when the control unit 210 detects a decrease in the DC bus voltage due to a decrease in the output voltage, it reduces the charging current and increases the DC bus voltage toward the target DC bus voltage. As the output voltage increases due to an increase in the DC bus voltage, the control unit 510 reduces the output current from I10 toward I12. Then, when the output voltage reaches Vp and the output current reaches I12, the output voltage and the change in the output voltage subside. In this way, when the charging current decreases by the amount of the decrease in the output current, the DC bus voltage increases to the target DC bus voltage and stabilizes.

[0092] As described above, in this embodiment, the control unit 210 controls the bidirectional DC / DC converter 300 so that the DC bus voltage is maintained at the DC target voltage, and when the output current of the DC / DC converter 500 exceeds a threshold value corresponding to the power obtainable from the solar cell 10, the control unit 510 controls the DC / DC converter 500 so that the output voltage of the DC / DC converter 500 decreases as the output current increases. In this embodiment, the output voltage of the DC / DC converter 500 does not change abruptly in response to a change in the output current of the DC / DC converter 500. Therefore, a change in the DC bus voltage accompanying this change in output voltage can be appropriately suppressed by adjusting the charging current or discharging current.

[0093] Therefore, according to this embodiment, the DC bus voltage can be stabilized without connecting a high-voltage, large-capacity capacitor to the DC bus 220. In other words, according to this embodiment, appropriate power storage can be achieved while suppressing an increase in size and cost of the power storage system 1000.

[0094] Furthermore, in this embodiment, the control unit 210 can grasp the output state of the DC / DC converter 500 by detecting the DC bus voltage and can adjust the charging current or discharging current according to this output state. That is, in this embodiment, cooperation between the bidirectional DC / DC converter 300 and the DC / DC converter 500 can be achieved simply by fluctuating the DC bus voltage and detecting this fluctuation. To achieve such cooperation, a configuration that enables communication between the control unit 210 and the control unit 510 or another control unit that can communicate with both the control unit 210 and the control unit 510 can be provided. However, this method requires time for communication, which may result in a decrease in the utilization rate of the generated power. Furthermore, this method requires communication lines, communication cables, etc., which may result in a complex system configuration and reduced portability.

[0095] Furthermore, in this embodiment, when the output current exceeds a threshold value, the DC / DC converter 500 is controlled so that the product of the output current and the output voltage remains substantially constant. Therefore, according to this embodiment, a decrease in the utilization rate of the generated power is suppressed.

[0096] Furthermore, in this embodiment, a switch element is provided that adjusts the direction and amount of current flowing between the power storage device 600 and the DC bus 220, and control of this switch element allows smooth switching between charging and discharging. Therefore, according to this embodiment, there is no time lag when switching between charging and discharging, and further stabilization of the DC bus voltage can be expected. Note that, as an alternative to the method of this embodiment, there is a method of controlling charging and discharging using a unidirectional DC / DC converter (not shown), a charger (not shown), and a discharger (not shown) instead of the bidirectional DC / DC converter 300. However, with this method, switching between the charger and the discharger takes time, making it difficult to stabilize the DC bus voltage.

[0097] Furthermore, in this embodiment, the DC / DC converter 500 can be electrically separated from the bidirectional DC / DC converter 300 and the bidirectional AC / DC converter 400. Therefore, according to this embodiment, it is easy to ensure the portability of the bidirectional DC / DC converter 300 and the bidirectional AC / DC converter 400.

[0098] Moreover, in this embodiment, the bidirectional DC / DC converter 300, the bidirectional AC / DC converter 400, and the power storage device 600 are housed in the case 700. Therefore, according to this embodiment, the portability of the bidirectional DC / DC converter 300, the bidirectional AC / DC converter 400, and the power storage device 600 is improved.

[0099] Furthermore, in this embodiment, wheels 704 are provided on the case 700. Therefore, according to this embodiment, the portability of the bidirectional DC / DC converter 300, the bidirectional AC / DC converter 400, and the power storage device 600 is further improved.

[0100] (Embodiment 2) In the first embodiment, an example has been described in which the bidirectional DC / DC converter 300, the bidirectional AC / DC converter 400, and the power storage device 600 are housed in the case 700. In the present embodiment, an example will be described in which the bidirectional DC / DC converter 300 and the bidirectional AC / DC converter 400 are housed in the case 700. Note that descriptions of the same configurations and processes as those in the first embodiment will be omitted or simplified.

[0101] 23 , a power storage system 1100 according to this embodiment includes a case 720 that houses a bidirectional DC / DC converter 300 and a bidirectional AC / DC converter 400. In this embodiment, a power storage device 600 is provided outside the case 720. The case 720 is provided with a DC input terminal 203 connected to a DC bus 220, an AC input / output terminal 202 connected to a terminal 402, and a DC input / output terminal 201 connected to a terminal 302.

[0102] DC input terminal 203 is provided in case 720 as, for example, a conductor included in plug 702 in FIG. 2. In this case, a conductor included in a power outlet (not shown) into which plug 702 is plugged corresponds to DC output terminal 501. Furthermore, AC input / output terminal 202 is provided in case 720 as, for example, a conductor included in plug 701 in FIG. 2. In this case, a conductor included in a power outlet (not shown) into which plug 701 is plugged corresponds to terminal 21. Alternatively, AC input / output terminal 202 is provided in case 720 as, for example, a conductor included in power outlet 703 in FIG. 2. In this case, a conductor included in a plug (not shown) into which power outlet 703 is plugged corresponds to terminal 31. DC input / output terminal 201 is provided in case 720 as, for example, a conductor included in a plug (not shown). In this case, a conductor included in a power outlet (not shown) into which this plug (not shown) is plugged corresponds to terminal 601.

[0103] In this embodiment, the bidirectional DC / DC converter 300 and the bidirectional AC / DC converter 400 are housed in the case 700. Therefore, according to this embodiment, the portability of the bidirectional DC / DC converter 300 and the bidirectional AC / DC converter 400 is improved.

[0104] (Variation) Although the embodiments of the present disclosure have been described above, various modifications and applications are possible when implementing the present disclosure. It is optional which parts of the configurations, functions, and operations described in the above embodiments are adopted in the present disclosure. Furthermore, in addition to the above-described configurations, functions, and operations, additional configurations, functions, and operations may also be adopted in the present disclosure. Furthermore, the above embodiments can be freely combined as appropriate. Furthermore, the number of components described in the above embodiments can be adjusted as appropriate. Furthermore, it goes without saying that the materials, sizes, electrical characteristics, and the like that can be adopted in the present disclosure are not limited to those shown in the above embodiments.

[0105] In the first embodiment, an example has been described in which, when the output current of the DC / DC converter 500 exceeds a threshold value corresponding to the power obtainable from the solar cell 10, the output voltage of the DC / DC converter 500 decreases in proportion to the increase in the output current. The method of decreasing the output voltage as the output current increases is not limited to this example. For example, the output voltage may be decreased as the output current increases so that the product of the output current and the output voltage remains constant. This configuration further increases the utilization rate of the power generated by the solar cell 10.

[0106] In the first embodiment, an example in which the target DC bus voltage is about 400 V has been described. However, various voltages can be used as the target DC bus voltage. For example, the target DC bus voltage Vp may be a voltage several tens of percent lower than Vh, which is set lower than 40 V. In this case, the voltage applied to the DC input terminal 203 is lower than 40 V. Therefore, a user without special qualifications can freely connect or disconnect the bidirectional inverter 200 and the DC / DC converter 500.

[0107] In the first embodiment, an example has been described in which the solar cell 10 is used as the power generating device. The power generating device may be a power generating device that generates electricity using various types of renewable energy such as sunlight, solar heat, hydraulic power, wind power, and geothermal power, or a fuel cell.

[0108] In the first embodiment, an example has been described in which control unit 210 and control unit 510 each include a microcomputer. Control unit 210 or control unit 510 does not necessarily have to include a microcomputer. For example, at least one of control unit 210 and control unit 510 may include an analog component such as an operational amplifier instead of a microcomputer. In the first embodiment, an example has been described in which IGBTs are used as switch elements. Other switch elements such as FETs (Field Effect Transistors) may also be used as switch elements.

[0109] The present disclosure allows various embodiments and modifications without departing from the broad spirit and scope of the present disclosure. Furthermore, the above-described embodiments are intended to explain the present disclosure and do not limit the scope of the present disclosure. That is, the scope of the present disclosure is defined by the claims, not the embodiments. Various modifications made within the scope of the claims and the meaning of equivalent disclosures are considered to be within the scope of the present disclosure. [Explanation of symbols]

[0110] 10. Solar Cells 20 AC system 21,31,204,206,301,302,401,402,502,503,504,601 terminals 30 AC load 100 Power conversion device 200 Bidirectional Inverter 201 DC input / output terminal 202,205 AC input / output terminal 203 DC input terminal 210,510 Control unit 211,213 adder 212 Constant voltage control section 214 Constant current control section 220 DC Bus 300 Bidirectional DC / DC Converter 311,312,411,412,413,414 IGBT 321,322,421,422,423,424,531,532,533,534,541,542,543,544,545,546 Diodes 330,430,551,552,553,554 inductors 340,440,561,562,563,564,565,566 capacitors 400 Bidirectional AC / DC Converter 500 DC / DC Converter 501 DC output terminal 511 Boost Converter 512 LLC Converter 521,522,523,524 N-channel MOSFET 555 transformer 600 Electricity storage device 700,720 cases 701,702 Plug 703 Outlet 704 wheels 1000,1100 Energy storage system

Claims

1. a DC / DC converter that converts the voltage of DC power from the power generation device and supplies the converted power to the DC bus; a bidirectional AC / DC converter including a first input / output terminal connected to the DC bus and a second input / output terminal connected to at least one of an AC system and an AC load; a bidirectional DC / DC converter including a third input / output terminal connected to the DC bus and a fourth input / output terminal connected to a power storage device; a first control unit that controls the bidirectional DC / DC converter to perform a discharging operation of converting a voltage of DC power from the power storage device and supplying the converted voltage to the DC bus, a charging operation of converting a voltage of DC power from the DC bus and supplying the converted voltage to the power storage device, and a voltage maintaining operation of maintaining a voltage of the DC bus at a predetermined first voltage, the DC / DC converter includes a second control unit that controls the DC / DC converter so that an output voltage of the DC / DC converter maintains a second voltage higher than the first voltage when an output current of the DC / DC converter does not exceed a threshold value corresponding to power obtainable from the power generation device, and that controls the DC / DC converter so that the output voltage decreases as the output current increases when the output current exceeds the threshold value. Power conversion device.

2. the second control unit controls the DC / DC converter so that, when the output current exceeds the threshold value, the product of the output current and the output voltage becomes substantially constant and the ratio of the decrease in the output voltage to the increase in the output current becomes constant. The power conversion device according to claim 1 .

3. the bidirectional DC / DC converter includes at least one switch element that adjusts the direction and amount of current flowing between the power storage device and the DC bus; the first control unit controls the at least one switch element so that, when switching from the discharging operation to the charging operation, the current from the power storage device to the DC bus decreases and then the current from the DC bus to the power storage device increases continuously, and so that, when switching from the charging operation to the discharging operation, the current from the DC bus to the power storage device decreases and then the current from the power storage device to the DC bus increases continuously. The power conversion device according to claim 1 or 2.

4. a DC input terminal connected to the DC bus; the DC / DC converter has a DC output terminal connectable to the DC input terminal, the DC / DC converter is electrically separable from the bidirectional AC / DC converter and the bidirectional DC / DC converter; The power conversion device according to any one of claims 1 to 3.

5. The power conversion device according to claim 4; the power storage device connected to the fourth input / output terminal; a power storage system including a case that houses the bidirectional AC / DC converter, the bidirectional DC / DC converter, and the power storage device, The case is provided with the DC input terminal and an AC input / output terminal connected to the second input / output terminal. Energy storage system.

6. The power conversion device according to claim 4; a power storage device connected to a DC input / output terminal connected to the fourth input / output terminal; a case that houses the bidirectional AC / DC converter and the bidirectional DC / DC converter, the case is provided with the DC input terminal, an AC input / output terminal connected to the second input / output terminal, and the DC input / output terminal; Energy storage system.

7. The case is provided with wheels. The power storage system according to claim 5 or 6.

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